Aerogeophysical evidence for active volcanism beneath the West Antarctic Ice Sheet

NASA Technical Reports Server (NTRS)

Blankenship, Donald D.; Bell, Robin E.; Hodge, Steven M.; Brozena, John M.; Behrendt, John C.

1993-01-01

Although it is widely understood that the collapse of the West Antarctic Ice Sheet (WAIS) would cause a global sea-level rise of 6 m, there continues to be considerable debate about the response of this ice sheet to climate change. The stability of the WAIS, which is characterized by a bed grounded well below sea level, may depend on geologically controlled conditions at the base, which are independent of climate. Ice streams moving up to 750 m/yr disperse material from the interior through to the oceans. As these ice streams tend to buffer the reservoir of slow-moving inland ice from exposure to oceanic degradation, understanding the ice-streaming process is important for evaluating WAIS stability. There is strong evidence that ice streams slide on a lubricating layer of water-saturated till. Development of this basal layer requires both water and easily eroded sediments. Active lithospheric extension may elevate regional heat flux, increase basal melting, and trigger ice streaming. If a geologically defined boundary with a sharp contrast in geothermal flux exists beneath the WAIS, ice streams may only be capable of operating as a buffer over a restricted region. Should ocean waters penetrate beyond this boundary, the ice-stream buffer would disappear, possibly triggering a collapse of the inland ice reservoir. Aerogeophysical evidence for active volcanism and elevated heat flux beneath the WAIS near the critical region where ice streaming begins is presented.

Balance of the West Antarctic Ice Sheet

NASA Technical Reports Server (NTRS)

2002-01-01

For several decades, measurements of the West Antarctic Ice Sheet showed it to be retreating rapidly. But new data derived from satellite-borne radar sensors show the ice sheet to be growing. Changing Antarctic ice sheets remains an area of high scientific interest, particularly in light of recent global warming concerns. These new findings are significant because scientists estimate that sea level would rise 5-6 meters (16-20 feet) if the ice sheet collapsed into the sea. Do these new measurements signal the end of the ice sheet's 10,000-year retreat? Or, are these new satellite data simply much more accurate than the sparse ice core and surface measurements that produced the previous estimates? Another possibility is that the ice accumulation may simply indicate that the ice sheet naturally expands and retreats in regular cycles. Cryologists will grapple with these questions, and many others, as they examine the new data. The image above depicts the region of West Antarctica where scientists measured ice speed. The fast-moving central ice streams are shown in red. Slower tributaries feeding the ice streams are shown in blue. Green areas depict slow-moving, stable areas. Thick black lines depict the areas that collect snowfall to feed their respective ice streams. Reference: Ian Joughin and Slawek Tulaczyk Science Jan 18 2002: 476-480. Image courtesy RADARSAT Antarctic Mapping Project

Ice Streams as the Critical Link Between the Interior Ice Reservoir of the Antarctic Ice Sheet and the Global Climate System - a WISSARD Perspective (Invited)

NASA Astrophysics Data System (ADS)

Tulaczyk, S. M.; Beem, L.; Walter, J. I.; Hossainzadeh, S.; Mankoff, K. D.

2010-12-01

Fast flowing ice streams represent crucial features of the Antarctic ice sheet because they provide discharge ‘valves’ for the interior ice reservoir and because their grounding lines are exposed to ocean thermal forcing. Even with no/little topographic control ice flow near the perimeter of a polar ice sheet self-organizes into discrete, fast-flowing ice streams. Within these features basal melting (i.e. lubrication for ice sliding) is sustained through elevated basal shear heating in a region of thin ice that would otherwise be characterized by basal freezing and slow ice motion. Because faster basal ice motion is typically associated with faster subglacial erosion, ice streams tend to localize themselves over time by carving troughs into underlying rocks and sediments. Debris generated by this erosional activity is carried to the continental shelf and/or continental slope where it may be deposited at very high rates, rivaling these associated with deposition by some of the largest rivers on Earth. In terms of their hydrologic and geological functions, Antarctic ice streams play pretty much the same role as rivers do on non-glaciated continents. However, understanding of their dynamics is still quite rudimentary, largely because of the relative inaccessibility of the key basal and marine boundaries of ice streams where pertinent measurements need to be made. The present elevated interest in predicting future contribution of Antarctica to global sea level changes is driving ambitious research programs aimed at scientific exploration of these poorly investigated environments that will play a key role in defining the response of the ice sheet to near future climate changes. We will review one of these programs, the Whillans Ice Stream Subglacial Access Research Drilling (WISSARD) with particular focus on its planned contributions to understanding of ice stream dynamics.

Reconstructing the last Irish Ice Sheet 2: a geomorphologically-driven model of ice sheet growth, retreat and dynamics

NASA Astrophysics Data System (ADS)

Greenwood, Sarah L.; Clark, Chris D.

2009-12-01

The ice sheet that once covered Ireland has a long history of investigation. Much prior work focussed on localised evidence-based reconstructions and ice-marginal dynamics and chronologies, with less attention paid to an ice sheet wide view of the first order properties of the ice sheet: centres of mass, ice divide structure, ice flow geometry and behaviour and changes thereof. In this paper we focus on the latter aspect and use our new, countrywide glacial geomorphological mapping of the Irish landscape (>39 000 landforms), and our analysis of the palaeo-glaciological significance of observed landform assemblages (article Part 1), to build an ice sheet reconstruction yielding these fundamental ice sheet properties. We present a seven stage model of ice sheet evolution, from initiation to demise, in the form of palaeo-geographic maps. An early incursion of ice from Scotland likely coalesced with local ice caps and spread in a south-westerly direction 200 km across Ireland. A semi-independent Irish Ice Sheet was then established during ice sheet growth, with a branching ice divide structure whose main axis migrated up to 140 km from the west coast towards the east. Ice stream systems converging on Donegal Bay in the west and funnelling through the North Channel and Irish Sea Basin in the east emerge as major flow components of the maximum stages of glaciation. Ice cover is reconstructed as extending to the continental shelf break. The Irish Ice Sheet became autonomous (i.e. separate from the British Ice Sheet) during deglaciation and fragmented into multiple ice masses, each decaying towards the west. Final sites of demise were likely over the mountains of Donegal, Leitrim and Connemara. Patterns of growth and decay of the ice sheet are shown to be radically different: asynchronous and asymmetric in both spatial and temporal domains. We implicate collapse of the ice stream system in the North Channel - Irish Sea Basin in driving such asymmetry, since rapid collapse would sever the ties between the British and Irish Ice Sheets and drive flow configuration changes in response. Enhanced calving and flow acceleration in response to rising relative sea level is speculated to have undermined the integrity of the ice stream system, precipitating its collapse and driving the reconstructed pattern of ice sheet evolution.

Palaeo-ice stream pathways in the easternmost Amundsen Sea Embayment, West Antarctica

NASA Astrophysics Data System (ADS)

Klages, Johann P.; Kuhn, Gerhard; Graham, Alastair G. C.; Smith, James A.; Hillenbrand, Claus-Dieter; Nitsche, Frank O.; Larter, Rob D.; Gohl, Karsten

2015-04-01

Multibeam swath bathymetry datasets collected over the past two decades have been compiled to identify palaeo-ice stream pathways in the easternmost Amundsen Sea Embayment. We mapped 3010 glacial landforms to reconstruct palaeo-ice flow in the ~250 km-long Abbot Glacial Trough that was occupied by a large palaeo-ice stream, fed by two tributaries (Cosgrove and Abbot) that reached the continental shelf edge during the last maximum ice-sheet advance. The mapping has enabled a clear differentiation between glacial landforms interpreted as indicative of wet- (e.g. mega-scale glacial lineations) and cold-based ice (e.g. hill-hole pairs) during the last glaciation of the continental shelf. Both the regions of fast palaeo-ice flow within the palaeo-ice stream troughs, and the regions of slow palaeo-ice flow on adjacent seafloor highs (referred to as inter-ice stream ridges) additionally record glacial landforms such as grounding-zone wedges and recessional moraines that indicate grounding line stillstands of the ice sheet during the last deglaciation from the shelf. As the palaeo-ice stream flowed along a trough with variable geometry and variable subglacial substrate, it appears that trough sections characterized by constrictions and outcropping hard substrate that changes the bed gradient, led the pace of grounding-line retreat to slow and subsequently pause, resulting in the deposition of grounding-zone wedges. The stepped retreat recorded within the Abbot Glacial Trough corresponds well to post-glacial stepped retreat interpreted for the neighbouring Pine Island-Thwaites Palaeo-Ice Stream trough, thus suggesting a uniform pattern of episodic retreat across the eastern Amundsen Sea Embayment. The correlation of episodic retreat features with geological boundaries further emphasises the significance of subglacial geology in steering ice stream flow. Our new geomorphological map of the easternmost Amundsen Sea Embayment resolves the pathways of palaeo-ice streams that were probably all active during the last maximum extent of the ice sheet on this part of the shelf, and reveals the style of postglacial grounding-line retreat. Both are important input variables in ice sheet models and therefore can be used for validating the reliability of these models.

From cyclic ice streaming to Heinrich-like events: the grow-and-surge instability in the Parallel Ice Sheet Model

NASA Astrophysics Data System (ADS)

Feldmann, Johannes; Levermann, Anders

2017-08-01

Here we report on a cyclic, physical ice-discharge instability in the Parallel Ice Sheet Model, simulating the flow of a three-dimensional, inherently buttressed ice-sheet-shelf system which periodically surges on a millennial timescale. The thermomechanically coupled model on 1 km horizontal resolution includes an enthalpy-based formulation of the thermodynamics, a nonlinear stress-balance-based sliding law and a very simple subglacial hydrology. The simulated unforced surging is characterized by rapid ice streaming through a bed trough, resulting in abrupt discharge of ice across the grounding line which is eventually calved into the ocean. We visualize the central feedbacks that dominate the subsequent phases of ice buildup, surge and stabilization which emerge from the interaction between ice dynamics, thermodynamics and the subglacial till layer. Results from the variation of surface mass balance and basal roughness suggest that ice sheets of medium thickness may be more susceptible to surging than relatively thin or thick ones for which the surge feedback loop is damped. We also investigate the influence of different basal sliding laws (ranging from purely plastic to nonlinear to linear) on possible surging. The presented mechanisms underlying our simulations of self-maintained, periodic ice growth and destabilization may play a role in large-scale ice-sheet surging, such as the surging of the Laurentide Ice Sheet, which is associated with Heinrich events, and ice-stream shutdown and reactivation, such as observed in the Siple Coast region of West Antarctica.

Subglacial hydrology and the formation of ice streams

PubMed Central

Kyrke-Smith, T. M; Katz, R. F; Fowler, A. C

2014-01-01

Antarctic ice streams are associated with pressurized subglacial meltwater but the role this water plays in the dynamics of the streams is not known. To address this, we present a model of subglacial water flow below ice sheets, and particularly below ice streams. The base-level flow is fed by subglacial melting and is presumed to take the form of a rough-bedded film, in which the ice is supported by larger clasts, but there is a millimetric water film which submerges the smaller particles. A model for the film is given by two coupled partial differential equations, representing mass conservation of water and ice closure. We assume that there is no sediment transport and solve for water film depth and effective pressure. This is coupled to a vertically integrated, higher order model for ice-sheet dynamics. If there is a sufficiently small amount of meltwater produced (e.g. if ice flux is low), the distributed film and ice sheet are stable, whereas for larger amounts of melt the ice–water system can become unstable, and ice streams form spontaneously as a consequence. We show that this can be explained in terms of a multi-valued sliding law, which arises from a simplified, one-dimensional analysis of the coupled model. PMID:24399921

Ice-sheet dynamics through the Quaternary on the mid-Norwegian continental margin inferred from 3D seismic data.

PubMed

Montelli, A; Dowdeswell, J A; Ottesen, D; Johansen, S E

2017-02-01

Reconstructing the evolution of ice sheets is critical to our understanding of the global environmental system, but most detailed palaeo-glaciological reconstructions have hitherto focused on the very recent history of ice sheets. Here, we present a three-dimensional (3D) reconstruction of the changing nature of ice-sheet derived sedimentary architecture through the Quaternary Ice Age of almost 3 Ma. An extensive geophysical record documents a marine-terminating, calving Fennoscandian Ice Sheet (FIS) margin present periodically on the mid-Norwegian shelf since the beginning of the Quaternary. Spatial and temporal variability of the FIS is illustrated by the gradual development of fast-flowing ice streams and associated intensification of focused glacial erosion and sedimentation since that time. Buried subglacial landforms reveal a complex and dynamic ice sheet, with converging palaeo-ice streams and several flow-switching events that may reflect major changes in topography and basal thermal regime. Lack of major subglacial meltwater channels suggests a largely distributed drainage system beneath the marine-terminating part of the FIS. This palaeo-environmental examination of the FIS provides a useful framework for ice-sheet modelling and shows that fragmentary preservation of buried surfaces and variability of ice-sheet dynamics should be taken into account when reconstructing glacial history from spatially limited datasets.

Improved age constraints for the retreat of the Irish Sea Ice Stream

NASA Astrophysics Data System (ADS)

Smedley, Rachel; Chiverrell, Richard; Duller, Geoff; Scourse, James; Small, David; Fabel, Derek; Burke, Matthew; Clarke, Chris; McCarroll, Danny; McCarron, Stephen; O'Cofaigh, Colm; Roberts, David

2016-04-01

BRITICE-CHRONO is a large (> 45 researchers) consortium project working to provide an extensive geochronological dataset constraining the rate of retreat of a number of ice streams of the British-Irish Ice Sheet following the Last Glacial Maximum. When complete, the large empirical dataset produced by BRITICE-CHRONO will be integrated into model simulations to better understand the behaviour of the British-Irish Ice Sheet in response to past climate change, and provide an analogue for contemporary ice sheets. A major feature of the British-Irish Ice Sheet was the dynamic Irish Sea Ice Stream, which drained a large proportion of the ice sheet and extended to the proposed southern limit of glaciation upon the Isles of Scilly (Scourse, 1991). This study will focus on a large suite of terrestrial samples that were collected along a transect of the Irish Sea basin, covering the line of ice retreat from the Isles of Scilly (50°N) in the south, to the Isle of Man (54°N) in the north; a distance of 500 km. Ages are determined for both the eastern and western margins of the Irish Sea using single-grain luminescence dating (39 samples) and terrestrial cosmogenic nuclide dating (10 samples). A Bayesian sequence model is then used in combination with the prior information determined for deglaciation to integrate the geochronological datasets, and assess retreat rates for the Irish Sea Ice Stream. Scourse, J.D., 1991. Late Pleistocene stratigraphy and palaeobotany of the Isles of Scilly. Philosophical Transactions of the Royal Society of London B334, 405 - 448.

Potential subglacial lake locations and meltwater drainage pathways beneath the Antarctic and Greenland ice sheets

NASA Astrophysics Data System (ADS)

Livingstone, S. J.; Clark, C. D.; Woodward, J.; Kingslake, J.

2013-11-01

We use the Shreve hydraulic potential equation as a simplified approach to investigate potential subglacial lake locations and meltwater drainage pathways beneath the Antarctic and Greenland ice sheets. We validate the method by demonstrating its ability to recall the locations of >60% of the known subglacial lakes beneath the Antarctic Ice Sheet. This is despite uncertainty in the ice-sheet bed elevation and our simplified modelling approach. However, we predict many more lakes than are observed. Hence we suggest that thousands of subglacial lakes remain to be found. Applying our technique to the Greenland Ice Sheet, where very few subglacial lakes have so far been observed, recalls 1607 potential lake locations, covering 1.2% of the bed. Our results will therefore provide suitable targets for geophysical surveys aimed at identifying lakes beneath Greenland. We also apply the technique to modelled past ice-sheet configurations and find that during deglaciation both ice sheets likely had more subglacial lakes at their beds. These lakes, inherited from past ice-sheet configurations, would not form under current surface conditions, but are able to persist, suggesting a retreating ice-sheet will have many more subglacial lakes than advancing ones. We also investigate subglacial drainage pathways of the present-day and former Greenland and Antarctic ice sheets. Key sectors of the ice sheets, such as the Siple Coast (Antarctica) and NE Greenland Ice Stream system, are suggested to have been susceptible to subglacial drainage switching. We discuss how our results impact our understanding of meltwater drainage, basal lubrication and ice-stream formation.

Using Sediment Provenance to Study Ice Streams in the Weddell Sea Embayment of Antarctica

NASA Astrophysics Data System (ADS)

Hemming, S. R.; Williams, T.; Boswell, S.; Licht, K.; Agrios, L.; Brachfeld, S. A.; van de Flierdt, T.; Kuhn, G.; Hillenbrand, C. D.; Zhai, X.

2016-12-01

The geochemical and geochronological fingerprint of rock debris eroded and carried by ice streams may be used to identify the provenance of iceberg-rafted debris (IRD) in the marine sediment record. During deglacial times it has been shown that there is an increase in IRD accumulation in marine sediments underlying the western limb of the Weddell Gyre. We seek to find the provenance of this IRD, identify the ice streams contributing to the IRD load, and interpret the geographic sequence of ice sheet retreat in the Weddell Sea embayment for the last three deglaciations. In December 2014 we conducted fieldwork to collect samples of rock and sediment debris carried by three of the major ice streams draining the Weddell Sea embayment: the Foundation Ice Stream, the Academy Glacier, and the Recovery Glacier. We sampled both modern moraines at the edges of the ice streams and older till on hillsides next to the ice streams. In addition to rocks representing the geology of local outcrops, we found that each of the three ice streams carries a characteristic set of erratic lithologies from further upstream, giving clues to the geology hidden under the ice sheet. Downstream, subglacial till and proximal glaciomarine sediment from existing core sites located at the edge of the Filchner and Ronne Ice Shelves, collected on past expeditions of the RV Polarstern, characterize the geochemical and geochronological fingerprint along ice flow lines extending from the ice streams. Finally, two deep-water RV Polarstern sites contain a continuous record of IRD sourced from the set of Weddell embayment ice streams over the last few glacial cycles. Here we present new 40Ar/39Ar hornblende and biotite thermochronological data from individual mineral grains, K-Ar from the silt fraction, and U-Pb zircon geochronology from the onshore tills and offshore sediments. Using this data we will discuss provenance matching between the IRD and the ice streams, and the possibilities for using provenance to understand ice sheet dynamics over the course of glacial cycles.

Ice streams of the Late Wisconsin Cordilleran Ice Sheet in western North America

NASA Astrophysics Data System (ADS)

Eyles, Nick; Arbelaez Moreno, Lina; Sookhan, Shane

2018-01-01

The Late Wisconsin Cordilleran Ice Sheet (CIS) of western North America is thought to have reached its maximum extent (∼2.5 × 106 km2) as late at c. 14.5 ka. Most (80%) of the ice sheet's bed consists of high mountains but its 'core zone' sited on plateaux of the Intermontane Belt of British Columbia and coterminous parts of the USA, shows broad swaths of subglacially-streamlined rock and sediment. Broad scale mapping from new digital imagery data identifies three subglacial bed types: 1) 'hard beds' of variably streamlined bedrock; 2) drumlinized 'soft beds' of deformation till reworked from antecedent sediment, and 3) 'mixed beds' of variably-streamlined bedrock protruding through drumlinized sediment. Drumlins on soft beds appear to be erosional features cut into till and antecedent sediments, and identify the catchment areas of paleo ice streams expressed downglacier as flow sets of megascale glacial lineations (MSGLs). 'Grooved' and 'cloned' drumlins appear to record the transition from drumlins to MSGLs. The location of paleo ice streams reflects topographic funneling of ice from plateau surfaces through outlet valleys and a soft bed that sustained fast flow; rock-cut MSGLs are also present locally on the floors of outlet valleys. CIS disintegrated in <1000 years shortly after c. 13.0 ka releasing very large volumes of meltwater and sediment to the Pacific coast. Abrupt deglaciation may reflect unsustainable calving of marine-based ice streams along the glacio-isostatically depressed coast; large deep 'fiord lakes' in the ice sheet's interior may have played an analogous role. Mapping of the broad scale distribution of bed types across the Cordilleran Ice Sheet provides key information for paleoglaciological modelling and also for understanding the beds of modern ice masses such as the Greenland Ice Sheet which is of a comparable topographic setting.

Bed roughness of palaeo-ice streams: insights and implications for contemporary ice sheet dynamics

NASA Astrophysics Data System (ADS)

Falcini, Francesca; Rippin, David; Selby, Katherine; Krabbendam, Maarten

2017-04-01

Bed roughness is the vertical variation of elevation along a horizontal transect. It is an important control on ice stream location and dynamics, with a correspondingly important role in determining the behaviour of ice sheets. Previous studies of bed roughness have been limited to insights derived from Radio Echo Sounding (RES) profiles across parts of Antarctica and Greenland. Such an approach has been necessary due to the inaccessibility of the underlying bed. This approach has led to important insights, such as identifying a general link between smooth beds and fast ice flow, as well as rough beds and slow ice flow. However, these insights are mainly derived from relatively coarse datasets, so that links between roughness and flow are generalised and rather simplistic. Here, we explore the use of DTMs from the well-preserved footprints of palaeo-ice streams, coupled with high resolution models of palaeo-ice flow, as a tool for investigating basal controls on the behaviour of contemporary, active ice streams in much greater detail. Initially, artificial transects were set up across the Minch palaeo-ice stream (NW Scotland) to mimic RES flight lines from past studies in Antarctica. We then explored how increasing data-resolution impacted upon the roughness measurements that were derived. Our work on the Minch palaeo-ice stream indicates that different roughness signatures are associated with different glacial landforms, and we discuss the potential for using these insights to infer, from RES-based roughness measurements, the occurrence of particular landform assemblages that may exist beneath contemporary ice sheets.

Terrestrial ice streams-a view from the lobe

USGS Publications Warehouse

Jennings, C.E.

2006-01-01

The glacial landforms of Minnesota are interpreted as the products of the lobate extensions of ice streams that issued from various ice sheds within the Laurentide Ice Sheet. Low-relief till plains, trough-shaped lowlands, boulder pavements, and streamlined forms make up the subglacial landsystem in Minnesota that is interpreted as having been formed by streaming ice. Extremely uniform tills are created subglacially in a way that remains somewhat mysterious. At the ice margins, thrust moraines and hummocky stagnation topography are more common than single-crested, simple moraines if the ice lobes had repeated advances. Subglacial drainage features are obscure up-ice but are present down-ice in the form of tunnel valleys, eskers, Spooner hills, and associated ice-marginal fans. Ice streaming may occur when basal shear stress is lowered as a result of high subglacial water pressure. Subglacial conditions that allow the retention of water will allow an ice lobe to extend far beyond the ice sheet as long as the ice shed also supports the advance by supplying adequate ice. Even with adequate ice flux, however, the advance of an ice lobe may be terminated, at least temporarily, if the subglacial water is drained, through tunnel valleys or perhaps a permeable substrate. Thrust moraines, and ice stagnation topography will result from sudden drainage. Although climate change is ultimately responsible for the accumulation of ice in the Laurentide Ice Sheet, the asynchronous advances and retreats of the ice lobes in the mid-continent are strongly overprinted by the internal dynamics of individual ice streams as well as the interaction of ice sheds, which obscure the climate signal. ?? 2005 Elsevier B.V. All rights reserved.

Equilibrium sensitivities of the Greenland ice sheet inferred from the adjoint of the three- dimensional thermo-mechanical model SICOPOLIS

NASA Astrophysics Data System (ADS)

Heimbach, P.; Bugnion, V.

2008-12-01

We present a new and original approach to understanding the sensitivity of the Greenland ice sheet to key model parameters and environmental conditions. At the heart of this approach is the use of an adjoint ice sheet model. MacAyeal (1992) introduced adjoints in the context of applying control theory to estimate basal sliding parameters (basal shear stress, basal friction) of an ice stream model which minimize a least-squares model vs. observation misfit. Since then, this method has become widespread to fit ice stream models to the increasing number and diversity of satellite observations, and to estimate uncertain model parameters. However, no attempt has been made to extend this method to comprehensive ice sheet models. Here, we present a first step toward moving beyond limiting the use of control theory to ice stream models. We have generated an adjoint of the three-dimensional thermo-mechanical ice sheet model SICOPOLIS of Greve (1997). The adjoint was generated using the automatic differentiation (AD) tool TAF. TAF generates exact source code representing the tangent linear and adjoint model of the parent model provided. Model sensitivities are given by the partial derivatives of a scalar-valued model diagnostic or "cost function" with respect to the controls, and can be efficiently calculated via the adjoint. An effort to generate an efficient adjoint with the newly developed open-source AD tool OpenAD is also under way. To gain insight into the adjoint solutions, we explore various cost functions, such as local and domain-integrated ice temperature, total ice volume or the velocity of ice at the margins of the ice sheet. Elements of our control space include initial cold ice temperatures, surface mass balance, as well as parameters such as appear in Glen's flow law, or in the surface degree-day or basal sliding parameterizations. Sensitivity maps provide a comprehensive view, and allow a quantification of where and to which variables the ice sheet model is most sensitive to. The model used in the present study includes simplifications in the model physics, parameterizations which rely on uncertain empirical constants, and is unable to capture fast ice streams. Nevertheless, as a proof-of-concept, this method can readily be extended to incorporate higher-order physics or parameterizations (or be applied to other models). It also opens the door to ice sheet state estimation: using the model's physics jointly with field and satellite observations to estimate a best estimate of the state of the ice sheets.

Role of ice sheet dynamics in the collapse of the early-Holocene Laurentide Ice Sheet

NASA Astrophysics Data System (ADS)

Matero, I. S. O.; Gregoire, L. J.; Cornford, S. L.; Ivanovic, R. F.

2017-12-01

The last stage of the deglaciation of the Laurentide Ice Sheet (LIS) during the early Holocene Thermal Maximum ( 9000 to 7000 years ago) provides an analogy and insight to the possible responses of contemporary ice sheets in a warming climate. What makes LIS particularly interesting is that meltwater from the collapse of an ice saddle over Hudson Bay was recently shown to be the primary forcing for the period of abrupt northern hemisphere cooling known as the 8.2 ka event. The evolution of the LIS during this period was likely influenced by its interaction with marginal lakes and the ocean, and its major ice stream, which exported ice towards Hudson Strait. Accurately simulating the early Holocene LIS evolution thus requires a model such as BISICLES, capable of accurately and efficiently resolving ice stream dynamics and grounding line migration thanks to the combined use of higher order physics and adaptive mesh refinement. We drive the BISICLES model using a positive degree day mass balance scheme with monthly precipitation and temperature from the HadCM3 climate model under climatic conditions from 10,000 to 8,000 years ago. We test the effect of varying the initial topographies and ice thicknesses from different timeslices in the ICE-6Gc reconstruction. We also test different parameterisations for the basal friction based on the thicknesses of the underlying sediments. These simulations evaluate the role of the Hudson Strait ice stream, ice sheet dynamics and interactions with the adjacent proglacial Lake Agassiz and North Atlantic Ocean in the collapse of the LIS. Our results highlight that the choice of parameterisation for basal friction has major effects on ice sheet dynamics and evolution.

Bedrock morphology reveals drainage network in northeast Baffin Bay

NASA Astrophysics Data System (ADS)

Slabon, Patricia; Dorschel, Boris; Jokat, Wilfried; Freire, Francis

2018-02-01

A subglacial drainage network underneath the paleo-ice sheet off West Greenland is revealed by a new compilation of high-resolution bathymetry data from Melville Bay, northeast Baffin Bay. This drainage network is an indicator for ice streaming and subglacial meltwater flow toward the outer shelf. Repeated ice sheet advances and retreats across the crystalline basement together with subglacial meltwater drainage had their impact in eroding overdeepened troughs along ice stream pathways. These overdeepenings indicate the location of a former ice sheet margin. The troughs inherit characteristics of glacial and subglacial meltwater erosion. Most of the troughs follow tectonic weakness zones such as faults and fractures in the crystalline bedrock. Many of these tectonic features correspond with the orientations of major fault axes in the Baffin Bay region. The troughs extend from the present (sub) glacial fjord systems at the Greenland coast and parallel modern outlet-glacier pathways. The fast flowing paleo-ice streams were likely accelerated from the meltwater flow as indicated by glacial landforms within and along the troughs. The ice streams flowed along narrow tributary troughs and merged to form large paleo-ice streams bedded in the major cross-shelf troughs of Melville Bay. Apart from the troughs, a rough seabed topography characterises the bedrock, and we see a sharp geomorphic transition where ice flowed onto sedimentary rock and deposits.

Topographic Steering of Enhanced Ice Flow at the Bottleneck Between East and West Antarctica

NASA Astrophysics Data System (ADS)

Winter, Kate; Ross, Neil; Ferraccioli, Fausto; Jordan, Tom A.; Corr, Hugh F. J.; Forsberg, René; Matsuoka, Kenichi; Olesen, Arne V.; Casal, Tania G.

2018-05-01

Hypothesized drawdown of the East Antarctic Ice Sheet through the "bottleneck" zone between East and West Antarctica would have significant impacts for a large proportion of the Antarctic Ice Sheet. Earth observation satellite orbits and a sparseness of radio echo sounding data have restricted investigations of basal boundary controls on ice flow in this region until now. New airborne radio echo sounding surveys reveal complex topography of high relief beneath the southernmost Weddell/Ross ice divide, with three subglacial troughs connecting interior Antarctica to the Foundation and Patuxent Ice Streams and Siple Coast ice streams. These troughs route enhanced ice flow through the interior of Antarctica but limit potential drawdown of the East Antarctic Ice Sheet through the bottleneck zone. In a thinning or retreating scenario, these topographically controlled corridors of enhanced flow could however drive ice divide migration and increase mass discharge from interior West Antarctica to the Southern Ocean.

Greenland ice cores tell tales on past sea level changes

NASA Astrophysics Data System (ADS)

Dahl-Jensen, D.

2017-12-01

All the deep ice cores drilled to the base of the Greenland ice sheet contain ice from the previous warm climate period, the Eemian 130-115 thousand years before present. This demonstrates the resilience of the Greenland ice sheet to a warming of 5 oC. Studies of basal material further reveal the presence of boreal forest over Greenland before ice covered Greenland. Conditions for Boreal forest implies temperatures at this time has been more than 10 oC warmer than the present. To compare the paleo-behavior of the Greenland ice sheet to the present in relation to sea level rise knowledge gabs include the reaction of ice streams to climate changes. To address this the international EGRIP-project is drilling an ice core in the center of the North East Greenland Ice Stream (NEGIS). The first results will be presented.

Geomorphology and till architecture of terrestrial palaeo-ice streams of the southwest Laurentide Ice Sheet: A borehole stratigraphic approach

NASA Astrophysics Data System (ADS)

Norris, Sophie L.; Evans, David J. A.; Cofaigh, Colm Ó.

2018-04-01

A multidimensional study, utilising geomorphological mapping and the analysis of regional borehole stratigraphy, is employed to elucidate the regional till architecture of terrestrial palaeo-ice streams relating to the Late Wisconsinan southwest Laurentide Ice Sheet. Detailed mapping over a 57,400 km2 area of southwestern Saskatchewan confirms previous reconstructions of a former southerly flowing ice stream, demarcated by a 800 km long corridor of megaflutes and mega-scale glacial lineations (Ice Stream 1) and cross cut by three, formerly southeast flowing ice streams (Ice Streams 2A, B and C). Analysis of the lithologic and geophysical characteristics of 197 borehole samples within these corridors reveals 17 stratigraphic units comprising multiple tills and associated stratified sediments overlying preglacial deposits, the till thicknesses varying with both topography and distance down corridor. Reconciling this regional till architecture with the surficial geomorphology reveals that surficial units are spatially consistent with a dynamic switch in flow direction, recorded by the cross cutting corridors of Ice Streams 1, 2A, B and C. The general thickening of tills towards lobate ice stream margins is consistent with subglacial deformation theory and variations in this pattern on a more localised scale are attributed to influences of subglacial topography including thickening at buried valley margins, thinning over uplands and thickening in overridden ice-marginal landforms.

Disintegration of a marine-based ice stream - evidence from the Norwegian Channel, north-eastern North Sea

NASA Astrophysics Data System (ADS)

Morén, Björn M.; Petter Sejrup, Hans; Hjelstuen, Berit O.; Haflidason, Haflidi; Schäuble, Cathrina; Borge, Marianne

2014-05-01

The Norwegian Channel Ice Stream repeatedly drained large part of the Fennoscandian Ice Sheet through Mid and Late Pleistocene glacial stages. During parts of Marine Isotope Stages 2 and 3, glacial ice from Fennoscandia and the British Isles coalesced in the central North Sea and the Norwegian Channel Ice Stream reached the shelf edge on multiple occasions. Through the last decades a large amount of acoustic and sediment core data have been collected from the Norwegian Channel, providing a good background for studies focussing on stability- and development-controlling parameters for marine-based ice streams, the retreat rate of the Norwegian Channel Ice Stream, and the behaviour of the Fennoscandian Ice Sheet. Further, this improved understanding can be used to develop more accurate numerical climate models and models which can be used to model ice-sheet behaviour of the past as well as the future. This study presents new acoustic records and data from sediment cores which contribute to a better understanding of the retreat pattern and the retreat rate of the last ice stream that occupied the Norwegian Channel. From bathymetric and TOPAS seismic data, mega-scale glacial lineations, grounding-zone wedges, and end moraines have been mapped, thereby allowing us to reconstruct the pro- and subglacial conditions at the time of the creation of these landforms. It is concluded that the whole Norwegian Channel was deglaciated in just over 1 000 years and that for most of this time the ice margin was located at positions reflected by depositional grounding-zone wedges. Further work will explore the influence of channel shape and feeding of ice from western Norwegian fjords on this retreat pattern through numerical modelling.

An Imminent Revolution in Modeling Interactions of Ice Sheets With Climate

NASA Astrophysics Data System (ADS)

Hughes, T.

2008-12-01

Modeling continental ice sheets was inaugurated by meteorologists William Budd and Uwe Radok, with mathematician Richard Jenssen, in 1971. Their model calculated the thermal and mechanical regime using measured surface accumulation rates, temperatures, and elevations, and bed topography. This top-down approach delivered a basal thermal regime of temperatures or melting rates for an assumed basal geothermal heat flux. When Philippe Huybrechts and others incorporated time, largely unknownpast surface conditions had a major effect on present basal thermal conditions. This approach produced ice-sheet models with only a slow response to external forcing, whereas the glacial geological record and climate records from ice and ocean cores show that ice sheets can have rapid changes in size and shape independent of external forcing. These top-down models were wholly inadequate for reconstructing former ice sheets at the LGM for CLIMAP in 1981. Ice-sheet areas,elevations, and volumes provided the albedo, surface topography, and sea-surface area as input to climate models. A bottom-up model based on dated glacial geology was developed to provide the areal extent and basal thermal regime of ice sheets at the LGM. Basal thermal conditions determined ice-bed coupling and therefore the elevation of ice sheets. High convex ice surfaces for slow sheet flow lower about 20 percent when a frozen bed becomes thawed. As further basal melting drowns bedrock bumps that "pin" basal ice, the ice surface becomes concave in fast stream flow that ends as low floating ice shelves at marine ice margins. A revolution in modeling interactions between glaciation, climate, and sea level is driven by new Greenland and Antarctic data from Earth-orbiting satellites, airborne and surface traverses, and deep drilling. We anticipate continuous data acquisition of surface albedo, accumulation/ablation rates, elevations, velocities, and temperatures over a whole ice sheet, mapping basal thermal conditions by radar, seismic, and magnetic profiling, and direct measurement of basal conditions by deep drilling and coring into the ice and the bed. These data allow calculating the geothermal heat flux and mapping flow of basal meltwater from geothermal sources to sinks at the termini of ice streams, which discharge up to 90 percent of the ice. James Fastook has a preliminary solution of the full momentum equation needed to model ice streams. Douglas MacAyeal has pioneered modeling catastrophic ice-shelf disintegration that releases "armadas" of icebergs into the world ocean, to extract heat from ocean surface water and thereby reduce the critical ocean-to-atmosphere heat exchange that drives global climate. Ice sheets are the only component of Earth's climate machine that can destroy itself-- swiftly--and thereby radically and rapidly alter global climate and sea level.

Constraining the Antarctic contribution to global sea-level change: ANDRILL and beyond

NASA Astrophysics Data System (ADS)

Naish, Timothy

2016-04-01

Observations, models and paleoclimate reconstructions suggest that Antarctica's marine-based ice sheets behave in an unstable manner with episodes of rapid retreat in response to warming climate. Understanding the processes involved in this "marine ice sheet instability" is key for improving estimates of Antarctic ice sheet contribution to future sea-level rise. Another motivating factor is that far-field sea-level reconstructions and ice sheet models imply global mean sea level (GMSL) was up to 20m and 10m higher, respectively, compared with present day, during the interglacials of the warm Pliocene (~4-3Ma) and Late Pleistocene (at ~400ka and 125ka). This was when atmospheric CO2 was between 280 and 400ppm and global average surface temperatures were 1 to 3°C warmer, suggesting polar ice sheets are highly sensitive to relatively modest increases in climate forcing. Such magnitudes of GMSL rise not only require near complete melt of the Greenland Ice Sheet and the West Antarctic Ice Sheet, but a substantial retreat of marine-based sectors of East Antarctic Ice Sheet. Recent geological drilling initiatives on the continental margin of Antarctica from both ship- (e.g. IODP; International Ocean Discovery Program) and ice-based (e.g. ANDRILL/Antarctic Geological Drilling) platforms have provided evidence supporting retreat of marine-based ice. However, without direct access through the ice sheet to archives preserved within sub-glacial sedimentary basins, the volume and extent of ice sheet retreat during past interglacials cannot be directly constrained. Sediment cores have been successfully recovered from beneath ice shelves by the ANDRILL Program and ice streams by the WISSARD (Whillans Ice Stream Sub-glacial Access Research Drilling) Project. Together with the potential of the new RAID (Rapid Access Ice Drill) initiative, these demonstrate the technological feasibility of accessing the subglacial bed and deeper sedimentary archives. In this talk I will outline the scientific challenges, some potential sub-glacial sedimentary targets, and a strategy for future drilling of sub-glacial sedimentary basins.

Constraining the Antarctic contribution to interglacial sea-level rise

NASA Astrophysics Data System (ADS)

Naish, T.; Mckay, R. M.; Barrett, P. J.; Levy, R. H.; Golledge, N. R.; Deconto, R. M.; Horgan, H. J.; Dunbar, G. B.

2015-12-01

Observations, models and paleoclimate reconstructions suggest that Antarctica's marine-based ice sheets behave in an unstable manner with episodes of rapid retreat in response to warming climate. Understanding the processes involved in this "marine ice sheet instability" is key for improving estimates of Antarctic ice sheet contribution to future sea-level rise. Another motivating factor is that far-field sea-level reconstructions and ice sheet models imply global mean sea level (GMSL) was up to 20m and 10m higher, respectively, compared with present day, during the interglacials of the warm Pliocene (~4-3Ma) and Late Pleistocene (at ~400ka and 125ka). This was when atmospheric CO2 was between 280 and 400ppm and global average surface temperatures were 1- 3°C warmer, suggesting polar ice sheets are highly sensitive to relatively modest increases in climate forcing. Such magnitudes of GMSL rise not only require near complete melt of the Greenland Ice Sheet and the West Antarctic Ice Sheet, but a substantial retreat of marine-based sectors of East Antarctic Ice Sheet. Recent geological drilling initiatives on the continental margin of Antarctica from both ship- (e.g. IODP; International Ocean Discovery Program) and ice-based (e.g. ANDRILL/Antarctic Geological Drilling) platforms have provided evidence supporting retreat of marine-based ice. However, without direct access through the ice sheet to archives preserved within sub-glacial sedimentary basins, the volume and extent of ice sheet retreat during past interglacials cannot be directly constrained. Sediment cores have been successfully recovered from beneath ice shelves by the ANDRILL Program and ice streams by the WISSARD (Whillans Ice Stream Sub-glacial Access Research Drilling) Project. Together with the potential of the new RAID (Rapid Access Ice Drill) initiative, these demonstrate the technological feasibility of accessing the subglacial bed and deeper sedimentary archives. In this talk I will outline the scientific challenges, some potential sub-glacial sedimentary targets, and a strategy for future drilling of sub-glacial sedimentary basins.

Cosmogenic exposure age constraints on deglaciation and flow behaviour of a marine-based ice stream in western Scotland, 21-16 ka

NASA Astrophysics Data System (ADS)

Small, David; Benetti, Sara; Dove, Dayton; Ballantyne, Colin K.; Fabel, Derek; Clark, Chris D.; Gheorghiu, Delia M.; Newall, Jennifer; Xu, Sheng

2017-07-01

Understanding how marine-based ice streams operated during episodes of deglaciation requires geochronological data that constrain both timing of deglaciation and changes in their flow behaviour, such as that from unconstrained ice streaming to topographically restricted flow. We present seventeen new 10Be exposure ages from glacial boulders and bedrock at sites in western Scotland within the area drained by the Hebrides Ice Stream, a marine-based ice stream that drained a large proportion of the former British-Irish Ice Sheet. Exposure ages from Tiree constrain deglaciation of a topographic high within the central zone of the ice stream, from which convergent flowsets were produced during ice streaming. These ages thus constrain thinning of the Hebrides Ice Stream, which, on the basis of supporting information, we infer to represent cessation of ice streaming at 20.6 ± 1.2 ka, 3-4 ka earlier than previously inferred. A period of more topographically restricted flow produced flow indicators superimposed on those relating to full ice stream conditions, and exposure ages from up-stream of these constrain deglaciation to 17.5 ± 1.0 ka. Complete deglaciation of the marine sector of the Hebrides Ice Stream occurred by 17-16 ka at which time the ice margin was located near the present coastline. Exposure ages from the southernmost Outer Hebrides (Mingulay and Barra) indicate deglaciation at 18.9 ± 1.0 and 17.1 ± 1.0 ka respectively, demonstrating that an independent ice cap persisted on the southern Outer Hebrides for 3-4 ka after initial ice stream deglaciation. This suggests that deglaciation of the Hebrides Ice Stream was focused along major submarine troughs. Collectively, our data constrain initial deglaciation and changes in flow regime of the Hebrides Ice Stream, final deglaciation of its marine sector, and deglaciation of the southern portion of the independent Outer Hebrides Ice Cap, providing chronological constraints on future numerical reconstructions of this key sector of the former British-Irish Ice Sheet.

Extensive Holocene ice sheet grounding line retreat and uplift-driven readvance in West Antarctica

NASA Astrophysics Data System (ADS)

Kingslake, J.; Scherer, R. P.; Albrecht, T.; Coenen, J. J.; Powell, R. D.; Reese, R.; Stansell, N.; Tulaczyk, S. M.; Whitehouse, P. L.

2017-12-01

The West Antarctic Ice Sheet (WAIS) reached its Last Glacial Maximum (LGM) extent 29-14 kyr before present. Numerical models used to project future ice-sheet contributions to sea-level rise exploit reconstructions of post-LGM ice mass loss to tune model parameterizations. Ice-sheet reconstructions are poorly constrained in areas where floating ice shelves or a lack of exposed geology obstruct conventional glacial-geological techniques. In the Weddell and Ross Sea sectors, ice-sheet reconstructions have traditionally assumed progressive grounding line (GL) retreat throughout the Holocene. Contrasting this view, using three distinct lines of evidence, we show that the GL retreated hundreds of kilometers inland of its present position, before glacial isostatic rebound during the Mid to Late Holocene caused the GL to readvance to its current position. Evidence for retreat and readvance during the last glacial termination includes (1) widespread radiocarbon in sediment cores recovered from beneath ice streams along the Siple and Gould Coasts, indicating marine exposure at least 200 km inland of the current GL, (2) ice-penetrating radar observations of relic crevasses and other englacial structures preserved in slow-moving grounded ice, indicating ice-shelf grounding and (3) an ensemble of new ice-sheet simulations showing widespread post-LGM retreat of the GL inland of its current location and later readvance. The model indicates that GL readvance across low slope ice-stream troughs requires uplift-driven grounding of the ice shelf on topographic highs (ice rises). Our findings highlight ice-shelf pinning points and lithospheric response to unloading as drivers of major ice-sheet fluctuations. Full WAIS collapse likely requires GL retreat well beyond its current position in the Ronne and Ross Sectors and linkage via Amundsen Sea sector glaciers.

Geological control of flow in the Institute and Möller Ice Streams, West Antarctica

NASA Astrophysics Data System (ADS)

Jordan, T. A.; Ferraccioli, F.; Ross, N.; Corr, H.; Bingham, R. G.; Rippin, D. M.; Le Brocq, A.; Siegert, M. J.

2012-12-01

The conditions at the base of an ice sheet influence its flow, and reflect the ongoing interaction between moving ice and the underlying geology. Critical influences on ice flow include subglacial topography, bed lithology, and geothermal heat flux. These factors are influenced either directly by local geology, or by the regional tectonic setting. Geophysical methods have been used in many parts of Antarctica, such as the Siple Coast, to reveal the role subglacial geology plays in influencing ice flow. Until recently, however, the Institute and Möller Ice Streams, which drain ~20% of the West Antarctic Ice Sheet into the Weddell Sea, were only covered by sparse airborne radar (~50 km line spacing), and reconnaissance aeromagnetic data, limiting our understanding of the geological template for this sector of the West Antarctic Ice Sheet. Here we present our geological interpretation of the first integrated aerogeophysical survey over the catchments of the Institute and Möller Ice Streams, which collected ~25,000 km of new aerogeophysical data during the 2010/11 field season. These new airborne radar, magnetic and gravity data reveals both the subglacial topography, and the subglacial geology. Our maps show the fastest flowing coastal part of the Institute Ice Stream crosses a sedimentary basin underlain by thinned continental crust. Further inland two distinct ice flow provinces are recognised: the Pagano Ice Flow Province, which follows the newly identified, ~75 km wide, sinistral strike-slip Pagano Fault Zone at the boundary between East and West Antarctica; and the Ellsworth Ice Flow Province, which is controlled by the Permo-Triassic structural grain of folded Middle Cambrian-Permian meta-sediments, and Jurassic granitic rocks which form significant subglacial highlands. Our new data highlight the importance of understanding subglacial geology when explaining the complex pattern of ice flow observed in the ice sheet interior.

Radiostratigraphy and age structure of the Greenland Ice Sheet

PubMed Central

MacGregor, Joseph A; Fahnestock, Mark A; Catania, Ginny A; Paden, John D; Prasad Gogineni, S; Young, S Keith; Rybarski, Susan C; Mabrey, Alexandria N; Wagman, Benjamin M; Morlighem, Mathieu

2015-01-01

Several decades of ice-penetrating radar surveys of the Greenland and Antarctic ice sheets have observed numerous widespread internal reflections. Analysis of this radiostratigraphy has produced valuable insights into ice sheet dynamics and motivates additional mapping of these reflections. Here we present a comprehensive deep radiostratigraphy of the Greenland Ice Sheet from airborne deep ice-penetrating radar data collected over Greenland by The University of Kansas between 1993 and 2013. To map this radiostratigraphy efficiently, we developed new techniques for predicting reflection slope from the phase recorded by coherent radars. When integrated along track, these slope fields predict the radiostratigraphy and simplify semiautomatic reflection tracing. Core-intersecting reflections were dated using synchronized depth-age relationships for six deep ice cores. Additional reflections were dated by matching reflections between transects and by extending reflection-inferred depth-age relationships using the local effective vertical strain rate. The oldest reflections, dating to the Eemian period, are found mostly in the northern part of the ice sheet. Within the onset regions of several fast-flowing outlet glaciers and ice streams, reflections typically do not conform to the bed topography. Disrupted radiostratigraphy is also observed in a region north of the Northeast Greenland Ice Stream that is not presently flowing rapidly. Dated reflections are used to generate a gridded age volume for most of the ice sheet and also to determine the depths of key climate transitions that were not observed directly. This radiostratigraphy provides a new constraint on the dynamics and history of the Greenland Ice Sheet. Key Points Phase information predicts reflection slope and simplifies reflection tracing Reflections can be dated away from ice cores using a simple ice flow model Radiostratigraphy is often disrupted near the onset of fast ice flow PMID:26213664

Evaluation of changes in atmospheric and oceanic fluxes during continental ice sheet retreat

NASA Astrophysics Data System (ADS)

Martin, J.; Martin, E. E.; Deuerling, K. M.

2017-12-01

Extensive land areas were exposed across North America, Eurasia, and to a lesser extent Greenland as continental ice sheets retreated following the last glacial maximum. A transect of watersheds from the coast to the western Greenland Ice Sheet (GrIS) provides an opportunity to evaluate possible changes in oceanic solute fluxes and atmospheric CO2 exchange as ice sheets retreat. We evaluate these fluxes in one proglacial watershed (draining ice sheet runoff) and four deglaciated watersheds (draining local precipitation and permafrost melt). Sr isotope ratios indicate bedrock near the coast has experienced greater weathering than near the ice sheet. A mass balance model of the major element composition of stream water indicates weathering in deglaciated watersheds is dominated by carbonic acid dissolution of carbonate minerals near the ice sheet that switches to carbonic acid alteration of silicate minerals near the coast. In addition, weathering by sulfuric acid, derived from oxidative dissolution of sulfide minerals, increases from the ice sheet to the coast. These changes in the weathered minerals and weathering acids impact CO2 sequestration associated with weathering. Weathering consumes 350 to 550 µmol CO2/L in watersheds near the ice sheet, but close to the coast, consumes only 15 µmol CO2/L in one watershed and sources 140 µmol CO2/L to the atmosphere at another coastal watershed. The decreasing CO2 weathering sink from the GrIS to coast reflects decreased carbonic acid weathering and increased sulfuric acid weathering of carbonate minerals. The proglacial stream shows downstream variations in composition from mixing of two water sources, with only minor in-stream weathering, which consumes < 0.1 µmol CO2/L. Discharge from the deglaciated watersheds is currently unknown but their higher solute concentrations and CO2 exchange than proglacial systems suggest deglaciated watersheds dominate atmospheric fluxes of CO2 and oceanic solute fluxes. These results imply that the initial CO2 drawdown associated with weathering of freshly exposed, fine-grained glacial sediment in deglaciated watersheds will decrease as the extent of weathering increases. As a result, weathering in this environment may become a source of atmospheric CO2 that could enhance CO2 induced global warming.

The Response of Ice Sheets to Climate Variability

NASA Astrophysics Data System (ADS)

Snow, K.; Goldberg, D. N.; Holland, P. R.; Jordan, J. R.; Arthern, R. J.; Jenkins, A.

2017-12-01

West Antarctic Ice Sheet loss is a significant contributor to sea level rise. While the ice loss is thought to be triggered by fluctuations in oceanic heat at the ice shelf bases, ice sheet response to ocean variability remains poorly understood. Using a synchronously coupled ice-ocean model permitting grounding line migration, this study evaluates the response of an ice sheet to periodic variations in ocean forcing. Resulting oscillations in grounded ice volume amplitude is shown to grow as a nonlinear function of ocean forcing period. This implies that slower oscillations in climatic forcing are disproportionately important to ice sheets. The ice shelf residence time offers a critical time scale, above which the ice response amplitude is a linear function of ocean forcing period and below which it is quadratic. These results highlight the sensitivity of West Antarctic ice streams to perturbations in heat fluxes occurring at decadal time scales.

The geomorphic signature of present ice-sheet flow in the radar-sounded subglacial record: Pine Island Glacier, West Antarctica

NASA Astrophysics Data System (ADS)

Bingham, R. G.; Davies, D.; King, E. C.; Vaughan, D. G.; Cornford, S. L.; Brisbourne, A.; Smith, A.; De Rydt, J.; Graham, A. G. C.; Spagnolo, M.

2016-12-01

Deglaciated landscapes and landforms are much used in the quest to reconstruct past ice-sheet behaviour, on the principle that aspects of landform shapes, sizes and relative associations "fossilise" palaeo-ice-sheet processes. Such techniques have been widely used around the margin of the marine West Antarctic Ice Sheet, taking advantage of bathymetric surveying techniques which have exposed a rich suite of landform assemblages across West Antarctica's continental shelf. Though these geomorphological interpretations are solidly grounded in glacial geological theory, there has, until now, been little ability to compare these deglaciated, and potentially postglacially-modified, landforms offshore with landforms currently situated (and potentially still evolving) beneath the contemporary ice sheet. This paper presents a widespread view of glacial landforms presently situated beneath 1-2 km of ice in multi-square-km "windows to the bed" distributed throughout the catchment of Pine Island Glacier, West Antarctica. Imaged over three field seasons between 2007 and 2013 by dedicated radar surveys designed specifically to capture landforms analogous to those surveyed offshore by bathymetric surveying, the results provide significant insights for the interpretation of palaeo-ice-stream landforms, and their use in modelling ice-ocean interactions around the fringes of marine ice sheets. We show that landform sizes, shapes and associations vary significantly around Pine Island Glacier's catchment. The key controls appear to be substrate composition, pre-existing tectonic structure, and longstanding spatial stability of the ice-stream's flow distribution. The findings offer crucial information for modelling ice coupling to the bed, which should feed through to wider efforts to reconstruct the past behaviour of this significant marine ice sheet using its palaeoglacial landforms.

Significance of Thermal Fluvial Incision and Bedrock Transfer due to Ice Advection on Greenland Ice Sheet Topography

NASA Astrophysics Data System (ADS)

Crozier, J. A.; Karlstrom, L.; Yang, K.

2017-12-01

Ice sheet surface topography reflects a complicated combination of processes that act directly upon the surface and that are products of ice advection. Using recently-available high resolution ice velocity, imagery, ice surface elevation, and bedrock elevation data sets, we seek to determine the domain of significance of two important processes - thermal fluvial incision and transfer of bedrock topography through the ice sheet - on controlling surface topography in the ablation zone. Evaluating such controls is important for understanding how melting of the GIS surface during the melt season may be directly imprinted in topography through supraglacial drainage networks, and indirectly imprinted through its contribution to basal sliding that affects bedrock transfer. We use methods developed by (Karlstrom and Yang, 2016) to identify supraglacial stream networks on the GIS, and use high resolution surface digital elevation models as well as gridded ice velocity and melt rate models to quantify surface processes. We implement a numerically efficient Fourier domain bedrock transfer function (Gudmundsson, 2003) to predict surface topography due to ice advection over bedrock topography obtained from radar. Despite a number of simplifying assumptions, the bedrock transfer function predicts the observed ice sheet surface in most regions of the GIS with ˜90% accuracy, regardless of the presence or absence of supraglacial drainage networks. This supports the hypothesis that bedrock is the most significant driver of ice surface topography on wavelengths similar to ice thickness. Ice surface topographic asymmetry on the GIS is common, with slopes in the direction of ice flow steeper than those faced opposite to ice flow, consistent with bedrock transfer theory. At smaller wavelengths, topography consistent with fluvial erosion by surface hydrologic features is evident. We quantify the effect of ice advection versus fluvial thermal erosion on supraglacial longitudinal stream profiles, as a function of location on the GIS (hence ice thickness and background melt rate) using spectral techniques to quantify longitudinal stream profiles. This work should provide a predictive guide for which processes are responsible for ice sheet topography scales from several m (DEM resolution) up to several ice thicknesses.

Whillans Ice Stream Subglacial Access Research Drilling (WISSARD): Integrative Study of Marine Ice Sheet Stability and Subglacial Life Habitats (Invited)

NASA Astrophysics Data System (ADS)

Tulaczyk, S. M.; Anandakrishnan, S.; Behar, A. E.; Christner, B. C.; Fisher, A. T.; Fricker, H. A.; Holland, D. M.; Jacobel, R. W.; Mikucki, J.; Mitchell, A. C.; Powell, R. D.; Priscu, J. C.; Scherer, R. P.; Severinghaus, J. P.

2009-12-01

The WISSARD project is a large, NSF-funded, interdisciplinary initiative focused on scientific drilling, exploration, and investigation of Antarctic subglacial aquatic environments. The project consists of three interrelated components: (1) LISSARD - Lake and Ice Stream Subglacial Access Research Drilling, (2) RAGES - Robotic Access to Grounding-zones for Exploration and Science, and (3) GBASE - GeomicroBiology of Antarctic Subglacial Environments). A number of previous studies in West Antarctica highlighted the importance of understanding ice sheet interactions with water, either at the basal boundary where ice streams come in contact with active subglacial hydrologic and geological systems or at the marine margin where the ice sheet is exposed to forcing from the global ocean and sedimentation. Recent biological investigations of Antarctic subglacial environments show that they provide a significant habitat for life and source of bacterial carbon in a setting that was previously thought to be inhospitable. Subglacial microbial ecosystems also enhance biogeochemical weathering, mobilizing elements from long term geological storage. The overarching scientific objective of WISSARD is to examine the subglacial hydrological system of West Antarctica in glaciological, geological, microbiological, geochemical, and oceanographic contexts. Direct sampling will yield seminal information on these systems and test the overarching hypothesis that active hydrological systems connect various subglacial environments and exert major control on ice sheet dynamics, subglacial sediment transfer, geochemistry, metabolic and phylogenetic diversity, and biogeochemical transformations and geological records of ice sheet history. Technological advances during WISSARD will provide the US-science community with a capability to access and study sub-ice sheet environments. Developing this technological infrastructure will benefit the broader science community and it will be available for future use. Furthermore, these projects will pioneer an approach implementing recommendations from the National Research Council committee on Principles of Environmental Stewardship for the Exploration and Study of Subglacial Environments.

Ocean forcing of Ice Sheet retreat in central west Greenland from LGM to the early Holocene

NASA Astrophysics Data System (ADS)

Jennings, Anne E.; Andrews, John T.; Ó Cofaigh, Colm; Onge, Guillaume St.; Sheldon, Christina; Belt, Simon T.; Cabedo-Sanz, Patricia; Hillaire-Marcel, Claude

2017-08-01

Three radiocarbon dated sediment cores from trough mouth fans on the central west Greenland continental slope were studied to determine the timing and processes of Greenland Ice Sheet (GIS) retreat from the shelf edge during the last deglaciation and to test the role of ocean forcing (i.e. warm ocean water) thereon. Analyses of lithofacies, quantitative x-ray diffraction mineralogy, benthic foraminiferal assemblages, the sea-ice biomarker IP25, and δ18 O of the planktonic foraminifera Neogloboquadrina pachyderma sinistral from sediments in the interval from 17.5-10.8 cal ka BP provide consistent evidence for ocean and ice sheet interactions during central west Greenland (CWG) deglaciation. The Disko and Uummannaq ice streams both retreated from the shelf edge after the last glacial maximum (LGM) under the influence of subsurface, warm Atlantic Water. The warm subsurface water was limited to depths below the ice stream grounding lines during the LGM, when the GIS terminated as a floating ice shelf in a sea-ice covered Baffin Bay. The deeper Uummannaq ice stream retreated first (ca. 17.1 cal ka BP), while the shallower Disko ice stream retreated at ca. 16.2 cal ka BP. The grounding lines were protected from accelerating mass loss (calving) by a buttressing ice shelf and by landward shallowing bathymetry on the outer shelf. Calving retreat was delayed until ca. 15.3 cal ka BP in the Uummannaq Trough and until 15.1 cal ka BP in the Disko Trough, during another interval of ocean warming. Instabilities in the Laurentide, Innuitian and Greenland ice sheets with outlets draining into northern Baffin Bay periodically released cold, fresh water that enhanced sea ice formation and slowed GIS melt. During the Younger Dryas, the CWG records document strong cooling, lack of GIS meltwater, and an increase in iceberg rafted material from northern Baffin Bay. The ice sheet remained in the cross-shelf troughs until the early Holocene, when it retreated rapidly by calving and strong melting under the influence of atmosphere and ocean warming and a steep reverse slope toward the deep fjords. We conclude that ocean warming played an important role in the palaeo-retreat dynamics of the GIS during the last deglaciation.

Discharge of water and sediment from ice-streams on the southeastern Laurentide Ice Sheet during Heinrich events: timing and magnitude

NASA Astrophysics Data System (ADS)

Rashid, H.; Piper, D.

2017-12-01

Several ice-streams on the southeastern sector of the Laurentide Ice Sheet discharged icebergs, meltwater, and fine-grained sediments into the North Atlantic during Heinrich (H) events. The principal contribution was through Hudson Strait, which is the only source clearly identified in H ice-rafted layers in the central North Atlantic. The role of direct supply of meltwater in modifying the Atlantic meridional circulation generally has been regarded as secondary. The relative chronology of discharge in different ice-streams is poorly known. Here, we re-assess these questions using continental margin cores constrained by high-resolution seismic profiles and multibeam bathymetry data. Relative importance of ice streams likely scales with cross-sectional area of their erosional troughs. On that basis, the Hudson Strait ice stream was twice as large as that in the Laurentian Channel and 3-4 times larger than smaller troughs. Several ice streams supplied petrographically and geochemically distinct sediment including black shales from Cumberland Sound, limestone and dolomite in particular proportions from Frobisher Bay and Hudson Strait, and red sandstones and shales ± carbonates from NE Newfoundland and Laurentian Channel. In several cases, detrital carbonate H layers derived predominantly from Hudson Strait are preceded by enhanced IRD deposition from smaller ice streams, e.g. deposits from Cumberland Sound on the Labrador slope, from NE Newfoundland in Orphan Basin, and from Laurentian Channel on the Nova Scotian margin. Gravel petrology indicates that Hudson Strait sources make up >90% of the ice-rafted component of distal H layers. H layers proximal to the Hudson Strait ice-streams are 4 to 12 meters thick compared to a few centimeters thick seaward of the Trinity Trough and Laurentian ice-streams, comparable to the thickness of the North Atlantic. This underscores the great importance of meltwater and suspended sediment close to ice stream outlets. Morphological features and plume deposits show that meltwater becomes much more abundant in more southerly ice streams and some local plume deposits contribute to the thickness of H layers. The contribution of freshwater from melting icebergs and from direct meltwater discharge are approximately similar during H events.

The role of membrane-like stresses in determining the stability and sensitivity of the Antarctic ice sheets: back pressure and grounding line motion.

PubMed

Hindmarsh, Richard C A

2006-07-15

Membrane stresses act along thin bodies which are relatively well lubricated on both surfaces. They operate in ice sheets because the bottom is either sliding, or is much less viscous than the top owing to stress and heat softening of the basal ice. Ice streams flow over very well lubricated beds, and are restrained at their sides. The ideal of the perfectly slippery bed is considered in this paper, and the propagation of mechanical effects along an ice stream considered by applying spatially varying horizontal body forces. Propagation distances depend sensitively on the rheological index, and can be very large for ice-type rheologies.A new analytical solution for ice-shelf profiles and grounded tractionless stream profiles is presented, which show blow up of the profile in a finite distance upstream at locations where the flux is non-zero. This is a feature of an earlier analytical solution for a floating shelf.The length scale of decay of membrane stresses from the grounding line is investigated through scale analysis. In ice sheets, such effects decay over distances of several tens of kilometres, creating a vertical boundary layer between sheet flow and shelf flow, where membrane stresses adjust. Bounded, physically reasonable steady surface profiles only exist conditionally in this boundary layer. Where bounded steady profiles exist, adjacent profile equilibria for the whole ice sheet corresponding to different grounded areas occur (neutral equilibrium). If no solution in the boundary layer can exist, the ice-sheet profile must change.The conditions for existence can be written in terms of whether the basal rate factor (sliding or internal deformation) is too large to permit a steady solution. The critical value depends extremely sensitively on ice velocity and the back stress applied at the grounding line. High ice velocity and high stress both favour the existence of solutions and stability. Changes in these parameters can cause the steady solution existence criterion to be traversed, and the ice-sheet dynamics to change.A finite difference model which represents both neutral equilibrium and the dynamical transition is presented, and preliminary investigations into its numerical sensitivity are carried out. Evidence for the existence of a long wavelength instability is presented through the solution of a numerical eigenproblem, which will hamper predictability.

Investigation of Controls on Ice Dynamics in Northeast Greenland from Ice-Thickness Change Record Using Ice Sheet System Model (ISSM)

NASA Astrophysics Data System (ADS)

Csatho, B. M.; Larour, E. Y.; Schenk, A. F.; Schlegel, N.; Duncan, K.

2015-12-01

We present a new, complete ice thickness change reconstruction of the NE sector of the Greenland Ice Sheet for 1978-2014, partitioned into changes due to surface processes and ice dynamics. Elevation changes are computed from all available stereoscopic DEMs, and laser altimetry data (ICESat, ATM, LVIS). Surface Mass Balance and firn-compaction estimates are from RACMO2.3. Originating nearly at the divide of the Greenland Ice Sheet (GrIS), the dynamically active North East Ice Stream (NEGIS) is capable of rapidly transmitting ice-marginal forcing far inland. Thus, NEGIS provides a possible mechanism for a rapid drawdown of ice from the ice sheet interior as marginal warming, thinning and retreat continues. Our altimetry record shows accelerating dynamic thinning of Zachariæ Isstrom, initially limited to the deepest part of the fjord near the calving front (1978-2000) and then extending at least 75 km inland. At the same time, changes over the Nioghalvfjerdsfjorden (N79) Glacier are negligible. We also detect localized large dynamic changes at higher elevations on the ice sheet. These thickness changes, often occurring at the onset of fast flow, could indicate rapid variations of basal lubrication due to rerouting of subglacial drainage. We investigate the possible causes of the observed spatiotemporal pattern of ice sheet elevation changes using the Ice Sheet System Model (ISSM). This work build on our previous studies examining the sensitivity of ice flow within the Northeast Greenland Ice Stream (NEGIS) to key fields, including ice viscosity, basal drag. We assimilate the new altimetry record into ISSM to improve the reconstruction of basal friction and ice viscosity. Finally, airborne geophysical (gravity, magnetic) and ice-penetrating radar data is examined to identify the potential geologic controls on the ice thickness change pattern. Our study provides the first comprehensive reconstruction of ice thickness changes for the entire NEGIS drainage basin during the last 40 years. Through the use of ISSM, we examine possible mechanism explaining the observed changes. The improved understanding gained through this research will contribute better projections of future ice loss from this most vulnerable region of the GrIS.

Decadal-scale sensitivity of Northeast Greenland ice flow to errors in surface mass balance using ISSM

NASA Astrophysics Data System (ADS)

Schlegel, N.-J.; Larour, E.; Seroussi, H.; Morlighem, M.; Box, J. E.

2013-06-01

The behavior of the Greenland Ice Sheet, which is considered a major contributor to sea level changes, is best understood on century and longer time scales. However, on decadal time scales, its response is less predictable due to the difficulty of modeling surface climate, as well as incomplete understanding of the dynamic processes responsible for ice flow. Therefore, it is imperative to understand how modeling advancements, such as increased spatial resolution or more comprehensive ice flow equations, might improve projections of ice sheet response to climatic trends. Here we examine how a finely resolved climate forcing influences a high-resolution ice stream model that considers longitudinal stresses. We simulate ice flow using a two-dimensional Shelfy-Stream Approximation implemented within the Ice Sheet System Model (ISSM) and use uncertainty quantification tools embedded within the model to calculate the sensitivity of ice flow within the Northeast Greenland Ice Stream to errors in surface mass balance (SMB) forcing. Our results suggest that the model tends to smooth ice velocities even when forced with extreme errors in SMB. Indeed, errors propagate linearly through the model, resulting in discharge uncertainty of 16% or 1.9 Gt/yr. We find that mass flux is most sensitive to local errors but is also affected by errors hundreds of kilometers away; thus, an accurate SMB map of the entire basin is critical for realistic simulation. Furthermore, sensitivity analyses indicate that SMB forcing needs to be provided at a resolution of at least 40 km.

Monitoring Supraglacial Streams over Three Months in Southwest Greenland

NASA Astrophysics Data System (ADS)

Muthyala, R.; Rennermalm, A.; Leidman, S. Z.; Cooper, M. G.; Cooley, S. W.; Smith, L. C.; van As, D.

2017-12-01

Supraglacial river networks are the most efficient conduits for evacuation of meltwater runoff produced on Greenland ice sheet. These rivers are prominent features on the ablation zone of southwest Greenland. However, little is known about the transport of meltwater through supraglacial stream network and most of the in-situ observations only capture a few days of streamflow. Here we report three months of observations of water level and discharge collected during summer of 2016, in two small supraglacial streams near the ice sheet margin in southwest Greenland. We also compare streamflow observations with meteorological data from a nearby automatic weather station. The two sites are very different, with the lower basin relatively steep, smooth and dark while the upper basin has rugged terrain and deeply incised stream channels. These catchment characteristics propagate to different relationships with meteorological parameters. For example, upper basin stream water levels show a strong covariance with surface temperature while the lower basin water levels do not. We also find differences in temporal variation of supraglacial stream water level, with the upper basin having two distinct peaks, in mid-June and mid-July, while the lower basin shows gradual decrease from June to August. Long-term supraglacial stream observations such as these will ultimately help assess how well surface mass balance models can simulate ice sheet runoff.

Malaspina Glacier: a modern analog to the Laurentide Glacier in New England

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gustavson, T.C.; Boothroyd, J.C.

1985-01-01

The land-based temperate Malaspina Glacier is a partial analog to the late Wisconsinan Laurentide Ice Sheet that occupied New England and adjacent areas. The Malaspina occupies a bedrock basin similar to basins occupied by the margin of the Laurentide Ice Sheet. Ice lobes of the Malaspina are similar in size to end moraine lobes in southern New England and Long Island,New York. Estimated ice temperature, ablation rates, surface slopes and meltwater discharge per unit of surface area for the Laurentide Ice Sheet are similar to those for the Malaspina Glacier. In a simple hydrologic-fluvial model for the Malaspina Glacier meltwatermore » moves towards the glacier bed and down-glacier along intercrystalline pathways, crevasses and moulins, and a series of tunnels. Regolith and bedrock at the glacier floor, which are eroded and transported by subglacial and englacial streams, are the sources of essentially all fluvio-lacustrine sediment on the Malaspina Foreland. Supraglacial eskers containing coarse gravels occur as much as 100 m above the glacier bed and are evidence that bedload can be lifted hydraulically. Subordinant amounts of sediment are contributed to outwash by small surface streams draining the ice margin. By analogy a similar hydrologic-fluvial system existed along the southeastern margin of the Laurentide Ice Sheet. Subglacial regolith and bedrock eroded from beneath the Laurentide Ice Sheet by meltwater was also the source of most glaciofluvial and glaciolacustrine deposits in southern New England, not sediment carried to the surface of the ice sheet along shear planes and washed off the glacier by meltwater.« less

A new bed elevation model for the Weddell Sea sector of the West Antarctic Ice Sheet

NASA Astrophysics Data System (ADS)

Jeofry, Hafeez; Ross, Neil; Corr, Hugh F. J.; Li, Jilu; Morlighem, Mathieu; Gogineni, Prasad; Siegert, Martin J.

2018-04-01

We present a new digital elevation model (DEM) of the bed, with a 1 km gridding, of the Weddell Sea (WS) sector of the West Antarctic Ice Sheet (WAIS). The DEM has a total area of ˜ 125 000 km2 covering the Institute, Möller and Foundation ice streams, as well as the Bungenstock ice rise. In comparison with the Bedmap2 product, our DEM includes new aerogeophysical datasets acquired by the Center for Remote Sensing of Ice Sheets (CReSIS) through the NASA Operation IceBridge (OIB) program in 2012, 2014 and 2016. We also improve bed elevation information from the single largest existing dataset in the region, collected by the British Antarctic Survey (BAS) Polarimetric radar Airborne Science Instrument (PASIN) in 2010-2011, from the relatively crude measurements determined in the field for quality control purposes used in Bedmap2. While the gross form of the new DEM is similar to Bedmap2, there are some notable differences. For example, the position and size of a deep subglacial trough (˜ 2 km below sea level) between the ice-sheet interior and the grounding line of the Foundation Ice Stream have been redefined. From the revised DEM, we are able to better derive the expected routing of basal water and, by comparison with that calculated using Bedmap2, we are able to assess regions where hydraulic flow is sensitive to change. Given the potential vulnerability of this sector to ocean-induced melting at the grounding line, especially in light of the improved definition of the Foundation Ice Stream trough, our revised DEM will be of value to ice-sheet modelling in efforts to quantify future glaciological changes in the region and, from this, the potential impact on global sea level. The new 1 km bed elevation product of the WS sector can be found at https://doi.org/10.5281/zenodo.1035488.

New insights into West Greenland ice sheet/stream dynamics during the last glacial cycle.

NASA Astrophysics Data System (ADS)

Roberts, David; Lane, Tim; Rea, Brice; Cofaigh, Colm O.; Jamieson, Stewart; Vieli, Andreas; Rodes, Angel

2015-04-01

Onshore and offshore geomorphological mapping and deglacial chronologies from West Greenland constrain the nature and magnitude of ice advance and decay of the Greenland Ice Sheet (GrIS) during the last glacial cycle. Several ice stream troughs are known to have fed ice to the shelf edge during the last glacial cycle. Their offshore expression suggests that many were coalescent systems fed by smaller outlet glaciers and ice streams onshore but their central flow pathways were also controlled by geology and preglacial topography. The bed morphology of these large ice streams shows they operated over soft, deforming beds with drumlins, mega-scale glacial lineations and grounding zone wedges marking an offshore transition from predominant areal scour onshore. Records of offshore deglacial chronology remain sparse but the Uummannaq and Disko Bugt ice stream corridors are now well constrained. The Uummannaq ice stream (UIS) completely deglaciated from the continental shelf between 14.8 ka and 11.0 ka in response to rising air temperatures, increasing JJA solar radiation and sea-level rise, but temporary standstills and the asynchronous retreat history of its feeder zones suggest that topography/bathymetry strongly modulated retreat rates as ice became 'locked' back into the coastal fjord system. Initial reconstructions of behaviour UIS discounted an oceanic role in early deglaciation and favoured retreat from the mid-shelf and inner-shelf prior to the Younger Dryas but both these concepts remain under investigation. In Disko Bugt, Jakobshavn Isbrae deglaciated later than the UIS and remained on the outer shelf during the Younger Dyras stadial (12.8 - 11.7 cal. kyrs BP) only reaching in the inner coast fjords at approximately 10.0 ka. The later deglaciation of the Disko system (despite similar external forcing mechanisms) was controlled by regional topographic/bathymetric contrasts in their respective trough morphologies. This hypothesis is supported by recent model output which indicates that non-linear retreat, grounding line stability and up-ice surface thinning is heavily influenced by both vertical and lateral constrictions in marine trough systems. While the offshore ice stream corridors are beginning to reveal their dynamic retreat history, knowledge of the inter-stream areas on the continental shelf remains very poor. The western, onshore sector of the GrIS has a much improved deglacial chronology derived from radiocarbon and new cosmogenic surface exposure dating undertaken in the last decade, but the deglacial history of wide swathes of the inner, mid and outer continental shelf remains completely unknown. The Hellefiske moraines on the West Greenland shelf were described in the late 1970's but little is known of ice sheet retreat behaviour across these areas. Understanding the deglacial signature of such regions is important if we are to use palaeo-reconstructions to understand ice sheet collapse/retreat mechanisms and to inform future model predictions.

Tracking millennial-scale Holocene glacial advance and retreat using osmium isotopes: Insights from the Greenland ice sheet

USGS Publications Warehouse

Rooney, Alan D.; Selby, David; Llyod, Jeremy M.; Roberts, David H.; Luckge, Andreas; Sageman, Bradley B.; Prouty, Nancy G.

2016-01-01

High-resolution Os isotope stratigraphy can aid in reconstructing Pleistocene ice sheet fluctuation and elucidating the role of local and regional weathering fluxes on the marine Os residence time. This paper presents new Os isotope data from ocean cores adjacent to the West Greenland ice sheet that have excellent chronological controls. Cores MSM-520 and DA00-06 represent distal to proximal sites adjacent to two West Greenland ice streams. Core MSM-520 has a steadily decreasing Os signal over the last 10 kyr (187Os/188Os = 1.35–0.81). In contrast, Os isotopes from core DA00-06 (proximal to the calving front of Jakobshavn Isbræ) highlight four stages of ice stream retreat and advance over the past 10 kyr (187Os/188Os = 2.31; 1.68; 2.09; 1.47). Our high-resolution chemostratigraphic records provide vital benchmarks for ice-sheet modelers as we attempt to better constrain the future response of major ice sheets to climate change. Variations in Os isotope composition from sediment and macro-algae (seaweed) sourced from regional and global settings serve to emphasize the overwhelming effect weathering sources have on seawater Os isotope composition. Further, these findings demonstrate that the residence time of Os is shorter than previous estimates of ∼104 yr.

The paradox of a long grounding during West Antarctic Ice Sheet retreat in Ross Sea.

PubMed

Bart, Philip J; Krogmeier, Benjamin J; Bart, Manon P; Tulaczyk, Slawek

2017-04-28

Marine geological data show that the West Antarctic Ice Sheet (WAIS) advanced to the eastern Ross Sea shelf edge during the Last Glacial Maximum (LGM) and eventually retreated ~1000 km to the current grounding-line position on the inner shelf. During the early deglacial, the WAIS deposited a voluminous stack of overlapping grounding zone wedges (GZWs) on the outer shelf of the Whales Deep Basin. The large sediment volume of the GZW cluster suggests that the grounding-line position of the paleo-Bindschadler Ice Stream was relatively stationary for a significant time interval. We used an upper bound estimate of paleo-sediment flux to investigate the lower bound duration over which the ice stream would have deposited sediment to account for the GZW volume. Our calculations show that the cluster represents more than three millennia of ice-stream sedimentation. This long duration grounding was probably facilitated by rapid GZW growth. The subsequent punctuated large-distance (~200 km) grounding-line retreat may have been a highly non-linear ice sheet response to relatively continuous external forcing such as gradual climate warming or sea-level rise. These findings indicate that reliable predictions of future WAIS retreat may require incorporation of realistic calculations of sediment erosion, transport and deposition.

The Influence of Subglacial Hydrology on Ice Stream Velocity in a Physical Model

NASA Astrophysics Data System (ADS)

Wagman, B. M.; Catania, G.; Buttles, J. L.

2011-12-01

We use a physical model to investigate how changes in subglacial hydrology affect ice motion in ice streams found in the West Antarctic Ice Sheet. Ice streams are modeled using silicone polymer placed over a thin water layer to simulate ice flow dominated by basal sliding. Dynamic similarity between modeled and natural ice streams is achieved through direct comparison of the glacier force balance using the conditions on Whillans Ice Stream (WIS) as our goal.This ice stream has a force balance that has evolved through time due to increased basal resistance. Currently, between 50-90% of the driving stress is supported by the ice stream shear margins [Stearns et al., JGlac 2005]. A similar force balance can be achieved in our model with a surface slope of 0.025. We test two hypotheses; 1) the distribution and thickness of the subglacial water layer influences the ice flow speed and thus the force balance and can reproduce the observed slowdown of WIS and; 2) shear margins are locations where transitions in water layer thickness occur.

Century-scale simulations of the response of the West Antarctic Ice Sheet to a warming climate

DOE PAGES

Cornford, S. L.; Martin, D. F.; Payne, A. J.; ...

2015-03-23

We use the BISICLES adaptive mesh ice sheet model to carry out one, two, and three century simulations of the fast-flowing ice streams of the West Antarctic Ice Sheet. Each of the simulations begins with a geometry and velocity close to present day observations, and evolves according to variation in meteoric ice accumulation, ice shelf melting, and mesh resolution. Future changes in accumulation and melt rates range from no change, through anomalies computed by atmosphere and ocean models driven by the E1 and A1B emissions scenarios, to spatially uniform melt rates anomalies that remove most of the ice shelves overmore » a few centuries. We find that variation in the resulting ice dynamics is dominated by the choice of initial conditions, ice shelf melt rate and mesh resolution, although ice accumulation affects the net change in volume above flotation to a similar degree. Given sufficient melt rates, we compute grounding line retreat over hundreds of kilometers in every major ice stream, but the ocean models do not predict such melt rates outside of the Amundsen Sea Embayment until after 2100. Sensitivity to mesh resolution is spurious, and we find that sub-kilometer resolution is needed along most regions of the grounding line to avoid systematic under-estimates of the retreat rate, although resolution requirements are more stringent in some regions – for example the Amundsen Sea Embayment – than others – such as the Möller and Institute ice streams.« less

Irish Ice Sheet dynamics during deglaciation of the central Irish Midlands: Evidence of ice streaming and surging from airborne LiDAR

NASA Astrophysics Data System (ADS)

Delaney, Catherine A.; McCarron, Stephen; Davis, Stephen

2018-04-01

High resolution digital terrain models (DTMs) generated from airborne LiDAR data and supplemented by field evidence are used to map glacial landform assemblages dating from the last glaciation (Midlandian glaciation; OI stages 2-3) in the central Irish Midlands. The DTMs reveal previously unrecognised low-amplitude landforms, including crevasse-squeeze ridges and mega-scale glacial lineations overprinted by conduit fills leading to ice-marginal subaqueous deposits. We interpret this landform assemblage as evidence for surging behaviour during ice recession. The data indicate that two separate phases of accelerated ice flow were followed by ice sheet stagnation during overall deglaciation. The second surge event was followed by a subglacial outburst flood, forming an intricate esker and crevasse-fill network. The data provide the first clear evidence that ice flow direction was eastward along the eastern watershed of the Shannon River basin, at odds with previous models, and raise the possibility that an ice stream existed in this area. Our work demonstrates the potential for airborne LiDAR surveys to produce detailed paleoglaciological reconstructions and to enhance our understanding of complex palaeo-ice sheet dynamics.

The Potsdam Parallel Ice Sheet Model (PISM-PIK) - Part 2: Dynamic equilibrium simulation of the Antarctic ice sheet

NASA Astrophysics Data System (ADS)

Martin, M. A.; Winkelmann, R.; Haseloff, M.; Albrecht, T.; Bueler, E.; Khroulev, C.; Levermann, A.

2010-08-01

We present a dynamic equilibrium simulation of the ice sheet-shelf system on Antarctica with the Potsdam Parallel Ice Sheet Model (PISM-PIK). The simulation is initialized with present-day conditions for topography and ice thickness and then run to steady state with constant present-day surface mass balance. Surface temperature and basal melt distribution are parameterized. Grounding lines and calving fronts are free to evolve, and their modeled equilibrium state is compared to observational data. A physically-motivated dynamic calving law based on horizontal spreading rates allows for realistic calving fronts for various types of shelves. Steady-state dynamics including surface velocity and ice flux are analyzed for whole Antarctica and the Ronne-Filchner and Ross ice shelf areas in particular. The results show that the different flow regimes in sheet and shelves, and the transition zone between them, are captured reasonably well, supporting the approach of superposition of SIA and SSA for the representation of fast motion of grounded ice. This approach also leads to a natural emergence of streams in this new 3-D marine ice sheet model.

Quaternary evolution of the Fennoscandian Ice Sheet from 3D seismic data

NASA Astrophysics Data System (ADS)

Montelli, A.; Dowdeswell, J. A.; Ottesen, D.; Johansen, S. E.

2016-12-01

The Quaternary seismic stratigraphy and architecture of the mid-Norwegian continental shelf and slope are investigated using extensive grids of marine 2D and 3D seismic reflection data that cover more than 100,000 km2 of the continental margin. At least 26 distinct regional palaeo-surfaces have been interpreted within the stratigraphy of the Quaternary Naust Formation on the mid-Norwegian margin. Multiple assemblages of buried glacigenic landforms are preserved within the Naust Formation across most of the study area, facilitating detailed palaeo-glaciological reconstructions. We document a marine-terminating, calving Fennoscandian Ice Sheet (FIS) margin present periodically on the Norwegian shelf since at least the beginning of the Quaternary. Elongate, streamlined landforms interpreted as mega-scale glacial lineations (MSGLs) have been found within the upper part of the Naust sequence N ( 1.9-1.6 Ma), sugesting the development of fast-flowing ice streams since that time. Shifts in the location of depocentres and direction of features indicative of fast ice-flow suggest that several reorganisations in the FIS drainage have occurred since 1.5 Ma. Subglacial landforms reveal a complex and dynamic ice sheet, with converging palaeo-ice streams and several flow-switching events that may reflect major changes in topography and internal ice-sheet structure. Lack of subglacial meltwater channels suggests a largely distributed, low-volume meltwater system that drained the FIS through permeable subglacial till without leaving much erosional evidence. This regional palaeo-environmental examination of the FIS provides a useful framework for ice-sheet modelling and shows that fragmentary preservation of buried surfaces and variability of ice-sheet dynamics should be taken into account when reconstructing glacial history from spatially limited datasets.

AVHRR imagery reveals Antarctic ice dynamics

NASA Technical Reports Server (NTRS)

Bindschadler, Robert A.; Vornberger, Patricia L.

1990-01-01

A portion of AVHRR data taken on December 5, 1987 at 06:15 GMT over a part of Antarctica is used here to show that many of the most significant dynamic features of ice sheets can be identified by a careful examination of AVHRR imagery. The relatively low resolution of this instrument makes it ideal for obtaining a broad view of the ice sheets, while its wide swath allows coverage of areas beyond the reach of high-resolution imagers either currently in orbit or planned. An interpretation is given of the present data, which cover the area of ice streams that drain the interior of the West Antarctic ice sheet into the Ross Ice Shelf.

The mass balance of the ice plain of Ice Stream B and Crary Ice Rise

NASA Technical Reports Server (NTRS)

Bindschadler, Robert

1993-01-01

The region in the mouth of Ice Stream B (the ice plain) and that in the vicinity of Crary Ice Rise are experiencing large and rapid changes. Based on velocity, ice thickness, and accumulation rate data, the patterns of net mass balance in these regions were calculated. Net mass balance, or the rate of ice thickness change, was calculated as the residual of all mass fluxes into and out of subregions (or boxes). Net mass balance provides a measure of the state of health of the ice sheet and clues to the current dynamics.

New age constraints for the Saalian glaciation in northern central Europe: Implications for the extent of ice sheets and related proglacial lake systems

NASA Astrophysics Data System (ADS)

Lang, Jörg; Lauer, Tobias; Winsemann, Jutta

2018-01-01

A comprehensive palaeogeographic reconstruction of ice sheets and related proglacial lake systems for the older Saalian glaciation in northern central Europe is presented, which is based on the integration of palaeo-ice flow data, till provenance, facies analysis, geomorphology and new luminescence ages of ice-marginal deposits. Three major ice advances with different ice-advance directions and source areas are indicated by palaeo-ice flow directions and till provenance. The first ice advance was characterised by a southwards directed ice flow and a dominance of clasts derived from southern Sweden. The second ice advance was initially characterised by an ice flow towards the southwest. Clasts are mainly derived from southern and central Sweden. The latest stage in the study area (third ice advance) was characterised by ice streaming (Hondsrug ice stream) in the west and a re-advance in the east. Clasts of this stage are mainly derived from eastern Fennoscandia. Numerical ages for the first ice advance are sparse, but may indicate a correlation with MIS 8 or early MIS 6. New pIRIR290 luminescence ages of ice-marginal deposits attributed to the second ice advance range from 175 ± 10 to 156 ± 24 ka and correlate with MIS 6. The ice sheets repeatedly blocked the main river-drainage pathways and led to the formation of extensive ice-dammed lakes. The formation of proglacial lakes was mainly controlled by ice-damming of river valleys and major bedrock spillways; therefore the lake levels and extends were very similar throughout the repeated ice advances. During deglaciation the lakes commonly increased in size and eventually drained successively towards the west and northwest into the Lower Rhine Embayment and the North Sea. Catastrophic lake-drainage events occurred when large overspill channels were suddenly opened. Ice-streaming at the end of the older Saalian glaciation was probably triggered by major lake-drainage events.

What can Subglacial Sediment Tell us About the Underlying Geology and the Dynamic of the West-Antarctic Ice Sheet?

NASA Astrophysics Data System (ADS)

Vogel, S. W.; Tulaczyk, S. M.; Carter, S.; Grunow, A.

2003-12-01

The West-Antarctic Ice Sheet (WAIS) is the second largest ice sheet in the world. Its dynamic is extensively studied due to the proposed threat of rapid disintegration and associated sea level rise (Mercer, 1971). Most of its ice drains through a few fast flowing (>100 m/yr) ice streams and outlet glaciers. Subglacial conditions in particular the distribution of basal water and the availability of subglacial sediment plays an important role for their location and extent. Subglacial geology in particular the distribution of sedimentary basin fill, providing material for a lubricating subglacial till layer, may pose a limit on the inland extent of the fast flowing ice stream. Subglacial volcanism and associated elevated geothermal heat fluxes may provide crucial subglacial melt water for ice stream lubrication. We have studied sediment from the base of the WAIS to elucidate questions about the existence of subglacial volcanism and to determine the provenance of the subglacial sediment. Within this study we measured clay mineralogy, sand petrography, magnetic and geochemical properties of subglacial and englacial sediment from different locations in the Ross Sea-catchment area of the WAIS. Our samples come from Whillans-, Kamb- and Bindschadler Ice Stream as well as from Siple Dome, Crary Ice Rise and Byrd Station. Most of our sediment samples represent samples of subglacial till, which in earlier studies have been characterized as reworked marine sediment of Cenozoic age. The englacial sediment samples come from basal ice. Our study so far has found no positive evidence for the existence of subglacial volcanism beneath the WAIS. The mineralogy as well as the REE-pattern of our samples correspond better with a crustal source for the sediment than Cenozoic basalts. The isotopic composition of our samples (Nd/Sm, Rb/Sr) show differences between individual ice streams locations as well as differences between different grain size fractions. TDM-ages range from ~900 Ma to 1800 Ma; ENd between -4 to -12 and 87Sr/86Sr ~0.715 to ~0.735. Our preliminary geochemical results so far point to rocks from outcrops in the upstream areas of the individual ice streams as provenance for their sediment (Horlick Mountains and Whitmore Mountains) with a possibly small East-Antarctic component.

The wide-spread presence of rib-like patterns in basal shear of ice streams detected by surface data inversion

NASA Astrophysics Data System (ADS)

Sergienko, O. V.

2013-12-01

The direct observations of the basal conditions under continental-scale ice sheets are logistically impossible. A possible approach to estimate conditions at the ice - bed interface is from surface observations by means of inverse methods. The recent advances in remote and ground-based observations have allowed to acquire a wealth observations from Greenland and Antarctic ice sheets. Using high-resolution data sets of ice surface and bed elevations and surface velocities, inversions for basal conditions have been performed for several ice streams in Greenland and Antarctica. The inversion results reveal the wide-spread presence of rib-like spatial structures in basal shear. The analysis of the hydraulic potential distribution shows that these rib-like structures co-locate with highs of the gradient of hydraulic potential. This suggests that subglacial water plays a role in the development and evolution of the basal shear ribs.

Ice flow in the Weddell Sea sector of West Antarctica as elucidated by radar-imaged internal layering

NASA Astrophysics Data System (ADS)

Bingham, R. G.; Rippin, D. M.; Karlsson, N. B.; Corr, H.; Ferraccioli, F.; Jordan, T. A.; Le Brocq, A.; Ross, N.; Wright, A.; Siegert, M. J.

2012-12-01

Radio-echo sounding (RES) across polar ice sheets reveals extensive, isochronous internal layers, whose stratigraphy, and especially their degree of continuity over multi-km distances, can inform us about both present ice flow and past ice-flow histories. Here, we bring together for the first time two recent advances in this field of cryospheric remote sensing to analyse ice flow into the Weddell Sea sector of West Antarctica. Firstly, we have developed a new quantitative routine for analysing the continuity of internal layers obtained over large areas of ice by airborne RES surveys - we term this routine the "Internal-Layering Continuity-Index (ILCI)". Secondly, in the austral season 2010-11 we acquired, by airborne RES survey, the first comprehensive dataset of deep internal layering across Institute and Möller Ice Streams, two of the more significant feeders of ice into the Filchner-Ronne Ice Shelf. Applying the ILCI to SAR-processed (migrated) RES profiles across Institute Ice Stream's catchment reveals two contrasting regions of internal-layering continuity behaviour. In the western portion of the catchment, where ice-stream tributaries incise deeply through the Ellsworth Subglacial Highlands, the continuity of internal layers is most disrupted across the present ice streams. We therefore interpret the ice-flow configuration in this western region as predominantly spatially stable over the lifetime of the ice. Further east, towards Möller Ice Stream, and towards the interior of the ice sheet, the ILCI does not closely match the present ice flow configuration, while across most of present-day Möller Ice Stream itself, the continuity of internal layers is generally low. We propose that the variation in continuity of internal layering across eastern Institute Ice Stream and the neighbouring Möller results primarily from two factors. Firstly, the noncorrespondence of some inland tributaries with internal-layering continuity acts as evidence for past spatial migration of those tributaries, with likely consequences for the relative positions of Institute and Möller Ice Streams over recent history. Secondly, the subglacial roughness, in part a function of the underlying geology across the region, imposes a strong influence on the continuity of the overlying deep internal layers, though whether it controls, or is a function of, ice flow, remains undetermined. We conclude that in the subglacially mountainous Ellsworth Subglacial Highlands sector, there is long-term stability in the spatial configuration of ice flow, but that elsewhere across Insitute and Möller Ice Streams, the ice-flow configuration has the potential to switch.

Sensitivity of simulated englacial isochrones to uncertain subglacial boundary conditions in central West Antarctica: Implications for detecting changes in ice dynamics

NASA Astrophysics Data System (ADS)

Muldoon, Gail; Jackson, Charles S.; Young, Duncan A.; Quartini, Enrica; Cavitte, Marie G. P.; Blankenship, Donald D.

2017-04-01

Information about the extent and dynamics of the West Antarctic Ice Sheet during past glaciations is preserved inside ice sheets themselves. Ice cores are capable of retrieving information about glacial history, but they are spatially sparse. Ice-penetrating radar, on the other hand, has been used to map large areas of the West Antarctic Ice Sheet and can be correlated to ice core chronologies. Englacial isochronous layers observed in ice-penetrating radar are the result of variations in ice composition, fabric, temperature and other factors. The shape of these isochronous surfaces is expected to encode information about past and present boundary conditions and ice dynamics. Dipping of englacial layers, for example, may reveal the presence of rapid ice flow through paleo ice streams or high geothermal heat flux. These layers therefore present a useful testbed for hypotheses about paleo ice sheet conditions. However, hypothesis testing requires careful consideration of the sensitivity of layer shape to the competing forces of ice sheet boundary conditions and ice dynamics over time. Controlled sensitivity tests are best completed using models, however ice sheet models generally do not have the capability of simulating layers in the presence of realistic boundary conditions. As such, modeling 3D englacial layers for comparison to observations is difficult and requires determination of a 3D ice velocity field. We present a method of post-processing simulated 3D ice sheet velocities into englacial isochronous layers using an advection scheme. We then test the sensitivity of layer geometry to uncertain boundary conditions, including heterogeneous subglacial geothermal flux and bedrock topography. By identifying areas of the ice sheet strongly influenced by boundary conditions, it may be possible to isolate the signature of paleo ice dynamics in the West Antarctic ice sheet.

Radar-imaged internal layering in the Weddell Sea sector of West Antarctica

NASA Astrophysics Data System (ADS)

Bingham, Robert G.; Rippin, David M.; Karlsson, Nanna B.; Corr, Hugh F. J.; Ferraccioli, Fausto; Jordan, Tom A.; Le Brocq, Anne M.; Ross, Neil; Wright, Andrew P.; Siegert, Martin J.

2013-04-01

Radio-echo sounding (RES) across polar ice sheets reveals extensive, isochronous internal layers, whose stratigraphy, and especially their degree of continuity over multi-km distances, can inform us about both present ice flow and past ice-flow histories. Here, we bring together for the first time two recent advances in this field of cryospheric remote sensing to analyse ice flow into the Weddell Sea sector of West Antarctica. Firstly, we have developed a new quantitative routine for analysing the continuity of internal layers obtained over large areas of ice by airborne RES surveys - we term this routine the "Internal-Layering Continuity-Index (ILCI)". Secondly, in the austral season 2010-11 we acquired, by airborne RES survey, the first comprehensive dataset of deep internal layering across Institute and Möller Ice Streams, two of the more significant feeders of ice into the Filchner-Ronne Ice Shelf. Applying the ILCI to SAR-processed (migrated) RES profiles across Institute Ice Stream's catchment reveals two contrasting regions of internal-layering continuity behaviour. In the western portion of the catchment, where ice-stream tributaries incise deeply through the Ellsworth Subglacial Highlands, the continuity of internal layers is most disrupted across the present ice streams. We therefore interpret the ice-flow configuration in this western region as predominantly spatially stable over the lifetime of the ice. Further east, towards Möller Ice Stream, and towards the interior of the ice sheet, the ILCI does not closely match the present ice flow configuration, while across most of present-day Möller Ice Stream itself, the continuity of internal layers is generally low. We propose that the variation in continuity of internal layering across eastern Institute Ice Stream and the neighbouring Möller results primarily from two factors. Firstly, the noncorrespondence of some inland tributaries with internal-layering continuity acts as evidence for past spatial migration of those tributaries, with likely consequences for the relative positions of Institute and Möller Ice Streams over recent history. Secondly, the subglacial roughness, in part a function of the underlying geology across the region, imposes a strong influence on the continuity of the overlying deep internal layers, though whether it controls, or is a function of, ice flow, remains undetermined. We conclude that in the subglacially mountainous Ellsworth Subglacial Highlands sector, there is long-term stability in the spatial configuration of ice flow, but that elsewhere across Insitute and Möller Ice Streams, the ice-flow configuration is not stable.

Ice stream motion facilitated by a shallow-deforming and accreting bed

PubMed Central

Spagnolo, Matteo; Phillips, Emrys; Piotrowski, Jan A.; Rea, Brice R.; Clark, Chris D.; Stokes, Chris R.; Carr, Simon J.; Ely, Jeremy C.; Ribolini, Adriano; Wysota, Wojciech; Szuman, Izabela

2016-01-01

Ice streams drain large portions of ice sheets and play a fundamental role in governing their response to atmospheric and oceanic forcing, with implications for sea-level change. The mechanisms that generate ice stream flow remain elusive. Basal sliding and/or bed deformation have been hypothesized, but ice stream beds are largely inaccessible. Here we present a comprehensive, multi-scale study of the internal structure of mega-scale glacial lineations (MSGLs) formed at the bed of a palaeo ice stream. Analyses were undertaken at macro- and microscales, using multiple techniques including X-ray tomography, thin sections and ground penetrating radar (GPR) acquisitions. Results reveal homogeneity in stratigraphy, kinematics, granulometry and petrography. The consistency of the physical and geological properties demonstrates a continuously accreting, shallow-deforming, bed and invariant basal conditions. This implies that ice stream basal motion on soft sediment beds during MSGL formation is accommodated by plastic deformation, facilitated by continuous sediment supply and an inefficient drainage system. PMID:26898399

Can Seismic Observations of Bed Conditions on Ice Streams Help Constrain Parameters in Ice Flow Models?

NASA Astrophysics Data System (ADS)

Kyrke-Smith, Teresa M.; Gudmundsson, G. Hilmar; Farrell, Patrick E.

2017-11-01

We investigate correlations between seismically derived estimates of basal acoustic impedance and basal slipperiness values obtained from a surface-to-bed inversion using a Stokes ice flow model. Using high-resolution measurements along several seismic profiles on Pine Island Glacier (PIG), we find no significant correlation at kilometer scale between acoustic impedance and either retrieved basal slipperiness or basal drag. However, there is a stronger correlation when comparing average values along the individual profiles. We hypothesize that the correlation appears at the length scales over which basal variations are important to large-scale ice sheet flow. Although the seismic technique is sensitive to the material properties of the bed, at present there is no clear way of incorporating high-resolution seismic measurements of bed properties on ice streams into ice flow models. We conclude that more theoretical work needs to be done before constraints on mechanical conditions at the ice-bed interface from acoustic impedance measurements can be of direct use to ice sheet models.

Thinning of the ice sheet in northwest Greenland over the past forty years.

PubMed

Paterson, W S; Reeh, N

2001-11-01

Thermal expansion of the oceans, as well as melting of glaciers, ice sheets and ice caps have been the main contributors to global sea level rise over the past century. The greatest uncertainty in predicting future sea level changes lies with our estimates of the mass balance of the ice sheets in Greenland and Antarctica. Satellite measurements have been used to determine changes in these ice sheets on short timescales, demonstrating that surface-elevation changes on timescales of decades or less result mainly from variations in snow accumulation. Here we present direct measurements of the changes in surface elevation between 1954 and 1995 on a traverse across the north Greenland ice sheet. Measurements over a time interval of this length should reflect changes in ice flow-the important quantity for predicting changes in sea level-relatively unperturbed by short-term fluctuations in snow accumulation. We find only small changes in the eastern part of the transect, except for some thickening of the north ice stream. On the west side, however, the thinning rates of the ice sheet are significantly higher and thinning extends to higher elevations than had been anticipated from previous studies.

The Potsdam Parallel Ice Sheet Model (PISM-PIK) - Part 2: Dynamic equilibrium simulation of the Antarctic ice sheet

NASA Astrophysics Data System (ADS)

Martin, M. A.; Winkelmann, R.; Haseloff, M.; Albrecht, T.; Bueler, E.; Khroulev, C.; Levermann, A.

2011-09-01

We present a dynamic equilibrium simulation of the ice sheet-shelf system on Antarctica with the Potsdam Parallel Ice Sheet Model (PISM-PIK). The simulation is initialized with present-day conditions for bed topography and ice thickness and then run to steady state with constant present-day surface mass balance. Surface temperature and sub-shelf basal melt distribution are parameterized. Grounding lines and calving fronts are free to evolve, and their modeled equilibrium state is compared to observational data. A physically-motivated calving law based on horizontal spreading rates allows for realistic calving fronts for various types of shelves. Steady-state dynamics including surface velocity and ice flux are analyzed for whole Antarctica and the Ronne-Filchner and Ross ice shelf areas in particular. The results show that the different flow regimes in sheet and shelves, and the transition zone between them, are captured reasonably well, supporting the approach of superposition of SIA and SSA for the representation of fast motion of grounded ice. This approach also leads to a natural emergence of sliding-dominated flow in stream-like features in this new 3-D marine ice sheet model.

Balance Velocities of the Greenland Ice Sheet

NASA Technical Reports Server (NTRS)

Joughin, Ian; Fahnestock, Mark; Ekholm, Simon; Kwok, Ron

1997-01-01

We present a map of balance velocities for the Greenland ice sheet. The resolution of the underlying DEM, which was derived primarily from radar altimetry data, yields far greater detail than earlier balance velocity estimates for Greenland. The velocity contours reveal in striking detail the location of an ice stream in northeastern Greenland, which was only recently discovered using satellite imagery. Enhanced flow associated with all of the major outlets is clearly visible, although small errors in the source data result in less accurate estimates of the absolute flow speeds. Nevertheless, the balance map is useful for ice-sheet modelling, mass balance studies, and field planning.

Antarctic glacial history from numerical models and continental margin sediments

USGS Publications Warehouse

Barker, P.F.; Barrett, P.J.; Cooper, A. K.; Huybrechts, P.

1999-01-01

The climate record of glacially transported sediments in prograded wedges around the Antarctic outer continental shelf, and their derivatives in continental rise drifts, may be combined to produce an Antarctic ice sheet history, using numerical models of ice sheet response to temperature and sea-level change. Examination of published models suggests several preliminary conclusions about ice sheet history. The ice sheet's present high sensitivity to sea-level change at short (orbital) periods was developed gradually as its size increased, replacing a declining sensitivity to temperature. Models suggest that the ice sheet grew abruptly to 40% (or possibly more) of its present size at the Eocene-Oligocene boundary, mainly as a result of its own temperature sensitivity. A large but more gradual middle Miocene change was externally driven, probably by development of the Antarctic Circumpolar Current (ACC) and Polar Front, provided that a few million years' delay can be explained. The Oligocene ice sheet varied considerably in size and areal extent, but the late Miocene ice sheet was more stable, though significantly warmer than today's. This difference probably relates to the confining effect of the Antarctic continental margin. Present-day numerical models of ice sheet development are sufficient to guide current sampling plans, but sea-ice formation, polar wander, basal topography and ice streaming can be identified as factors meriting additional modelling effort in the future.

Instability of the Antarctic Ross Sea Embayment as climate warms

NASA Astrophysics Data System (ADS)

Hughes, Terence; Zhao, Zihong; Hintz, Raymond; Fastook, James

2017-06-01

Collapse of the Antarctic Ice Sheet since the Last Glacial Maximum 18,000 years ago is most pronounced in the Ross Sea Embayment, which is partly ice-free during Antarctic summers, thereby breaching the O-ring of ice shelves and sea ice surrounding Antarctica that stabilizes the ice sheet. The O-ring may have vanished during Early Holocene (5000 to 3000 B.C.), Roman (1 to 400 A.D.), and Medieval (900 to 1300 A.D.) warm periods and reappeared during the Little Ice Age (1300 to 1900 A.D.). We postulate further collapse in the embayment during the post-1900 warming may be forestalled because East Antarctic outlet glaciers "nail" the Ross Ice Shelf to the Transantarctic Mountains so it can resist the push from West Antarctic ice streams. Our hypothesis is examined for Byrd Glacier and a static ice shelf using three modeling experiments having plastic, viscous, and viscoplastic solutions as more data and improved modeling became available. Observed crevasse patterns were not reproduced. A new research study is needed to model a dynamic Ross Ice Shelf with all its feeder ice streams, outlet glaciers, and ice calving dynamics in three dimensions over time to fully test our hypothesis. The required model must allow accelerated calving if further warming melts sea ice and discerps the ice shelf. Calving must then successively pull the outlet glacier "nails" so collapse of the marine West Antarctic Ice Sheet proceeds to completion.

Aerogeophysical evidence for complex subglacial geology below the Rutford drainage basin,WestAntarctica

NASA Astrophysics Data System (ADS)

Jones, P.; Ferraccioli, F.; Corr, H.; Smith, A. M.; King, E.; Vaughan, D.

2003-12-01

A significant part of the West Antarctic Ice Sheet appears to be imposed upon a complex and still largely unknown continental rift system, perhaps featuring sedimentary basins, thin crust and high heat flow. Subglacial geology has been postulated to strongly modulate the dynamics and stability of the ice sheet itself. Specifically, recent aerogeophysics collected over central West Antarctica at edge of the Whitmore Mountains crustal block show that narrow subglacial rift basins with thick sedimentary infill may control the onsets and lateral margins of ice streams. The British Antarctic Survey flew an aerogeophysical survey during the 2001-02 field season: the main aim was to investigate what factors control the location and dynamics of the onset region of the Rutford Ice stream. Airborne radar, aerogravity and aeromagnetic data were simultaneously collected over the drainage basin of the Rutford Ice Stream. The new bedrock elevation grid for the area shows that the Rutford Ice Stream is constrained by a deep bedrock trough with a N-S to NE-SW trend. The onset region appears however to lie within an E-W bedrock trough at the edge of the Ellsworth Mountains crustal block. Bouguer gravity maps do not reveal typical signatures for a coincident deep rift basin at this location. However, a sharp NE-SW trending gradient, likely separating crustal blocks with contrasting crustal thickness is revealed. Aeromagnetic data image NE-SW trends north of the Rutford Ice Stream. In the onset region, these trends appear to be truncated by a NNW-SSE trend, lying on strike with the Ellsworth Mountains. Hence, the new aerogeophysical data suggests greater complexity in the subglacial geology and structure of an onset region of an ice stream compared to previous investigations.

Ice stream reorganization and glacial retreat on the northwest Greenland shelf

NASA Astrophysics Data System (ADS)

Newton, A. M. W.; Knutz, P. C.; Huuse, M.; Gannon, P.; Brocklehurst, S. H.; Clausen, O. R.; Gong, Y.

2017-08-01

Understanding conditions at the grounding-line of marine-based ice sheets is essential for understanding ice sheet evolution. Offshore northwest Greenland, knowledge of the Last Glacial Maximum (LGM) ice sheet extent in Melville Bugt was previously based on sparse geological evidence. This study uses multibeam bathymetry, combined with 2-D and 3-D seismic reflection data, to present a detailed landform record from Melville Bugt. Seabed landforms include mega-scale glacial lineations, grounding-zone wedges, iceberg scours, and a lateral shear margin moraine, formed during the last glacial cycle. The geomorphology indicates that the LGM ice sheet reached the shelf edge before undergoing flow reorganization. After retreat of 80 km across the outer shelf, the margin stabilized in a mid-shelf position, possibly during the Younger Dryas (12.9-11.7 ka). The ice sheet then decoupled from the seafloor and retreated to a coast-proximal position. This landform record provides an important constraint on deglaciation history offshore northwest Greenland.

The Microseismicity of Glacier Sliding

NASA Astrophysics Data System (ADS)

Walter, Fabian; Röösli, Claudia; Kissling, Edi

2017-04-01

Our understanding of glacier and ice sheet basal motion remains incomplete. The past decades have witnessed a shift away from initially proposed hard bed theories towards soft, till-laden beds, which deform and thus participate in basal motion. The theoretical treatment of deformable beds is subject to debate, yet our capability to predict ice sheet flow and ultimately sea level rise is contingent upon correct parameterization of basal motion (Ritz et al., 2015). Both hard and soft bed theories neglect frictional sliding across distinct basal fault planes and elastic deformation in response to sudden dislocation. Over recent years, this view has been repeatedly challenged as more and more studies report seismogenic faulting associated with basal sliding. For instance, large parts of the Whillans Ice Stream at Antarctica's Siple Coast move nearly exclusively during sudden sliding episodes (Wiens et al., 2008). This "stick-slip motion" is difficult to explain with traditional glacier sliding theories but more analogous to earthquake dislocation on tectonic faults. Although the Whillans Ice Stream motion may be an extreme example, there exists evidence for much smaller microseismic stick-slip events beneath the Greenland Ice Sheet and non-polar glaciers (Podolskiy and Walter, 2016). This raises the question how relevant and widespread the stick-slip phenomenon is and if it is necessary to include it into ice sheet models. Here we discuss recent seismic deployments, which focused on detection of stick-slip events beneath the Greenland Ice Sheet and European Alpine Glaciers. For all deployments, a considerable challenge lies in detection of stick-slip seismograms in the presence of a dominant background seismicity associated with surface crevassing. Nevertheless, automatic search algorithms and waveform characteristics provide important insights into temporal variation of stick-slip activity as well as information about fault plane geometry and co-seismic sliding direction. REFERENCES E.A. Podolskiy and F. Walter (2016). Cryo-seismology. Reviews of Geophysics. Ritz, C., Edwards, T. L., Durand, G., Payne, A. J., Peyaud, V., & Hindmarsh, R. C. (2015). Potential sea-level rise from Antarctic ice-sheet instability constrained by observations. Nature, 528(7580), 115-118. Wiens, D. A., Anandakrishnan, S., Winberry, J. P., & King, M. A. (2008). Simultaneous teleseismic and geodetic observations of the stick-slip motion of an Antarctic ice stream. Nature, 453(7196), 770-774.

Controls on the early Holocene collapse of the Bothnian Sea Ice Stream

NASA Astrophysics Data System (ADS)

Clason, Caroline C.; Greenwood, Sarah L.; Selmes, Nick; Lea, James M.; Jamieson, Stewart S. R.; Nick, Faezeh M.; Holmlund, Per

2016-12-01

New high-resolution multibeam data in the Gulf of Bothnia reveal for the first time the subglacial environment of a Bothnian Sea Ice Stream. The geomorphological record suggests that increased meltwater production may have been important in driving rapid retreat of Bothnian Sea Ice during deglaciation. Here we apply a well-established, one-dimensional flow line model to simulate ice flow through the Gulf of Bothnia and investigate controls on retreat of the ice stream during the post-Younger Dryas deglaciation of the Fennoscandian Ice Sheet. The relative influence of atmospheric and marine forcings are investigated, with the modeled ice stream exhibiting much greater sensitivity to surface melting, implemented through surface mass balance and hydrofracture-induced calving, than to submarine melting or relative sea level change. Such sensitivity is supported by the presence of extensive meltwater features in the geomorphological record. The modeled ice stream does not demonstrate significant sensitivity to changes in prescribed ice stream width or overall bed slope, but local variations in basal topography and ice stream width result in nonlinear retreat of the grounding line, notably demonstrating points of short-lived retreat slowdown on reverse bed slopes. Retreat of the ice stream was most likely governed by increased ice surface meltwater production, with the modeled retreat rate less sensitive to marine forcings despite the marine setting.

Analogue modelling of the influence of ice shelf collapse on the flow of ice sheets grounded below sea-level

NASA Astrophysics Data System (ADS)

Corti, Giacomo; Zeoli, Antonio

2016-04-01

The sudden breakup of ice shelves is expected to result in significant acceleration of inland glaciers, a process related to the removal of the buttressing effect exerted by the ice shelf on the tributary glaciers. This effect has been tested in previous analogue models, which however applied to ice sheets grounded above sea level (e.g., East Antarctic Ice Sheet; Antarctic Peninsula and the Larsen Ice Shelf). In this work we expand these previous results by performing small-scale laboratory models that analyse the influence of ice shelf collapse on the flow of ice streams draining an ice sheet grounded below sea level (e.g., the West Antarctic Ice Sheet). The analogue models, with dimensions (width, length, thickness) of 120x70x1.5cm were performed at the Tectonic Modelling Laboratory of CNR-IGG of Florence, Italy, by using Polydimethilsyloxane (PDMS) as analogue for the flowing ice. This transparent, Newtonian silicone has been shown to well approximate the rheology of natural ice. The silicone was allowed to flow into a water reservoir simulating natural conditions in which ice streams flow into the sea, terminating in extensive ice shelves which act as a buttress for their glaciers and slow their flow. The geometric scaling ratio was 10(-5), such that 1cm in the models simulated 1km in nature; velocity of PDMS (a few mm per hour) simulated natural velocities of 100-1000 m/year. Instability of glacier flow was induced by manually removing a basal silicone platform (floating on water) exerting backstresses to the flowing analogue glacier: the simple set-up adopted in the experiments isolates the effect of the removal of the buttressing effect that the floating platform exerts on the flowing glaciers, thus offering insights into the influence of this parameter on the flow perturbations resulting from a collapse event. The experimental results showed a significant increase in glacier velocity close to its outlet following ice shelf breakup, a process similar to what observed in previous models. This transient effect did not significantly propagate upstream towards the inner parts of ice sheet, and rapidly decayed with time. The process was also accompanied by significant ice thinning. Models results suggest that the ice sheet is almost unaffected by flow perturbations induced by ice shelf collapse, unless other processes (e.g., grounding line instability induced by warm water penetration) are involved.

Reactivation of Kamb Ice Stream tributaries triggers century-scale reorganization of Siple Coast ice flow in West Antarctica

DOE PAGES

Bougamont, M.; Christoffersen, P.; Price, S. F.; ...

2015-10-21

Ongoing, centennial-scale flow variability within the Ross ice streams of West Antarctica suggests that the present-day positive mass balance in this region may reverse in the future. Here we use a three-dimensional ice sheet model to simulate ice flow in this region over 250 years. The flow responds to changing basal properties, as a subglacial till layer interacts with water transported in an active subglacial hydrological system. We show that a persistent weak bed beneath the tributaries of the dormant Kamb Ice Stream is a source of internal ice flow instability, which reorganizes all ice streams in this region, leadingmore » to a reduced (positive) mass balance within decades and a net loss of ice within two centuries. This hitherto unaccounted for flow variability could raise sea level by 5 mm this century. Furthermore, better constraints on future sea level change from this region will require improved estimates of geothermal heat flux and subglacial water transport.« less

The feasibility of imaging subglacial hydrology beneath ice streams with ground-based electromagnetics

NASA Astrophysics Data System (ADS)

Siegfried, M. R.; Key, K.

2017-12-01

Subglacial hydrologic systems in Antarctica and Greenland play a fundamental role in ice-sheet dynamics, yet critical aspects of these systems remain poorly understood due to a lack of observations. Ground-based electromagnetic (EM) geophysical methods are established for mapping groundwater in many environments, but have never been applied to imaging lakes beneath ice sheets. Here we study the feasibility of passive and active source EM imaging for quantifying the nature of subglacial water systems beneath ice streams, with an emphasis on the interfaces between ice and basal meltwater, as well as deeper groundwater in the underlying sediments. Specifically, we look at the passive magnetotelluric method and active-source EM methods that use a large loop transmitter and receivers that measure either frequency-domain or transient soundings. We describe a suite of model studies that exam the data sensitivity as a function of ice thickness, water conductivity and hydrologic system geometry for models representative of a subglacial lake and a grounding zone estuary. We show that EM data are directly sensitive to groundwater and can image its lateral and depth extent. By combining the conductivity obtained from EM data with ice thickness and geological structure from conventional geophysical techniques such as ground-penetrating radar and active seismic techniques, EM data have the potential to provide new insights on the interaction between ice, rock, and water at critical ice-sheet boundaries.

Bacteria beneath the West Antarctic ice sheet.

PubMed

Lanoil, Brian; Skidmore, Mark; Priscu, John C; Han, Sukkyun; Foo, Wilson; Vogel, Stefan W; Tulaczyk, Slawek; Engelhardt, Hermann

2009-03-01

Subglacial environments, particularly those that lie beneath polar ice sheets, are beginning to be recognized as an important part of Earth's biosphere. However, except for indirect indications of microbial assemblages in subglacial Lake Vostok, Antarctica, no sub-ice sheet environments have been shown to support microbial ecosystems. Here we report 16S rRNA gene and isolate diversity in sediments collected from beneath the Kamb Ice Stream, West Antarctic Ice Sheet and stored for 15 months at 4 degrees C. This is the first report of microbes in samples from the sediment environment beneath the Antarctic Ice Sheet. The cells were abundant ( approximately 10(7) cells g(-1)) but displayed low diversity (only five phylotypes), likely as a result of enrichment during storage. Isolates were cold tolerant and the 16S rRNA gene diversity was a simplified version of that found in subglacial alpine and Arctic sediments and water. Although in situ cell abundance and the extent of wet sediments beneath the Antarctic ice sheet can only be roughly extrapolated on the basis of this sample, it is clear that the subglacial ecosystem contains a significant and previously unrecognized pool of microbial cells and associated organic carbon that could potentially have significant implications for global geochemical processes.

Widespread Refreezing of Both Surface and Basal Melt Water Beneath the Greenland Ice Sheet

NASA Astrophysics Data System (ADS)

Bell, R. E.; Tinto, K. J.; Das, I.; Wolovick, M.; Chu, W.; Creyts, T. T.; Frearson, N.

2013-12-01

The isotopically and chemically distinct, bubble-free ice observed along the Greenland Ice Sheet margin both in the Russell Glacier and north of Jacobshavn must have formed when water froze from subglacial networks. Where this refreezing occurs and what impact it has on ice sheet processes remain unclear. We use airborne radar data to demonstrate that freeze-on to the ice sheet base and associated deformation produce large ice units up to 700 m thick throughout northern Greenland. Along the ice sheet margin, in the ablation zone, surface meltwater, delivered via moulins, refreezes to the ice sheet base over rugged topography. In the interior, water melted from the ice sheet base is refrozen and surrounded by folded ice. A significant fraction of the ice sheet is modified by basal freeze-on and associated deformation. For the Eqip and Petermann catchments, representing the ice sheet margin and interior respectively, extensive airborne radar datasets show that 10%-13% of the base of the ice sheet and up to a third of the catchment width is modified by basal freeze-on. The interior units develop over relatively subdued topography with modest water flux from basal melt where conductive cooling likely dominates. Steps in the bed topography associated with subglacial valley networks may foster glaciohydraulic supercooling. The ablation zone units develop where both surface melt and crevassing are widespread and large volumes of surface meltwater will reach the base of the ice sheet. The relatively steep topography at the upslope edge of the ablation zone units combined with the larger water flux suggests that supercooling plays a greater role in their formation. The ice qualities of the ablation zone units should reflect the relatively fresh surface melt whereas the chemistry of the interior units should reflect solute-rich basal melt. Changes in basal conditions such as the presence of till patches may contribute to the formation of the large basal units near the Northeast Ice Stream. The contrasting rheology of glacial and interglacial ice may also enhance the deformation associated with freeze-on beneath large ice sheets. The occurrence of basal units both in the ice sheet interior and in the thermally very different ablation zone indicates refreezing is widespread and can occur in many environments beneath an ice sheet. This process appears to influence the morphology and behavior of the ice sheet from top to bottom.

Eastern Ross Ice Sheet Deglacial History inferred from the Roosevelt Island Ice Core

NASA Astrophysics Data System (ADS)

Fudge, T. J.; Buizert, C.; Lee, J.; Waddington, E. D.; Bertler, N. A. N.; Conway, H.; Brook, E.; Severinghaus, J. P.

2017-12-01

The Ross Ice Sheet drains large portions of both West and East Antarctica. Understanding the retreat of the Ross Ice Sheet following the Last Glacial Maximum is particularly difficult in the eastern Ross area where there is no exposed rock and the Ross Ice Shelf prevents extensive bathymetric mapping. Coastal domes, by preserving old ice, can be used to infer the establishment of grounded ice and be used to infer past ice thickness. Here we focus on Roosevelt Island, in the eastern Ross Sea, where the Roosevelt Island Climate Evolution project recently completed an ice core to bedrock. Using ice-flow modeling constrained by the depth-age relationship and an independent estimate of accumulation rate from firn-densification measurements and modeling, we infer ice thickness histories for the LGM (20ka) to present. Preliminary results indicate thinning of 300m between 15ka and 12ka is required. This is similar to the amount and timing of thinning inferred at Siple Dome, in the central Ross Sea (Waddington et al., 2005; Price et al., 2007) and supports the presence of active ice streams throughout the Ross Ice Sheet advance during the LGM.

Extensive retreat and re-advance of the West Antarctic Ice Sheet during the Holocene.

PubMed

Kingslake, J; Scherer, R P; Albrecht, T; Coenen, J; Powell, R D; Reese, R; Stansell, N D; Tulaczyk, S; Wearing, M G; Whitehouse, P L

2018-06-01

To predict the future contributions of the Antarctic ice sheets to sea-level rise, numerical models use reconstructions of past ice-sheet retreat after the Last Glacial Maximum to tune model parameters 1 . Reconstructions of the West Antarctic Ice Sheet have assumed that it retreated progressively throughout the Holocene epoch (the past 11,500 years or so) 2-4 . Here we show, however, that over this period the grounding line of the West Antarctic Ice Sheet (which marks the point at which it is no longer in contact with the ground and becomes a floating ice shelf) retreated several hundred kilometres inland of today's grounding line, before isostatic rebound caused it to re-advance to its present position. Our evidence includes, first, radiocarbon dating of sediment cores recovered from beneath the ice streams of the Ross Sea sector, indicating widespread Holocene marine exposure; and second, ice-penetrating radar observations of englacial structure in the Weddell Sea sector, indicating ice-shelf grounding. We explore the implications of these findings with an ice-sheet model. Modelled re-advance of the grounding line in the Holocene requires ice-shelf grounding caused by isostatic rebound. Our findings overturn the assumption of progressive retreat of the grounding line during the Holocene in West Antarctica, and corroborate previous suggestions of ice-sheet re-advance 5 . Rebound-driven stabilizing processes were apparently able to halt and reverse climate-initiated ice loss. Whether these processes can reverse present-day ice loss 6 on millennial timescales will depend on bedrock topography and mantle viscosity-parameters that are difficult to measure and to incorporate into ice-sheet models.

Long-term record of Barents Sea Ice Sheet advance to the shelf edge from a 140,000 year record

NASA Astrophysics Data System (ADS)

Pope, Ed L.; Talling, Peter J.; Hunt, James E.; Dowdeswell, Julian A.; Allin, Joshua R.; Cartigny, Matthieu J. B.; Long, David; Mozzato, Alessandro; Stanford, Jennifer D.; Tappin, David R.; Watts, Millie

2016-10-01

The full-glacial extent and deglacial behaviour of marine-based ice sheets, such as the Barents Sea Ice Sheet, is well documented since the Last Glacial Maximum about 20,000 years ago. However, reworking of older sea-floor sediments and landforms during repeated Quaternary advances across the shelf typically obscures their longer-term behaviour, which hampers our understanding. Here, we provide the first detailed long-term record of Barents Sea Ice Sheet advances, using the timing of debris-flows on the Bear Island Trough-Mouth Fan. Ice advanced to the shelf edge during four distinct periods over the last 140,000 years. By far the largest sediment volumes were delivered during the oldest advance more than 128,000 years ago. Later advances occurred from 68,000 to 60,000, 39,400 to 36,000 and 26,000 to 20,900 years before present. The debris-flows indicate that the dynamics of the Saalian and the Weichselian Barents Sea Ice Sheet were very different. The repeated ice advance and retreat cycles during the Weichselian were shorter lived than those seen in the Saalian. Sediment composition shows the configuration of the ice sheet was also different between the two glacial periods, implying that the ice feeding the Bear Island Ice stream came predominantly from Scandinavia during the Saalian, whilst it drained more ice from east of Svalbard during the Weichselian.

Fire beneath the ice

DOE Office of Scientific and Technical Information (OSTI.GOV)

Monastersky, R.

1993-02-13

A volcano discovered six years ago by researchers Blankenship and Bell under Antarctica poses questions about a potential climatic catastrophe. The researchers claim that the volcano is still active, erupting occasionally and growing. A circular depression on the surface of the ice sheet has ice flowing into it and is used to provide a portrait of the heat source. The volcano is on a critical transition zone within West Antarctica with fast flowing ice streams directly downhill. Work by Blankenship shows that a soft layer of water-logged sediments called till provide the lubricating layer on the underside of the icemore » streams. Volcanos may provide the source of this till. The ice streams buffer the thick interior ice from the ocean and no one know what will happen if the ice streams continue to shorten. These researchers believe their results indicate that the stability of West Antarctica ultimately depends less on the current climate than on the location of heat and sediments under the ice and the legacy of past climatic changes.« less

Upper Ocean Circulation in the Glacial Northeast Atlantic during Heinrich Stadials Ice-Sheet Retreat

NASA Astrophysics Data System (ADS)

Toucanne, S.; Soulet, G.; Bosq, M.; Marjolaine, S.; Zaragosi, S.; Bourillet, J. F.; Bayon, G.

2016-12-01

Intermediate ocean water variability is involved in climate changes over geological timescales. As a prominent example, changes in North Atlantic subsurface water properties (including warming) during Heinrich Stadials may have triggered the so-called Heinrich events through ice-shelf loss and attendant ice-stream acceleration. While the origin of Heinrich Stadials and subsequent iceberg calving remains controversial, paleoceanographic research efforts mainly focus on the deep Atlantic overturning, leaving the upper ocean largely unexplored. To further evaluate variability in upper ocean circulation and its possible relationship with ice-sheet instabilities, a depth-transect of eight cores (BOBGEO and GITAN-TANDEM cruises) from the Northeast Atlantic (down to 2 km water depth) have been used to investigate kinematic and chemical changes in the upper ocean during the last glacial period. Our results reveal that near-bottom flow speeds (reconstructed by using sortable silt mean grain-size and X-ray fluorescence core-scanner Zr/Rb ratio) and water-masses chemistry (carbon and neodymium isotopes performed on foraminifera) substantially changed in phase with the millennial-scale climate changes recognized in the ice-core records. Our results are compared with paleoceanographic reconstructions of the 'Western Boundary Undercurrent' in order to discuss regional hydrographic differences at both sides of the North Atlantic, as well as with the fluctuations of both the marine- (through ice-rafted debris) and terrestrial-terminating ice-streams (through meltwater discharges) of the circum-Atlantic ice-sheets. Particular attention will be given to the Heinrich Stadials and concomitant Channel River meltwater discharges into the Northeast Atlantic in response to the melting of the European Ice-Sheet. This comparison helps to disentangle the cryosphere-ocean interactions throughout the last ice age, and the sequence of events occurring in the course of the Heinrich Stadials.

New aerogeophysical data reveal the extent of the Weddell Sea Rift beneath the Institute and Möller ice streams

NASA Astrophysics Data System (ADS)

Jordan, T. A.; Ferraccioli, F.; Siegert, M. J.; Ross, N.; Corr, H.; Bingham, R. G.; Rippin, D. M.; Le Brocq, A. M.

2011-12-01

Significant continental rifting associated with Gondwana breakup has been widely recognised in the Weddell Sea region. However, plate reconstructions and the extent of this rift system onshore beneath the West Antarctic Ice Sheet (WAIS) are ambiguous, due to the paucity of modern geophysical data across the Institute and Möller ice stream catchments. Understanding this region is key to unravelling Gondwana breakup and the possible kinematic links between the Weddell Sea and the West Antarctic Rift System. The nature of the underlying tectonic structure is also critical, as it provides the template for ice-flow draining ~20% of the West Antarctic Ice Sheet (WAIS). During the 2010/11 Antarctic field season ~25,000 km of new airborne radar, aerogravity and aeromagnetic data were collected to help unveil the crustal structure and geological boundary conditions beneath the Institute and Möller ice streams. Our new potential field maps delineate varied subglacial geology beneath the glacial catchments, including Jurassic intrusive rocks, sedimentary basins, and Precambrian basement rocks of the Ellsworth Mountains. Inversion of airborne gravity data reveal significant crustal thinning directly beneath the faster flowing coastal parts of the Institute and Möller ice streams. We suggest that continental rifting focussed along the Weddell Sea margin of the Ellsworth-Whitmore Mountains block, providing geological controls for the fast flowing ice streams of the Weddell Sea Embayment. Further to the south we suggest that strike-slip motion between the East Antarctica and the Ellsworth-Whitmore Mountains block may provide a kinematic link between Cretaceous-Cenozoic extension in the West Antarctic Rift System and deformation in the Weddell Sea Embayment.

Implications of 36Cl exposure ages from Skye, northwest Scotland for the timing of ice stream deglaciation and deglacial ice dynamics

NASA Astrophysics Data System (ADS)

Small, David; Rinterknecht, Vincent; Austin, William E. N.; Bates, Richard; Benn, Douglas I.; Scourse, James D.; Bourlès, Didier L.; Hibbert, Fiona D.

2016-10-01

Geochronological constraints on the deglaciation of former marine based ice streams provide information on the rates and modes by which marine based ice sheets have responded to external forcing factors such as climate change. This paper presents new 36Cl cosmic ray exposure dating from boulders located on two moraines (Glen Brittle and Loch Scavaig) in southern Skye, northwest Scotland. Ages from the Glen Brittle moraines constrain deglaciation of a major marine terminating ice stream, the Barra-Donegal Ice Stream that drained the former British-Irish Ice Sheet, depending on choice of production method and scaling model this occurred 19.9 ± 1.5-17.6 ± 1.3 ka ago. We compare this timing of deglaciation to existing geochronological data and changes in a variety of potential forcing factors constrained through proxy records and numerical models to determine what deglaciation age is most consistent with existing evidence. Another small section of moraine, the Scavaig moraine, is traced offshore through multibeam swath-bathymetry and interpreted as delimiting a later stillstand/readvance stage following ice stream deglaciation. Additional cosmic ray exposure dating from the onshore portion of this moraine indicate that it was deposited 16.3 ± 1.3-15.2 ± 0.9 ka ago. When calculated using the most up-to-date scaling scheme this time of deposition is, within uncertainty, the same as the timing of a widely identified readvance, the Wester Ross Readvance, observed elsewhere in northwest Scotland. This extends the area over which this readvance has potentially occurred, reinforcing the view that it was climatically forced.

Sensing the bed-rock movement due to ice unloading from space using InSAR time-series

NASA Astrophysics Data System (ADS)

Zhao, W.; Amelung, F.; Dixon, T. H.; Wdowinski, S.

2014-12-01

Ice-sheets in the Arctic region are retreating rapidly since late 1990s. Typical ice loss rates are 0.5 - 1 m/yr at the Canadian Arctic Archipelago, ~ 1 m/yr at the Icelandic ice sheets, and several meters per year at the edge of Greenland ice sheet. Such load decreasing causes measurable (several millimeter per year) deformation of the Earth's crust from Synthetic Aperture Radar Interferometry (InSAR). Using small baseline time-series analysis, this signal is retrieved after noises such as orbit error, atmospheric delay and DEM error being removed. We present results from Vatnajokull ice cap, Petermann glacier and Barnes ice cap using ERS, Envisat and TerraSAR-X data. Up to 2 cm/yr relative radar line-of-sight displacement is detected. The pattern of deformation matches the shape of ice sheet very well. The result in Iceland was used to develop a new model for the ice mass balance estimation from 1995 to 2010. Other applications of this kind of technique include validation of ICESat or GRACE based ice sheet model, Earth's rheology (Young's modulus, viscosity and so on). Moreover, we find a narrow (~ 1km) uplift zone close to the periglacial area of Petermann glacier which may due to a special rheology under the ice stream.

Ice-Sheet Glaciation of the Puget lowland, Washington, during the Vashon Stade (late pleistocene)

USGS Publications Warehouse

Thorson, R.M.

1980-01-01

During the Vashon Stade of the Fraser Glaciation, about 15,000-13,000 yr B.P., a lobe of the Cordilleran Ice Sheet occupied the Puget lowland of western Washington. At its maximum extent about 14,000 yr ago, the ice sheet extended across the Puget lowland between the Cascade Range and Olympic Mountains and terminated about 80 km south of Seattle. Meltwater streams drained southwest to the Pacific Ocean and built broad outwash trains south of the ice margin. Reconstructed longitudinal profiles for the Puget lobe at its maximum extent are similar to the modern profile of Malaspina Glacier, Alaska, suggesting that the ice sheet may have been in a near-equilibrium state at the glacial maximum. Progressive northward retreat from the terminal zone was accompanied by the development of ice-marginal streams and proglacial lakes that drained southward during initial retreat, but northward during late Vashon time. Relatively rapid retreat of the Juan de Fuca lobe may have contributed to partial stagnation of the northwestern part of the Puget lobe. Final destruction of the Puget lobe occurred when the ice retreated north of Admiralty Inlet. The sea entered the Puget lowland at this time, allowing the deposition of glacial-marine sediments which now occur as high as 50 m altitude. These deposits, together with ice-marginal meltwater channels presumed to have formed above sea level during deglaciation, suggest that a significant amount of postglacial isostatic and(or) tectonic deformation has occurred in the Puget lowland since deglaciation. ?? 1980.

Timing, variability and sediment provenance of the Norwegian Channel Ice Stream during the Last Glacial Maximum

NASA Astrophysics Data System (ADS)

Becker, L. W. M.; Sejrup, H. P.; Hjelstuen, B. O. B.; Haflidason, H.

2016-12-01

The extent of the NW European ice sheet during the Last Glacial Maximum is fairly well constrained to, at least in periods, the shelf edge. However, the exact timing and varying activity of the largest ice stream, the Norwegian Channel Ice Stream (NCIS), remains uncertain. We here present three sediment records, recovered proximal and distal to the upper NW European continental slope. All age models for the cores are constructed in the same way and based solely on 14C dating of planktonic foraminifera. The sand-sized sediments in the discussed cores is believed to be primarily transported by ice rafting. All records suggest ice streaming activity between 25.8 and 18.5 ka BP. However, the core proximal to the mouth of the Norwegian Channel (NC) shows distinct periods of activity and periods of very little coarse sediment input. Out of this there appear to be at least three well-defined periods of ice streaming activity which lasted each for 1.5 to 2 ka, with "pauses" of several hundred years in between. The same core shows a conspicuous variation in several proxies and sediment colour within the first peak of ice stream activity, compared to the second and third peak. The light grey colour of the sediment was earlier attributed to Triassic chalk grains, yet all "chalk" grains are in fact mollusc fragments. The low magnetic susceptibility values, the high Ca, high Sr and low Fe content compared to the other peaks suggests a different provenance for the material of the first peak. We suggest therefore, that the origin of this material is rather the British Irish Ice Sheet (BIIS) and not the Fennoscandian Ice Sheet (FIS). Earlier studies have shown an extent of the BIIS at least to the NC, whereas ice from the FIS likely stayed within the boundaries of the NC. A possible scenario for the different provenance could therefore be the build-up of the BIIS into the NC until it merged with the FIS. At this point the BIIS calved off the shelf edge southwest of the mouth of the NC, delivering material with BIIS origin to the proximal cores. The NCIS became as such possibly only active from the second `push' of material ( 23.0 to 18.5 ka BP). This is in agreement with the relatively low accumulation rates during the first peak and the input of coarse sediments in a southern, slightly more distal core, only during the first peak.

West-Antarctic Ice Streams: Analog to Ice Flow in Channels on Mars

NASA Technical Reports Server (NTRS)

Lucchitta, B. K.

1997-01-01

Sounding of the sea floor in front of the Ross Ice Shelf in Antarctica recently revealed large persistent patterns of longitudinal megaflutes and drumlinoid forms, which are interpreted to have formed at the base of ice streams during the list glacial advance. The flutes bear remarkable resemblance to longitudinal grooves and highly elongated streamlined islands found on the floors of some large martian channels, called outflow channels. ln addition, other similarities exist between Antarctic ice streams and outflow channels. Ice streams are 30 to 80 km wide and hundreds of kilometers long, as are the martian channels. Ice stream beds are below sea level. Floors of many martian outflow channels lie below martian datum, which may have been close to or below past martian sea levels. The Antarctic ice stream bed gradient is flat and locally may go uphill, and surface slopes are exceptionally low. So are gradients of martian channels. The depth to the bed in ice streams is 1 to 1.5 km. At bankful stage, the depth of the fluid in outflow channels would have been 1 to 2 km. These similarities suggest that the martian outflow channels, whose origin is commonly attributed to gigantic catastrophic floods, were locally filled by ice that left a conspicuous morphologic imprint. Unlike the West-Antarctic-ice streams, which discharge ice from an ice sheet, ice in the martian channels came from water erupting from the ground. In the cold martian environment, this water, if of moderate volume, would eventually freeze. Thus it may have formed icings on springs, ice dams and jams on constrictions in the channel path, or frozen pools. Given sufficient thickness and downhill surface gradient, these ice masses would have moved; and given the right conditions, they could have moved like Antarctic ice streams.

A balanced water layer concept for subglacial hydrology in large scale ice sheet models

NASA Astrophysics Data System (ADS)

Goeller, S.; Thoma, M.; Grosfeld, K.; Miller, H.

2012-12-01

There is currently no doubt about the existence of a wide-spread hydrological network under the Antarctic ice sheet, which lubricates the ice base and thus leads to increased ice velocities. Consequently, ice models should incorporate basal hydrology to obtain meaningful results for future ice dynamics and their contribution to global sea level rise. Here, we introduce the balanced water layer concept, covering two prominent subglacial hydrological features for ice sheet modeling on a continental scale: the evolution of subglacial lakes and balance water fluxes. We couple it to the thermomechanical ice-flow model RIMBAY and apply it to a synthetic model domain inspired by the Gamburtsev Mountains, Antarctica. In our experiments we demonstrate the dynamic generation of subglacial lakes and their impact on the velocity field of the overlaying ice sheet, resulting in a negative ice mass balance. Furthermore, we introduce an elementary parametrization of the water flux-basal sliding coupling and reveal the predominance of the ice loss through the resulting ice streams against the stabilizing influence of less hydrologically active areas. We point out, that established balance flux schemes quantify these effects only partially as their ability to store subglacial water is lacking.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cornford, S. L.; Martin, D. F.; Lee, V.

At least in conventional hydrostatic ice-sheet models, the numerical error associated with grounding line dynamics can be reduced by modifications to the discretization scheme. These involve altering the integration formulae for the basal traction and/or driving stress close to the grounding line and exhibit lower – if still first-order – error in the MISMIP3d experiments. MISMIP3d may not represent the variety of real ice streams, in that it lacks strong lateral stresses, and imposes a large basal traction at the grounding line. We study resolution sensitivity in the context of extreme forcing simulations of the entire Antarctic ice sheet, using the BISICLES adaptive mesh ice-sheet model with two schemes: the original treatment, and a scheme, which modifies the discretization of the basal traction. The second scheme does indeed improve accuracy – by around a factor of two – for a given mesh spacing, butmore » $$\\lesssim 1$$ km resolution is still necessary. For example, in coarser resolution simulations Thwaites Glacier retreats so slowly that other ice streams divert its trunk. In contrast, with $$\\lesssim 1$$ km meshes, the same glacier retreats far more quickly and triggers the final phase of West Antarctic collapse a century before any such diversion can take place.« less

Ice sheet-ocean interactions and sea level change

NASA Astrophysics Data System (ADS)

Heimbach, Patrick

2014-03-01

Mass loss from the Greenland and Antarctic ice sheets has increased rapidly since the mid-1990s. Their combined loss now accounts for about one-third of global sea level rise. In Greenland, a growing body of evidence points to the marine margins of these glaciers as the region from which this dynamic response originated. Similarly, ice streams in West Antarctica that feed vast floating ice shelves have exhibited large decadal changes. We review observational evidence and present physical mechanisms that might explain the observed changes, in particular in the context of ice sheet-ocean interactions. Processes involve cover 7 orders of magnitudes of scales, ranging from mm boundary-layer processes to basin-scale coupled atmosphere-ocean variability. We discuss observational needs to fill the gap in our mechanistic understanding.

Grounding Zones, Subglacial Lakes, and Dynamics of an Antarctic Ice Stream: The WISSARD Glaciological Experiment

NASA Astrophysics Data System (ADS)

Tulaczyk, S. M.; Schwartz, S. Y.; Fisher, A. T.; Powell, R. D.; Fricker, H. A.; Anandakrishnan, S.; Horgan, H. J.; Scherer, R. P.; Walter, J. I.; Siegfried, M. R.; Mikucki, J.; Christianson, K.; Beem, L.; Mankoff, K. D.; Carter, S. P.; Hodson, T. O.; Marsh, O.; Barcheck, C. G.; Branecky, C.; Neuhaus, S.; Jacobel, R. W.

2015-12-01

Interactions of West Antarctic ice streams with meltwater at their beds, and with seawater at their grounding lines, are widely considered to be the primary drivers of ice stream flow variability on different timescales. Understanding of processes controlling ice flow variability is needed to build quantitative models of the Antarctic Ice Sheet that can be used to help predict its future behavior and to reconstruct its past evolution. The ice plain of Whillans Ice Stream provides a natural glaciological laboratory for investigations of Antarctic ice flow dynamics because of its highly variable flow rate modulated by tidal processes and fill-drain cycles of subglacial lakes. Moreover, this part of Antarctica has one of the longest time series of glaciological observations, which can be used to put recently acquired datasets in a multi-decadal context. Since 2007 Whillans Ice Stream has been the focus of a regional glaciological experiment, which included surface GPS and passive-source seismic sensors, radar and seismic imaging of subglacial properties, as well as deep borehole geophysical sensors. This experiment was possible thanks to the NSF-funded multidisciplinary WISSARD project (Whillans Ice Stream Subglacial Access Research Drilling). Here we will review the datasets collected during the WISSARD glaciological experiment and report on selected results pertaining to interactions of this ice stream with water at its bed and its grounding line.

Evidence for a substantial West Antarctic ice sheet contribution to meltwater pulses and abrupt global sea level rise

NASA Astrophysics Data System (ADS)

Fogwill, C. J.; Turney, C. S.; Golledge, N. R.; Etheridge, D. M.; Rubino, M.; Thornton, D.; Woodward, J.; Winter, K.; van Ommen, T. D.; Moy, A. D.; Curran, M. A.; Rootes, C.; Rivera, A.; Millman, H.

2015-12-01

During the last deglaciation (21,000 to 7,000years ago) global sea level rise was punctuated by several abrupt meltwater spikes triggered by the retreat of ice sheets and glaciers world-wide. However, the debate regarding the relative timing, geographical source and the physical mechanisms driving these rapid increases in sea level has catalyzed debate critical to predicting future sea level rise and climate. Here we present a unique record of West Antarctic Ice Sheet elevation change derived from the Patriot Hills blue ice area, located close to the modern day grounding line of the Institute Ice Stream in the Weddell Sea Embayment. Combined isotopic signatures and gas volume analysis from the ice allows us to develop a record of local ice sheet palaeo-altitude that is assessed against independent regional high-resolution ice sheet modeling studies, allowing us to demonstrate that past ice sheet elevations across this sector of the WSE were considerably higher than those suggested by current terrestrial reconstructions. We argue that ice in the WSE had a significant influence on both pre and post LGM sea level rise including MWP-1A (~14.6 ka) and during MWP-1B (11.7-11.6 ka), reconciling past sea level rise and demonstrating for the first time that this sector of the WAIS made a significant and direct contribution to post LGM sea level rise.

Adaptive mesh refinement versus subgrid friction interpolation in simulations of Antarctic ice dynamics

DOE PAGES

Cornford, S. L.; Martin, D. F.; Lee, V.; ...

2016-05-13

At least in conventional hydrostatic ice-sheet models, the numerical error associated with grounding line dynamics can be reduced by modifications to the discretization scheme. These involve altering the integration formulae for the basal traction and/or driving stress close to the grounding line and exhibit lower – if still first-order – error in the MISMIP3d experiments. MISMIP3d may not represent the variety of real ice streams, in that it lacks strong lateral stresses, and imposes a large basal traction at the grounding line. We study resolution sensitivity in the context of extreme forcing simulations of the entire Antarctic ice sheet, using the BISICLES adaptive mesh ice-sheet model with two schemes: the original treatment, and a scheme, which modifies the discretization of the basal traction. The second scheme does indeed improve accuracy – by around a factor of two – for a given mesh spacing, butmore » $$\\lesssim 1$$ km resolution is still necessary. For example, in coarser resolution simulations Thwaites Glacier retreats so slowly that other ice streams divert its trunk. In contrast, with $$\\lesssim 1$$ km meshes, the same glacier retreats far more quickly and triggers the final phase of West Antarctic collapse a century before any such diversion can take place.« less

Reconstructing the post-LGM decay of the Eurasian Ice Sheets with Ice Sheet Models; data-model comparison and focus on the Storfjorden (Svalbard) ice stream dynamics history

NASA Astrophysics Data System (ADS)

Petrini, Michele; Kirchner, Nina; Colleoni, Florence; Camerlenghi, Angelo; Rebesco, Michele; Lucchi, Renata G.; Forte, Emanuele; Colucci, Renato R.

2017-04-01

The challenge of reconstructing palaeo-ice sheets past growth and decay represent a critical task to better understand mechanisms of present and future global climate change. Last Glacial Maximum (LGM), and the subsequent deglaciation until Pre-Industrial time (PI) represent an excellent testing ground for numerical Ice Sheet Models (ISMs), due to the abundant data available that can be used in an ISM as boundary conditions, forcings or constraints to test the ISMs results. In our study, we simulate with ISMs the post-LGM decay of the Eurasian Ice Sheets, with a focus on the marine-based Svalbard-Barents Sea-Kara Sea Ice Sheet. In particular, we aim to reconstruct the Storfjorden ice stream dynamics history by comparing the model results with the marine geological data (MSGLs, GZWs, sediment cores analysis) available from the area, e.g., Pedrosa et al. 2011, Rebesco et al. 2011, 2013, Lucchi et al. 2013. Two hybrid SIA/SSA ISMs are employed, GRISLI, Ritz et al. 2001, and PSU, Pollard&DeConto 2012. These models differ mainly in the complexity with which grounding line migration is treated. Climate forcing is interpolated by means of climate indexes between LGM and PI climate. Regional climate indexes are constructed based on the non-accelerated deglaciation transient experiment carried out with CCSM3, Liu et al. 2009. Indexes representative of the climate evolution over Siberia, Svalbard and Scandinavia are employed. The impact of such refined representation as opposed to the common use of the NGRIP δ18O index for transient experiments is analysed. In this study, the ice-ocean interaction is crucial to reconstruct the Storfjorden ice stream dynamics history. To investigate the sensitivity of the ice shelf/stream retreat to ocean temperature, we allow for a space-time variation of basal melting under the ice shelves by testing two-equations implementations based on Martin et al. 2011 forced with simulated ocean temperature and salinity from the TraCE-21ka coupled climate simulation. In this presentation, we will show work in progress, address open issues, and sketch future work. In particular, we invite the community to suggest possibilities for model-data comparison and integration. Liu, Z., Otto-Bliesner, B.L., He, F., Brady, E.C., Tomas, R., Clark, P.U., Carlson, A.E., Lynch-Stieglitz, J., Curry, W., Brook, E. and Erickson, D., 2009. Transient simulation of last deglaciation with a new mechanism for Bólling-Alleród warming. Science, 325(5938), pp.310-314. Lucchi, R.G., Camerlenghi, A., Rebesco, M., Colmenero-Hidalgo, E., Sierro, F.J., Sagnotti, L., Urgeles, R., Melis, R., Morigi, C., Bárcena, M.A. and Giorgetti, G., 2013. Postglacial sedimentary processes on the Storfjorden and Kveithola trough mouth fans: Significance of extreme glacimarine sedimentation. Global and planetary change, 111, pp.309-326. Martin, M.A., Winkelmann, R., Haseloff, M., Albrecht, T., Bueler, E., Khroulev, C. and Levermann, A., 2011. The Potsdam Parallel Ice Sheet Model (PISM-PIK)-Part 2: Dynamic equilibrium simulation of the Antarctic ice sheet. The Cryosphere, 5(3), pp.727-740. Pedrosa, M.T., Camerlenghi, A., De Mol, B., Urgeles, R., Rebesco, M. and Lucchi, R.G., 2011. Seabed morphology and shallow sedimentary structure of the Storfjorden and Kveithola trough-mouth fans (north west Barents Sea). Marine Geology, 286(1), pp.65-81. Pollard, D. and DeConto, R.M., 2012. Description of a hybrid ice sheet-shelf model, and application to Antarctica. Geoscientific Model Development, 5(5), pp.1273-1295. Rebesco, M., Liu, Y., Camerlenghi, A., Winsborrow, M., Laberg, J.S., Caburlotto, A., Diviacco, P., Accettella, D., Sauli, C., Wardell, N. and Tomini, I., 2011. Deglaciation of the western margin of the Barents Sea Ice Sheet-a swath bathymetric and sub-bottom seismic study from the Kveithola Trough. Marine Geology, 279(1), pp.141-147. Rebesco, M., Laberg, J., Pedrosa, M., Camerlenghi, A., Lucchi, R., Zgur, F. and Wardell, N., 2013. Onset and growth of Trough-Mouth Fans on the North-Western Barents Sea margin e implications for the evolution of the Barents Sea/Svalbard Ice Sheet. Quaternary Science Reviews, 30, pp.1-8. Ritz, C., Rommelaere, V. and Dumas, C., 2001. Modeling the evolution of Antarctic ice sheet over the last 420,000 years: Implications for altitude changes in the Vostok region. Journal of Geophysical Research: Atmospheres, 106(D23), pp.31943-31964.

Microbial processes at the beds of glaciers and ice sheets: a look at life below the Whillans Ice Stream

NASA Astrophysics Data System (ADS)

Mikucki, J.; Campen, R.; Vancleave, S.; Scherer, R. P.; Coenen, J. J.; Powell, R. D.; Tulaczyk, S. M.

2017-12-01

Groundwater, saturated sediments and hundreds of subglacial lakes exist below the ice sheets of Antarctica. The few Antarctic subglacial environments sampled to date all contain viable microorganisms. This is a significant finding because microbes are known to be key in mediating biogeochemical cycles. In sediments, microbial metabolic activity can also result in byproducts or direct interactions with sediment particles that influence the physical and geochemical characteristics of the matrix they inhabit. Subglacial Lake Whillans (SLW), a fresh water lake under the Whillans Ice Stream that drains into the Ross Sea at its grounding zone, was recently sampled as part of the NSF-funded Whillans Ice Stream Subglacial Access Research Drilling (WISSARD) project. Sediments from both SLW and its grounding zone contain microbial taxa related to iron, sulfur, nitrogen and methane oxidizers. In addition to molecular data, biogeochemical measurements and culture based experiments on Whillans sediments support the notion that the system is chemosynthetic with energy derived in part by cycling inorganic compounds. Etch pitting and mineral precipitates on fossil sponge spicules suggest that spicules may also provide microbial nutrients in these environments. Perhaps the most widespread microbial process that affects sediment structure and mineral weathering is the production of extra polymeric substances (EPS). Several phylogenetic groups detected in Whillans sediments are known to produce EPS and we have observed its production in pure cultures enriched directly from these sediments. Our data sheds light on how microbial life persists below the Antarctic Ice Sheet despite extended isolation in icy darkness, and how these microbes may be shaping their environment.

Sustained High Basal Motion of the Greenland Ice Sheet Revealed by Borehole Deformation

NASA Technical Reports Server (NTRS)

Ryser, Claudia; Luthi, Martin P.; Andrews, Lauren C.; Hoffman, Matthew, J.; Catania, Ginny A.; Hawley, Robert L.; Neumann, Thomas A.; Kristensen, Steen S.

2014-01-01

Ice deformation and basal motion characterize the dynamical behavior of the Greenland ice sheet (GrIS). We evaluate the contribution of basal motion from ice deformation measurements in boreholes drilled to the bed at two sites in the western marginal zone of the GrIS. We find a sustained high amount of basal motion contribution to surface velocity of 44-73 percent in winter, and up to 90 percent in summer. Measured ice deformation rates show an unexpected variation with depth that can be explained with the help of an ice-flow model as a consequence of stress transfer from slippery to sticky areas. This effect necessitates the use of high-order ice-flow models, not only in regions of fast-flowing ice streams but in all temperate-based areas of the GrIS. The agreement between modeled and measured deformation rates confirms that the recommended values of the temperature-dependent flow rate factor A are a good choice for ice-sheet models.

NASA Spacecraft Images Massive Crack in Antarctica Pine Island Glacier

NASA Image and Video Library

2011-11-15

This image from NASA Terra spacecraft shows a massive crack across the Pine Island Glacier, a major ice stream that drains the West Antarctic Ice Sheet. Eventually, the crack will extend all the way across the glacier.

The Potsdam Parallel Ice Sheet Model (PISM-PIK) - Part 1: Model description

NASA Astrophysics Data System (ADS)

Winkelmann, R.; Martin, M. A.; Haseloff, M.; Albrecht, T.; Bueler, E.; Khroulev, C.; Levermann, A.

2011-09-01

We present the Potsdam Parallel Ice Sheet Model (PISM-PIK), developed at the Potsdam Institute for Climate Impact Research to be used for simulations of large-scale ice sheet-shelf systems. It is derived from the Parallel Ice Sheet Model (Bueler and Brown, 2009). Velocities are calculated by superposition of two shallow stress balance approximations within the entire ice covered region: the shallow ice approximation (SIA) is dominant in grounded regions and accounts for shear deformation parallel to the geoid. The plug-flow type shallow shelf approximation (SSA) dominates the velocity field in ice shelf regions and serves as a basal sliding velocity in grounded regions. Ice streams can be identified diagnostically as regions with a significant contribution of membrane stresses to the local momentum balance. All lateral boundaries in PISM-PIK are free to evolve, including the grounding line and ice fronts. Ice shelf margins in particular are modeled using Neumann boundary conditions for the SSA equations, reflecting a hydrostatic stress imbalance along the vertical calving face. The ice front position is modeled using a subgrid-scale representation of calving front motion (Albrecht et al., 2011) and a physically-motivated calving law based on horizontal spreading rates. The model is tested in experiments from the Marine Ice Sheet Model Intercomparison Project (MISMIP). A dynamic equilibrium simulation of Antarctica under present-day conditions is presented in Martin et al. (2011).

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bougamont, M.; Christoffersen, P.; Price, S. F.

Ongoing, centennial-scale flow variability within the Ross ice streams of West Antarctica suggests that the present-day positive mass balance in this region may reverse in the future. Here we use a three-dimensional ice sheet model to simulate ice flow in this region over 250 years. The flow responds to changing basal properties, as a subglacial till layer interacts with water transported in an active subglacial hydrological system. We show that a persistent weak bed beneath the tributaries of the dormant Kamb Ice Stream is a source of internal ice flow instability, which reorganizes all ice streams in this region, leadingmore » to a reduced (positive) mass balance within decades and a net loss of ice within two centuries. This hitherto unaccounted for flow variability could raise sea level by 5 mm this century. Furthermore, better constraints on future sea level change from this region will require improved estimates of geothermal heat flux and subglacial water transport.« less

Glacially-megalineated limestone terrain of Anticosti Island, Gulf of St. Lawrence, Canada; onset zone of the Laurentian Channel Ice Stream

NASA Astrophysics Data System (ADS)

Eyles, Nick; Putkinen, Niko

2014-03-01

Anticosti is a large elongate island (240 km long, 60 km wide) in eastern Canada within the northern part of a deep water trough (Gulf of St. Lawrence) that terminates at the Atlantic continental shelf edge. The island's Pleistocene glaciological significance is that its long axis lay transverse to ice from the Quebec and Labrador sectors of the Laurentide Ice Sheet moving south from the relatively high-standing Canadian Shield. Recent glaciological reconstructions place a fast-flowing ice stream along the axis of the Gulf of St. Lawrence but supporting geologic evidence in terms of recognizing its hard-bedded onset zone and downstream streamlined soft bed is limited. Anticosti Island consists of gently southward-dipping limestone plains composed of Ordovician and Silurian limestones (Vaureal, Becscie and Jupiter formations) with north-facing escarpments transverse to regional ice flow. Glacial deposits are largely absent and limestone plains in the higher central plateau of the island retain a relict apparently ‘preglacial’ drainage system consisting of deeply-incised dendritic bedrock valleys. In contrast, the bedrock geomorphology of the lower lying western and eastern limestone plains of the island is strikingly different having been extensively modified by glacial erosion. Escarpments are glacially megalineated with a distinct ‘zig-zag’ planform reflecting northward-projecting bullet-shaped ‘noses’ (identified as rock drumlins) up to 2 km wide at their base and 4 km in length with rare megagrooved upper surfaces. Drumlins are separated by southward-closing, funnel-shaped ‘through valleys’ where former dendritic valleys have been extensively altered by the streaming of basal ice through gaps in the escarpments. Glacially-megalineated bedrock terrain such as on the western and eastern flanks of Anticosti Island is elsewhere associated with the hard-bedded onset zones of fast flowing ice streams and provides important ground truth for the postulated Laurentian Channel Ice Stream (LCIS) within the Gulf of St. Lawrence sector of the Laurentide Ice Sheet.

Global ice sheet/RSL simulations using the higher-order Ice Sheet System Model.

NASA Astrophysics Data System (ADS)

Larour, E. Y.; Ivins, E. R.; Adhikari, S.; Schlegel, N.; Seroussi, H. L.; Morlighem, M.

2017-12-01

Relative sea-level rise is driven by processes that are intimately linked to the evolution ofglacial areas and ice sheets in particular. So far, most Earth System models capable of projecting theevolution of RSL on decadal to centennial time scales have relied on offline interactions between RSL andice sheets. In particular, grounding line and calving front dynamics have not been modeled in a way that istightly coupled with Elasto-Static Adjustment (ESA) and/or Glacial-Isostatic Adjustment (GIA). Here, we presenta new simulation of the entire Earth System in which both Greenland and Antarctica ice sheets are tightly coupledto an RSL model that includes both ESA and GIA at resolutions and time scales compatible with processes suchas grounding line dynamics for Antarctica ice shelves and calving front dynamics for Greenland marine-terminatingglaciers. The simulations rely on the Ice Sheet System Model (ISSM) and show the impact of higher-orderice flow dynamics and coupling feedbacks between ice flow and RSL. We quantify the exact impact of ESA andGIA inclusion on grounding line evolution for large ice shelves such as the Ronne and Ross ice shelves, as well asthe Agasea Embayment ice streams, and demonstate how offline vs online RSL simulations diverge in the long run,and the consequences for predictions of sea-level rise.This work was performed at the California Institute of Technology's Jet Propulsion Laboratory undera contract with the National Aeronautics and Space Administration's Cryosphere Science Program.

Efficient meltwater drainage through supraglacial streams and rivers on the southwest Greenland ice sheet.

PubMed

Smith, Laurence C; Chu, Vena W; Yang, Kang; Gleason, Colin J; Pitcher, Lincoln H; Rennermalm, Asa K; Legleiter, Carl J; Behar, Alberto E; Overstreet, Brandon T; Moustafa, Samiah E; Tedesco, Marco; Forster, Richard R; LeWinter, Adam L; Finnegan, David C; Sheng, Yongwei; Balog, James

2015-01-27

Thermally incised meltwater channels that flow each summer across melt-prone surfaces of the Greenland ice sheet have received little direct study. We use high-resolution WorldView-1/2 satellite mapping and in situ measurements to characterize supraglacial water storage, drainage pattern, and discharge across 6,812 km(2) of southwest Greenland in July 2012, after a record melt event. Efficient surface drainage was routed through 523 high-order stream/river channel networks, all of which terminated in moulins before reaching the ice edge. Low surface water storage (3.6 ± 0.9 cm), negligible impoundment by supraglacial lakes or topographic depressions, and high discharge to moulins (2.54-2.81 cm⋅d(-1)) indicate that the surface drainage system conveyed its own storage volume every <2 d to the bed. Moulin discharges mapped inside ∼52% of the source ice watershed for Isortoq, a major proglacial river, totaled ∼41-98% of observed proglacial discharge, highlighting the importance of supraglacial river drainage to true outflow from the ice edge. However, Isortoq discharges tended lower than runoff simulations from the Modèle Atmosphérique Régional (MAR) regional climate model (0.056-0.112 km(3)⋅d(-1) vs. ∼0.103 km(3)⋅d(-1)), and when integrated over the melt season, totaled just 37-75% of MAR, suggesting nontrivial subglacial water storage even in this melt-prone region of the ice sheet. We conclude that (i) the interior surface of the ice sheet can be efficiently drained under optimal conditions, (ii) that digital elevation models alone cannot fully describe supraglacial drainage and its connection to subglacial systems, and (iii) that predicting outflow from climate models alone, without recognition of subglacial processes, may overestimate true meltwater export from the ice sheet to the ocean.

Efficient meltwater drainage through supraglacial streams and rivers on the southwest Greenland ice sheet

PubMed Central

Smith, Laurence C.; Chu, Vena W.; Yang, Kang; Gleason, Colin J.; Pitcher, Lincoln H.; Rennermalm, Asa K.; Legleiter, Carl J.; Behar, Alberto E.; Overstreet, Brandon T.; Moustafa, Samiah E.; Tedesco, Marco; Forster, Richard R.; LeWinter, Adam L.; Finnegan, David C.; Sheng, Yongwei; Balog, James

2015-01-01

Thermally incised meltwater channels that flow each summer across melt-prone surfaces of the Greenland ice sheet have received little direct study. We use high-resolution WorldView-1/2 satellite mapping and in situ measurements to characterize supraglacial water storage, drainage pattern, and discharge across 6,812 km2 of southwest Greenland in July 2012, after a record melt event. Efficient surface drainage was routed through 523 high-order stream/river channel networks, all of which terminated in moulins before reaching the ice edge. Low surface water storage (3.6 ± 0.9 cm), negligible impoundment by supraglacial lakes or topographic depressions, and high discharge to moulins (2.54–2.81 cm⋅d−1) indicate that the surface drainage system conveyed its own storage volume every <2 d to the bed. Moulin discharges mapped inside ∼52% of the source ice watershed for Isortoq, a major proglacial river, totaled ∼41–98% of observed proglacial discharge, highlighting the importance of supraglacial river drainage to true outflow from the ice edge. However, Isortoq discharges tended lower than runoff simulations from the Modèle Atmosphérique Régional (MAR) regional climate model (0.056–0.112 km3⋅d−1 vs. ∼0.103 km3⋅d−1), and when integrated over the melt season, totaled just 37–75% of MAR, suggesting nontrivial subglacial water storage even in this melt-prone region of the ice sheet. We conclude that (i) the interior surface of the ice sheet can be efficiently drained under optimal conditions, (ii) that digital elevation models alone cannot fully describe supraglacial drainage and its connection to subglacial systems, and (iii) that predicting outflow from climate models alone, without recognition of subglacial processes, may overestimate true meltwater export from the ice sheet to the ocean. PMID:25583477

Ice cover affects the growth of a stream-dwelling fish.

PubMed

Watz, Johan; Bergman, Eva; Piccolo, John J; Greenberg, Larry

2016-05-01

Protection provided by shelter is important for survival and affects the time and energy budgets of animals. It has been suggested that in fresh waters at high latitudes and altitudes, surface ice during winter functions as overhead cover for fish, reducing the predation risk from terrestrial piscivores. We simulated ice cover by suspending plastic sheeting over five 30-m-long stream sections in a boreal forest stream and examined its effects on the growth and habitat use of brown trout (Salmo trutta) during winter. Trout that spent the winter under the artificial ice cover grew more than those in the control (uncovered) sections. Moreover, tracking of trout tagged with passive integrated transponders showed that in the absence of the artificial ice cover, habitat use during the day was restricted to the stream edges, often under undercut banks, whereas under the simulated ice cover condition, trout used the entire width of the stream. These results indicate that the presence of surface ice cover may improve the energetic status and broaden habitat use of stream fish during winter. It is therefore likely that reductions in the duration and extent of ice cover due to climate change will alter time and energy budgets, with potentially negative effects on fish production.

Sensitivities of Greenland ice sheet volume inferred from an ice sheet adjoint model

NASA Astrophysics Data System (ADS)

Heimbach, P.; Bugnion, V.

2009-04-01

We present a new and original approach to understanding the sensitivity of the Greenland ice sheet to key model parameters and environmental conditions. At the heart of this approach is the use of an adjoint ice sheet model. Since its introduction by MacAyeal (1992), the adjoint method has become widespread to fit ice stream models to the increasing number and diversity of satellite observations, and to estimate uncertain model parameters such as basal conditions. However, no attempt has been made to extend this method to comprehensive ice sheet models. As a first step toward the use of adjoints of comprehensive three-dimensional ice sheet models we have generated an adjoint of the ice sheet model SICOPOLIS of Greve (1997). The adjoint was generated by means of the automatic differentiation (AD) tool TAF. The AD tool generates exact source code representing the tangent linear and adjoint model of the nonlinear parent model provided. Model sensitivities are given by the partial derivatives of a scalar-valued model diagnostic with respect to the controls, and can be efficiently calculated via the adjoint. By way of example, we determine the sensitivity of the total Greenland ice volume to various control variables, such as spatial fields of basal flow parameters, surface and basal forcings, and initial conditions. Reliability of the adjoint was tested through finite-difference perturbation calculations for various control variables and perturbation regions. Besides confirming qualitative aspects of ice sheet sensitivities, such as expected regional variations, we detect regions where model sensitivities are seemingly unexpected or counter-intuitive, albeit ``real'' in the sense of actual model behavior. An example is inferred regions where sensitivities of ice sheet volume to basal sliding coefficient are positive, i.e. where a local increase in basal sliding parameter increases the ice sheet volume. Similarly, positive ice temperature sensitivities in certain parts of the ice sheet are found (in most regions it is negativ, i.e. an increase in temperature decreases ice sheet volume), the detection of which seems highly unlikely if only conventional perturbation experiments had been used. An effort to generate an efficient adjoint with the newly developed open-source AD tool OpenAD is also under way. Available adjoint code generation tools now open up a variety of novel model applications, notably with regard to sensitivity and uncertainty analyses and ice sheet state estimation or data assimilation.

Insights into Spatial Sensitivities of Ice Mass Response to Environmental Change from the SeaRISE Ice Sheet Modeling Project I: Antarctica

NASA Technical Reports Server (NTRS)

Nowicki, Sophie; Bindschadler, Robert A.; Abe-Ouchi, Ayako; Aschwanden, Andy; Bueler, Ed; Choi, Hyengu; Fastook, Jim; Granzow, Glen; Greve, Ralf; Gutowski, Gail;

New marine geophysical and sediment record of the Northeast Greenland Ice Stream.

NASA Astrophysics Data System (ADS)

Callard, L.; Roberts, D. H.; O'Cofaigh, C.; Lloyd, J. M.; Smith, J. A.; Dorschel, B.

2017-12-01

The NE Greenland Ice Stream (NEGIS) drains 16% of the Greenland Ice Sheet (GrIS) and has a sea-level equivalent of 1.1-1.4 m. Stabilised by two floating ice shelves, 79N and Zachariae Isstrom, until recently it has shown little response to increased atmospheric and oceanic warming. However, since 2010 it has experienced an accelerated rate of grounding line retreat ( 4 km) and significant ice shelf loss that indicates that this sector of the GrIS is now responding to current oceanic and/or climatic change and has the potential to be a major contributor to future global sea-level rise. The project `NEGIS', a collaboration between Durham University and AWI, aims to reconstruct the history of the NE Greenland Ice Stream from the Last Glacial Maximum (LGM) to present using both onshore and offshore geological archives to better understand past ice stream response to a warming climate. This contribution presents results and interpretations from an offshore dataset collected on the RV Polarstern, cruises PS100 and PS109 in 2016 and 2017. Gravity and box cores, supplemented by swath bathymetric and sub-bottom profiler data, were acquired and initial core analysis including x-radiographs and MSCL data logging has been performed. Data collection focused principally in the Norske Trough and the area directly in front of the 79N ice shelf, a sub-ice shelf environment as recently as two years ago. On the outer shelf streamlined subglacial bedforms, grounding-zone wedges and moraines as well as overconsolidated subglacial tills, record an extensive ice sheet advance to the shelf edge. On the inner shelf and in front of the 79N ice shelf, deep, glacially-eroded bedrock basins are infilled with stratified sediment. The stratified muds represent deglacial and Holocene glacimarine sedimentation, and capture the recent transition from sub-ice shelf to shelf free conditions. Multiproxy palaeoenvironmental reconstructions, including foraminifera and diatom analysis, and radiocarbon dating are used to constrain the timing and mechanism of retreat.

The last glaciation of Bear Peninsula, central Amundsen Sea Embayment of Antarctica: Constraints on timing and duration revealed by in situ cosmogenic 14C and 10Be dating

NASA Astrophysics Data System (ADS)

Johnson, Joanne S.; Smith, James A.; Schaefer, Joerg M.; Young, Nicolás E.; Goehring, Brent M.; Hillenbrand, Claus-Dieter; Lamp, Jennifer L.; Finkel, Robert C.; Gohl, Karsten

2017-12-01

Ice streams in the Pine Island-Thwaites region of West Antarctica currently dominate contributions to sea level rise from the Antarctic ice sheet. Predictions of future ice-mass loss from this area rely on physical models that are validated with geological constraints on past extent, thickness and timing of ice cover. However, terrestrial records of ice sheet history from the region remain sparse, resulting in significant model uncertainties. We report glacial-geological evidence for the duration and timing of the last glaciation of Hunt Bluff, in the central Amundsen Sea Embayment. A multi-nuclide approach was used, measuring cosmogenic 10Be and in situ14C in bedrock surfaces and a perched erratic cobble. Bedrock 10Be ages (118-144 ka) reflect multiple periods of exposure and ice-cover, not continuous exposure since the last interglacial as had previously been hypothesized. In situ14C dating suggests that the last glaciation of Hunt Bluff did not start until 21.1 ± 5.8 ka - probably during the Last Glacial Maximum - and finished by 9.6 ± 0.9 ka, at the same time as ice sheet retreat from the continental shelf was complete. Thickening of ice at Hunt Bluff most likely post-dated the maximum extent of grounded ice on the outer continental shelf. Flow re-organisation provides a possible explanation for this, with the date for onset of ice-cover at Hunt Bluff providing a minimum age for the timing of convergence of the Dotson and Getz tributaries to form a single palaeo-ice stream. This is the first time that timing of onset of ice cover has been constrained in the Amundsen Sea Embayment.

Possible contribution of ice-sheet/lithosphere interactions to past glaciological changes in Greenland

NASA Astrophysics Data System (ADS)

Alley, R. B.; Parizek, B. R.; Anandakrishnan, S.; Pollard, D.; Stevens, N. T.; Pourpoint, M.

2017-12-01

Ice-lithosphere interactions may have influenced the history of ice-sheet sensitivity to climate change. The Greenland ice sheet (GIS) is sensitive to warming, and is likely to be largely removed if subjected to relatively small additional temperature increases. The recent report (Schaefer et al., 2016, Nature) of near-complete GIS removal under modest Pleistocene forcing suggests that GIS sensitivity may be even greater than generally modeled, but lack of major Holocene retreat is more consistent with existing models. As shown by Stevens et al. (2016, JGR), peak lithospheric flexural stresses associated with ice-age GIS cycling are of the same order as dike-driving stresses in plutonic systems, and migrate over ice-age cycles. The full analysis by Stevens et al. suggests the possibility that the onset of cyclic ice-sheet loading allowed deep melt associated with the passage of the Icelandic hot spot beneath Greenland to work up though the crust to or near the base of the ice sheet, helping explain the anomalous geothermal heat fluxes observed at the head of the Northeast Greenland Ice Stream and elsewhere in the northern part of GIS. If ice-age cycling aided extraction of an existing reservoir of melted rock, then geothermal heat flux would have risen with the onset of extraction and migration, but with a subsequent fall associated with reservoir depletion. Simple parameterized flow-model simulations confirm intuition that a higher geothermal flux makes deglaciation easier, with the northern part of the ice sheet especially important. Large uncertainties remain in quantification, but we suggest the hypothesis that, following the onset of ice-age cycling, deglaciation of the GIS first became easier and then more difficult in response to feedbacks involving the ice sheet and the geological system beneath. In turn, this suggests that evidence of past deglaciation under moderate forcing is consistent with existing ice-sheet models.

The Potsdam Parallel Ice Sheet Model (PISM-PIK) - Part 1: Model description

NASA Astrophysics Data System (ADS)

Winkelmann, R.; Martin, M. A.; Haseloff, M.; Albrecht, T.; Bueler, E.; Khroulev, C.; Levermann, A.

2010-08-01

We present the Potsdam Parallel Ice Sheet Model (PISM-PIK), developed at the Potsdam Institute for Climate Impact Research to be used for simulations of large-scale ice sheet-shelf systems. It is derived from the Parallel Ice Sheet Model (Bueler and Brown, 2009). Velocities are calculated by superposition of two shallow stress balance approximations within the entire ice covered region: the shallow ice approximation (SIA) is dominant in grounded regions and accounts for shear deformation parallel to the geoid. The plug-flow type shallow shelf approximation (SSA) dominates the velocity field in ice shelf regions and serves as a basal sliding velocity in grounded regions. Ice streams naturally emerge through this approach and can be identified diagnostically as regions with a significant contribution of membrane stresses to the local momentum balance. All lateral boundaries in PISM-PIK are free to evolve, including the grounding line and ice fronts. Ice shelf margins in particular are modeled using Neumann boundary conditions for the SSA equations, reflecting a hydrostatic stress imbalance along the vertical calving face. The ice front position is modeled using a subgrid scale representation of calving front motion (Albrecht et al., 2010) and a physically motivated dynamic calving law based on horizontal spreading rates. The model is validated within the Marine Ice Sheet Model Intercomparison Project (MISMIP) and is used for a dynamic equilibrium simulation of Antarctica under present-day conditions in the second part of this paper (Martin et al., 2010).

Modelling water flow under glaciers and ice sheets

PubMed Central

Flowers, Gwenn E.

2015-01-01

Recent observations of dynamic water systems beneath the Greenland and Antarctic ice sheets have sparked renewed interest in modelling subglacial drainage. The foundations of today's models were laid decades ago, inspired by measurements from mountain glaciers, discovery of the modern ice streams and the study of landscapes evacuated by former ice sheets. Models have progressed from strict adherence to the principles of groundwater flow, to the incorporation of flow ‘elements’ specific to the subglacial environment, to sophisticated two-dimensional representations of interacting distributed and channelized drainage. Although presently in a state of rapid development, subglacial drainage models, when coupled to models of ice flow, are now able to reproduce many of the canonical phenomena that characterize this coupled system. Model calibration remains generally out of reach, whereas widespread application of these models to large problems and real geometries awaits the next level of development. PMID:27547082

Modelling water flow under glaciers and ice sheets.

PubMed

Flowers, Gwenn E

2015-04-08

Recent observations of dynamic water systems beneath the Greenland and Antarctic ice sheets have sparked renewed interest in modelling subglacial drainage. The foundations of today's models were laid decades ago, inspired by measurements from mountain glaciers, discovery of the modern ice streams and the study of landscapes evacuated by former ice sheets. Models have progressed from strict adherence to the principles of groundwater flow, to the incorporation of flow 'elements' specific to the subglacial environment, to sophisticated two-dimensional representations of interacting distributed and channelized drainage. Although presently in a state of rapid development, subglacial drainage models, when coupled to models of ice flow, are now able to reproduce many of the canonical phenomena that characterize this coupled system. Model calibration remains generally out of reach, whereas widespread application of these models to large problems and real geometries awaits the next level of development.

Rapid grounding line migration induced by internal variability of a marine-terminating ice stream

NASA Astrophysics Data System (ADS)

Robel, A.; Schoof, C.; Tziperman, E.

2013-12-01

Numerous studies have found significant variability in the velocity of ice streams to be a prominent feature of geomorphologic records in the Siple Coast (Catania et al. 2012) and other regions in West Antarctica (Dowdeswell et al. 2008). Observations indicate that grounding line position is strongly influenced by ice stream variability, producing rapid grounding line migration in the recent past (Catania et al. 2006) and the modern (Joughin & Tulaczyk 2002). We analyze the interaction of grounding line mass flux and position in a marine-terminating ice stream using a stretch-coordinate flowline model. This model is based on that described in Schoof (2007), with a mesh refined near the grounding line to ensure accurate resolution of the mechanical transition zone. Here we have added lateral shear stress (Dupont & Alley 2005) and an undrained plastic bed (Tulaczyk et al. 2000). The parameter dependence of ice stream variability seen in this model compares favorably to both simpler (Robel et al. 2013) and more complex (van der Wel et al. 2013) models, though with some key differences. We find that thermally-induced internal ice stream variability can cause very rapid grounding line migration even in the absence of retrograde bed slopes or external forcing. Activation waves propagate along the ice stream length and trigger periods of rapid grounding line migration. We compare the behavior of the grounding line due to internal ice stream variability to changes triggered externally at the grounding line such as the rapid disintegration of buttressing ice shelves. Implications for Heinrich events and the Marine Ice Sheet Instability are discussed.

Widespread surface meltwater drainage in Antarctica

NASA Astrophysics Data System (ADS)

Kingslake, J.; Ely, J.; Das, I.; Bell, R. E.

2016-12-01

Surface meltwater is thought to cause ice-shelf disintegration, which accelerates the contribution of ice sheets to sea-level rise. Antarctic surface melting is predicted to increase and trigger further ice-shelf disintegration during this century. These climate-change impacts could be modulated by an active hydrological network analogous to the one in operation in Greenland. Despite some observations of Antarctic surface and sub-surface hydrological systems, large-scale active surface drainage in Antarctica has rarely been studied. We use satellite imagery and aerial photography to reveal widespread active hydrology on the surface of the Antarctic Ice Sheet as far south as 85o and as high as 1800 m a.s.l., often near mountain peaks that protrude through the ice (nunataks) and relatively low-albedo `blue-ice areas'. Despite predominantly sub-zero regional air temperatures, as simulated by a regional climate model, Antarctic active drainage has persisted for decades, transporting water through surface streams and feeding vast melt ponds up to 80 km long. Drainage networks (the largest are over 100 km in length) form on flat ice shelves, steep outlet glaciers and ice-sheet flanks across the West and East Antarctica Ice Sheets. Motivated by the proximity of many drainage systems to low-albedo rock and blue-ice areas, we hypothesize a positive feedback between exposed-rock extent, BIA formation, melting and ice-sheet thinning. This feedback relies on drainage moving water long distances from areas near exposed rock, across the grounding line onto and across ice shelves - a process we observe, but had previously thought to be unlikely in Antarctica. This work highlights previously-overlooked processes, not captured by current regional-scale models, which may accelerate the retreat of the Antarctic Ice Sheet.

The Sensitivity of the Greenland Ice Sheet to Glacial-Interglacial Oceanic Forcing

NASA Astrophysics Data System (ADS)

Tabone, I.; Blasco Navarro, J.; Robinson, A.; Alvarez-Solas, J.; Montoya, M.

2017-12-01

Up to now, the scientific community has mainly focused on the sensitivity of the Greenland Ice Sheet (GrIS) to atmospheric variations. However, several studies suggest that the enhanced ice mass loss experienced by the GrIS in the past decades is directly connected to the increasing North Atlantic temperatures. Melting of GrIS outlet glaciers triggers grounding-line retreat increasing ice discharge into the ocean. This new evidence leads to consider the ocean as a relevant driver to be taken into account when modeling the evolution of the GrIS. The ice-ocean interaction is a primary factor controling not only the likely future retreat of GrIS outlet glaciers, or the huge ice loss in past warming climates, but also, and more strongly, the past GrIS glacial expansion. The latter assumption is supported by reconstructions which propose the GrIS to be fully marine-based during glacials, and thus more exposed to the influence of the ocean. Here, for the first time, we investigate the response of the GrIS to past oceanic changes using a three-dimensional hybrid ice-sheet/ice-shelf model, which combines the Shallow Ice Approximation (SIA) for slow grounded ice sheets and the Shallow Shelf Approximation (SSA) in ice shelves and ice streams. The model accounts for a time-dependent parametrisation of the marine basal melting rate, which is used to reproduce past oceanic variations. In this work simulations of the last two glacial cycles are performed. Our results show that the GrIS is very sensitive to the ocean-triggered submarine melting (freezing). Mild oceanic temperature variations lead to a rapid retreat (expansion) of the GrIS margins, which, inducing a dynamic adjustment of the grounded ice sheet, drive the evolution of the whole ice sheet. Our results strongly suggest the need to consider the ocean as an active forcing in paleo ice sheet models.

Patterns of variability in steady- and non steady-state Ross Ice Shelf flow

NASA Astrophysics Data System (ADS)

Campbell, A. J.; Hulbe, C. L.; Scambos, T. A.; Klinger, M. J.; Lee, C. K.

2016-12-01

Ice shelves are gateways through which climate change can be transmitted from the ocean or atmosphere to a grounded ice sheet. It is thus important to separate patterns of ice shelf change driven internally (from the ice sheet) and patterns driven externally (by the ocean or atmosphere) so that modern observations can be viewed in an appropriate context. Here, we focus on the Ross Ice Shelf (RIS), a major component of the West Antarctic Ice Sheet system and a feature known to experience variable ice flux from tributary ice streams and glaciers, for example, ice stream stagnation and glacier surges. We perturb a model of the Ross Ice Shelf with periodic influx variations, ice rise and ice plain grounding events, and iceberg calving in order to generate transients in the ice shelf flow and thickness. Characteristic patterns associated with those perturbations are identified using empirical orthogonal functions (EOFs). The leading EOFs reveal shelf-wide pattern of response to local perturbations that can be interpreted in terms of coupled mass and momentum balance. For example, speed changes on Byrd Glacier cause both thinning and thickening in a broad region that extends to Roosevelt Island. We calculate decay times at various locations for various perturbations and find that mutli-decadal to century time scales are typical. Unique identification of responses to particular forcings may thus be difficlult to achieve and flow divergence cannot be assumed to be constant when interpreting observed changes in ice thickness. In reality, perturbations to the ice shelf do not occur individually, rather the ice shelf contains a history of boundary perturbations. To explore the degree individual perturbations are seperable from their ensemble, EOFs from individual events are combined in pairs and compared against experiments with the same periodic perturbations pairs. Residuals between these EOFs reveal the degree interaction between between disctinct perturbations.

The response of grounded ice to ocean temperature forcing in a coupled ice sheet-ice shelf-ocean cavity model

NASA Astrophysics Data System (ADS)

Goldberg, D. N.; Little, C. M.; Sergienko, O. V.; Gnanadesikan, A.

2010-12-01

Ice shelves provide a pathway for the heat content of the ocean to influence continental ice sheets. Changes in the rate or location of basal melting can alter their geometry and effect changes in stress conditions at the grounding line, leading to a grounded ice response. Recent observations of ice streams and ice shelves in the Amundsen Sea sector of West Antarctica have been consistent with this story. On the other hand, ice dynamics in the grounding zone control flux into the shelf and thus ice shelf geometry, which has a strong influence on the circulation in the cavity beneath the shelf. Thus the coupling between the two systems, ocean and ice sheet-ice shelf, can be quite strong. We examine the response of the ice sheet-ice shelf-ocean cavity system to changes in ocean temperature using a recently developed coupled model. The coupled model consists a 3-D ocean model (GFDL's Generalized Ocean Layered Dynamics model, or GOLD) to a two-dimensional ice sheet-ice shelf model (Goldberg et al, 2009), and allows for changing cavity geometry and a migrating grounding line. Steady states of the coupled system are found even under considerable forcing. The ice shelf morphology and basal melt rate patterns of the steady states exhibit detailed structure, and furthermore seem to be unique and robust. The relationship between temperature forcing and area-averaged melt rate is influenced by the response of ice shelf morphology to thermal forcing, and is found to be sublinear in the range of forcing considered. However, results suggest that area-averaged melt rate is not the best predictor of overall system response, as grounding line stability depends on local aspects of the basal melt field. Goldberg, D N, D M Holland and C G Schoof, 2009. Grounding line movement and ice shelf buttressing in marine ice sheets, Journal of Geophysical Research-Earth Surfaces, 114, F04026.

High geothermal heat flux measured below the West Antarctic Ice Sheet.

PubMed

Fisher, Andrew T; Mankoff, Kenneth D; Tulaczyk, Slawek M; Tyler, Scott W; Foley, Neil

2015-07-01

The geothermal heat flux is a critical thermal boundary condition that influences the melting, flow, and mass balance of ice sheets, but measurements of this parameter are difficult to make in ice-covered regions. We report the first direct measurement of geothermal heat flux into the base of the West Antarctic Ice Sheet (WAIS), below Subglacial Lake Whillans, determined from the thermal gradient and the thermal conductivity of sediment under the lake. The heat flux at this site is 285 ± 80 mW/m(2), significantly higher than the continental and regional averages estimated for this site using regional geophysical and glaciological models. Independent temperature measurements in the ice indicate an upward heat flux through the WAIS of 105 ± 13 mW/m(2). The difference between these heat flux values could contribute to basal melting and/or be advected from Subglacial Lake Whillans by flowing water. The high geothermal heat flux may help to explain why ice streams and subglacial lakes are so abundant and dynamic in this region.

Improving Altimetry Height-change Retrieval on the Fringes of the Antarctic Ice Sheet

NASA Astrophysics Data System (ADS)

Paolo, F. S.; Nilsson, J.; Gardner, A. S.

2017-12-01

Projections of sea-level change over the next century are highly uncertain, in part, due to insufficient understanding of ice-sheet sensitivity to changes in oceanic and atmospheric circulation. This limitation is, to a large degree, related to the lack of long and continuous observational records covering critical regions along the ice-sheet margins where the ice interacts with the ocean. Of particular importance are accurate records of changes in ice thickness that provide information on how mass fluctuates on the floating extensions of ice streams and glaciers through which the ice-sheet drains. These changes can modify the stability of the grounded ice sheet through changing back-stress, for example, through loss of ice-shelf buttressing. Here, we synthetize 25+ years of satellite altimetry observations to extend the time span and improve the resolution and accuracy of the existing record of Antarctic floating ice thickness. We incorporate data from ESA's ERS-1, ERS-2, Envisat and Cryosat-2 radar altimeters (1992-present) and NASA's ICESat laser altimeter (2003-2009) and Operation IceBridge surveys (2009-present); with plans to include ICESat-2 data soon after its launch in September 2018. Towards this effort, we revisit some of the main corrections applied to altimeter data, such as minimization of the difference between measurements from radar and laser systems; and we improve the approach for the synthesis of heterogeneous measurements of ice-surface topography and uncertainty estimation. We report on our progress in constructing this long-term and homogeneous record, with a particular focus on the floating ice shelves.

Evolution of a Greenland Ice sheet Including Shelves and Regional Sea Level Variations

NASA Astrophysics Data System (ADS)

Bradley, Sarah; Reerink, Thomas; van de Wal, Roderik S. W.; Helsen, Michiel; Goelzer, Heiko

2016-04-01

Observational evidence, including offshore moraines and marine sediment cores infer that at the Last Glacial maximum (LGM) the Greenland ice sheet (GIS) grounded out across the Davis Strait into Baffin Bay, with fast flowing ice streams extending out to the continental shelf break along the NW margin. These observations lead to a number of questions as to weather the GIS and Laurentide ice sheet (LIS) coalesced during glacial maximums, and if so, did a significant ice shelf develop across Baffin Bay and how would such a configuration impact on the relative contribution of these ice sheets to eustatic sea level (ESL). Most previous paleo ice sheet modelling simulations of the GIS recreated an ice sheet that either did not extend out onto the continental shelf or utilised a simplified marine ice parameterisation to recreate an extended GIS, and therefore did not fully include ice shelf dynamics. In this study we simulate the evolution of the GIS from 220 kyr BP to present day using IMAU-ice; a 3D thermodynamical ice sheet model which fully accounts for grounded and floating ice, calculates grounding line migration and ice shelf dynamics. As there are few observational estimates of the long-term (yrs) sub marine basal melting rates (mbm) for the GIS, we developed a mbm parameterization within IMAU-ice controlled primarily by changes in paleo water depth. We also investigate the influence of the LIS on the GIS evolution by including relative sea level forcing's derived from a Glacial Isostatic Adjustment model. We will present results of how changes in the mbm directly impacts on the ice sheet dynamics, timing and spatial extent of the GIS at the glacial maximums, but also on the rate of retreat and spatial extent at the Last interglacial (LIG) minimum. Results indicate that with the inclusion of ice shelf dynamics, a larger GIS is generated which is grounded out into Davis strait, up to a water depth of -750 m, but significantly reduces the GIS contribution to Last interglacial ESL.

Evolution of a Greenland Ice sheet Including Shelves and Regional Sea Level Variations

NASA Astrophysics Data System (ADS)

Bradley, S.; Reerink, T.; Vandewal, R.; Helsen, M.

2015-12-01

Observational evidence, including offshore moraines and marine sediment cores infer that at the Last Glacial maximum (LGM) the Greenland ice sheet (GIS) grounded out across the Davis Strait into Baffin Bay, with fast flowing ice streams extending out to the continental shelf break along the NW margin. These observations lead to a number of questions as to weather the GIS and Laurentide ice sheet (LIS) coalesced during glacial maximums, and if so, did a significant ice shelf develop across Baffin Bay and how would such a configuration impact on the relative contribution of these ice sheets to eustatic sea level (ESL). Most previous paleo ice sheet modelling simulations of the GIS recreated an ice sheet that either did not extend out onto the continental shelf or utilised a simplified marine ice parameterisation to recreate an extended GIS, and therefore did not fully include ice shelf dynamics. In this study we simulate the evolution of the GIS from 220 kyr BP to present day using IMAU-ice; a 3D thermodynamical ice sheet model which fully accounts for grounded and floating ice, calculates grounding line migration and ice shelf dynamics. There is few observational estimates of long-term (yrs) sub marine basal melting rates (mbm) for the GIS. Therefore we investigate a range of relationships to constrain the spatial and temporal parameterisation of mbm within IMAU-ice related to changes in paleo water depth, driven by changes in relative sea level and ocean temperature. We will present results of how changes in the mbm directly impacts on the ice sheet dynamics, timing and spatial extent of the GIS at the glacial maximums, but also on the rate of retreat and spatial extent at the Last interglacial (LIG) minimum. Initial results indicate that with the inclusion of ice shelf dynamics, a larger GIS is generated which is grounded out into Davis strait, up to a water depth of -750 m, but the total contribution to LIG ESL is reduced by up to 0.6 m.

Antarctic subglacial groundwater: measurement concept and potential influence on ice flow

NASA Astrophysics Data System (ADS)

Kulessa, Bernd; Siegert, Martin; Bougamont, Marion; Christoffersen, Poul; Key, Kerry; Andersen, Kristoffer; Booth, Adam; Smith, Andrew

2017-04-01

Is groundwater abundant in Antarctica and does it modulate ice flow? Answering this question matters because ice streams flow by gliding over a wet substrate of till. Water fed to ice-stream beds thus influences ice-sheet dynamics and, potentially, sea-level rise. It is recognised that both till and the sedimentary basins from which it originates are porous and could host a reservoir of mobile groundwater that interacts with the subglacial interfacial system. According to recent numerical modelling up to half of all water available for basal lubrication, and time lags between hydrological forcing and ice-sheet response as long as millennia, may have been overlooked in models of ice flow. Here, we review evidence in support of Antarctic groundwater and propose how it can be measured to ascertain the extent to which it modulates ice flow. We present new seismoelectric soundings of subglacial till, and new magnetotelluric and transient electromagnetic forward models of subglacial groundwater reservoirs. We demonstrate that multi-facetted and integrated geophysical datasets can detect, delineate and quantify the groundwater contents of subglacial sedimentary basins and, potentially, monitor groundwater exchange rates between subglacial till layers. We thus describe a new area of glaciological investigation and how it should progress in future.

Initial results from geophysical surveys and shallow coring of the Northeast Greenland Ice Stream (NEGIS)

NASA Astrophysics Data System (ADS)

Vallelonga, P.; Christianson, K.; Alley, R. B.; Anandakrishnan, S.; Christian, J. E. M.; Dahl-Jensen, D.; Gkinis, V.; Holme, C.; Jacobel, R. W.; Karlsson, N. B.; Keisling, B. A.; Kipfstuhl, S.; Kjær, H. A.; Kristensen, M. E. L.; Muto, A.; Peters, L. E.; Popp, T.; Riverman, K. L.; Svensson, A. M.; Tibuleac, C.; Vinther, B. M.; Weng, Y.; Winstrup, M.

2014-07-01

The Northeast Greenland Ice Stream (NEGIS) is the sole interior Greenlandic ice stream. Fast flow initiates near the summit dome, and the ice stream terminates approximately 1000 km downstream in three large outlet glaciers that calve into the Greenland Sea. To better understand this important system, in the summer of 2012 we drilled a 67 m firn core and conducted ground-based radio-echo sounding (RES) and active-source seismic surveys at a site approximately 150 km downstream from the onset of streaming flow (NEGIS firn core, 75°37.61' N, 35°56.49' W). The site is representative of the upper part of the ice stream, while also being in a crevasse-free area for safe surface operations. Annual cycles were observed for insoluble dust, sodium and ammonium concentrations and for electrolytic conductivity, allowing a seasonally resolved chronology covering the past 400 yr. Annual layer thicknesses averaged 0.11 m ice equivalent (i.e.) for the period 1607-2011, although accumulation varied between 0.08 and 0.14 m i.e., likely due to flow-related changes in surface topography. Tracing of RES layers from the NGRIP (North Greenland Ice Core Project) ice core site shows that the ice at NEGIS preserves a climatic record of at least the past 51 kyr. We demonstrate that deep ice core drilling in this location can provide a reliable Holocene and late-glacial climate record, as well as helping to constrain the past dynamics and ice-lithosphere interactions of the Greenland Ice Sheet.

A synthesis of the basal thermal state of the Greenland Ice Sheet

PubMed Central

MacGregor, Joseph A.; Fahnestock, Mark A.; Catania, Ginny A.; Aschwanden, Andy; Clow, Gary D.; Colgan, William T.; Gogineni, S. Prasad; Morlighem, Mathieu; Nowicki, Sophie M. J.; Paden, John D.; Price, Stephen F.; Seroussi, Hélène

2017-01-01

The basal thermal state of an ice sheet (frozen or thawed) is an important control upon its evolution, dynamics and response to external forcings. However, this state can only be observed directly within sparse boreholes or inferred conclusively from the presence of subglacial lakes. Here we synthesize spatially extensive inferences of the basal thermal state of the Greenland Ice Sheet to better constrain this state. Existing inferences include outputs from the eight thermomechanical ice-flow models included in the SeaRISE effort. New remote-sensing inferences of the basal thermal state are derived from Holocene radiostratigraphy, modern surface velocity and MODIS imagery. Both thermomechanical modeling and remote inferences generally agree that the Northeast Greenland Ice Stream and large portions of the southwestern ice-drainage systems are thawed at the bed, whereas the bed beneath the central ice divides, particularly their west-facing slopes, is frozen. Elsewhere, there is poor agreement regarding the basal thermal state. Both models and remote inferences rarely represent the borehole-observed basal thermal state accurately near NorthGRIP and DYE-3. This synthesis identifies a large portion of the Greenland Ice Sheet (about one third by area) where additional observations would most improve knowledge of its overall basal thermal state. PMID:28163988

A synthesis of the basal thermal state of the Greenland Ice Sheet.

PubMed

MacGregor, Joseph A; Fahnestock, Mark A; Catania, Ginny A; Aschwanden, Andy; Clow, Gary D; Colgan, William T; Gogineni, S Prasad; Morlighem, Mathieu; Nowicki, Sophie M J; Paden, John D; Price, Stephen F; Seroussi, Hélène

2016-08-10

The basal thermal state of an ice sheet (frozen or thawed) is an important control upon its evolution, dynamics and response to external forcings. However, this state can only be observed directly within sparse boreholes or inferred conclusively from the presence of subglacial lakes. Here we synthesize spatially extensive inferences of the basal thermal state of the Greenland Ice Sheet to better constrain this state. Existing inferences include outputs from the eight thermomechanical ice-flow models included in the SeaRISE effort. New remote-sensing inferences of the basal thermal state are derived from Holocene radiostratigraphy, modern surface velocity and MODIS imagery. Both thermomechanical modeling and remote inferences generally agree that the Northeast Greenland Ice Stream and large portions of the southwestern ice-drainage systems are thawed at the bed, whereas the bed beneath the central ice divides, particularly their west-facing slopes, is frozen. Elsewhere, there is poor agreement regarding the basal thermal state. Both models and remote inferences rarely represent the borehole-observed basal thermal state accurately near NorthGRIP and DYE-3. This synthesis identifies a large portion of the Greenland Ice Sheet (about one third by area) where additional observations would most improve knowledge of its overall basal thermal state.

A synthesis of the basal thermal state of the Greenland Ice Sheet

USGS Publications Warehouse

MacGregor, Joseph A; Fahnestock, Mark A; Catania, Ginny A; Aschwanden, Andy; Clow, Gary D.; Colgan, William T.; Gogineni, Prasad S.; Morlighem, Mathieu; Nowicki, Sophie M .J.; Paden, John D; Price, Stephen F.; Seroussi, Helene

2016-01-01

The basal thermal state of an ice sheet (frozen or thawed) is an important control upon its evolution, dynamics and response to external forcings. However, this state can only be observed directly within sparse boreholes or inferred conclusively from the presence of subglacial lakes. Here we synthesize spatially extensive inferences of the basal thermal state of the Greenland Ice Sheet to better constrain this state. Existing inferences include outputs from the eight thermomechanical ice-flow models included in the SeaRISE effort. New remote-sensing inferences of the basal thermal state are derived from Holocene radiostratigraphy, modern surface velocity and MODIS imagery. Both thermomechanical modeling and remote inferences generally agree that the Northeast Greenland Ice Stream and large portions of the southwestern ice-drainage systems are thawed at the bed, whereas the bed beneath the central ice divides, particularly their west-facing slopes, is frozen. Elsewhere, there is poor agreement regarding the basal thermal state. Both models and remote inferences rarely represent the borehole-observed basal thermal state accurately near NorthGRIP and DYE-3. This synthesis identifies a large portion of the Greenland Ice Sheet (about one third by area) where additional observations would most improve knowledge of its overall basal thermal state.

Deglacial history of the Pensacola Mountains, Antarctica from glacial geomorphology and cosmogenic nuclide surface exposure dating

NASA Astrophysics Data System (ADS)

Bentley, M. J.; Hein, A. S.; Sugden, D. E.; Whitehouse, P. L.; Shanks, R.; Xu, S.; Freeman, S. P. H. T.

2017-02-01

The retreat history of the Antarctic Ice Sheet is important for understanding rapid deglaciation, as well as to constrain numerical ice sheet models and ice loading models required for glacial isostatic adjustment modelling. There is particular debate about the extent of grounded ice in the Weddell Sea embayment at the Last Glacial Maximum, and its subsequent deglacial history. Here we provide a new dataset of geomorphological observations and cosmogenic nuclide surface exposure ages of erratic samples that constrain the deglacial history of the Pensacola Mountains, adjacent to the present day Foundation Ice Stream and Academy Glacier in the southern Weddell Sea embayment. We show there is evidence of at least two glaciations, the first of which was relatively old and warm-based, and a more recent cold-based glaciation. During the most recent glaciation ice thickened by at least 450 m in the Williams Hills and at least 380 m on Mt Bragg. Progressive thinning from these sites was well underway by 10 ka BP and ice reached present levels by 2.5 ka BP, and is broadly similar to the relatively modest thinning histories in the southern Ellsworth Mountains. The thinning history is consistent with, but does not mandate, a Late Holocene retreat of the grounding line to a smaller-than-present configuration, as has been recently hypothesized based on ice sheet and glacial isostatic modelling. The data also show that clasts with complex exposure histories are pervasive and that clast recycling is highly site-dependent. These new data provide constraints on a reconstruction of the retreat history of the formerly-expanded Foundation Ice Stream, derived using a numerical flowband model.

Initial results from geophysical surveys and shallow coring of the Northeast Greenland Ice Stream (NEGIS)

NASA Astrophysics Data System (ADS)

Vallelonga, P.; Christianson, K.; Alley, R. B.; Anandakrishnan, S.; Christian, J. E. M.; Dahl-Jensen, D.; Gkinis, V.; Holme, C.; Jacobel, R. W.; Karlsson, N.; Keisling, B. A.; Kipfstuhl, S.; Kjær, H. A.; Kristensen, M. E. L.; Muto, A.; Peters, L. E.; Popp, T.; Riverman, K. L.; Svensson, A. M.; Tibuleac, C.; Vinther, B. M.; Weng, Y.; Winstrup, M.

2014-01-01

The Northeast Greenland Ice Stream (NEGIS) is the sole interior Greenlandic ice stream. Fast flow initiates near the summit dome, and the ice stream terminates approximately 1000 km downstream in three large outlet glaciers that calve into the Greenland Sea. To better understand this important system, in the summer of 2012 we drilled a 67 m firn core and conducted ground-based radio-echo sounding (RES) and active-source seismic surveys at a site approximately 150 km downstream from the onset of streaming flow (NEGIS firn core, 75° 37.61' N, 35°56.49' W). The site is representative of the upper part of the ice stream, while also being in a crevasse-free area for safe surface operations. Annual cycles were observed for insoluble dust, sodium and ammonium concentrations and for electrolytic conductivity, allowing a seasonally resolved chronology covering the past 400 yr. Annual layer thicknesses averaged 0.11 m ice equivalent (i.e.) for the period 1607-2011, although accumulation varied between 0.08 and 0.14 m i.e., likely due to flow-related changes in surface topography. Tracing of RES layers from the NGRIP ice core site shows that the ice at NEGIS preserves a climatic record of at least the past 51 kyr. We demonstrate that a deep ice core drilling in this location can provide a reliable Holocene and late-glacial climate record, as well as helping to constrain the past dynamics and ice-lithosphere interactions of the Greenland Ice Sheet.

Reconstruction of North American drainage basins and river discharge since the Last Glacial Maximum

NASA Astrophysics Data System (ADS)

Wickert, Andrew D.

2016-11-01

Over the last glacial cycle, ice sheets and the resultant glacial isostatic adjustment (GIA) rearranged river systems. As these riverine threads that tied the ice sheets to the sea were stretched, severed, and restructured, they also shrank and swelled with the pulse of meltwater inputs and time-varying drainage basin areas, and sometimes delivered enough meltwater to the oceans in the right places to influence global climate. Here I present a general method to compute past river flow paths, drainage basin geometries, and river discharges, by combining models of past ice sheets, glacial isostatic adjustment, and climate. The result is a time series of synthetic paleohydrographs and drainage basin maps from the Last Glacial Maximum to present for nine major drainage basins - the Mississippi, Rio Grande, Colorado, Columbia, Mackenzie, Hudson Bay, Saint Lawrence, Hudson, and Susquehanna/Chesapeake Bay. These are based on five published reconstructions of the North American ice sheets. I compare these maps with drainage reconstructions and discharge histories based on a review of observational evidence, including river deposits and terraces, isotopic records, mineral provenance markers, glacial moraine histories, and evidence of ice stream and tunnel valley flow directions. The sharp boundaries of the reconstructed past drainage basins complement the flexurally smoothed GIA signal that is more often used to validate ice-sheet reconstructions, and provide a complementary framework to reduce nonuniqueness in model reconstructions of the North American ice-sheet complex.

Spatial Patterns of Long-Term Erosion Rates Beneath the Marine West Antarctic Ice Sheet: Insights into the Physics of Continental Scale Glacial Erosion from a Comparison with the Ice-Velocity Field

NASA Astrophysics Data System (ADS)

Howat, I. M.; Tulaczyk, S.; Mac Gregor, K.; Joughin, I.

2001-12-01

As part of the effort to build quantitative models of glacial erosion and sedimentation, it is particularly important to construct scaled relations between erosion, transport, and sedimentation rates and appropriate glaciological variables (e.g., ice velocity). Recent acquisition of bed topography and ice velocity data for the marine West Antarctic Ice Sheet (WAIS)[Joughin et al., 1999; Lythe et al., in press] provides an unprecedented opportunity to investigate continental-scale patterns of glacial erosion and their relationship to the ice velocity field. Utilizing this data, we construct a map of estimated long-term erosion rates beneath the WAIS. In order to calculate long-term erosion rates from the available data, we assume that: (1) the ice sheet has been present for ~5 mill. years, (2) the initial topography beneath the WAIS was that of a typical ( ~200 m.b.s.l.) continental shelf, and (3) the present topography is near local isostatic equilibrium (Airy type). The map of long-term erosion rates constructed in this fashion shows an intriguing pattern of relatively high rates (of the order of 0.1 mm/yr) concentrated beneath modern ice stream tributaries (ice velocity ~100 m/yr), but much lower erosion rates (of the order of 0.01 mm/yr) beneath both the modern fast-moving ice streams ( ~400 m/yr.) and the slow-moving parts of the ice sheet ( ~10 m/yr). This lack of clear correlation between the estimated erosion rates and ice velocity is somewhat unexpected given that both observational and theoretical studies have shown that bedrock erosion rates beneath mountain glaciers can often be calculated by multiplying the basal sliding velocity by a constant (typically of the order of ~10^-4)(Humphrey and Raymond, 1993 and Mac Gregor et al., 2000). We obtain an improved match between estimated erosion rates and bed topography by calculating erosion rates using horizontal gradients within the ice velocity field rather than the magnitude of ice velocity, as consistent with the steady state deforming till model of Cuffey and Alley (1997). Therefore, we hypothesize that the erosional system beneath the WAIS, which has overridden a thick layer of erodible, Tertiary marine sediments (Studinger et al., in press), is 'transport limited' and that the horizontal gradients in ice velocity and till flux have the predominant control over spatial patterns of subglacial erosion and deposition rates. In contrast, past studies of erosional systems have concentrated on mountain glaciers that derive their debris through erosion of hard bedrock. In those cases, the erosional system may be 'production limited' because erosion rates scale with dissipation of gravitational energy, represented by the velocity-times-constant equation. Thus, this concept of a 'transport limited' system represents a deviation from past thinking regarding the dynamics of bed erosion, and may be unique to marine-based ice sheets. Using this concept as a base, we will construct more accurately parameterized models to better define the relationship between the dynamics of ice streams and the character of the sub glacial bed.

Constraining Future Sea Level Rise Estimates from the Amundsen Sea Embayment, West Antarctica

NASA Astrophysics Data System (ADS)

Nias, I.; Cornford, S. L.; Edwards, T.; Gourmelen, N.; Payne, A. J.

2016-12-01

The Amundsen Sea Embayment (ASE) is the primary source of mass loss from the West Antarctic Ice Sheet. The catchment is particularly susceptible to grounding line retreat, because the ice sheet is grounded on bedrock that is below sea level and deepening towards its interior. Mass loss from the ASE ice streams, which include Pine Island, Thwaites and Smith glaciers, is a major uncertainty on future sea level rise, and understanding the dynamics of these ice streams is essential to constraining this uncertainty. The aim of this study is to construct a distribution of future ASE sea level contributions from an ensemble of ice sheet model simulations and observations of surface elevation change. A 284 member ensemble was performed using BISICLES, a vertically-integrated ice flow model with adaptive mesh refinement. Within the ensemble parameters associated with basal traction, ice rheology and sub-shelf melt rate were perturbed, and the effect of bed topography and sliding law were also investigated. Initially each configuration was run to 50 model years. Satellite observations of surface height change were then used within a Bayesian framework to assign likelihoods to each ensemble member. Simulations that better reproduced the current thinning patterns across the catchment were given a higher score. The resulting posterior distribution of sea level contributions is narrower than the prior distribution, although the central estimates of sea level rise are similar between the prior and posterior. The most extreme simulations were eliminated and the remaining ensemble members were extended to 200 years, using a simple melt rate forcing.

Geomorphology of ice stream beds: recent progress and future challenges

NASA Astrophysics Data System (ADS)

Stokes, Chris R.

2016-04-01

Ice sheets lose mass primarily by melting and discharge via rapidly-flowing ice streams. Surface and basal melting (e.g. of ice shelves) are closely linked to atmospheric and oceanic conditions, but the mechanisms that drive changes in ice stream discharge are more complex; and are influenced by conditions at their bed which can sustain, enhance or inhibit their motion. Although explicit comparisons are rare, the ice-bed interface is similar to the 'boundary layer' in fluvial and aeolian environments, where shear stresses (both basal and lateral in the case of ice streams) oppose the flow of the overlying medium. The analogy extends further because processes within the boundary layer create a distinctive geomorphology (and roughness) that is characterised by subglacial bedforms that resemble features in fluvial and aeolian environments. Their creation results from erosion, transport and deposition of sediment which is poorly constrained, but which is intimately linked to the mechanisms through which ice streams are able to flow rapidly. The study of ice stream geomorphology is, therefore, critical to our understanding of their dynamics. Despite difficulty in observing the subglacial environment of active ice streams, our understanding of their geomorphology has grown rapidly in the last three decades, from almost complete ignorance to a detailed knowledge of their geomorphological products. This has been brought about by two main approaches: (i) geophysical investigation of modern (active) ice streams, and (ii) sedimentological and geomorphological investigation of palaeo-ice stream beds. The aim of this paper is to review progress in these two areas, highlight the key questions that remain, and discuss the opportunities that are likely to arise that will enable them to be addressed. It is clear that whilst these two main approaches have led to important advances, they have often been viewed as separate sub-disciplines, with minimal cross-pollination of ideas and concepts, particularly with respect to how landforms can be securely linked to subglacial processes and ice dynamics. However, recent developments in numerical modelling of the subglacial environment are beginning to offer new opportunities to tackle this issue and observations from both modern and palaeo-ice streams will be critical to constrain and validate such modelling.

Investigating the Greenland ice sheet evolution under changing climate using a three-dimensional full-Stokes model

NASA Astrophysics Data System (ADS)

Seddik, H.; Greve, R.; Zwinger, T.; Gillet-Chaulet, F.; Gagliardini, O.

2010-12-01

A three-dimensional, thermo-mechanically coupled model is applied to the Greenland ice sheet. The model implements the full-Stokes equations for the ice dynamics, and the system is solved with the finite-element method (FEM) using the open source multi-physics package Elmer (http://www.csc.fi/elmer/). The finite-element mesh for the computational domain has been created using the Greenland surface and bedrock DEM data with a spatial resolution of 5 km (SeaRise community effort, based on Bamber and others, 2001). The study is particularly aimed at better understanding the ice dynamics near the major Greenland ice streams. The meshing procedure starts with the bedrock footprint where a mesh with triangle elements and a resolution of 5 km is constructed. Since the resulting mesh is unnecessarily dense in areas with slow ice dynamics, an anisotropic mesh adaptation procedure has been introduced. Using the measured surface velocities to evaluate the Hessian matrix of the velocities, a metric tensor is computed at the mesh vertices in order to define the adaptation scheme. The resulting meshed footprint obtained with the automatic tool YAMS shows a high density of elements in the vicinities of the North-East Greenland Ice Stream (NEGIS), the Jakobshavn ice stream (JIS) and the Kangerdlugssuaq (KL) and Helheim (HH) glaciers. On the other hand, elements with a coarser resolution are generated away from the ice streams and domain margins. The final three-dimensional mesh is obtained by extruding the 2D footprint with 21 vertical layers, so that the resulting mesh contains 400860 wedge elements and 233583 nodes. The numerical solution of the Stokes and the heat transfer equations involves direct and iterative solvers depending on the simulation case, and both methods are coupled with stabilization procedures. The boundary conditions are such that the temperature at the surface uses the present-day mean annual air temperature given by a parameterization or directly from the available data, the geothermal heat flux at the bedrock is prescribed as spatially constant and the lateral sides are open boundaries. A non-linear Weertman law is used for the basal sliding. The project goal is to better assess the effects of dynamical changes of the Greenland ice sheet on sea level rise under global-warming conditions. Hence, the simulations have been conducted in order to investigate the ice sheet evolution using the climate forcing experiments defined in the SeaRISE project. For that purpose, four different experiments have been conducted, (i) constant climate control run beginning at present (epoch 2004-1-1 0:0:0) and running up to 500 years holding the climate constant to its present state, (ii) constant climate forcing with increased basal lubrication, (iii) AR4 climate run forced by anomalies derived from results given in the IPCC 4th Assessment Report (AR4) for the A1B emission scenario, (iv) AR4 climate run with increased basal lubrication.

Ice-sheet modelling accelerated by graphics cards

NASA Astrophysics Data System (ADS)

Brædstrup, Christian Fredborg; Damsgaard, Anders; Egholm, David Lundbek

2014-11-01

Studies of glaciers and ice sheets have increased the demand for high performance numerical ice flow models over the past decades. When exploring the highly non-linear dynamics of fast flowing glaciers and ice streams, or when coupling multiple flow processes for ice, water, and sediment, researchers are often forced to use super-computing clusters. As an alternative to conventional high-performance computing hardware, the Graphical Processing Unit (GPU) is capable of massively parallel computing while retaining a compact design and low cost. In this study, we present a strategy for accelerating a higher-order ice flow model using a GPU. By applying the newest GPU hardware, we achieve up to 180× speedup compared to a similar but serial CPU implementation. Our results suggest that GPU acceleration is a competitive option for ice-flow modelling when compared to CPU-optimised algorithms parallelised by the OpenMP or Message Passing Interface (MPI) protocols.

Deglaciation of the Western Margin of the Barents Sea Ice Sheet - a Swath Bathymetric and Sub-Bottom Seismic Study from Eglacom Nice-Streams Data in the Kveithola Trough

NASA Astrophysics Data System (ADS)

Rebesco, M.; Liu, Y.; Camerlenghi, A.; Winsborrow, M. C.; Laberg, J.; Caburlotto, A.; Diviacco, P.; Accettella, D.; Sauli, C.; Wardell, N.

2010-12-01

IPY Activity N. 367 focusing on Neogene ice streams and sedimentary processes on high- latitude continental margins (NICE-STREAMS) resulted in two coordinated cruises on the adjacent Storfjorden and Kveithola trough-mouth fans in the NW Barents Sea: SVAIS Cruise of BIO Hespérides, summer 2007, and EGLACOM Cruise of Cruise R/V OGS-Explora, summer 2008. The objectives were to acquire a high-resolution set of bathymetric, seismic and sediment core data in order to decipher the Neogene architectural development of the glacially-dominated NW Barents Sea continental margin in response to natural climate change. The paleo-ice streams drained ice from southern Spitsbergen, Spitsbergen Bank, and Bear Island. The short distance from the ice source to the calving front produced a short residence time of ice, and therefore a rapid response to climatic changes. We describe here the EGLACOM data collected within the Kveithola Trough, an E-W trending glacial trough in the NW Barents Sea, NW of the Bear Island. Swath bathymetry shows that the seafloor is characterised by E-W trending mega-scale glacial lineations (MSGL) that record a fast flowing ice stream draining the Svalbard/Barents Sea Ice Sheet (SBIS) during the Last Glacial Maximum (LGM). MSGL are overprinted by transverse sediment ridges about 15 km apart which give rise to a staircase axial profile of the trough. Such transverse ridges are interpreted as grounding-zone wedges (GZW) formed by deposition of unconsolidated, saturated subglacial till during episodic ice stream retreat. Sub-bottom (CHIRP) and multi-channel reflection seismic data show that present-day morphology is largely inherited from the palaeo-seafloor topography at the time of deposition of the transverse ridges, overlain by a draping glaciomarine unit up to over 15 m thick. Our data allow the reconstruction of depositional processes that accompanied the deglaciation of the Spitsbergen Bank area. The sedimentary drape deposited on top of the GZWs which accumulated at a very high rate in the order of 1-1.5 m ka-1 has a potential to preserve a high resolution palaeoclimatic record of the deglaciation and post-glacial condition in this sector of the Barents Sea.

Did ice streams carve martian outflow channels?

USGS Publications Warehouse

Lucchitta, B.K.; Anderson, D.M.; Shoji, H.

1981-01-01

Outflow channels on Mars1 are long sinuous linear depressions that occur mostly in the equatorial area (??30?? lat.). They differ from small valley networks2 by being larger and arising full born from chaotic terrains. Outflow channels resemble terrestrial stream beds, and their origin has generally been attributed to water3-5 in catastrophic floods6,7 or mudflows8. The catastrophic-flood hypothesis is derived primarily from the morphological similarities of martian outflow channels and features created by the catastrophic Spokane flood that formed the Washington scablands. These similarities have been documented extensively3,6,7, but differences of scale remain a major problemmartian channel features are on the average much larger than their proposed terrestrial analogues. We examine here the problem of channel origin from the perspective of erosional characteristics and the resultant landf orms created by former and present-day ice streams and glaciers on Earth. From morphologic comparisons, an ice-stream origin seems equally well suited to explain the occurrences and form of the outflow channels on Mars, and in contrast with the hydraulic hypothesis, ice streams and ice sheets produce terrestrial features of the same scale as those observed on Mars. ?? 1981 Nature Publishing Group.

Ice stream behaviour in the western sector of the North Sea during the end of the last glacial cycle

NASA Astrophysics Data System (ADS)

Roberts, David; Evans, David; Clark, Chris; Bateman, Mark; Livingstone, Stephen; Medialdea, Alicia; Cofaigh, Colm O.; Grimoldi, Elena; Callard, Louise; Dove, Dayton; Stewart, Heather; Davies, Bethan; Chiverell, Richard

2016-04-01

During the last glacial cycle the East coast of the UK was overrun by the British-Irish Ice Sheet (BIIS) flowing eastwards and southwards. In recent years it has become evident that several ice streams including the Tweed, Tyne, and Stainmore Gap ice streams, as well as the late stage North Sea Lobe (NSL), all played a role in shaping the glacial landscape during this period, but understanding the flow phasing of these ice streams during advance and collapse has proved challenging. Here we present new data from the seafloor collected during recent work undertaken by the Britice Chrono and Glanam project teams during cruise JC123 in the North Sea. Sub-bottom seafloor data together with new swath data clearly show that the final phases of the collapse of the NSL were controlled by ice sourced from the Firth of Forth ice stream which deglaciated in a NNW trajectory. Other ice streams being fed from the west (e.g. Stainmore, Tyne, Tweed) were not influential in final phase ice retreat from the southern North Sea. The Forth ice imprint is characterised by several grounding zone/till wedges marking dynamic, oscillatory retreat of the ice as it retreated along an offshore corridor between North Yorkshire and Northumberland. Repeated packages of tills, ice marginal and glaciomarine sediments, which drape glacially scoured bedrock terrain and drumlins along this corridor, point to marine inundation accompanying ice retreat. New TCN ages suggest decoupling of the Tyne Gap ice stream and NSL between 17.8 and 16.5 ka and this coincides with rapid, regional collapse of the NSL between 17.2 and 16.0 ka along the Yorkshire and Durham coasts (new OSL ages; Britice Chrono). Hence, both the central and northern sectors of the BIIS were being strongly influenced by marine margin instability during the latter phases of the last glacial cycle.

Decadal-Scale Response of the Antarctic Ice sheet to a Warming Ocean using the POPSICLES Coupled Ice Sheet-Ocean model

NASA Astrophysics Data System (ADS)

Martin, D. F.; Asay-Davis, X.; Cornford, S. L.; Price, S. F.; Ng, E. G.; Collins, W.

2015-12-01

We present POPSICLES simulation results covering the full Antarctic Ice Sheet and the Southern Ocean spanning the period from 1990 to 2010. We use the CORE v. 2 interannual forcing data to force the ocean model. Simulations are performed at 0.1o(~5 km) ocean resolution with adaptive ice sheet resolution as fine as 500 m to adequately resolve the grounding line dynamics. We discuss the effect of improved ocean mixing and subshelf bathymetry (vs. the standard Bedmap2 bathymetry) on the behavior of the coupled system, comparing time-averaged melt rates below a number of major ice shelves with those reported in the literature. We also present seasonal variability and decadal melting trends from several Antarctic regions, along with the response of the ice shelves and the consequent dynamic response of the grounded ice sheet.POPSICLES couples the POP2x ocean model, a modified version of the Parallel Ocean Program, and the BISICLES ice-sheet model. POP2x includes sub-ice-shelf circulation using partial top cells and the commonly used three-equation boundary layer physics. Standalone POP2x output compares well with standard ice-ocean test cases (e.g., ISOMIP) and other continental-scale simulations and melt-rate observations. BISICLES makes use of adaptive mesh refinement and a 1st-order accurate momentum balance similar to the L1L2 model of Schoof and Hindmarsh to accurately model regions of dynamic complexity, such as ice streams, outlet glaciers, and grounding lines. Results of BISICLES simulations have compared favorably to comparable simulations with a Stokes momentum balance in both idealized tests (MISMIP-3d) and realistic configurations.The figure shows the BISICLES-computed vertically-integrated grounded ice velocity field 5 years into a 20-year coupled full-continent Antarctic-Southern-Ocean simulation. Submarine melt rates are painted onto the surface of the floating ice shelves. Grounding lines are shown in green.

High geothermal heat flux measured below the West Antarctic Ice Sheet

PubMed Central

Fisher, Andrew T.; Mankoff, Kenneth D.; Tulaczyk, Slawek M.; Tyler, Scott W.; Foley, Neil

2015-01-01

The geothermal heat flux is a critical thermal boundary condition that influences the melting, flow, and mass balance of ice sheets, but measurements of this parameter are difficult to make in ice-covered regions. We report the first direct measurement of geothermal heat flux into the base of the West Antarctic Ice Sheet (WAIS), below Subglacial Lake Whillans, determined from the thermal gradient and the thermal conductivity of sediment under the lake. The heat flux at this site is 285 ± 80 mW/m2, significantly higher than the continental and regional averages estimated for this site using regional geophysical and glaciological models. Independent temperature measurements in the ice indicate an upward heat flux through the WAIS of 105 ± 13 mW/m2. The difference between these heat flux values could contribute to basal melting and/or be advected from Subglacial Lake Whillans by flowing water. The high geothermal heat flux may help to explain why ice streams and subglacial lakes are so abundant and dynamic in this region. PMID:26601210

Predicting uncertainty in future marine ice sheet volume using Bayesian statistical methods

NASA Astrophysics Data System (ADS)

Davis, A. D.

2015-12-01

The marine ice instability can trigger rapid retreat of marine ice streams. Recent observations suggest that marine ice systems in West Antarctica have begun retreating. However, unknown ice dynamics, computationally intensive mathematical models, and uncertain parameters in these models make predicting retreat rate and ice volume difficult. In this work, we fuse current observational data with ice stream/shelf models to develop probabilistic predictions of future grounded ice sheet volume. Given observational data (e.g., thickness, surface elevation, and velocity) and a forward model that relates uncertain parameters (e.g., basal friction and basal topography) to these observations, we use a Bayesian framework to define a posterior distribution over the parameters. A stochastic predictive model then propagates uncertainties in these parameters to uncertainty in a particular quantity of interest (QoI)---here, the volume of grounded ice at a specified future time. While the Bayesian approach can in principle characterize the posterior predictive distribution of the QoI, the computational cost of both the forward and predictive models makes this effort prohibitively expensive. To tackle this challenge, we introduce a new Markov chain Monte Carlo method that constructs convergent approximations of the QoI target density in an online fashion, yielding accurate characterizations of future ice sheet volume at significantly reduced computational cost.Our second goal is to attribute uncertainty in these Bayesian predictions to uncertainties in particular parameters. Doing so can help target data collection, for the purpose of constraining the parameters that contribute most strongly to uncertainty in the future volume of grounded ice. For instance, smaller uncertainties in parameters to which the QoI is highly sensitive may account for more variability in the prediction than larger uncertainties in parameters to which the QoI is less sensitive. We use global sensitivity analysis to help answer this question, and make the computation of sensitivity indices computationally tractable using a combination of polynomial chaos and Monte Carlo techniques.

Continental-scale transport of sediments by the Baltic Ice Stream elucidated by coupled grain size and Nd provenance analyses

NASA Astrophysics Data System (ADS)

Boswell, Steven M.; Toucanne, Samuel; Creyts, Timothy T.; Hemming, Sidney R.

2018-05-01

We introduce a methodology for determining the transport distance of subglacially comminuted and entrained sediments. We pilot this method on sediments from the terminal margin of the Baltic Ice Stream, the largest ice stream of the Fennoscandian Ice Sheet during the Last Glacial Maximum. A strong correlation (R2 = 0.83) between the εNd and latitudes of circum-Baltic river sediments enables us to use εNd as a calibrated measure of distance. The proportion of subglacially transported sediments in a sample is estimated from grain size ratios in the silt fraction (<63 μm). Coupled εNd and grain size analyses reveal a common erosion source for the Baltic Ice Stream sediments located near the Åland sill, more than 850 km upstream from the terminal moraines. This result is in agreement with both numerical modeling and geomorphological investigations of Fennoscandinavian erosion, and is consistent with rapid ice flow into the Baltic basins prior to the Last Glacial Maximum. The methodology introduced here could be used to infer the distances of glacigenic sediment transport from Late Pleistocene and earlier glaciations.

Sedimentology and chronology of the advance and retreat of the last British-Irish Ice Sheet on the continental shelf west of Ireland

NASA Astrophysics Data System (ADS)

Peters, Jared L.; Benetti, Sara; Dunlop, Paul; Ó Cofaigh, Colm; Moreton, Steven G.; Wheeler, Andrew J.; Clark, Christopher D.

2016-05-01

The last British-Irish Ice Sheet (BIIS) had extensive marine-terminating margins and was drained by multiple large ice streams and is thus a useful analogue for marine-based areas of modern ice sheets. However, despite recent advances from investigating the offshore record of the BIIS, the dynamic history of its marine margins, which would have been sensitive to external forcing(s), remain inadequately understood. This study is the first reconstruction of the retreat dynamics and chronology of the western, marine-terminating, margin of the last (Late Midlandian) BIIS. Analyses of shelf geomorphology and core sedimentology and chronology enable a reconstruction of the Late Midlandian history of the BIIS west of Ireland, from initial advance to final retreat onshore. Five AMS radiocarbon dates from marine cores constrain the timing of retreat and associated readvances during deglaciation. The BIIS advanced without streaming or surging, depositing a bed of highly consolidated subglacial traction till, and reached to within ∼20 km of the shelf break by ∼24,000 Cal BP. Ice margin retreat was likely preceded by thinning, grounding zone retreat and ice shelf formation on the outer shelf by ∼22,000 Cal BP. This ice shelf persisted for ≤2500 years, while retreating at a minimum rate of ∼24 m/yr and buttressing a >150-km long, 20-km wide, bathymetrically-controlled grounding zone. A large (∼150 km long), arcuate, flat-topped grounding-zone wedge, termed here the Galway Lobe Grounding-Zone Wedge (GLGZW), was deposited below this ice shelf and records a significant stillstand in BIIS retreat. Geomorphic relationships indicate that the BIIS experienced continued thinning during its retreat across the shelf, which led to increased topographic influence on its flow dynamics following ice shelf break up and grounding zone retreat past the GLGZW. At this stage of retreat the western BIIS was comprised of several discrete, asynchronous lobes that underwent several readvances. Sedimentary evidence of dilatant till deposition suggests that the readvances may have been rapid and possibly associated with ice streaming or surging. The largest lobe extended offshore from Galway Bay and deposited the Galway Lobe Readvance Moraine by <18,500 Cal BP. Further to the north, an ice lobe readvanced at least 50 km offshore from Killary Harbour, possibly by ≤15,100 Cal BP. The existing chronology currently does not allow us to determine conclusively whether these readvances were a glaciodynamic (internally-driven) response of the ice sheet during deglaciation or were climatically-driven. Following the <18,500 Cal BP readvance, the Galway Lobe experienced accelerated eastward retreat at an estimated rate of ∼113 m/yr.

Lithology and chronology of ice-sheet fluctuations (magnetic susceptibility of cores from the western Ross Sea)

NASA Technical Reports Server (NTRS)

Jennings, Anne E.

1993-01-01

The goals of the marine geology part of WAIS include reconstructing the chronology and areal extent of ice-sheet fluctuations and understanding the climatic and oceanographic influences on ice-sheet history. As an initial step toward attaining these goals, down-core volume magnetic susceptibility (MS) logs of piston cores from three N-S transects in the western Ross Sea are compared. The core transects are within separate petrographic provinces based on analyses of till composition. The provinces are thought to reflect the previous locations of ice streams on the shelf during the last glaciation. Magnetic susceptibility is a function of magnetic mineral composition, sediment texture, and sediment density. It is applied in the western Ross Sea for two purposes: (1) to determine whether MS data differentiates the three transects (i.e., flow lines), and thus can be used to make paleodrainage reconstructions of the late Wisconsinan ice sheet; and (2) to determine whether the MS data can aid in distinguishing basal till diamictons from diamictons of glacial-marine origin and thus, aid paleoenvironmental interpretations. A comparison of the combined data of cores in each transect is presented.

Glacimarine Sedimentary Processes and Facies on the Polar North Atlantic Margins

NASA Astrophysics Data System (ADS)

Dowdeswell, J. A.; Elverhfi, A.; Spielhagen, R.

Major contrasts in the glaciological, oceanic and atmospheric parameters affecting the Polar North Atlantic, both over space between its eastern and western margins, and through time from full glacial to interglacial conditions, have lead to the deposition of a wide variety of sedimentary facies in these ice-influenced seas. The dynamics of the glaciers and ice sheets on the hinterlands surrounding the Polar North Atlantic have exterted a major influence on the processes, rates and patterns of sedimentation on the continental margins of the Norwegian and Greenland seas over the Late Cenozoic. The western margin is influenced by the cold East Greenland Current and the Svalbard margin by the northernmost extent of the warm North Atlantic Drift and the passage of relatively warm cyclonic air masses. In the fjords of Spitsbergen and the northwestern Barents Sea, glacial meltwater is dominant in delivering sediments. In the fjords of East Greenland the large numbers of icebergs produced from fast-flowing outlets of the Greenland Ice Sheet play a more significant role in sedimentation. During full glacials, sediments are delivered to the shelf break from fast-flowing ice streams, which drain huge basins within the parent ice sheet. Large prograding fans located on the continental slope offshore of these ice streams are made up of stacked debris flows. Large-scale mass failures, turbidity currents, and gas-escape structures also rework debris in continental slope and shelf settings. Even during interglacials, both the margins and the deep ocean basins beyond them retain a glacimarine overprint derived from debris in far-travelled icebergs and sea ice. Under full glacial conditions, the glacier influence is correspondingly stronger, and this is reflected in the glacial and glacimarine facies deposited at these times.

Insights into the Geographic Sequence of Deglaciation in the Weddell Sea Embayment by Provenance of Ice-Rafted Debris

NASA Astrophysics Data System (ADS)

Williams, T.; Hemming, S. R.; Licht, K.; Agrios, L.; Brachfeld, S. A.; van de Flierdt, T.; Hillenbrand, C. D.; Ehrmann, W. U.; Zhai, X.; Cai, Y.; Corley, A. D.; Kuhn, G.

2017-12-01

The geochemical and geochronological fingerprint of rock debris eroded and carried by ice streams may be used to identify the provenance of iceberg-rafted debris (IRD) in the marine sediment record. During ice retreat following glacial maxima, it has been shown that there is an increase in IRD accumulation in marine sediments underlying the western limb of the Weddell Gyre. Here we present IRD provenance records from sediment core PS1571-1 in the NW Weddell Sea, and interpret these records in terms of the geographic sequence of ice sheet retreat in the Weddell Sea embayment during the most recent deglaciation. We first characterize the source areas of eroded debris around the Weddell Sea Embayment, using published mapping of the embayment and new material from: 1. Till in modern moraines at the edges of ice streams, including the Foundation Ice Stream, the Academy Glacier, and the Recovery Glacier; and 2. Subglacial till and proximal glaciomarine sediment from existing cores located along the front of the Filchner and Ronne Ice Shelves, collected on past expeditions of the RV Polarstern. The analyses on these samples include 40Ar/39Ar hornblende and biotite thermochronology and U-Pb zircon geochronology on individual mineral grains, and K-Ar thermochronology, Nd isotopes, and clay mineralogy on the clay grain size fraction. Results so far indicate that samples along the front of the Filchner and Ronne Ice Shelves record the geochemical and geochronological fingerprint that would be expected from tracing ice flow lines back to the bedrock terranes. The Ronne (west), Hughes (central), and Filchner (east) sectors have distinguishable provenance source signatures, and further subdivision is possible. In core PS1571-1, downcore IRD provenance changes reflect iceberg output and ice sheet retreat from the different sectors of the embayment through the last deglaciation. The detrital provenance method of interpreting the geographic sequence of ice retreat can equally be applied to previous deglaciations of the Weddell Sea Embayment.

CUDA GPU based full-Stokes finite difference modelling of glaciers

NASA Astrophysics Data System (ADS)

Brædstrup, C. F.; Egholm, D. L.

2012-04-01

Many have stressed the limitations of using the shallow shelf and shallow ice approximations when modelling ice streams or surging glaciers. Using a full-stokes approach requires either large amounts of computer power or time and is therefore seldom an option for most glaciologists. Recent advances in graphics card (GPU) technology for high performance computing have proven extremely efficient in accelerating many large scale scientific computations. The general purpose GPU (GPGPU) technology is cheap, has a low power consumption and fits into a normal desktop computer. It could therefore provide a powerful tool for many glaciologists. Our full-stokes ice sheet model implements a Red-Black Gauss-Seidel iterative linear solver to solve the full stokes equations. This technique has proven very effective when applied to the stokes equation in geodynamics problems, and should therefore also preform well in glaciological flow probems. The Gauss-Seidel iterator is known to be robust but several other linear solvers have a much faster convergence. To aid convergence, the solver uses a multigrid approach where values are interpolated and extrapolated between different grid resolutions to minimize the short wavelength errors efficiently. This reduces the iteration count by several orders of magnitude. The run-time is further reduced by using the GPGPU technology where each card has up to 448 cores. Researchers utilizing the GPGPU technique in other areas have reported between 2 - 11 times speedup compared to multicore CPU implementations on similar problems. The goal of these initial investigations into the possible usage of GPGPU technology in glacial modelling is to apply the enhanced resolution of a full-stokes solver to ice streams and surging glaciers. This is a area of growing interest because ice streams are the main drainage conjugates for large ice sheets. It is therefore crucial to understand this streaming behavior and it's impact up-ice.

Coastal-change and glaciological map of the Saunders Coast area, Antarctica; 1972-1997

USGS Publications Warehouse

Swithinbank, Charles; Williams, Richard S.; Ferrigno, Jane G.; Foley, Kevin M.; Hallam, Cheryl A.; Rosanova, Christine E.

2003-01-01

Changes in the area and volume of polar ice sheets are intricately linked to changes in global climate, and the resulting changes in sea level may severely impact the densely populated coastal regions on Earth. Melting of the West Antarctic part alone of the Antarctic ice sheet could cause a sea-level rise of approximately 6 meters (m). The potential sea-level rise after melting of the entire Antarctic ice sheet is estimated to be 65 m (Lythe and others, 2001) to 73 m (Williams and Hall, 1993). In spite of its importance, the mass balance (the net volumetric gain or loss) of the Antarctic ice sheet is poorly known; it is not known for certain whether the ice sheet is growing or shrinking. In a review paper, Rignot and Thomas (2002) concluded that the West Antarctic part of the Antarctic ice sheet is probably becoming thinner overall; although the western part is thickening, the northern part is thinning. Joughin and Tulaczyk (2002), based on analysis of ice-flow velocities derived from synthetic aperture radar, concluded that most of the Ross ice streams (ice streams on the east side of the Ross Ice Shelf) have a positive mass balance. The mass balance of the East Antarctic is unknown, but thought to be in near equilibrium. Measurement of changes in area and mass balance of the Antarctic ice sheet was given a very high priority in recommendations by the Polar Research Board of the National Research Council (1986), in subsequent recommendations by the Scientific Committee on Antarctic Research (SCAR) (1989, 1993), and by the National Science Foundation?s (1990) Division of Polar Programs. On the basis of these recommendations, the U.S. Geological Survey (USGS) decided that the archive of early 1970s Landsat 1, 2, and 3 Multispectral Scanner (MSS) images of Antarctica and the subsequent repeat coverage made possible with Landsat and other satellite images provided an excellent means of documenting changes in the coastline of Antarctica (Ferrigno and Gould, 1987). The availability of this information provided the impetus for carrying out a comprehensive analysis of the glaciological features of the coastal regions and changes in ice fronts of Antarctica (Swithinbank, 1988; Williams and Ferrigno, 1988). The project was later modified to include Landsat 4 and 5 MSS and Thematic Mapper (TM) (and in some areas Landsat 7 Enhanced Thematic Mapper Plus (ETM+)), RADARSAT images, and other data where available, to compare changes over a 20- to 25- or 30-year time interval (or longer where data were available, as in the Antarctic Peninsula). The results of the analysis are being used to produce a digital database and a series of USGS Geologic Investigations Series Maps consisting of 24 maps at 1:1,000,000 scale and 1 map at 1:5,000,000 scale, in both paper and digital format (Williams and others, 1995; Williams and Ferrigno, 1998; and Ferrigno and others, 2002).

Coastal-change and glaciological map of the Eights Coast area, Antarctica, 1972-2001

USGS Publications Warehouse

Swithinbank, Charles; Williams, Richard S.; Ferrigno, Jane G.; Foley, Kevin M.; Rosanova, Christine E.; Dailide, Lina M.

2004-01-01

Changes in the area and volume of polar ice sheets are intricately linked to changes in global climate, and the resulting changes in sea level may severely impact the densely populated coastal regions on Earth. Melting of the West Antarctic part alone of the Antarctic ice sheet could cause a sea-level rise of approximately 6 meters (m). The potential sea-level rise after melting of the entire Antarctic ice sheet is estimated to be 65 m (Lythe and others, 2001) to 73 m (Williams and Hall, 1993). In spite of its importance, the mass balance (the net volumetric gain or loss) of the Antarctic ice sheet is poorly known; it is not known for certain whether the ice sheet is growing or shrinking. In a review paper, Rignot and Thomas (2002) concluded that the West Antarctic part of the Antarctic ice sheet is probably becoming thinner overall; although the western part is thickening, the northern part is thinning. Joughin and Tulaczyk (2002), based on analysis of ice-flow velocities derived from synthetic aperture radar, concluded that most of the Ross ice streams (ice streams on the east side of the Ross Ice Shelf) have a positive mass balance. The mass balance of the East Antarctic is unknown, but thought to be in near equilibrium. Measurement of changes in area and mass balance of the Antarctic ice sheet was given a very high priority in recommendations by the Polar Research Board of the National Research Council (1986), in subsequent recommendations by the Scientific Committee on Antarctic Research (SCAR) (1989, 1993), and by the National Science Foundation's (1990) Division of Polar Programs. On the basis of these recommendations, the U.S. Geological Survey (USGS) decided that the archive of early 1970s Landsat 1, 2, and 3 Multispectral Scanner (MSS) images of Antarctica and the subsequent repeat coverage made possible with Landsat and other satellite images provided an excellent means of documenting changes in the coastline of Antarctica (Ferrigno and Gould, 1987). The availability of this information provided the impetus for carrying out a comprehensive analysis of the glaciological features of the coastal regions and changes in ice fronts of Antarctica (Swithinbank, 1988; Williams and Ferrigno, 1988). The project was later modified to include Landsat 4 and 5 MSS and Thematic Mapper (TM) (and in some areas Landsat 7 Enhanced Thematic Mapper Plus (ETM+)), RADARSAT images, and other data where available, to compare changes over a 20- to 25- or 30-year time interval (or longer where data were available, as in the Antarctic Peninsula). The results of the analysis are being used to produce a digital database and a series of USGS Geologic Investigations Series Maps consisting of 24 maps at 1:1,000,000 scale and 1 map at 1:5,000,000 scale, in both paper and digital format (Williams and others, 1995; Williams and Ferrigno, 1998; and Ferrigno and others, 2002).

Coastal-change and glaciological maps of Antarctica

USGS Publications Warehouse

Williams, Richard S.

2004-01-01

Changes in the area and volume of polar ice sheets are intricately linked to changes in global climate, and the resulting changes in sea level may severely impact the densely populated coastal regions on Earth. Melting of the West Antarctic part alone of the Antarctic ice sheet could cause a sea-level rise of approximately 6 meters (m). The potential sea-level rise after melting of the entire Antarctic ice sheet is estimated to be 65 m (Lythe and others, 2001) to 73 m (Williams and Hall, 1993). In spite of its importance, the mass balance (the net volumetric gain or loss) of the Antarctic ice sheet is poorly known; it is not known for certain whether the ice sheet is growing or shrinking. In a review paper, Rignot and Thomas (2002) concluded that the West Antarctic part of the Antarctic ice sheet is probably becoming thinner overall; although the western part is thickening, the northern part is thinning. Joughin and Tulaczyk (2002), based on analysis of ice-flow velocities derived from synthetic aperture radar, concluded that most of the Ross ice streams (ice streams on the east side of the Ross Ice Shelf) have a positive mass balance. The mass balance of the East Antarctic is unknown, but thought to be in near equilibrium. Measurement of changes in area and mass balance of the Antarctic ice sheet was given a very high priority in recommendations by the Polar Research Board of the National Research Council (1986), in subsequent recommendations by the Scientific Committee on Antarctic Research (SCAR) (1989, 1993), and by the National Science Foundation's (1990) Division of Polar Programs. On the basis of these recommendations, the U.S. Geological Survey (USGS) decided that the archive of early 1970s Landsat 1, 2, and 3 Multispectral Scanner (MSS) images of Antarctica and the subsequent repeat coverage made possible with Landsat and other satellite images provided an excellent means of documenting changes in the coastline of Antarctica (Ferrigno and Gould, 1987). The availability of this information provided the impetus for carrying out a comprehensive analysis of the glaciological features of the coastal regions and changes in ice fronts of Antarctica (Swithinbank, 1988; Williams and Ferrigno, 1988). The project was later modified to include Landsat 4 and 5 MSS and Thematic Mapper (TM) (and in some areas Landsat 7 Enhanced Thematic Mapper Plus (ETM+)), RADARSAT images, and other data where available, to compare changes over a 20- to 25- or 30-year time interval (or longer where data were available, as in the Antarctic Peninsula). The results of the analysis are being used to produce a digital database and a series of USGS Geologic Investigations Series Maps consisting of 24 maps at 1:1,000,000 scale and 1 map at 1:5,000,000 scale, in both paper and digital format (Williams and others, 1995; Williams and Ferrigno, 1998; and Ferrigno and others, 2002).

East Antarctic ice-sheet dynamics between 5.2 and 0 Ma from a high-resolution terrigenous particle size record, ODP Site 1165, Prydz Bay-Cooperation Sea

USGS Publications Warehouse

Passchier, S.

2007-01-01

This paper discusses a 5.2-0 Ma high-resolution terrigenous particle size record recovered from a sediment drift off East Antarctica. The particle size properties of Hole 1165B are interpreted in the context of previously acquired data on a continental shelf to slope transect drilled by ODP Leg 188 in Prydz Bay and the Cooperation Sea. The new data indicate that the Lambert ice stream stayed predominantly landward of the shelf break in the early Pliocene (5.2-3.5 Ma) with periods of ice sheet recession on land. The middle Pliocene (3.5-3.1 Ma) is characterized as major ice expansion during glacials with deposition of laminated clays from meltwater plumes on the continental rise, alternating with periods of ice recession. A change in sedimentary facies and a decrease in sedimentation rates occurred at ~3.1 Ma indicating a more retreated Lambert Glacier. Between 2.5 and 1 Ma the ice stream was generally stable and had become cold-based with ice flow in a glacial trough extending to the shelf break. Three-four large pulses of coarse-grained glacigenic debris mark the record at ~1 Ma. These are interpreted as extensive calving due to decoupling of the marine terminus from its bed in response to Northern Hemisphere deglaciations and associated sea level rises.

Antarctic ice sheet mass loss estimates using Modified Antarctic Mapping Mission surface flow observations

NASA Astrophysics Data System (ADS)

Ren, Diandong; Leslie, Lance M.; Lynch, Mervyn J.

2013-03-01

The long residence time of ice and the relatively gentle slopes of the Antarctica Ice Sheet make basal sliding a unique positive feedback mechanism in enhancing ice discharge along preferred routes. The highly organized ice stream channels extending to the interior from the lower reach of the outlets are a manifestation of the role of basal granular material in enhancing the ice flow. In this study, constraining the model-simulated year 2000 ice flow fields with surface velocities obtained from InSAR measurements permits retrieval of the basal sliding parameters. Forward integrations of the ice model driven by atmospheric and oceanic parameters from coupled general circulation models under different emission scenarios provide a range of estimates of total ice mass loss during the 21st century. The total mass loss rate has a small intermodel and interscenario spread, rising from approximately -160 km3/yr at present to approximately -220 km3/yr by 2100. The accelerated mass loss rate of the Antarctica Ice Sheet in a warming climate is due primarily to a dynamic response in the form of an increase in ice flow speed. Ice shelves contribute to this feedback through a reduced buttressing effect due to more frequent systematic, tabular calving events. For example, by 2100 the Ross Ice Shelf is projected to shed 40 km3 during each systematic tabular calving. After the frontal section's attrition, the remaining shelf will rebound. Consequently, the submerged cross-sectional area will reduce, as will the buttressing stress. Longitudinal differential warming of ocean temperature contributes to tabular calving. Because of the prevalence of fringe ice shelves, oceanic effects likely will play a very important role in the future mass balance of the Antarctica Ice Sheet, under a possible future warming climate.

Jakobshavn Isbrae, Greenland: DEMs, orthophotos, surface velocities, and ice loss derived from photogrammetric re-analysis of July 1985 repeat aerial photography

NASA Astrophysics Data System (ADS)

Motyka, R.; Fahnestock, M.; Howat, I.; Truffer, M.; Brecher, H.; Luethi, M.

2008-12-01

Jakobshavn Isbrae drains about 7 % of the Greenland Ice Sheet and is the ice sheet's largest outlet glacier. Two sets of high elevation (~13,500 m), high resolution (2 m) aerial photographs of Jakobshavn Isbrae were obtained about two weeks apart during July 1985 (Fastook et al, 1995). These historic photo sets have become increasingly important for documenting and understanding the dynamic state of this outlet stream prior to the rapid retreat and massive ice loss that began in 1998 and continues today. The original photogrammetric analysis of this imagery is summarized in Fastook et al. (1995). They derived a coarse DEM (3 km grid spacing) covering an area of approximately 100 km x 100 km by interpolating several hundred positions determined manually from block-aerial triangulation. We have re-analyzed these photos sets using digital photogrammetry (BAE Socet Set©) and significantly improved DEM quality and resolution (20, 50, and 100 m grids). The DEMs were in turn used to produce high quality orthophoto mosaics. Comparing our 1985 DEM to a DEM we derived from May 2006 NASA ATM measurements showed a total ice volume loss of ~ 105 km3 over the lower drainage area; almost all of this loss has occurred since 1997. Ice stream surface velocities derived from the 1985 orthomosaics showed speeds of 20 m/d on the floating tongue, diminishing to 5 m/d at 50 km further upstream. Velocities have since nearly doubled along the ice stream during its current retreat. Fastook, J.L., H.H. Brecher, and T.J. Hughes, 1995. J.of Glaciol. 11 (137), 161-173.

Geoengineering Marine Ice Sheets

NASA Astrophysics Data System (ADS)

Wolovick, M.

2017-12-01

Mass loss from Greenland and Antarctica is highly sensitive to the presence of warm ocean water that drives melting at the grounding line. Rapid melting near the grounding line causes ice shelf thinning, loss of buttressing, flow acceleration, grounding line retreat, and ultimately mass loss and sea-level rise. If the grounding line enters a section of overdeepened bed the ice sheet may even enter a runaway collapse via the marine ice sheet instability. The warm water that triggers this process resides offshore at depth and accesses the grounding line through deep troughs in the continental shelf. In Greenland, warm water transport is further constricted through narrow fjords. Here, I propose blocking warm water transport through these choke points with an artificial sill. Using a simple width- and depth-averaged model of ice stream flow coupled to a buoyant-plume model of ocean melting, I find that grounding line retreat and sea level rise can be delayed or reversed for hundreds of years if warm water is prevented from accessing the grounding line at depth. Blocking of warm water from the sub-ice cavity causes ice shelf thickening, increased buttressing, and grounding line readvance. The increase in buttressing is greatly magnified if the thickened ice shelf regrounds on a bathymetric high or on the artificial sill itself. In some experiments for Thwaites Glacier the grounding line is able to recover from a severely retreated state over 100 km behind its present-day position. Such a dramatic recovery demonstrates that it is possible, at least in principle, to stop and reverse an ongoing marine ice sheet collapse. If the ice shelf regrounds on the artificial sill itself, erosion of the sill beneath the grounded ice could reduce the effectiveness of the intervention. However, experiments including sill erosion suggest that even a very weak sill (1 kPa) could delay a collapse for centuries. The scale of the artificial sills in Greenlandic fjords is comparable to existing large public works, while in Antarctica they are one to two orders of magnitude larger. However, this is still small in comparison to the global disruption that would be caused by a collapse of West Antarctica. Marine-terminating ice streams are high-leverage points in the climate system, where global impacts can be achieved through local intervention.

Ice sheet systems and sea level change.

NASA Astrophysics Data System (ADS)

Rignot, E. J.

2015-12-01

Modern views of ice sheets provided by satellites, airborne surveys, in situ data and paleoclimate records while transformative of glaciology have not fundamentally changed concerns about ice sheet stability and collapse that emerged in the 1970's. Motivated by the desire to learn more about ice sheets using new technologies, we stumbled on an unexplored field of science and witnessed surprising changes before realizing that most were coming too fast, soon and large. Ice sheets are integrant part of the Earth system; they interact vigorously with the atmosphere and the oceans, yet most of this interaction is not part of current global climate models. Since we have never witnessed the collapse of a marine ice sheet, observations and exploration remain critical sentinels. At present, these observations suggest that Antarctica and Greenland have been launched into a path of multi-meter sea level rise caused by rapid climate warming. While the current loss of ice sheet mass to the ocean remains a trickle, every mm of sea level change will take centuries of climate reversal to get back, several major marine-terminating sectors have been pushed out of equilibrium, and ice shelves are irremediably being lost. As glaciers retreat from their salty, warm, oceanic margins, they will melt away and retreat slower, but concerns remain about sea level change from vastly marine-based sectors: 2-m sea level equivalent in Greenland and 23-m in Antarctica. Significant changes affect 2/4 marine-based sectors in Greenland - Jakobshavn Isb. and the northeast stream - with Petermann Gl. not far behind. Major changes have affected the Amundsen Sea sector of West Antarctica since the 1980s. Smaller yet significant changes affect the marine-based Wilkes Land sector of East Antarctica, a reminder that not all marine-based ice is in West Antarctica. Major advances in reducing uncertainties in sea level projections will require massive, interdisciplinary efforts that are not currently in place but are getting there. Projection scenarios are overwhelmingly conservative, pushed up by observations, awaiting more detailed knowledge of ocean circulation, winds, ice-ocean interaction, and mechanics of rapid ice fracture, not to mention the mere definition of static boundaries (ice thickness and sea floor bathymetry).

Turning up the Heat on the Antarctic Ice Sheet (From Below): Challenges and Near-Term Opportunities for Measuring Antarctic Geothermal Fluxes (Invited)

NASA Astrophysics Data System (ADS)

Tulaczyk, S. M.; Hossainzadeh, S.

2010-12-01

Antarctic heat flow plays an important role in determining the rate of meltwater production at the base of the Antarctic ice sheet. Basal meltwater represents a key control on ice sheet mass balance, Antarctic geochemical fluxes into the Southern Ocean, and subglacial microbial habitats. However, direct measurements of heat flow are difficult in glaciated terrains. Vertical temperature profiles determined in ice boreholes are influenced by thermal energy fluxes associated with basal melting/freezing and have to be used with caution when calculating geothermal flux rates. Two published continent-wide geophysical estimates of Antarctic geothermal fluxes provide valuable databases but are not fully consistent with each other and need to be verified by direct subglacial measurements. Planned drilling into Antarctic subglacial environments will offer the opportunity to perform such measurements. Determination of temperature gradients in sedimentary sequences resting at the bottom of subglacial lakes will offer particularly useful insights. Temperature profiles in such environments will not be thermally or mechanically disturbed as it may be the case in till layers proximal to a sliding ice base. We will review plans for making such measurements as part of the WISSARD (Whillans Ice Stream Subglacial Access Research Drilling) project, which is scheduled to penetrate the West Antarctic ice sheet in 2012-13 and 2013-14.

Heinrich-type glacial surges in a low-order dynamical climate model

DOE Office of Scientific and Technical Information (OSTI.GOV)

Verbitsky, M.; Saltzman, B.

1994-07-01

Recent studies suggest the occurrence of sporadic episodes during which the ice streams that discharge ice sheets become enormously active, producing large numbers of icebergs (reflected in North Atlantic sea cores as {open_quotes}Heinrich events{close_quotes}) and possibly causing the partial collapse of the ice sheets. To simulate the mechanism of implied internal thermo-hydrodynamical instability in the context of a more general paleoclimate dynamics model (PDM), a new sliding-catastrophe function that can account for ice-sheet surges in terms of the thickness, density, viscosity, heat-capacity. and heat-conductivity of ice is introduced. Analysis suggests these events might be of three possible kinds: the firstmore » occurs in periods of glacial maximum when temperature conditions on the ice surface are extremely cold, but internal friction within bottom boundary layer is also at its maximum and is strong enough to melt ice and cause its surge. The second may happen during an interglacial, when the ice thickness is small but relatively warm climate conditions on the upper surface of ice can be easily advected with the flow of ice to the bottom where even a small additional heating due to friction may cause melting. The third and, perhaps, most interesting type is one that may occur during ice sheet growth: in this period particles of ice reaching the bottom {open_quotes}remember{close_quotes} the warm temperature conditions of the previous interglacial and additional heating due to increasing friction associated with the growing ice sheet may again cause melting. This third introduces the interesting possibility that earlier CO{sub 2} concentrations may be as important for the present-day climate as its current value. According to our model the climate system seems more vulnerable to surges during the penultimate interglacial period than in present one contributing to an explanation of the recent results of the Greenland Ice Core Project. 18 refs., 3 figs., 1 tab.« less

Bellingshausen Sea

NASA Image and Video Library

2017-12-08

A small hole in the clouds revealed newly formed sea ice in the Bellingshausen Sea next to an ice berg on Nov. 5, 2014 flight. Image Credit: NASA/Digital Mapping System NASA’s Operation IceBridge collected some rare images on a flight out of Punta Arenas, Chile on Nov. 5, 2014, on a science flight over western Antarctica dubbed Ferrigno-Alison-Abbott 01. The crew snapped a few shots of a calving front of the Antarctic ice sheet. This particular flight plan was designed to collect data on changes in ice elevation along the coast near the Ferrigno and Alison ice streams, on the Abbot Ice Shelf, and grounded ice along the Eights Coast.

Mountain Peaks

NASA Image and Video Library

2014-11-10

Mountain peaks through the ice cover on Thurston Island off of western Antarctica as seen on the IceBridge flight on Nov. 5, 2014. Image Credit: NASA/Jim Yungel NASA’s Operation IceBridge collected some rare images on a flight out of Punta Arenas, Chile on Nov. 5, 2014, on a science flight over western Antarctica dubbed Ferrigno-Alison-Abbott 01. The crew snapped a few shots of a calving front of the Antarctic ice sheet. This particular flight plan was designed to collect data on changes in ice elevation along the coast near the Ferrigno and Alison ice streams, on the Abbot Ice Shelf, and grounded ice along the Eights Coast.

Pathways of warm water to the Northeast Greenland outlet glaciers

NASA Astrophysics Data System (ADS)

Schaffer, Janin; Timmermann, Ralph; Kanzow, Torsten; Arndt, Jan Erik; Mayer, Christoph; Schauer, Ursula

2015-04-01

The ocean plays an important role in modulating the mass balance of the Greenland Ice Sheet by delivering heat to the marine-terminating outlet glaciers surrounding the Greenland coast. The warming and accumulation of Atlantic Water in the subpolar North Atlantic has been suggested to be a potential driver of the glaciers' retreat over the last decades. The shelf regions thus play a critical role for the transport of Atlantic Water towards the glaciers, but also for the transfer of freshwater towards the deep ocean. A key region for the mass balance of the Greenland Ice Sheet is the Northeast Greenland Ice Stream. This large ice stream drains the second-largest basin of the Greenland Ice Sheet and feeds three outlet glaciers. The largest one is Nioghalvfjerdsfjorden (79°N-Glacier) featuring an 80 km long floating ice tongue. Both the ocean circulation on the continental shelf off Northeast Greenland and the circulation in the cavity below the ice tongue are weakly constrained so far. In order to study the relevant processes of glacier-ocean interaction we combine observations and model work. Here we focus on historic and recent hydrographic observations and on the complex bathymetry in the Northeast Greenland shelf region, which is thought to steer the flux of warm Atlantic water onto the continental shelf and into the sub-ice cavity beneath the 79°N-Glacier. We present a new global topography data set, RTopo-2, which includes the most recent surveys on the Northeast Greenland continental shelf and provides a detailed bathymetry for all around Greenland. In addition, RTopo-2 contains ice and bedrock surface topographies for Greenland and Antarctica. Based on the updated ocean bathymetry and a variety of hydrographic observations we show the water mass distribution on the continental shelf off Northeast Greenland. These maps enable us to discuss possible supply pathways of warm modified Atlantic waters on the continental shelf and thus potential ways of heat transport towards the base of the 79°N-Glacier.

The build-up, configuration, and dynamical sensitivity of the Eurasian ice-sheet complex to Late Weichselian climatic and oceanic forcing

NASA Astrophysics Data System (ADS)

Patton, Henry; Hubbard, Alun; Andreassen, Karin; Winsborrow, Monica; Stroeven, Arjen P.

2016-12-01

The Eurasian ice-sheet complex (EISC) was the third largest ice mass during the Last Glacial Maximum (LGM), after the Antarctic and North American ice sheets. Despite its global significance, a comprehensive account of its evolution from independent nucleation centres to its maximum extent is conspicuously lacking. Here, a first-order, thermomechanical model, robustly constrained by empirical evidence, is used to investigate the dynamics of the EISC throughout its build-up to its maximum configuration. The ice flow model is coupled to a reference climate and applied at 10 km spatial resolution across a domain that includes the three main spreading centres of the Celtic, Fennoscandian and Barents Sea ice sheets. The model is forced with the NGRIP palaeo-isotope curve from 37 ka BP onwards and model skill is assessed against collated flowsets, marginal moraines, exposure ages and relative sea-level history. The evolution of the EISC to its LGM configuration was complex and asynchronous; the western, maritime margins of the Fennoscandian and Celtic ice sheets responded rapidly and advanced across their continental shelves by 29 ka BP, yet the maximum aerial extent (5.48 × 106 km2) and volume (7.18 × 106 km3) of the ice complex was attained some 6 ka later at c. 22.7 ka BP. This maximum stand was short-lived as the North Sea and Atlantic margins were already in retreat whilst eastern margins were still advancing up until c. 20 ka BP. High rates of basal erosion are modelled beneath ice streams and outlet glaciers draining the Celtic and Fennoscandian ice sheets with extensive preservation elsewhere due to frozen subglacial conditions, including much of the Barents and Kara seas. Here, and elsewhere across the Norwegian shelf and North Sea, high pressure subglacial conditions would have promoted localised gas hydrate formation.

Coastal-change and glaciological map of the Ronne Ice Shelf area, Antarctica, 1974-2002

USGS Publications Warehouse

Ferrigno, Jane G.; Foley, K.M.; Swithinbank, C.; Williams, R.S.; Dalide, L.M.

2005-01-01

Changes in the area and volume of polar ice sheets are intricately linked to changes in global climate, and the resulting changes in sea level may severely impact the densely populated coastal regions on Earth. Melting of the West Antarctic part alone of the Antarctic ice sheet could cause a sea-level rise of approximately 6 meters (m). The potential sea-level rise after melting of the entire Antarctic ice sheet is estimated to be 65 m (Lythe and others, 2001) to 73 m (Williams and Hall, 1993). In spite of its importance, the mass balance (the net volumetric gain or loss) of the Antarctic ice sheet is poorly known; it is not known for certain whether the ice sheet is growing or shrinking. In a review paper, Rignot and Thomas (2002) concluded that the West Antarctic part of the Antarctic ice sheet is probably becoming thinner overall; although it is thickening in the west, it is thinning in the north. Joughin and Tulaczyk (2002), on the basis of analysis of ice-flow velocities derived from synthetic aperture radar, concluded that most of the Ross ice streams (ice streams on the east side of the Ross Ice Shelf) have a positive mass balance, whereas Rignot and others (in press) infer even larger negative mass balance for glaciers flowing northward into the Amundsen Sea, a trend suggested by Swithinbank and others (2003a,b, 2004). The mass balance of the East Antarctic part of the Antarctic ice sheet is unknown, but thought to be in near equilibrium. Measurement of changes in area and mass balance of the Antarctic ice sheet was given a very high priority in recommendations by the Polar Research Board of the National Research Council (1986), in subsequent recommendations by the Scientific Committee on Antarctic Research (SCAR) (1989, 1993), and by the National Science Foundation's (1990) Division of Polar Pro-grams. On the basis of these recommendations, the U.S. Geo-logical Survey (USGS) decided that the archive of early 1970s Landsat 1, 2, and 3 Multispectral Scanner (MSS) images of Ant-arctica and the subsequent repeat coverage made possible with Landsat and other satellite images provided an excellent means of documenting changes in the coastline of Antarctica (Ferrigno and Gould, 1987). The availability of this information provided the impetus for carrying out a comprehensive analysis of the glaciological features of the coastal regions and changes in ice fronts of Antarctica (Swithinbank, 1988; Williams and Ferrigno, 1988). The project was later modified to include Landsat 4 and 5 MSS and Thematic Mapper (TM) (and in some areas Landsat 7 Enhanced Thematic Mapper Plus (ETM+)), RADARSAT images, and other data where available, to compare changes during a 20- to 25- or 30-year time interval (or longer where data were available, as in the Antarctic Peninsula). The results of the analysis are being used to produce a digital database and a series of USGS Geologic Investigations Series Maps (I-2600) consisting of 23 maps at 1:1,000,000 scale and 1 map at 1:5,000,000 scale, in both paper and digital format (Williams and others, 1995; Williams and Ferrigno, 1998; Ferrigno and others, 2002) (available online at http://www.glaciers.er.usgs.gov).

Ice-shelf collapse from subsurface warming as a trigger for Heinrich events

PubMed Central

Marcott, Shaun A.; Clark, Peter U.; Padman, Laurie; Klinkhammer, Gary P.; Springer, Scott R.; Liu, Zhengyu; Otto-Bliesner, Bette L.; Carlson, Anders E.; Ungerer, Andy; Padman, June; He, Feng; Cheng, Jun; Schmittner, Andreas

2011-01-01

Episodic iceberg-discharge events from the Hudson Strait Ice Stream (HSIS) of the Laurentide Ice Sheet, referred to as Heinrich events, are commonly attributed to internal ice-sheet instabilities, but their systematic occurrence at the culmination of a large reduction in the Atlantic meridional overturning circulation (AMOC) indicates a climate control. We report Mg/Ca data on benthic foraminifera from an intermediate-depth site in the northwest Atlantic and results from a climate-model simulation that reveal basin-wide subsurface warming at the same time as large reductions in the AMOC, with temperature increasing by approximately 2 °C over a 1–2 kyr interval prior to a Heinrich event. In simulations with an ocean model coupled to a thermodynamically active ice shelf, the increase in subsurface temperature increases basal melt rate under an ice shelf fronting the HSIS by a factor of approximately 6. By analogy with recent observations in Antarctica, the resulting ice-shelf loss and attendant HSIS acceleration would produce a Heinrich event. PMID:21808034

The northern Uummannaq Ice Stream System, West Greenland: ice dynamics and and controls upon deglaciation

NASA Astrophysics Data System (ADS)

Lane, Timothy; Roberts, David; Rea, Brice; Cofaigh, Colm Ó.; Vieli, Andreas

2013-04-01

At the Last Glacial Maximum (LGM), the Uummannaq Ice Stream System comprised a series coalescent outlet glaciers which extended along the trough to the shelf edge, draining a large proportion of the West Greenland Ice Sheet. Geomorphological mapping, terrestrial cosmogenic nuclide (TCN) exposure dating, and radiocarbon dating constrain warm-based ice stream activity in the north of the system to 1400 m a.s.l. during the LGM. Intervening plateaux areas (~ 2000 m a.s.l.) either remained ice free, or were covered by cold-based icefields, preventing diffluent or confluent flow throughout the inner to outer fjord region. Beyond the fjords, a topographic sill north of Ubekendt Ejland prevented the majority of westward ice flow, forcing it south through Igdlorssuit Sund, and into the Uummannaq Trough. Here it coalesced with ice from the south, forming the trunk zone of the UISS. Deglaciation of the UISS began at 14.9 cal. ka BP, rapidly retreating through the overdeepened Uummannaq Trough. Once beyond Ubekendt Ejland, the northern UISS retreated northwards, separating from the south. Retreat continued, and ice reached the present fjord confines in northern Uummannaq by 11.6 kyr. Both geomorphological (termino-lateral moraines) and geochronological (14C and TCN) data provide evidence for an ice marginal stabilisation at within Karrat-Rink Fjord, at Karrat Island, from 11.6-6.9 kyr. The Karrat moraines appear similar in both fjord position and form to 'Fjord Stade' moraines identified throughout West Greenland. Though chronologies constraining moraine formation are overlapping (Fjord Stade moraines - 9.3-8.2 kyr, Karrat moraines - 11.6-6.9 kyr), these moraines have not been correlated. This ice margin stabilisation was able to persist during the Holocene Thermal Maximum (~7.2 - 5 kyr). It overrode climatic and oceanic forcings, remaining on Karrat Island throughout peaks of air temperature and relative sea-level, and during the influx of the warm West Greenland Current into the Uummannaq region. Based upon analysis of fjord bathymetry and width, this ice marginal stabilisation has been shown to have been caused by increases in topographic constriction at Karrat Island. The location of the marginal stillstand is coincident with a dramatic narrowing of fjord width and bed shallowing. These increases in local lateral resistance reduces the ice flux necessary to maintain a stable grounding line, leading to ice margin stabilisation. This acted to negate the effects of the Holocene Thermal Maximum. Following this stabilisation, retreat within Rink-Karrat Fjord continued, driven by calving into the overdeepened Rink Fjord. Rink Isbræ reached its present ice margin or beyond after 5 kyr, during the Neoglacial. In contrast, the southern UISS reached its present margin at 8.7 kyr and Jakobshavn Isbræ reached its margin by 7 kyr. This work therefore provides compelling evidence for topographically forced asynchronous, non-linear ice stream retreat between outlet glaciers in West Greenland. In addition, it has major implications for our understanding and reconstruction of mid-Holocene ice sheet extent, and ice sheet dynamics during the Holocene Thermal Maximum to Neoglacial switch.

Present-day Circum-Antarctic Simulations using the POPSICLES Coupled Ice Sheet-Ocean Model

NASA Astrophysics Data System (ADS)

Asay-Davis, X.; Martin, D. F.; Price, S. F.; Maltrud, M. E.; Collins, W.

2014-12-01

We present POPSICLES simulation results covering the full Antarctic Ice Sheet and the Southern Ocean spanning the period 1990 to 2010. Simulations are performed at 0.1o (~5 km) ocean resolution and with adaptive ice-sheet model resolution as fine as 500 m. We compare time-averaged melt rates below a number of major ice shelves with those reported by Rignot et al. (2013) as well as other recent studies. We also present seasonal variability and decadal trends in submarine melting from several Antarctic regions. Finally, we explore the influence on basal melting and system dynamics resulting from two different choices of climate forcing: a "normal-year" climatology and the CORE v. 2 forcing data (Large and Yeager 2008).POPSICLES couples the POP2x ocean model, a modified version of the Parallel Ocean Program (Smith and Gent, 2002), and the BISICLES ice-sheet model (Cornford et al., 2012). POP2x includes sub-ice-shelf circulation using partial top cells (Losch, 2008) and boundary layer physics following Holland and Jenkins (1999), Jenkins (2001), and Jenkins et al. (2010). Standalone POP2x output compares well with standard ice-ocean test cases (e.g., ISOMIP; Losch, 2008) and other continental-scale simulations and melt-rate observations (Kimura et al., 2013; Rignot et al., 2013). BISICLES makes use of adaptive mesh refinement and a 1st-order accurate momentum balance similar to the L1L2 model of Schoof and Hindmarsh (2009) to accurately model regions of dynamic complexity, such as ice streams, outlet glaciers, and grounding lines. Results of BISICLES simulations have compared favorably to comparable simulations with a Stokes momentum balance in both idealized tests (MISMIP-3D; Pattyn et al., 2013) and realistic configurations (Favier et al. 2014).A companion presentation, "Response of the Antarctic Ice Sheet to ocean forcing using the POPSICLES coupled ice sheet-ocean model" in session C024 covers the ice-sheet response to these melt rates in the coupled simulation. The figure shows eddy activity in the vertically integrated (barotropic) velocity nearly six years into a POPSICLES simulation of the Antarctic region.

Palaeoglaciology of the Alexander Island ice cap, western Antarctic Peninsula, reconstructed from marine geophysical and core data

NASA Astrophysics Data System (ADS)

Graham, Alastair G. C.; Smith, James A.

2012-03-01

The glacial history of the continental shelf northwest of Alexander Island is not well known, due mainly to a lack of targeted marine data on Antarctica's palaeo-ice sheets in their inter-ice-stream areas. Recently it has been argued that the region was ice-free at the Last Glacial Maximum (LGM) and thus a potential site for glacial refugia. In this paper, multibeam swath bathymetry, sub-bottom profiles and sediment cores are used to map the Alexander Island sector of the Antarctic Peninsula margin, in order to reconstruct the shelf's palaeoglaciology. Sea-floor bedforms provide evidence that an independent ice cap persisted on Alexander Island through the LGM and deglaciation. We show that this ice cap drained via two major, previously-undescribed tidewater outlets (Rothschild and Charcot Glaciers) sourced from an ice dome centred over the west of the island and near-shore areas. The glaciers grounded along deep, fjord-like cross-shelf troughs to within at least ˜10-20 km of the shelf edge, and probably reached the shelf break. Only one small outer-shelf zone appears to have remained free of ice throughout an otherwise extensive LGM. During retreat, grounding-line geomorphology indicates periodic stabilisation of Charcot Glacier on the mid-shelf after 13,500 cal yrs BP, while Rothschild Glacier retreated across its mid-shelf by 14,450 cal yrs BP. The timing of these events is in phase with retreat in nearby Marguerite Trough, and we take this as evidence of a common history and forcing with the Antarctic Peninsula Ice Sheet. The fine details of ice flow documented by our new reconstruction highlight the importance of capturing complex ice flow patterns in models (e.g. in inter-stream areas), for understanding how region-specific parts of Antarctica may change in the future. Moreover, the reconstruction shows that glacial refugia, if present, cannot have been extensive on the Alexander Island shelf at the LGM as indicated by previous biological studies; instead, we argue that any ice-free refugia were probably restricted to isolated outer-shelf pockets, that opened, closed, or were maintained through diachronous ice-sheet advance and retreat.

Reconstruction of the extent and variability of late Quaternary ice sheets and Arctic sea ice: Insights from new mineralogical and geochemical proxy records

NASA Astrophysics Data System (ADS)

Stein, R. H.; Niessen, F.; Fahl, K.; Forwick, M.; Kudriavtseva, A.; Ponomarenko, E.; Prim, A. K.; Quatmann-Hense, A.; Spielhagen, R. F.; Zou, H.

2016-12-01

The Arctic Ocean and surrounding continents are key areas within the Earth system and very sensitive to present and past climate change. In this context, the timing and extent of circum-Arctic ice sheets and its interaction with oceanic and sea-ice dynamics are major interest and focus of international research. New sediment cores recovered during the Polarstern Expeditions PS87 (Lomonosov Ridge/2014) and PS93.1 (Fram Strait/2015) together with several sediment cores available from previous Polarstern expeditions allow to carry out a detailed sedimentological and geochemical study that may help to unravel the changes in Arctic sea ice and circum-Arctic ice sheets during late Quaternary times. Our new data include biomarkers indicative for past sea-ice extent, phytoplankton productivity and terrigenous input as well as grain size, physical property, XRD and XRF data indicative for sources and pathways of terrigenous sediments (ice-rafted debris/IRD) related to glaciations in Eurasia, East Siberia, Canada and Greenland. Here, we present examples from selected sediment cores that give new insights into the timing and extent of sea ice and glaciations during MIS 6 to MIS 2. To highlight one example: SE-NW oriented, streamlined landforms have been mapped on top of the southern Lomonosov Ridge (LR) at water depths between 800 and 1000 m over long distances during Polarstern Expedition PS87, interpreted to be glacial lineations that formed beneath grounded ice sheets and ice streams. The orientations of the lineations identified are similar to those on the East Siberian continental margin, suggesting an East Siberian Chukchi Ice Sheet extended far to the north on LR during times of extreme Quaternary glaciations. Based on our new biomarker records from Core PS2757 (located on LR near 81°N) indicating a MIS 6 ice-edge situation with some open-water phytoplankton productivity, the glacial erosional event should have been older than MIS 6 (e.g., MIS 12?).

Anatomy of Heinrich Layer 1 and its role in the last deglaciation

NASA Astrophysics Data System (ADS)

Hodell, David A.; Nicholl, Joseph A.; Bontognali, Tomaso R. R.; Danino, Steffan; Dorador, Javier; Dowdeswell, Julian A.; Einsle, Joshua; Kuhlmann, Holger; Martrat, Belen; Mleneck-Vautravers, Maryline J.; Rodríguez-Tovar, Francisco Javier; Röhl, Ursula

2017-03-01

X-ray fluorescence (XRF) core scanning and X-ray computed tomography data were measured every 1 mm to study the structure of Heinrich Event 1 during the last deglaciation at International Ocean Discovery Program Site U1308. Heinrich Layer 1 comprises two distinct layers of ice-rafted detritus (IRD), which are rich in detrital carbonate (DC) and poor in foraminifera. Each DC layer consists of poorly sorted, coarse-grained clasts of IRD embedded in a dense, fine-grained matrix of glacial rock flour that is partially cemented. The radiocarbon ages of foraminifera at the base of the two layers indicate a difference of 1400 14C years, suggesting that they are two distinct events, but the calendar ages depend upon assumptions made for surface reservoir ages. The double peak indicates at least two distinct stages of discharge of the ice streams that drained the Laurentide Ice Sheet through Hudson Strait during HE1 or, alternatively, the discharge of two independent ice streams containing detrital carbonate. Heinrich Event 1.1 was the larger of the two events and began at 16.2 ka (15.5-17.1 ka) when the polar North Atlantic was already cold and Atlantic Meridional Overturning Circulation (AMOC) weakened. The younger peak (H1.2) at 15.1 ka (14.3 to 15.9 ka) was a weaker event than H1.1 that was accompanied by minor cooling. Our results support a complex history for Heinrich Stadial 1 (HS1) with reduction in AMOC during the early part ( 20-16.2 ka) possibly driven by melting of European ice sheets, whereas the Laurentide Ice Sheet assumed a greater role during the latter half ( 16.2-14.7 ka).

A microbial ecosystem beneath the West Antarctic ice sheet.

PubMed

Christner, Brent C; Priscu, John C; Achberger, Amanda M; Barbante, Carlo; Carter, Sasha P; Christianson, Knut; Michaud, Alexander B; Mikucki, Jill A; Mitchell, Andrew C; Skidmore, Mark L; Vick-Majors, Trista J

2014-08-21

Liquid water has been known to occur beneath the Antarctic ice sheet for more than 40 years, but only recently have these subglacial aqueous environments been recognized as microbial ecosystems that may influence biogeochemical transformations on a global scale. Here we present the first geomicrobiological description of water and surficial sediments obtained from direct sampling of a subglacial Antarctic lake. Subglacial Lake Whillans (SLW) lies beneath approximately 800 m of ice on the lower portion of the Whillans Ice Stream (WIS) in West Antarctica and is part of an extensive and evolving subglacial drainage network. The water column of SLW contained metabolically active microorganisms and was derived primarily from glacial ice melt with solute sources from lithogenic weathering and a minor seawater component. Heterotrophic and autotrophic production data together with small subunit ribosomal RNA gene sequencing and biogeochemical data indicate that SLW is a chemosynthetically driven ecosystem inhabited by a diverse assemblage of bacteria and archaea. Our results confirm that aquatic environments beneath the Antarctic ice sheet support viable microbial ecosystems, corroborating previous reports suggesting that they contain globally relevant pools of carbon and microbes that can mobilize elements from the lithosphere and influence Southern Ocean geochemical and biological systems.

Overview of Ice-Sheet Mass Balance and Dynamics from ICESat Measurements

NASA Technical Reports Server (NTRS)

Zwally, H. Jay

2010-01-01

The primary purpose of the ICESat mission was to determine the present-day mass balance of the Greenland and Antarctic ice sheets, identify changes that may be occurring in the surface-mass flux and ice dynamics, and estimate their contributions to global sea-level rise. Although ICESat's three lasers were planned to make continuous measurements for 3 to 5 years, the mission was re-planned to operate in 33-day campaigns 2 to 3 times each year following failure of the first laser after 36 days. Seventeen campaigns were conducted with the last one in the Fall of 2009. Mass balance maps derived from measured ice-sheet elevation changes show that the mass loss from Greenland has increased significantly to about 170 Gt/yr for 2003 to 2007 from a state of near balance in the 1990's. Increased losses (189 Gt/yr) from melting and dynamic thinning are over seven times larger'than increased gains (25 gt/yr) from precipitation. Parts of the West Antarctic ice sheet and the Antarctic Peninsula are losing mass at an increasing rate, but other parts of West Antarctica and the East Antarctic ice sheet are gaining mass at an increasing rate. Increased losses of 35 Gt/yr in Pine Island, Thwaites-Smith, and Marie-Bryd.Coast are more than balanced by gains in base of Peninsula and ice stream C, D, & E systems. From the 1992-2002 to 2003-2007 period, the overall mass balance for Antarctica changed from a loss of about 60 Gt/yr to near balance or slightly positive.

Warm ocean surface led to ice margin retreat in central-eastern Baffin Bay during the Younger Dryas

NASA Astrophysics Data System (ADS)

Oksman, Mimmi; Weckström, Kaarina; Miettinen, Arto; Juggins, Stephen; Divine, Dmitry; Jackson, Rebecca; Korsgaard, Niels J.; Telford, Richard; Kucera, Michal

2017-04-01

The Greenland ice sheet stability is linked to fast-flowing ice streams that are influenced by sea surface temperatures (SSTs) at their front. One of the largest ice streams in West Greenland is the Jakobshavn Isbræ, which has been shown to have collapsed at ca. 12.2 kyr BP in the middle of the Younger Dryas (YD) cold period (12.9-11.7 kyr BP). The cause for this collapse is still unknown yet hypotheses, such as warm Atlantic water inflow, have been put forward to explain it. Here we present the first diatom-based high-resolution reconstruction of sea surface conditions in the central-eastern Baffin Bay between 14.0 and 10.2 kyr BP. The sea surface temperatures reveal warmer conditions beginning at ca. 13.4 kyr BP and leading to intensive calving and iceberg discharge from Jakobshavn Isbræ visible as increased sedimentation rates and deposition of coarse-grained material in our sediment stratigraphy. The warm YD ocean surface conditions in Baffin Bay are out of phase with the δ18O record from the North Greenland Ice Core Project (NGRIP) and other SST records from northern North-Atlantic. We show that the ocean has had significant interactions with the Greenland ice sheet in the past and emphasize its importance under the current warming of the North Atlantic.

Thurston Island

NASA Image and Video Library

2014-11-10

Snow is blown off of the Thurston Island calving front off of western Antarctica as seen on the IceBridge flight on Nov. 5, 2014. Image Credit: NASA/Jim Yungel NASA’s Operation IceBridge collected some rare images on a flight out of Punta Arenas, Chile on Nov. 5, 2014, on a science flight over western Antarctica dubbed Ferrigno-Alison-Abbott 01. The crew snapped a few shots of a calving front of the Antarctic ice sheet. This particular flight plan was designed to collect data on changes in ice elevation along the coast near the Ferrigno and Alison ice streams, on the Abbot Ice Shelf, and grounded ice along the Eights Coast.

Thurston Island Calving

NASA Image and Video Library

2014-11-10

A sea of icebergs float near the Thurston Island calving front off of western Antarctica as seen on the IceBridge flight on Nov. 5, 2014. Image Credit: NASA/Jim Yungel NASA’s Operation IceBridge collected some rare images on a flight out of Punta Arenas, Chile on Nov. 5, 2014, on a science flight over western Antarctica dubbed Ferrigno-Alison-Abbott 01. The crew snapped a few shots of a calving front of the Antarctic ice sheet. This particular flight plan was designed to collect data on changes in ice elevation along the coast near the Ferrigno and Alison ice streams, on the Abbot Ice Shelf, and grounded ice along the Eights Coast.

Seismic stratigraphic architecture of the Disko Bay trough-mouth fan system, West Greenland

NASA Astrophysics Data System (ADS)

Hofmann, Julia C.; Knutz, Paul C.

2015-04-01

Spatial and temporal changes of the Greenland Ice Sheet on the continental shelf bordering Baffin Bay remain poorly constrained. Then as now, fast-flowing ice streams and outlet glaciers have played a key role for the mass balance and stability of polar ice sheets. Despite their significance for Greenland Ice Sheet dynamics and evolution, our understanding of their long-term behaviour is limited. The central West Greenland margin is characterized by a broad continental shelf where a series of troughs extend from fjords to the shelf margin, acting as focal points for trough-mouth fan (TMF) accummulations. The sea-ward bulging morphology and abrupt shelf-break of these major depositional systems is generated by prograding depocentres that formed during glacial maxima when ice streams reached the shelf edge, delivering large amounts of subglacial sediment onto the continental slope (Ó Cofaigh et al., 2013). The aim of this study is to unravel the seismic stratigraphic architecture and depositional processes of the Disko Bay TMF, aerially the largest single sedimentary system in West Greenland, using 2D and 3D seismic reflection data, seabed bathymetry and stratigraphic information from exploration well Hellefisk-1. The south-west Disko Bay is intersected by a deep, narrow trough, Egedesminde Dyb, which extends towards the southwest and links to the shallower and broader cross-shelf Disko Trough (maximum water depths of > 1000 m and a trough length of c. 370 km). Another trough-like depression (trough length of c. 120 km) in the northern part of the TMF, indicating a previous position of the ice stream, can be distinguished on the seabed topographic map and the seismic images. The Disko Bay TMF itself extends from the shelf edge down to the abyssal plain (abyssal floor depths of 2000 m) of the southern Baffin Bay. Based on seismic stratigraphic configurations relating to reflection terminations, erosive patterns and seismic facies (Mitchum et al., 1977), the TMF succession has been divided into five seismic units, each representing different stages in the progradational accumulation of the TMF system. This poster and ongoing study will discuss how the ice-stream flow switching is linked to changes in depocentres of sedimentary sequences and further investigate the major controls, e.g. ice-sheet dynamics, ocean-climate changes, tectonic forcing and subglacial geology, that determined the evolution of the Disko Bay TMF. Essencial bibliography Mitchum, R.M. Jr., Vail, P.R., Sangree, J.B., 1977. Seismic stratigraphy and global changes of sea level, Part 6: Stratigraphic interpretation of seismic reflection patterns in depositional sequences. AAPG Memoir 26, 117-133. Ó Cofaigh, C., Andrews, J.T., Jennings, A.E., Dowdeswell, J.A., Hogan, K.A., Kilfeather, A.A., Sheldon, C., 2013. Glacimarine lithofacies, provenance and depositional processes on a West Greenland trough-mouth fan. Journal of Quaternary Science, 28(1), 13-26.

Bayesian prediction of future ice sheet volume using local approximation Markov chain Monte Carlo methods

NASA Astrophysics Data System (ADS)

Davis, A. D.; Heimbach, P.; Marzouk, Y.

2017-12-01

We develop a Bayesian inverse modeling framework for predicting future ice sheet volume with associated formal uncertainty estimates. Marine ice sheets are drained by fast-flowing ice streams, which we simulate using a flowline model. Flowline models depend on geometric parameters (e.g., basal topography), parameterized physical processes (e.g., calving laws and basal sliding), and climate parameters (e.g., surface mass balance), most of which are unknown or uncertain. Given observations of ice surface velocity and thickness, we define a Bayesian posterior distribution over static parameters, such as basal topography. We also define a parameterized distribution over variable parameters, such as future surface mass balance, which we assume are not informed by the data. Hyperparameters are used to represent climate change scenarios, and sampling their distributions mimics internal variation. For example, a warming climate corresponds to increasing mean surface mass balance but an individual sample may have periods of increasing or decreasing surface mass balance. We characterize the predictive distribution of ice volume by evaluating the flowline model given samples from the posterior distribution and the distribution over variable parameters. Finally, we determine the effect of climate change on future ice sheet volume by investigating how changing the hyperparameters affects the predictive distribution. We use state-of-the-art Bayesian computation to address computational feasibility. Characterizing the posterior distribution (using Markov chain Monte Carlo), sampling the full range of variable parameters and evaluating the predictive model is prohibitively expensive. Furthermore, the required resolution of the inferred basal topography may be very high, which is often challenging for sampling methods. Instead, we leverage regularity in the predictive distribution to build a computationally cheaper surrogate over the low dimensional quantity of interest (future ice sheet volume). Continual surrogate refinement guarantees asymptotic sampling from the predictive distribution. Directly characterizing the predictive distribution in this way allows us to assess the ice sheet's sensitivity to climate variability and change.

Wisconsinan and early Holocene glacial dynamics of Cumberland Peninsula, Baffin Island, Arctic Canada

NASA Astrophysics Data System (ADS)

Margreth, Annina; Gosse, John C.; Dyke, Arthur S.

2017-07-01

Three glacier systems-an ice sheet with a large marine-based ice stream, an ice cap, and an alpine glacier complex-coalesced on Cumberland Peninsula during the Late Wisconsinan. We combine high-resolution mapping of glacial deposits with new cosmogenic nuclide and radiocarbon age determinations to constrain the history and dynamics of each system. During the Middle Wisconsinan (Oxygen Isotope Stage 3, OIS-3) the Cumberland Sound Ice Stream of the Laurentide Ice Sheet retreated well back into Cumberland Sound and the alpine ice retreated at least to fiord-head positions, a more significant recession than previously documented. The advance to maximal OIS-2 ice positions beyond the mouth of Cumberland Sound and beyond most stretches of coastline remains undated. Partial preservation of an over-ridden OIS-3 glaciomarine delta in a fiord-side position suggests that even fiord ice was weakly erosive in places. Moraines formed during deglaciation represent stillstands and re-advances during three major cold events: H-1 (14.6 ka), Younger Dryas (12.9-11.7 ka), and Cockburn (9.5 ka). Distinctly different responses of the three glacial systems are evident, with the alpine system responding most sensitively to Bølling-Allerød warming whereas the larger systems retreated mainly during Pre-Boreal warming. While the larger ice masses were mainly influenced by internal dynamics, the smaller alpine glacier system responded sensitively to local climate effects. Asymmetrical recession of the alpine glacier complex indicates topoclimatic control on deglaciation and perhaps migration of the accumulation area toward moisture source.

Differences in ice retreat across Pine Island Bay, West Antarctica, since the Last Glacial Maximum: Indications from multichannel seismic reflection data

USGS Publications Warehouse

Uenzelmann-Neben, G.; Gohl, K.; Larter, R.D.; Schlüter, P.

2007-01-01

An understanding of the glacial history of Pine Island Bay (PIB) is essential for refining models of the future stability of the West Antarctic Ice Sheet (WAIS). New multichannel seismic reflection data from inner PIB are interpreted in context of previously published reconstructions for the retreat history in this area since the Last Glacial Maximum. Differences in the behavior of the ice sheet during deglaciation are shown to exist for the western and eastern parts of PIB. While we can identify only a thin veneer of sedimentary deposits in western PIB, eastern PIB shows sedimentary layers ≤ 400 msTWT. This is interpreted as a result of differences in ice retreat: a fast ice retreat in western PIB accompanied by rapid basal melting led to production of large meltwater streams, a slower ice retreat in eastern PIB is most probably the result of smaller drainage basins resulting in less meltwater production.

Early glaciation already during the Early Miocene in the Amundsen Sea, Southern Pacific: Indications from the distribution of sedimentary sequences

NASA Astrophysics Data System (ADS)

Uenzelmann-Neben, Gabriele; Gohl, Karsten

2014-09-01

The distribution and internal architecture of seismostratigraphic sequences observed on the Antarctic continental slope and rise are results of sediment transport and deposition by bottom currents and ice sheets. Analysis of seismic reflection data allows to reconstruct sediment input and sediment transport patterns and to infer past changes in climate and oceanography. We observe four seismostratigraphic units which show distinct differences in location and shape of their depocentres and which accumulated at variable sedimentation rates. We used an age-depth model based on DSDP Leg 35 Site 324 for the Plio/Pleistocene and a correlation with seismic reflection characteristics from the Ross and Bellingshausen Seas, which unfortunately has large uncertainties. For the period before 21 Ma, we interpret low energy input of detritus via a palaeo-delta originating in an area of the Amundsen Sea shelf, where a palaeo-ice stream trough (Pine Island Trough East, PITE) is located today, and deposition of this material on the continental rise under sea ice coverage. For the period 21-14.1 Ma we postulate glacial erosion for the hinterland of this part of West Antarctica, which resulted in a larger depocentre and an increase in mass transport deposits. Warming during the Mid Miocene Climatic Optimum resulted in a polythermal ice sheet and led to a higher sediment supply along a broad front but with a focus via two palaeo-ice stream troughs, PITE and Abbot Trough (AT). Most of the glaciogenic debris was transported onto the eastern Amundsen Sea rise where it was shaped into levee-drifts by a re-circulating bottom current. A reduced sediment accumulation in the deep-sea subsequent to the onset of climatic cooling after 14 Ma indicates a reduced sediment supply probably in response to a colder and drier ice sheet. A dynamic ice sheet since 4 Ma delivered material offshore mainly via AT and Pine Island Trough West (PITW). Interaction of this glaciogenic detritus with a west-setting bottom current resulted in the continued formation of levee-drifts in the eastern and central Amundsen Sea.

Modelling West Antarctic ice sheet growth and collapse through the past five million years.

PubMed

Pollard, David; DeConto, Robert M

2009-03-19

The West Antarctic ice sheet (WAIS), with ice volume equivalent to approximately 5 m of sea level, has long been considered capable of past and future catastrophic collapse. Today, the ice sheet is fringed by vulnerable floating ice shelves that buttress the fast flow of inland ice streams. Grounding lines are several hundred metres below sea level and the bed deepens upstream, raising the prospect of runaway retreat. Projections of future WAIS behaviour have been hampered by limited understanding of past variations and their underlying forcing mechanisms. Its variation since the Last Glacial Maximum is best known, with grounding lines advancing to the continental-shelf edges around approximately 15 kyr ago before retreating to near-modern locations by approximately 3 kyr ago. Prior collapses during the warmth of the early Pliocene epoch and some Pleistocene interglacials have been suggested indirectly from records of sea level and deep-sea-core isotopes, and by the discovery of open-ocean diatoms in subglacial sediments. Until now, however, little direct evidence of such behaviour has been available. Here we use a combined ice sheet/ice shelf model capable of high-resolution nesting with a new treatment of grounding-line dynamics and ice-shelf buttressing to simulate Antarctic ice sheet variations over the past five million years. Modelled WAIS variations range from full glacial extents with grounding lines near the continental shelf break, intermediate states similar to modern, and brief but dramatic retreats, leaving only small, isolated ice caps on West Antarctic islands. Transitions between glacial, intermediate and collapsed states are relatively rapid, taking one to several thousand years. Our simulation is in good agreement with a new sediment record (ANDRILL AND-1B) recovered from the western Ross Sea, indicating a long-term trend from more frequently collapsed to more glaciated states, dominant 40-kyr cyclicity in the Pliocene, and major retreats at marine isotope stage 31 ( approximately 1.07 Myr ago) and other super-interglacials.

2014_11_05_uss_constellation

NASA Image and Video Library

2014-11-10

NASA’s Operation IceBridge collected some rare images on a flight out of Punta Arenas, Chile on Nov. 5, 2014, on a science flight over western Antarctica dubbed Ferrigno-Alison-Abbott 01. Following a routine calibration pass over Punta Arenas airport, the NASA DC-8 overflew the USS Constellation which is being towed for demolition after 53 yeas of service. The crew then snapped a few shots of a calving front of the Antarctic ice sheet. This particular flight plan was designed to collect data on changes in ice elevation along the coast near the Ferrigno and Alison ice streams, on the Abbot Ice Shelf, and grounded ice along the Eights Coast.

Thurston Island

NASA Image and Video Library

2017-12-08

The Thurston Island calving front off of western Antarctica as seen from the window of NASA's DC-8 on Nov. 5, 2014. Image Credit: NASA/Jim Yungel NASA’s Operation IceBridge collected some rare images on a flight out of Punta Arenas, Chile on Nov. 5, 2014, on a science flight over western Antarctica dubbed Ferrigno-Alison-Abbott 01. The crew snapped a few shots of a calving front of the Antarctic ice sheet. This particular flight plan was designed to collect data on changes in ice elevation along the coast near the Ferrigno and Alison ice streams, on the Abbot Ice Shelf, and grounded ice along the Eights Coast.

3D full-Stokes modeling of the grounding line dynamics of Thwaites Glacier, West Antarctica

NASA Astrophysics Data System (ADS)

Yu, H.; Rignot, E. J.; Morlighem, M.; Seroussi, H. L.

2016-12-01

Thwaites Glacier (TG) is the broadest and second largest ice stream in the West Antarctica. Satellite observations have revealed rapid grounding line retreat and mass loss of this glacier in the past few decades, which has been attributed to the enhanced basal melting in the Amundsen Sea Embayment. With a retrograde bed configuration, TG is on the verge of collapse according to the marine ice sheet instability theory. Here, we use the UCI/JPL Ice Sheet System Model (ISSM) to simulate the grounding line position of TG to determine its stability, rate of retreat and sensitivity to enhanced basal melting using a three-dimensional full-Stokes numerical model. Simulations with simplified models (Higher Order (HO), and Shelfy-Stream Approximation (SSA)) are also conducted for comparison. We first validate our full Stokes model by conducting MISMIP3D experiments. Then we applied the model to TG using new bed elevation dataset combining IceBridge (OIB) gravity data, OIB ice thickness, ice flow vectors from interferometry and a mass conservation method at 450 m spacing. Basal friction coefficient and ice rheology of floating ice are inferred to match observed surface velocity. We find that the grounding line is capable of retreating at rate of 1km/yr under current forcing and that the glacier's sensitivity to melt is higher in the Stokes model than HO or SSA, which means that projections using SSA or HO might underestimate the future rate of retreat of the glacier. This work has been performed at UC Irvine and Caltech's Jet Propulsion Laboratory under a contract with NASA's Cryospheric Science Program.

A Tale of Two Forcings: Present-Day Coupled Antarctic Ice-sheet/Southern Ocean dynamics using the POPSICLES model.

NASA Astrophysics Data System (ADS)

Martin, Daniel; Asay-Davis, Xylar; Cornford, Stephen; Price, Stephen; Ng, Esmond; Collins, William

2015-04-01

We present POPSICLES simulation results covering the full Antarctic Ice Sheet and the Southern Ocean spanning the period 1990 to 2010 resulting from two different choices of climate forcing: a 'normal-year' climatology and the CORE v. 2 interannual forcing data (Large and Yeager 2008). Simulations are performed at 0.1o (~5 km) ocean resolution and adaptive ice sheet resolution as fine as 500 m. We compare time-averaged melt rates below a number of major ice shelves with those reported by Rignot et al. (2013) as well as other recent studies. We also present seasonal variability and decadal melting trends from several Antarctic regions, along with the response of the ice shelves and consequent dynamics of the grounded ice sheet. POPSICLES couples the POP2x ocean model, a modified version of the Parallel Ocean Program (Smith and Gent, 2002), and the BISICLES ice-sheet model (Cornford et al., 2012). POP2x includes sub-ice-shelf circulation using partial top cells (Losch, 2008) and boundary layer physics following Holland and Jenkins (1999), Jenkins (2001), and Jenkins et al. (2010). Standalone POP2x output compares well with standard ice-ocean test cases (e.g., ISOMIP; Losch, 2008) and other continental-scale simulations and melt-rate observations (Kimura et al., 2013; Rignot et al., 2013). BISICLES makes use of adaptive mesh refinement and a 1st-order accurate momentum balance similar to the L1L2 model of Schoof and Hindmarsh (2009) to accurately model regions of dynamic complexity, such as ice streams, outlet glaciers, and grounding lines. Results of BISICLES simulations have compared favorably to comparable simulations with a Stokes momentum balance in both idealized tests (MISMIP-3d; Pattyn et al., 2013) and realistic configurations (Favier et al. 2014).

Coastal-Change and Glaciological Map of the Northern Ross Ice Shelf Area, Antarctica: 1962-2004

USGS Publications Warehouse

Ferrigno, Jane G.; Foley, Kevin M.; Swithinbank, Charles; Williams, Richard S.

2007-01-01

Changes in the area and volume of polar ice sheets are intricately linked to changes in global climate, and the resulting changes in sea level could severely impact the densely populated coastal regions on Earth. Melting of the West Antarctic part alone of the Antarctic ice sheet would cause a sea-level rise of approximately 6 meters (m). The potential sea-level rise after melting of the entire Antarctic ice sheet is estimated to be 65 m (Lythe and others, 2001) to 73 m (Williams and Hall, 1993). The mass balance (the net volumetric gain or loss) of the Antarctic ice sheet is highly complex, responding differently to different conditions in each region (Vaughan, 2005). In a review paper, Rignot and Thomas (2002) concluded that the West Antarctic ice sheet is probably becoming thinner overall; although it is thickening in the west, it is thinning in the north. Thomas and others (2004), on the basis of aircraft and satellite laser altimetry surveys, believe the thinning may be accelerating. Joughin and Tulaczyk (2002), on the basis of analysis of ice-flow velocities derived from synthetic aperture radar, concluded that most of the Ross ice streams (ice streams on the east side of the Ross Ice Shelf) have a positive mass balance, whereas Rignot and others (2004) infer even larger negative mass balance for glaciers flowing northward into the Amundsen Sea, a trend suggested by Swithinbank and others (2003a,b; 2004). The mass balance of the East Antarctic ice sheet is thought by Davis and others (2005) to be strongly positive on the basis of the change in satellite altimetry measurements made between 1992 and 2003. Measurement of changes in area and mass balance of the Antarctic ice sheet was given a very high priority in recommendations by the Polar Research Board of the National Research Council (1986), in subsequent recommendations by the Scientific Committee on Antarctic Research (SCAR) (1989, 1993), and by the National Science Foundation?s (1990) Division of Polar Programs. On the basis of these recommendations, the U.S. Geological Survey (USGS) decided that the archive of early 1970s Landsat 1, 2, and 3 Multispectral Scanner (MSS) images of Antarctica and the subsequent repeat coverage made possible with Landsat and other satellite images provided an excellent means of documenting changes in the coastline of Antarctica (Ferrigno and Gould, 1987). The availability of this information provided the impetus for carrying out a comprehensive analysis of the glaciological features of the coastal regions and changes in ice fronts of Antarctica (Swithinbank, 1988; Williams and Ferrigno, 1988). The project was later modified to include Landsat 4 and 5 MSS and Thematic Mapper (TM) images (and in some areas Landsat 7 Enhanced Thematic Mapper Plus [ETM+] images), RADARSAT images, and other data where available, in order to compare changes that occurred during a 20- to 25- or 30-year time interval (or longer where data were available, as in the Antarctic Peninsula). The results of the analysis are being used to produce a digital database and a series of USGS Geologic Investigations Series Maps (I?2600) (Williams and others, 1995; Williams and Ferrigno, 1998; Ferrigno and others, 2002) (available online at http://www.glaciers.er.usgs.gov).

Simulations of Antarctic ice shelves and the Southern Ocean in the POP2x ocean model coupled with the BISICLES ice-sheet model

NASA Astrophysics Data System (ADS)

Asay-Davis, Xylar; Martin, Daniel; Price, Stephen; Maltrud, Mathew

2014-05-01

We present initial results from Antarctic, ice-ocean coupled simulations using large-scale ocean circulation and ice-sheet evolution models. This presentation focuses on the ocean model, POP2x, which is a modified version of POP, a fully eddying, global-scale ocean model (Smith and Gent, 2002). POP2x allows for circulation beneath ice shelf cavities using the method of partial top cells (Losch, 2008). Boundary layer physics, which control fresh water and salt exchange at the ice-ocean interface, are implemented following Holland and Jenkins (1999), Jenkins (2001), and Jenkins et al. (2010). Standalone POP2x output compares well with standard ice-ocean test cases (e.g., ISOMIP; Losch, 2008) and other continental-scale simulations and melt-rate observations (Kimura et al., 2013; Rignot et al., 2013) and with results from other idealized ice-ocean coupling test cases (e.g., Goldberg et al., 2012). A companion presentation, 'Fully resolved whole-continent Antarctica simulations using the BISICLES AMR ice sheet model coupled with the POP2x Ocean Model', concentrates more on the ice-sheet model, BISICLES (Cornford et al., 2012), which includes a 1st-order accurate momentum balance (L1L2) and uses block structured, adaptive-mesh refinement to more accurately model regions of dynamic complexity, such as ice streams, outlet glaciers, and grounding lines. For idealized test cases focused on marine-ice sheet dynamics, BISICLES output compares very favorably relative to simulations based on the full, nonlinear Stokes momentum balance (MISMIP-3d; Pattyn et al., 2013). Here, we present large-scale (Southern Ocean) simulations using POP2x at 0.1 degree resolution with fixed ice shelf geometries, which are used to obtain and validate modeled submarine melt rates against observations. These melt rates are, in turn, used to force evolution of the BISICLES model. An offline-coupling scheme, which we compare with the ice-ocean coupling work of Goldberg et al. (2012), is then used to sequentially update the sub-shelf cavity geometry seen by POP2x.

Microbially driven export of labile organic carbon from the Greenland ice sheet

NASA Astrophysics Data System (ADS)

Musilova, Michaela; Tranter, Martyn; Wadham, Jemma; Telling, Jon; Tedstone, Andrew; Anesio, Alexandre M.

2017-04-01

Glaciers and ice sheets are significant sources of dissolved organic carbon and nutrients to downstream subglacial and marine ecosystems. Climatically driven increases in glacial runoff are expected to intensify the impact of exported nutrients on local and regional downstream environments. However, the origin and bioreactivity of dissolved organic carbon from glacier surfaces are not fully understood. Here, we present simultaneous measurements of gross primary production, community respiration, dissolved organic carbon composition and export from different surface habitats of the Greenland ice sheet, throughout the ablation season. We found that microbial production was significantly correlated with the concentration of labile dissolved organic species in glacier surface meltwater. Further, we determined that freely available organic compounds made up 62% of the dissolved organic carbon exported from the glacier surface through streams. We therefore conclude that microbial communities are the primary driver for labile dissolved organic carbon production and recycling on glacier surfaces, and that glacier dissolved organic carbon export is dependent on active microbial processes during the melt season.

Thermodynamic and Dynamic Causes of Pluvial Conditions During the Last Glacial Maximum in Western North America

NASA Astrophysics Data System (ADS)

Morrill, Carrie; Lowry, Daniel P.; Hoell, Andrew

2018-01-01

During the last glacial period, precipitation minus evaporation increased across the currently arid western United States. These pluvial conditions have been commonly explained for decades by a southward deflection of the jet stream by the Laurentide Ice Sheet. Here analysis of state-of-the-art coupled climate models shows that effects of the Laurentide Ice Sheet on the mean circulation were more important than storm track changes in generating wet conditions. Namely, strong cooling by the ice sheet significantly reduced humidity over land, increasing moisture advection in the westerlies due to steepened humidity gradients. Additionally, the removal of moisture from the atmosphere by mass divergence associated with the subtropical high was diminished at the Last Glacial Maximum compared to present. These same dynamic and thermodynamic factors, working in the opposite direction, are projected to cause regional drying in western North America under increased greenhouse gas concentrations, indicating continuity from past to future in the mechanisms altering hydroclimate.

Characterizing the Siple Coast Ice Stream System using Satellite Images, Improved Topography, and Integrated Aerogeophysical Measurements

NASA Technical Reports Server (NTRS)

Scambos, Ted

2003-01-01

A technique for improving elevation maps of the polar ice sheets has been developed using AVHRR images. The technique is based on 'photoclinometry' or 'shape from shading', a technique used in the past for mapping planetary surfaces where little elevation information was available. The fundamental idea behind photoclinometry is using the brightness of imaged areas to infer their surface slope in the sun-illuminated direction. Our version of the method relies on a calibration of the images based on an existing lower-resolution digital elevation model (DEM), and then using the images to improve the input DEM resolution to the scale of the image data. Most current DEMs covering the ice sheets are based on Radar altimetry data, and have an inherent resolution of 10 to 25 km at best - although the grid scale of the DEM is often finer. These DEMs are highly accurate (to less than 1 meter); but they report the mean elevation of a broad area, thus erasing smaller features of glaciological interest. AVHRR image data, when accurately geolocated and calibrated, provides surface slope measurements (based on the pixel brightness under known lighting conditions) every approximately 1.1 km. The limitations of the technique are noisiness in the image data, small variations in the albedo of the snow surface, and the integration technique used to create an elevation field from the image-derived slopes. Our study applied the technique to several ice sheet areas having some elevation data; Greenland, the Amery Ice Shelf, the Institute Ice Stream, and the Siple Coast. For the latter, the input data set was laser-altimetry data collected under NSF's SOAR Facility (Support Office for Aerogeophysical Research) over the onset area of the Siple Coast. Over the course of the grant, the technique was greatly improved and modified, significantly improving accuracy and reducing noise from the images. Several publications resulted from the work, and a follow-on proposal to NASA has been submitted to apply the same method to MODIS data using ICESat and other elevation input information. This follow-on grant will explore two applications that are facilitated by the improved surface morphology characterizations of the ice sheets: accumulation and temperature variations near small undulations in the ice.

E-tracers: A New Technique for Wireless Sensing Under Ice Sheets

NASA Astrophysics Data System (ADS)

Burrow, S.; Wadham, J. L.; Salter, M.; Barnes, R.

2009-12-01

A significant hurdle to the understanding of ice sheet basal hydrology and its coupling with ice motion is the difficulty in making in-situ measurements along a flow path. While dye tracing techniques may be used in small glaciers to determine transit times of surface melt water through the sub-glacial system, they provide no information on in situ conditions (e.g. pressure) and are ineffective at ice-sheet scale where dilution is high. The use of tethered sensor packages is complicated by the long lengths (~100’s m) and torturous path of the moulins and conduits within ice sheets. Recent attempts to pass solid objects (rubber ducks) and other sensor packages through glacial moulins have confirmed the difficultly in deploying sensors into the sub glacial environment. Here, we report the first successful deployment and recovery of compact, electronic units to moulins up to 7 km from the margin of a large land-terminating Greenland outlet. The technique uses RF (Radio Frequency) location to create an electronic tracer (an ‘e-tracer’) enabling a data-logging sensor package to be located in the pro-glacial flood plain once it has passed through the ice sheet. A number of individual packages are used in each deployment mitigating for the risk that some may become stuck within the moulin or lodge in an inaccessible part of the floodplain. In preliminary tests on the Leverett glacier in West Greenland during August 2009 we have demonstrated that this technique can be used to locate and retrieve dummy sensor packages: 50% and 20% of the dummy sensor packages introduced to moulins at 1 and 7 km from the ice sheet terminus respectively, emerged in the sub-glacial stream. It was possible to effectively detect the e-tracer units (which broadcast on 151MHz with 10mW of power) over a horizontal range of up to 5km across the pro-glacial floodplain and locate them to a high accuracy, allowing visual recognition and manual recovery. These performance statistics give this technique strong potential for investigating in-situ conditions along a flow path at ice sheet scale.

The role of surface-to-bed meltwater transfer events on the evolution of the Scandinavian Ice Sheet during the Weichselian

NASA Astrophysics Data System (ADS)

Clason, C.; Holmlund, P.; Applegate, P. J.; Strömberg, B.

2012-12-01

Inclusion of surface-to-bed meltwater transfer in the ice sheet model SICOPOLIS may help explain enigmatic erosional features, remnant of the last-glacial Scandinavian Ice Sheet (SIS), off Sweden's east coast. Modelling of ice sheets has largely neglected specific transfer of meltwater from the ice surface to the subglacial system, yet numerous studies on Greenland reveal dynamic response to surface meltwater generation and lake drainages, alluding to the importance of meltwater transfer for ice sheet response to climate change. Geologic evidence suggests the SIS experienced a number of oscillations during its evolution, characterised by variability in areas of fast flow, likely driven by changes in the thermal regime and fluctuating basal water pressure. SICOPOLIS accounts for polythermal conditions by applying a Weertman-type sliding law where basal ice is temperate. Furthermore, a first approximation of the surface meltwater effect on basal sliding is implemented within the SICOPOLIS Greenland domain, dependent on ice thickness and runoff. Field studies within the Swedish Archipelago have revealed numerous meltwater erosion features, including polished flutes. These flutes are deeper than the glacial striations in the area, and are both younger than and oriented differently to the youngest striae. Significant quantities of meltwater would have been necessary to erode such features, and large deposits of silt and clay in the surrounding area reinforce that meltwater was in good supply. Given the scattered distribution of polished fluting sites, access of meltwater to the bed through fracture penetration and lake drainage may have been instrumental in the localised nature of the sites. Driven by the geological evidence, SICOPOLIS is modified to include the surface meltwater effect within the Scandinavian domain. We aim to evaluate the role of meltwater transfer on the evolution of the SIS during the Weichselian, with particular focus on the area of the theorised Baltic Ice Stream.

Coastal-change and glaciological map of the Trinity Peninsula area and south Shetland Islands, Antarctica: 1843-2001: Chapter A in Coastal-change and glaciological maps of Antarctica

USGS Publications Warehouse

Ferrigno, Jane G.; Cook, Alison J.; Foley, Kevin M.; Williams, Richard S.; Swithinbank, Charles; Fox, Adrian J.; Thomson, Janet W.; Sievers, Jorn

2006-01-01

Changes in the area and volume of polar ice sheets are intricately linked to changes in global climate, and the resulting changes in sea level could severely impact the densely populated coastal regions on Earth. Melting of the West Antarctic part alone of the Antarctic ice sheet would cause a sea-level rise of approximately 6 meters (m). The potential sea-level rise after melting of the entire Antarctic ice sheet is estimated to be 65 m (Lythe and others, 2001) to 73 m (Williams and Hall, 1993). In addition to its importance, the mass balance (the net volumetric gain or loss) of the Antarctic ice sheet is highly complex, responding differently to different conditions in each region (Vaughan, 2005). In a review paper, Rignot and Thomas (2002) concluded that the West Antarctic ice sheet is probably becoming thinner overall; although it is thickening in the west, it is thinning in the north. Thomas and others (2004), on the basis of aircraft and satellite laser altimetry surveys, believe the thinning may be accelerating. Joughin and Tulaczyk (2002), on the basis of analysis of ice-flow velocities derived from synthetic aperture radar, concluded that most of the Ross ice streams (ice streams on the east side of the Ross Ice Shelf) have a positive mass balance, whereas Rignot and others (2004) infer even larger negative mass balance for glaciers flowing northward into the Amundsen Sea, a trend suggested by Swithinbank and others (2003a,b, 2004). The mass balance of the East Antarctic ice sheet is thought by Davis and others (2005) to be strongly positive on the basis of the change in satellite altimetry measurements made between 1992 and 2003. Measurement of changes in area and mass balance of the Antarctic ice sheet was given a very high priority in recommendations by the Polar Research Board of the National Research Council (1986), in subsequent recommendations by the Scientific Committee on Antarctic Research (SCAR) (1989, 1993), and by the National Science Foundation's (1990) Division of Polar Programs. On the basis of these recommendations, the U.S. Geological Survey (USGS) decided that the archive of early 1970s Landsat 1, 2, and 3 Multispectral Scanner (MSS) images of Antarctica and the subsequent repeat coverage made possible with Landsat and other satellite images provided an excellent means of documenting changes in the coastline of Antarctica (Ferrigno and Gould, 1987). The availability of this information provided the impetus for carrying out a comprehensive analysis of the glaciological features of the coastal regions and changes in ice fronts of Antarctica (Swithinbank, 1988; Williams and Ferrigno, 1988). The project was later modified to include Landsat 4 and 5 MSS and Thematic Mapper (TM) [and in some areas Landsat 7 Enhanced Thematic Mapper Plus (ETM+)], RADARSAT images, and other data where available, to compare changes that occurred during a 20- to 25- or 30-year time interval (or longer where data were available, as in the Antarctic Peninsula). The results of the analysis are being used to produce a digital database and a series of USGS Geologic Investigations Series Maps (I–2600) consisting of 23 maps at 1:1,000,000 scale and 1 map at 1:5,000,000 scale, in both paper and digital format (Williams and others, 1995; Williams and Ferrigno, 1998; Ferrigno and others, 2002).

Automated drumlin shape and volume estimation using high resolution LiDAR imagery (Curvature Based Relief Separation): A test from the Wadena Drumlin Field, Minnesota

NASA Astrophysics Data System (ADS)

Yu, Peter; Eyles, Nick; Sookhan, Shane

2015-10-01

Resolving the origin(s) of drumlins and related megaridges in areas of megascale glacial lineations (MSGL) left by paleo-ice sheets is critical to understanding how ancient ice sheets interacted with their sediment beds. MSGL is now linked with fast-flowing ice streams but there is a broad range of erosional and depositional models. Further progress is reliant on constraining fluxes of subglacial sediment at the ice sheet base which in turn is dependent on morphological data such as landform shape and elongation and most importantly landform volume. Past practice in determining shape has employed a broad range of geomorphological methods from strictly visualisation techniques to more complex semi-automated and automated drumlin extraction methods. This paper reviews and builds on currently available visualisation, semi-automated and automated extraction methods and presents a new, Curvature Based Relief Separation (CBRS) technique; for drumlin mapping. This uses curvature analysis to generate a base level from which topography can be normalized and drumlin volume can be derived. This methodology is tested using a high resolution (3 m) LiDAR elevation dataset from the Wadena Drumlin Field, Minnesota, USA, which was constructed by the Wadena Lobe of the Laurentide Ice Sheet ca. 20,000 years ago and which as a whole contains 2000 drumlins across an area of 7500 km2. This analysis demonstrates that CBRS provides an objective and robust procedure for automated drumlin extraction. There is strong agreement with manually selected landforms but the method is also capable of resolving features that were not detectable manually thereby considerably expanding the known population of streamlined landforms. CBRS provides an effective automatic method for visualisation of large areas of the streamlined beds of former ice sheets and for modelling sediment fluxes below ice sheets.

3D-seismic observations of Late Pleistocene glacial dynamics on the central West Greenland margin

NASA Astrophysics Data System (ADS)

Hofmann, Julia; Knutz, Paul; Cofaigh, Colm Ó.

2016-04-01

Fast-flowing ice streams and outlet glaciers exert a major control on glacial discharge from contemporary and palaeo ice sheets. Improving our understanding of the extent and dynamic behaviour of these palaeo-ice streams is therefore crucial for predictions of the response of ice sheets to present and future climate warming and the associated implications for global sea level. This poster presents results from two 3D-seismic surveys located on the shelf adjoining the Disko Bay trough-mouth fan (TMF), one of the largest glacial outlet systems in Greenland. Located at the seaward terminus of the c. 370 km long cross-shelf Disko Trough, the Disko Bay TMF was generated by highly efficient subglacial sediment delivery onto the continental slopes during repeated ice-stream advances. A variety of submarine glacial landform assemblages are recognised on the seabed reflecting past ice-stream activity presumably related to glacial-interglacial cycles. The 3D-seismic volumes cover the shallow banks located north and south of the Disko Trough. The focus of this study is the seabed and the uppermost stratigraphic interval associated with the Late Stage of TMF development, presumably covering the late Pleistocene (Hofmann et al., submitted). Seabed morphologies include multiple sets of ridges up to 20 m high that extend in NW-SE direction for c. 30 km, and cross-cutting curvilinear furrows with maximum lengths of c. 9 km and average depths of c. 4.5 m. Back-stepping, arcuate scarps facing NW define the shelf break on the northern survey, comprising average widths of c. 4.5 km and incision depths of c. 27.5 m. The large transverse ridge features on the southern survey are likely ice-marginal and are interpreted as terminal moraine ridges recording the existence of a shelf-edge terminating, grounded Late Weichselian ice sheet. The furrows, most prominent on the outer shelf adjoining the shallow banks and partly incising the moraine ridges, are interpreted as iceberg ploughmarks suggesting the transition between grounded ice and a glacimarine setting. The back-stepping scarps are suggestive of slide scars that were created as a result of mass movement induced by instabilities along the NW slope. The buried section contains morphologies indicating an asymmetric feature with a steeper side facing south. It comprises a thickness of c. 100 m and a length of c. 28 km. The detailed surface observations and seismic geometries suggest that the northern area represents a relict grounding-zone wedge (GZW). The wedge is covered by stratified deposits suggesting that it was at least occasionally submarine after its formation and may have served as pinning-point for floating ice shelves during periods of the Late TMF Stage. Important implications of the study are the intermittent development of floating ice shelves during the course of the Late Stage of TMF development and the presence of shelf-edge terminating grounded Late Weichselian ice outside of the troughs. Hofmann, J.C., Knutz, P.C., Nielsen, T., Kuijpers, A., submitted. Seismic architecture and evolution of the Disko Bay trough-mouth fan, central West Greenland margin. Quaternary Science Reviews.

High geothermal heat flux in close proximity to the Northeast Greenland Ice Stream.

PubMed

Rysgaard, Søren; Bendtsen, Jørgen; Mortensen, John; Sejr, Mikael K

2018-01-22

The Greenland ice sheet (GIS) is losing mass at an increasing rate due to surface melt and flow acceleration in outlet glaciers. Currently, there is a large disagreement between observed and simulated ice flow, which may arise from inaccurate parameterization of basal motion, subglacial hydrology or geothermal heat sources. Recently it was suggested that there may be a hidden heat source beneath GIS caused by a higher than expected geothermal heat flux (GHF) from the Earth's interior. Here we present the first direct measurements of GHF from beneath a deep fjord basin in Northeast Greenland. Temperature and salinity time series (2005-2015) in the deep stagnant basin water are used to quantify a GHF of 93 ± 21 mW m -2 which confirm previous indirect estimated values below GIS. A compilation of heat flux recordings from Greenland show the existence of geothermal heat sources beneath GIS and could explain high glacial ice speed areas such as the Northeast Greenland ice stream.

Sedimentary response to ice stream advance and retreat on the Storfjorden Trough Mouth Fan (NW Barents Sea), during Late Weichselian

NASA Astrophysics Data System (ADS)

Pedrosa, Mayte; Camerlengui, Angelo; de Mol, Ben; Lucchi, Renata. G.; Úrgeles, Roger; Rebesco, Michele; Winsborrow, Monica; Laberg, Jan. S.; Andreassen, Karin; Accettella, Daniela

2010-05-01

This seafloor morphological study of the Storfjorden Trough Mouth Fan (TMF) (offshore Svalbard, NW Barents Sea) is based on new multibeam bathymetry and chirp sub-bottom profiler data acquired in 2007 during the BIO Hespérides cruise SVAIS that provides an unprecedented image of the sedimentary processes that accompanied the last advance and retreat of the Storfjorden Ice Stream. Compared to other glacial-marine sedimentary systems (such as the adjacent Bjørnøyrenna TMF), the Storfjorden TMF system is small and associated to a relatively small terrestrial ice sheet, approximately 40.000 km2, with local provenance from Svalbard and the Spitsbergen Bank. Due to this short distance from the ice source to the calving areas and the resulting short residence time of ice in the ice sheet, therefore the glacio -marine system of the Storfjorden reacts rapidly to climatic changes. The Storfjorden continental slope is characterized by three depositional lobes, produced by focused sedimentation at the terminus of ice streams that have changed their location with time. The superficial morphology features associated to the two northernmost lobes are straight gullies in the upper slope, and debris lobes starting from the midslope onwards. The seafloor expression of the southernmost lobe, adjacent to the much smaller Kveithola TMF, demonstrate almost no gully incisions and is dominated by the widespread occurrence of small-scale submarine landslides. The subbottom profiles illustrate that sediment failures occurred throughout the Late Neogene evolution of the southern Storfjorden and Kveithola margin, including large-scale mass transport deposits of up to 200 m thick. Seismic facies of the Neogene sequence shows an alternation of glacigenic debris flows and laminated sediment drape inferred to be plumites. Gullies incising glacigenic debris flows at the surface and subsurface and are filled by an interglacial drape sequence. The gullies are formed during each deglaciation phase, most likely by the erosive action of short-lived high density currents originated by sediment-loaded subglacial melt water discharge.At the outer continental shelf of the southernmost lobe a striking fresh linear straight, which has a width of 1, 5 kilometres and cut the morainal deposits. These features are interpreted as mega-scale glacial lineations, which are tentatively attributed to mega-iceberg scours. These lineations are witness the latest advances of the Storfjorden ice streams before the final retreat which was located at the southernmost lobe. One of the main pre-conditioning factors to slope instability on the southern part of the Storfjorden TMF is identified as high sedimentation rate plumites deposited on the middle-upper continental slope by glacial melt water plumes. This study is part of the SVAIS project (funded by the Spanish IPY), that has a main objective to improve the understanding and the relationship between sedimentation and ice sheet dynamics under natural climatic changes.

Common-midpoint radar surveys of ice sheets: a tool for better ice and bed property inversions

NASA Astrophysics Data System (ADS)

Holschuh, N.; Christianson, K.; Anandakrishnan, S.; Alley, R. B.; Jacobel, R. W.

2016-12-01

In response to the demand for observationally derived boundary conditions in ice-sheet models, geophysicists are striving to more quantitatively interpret the reflection amplitudes of ice penetrating radar data. Inversions for ice-flow parameters and basal properties typically use common-offset radar data, which contain a single observation of bed reflection amplitude at each location in the survey; however, the radar equation has more than one unknown - ice temperature, subglacial water content, and bedrock roughness cannot be uniquely determined without additional constraints. In this study, we adapt traditional seismic property inversion techniques to radar data, using additional information collected with a common-midpoint (CMP) radar survey geometry (which varies the source-receiver offset for each subsurface target). Using two of the first common-midpoint ice-penetrating radar data sets collected over thick ice in Antarctica and Greenland, we test the hypothesis that these data can be used to disentangle the contributions of ice conductivity and bed permittivity to the received reflection amplitudes. We focus specifically on the corrections for the angular dependence of antenna gain and surface reflectivity, refractive focusing effects, and surface scattering losses. Inferred temperature profiles, derived from the constrained ice conductivities at Kamb Ice Stream and the North East Greenland Ice Stream, suggest higher than expected depth-integrated temperatures, as well as non-physical depth trends (with elevated temperatures near the surface). We hypothesize that this is driven in part by offset-dependent interferences between the sub-wavelength layers that make up a single nadir reflection, and present a convolutional model that describes how this interference might systematically reduce reflection power with offset (thereby elevating the inferred attenuation rate). If these additional offset-dependent power losses can be isolated and removed, common-midpoint profiles could provide a promising new way to calibrate property inversions that use the more laterally extensive, airborne, common-offset radar surveys.

Ice-Borehole Probe

NASA Technical Reports Server (NTRS)

Behar, Alberto; Carsey, Frank; Lane, Arthur; Engelhardt, Herman

2006-01-01

An instrumentation system has been developed for studying interactions between a glacier or ice sheet and the underlying rock and/or soil. Prior borehole imaging systems have been used in well-drilling and mineral-exploration applications and for studying relatively thin valley glaciers, but have not been used for studying thick ice sheets like those of Antarctica. The system includes a cylindrical imaging probe that is lowered into a hole that has been bored through the ice to the ice/bedrock interface by use of an established hot-water-jet technique. The images acquired by the cameras yield information on the movement of the ice relative to the bedrock and on visible features of the lower structure of the ice sheet, including ice layers formed at different times, bubbles, and mineralogical inclusions. At the time of reporting the information for this article, the system was just deployed in two boreholes on the Amery ice shelf in East Antarctica and after successful 2000 2001 deployments in 4 boreholes at Ice Stream C, West Antarctica, and in 2002 at Black Rapids Glacier, Alaska. The probe is designed to operate at temperatures from 40 to +40 C and to withstand the cold, wet, high-pressure [130-atm (13.20-MPa)] environment at the bottom of a water-filled borehole in ice as deep as 1.6 km. A current version is being outfitted to service 2.4-km-deep boreholes at the Rutford Ice Stream in West Antarctica. The probe (see figure) contains a sidelooking charge-coupled-device (CCD) camera that generates both a real-time analog video signal and a sequence of still-image data, and contains a digital videotape recorder. The probe also contains a downward-looking CCD analog video camera, plus halogen lamps to illuminate the fields of view of both cameras. The analog video outputs of the cameras are converted to optical signals that are transmitted to a surface station via optical fibers in a cable. Electric power is supplied to the probe through wires in the cable at a potential of 170 VDC. A DC-to-DC converter steps the supply down to 12 VDC for the lights, cameras, and image-data-transmission circuitry. Heat generated by dissipation of electric power in the probe is removed simply by conduction through the probe housing to the visible features of the lower structure of the ice sheet, including ice layers formed at different times, bubbles, and mineralogical inclusions. At the time of reporting the information for this article, the system was just deployed in two boreholes on the Amery ice shelf in East Antarctica and after successful 2000 2001 deployments in 4 boreholes at Ice Stream C, West Antarctica, and in 2002 at Black Rapids Glacier, Alaska. The probe is designed to operate at temperatures from 40 to +40 C and to withstand the cold, wet, high-pressure [130-atm (13.20-MPa)] environment at the bottom of a water-filled borehole in ice as deep as 1.6 km. A current version is being outfitted to service 2.4-km-deep boreholes at the Rutford Ice Stream in West Antarctica. The probe (see figure) contains a sidelooking charge-coupled-device (CCD) camera that generates both a real-time analog video signal and a sequence of still-image data, and contains a digital videotape recorder. The probe also contains a downward-looking CCD analog video camera, plus halogen lamps to illuminate the fields of view of both cameras. The analog video outputs of the cameras are converted to optical signals that are transmitted to a surface station via optical fibers in a cable. Electric power is supplied to the probe through wires in the cable at a potential of 170 VDC. A DC-to-DC converter steps the supply down to 12 VDC for the lights, cameras, and image-datatransmission circuitry. Heat generated by dissipation of electric power in the probe is removed simply by conduction through the probe housing to the visible features of the lower structure of the ice sheet, including ice layers formed at different times, bubbles, and mineralogical inclusions. At thime of reporting the information for this article, the system was just deployed in two boreholes on the Amery ice shelf in East Antarctica and after successful 2000 2001 deployments in 4 boreholes at Ice Stream C, West Antarctica, and in 2002 at Black Rapids Glacier, Alaska. The probe is designed to operate at temperatures from 40 to +40 C and to withstand the cold, wet, high-pressure [130-atm (13.20-MPa)] environment at the bottom of a water-filled borehole in ice as deep as 1.6 km. A current version is being outfitted to service 2.4-km-deep boreholes at the Rutford Ice Stream in West Antarctica. The probe (see figure) contains a sidelooking charge-coupled-device (CCD) camera that generates both a real-time analog video signal and a sequence of still-image data, and contains a digital videotape recorder. The probe also contains a downward-looking CCD analog video camera, plus halogen lamps to illuminate the fields of view of both cameras. The analog video outputs of the cameras are converted to optical signals that are transmitted to a surface station via optical fibers in a cable. Electric power is supplied to the probe through wires in the cable at a potential of 170 VDC. A DC-to-DC converter steps the supply down to 12 VDC for the lights, cameras, and image-datatransmission circuitry. Heat generated by dissipation of electric power in the probe is removed simply by conduction through the probe housing to the adjacent water and ice.

Documenting Melting Features of the Greenland Ice Sheet

NASA Astrophysics Data System (ADS)

Tedesco, M.

2011-12-01

There is an increasing interest in studying the Greenland Ice Sheet, its hydrology and dynamics over the short term and longer term because of the potential impact of a warming Arctic. Major studies concern about whether increased surface melting will lead to changes in production of supraglacial lakes and subglacial water pressures and hence , potentially, rates of ice movement. In this talk I will show movies recorded over the past three years form fieldwork activities carried out over the West Greenland ice sheet. In particular, I will project and comment movies concerning surface streams and supraglacial lakes, as the one at http://www.youtube.com/watch?v=QbuFphwJn4c. I will discuss the importance of observing such phenomena and how the recorded videos can be used to summarize scientific studies and communicate the relevance of scientific findings. I will also show, for the first time, the video of the drainage of a supraglacial lake, an event during which a lake ~ 6 m deep and ~ 1 km drained in ~ 1.5 hours. This section of the movie is under development as video material was collected during our latest expedition in June 2011.

The role of the margins in ice stream dynamics

NASA Technical Reports Server (NTRS)

Echelmeyer, Keith; Harrison, William

1993-01-01

At first glance, it would appear that the bed of the active ice stream plays a much more important role in the overall force balance than do the margins, especially because the ratio of the half-width to depth for a typical ice stream is large (15:1 to 50:1). On the other hand, recent observations indicate that at least part of the ice stream is underlain by a layer of very weak till (shear strength about 2 kPa), and this weak basal layer would then imply that some or all of the resistive drag is transferred to the margins. In order to address this question, a detailed velocity profile near Upstream B Camp, which extends from the center of the ice stream, across the chaotic shear margin, and onto the Unicorn, which is part of the slow-moving ice sheet was measured. Comparison of this observed velocity profile with finite-element models of flow shows several interesting features. First, the shear stress at the margin is on the order of 130 kPa, while the mean value along the bed is about 15 kPa. Integration of these stresses along the boundaries indicates that the margins provide 40 to 50 percent, and the bed, 60 to 40 percent of the total resistive drag needed to balance the gravitational driving stress in this region. (The range of values represents calculations for different values of surface slope.) Second, the mean basal stress predicted by the models shows that the entire bed cannot be blanketed by the weak till observed beneath upstream B - instead there must be a distribution of weak till and 'sticky spots' (e.g., 85 percent till and 15 percent sticky spots of resistive stress equal to 100 kPa). If more of the bed were composed of weak till, then the modeled velocity would not match that observed. Third, the ice must exhibit an increasing enhancement factor as the margins are approached (E equals 10 in the chaotic zone), in keeping with laboratory measurements on ice under prolonged shear strain. Also, there is either a narrow zone of somewhat stiffer ice (E equals 5) outward of the shear margin, or the bed is frozen there. And last, the high shear stress and strain rate found at the margin are likely to cause significant viscous heating (q) in the marginal ice. The increase in temperature is proportional to qX/u, where X is the width of the shear zone and u is the transverse velocity component bringing cold ice in from the ice sheet outside the shear zone. Near upstream B, this heating is likely to cause an increase in temperature of 4 to 10 K. Plans are to measure this temperature increase in a series of bore holes near the margin during the 1992-93 field season, as well as to provide a more detailed description of the velocity field there.

The role of the margins in ice stream dynamics

NASA Astrophysics Data System (ADS)

Echelmeyer, Keith; Harrison, William

1993-07-01

At first glance, it would appear that the bed of the active ice stream plays a much more important role in the overall force balance than do the margins, especially because the ratio of the half-width to depth for a typical ice stream is large (15:1 to 50:1). On the other hand, recent observations indicate that at least part of the ice stream is underlain by a layer of very weak till (shear strength about 2 kPa), and this weak basal layer would then imply that some or all of the resistive drag is transferred to the margins. In order to address this question, a detailed velocity profile near Upstream B Camp, which extends from the center of the ice stream, across the chaotic shear margin, and onto the Unicorn, which is part of the slow-moving ice sheet was measured. Comparison of this observed velocity profile with finite-element models of flow shows several interesting features. First, the shear stress at the margin is on the order of 130 kPa, while the mean value along the bed is about 15 kPa. Integration of these stresses along the boundaries indicates that the margins provide 40 to 50 percent, and the bed, 60 to 40 percent of the total resistive drag needed to balance the gravitational driving stress in this region. (The range of values represents calculations for different values of surface slope.) Second, the mean basal stress predicted by the models shows that the entire bed cannot be blanketed by the weak till observed beneath upstream B - instead there must be a distribution of weak till and 'sticky spots' (e.g., 85 percent till and 15 percent sticky spots of resistive stress equal to 100 kPa). If more of the bed were composed of weak till, then the modeled velocity would not match that observed. Third, the ice must exhibit an increasing enhancement factor as the margins are approached (E equals 10 in the chaotic zone), in keeping with laboratory measurements on ice under prolonged shear strain. Also, there is either a narrow zone of somewhat stiffer ice (E equals 5) outward of the shear margin, or the bed is frozen there. And last, the high shear stress and strain rate found at the margin are likely to cause significant viscous heating (q) in the marginal ice. The increase in temperature is proportional to qX/u, where X is the width of the shear zone and u is the transverse velocity component bringing cold ice in from the ice sheet outside the shear zone. Near upstream B, this heating is likely to cause an increase in temperature of 4 to 10 K. Plans are to measure this temperature increase in a series of bore holes near the margin during the 1992-93 field season, as well as to provide a more detailed description of the velocity field there.

Mass Balance of the West Antarctic Ice-Sheet from ICESat Measurements

NASA Technical Reports Server (NTRS)

Zwally, H. Jay; Li, Jun; Robins, John; Saba, Jack L.; Yi, Donghui

2011-01-01

Mass balance estimates for 2003-2008 are derived from ICESat laser altimetry and compared with estimates for 1992-2002 derived from ERS radar altimetry. The net mass balance of 3 drainage systems (Pine Island, Thwaites/Smith, and the coast of Marie Bryd) for 2003-2008 is a loss of 100 Gt/yr, which increased from a loss of 70 Gt/yr for the earlier period. The DS including the Bindschadler and MacAyeal ice streams draining into the Ross Ice Shelf has a mass gain of 11 Gt/yr for 2003-2008, compared to an earlier loss of 70 Gt/yr. The DS including the Whillans and Kamb ice streams has a mass gain of 12 Gt/yr, including a significant thickening on the upper part of the Kamb DS, compared to a earlier gain of 6 Gt/yr (includes interpolation for a large portion of the DS). The other two DS discharging into the Ronne Ice Shelf and the northern Ellsworth Coast have a mass gain of 39 Gt/yr, compared to a gain of 4 Gt/yr for the earlier period. Overall, the increased losses of 30 Gt/yr in the Pine Island, Thwaites/Smith, and the coast of Marie Bryd DSs are exceeded by increased gains of 59 Gt/yr in the other 4 DS. Overall, the mass loss from the West Antarctic ice sheet has decreased to 38 Gt/yr from the earlier loss of 67 Gt/yr, reducing the contribution to sea level rise to 0.11 mm/yr from 0.19 mm/yr

Could a new ice core offer an insight into the stability of the West Antarctic Ice Sheet during the last interglacial?

NASA Astrophysics Data System (ADS)

Mulvaney, R.; Hindmarsh, R. C.

2013-12-01

Vaughan et al., in their 2011 paper 'Potential Seaways across West Antarctica' (Geochem. Geophys. Geosyst., 12, Q10004, doi:10.1029/2011GC003688), offer the intriguing prospect that substantial ice loss from the West Antarctic Ice Sheet during the previous interglacial period might have resulted in the opening of a seaway between the Weddell Sea and the Amundsen Sea. One of their potential seaways passes between the south western corner of the present Ronne Ice Shelf and the Pine Island Bay, through what is currently the course of the Rutford Ice Stream, between the Ellsworth Mountains and the Fletcher Promontory. To investigate whether this seaway could have existed (and to recover a paleoclimate and ice sheet history from the Weddell Sea), a team from the British Antarctic Survey and the Laboratoire de Glaciologie et Géophysique de l'Environnement drilled an ice core from a close to a topographic dome in the ice surface on the Fletcher Promontory in January 2012, reaching the bedrock at 654.3m depth from the surface. The site was selected to penetrate directly through the centre of a Raymond cupola observed in internal radar reflections from the ice sheet, with the intention that this would ensure we obtained the oldest ice available from the Fletcher Promontory. The basal ice sheet temperature measured was -18°C, implying the oldest ice would not have melted away from the base, while the configuration of the Raymond cupola in the radar horizons suggested stability in the ice dome topography during the majority of the Holocene. Our hypothesis is that chemical analysis of the ice core will reveal whether the site was ever relatively close to open sea water or ice shelf in the Rutford channel 20 km distant, rather than the current 700 km distance to sea ice/open water in either the Weddell Sea or the Amundsen Sea. While we do not yet have the chemistry data to test this hypothesis, in this poster we will discuss whether there is in reality any potential local meteoric ice remaining from the previous interglacial that could provide the evidence we need. We show likely age-depth models in an ice dome with a pronounced Raymond cupola and flat bedrock. The evidence from the stable water isotope temperature history from the site shows the Last Glacial Maximum/Holocene boundary substantially above the bedrock, implying the possibility of much older ice in the lowest ice layers.

Luminescence dating of paleolake deltas and glacial deposits in Garwood Valley, Antarctica: Implications for climate, Ross ice sheet dynamics, and paleolake duration

USGS Publications Warehouse

Levy, Joseph S.; Rittenour, Tammy M.; Fountain, Andrew G.; O'Connor, Jim E.

2017-01-01

The formation of perched deltas and other lacustrine deposits in the McMurdo Dry Valleys of Antarctica is widely considered to be evidence of valley-filling lakes dammed by the grounded Ross Sea ice sheet during the local Last Glacial Maximum, with lake drainage interpreted as a record of grounding line retreat. We used luminescence dating to determine the age of paleolake deltas and glacial tills in Garwood Valley, a coastal dry valley that opens to the Ross Sea. Luminescence ages are stratigraphically consistent with radiocarbon results from algal mats within the same delta deposits but suggest radiocarbon dates from lacustrine carbonates may overestimate deposit ages by thousands of years. Results suggest that late Holocene delta deposition into paleolake Howard in Garwood Valley persisted until ca. 3.5 ka. This is significantly younger than the date when grounded ice is thought to have retreated from the Ross Sea. Our evidence suggests that the local, stranded ice-cored till topography in Garwood Valley, rather than regional ice-sheet dynamics, may have controlled lake levels for some McMurdo Dry Valleys paleolakes. Age control from the supraglacial Ross Sea drift suggests grounding and up-valley advance of the Ross Sea ice sheet into Garwood valley during marine oxygen isotope stage (MIS) 4 (71–78 ka) and the local Last Glacial Maximum (9–10 ka). This work demonstrates the power of combining luminescence dating with existing radiocarbon data sets to improve understanding of the relationships among paleolake formation, glacial position, and stream discharge in response to climate change.

A new full-Stokes model as a tool for basal inversions.

NASA Astrophysics Data System (ADS)

Kyrke-Smith, Teresa M.; Hilmar Gudmundsson, G.; Farrell, Patrick E.

2016-04-01

High resolution models of ice sheet dynamics are required to make accurate predictions of the future mass balance of ice sheets. These require knowledge of flow conditions at the bed of the ice, however, the inaccessibility of the bed means there exist few observational constraints. Inverse methods are therefore commonly used to obtain information about the nature of basal control using given surface observations. We present a new 3D Stokes solver written using FEniCS with the potential to carry out second-order inversions for basal slipperiness. We will be applying the model to Pine Island Glacier, Antarctica. Pine Island Glacier is one of the fastest flowing and most rapidly changing ice streams in Antarctica, and is currently contributing to sea-level rise at an increasing rate. Recent field seasons as part of the iSTAR project have acquired high-resolution in-situ geophysical measurements; results from our model will be compared with these to try and increase understanding about the conditions at the bed of Pine Island Glacier.

Using Airborne Radar Stratigraphy to Model Surface Accumulation Anomaly and Basal Control over Deformed Basal Ice in Greenland

NASA Astrophysics Data System (ADS)

Das, I.; Bell, R. E.; Creyts, T. T.; Wolovick, M.

2013-12-01

Large deformed ice structures have been imaged at the base of northern Greenland ice sheet by IceBridge airborne radar. Numerous deformed structures lie along the base of both Petermann Glacier and Northeast Ice stream catchments covering 10-13% of the catchment area. These structures may be combinations of basal freeze-on and folded ice that overturns and inverts stratigraphy. In the interior, where the ice velocity is low, the radar imaged height of the deformed structures are frequently a significant fraction of the ice thickness. They are related to basal freeze on and stick-slip at the base of the ice sheet and may be triggered by subglacial water, sediments or local geological conditions. The larger ones (at times up to 700 m thick and 140 km long) perturb the ice stratigraphy and create prominent undulations on the ice surface and modify the local surface mass balance. Here, we investigate the relationship between the deformed structures and surface processes using shallow and deep ice radar stratigraphy. The surface undulations caused by the deformed structures modulate the pattern of local surface snow accumulation. Using normalized differences of several near-surface stratigraphic layers, we have calculated the accumulation anomaly over these deformed structures. The accumulation anomalies can be as high as 20% of the local surface accumulation over some of the larger surface depressions caused by these deformed structures. We observe distinct differences in the phases of the near-surface internal layers on the Petermann and Northeast catchments. These differences indicate that the deformed bodies over Petermann are controlled by conditions at the bed different from the Northeast Ice stream. The distinctly different near-surface stratigraphy over the deformed structures in the Petermann and Northeast catchments have opened up a number of questions including their formation and how they influence the ice dynamics, ice stratigraphy and surface mass balance. In this study we will model the different physical conditions at the bed and ice rheology from their distinct signatures in the near-surface strata. The results will identify the distinct mechanisms that form these bodies and their control over the surface morphology and snow accumulation.

DEM, tide and velocity over sulzberger ice shelf, West Antarctica

USGS Publications Warehouse

Baek, S.; Shum, C.K.; Lee, H.; Yi, Y.; Kwoun, Oh-Ig; Lu, Z.; Braun, Andreas

2005-01-01

Arctic and Antarctic ice sheets preserve more than 77% of the global fresh water and could raise global sea level by several meters if completely melted. Ocean tides near and under ice shelves shifts the grounding line position significantly and are one of current limitations to study glacier dynamics and mass balance. The Sulzberger ice shelf is an area of ice mass flux change in West Antarctica and has not yet been well studied. In this study, we use repeat-pass synthetic aperture radar (SAR) interferometry data from the ERS-1 and ERS-2 tandem missions for generation of a high-resolution (60-m) Digital Elevation Model (DEM) including tidal deformation detection and ice stream velocity of the Sulzberger Ice Shelf. Other satellite data such as laser altimeter measurements with fine foot-prints (70-m) from NASA's ICESat are used for validation and analyses. The resulting DEM has an accuracy of-0.57??5.88 m and is demonstrated to be useful for grounding line detection and ice mass balance studies. The deformation observed by InSAR is found to be primarily due to ocean tides and atmospheric pressure. The 2-D ice stream velocities computed agree qualitatively with previous methods on part of the Ice Shelf from passive microwave remote-sensing data (i.e., LANDSAT). ?? 2005 IEEE.

Asynchronous North Atlantic iceberg discharges during the last glacial period explained through ocean circulation changes

NASA Astrophysics Data System (ADS)

Montoya, M.; Banderas, R.; Alvarez-Solas, J.; Robinson, A.

2017-12-01

Heinrich events (HEs) are episodes of increased ice-rafted debris (IRD) deposition in the North Atlantic Ocean that took place during stadials of the last glacial period, and are interpreted as massive iceberg discharges from the Laurentide Ice Sheet (LIS). IRD originating from the Fennoscandian ice sheet (FIS) accompany HEs during stadials, but enhanced calving has also been reported, however, during interstadials. While a number of mechanisms have been proposed to explain HEs involving the LIS, the role of the FIS during these events has not received much attention from a modeling perspective. Thus, a consistent explanation for the asynchronous occurrence of enhanced IRD throughout the North Atlantic is lacking. Here we investigate the response of the FIS to millennial-scale climate variability during the last glacial period. We use a hybrid three-dimensional thermomechanical ice-sheet model forced offline through a novel perturbative approach accounting for a more realistic treatment of millennial-scale climatic variability, including both the atmospheric and the oceanic components. Our results show that the FIS responds with enhanced iceberg discharges in phase with interstadial warmings in the North Atlantic. Separating the atmospheric and oceanic effects demonstrates the major role of the ocean in controlling the dynamics of the FIS on millennial timescales. While the atmospheric forcing alone is only able to produce modest iceberg discharges (< 0.02 Sv), the warmer oceanic surface waters lead to much higher rates of iceberg surges (ca. 0.1 Sv) as a result of relatively high basal melting rates within the margins of the ice sheet through the reactivation of ice streams in the northeastern (NE) part of the ice sheet. Together with previous work our results provide a consistent explanation for the asynchronous response of the LIS and the FIS to glacial abrupt climate changes. Finally, they support the notion that the FIS is a likely candidate to produce iceberg discharges during interstadials as suggested by IRD in the region.

IceBridge Provides Novel Evidence for Thick Units of Basal Freeze-on Ice Along Petermann Glacier, Greenland

NASA Astrophysics Data System (ADS)

Bell, R. E.; Tinto, K. J.; Wolovick, M.; Block, A. E.; Frearson, N.; Das, I.; Abdi, A.; Creyts, T. T.; Cochran, J. R.; Csatho, B. M.; Babonis, G. S.

2011-12-01

The Petermann Glacier, one of the major outlet glaciers in Greenland, drains six percent of the Greenland ice from a basin largely below sea level. Petermann Glacier and its large ice shelf may be susceptible to increased change as the waters along the Greenland margin warm. The 2010 and 2011 Operation IceBridge mission, acquired a comprehensive aerogeophysical data set over the Petermann Glacier that provides insights into the ice sheet structure. This analysis employs most of the data streams acquired by the Icebridge platform including ice-penetrating radar, laser altimetry, gravity and magnetics. An orthogonal 10 km grid extends from 60 km upstream of the grounding line to 240 km inland. The ice velocities in the region range from <50m/yr to >200m/yr. On the interior lines the internal layers are pulled down over 2-3 km wide regions. Up to 400m of ice from the base of the ice sheet appears to be absent in these regions. We interpret these pulled down regions as basal melt. These melt regions are mainly located along the upstream side of a 80 km wide east-west trending topographic ridge that separates the interior ice from the Petermann Fjord. The IceBridge magnetic data indicates that this broad flat ridge is the boundary between the Franklinian Basins and the Ellsmerian Foldbelt to the north. Downstream of these pull-down layers we have identified 4 distinct packages of ice that thicken downstream and are characterized by a strong upper reflector. These packages develop at the base of the ice sheet and reach thicknesses of 500-700m over distances of 10-20 km. These basal packages can be traced for 30-100 km following the direction of flow, and may be present close to the grounding line. These basal reflectors deflect the overlying internal layers upward indicating the addition of ice to the base of the ice sheet. The IceBridge gravity data indicates that these features are probably not off-nadir topography since these would show up as around 30mGal anomalies in the gravity data, and no such signature exists. We interpret these basal features in the radar data as ice. The geometry of these packages is very similar to the frozen-on ice packages imaged along the margins of the Gamburtsev Mountains, East Antarctica. We have interpreted these packages as bodies of accreted ice sourced from the upstream melting. Similar packages of basal ice up to 1200m thick are found throughout the margins of the Greenland ice sheet north of 70°N. The accretion process is modifying the base of the ice sheet in the onset regions of Petermann and other outlet glaciers in Greenland. Any change in rheology between basal and meteoric ice is likely to influence ice flow fostering enhanced melting and deformation of the basal ice. These frozen-on basal ice packages may be more susceptible to melt when ice sheet goes afloat and may be the origin of the elongate melt channels in the ice shelf. Accretion may influence both ice flow in outlet glaciers and melt rates at the grounding line and the adjacent ice shelves.

Firn thickness variations across the Northeast Greenland Ice Stream margins indicating nonlinear densification rates

NASA Astrophysics Data System (ADS)

Riverman, K. L.; Anandakrishnan, S.; Alley, R. B.; Peters, L. E.; Christianson, K. A.; Muto, A.

2013-12-01

Northeast Greenland Ice Stream (NEGIS) is the largest ice stream in Greenland, draining approximately 8.4% of the ice sheet's area. The flow pattern and stability mechanism of this ice stream are unique to others in Greenland and Antarctica, and merit further study to ascertain the sensitivity of this ice stream to future climate change. Geophysical methods are valuable tools for this application, but their results are sensitive to the structure of the firn and any spatial variations in firn properties across a given study region. Here we present firn data from a 40-km-long seismic profile across the upper reaches of NEGIS, collected in the summer of 2012 as part of an integrated ground-based geophysical survey. We find considerable variations in firn thickness that are coincident with the ice stream shear margins, where a thinner firn layer is present within the margins, and a thicker, more uniform firn layer is present elsewhere in our study region. Higher accumulation rates in the marginal surface troughs due to drift-snow trapping can account for some of this increased densification; however, our seismic results also highlight enhanced anisotropy within the firn and upper ice column that is confined to narrow bands within the shear margins. We thus interpret these large firn thickness variations and abrupt changes in anisotropy as indicators of firn densification dependent on the effective stress state as well as the overburden pressure, suggesting that the strain rate increases nonlinearly with stress across the shear margins. A GPS strain grid maintained for three weeks across both margins observed strong side shearing, with rapid stretching and then compression along particle paths, indicating large deviatoric stresses in the margins. This work demonstrates the importance of developing a high-resolution firn densification model when conducting geophysical field work in regions possessing a complex ice flow history; it also motivates the need for a more detailed firn densification study along NEGIS to better understand the evolution of these abrupt structural variations within the firn.

Ross Sea paleo-ice sheet drainage and deglacial history during and since the LGM

NASA Astrophysics Data System (ADS)

Anderson, John B.; Conway, Howard; Bart, Philip J.; Witus, Alexandra E.; Greenwood, Sarah L.; McKay, Robert M.; Hall, Brenda L.; Ackert, Robert P.; Licht, Kathy; Jakobsson, Martin; Stone, John O.

2014-09-01

Onshore and offshore studies show that an expanded, grounded ice sheet occupied the Ross Sea Embayment during the Last Glacial Maximum (LGM). Results from studies of till provenance and the orientation of geomorphic features on the continental shelf show that more than half of the grounded ice sheet consisted of East Antarctic ice flowing through Transantarctic Mountain (TAM) outlet glaciers; the remainder came from West Antarctica. Terrestrial data indicate little or no thickening in the upper catchment regions in both West and East Antarctica during the LGM. In contrast, evidence from the mouths of the southern and central TAM outlet glaciers indicate surface elevations between 1000 m and 1100 m (above present-day sea level). Farther north along the western margin of the Ross Ice Sheet, surface elevations reached 720 m on Ross Island, and 400 m at Terra Nova Bay. Evidence from Marie Byrd Land at the eastern margin of the ice sheet indicates that the elevation near the present-day grounding line was more than 800 m asl, while at Siple Dome in the central Ross Embayment, the surface elevation was about 950 m asl. Farther north, evidence that the ice sheet was grounded on the middle and the outer continental shelf during the LGM implies that surface elevations had to be at least 100 m above the LGM sea level. The apparent low surface profile and implied low basal shear stress in the central and eastern embayment suggests that although the ice streams may have slowed during the LGM, they remained active. Ice-sheet retreat from the western Ross Embayment during the Holocene is constrained by marine and terrestrial data. Ages from marine sediments suggest that the grounding line had retreated from its LGM outer shelf location only a few tens of kilometer to a location south of Coulman Island by ˜13 ka BP. The ice sheet margin was located in the vicinity of the Drygalski Ice Tongue by ˜11 ka BP, just north of Ross Island by ˜7.8 ka BP, and near Hatherton Glacier by ˜6.8 ka BP. Farther south, 10Be exposure ages from glacial erratics on nunataks near the mouths of Reedy, Scott and Beardmore Glaciers indicate thinning during the mid to late Holocene, but the grounding line did not reach its present position until 2 to 3 ka BP. Marine dates, which are almost exclusively Acid Insoluble Organic (AIO) dates, are consistently older than those derived from terrestrial data. However, even these ages indicate that the ice sheet experienced significant retreat after ˜13 ka BP. Geomorphic features indicate that during the final stages of ice sheet retreat ice flowing through the TAM remained grounded on the shallow western margin of Ross Sea. The timing of retreat from the central Ross Sea remains unresolved; the simplest reconstruction is to assume that the grounding line here started to retreat from the continental shelf more or less in step with the retreat from the western and eastern sectors. An alternative hypothesis, which relies on the validity of radiocarbon ages from marine sediments, is that grounded ice had retreated from the outer continental shelf prior to the LGM. More reliable ages from marine sediments in the central Ross Embayment are needed to test and validate this hypothesis.

Glacier seismology: eavesdropping on the ice-bed interface

NASA Astrophysics Data System (ADS)

Walter, F.; Röösli, C.

2015-12-01

Glacier sliding plays a central role in ice dynamics. A number of remote sensing and deep drilling initiatives have therefore focused on the ice-bed interface. Although these techniques have provided valuable insights into bed properties, they do not supply theorists with data of sufficient temporal and spatial resolution to rigorously test mathematical sliding laws. As an alternative, passive seismic techniques have gained popularity in glacier monitoring. Analysis of glacier-related seismic sources ('icequakes') has become a useful technique to study inaccessible regions of the cryosphere, including the ice-bed interface. Seismic monitoring networks on the polar ice sheets have shown that ice sliding is not only a smooth process involving viscous deformation and regelation of basal ice layers. Instead, ice streams exhibit sudden slip episodes over their beds and intermittent phases of partial or complete stagnation. Here we discuss new and recently published discoveries of basal seismic sources beneath various glacial bodies. We revisit basal seismicity of hard-bedded Alpine glaciers, which is not the result of pure stick-slip motion. Sudden changes in seismicity suggest that the local configuration of the subglacial drainage system undergoes changes on sub daily time scales. Accordingly, such observations place constraints on basal resistance and sliding of hard-bedded glaciers. In contrast, certain clusters of stick-slip dislocations associated with micro seismicity beneath the Greenland ice sheet undergo diurnal variations in magnitudes and inter event times. This is best explained with a soft till bed, which hosts the shear dislocations and whose strength varies in response to changes in subglacial water pressure. These results suggest that analysis of basal icequakes is well suited for characterizing glacier and ice sheet beds. Future studies should address the relative importance between "smooth" and seismogenic sliding in different glacial environments.

Ross Sea Till Properties: Implications for Ice Sheet Bed Interaction

NASA Astrophysics Data System (ADS)

Halberstadt, A. R.; Anderson, J. B.; Simkins, L.; Prothro, L. O.; Bart, P. J.

2015-12-01

Since the discovery of a pervasive shearing till layer underlying Ice Stream B, the scientific community has categorized subglacial diamictons as either deformation till or lodgement till primarily based on shear strength. Deformation till is associated with streaming ice, formed through subglacial deformation of unconsolidated sediments. Lodgement till is believed to be deposited by the plastering of sediment entrained at the base of slow-flowing ice onto a rigid bed. Unfortunately, there has been a paucity of quantitative data on the spatial distribution of shear strength across the continental shelf. Cores collected from the Ross Sea on cruises NBP1502 and NBP9902 provide a rich dataset that can be used to interpret till shear strength variability. Till strengths are analyzed within the context of: (1) geologic substrate; (2) water content and other geotechnical properties; (3) ice sheet retreat history; and (4) geomorphic framework. Tills display a continuum of shear strengths rather than a bimodal distribution, suggesting that shear strength cannot be used to distinguish between lodgement and deformation till. Where the substrate below the LGM unconformity is comprised of older lithified deposits, till shear strengths are both highly variable within the till unit, as well as highly variable between cores. Conversely, where ice streams flowed across unconsolidated Plio-Pleistocene deposits, shear strengths are low and less variable within the unit and between cores. This suggests greater homogenization of cannibalized tills, and possibly a deeper pervasive shear layer. Coarser-grained tills are observed on banks and bank slopes, with finer tills in troughs. Highly variable and more poorly sorted tills are found in close proximity to sediment-based subglacial meltwater channels, attesting to a change in ice-bed interaction as subglacial water increases. Pellets (rounded sedimentary clasts of till matrix) are observed in Ross Sea cores, suggesting a history of deformation responsible for pellet formation. Till strength was measured in a variety of environments, including mega-scale lineations and grounding zone wedges; ongoing work focuses on evaluating till shear strengths within a geomorphic context. These analyses are used to re-evaluate till genesis, transport, and characterization.

Implications for the crustal Architecture in West Antarctica revealed by the means of depth-to-the-bottom of the magnetic source (DBMS) mapping and 3D FEM geothermal heat flux models

NASA Astrophysics Data System (ADS)

Dziadek, Ricarda; Gohl, Karsten; Kaul, Norbert

2017-04-01

The West Antarctic Rift System (WARS) is one of the largest rift systems in the world, which displays unique coupled relationships between tectonic processes and ice sheet dynamics. Palaeo-ice streams have eroded troughs across the Amundsen Sea Embayment (ASE) that today route warm ocean deep water to the West Antarctic Ice Sheet (WAIS) grounding zone and reinforce dynamic ice sheet thinning. Rift basins, which cut across West Antarctica's landward-sloping shelves, promote ice sheet instability. Young, continental rift systems are regions with significantly elevated geothermal heat flux (GHF), because the transient thermal perturbation to the lithosphere caused by rifting requires 100 m.y. to reach long-term thermal equilibrium. The GHF in this region is, especially on small scales, poorly constrained and suspected to be heterogeneous as a reflection of the distribution of tectonic and volcanic activity along the complex branching geometry of the WARS, which reflects its multi-stage history and structural inheritance. We investigate the crustal architecture and the possible effects of rifting history from the WARS on the ASE ice sheet dynamics, by the use of depth-to-the-bottom of the magnetic source (DBMS) estimates. These are based on airborne-magnetic anomaly data and provide an additional insight into the deeper crustal properties. With the DBMS estimates we reveal spatial changes at the bottom of the igneous crust and the thickness of the magnetic layer, which can be further incorporated into tectonic interpretations. The DBMS also marks an important temperature transition zone of approximately 580°C and therefore serves as a boundary condition for our numerical FEM models in 2D and 3D. On balance, and by comparison to global values, we find average GHF of 90 mWm-2 with spatial variations due to crustal heterogeneities and volcanic activities. This estimate is 30% more than commonly used in ice sheet models in the ASE region.

Triangular-shaped landforms reveal subglacial drainage routes in SW Finland

NASA Astrophysics Data System (ADS)

Mäkinen, J.; Kajuutti, K.; Palmu, J.-P.; Ojala, A.; Ahokangas, E.

2017-05-01

The aim of this study is to present the first evidence of triangular-shaped till landforms and related erosional features indicative of subglacial drainage within the ice stream bed of the Scandinavian ice sheet in Finland. Previously unidentified grouped patterns of Quaternary deposits with triangular landforms can be recognized from LiDAR-based DEMs. The triangular landforms occur as segments within geomorphologically distinguishable routes that are associated with eskers. The morphological and sedimentological characteristics as well as the distribution of the triangular landforms are interpreted to involve the creep of saturated deforming till, flow and pressure fluctuations of subglacial meltwater associated with meltwater erosion. There are no existing models for the formation of this kind of large-scale drainage systems, but we claim that they represent an efficient drainage system for subglacial meltwater transfer under high pressure conditions. Our hypothesis is that the routed, large-scale subglacial drainage systems described herein form a continuum between channelized (eskers) and more widely spread small-scale distributed subglacial drainage. Moreover, the transition from the conduit dominated drainage to triangular-shaped subglacial landforms takes place about 50-60 km from the ice margin. We provide an important contribution towards a more realistic representation of ice sheet hydrological drainage systems that could be used to improve paleoglaciological models and to simulate likely responses of ice sheets to increased meltwater production.

A Microbial Community in Sediments Beneath the Western Antarctic Ice Sheet, Ice Stream C (Kamb)

NASA Astrophysics Data System (ADS)

Skidmore, M.; Han, S.; Foo, W.; Bui, D.; Lanoil, B.

2004-12-01

In 2000, an ice-drilling project focusing on the "sticky spot" of Ice Stream C recovered cores of sub-glacial sediments from beneath the Western Antarctic Ice Sheet. We have characterized several chemical and microbiological parameters of the sole intact sediment core. Pore waters extracted from these sediments were brackish and some were supersaturated with respect to calcite. Ion chromatography demonstrated the presence of several organic acids at low, but detectable, levels in the pore water. DAPI direct cell counts were approximately 107 cells g-1. Aerobic viable plate counts were much lower than direct cell counts; however, they were two orders of magnitude higher on plates incubated at low temperature (4 ° C; 3.63 x 105 CFU ml-1) than at higher temperatures (ca. 22° C; 1.5 x 103 CFU ml-1); no colonies were detected on plates incubated anaerobically at either temperature. 16S rDNA clone library analysis indicates extremely limited bacterial diversity in these samples: six phylogenetic clades were detected. The three dominant bacterial phylogenetic clades in the clone libraries (252 clones total) were most closely related to Thiobacillus thioparus (180 clones), Polaromonas vacuolata (34 clones), and Gallionella ferruginea (35 clones) and their relatives; one clone each represented the other three phylogenetic clades (most closely related to Ralstonia pickettii, Lysobacter antibioticus, and Xylella fastidiosa, respectively). These sequences match closely with sequences previously obtained from other subglacial environments in Alaska, Ellesmere Island, Canada and New Zealand. Implications of this microbial community to subglacial chemistry and microbial biogeography will be discussed.

Post-LGM Grounding-Line Positions of the Bindschadler Paleo Ice Stream in the Ross Sea Embayment, Antarctica

NASA Astrophysics Data System (ADS)

Bart, Philip J.; Anderson, John B.; Nitsche, Frank

2017-10-01

The West Antarctic Ice Sheet (WAIS) retreated more than 1,000 km since last grounding at the Ross Sea outer continental shelf. Here we show an interpretation of former grounding line positions from a new large-area multibeam survey and a regional grid of chirp cross-sectional data from the Whales Deep Basin in eastern Ross Sea. The basin is a paleo-glacial trough that was occupied by the Bindschadler Ice Stream when grounded ice advanced to the shelf edge during the Last Glacial Maximum. These new geophysical data provide unambiguous evidence that the WAIS occupied at least seven grounding line positions within 60 km of the shelf edge. Four of seven grounding zone wedges (GZWs) are partly exposed over large areas of the trough. The overlapping stratal arrangement created a large-volume compound GZW. Some of the groundings involved local readvance of the grounding line. Subsequent to these seven outer continental shelf groundings, the ice sheet retreated more than 200 km towards Roosevelt Island on the middle continental shelf. The major retreat across the middle continental shelf is recorded by small-scale moraine ridges that mantle the top of GZW7, and these are suggestive of relatively continuous grounding line recession. The results indicate that retreat was considerably more complex than was possible to reconstruct with reconnaissance-level data. The added details are important to climate models, which must first be able to reproduce the recent retreat pattern in all of its complexities to improve confidence in model predictions of the system's future response.

Hydrogeologic Controls on Water Dynamics in a Discontinuous Permafrost, Lake-Rich Landscape

NASA Astrophysics Data System (ADS)

Walvoord, M. A.; Briggs, M. A.; Day-Lewis, F. D.; Jepsen, S. M.; Lane, J. W., Jr.; McKenzie, J. M.; Minsley, B. J.; Striegl, R. G.; Voss, C. I.; Wellman, T. P.

2014-12-01

Glacier sliding plays a central role in ice dynamics. A number of remote sensing and deep drilling initiatives have therefore focused on the ice-bed interface. Although these techniques have provided valuable insights into bed properties, they do not supply theorists with data of sufficient temporal and spatial resolution to rigorously test mathematical sliding laws. As an alternative, passive seismic techniques have gained popularity in glacier monitoring. Analysis of glacier-related seismic sources ('icequakes') has become a useful technique to study inaccessible regions of the cryosphere, including the ice-bed interface. Seismic monitoring networks on the polar ice sheets have shown that ice sliding is not only a smooth process involving viscous deformation and regelation of basal ice layers. Instead, ice streams exhibit sudden slip episodes over their beds and intermittent phases of partial or complete stagnation. Here we discuss new and recently published discoveries of basal seismic sources beneath various glacial bodies. We revisit basal seismicity of hard-bedded Alpine glaciers, which is not the result of pure stick-slip motion. Sudden changes in seismicity suggest that the local configuration of the subglacial drainage system undergoes changes on sub daily time scales. Accordingly, such observations place constraints on basal resistance and sliding of hard-bedded glaciers. In contrast, certain clusters of stick-slip dislocations associated with micro seismicity beneath the Greenland ice sheet undergo diurnal variations in magnitudes and inter event times. This is best explained with a soft till bed, which hosts the shear dislocations and whose strength varies in response to changes in subglacial water pressure. These results suggest that analysis of basal icequakes is well suited for characterizing glacier and ice sheet beds. Future studies should address the relative importance between "smooth" and seismogenic sliding in different glacial environments.

Data assimilation of surface altimetry on the North-Easter Ice Stream using the Ice Sheet System Model (ISSM)

NASA Astrophysics Data System (ADS)

Larour, Eric; Utke, Jean; Morlighem, Mathieu; Seroussi, Helene; Csatho, Beata; Schenk, Anton; Rignot, Eric; Khazendar, Ala

2014-05-01

Extensive surface altimetry data has been collected on polar ice sheets over the past decades, following missions such as Envisat and IceSat. This data record will further increase in size with the new CryoSat mission, the ongoing Operation IceBridge Mission and the soon to launch IceSat-2 mission. In order to make the best use of these dataset, ice flow models need to improve on the way they ingest surface altimetry to infer: 1) parameterizations of poorly known physical processes such as basal friction; 2) boundary conditions such as Surface Mass Balance (SMB). Ad-hoc sensitivity studies and adjoint-based inversions have so far been the way ice sheet models have attempted to resolve the impact of 1) on their results. As for boundary conditions or the lack thereof, most studies assume that they are a fixed quantity, which, though prone to large errors from the measurement itself, is not varied according to the simulated results. Here, we propose a method based on automatic differentiation to improve boundary conditions at the base and surface of the ice sheet during a short-term transient run for which surface altimetry observations are available. The method relies on minimizing a cost-function, the best fit between modeled surface evolution and surface altimetry observations, using gradients that are computed for each time step from automatic differentiation of the ISSM (Ice Sheet System Model) code. The approach relies on overloaded operators using the ADOLC (Automatic Differentiation by OverLoading in C++) package. It is applied to the 79 North Glacier, Greenland, for a short term transient spanning a couple of decades before the start of the retreat of the Zachariae Isstrom outlet glacier. Our results show adjustments required on the basal friction and the SMB of the whole basin to best fit surface altimetry observations, along with sensitivities each one of these parameters has on the overall cost function. Our approach presents a pathway towards assimilating multiple datasets in transient ice flow models of Greenland and Antarctica, which will become increasingly important as the amount of available observations becomes too large to assess on a case by case basis. This work was performed at the California Institute of Technology's Jet Propulsion Laboratory under a contract with the National Aeronautics and Space Administration's Cryosphere Science Program.

Can we Relate Basal Ice Mechanics to Seismic Observations of the Bed?

NASA Astrophysics Data System (ADS)

Kyrke-Smith, T.; Gudmundsson, G. H.; Farrell, P. E.

2017-12-01

We compare results from two different methods of quanitfying basal ice conditions, by investigating correlations between seismically-derived estimates of basal acoustic impedance and basal slipperiness values obtained from a surface-to-bed inversion of a Stokes ice flow model. Using high-resolution measurements taken along several seismic profiles on Pine Island Glacier (PIG), we find no correlation between acoustic impedance and retrieved basal slipperiness wihtin each individual profile. However, there is a correlation when comparing averaged values across each distinct profile. Nevertheless, there is no clear way of incorporating seismic measurements of bed properties on ice streams into ice flow models. We conclude that more theoretical work needs done before constraints on mechanical conditions at the ice-bed interface from acoustic impedance measurements can be of direct use to ice sheet models.

Antarctic ice shelf potentially stabilized by export of meltwater in surface river.

PubMed

Bell, Robin E; Chu, Winnie; Kingslake, Jonathan; Das, Indrani; Tedesco, Marco; Tinto, Kirsty J; Zappa, Christopher J; Frezzotti, Massimo; Boghosian, Alexandra; Lee, Won Sang

2017-04-19

Meltwater stored in ponds and crevasses can weaken and fracture ice shelves, triggering their rapid disintegration. This ice-shelf collapse results in an increased flux of ice from adjacent glaciers and ice streams, thereby raising sea level globally. However, surface rivers forming on ice shelves could potentially export stored meltwater and prevent its destructive effects. Here we present evidence for persistent active drainage networks-interconnected streams, ponds and rivers-on the Nansen Ice Shelf in Antarctica that export a large fraction of the ice shelf's meltwater into the ocean. We find that active drainage has exported water off the ice surface through waterfalls and dolines for more than a century. The surface river terminates in a 130-metre-wide waterfall that can export the entire annual surface melt over the course of seven days. During warmer melt seasons, these drainage networks adapt to changing environmental conditions by remaining active for longer and exporting more water. Similar networks are present on the ice shelf in front of Petermann Glacier, Greenland, but other systems, such as on the Larsen C and Amery Ice Shelves, retain surface water at present. The underlying reasons for export versus retention remain unclear. Nonetheless our results suggest that, in a future warming climate, surface rivers could export melt off the large ice shelves surrounding Antarctica-contrary to present Antarctic ice-sheet models, which assume that meltwater is stored on the ice surface where it triggers ice-shelf disintegration.

Antarctic Ice Shelf Potentially Stabilized by Export of Meltwater in Surface River

NASA Technical Reports Server (NTRS)

Bell, Robin E.; Chu, Winnie; Kingslake, Jonathan; Das, Indrani; Tedesco, Marco; Tinto, Kirsty J.; Zappa, Christopher J.; Frezzotti, Massimo; Boghosian, Alexandra; Lee, Won Sang

2017-01-01

Meltwater stored in ponds and crevasses can weaken and fracture ice shelves, triggering their rapid disintegration. This ice-shelf collapse results in an increased flux of ice from adjacent glaciers and ice streams, thereby raising sea level globally. However, surface rivers forming on ice shelves could potentially export stored meltwater and prevent its destructive effects. Here we present evidence for persistent active drainage networks-interconnected streams, ponds and rivers-on the Nansen Ice Shelf in Antarctica that export a large fraction of the ice shelf's meltwater into the ocean. We find that active drainage has exported water off the ice surface through waterfalls and dolines for more than a century. The surface river terminates in a 130-metre-wide waterfall that can export the entire annual surface melt over the course of seven days. During warmer melt seasons, these drainage networks adapt to changing environmental conditions by remaining active for longer and exporting more water. Similar networks are present on the ice shelf in front of Petermann Glacier, Greenland, but other systems, such as on the Larsen C and Amery Ice Shelves, retain surface water at present. The underlying reasons for export versus retention remain unclear. Nonetheless our results suggest that, in a future warming climate, surface rivers could export melt off the large ice shelves surrounding Antarctica-contrary to present Antarctic ice-sheet models, which assume that meltwater is stored on the ice surface where it triggers ice-shelf disintegration.

Antarctic ice shelf potentially stabilized by export of meltwater in surface river

NASA Astrophysics Data System (ADS)

Bell, Robin E.; Chu, Winnie; Kingslake, Jonathan; Das, Indrani; Tedesco, Marco; Tinto, Kirsty J.; Zappa, Christopher J.; Frezzotti, Massimo; Boghosian, Alexandra; Lee, Won Sang

2017-04-01

Meltwater stored in ponds and crevasses can weaken and fracture ice shelves, triggering their rapid disintegration. This ice-shelf collapse results in an increased flux of ice from adjacent glaciers and ice streams, thereby raising sea level globally. However, surface rivers forming on ice shelves could potentially export stored meltwater and prevent its destructive effects. Here we present evidence for persistent active drainage networks—interconnected streams, ponds and rivers—on the Nansen Ice Shelf in Antarctica that export a large fraction of the ice shelf’s meltwater into the ocean. We find that active drainage has exported water off the ice surface through waterfalls and dolines for more than a century. The surface river terminates in a 130-metre-wide waterfall that can export the entire annual surface melt over the course of seven days. During warmer melt seasons, these drainage networks adapt to changing environmental conditions by remaining active for longer and exporting more water. Similar networks are present on the ice shelf in front of Petermann Glacier, Greenland, but other systems, such as on the Larsen C and Amery Ice Shelves, retain surface water at present. The underlying reasons for export versus retention remain unclear. Nonetheless our results suggest that, in a future warming climate, surface rivers could export melt off the large ice shelves surrounding Antarctica—contrary to present Antarctic ice-sheet models, which assume that meltwater is stored on the ice surface where it triggers ice-shelf disintegration.

Mapping seabed geomorphology in the Inner Hebrides, Scotland; Bathymetric records of ice streaming and retreat

NASA Astrophysics Data System (ADS)

Dove, Dayton; Finlayson, Andrew; Bradwell, Tom; Arosio, Riccardo; Howe, John

2014-05-01

Approximately 7,000 km² of new bathymetry have been stitched together with onshore airborne radar data, both gridded at 5m resolution, to map and describe the submarine glacial landscape of the Inner Hebrides sector of the former British-Irish Ice Sheet (BIIS). As part of the MAREMAP Project (http://www.maremap.ac.uk), and to build on previous work (Howe et al., 2012), we are using recently acquired swath bathymetry data, collected primarily by the UKHO Civil Hydrography Programme, to characterise the geomorphology, sea-bed sediments, and bedrock geology of the Inner Hebrides region. Mapping has revealed an extensive array of well-preserved glacigenic landforms on the seabed associated with key stages of ice flow and retreat of the BIIS following the Last Glacial Maximum. On multiple submarine rock platforms and within overdeepened troughs, diverse assemblages of glacially streamlined landforms are present, forming a geomorphic continuum between rock drumlins and mega-flutes. Superimposed streamlined bedforms indicate different phases of fast flow at the ice sheet bed, and the convergence of flow sets suggest that ice sheet flow was organised into faster flowing topographically controlled corridors. Across the region, the streamlined landforms occur within a geographically controlled zone, semi-independent of the underlying geology. This is consistent with the onset zone of the Hebrides Ice Stream, as previously postulated (Howe et al., 2012). Submarine moraine ridges are observed widely across the survey area: within sea lochs, atop rock platforms and superimposed on glacially streamlined bedforms, as well as pinned to topographic highs (i.e. islands). Some retreat patterns reveal clear glacial recession towards respective catchments, while others are more ambiguous and are the focus of ongoing work. The bathymetry data notably reveal more geomorphic evidence of glaciation than adjacent land records, thus providing the opportunity to reassess onshore mapping where clear offshore examples may provide insights into poorly understood terrestrial geological and geomorphological features. And importantly, these new data provide the opportunity to greatly improve offshore geology maps of the region, which are in increasing demand by governmental, commercial, and conservation groups.

Late Weichselian ice-sheet dynamics and deglaciation history of the northern Svalbard margin

NASA Astrophysics Data System (ADS)

Fransner, O.; Noormets, R. R. N. N.; Flink, A.; Hogan, K.; Dowdeswell, J. A.; O'Regan, M.; Jakobsson, M.

2016-12-01

The glacial evolution of the northern Svalbard margin is poorly known compared with the western margin. Gravity cores, swath bathymetric, sub-bottom acoustic and 2D airgun data are used to investigate the Late Weichselian Svalbard-Barents Ice Sheet history on the northern Svalbard margin. Prograding sequences in Kvitøya and Albertini trough mouths (TMs) indicate ice streaming to the shelf edge multiple times during the Quaternary. While Kvitøya Trough has an associated trough-mouth fan (TMF), Albertini TM is cut back into the shelf edge. Down-faulted bedrock below Albertini TM suggests larger sediment accommodation space there, explaining the absence of a TMF. The bathymetry indicates that ice flow in Albertini Trough was sourced from Duvefjorden and Albertinibukta. Exposed crystalline bedrock likely kept the two ice flows separated before merging north of Karl XII-Øya. Subglacial landforms in Rijpfjorden and Duvefjorden indicate that both fjords accommodated northward-flowing ice streams during the LGM. The deeper fjord basin and higher elongation ratios of landforms in Duvefjorden suggest a more focused and/or larger ice flow there. Easily erodible sedimentary rocks are common in Duvefjorden, which may explain different ice flow dynamics in these fjords. Kvitøya TMF is flanked by gullies, probably formed through erosive downslope gravity flows triggered by sediment-laden meltwater during early deglaciation. Glacial landforms in Albertini Trough comprise retreat-related landforms indicating slow deglaciation. Iceberg scours in Albertini Trough suggest the importance of calving for mass-loss. Sets of De Geer moraines in Rijpfjorden imply that slow, grounded retreat continued in <210 m water depth. Lack of retreat-related landforms in deeper areas of Rijpfjorden and in Duvefjorden indicates floating glacier fronts influenced by calving. 14C ages suggest that deglaciation of inner Rijpfjorden and central Duvefjorden were complete before 10,434 cal a BP and 10,779 cal a BP respectively.

Buttressing and stability of marine Ice sheets

NASA Astrophysics Data System (ADS)

Goldberg, D.; Holland, D. M.; Schoof, C.

2009-04-01

The West Antarctic Ice Sheet is marine in nature, meaning most of its base is below sea level. At the grounding line (where it becomes thin enough to float), its outlet streams flow into large ice shelves. Gravitational stress in the shelf is transmitted back to the grounding line, and largely balanced by basal friction in the transition zone. The details of this force balance control the evolution of both the thickness and grounded extent of the ice sheet, and can lead to Weertman's (1974) Marine Instability for a foredeepened bedrock (one that deepens inland). However, the presence of rigid sidewalls and locally grounded regions in the shelf can reduce the longitudinal stresses felt at the grounding line (a phenomenon called buttressing). Thomas (1979) and others pointed out that Marine Instability may be lessened or reversed by ice shelf buttressing. When modelling marine ice sheets numerically, the physics of the grounded-to-floating transition must be represented and the associated small length scales must be resolved (Schoof, 2007). Failing to do so can result in nonphysical or numerically inconsistent behavior (Vieli and Payne, 2005). While several methods have been developed to treat these issues (Vieli and Payne, 2005; Pattyn et al, 2006; Schoof, 2007) they are limited to flowline models. We present a model that represents the physics of the grounded-to-floating transition in a time-dependent three-dimensional marine ice sheet, using mesh adaption to resolve the transition zone. We show that in the special case of a two-dimensional sheet our model reproduces the theoretical results of the MISMIP experiments, and that it produces robust results when both horizontal dimensions are resolved. In idealized experiments in a channel with rigid sidewalls and a foredeepened bed, we narrow the channel to determine whether buttressing is sufficient to reverse instability. We find that for strong beds (high friction coefficients), while the timescales and dynamics are affected greatly by buttressing, true stability reversal is still not seen even as the channel becomes quite narrow. However, for weaker beds, stability is seen, though it can be shown that sufficient ocean melting can still cause collapse. Experiments with more complicated bed topography show that locally grounded areas (ice rises) have similar dynamic effects to rigid sidewalls.

Massive subtropical icebergs and freshwater forcing of climate

NASA Astrophysics Data System (ADS)

Condron, Alan; Hill, Jenna

2014-05-01

High resolution seafloor mapping shows incredible evidence that massive (>300m thick) icebergs drifted more than 5,000 km along the United States continental margin to southern Florida during the last deglaciation. Here we discuss how the discovery of icebergs in this location highlights a previously unknown ocean circulation pathway capable of transporting icebergs and meltwater from the Northern Hemisphere ice sheets directly to the subtropical North Atlantic. This pathway questions the classical idea that freshwater forcing from meltwater floods and icebergs occurred primarily over the subpolar North Atlantic (50N - 70N), with little penetration to subtropical latitudes, south of 40N. Using a sophisticated, high-resolution (1/6 deg.) ocean model, capable of resolving the circulation of the coastal ocean in detail, we show that icebergs off the coast of Florida likely calved from ice streams in the Gulf of St Lawrence and Hudson Bay. We find that icebergs can only drift south of Cape Hatteras, and overcome the northward flow of the Gulf Stream, when they are entrained in a narrow, southward-flowing, coastal meltwater flood originating from the Laurentide Ice Sheet. This cold meltwater increases iceberg survival in the warm subtropics and flows in the opposite direction to the Gulf Stream along the coast, allowing icebergs to drift to southern Florida in less than 4 months. We conclude that during the last deglaciation, icebergs drifted south in massive meltwater floods that delivered freshwater to the subtropical North Atlantic. Our findings have important implications for understanding how changes in freshwater forcing triggered past abrupt climate change.

Timing of the Northern Prince Gustav Ice Stream retreat and the deglaciation of northern James Ross Island, Antarctic Peninsula during the last glacial-interglacial transition

NASA Astrophysics Data System (ADS)

Nývlt, Daniel; Braucher, Régis; Engel, Zbyněk; Mlčoch, Bedřich

2014-09-01

The Northern Prince Gustav Ice Stream located in Prince Gustav Channel, drained the northeastern portion of the Antarctic Peninsula Ice Sheet during the last glacial maximum. Here we present a chronology of its retreat based on in situ produced cosmogenic 10Be from erratic boulders at Cape Lachman, northern James Ross Island. Schmidt hammer testing was adopted to assess the weathering state of erratic boulders in order to better interpret excess cosmogenic 10Be from cumulative periods of pre-exposure or earlier release from the glacier. The weighted mean exposure age of five boulders based on Schmidt hammer data is 12.9 ± 1.2 ka representing the beginning of the deglaciation of lower-lying areas (< 60 m a.s.l.) of the northern James Ross Island, when Northern Prince Gustav Ice Stream split from the remaining James Ross Island ice cover. This age represents the minimum age of the transition from grounded ice stream to floating ice shelf in the middle continental shelf areas of the northern Prince Gustav Channel. The remaining ice cover located at higher elevations of northern James Ross Island retreated during the early Holocene due to gradual decay of terrestrial ice and increase of equilibrium line altitude. Schmidt hammer R-values are inversely correlated with 10Be exposure ages and could be used as a proxy for exposure history of individual granite boulders in this region and favour the hypothesis of earlier release of boulders with excessive 10Be concentrations from glacier directly at this site. These data provide evidences for an earlier deglaciation of northern James Ross Island when compared with other recently presented cosmogenic nuclide based deglaciation chronologies, but this timing coincides with rapid increase of atmospheric temperature in this marginal part of Antarctica.

Towards a Copernicus Service for Monitoring Polar Ice Sheet Velocity and Discharge using Sentinel-1A and 1B SAR

NASA Astrophysics Data System (ADS)

Wuite, Jan; Nagler, Thomas; Hetzenecker, Markus; Blumthaler, Ursula; Ossowska, Joanna; Rott, Helmut

2017-04-01

The enhanced imaging capabilities of Sentinel-1A and 1B and the systematic acquisition planning of polar regions by ESA form the basis for the development and implementation of an operational system for monitoring ice dynamics and discharge of Antarctica, Greenland and other polar ice caps. Within the framework of the ESA CCI and the Austrian ASAP/FFG programs we implemented an automatic system for generation of ice velocity maps from repeat pass Sentinel-1 Terrain Observation by Progressive Scans (TOPS) mode data applying iterative offset tracking using both coherent and incoherent image cross-correlation. Greenland's margins are monitored by 6 tracks continuously since mid of 2015 with 12 days repeat observations using Sentinel-1A. With the twin satellite Sentinel-1B, launched in April 2016, the repeat acquisition period is reduced to only 6 days allowing frequent velocity retrievals - even in regions with high accumulation rates and very fast flow - and providing insight for studying short-term variations of ice flow and discharge. The Sentinel-1 ice velocity products continue the sparse coverage in time and space of previous velocity mapping efforts. The annual Greenland wide winter acquisition campaigns of 4 to 6 repeat track observations, acquired within a few weeks, provide nearly gapless and seamless ice sheet wide flow velocity maps on a yearly basis which are important for ice sheet modelling purposes and accurate mass balance assessments. An Antarctic ice sheet wide ice velocity map (with polar gap) was generated from Sentinel-1A data, acquired within 8 months, providing an important benchmark for gauging future changes in ice dynamics. For regions with significant warming continuous monitoring of ice streams with 6 to 12-day repeat intervals, exploiting both satellites, is ongoing to detect changes of ice flow as indicators of climate change. We present annual ice sheet wide velocity maps of Greenland from 2014/15 to 2016/17 and Antarctica from 2015/16 as well as dense time series of short-term velocity changes of outlet glaciers since 2014. We will highlight the improvements of the dual satellite constellation of Sentinel-1A and 1B, in particular for fast moving glaciers and regions with high accumulation rates. Derived surface velocities are combined with ice thickness from airborne Radio Echo Sounding data to compute ice discharge and its short-term variation across flux gates of major outlet glaciers in Greenland and Antarctica. Ice velocity maps, including dense time series for outlet glaciers, and ice discharge products are made available to registered users through our webtool at cryoportal.enveo.at.

Caltech water-ice dusty plasma: preliminary results

NASA Astrophysics Data System (ADS)

Bellan, Paul; Chai, Kilbyoung

2013-10-01

A water-ice dusty plasma laboratory experiment has begun operation at Caltech. As in Ref., a 1-5 watt parallel-plate 13.56 MHz rf discharge plasma has LN2-cooled electrodes that cool the neutral background gas to cryogenic temperatures. However, instead of creating water vapor by in-situ deuterium-oxygen bonding, here the neutral gas is argon and water vapor is added in a controlled fashion. Ice grains spontaneously form after a few seconds. Photography with a HeNe line filter of a sheet of HeNe laser light sheet illuminating a cross section of dust grains shows a large scale whorl pattern composed of concentric sub-whorls having wave-like spatially varying intensity. Each sub-whorl is composed of very evenly separated fine-scale stream-lines indicating that the ice grains move in self-organized lanes like automobiles on a multi-line highway. HeNe laser extinction together with an estimate of dust density from the intergrain spacing in photographs indicates a 5 micron nominal dust grain radius. HeNe laser diffraction patterns indicate the ice dust grains are large and ellipsoidal at low pressure (200 mT) but small and spheroidal at high pressure (>600 mT). Supported by USDOE.

Mind the Gap: Reconstructing the timing and consequences of the blockage of the Humber Gap by the last British-Irish Ice Sheet

NASA Astrophysics Data System (ADS)

Bateman, Mark; Evans, David; Roberts, David; Ely, Jeremy; Medialdea, Alicia; Clark, Chris

2017-04-01

The Eastern England terrestrial glacial sequences are critical to the spatial and temporal reconstruction of the last British-Irish Ice sheet (BIIS). Understanding the Humber Gap area is key as its blockage by ice created the extensive proglacial lakes. Here we use the glacial geomorphology and luminescence based chronologies from the Humber Gap region to establish the extent and thickness of the North Sea Lobe (NSL) of the BIIS. From this we establish the initial maximal ice advance occurred regionally at 21.2 ka. The NSL retreated off-shore 18 ka (Stage 2). Punctuated in stages in the south of the region whilst in the north retreat was initially rapid before a series of near synchronous ice-advances occurred at 16.8 ka (Stage 3). Full withdrawal of BIIS ice occurred prior to 15 ka (Stage 4). Geomorphic mapping and stratigraphy confirms the existence of a proto Lake Humber in Stage 1 which persisted to Stage 3 expanding eastward as the NSL ice retreated. It appears wherever during the advance and retreat of the NSL ice encountered low topography and reverse gradients proglacial lakes commonly formed. These lakes through ice draw down and associated streaming/surging may in part explain the dynamism of the parts of the NSL. The above record of ice-dammed lakes provides an analogue for now off-shore parts of the BIIS where it advanced as number of asynchronous lowland lobes.

Crevasse Patterns and Grounding Line Change Along the Siple and Gould Coasts, West Antarctica

NASA Astrophysics Data System (ADS)

Hulbe, C. L.; Fahnestock, M. A.

2003-12-01

Crevasses and strealklines observed in composite MODIS imagery of the Ross Ice Shelf have been used to infer changes in flow across the transition from ice sheet to ice shelf. We focus on changes in crevasse type and orientation as a guide to recent (100s of years) changes grounding line dynamics and location at the now-quiescent Kamb, and fast flowing Whillans and Mercer Ice Stream outlets. Across the grounding line of a rapidly flowing ice stream, the transition in the basal stress condition is slight so few (if any) crevasses are formed. In contrast, along-flow tension is relatively large across downstream no-slip/slip transitions (i.e. the downstream ends of ice rises and interstream ridges, and the current Kamb grounding line) and will produce crevasses transverse to flow. This is distinctly different from the upstream pointing orientation of crevasses that form due to shear at lateral boundaries. At a no-slip/slip grounding line that is transverse to flow, only tensional crevasses may form so the presence of other crevasse types in the ice stream effluent, or the transition from one type to another, indicates a change in flow style. The Kamb Ice Stream grounding line is now generating transverse crevasses while most of the Mercer/Whillans ice plain grounding line is not. The southern end of the current Kamb grounding line was established as a no-slip/slip boundary sometime after Steershead became an ice rise, as evidenced by the change from shear crevasses to tension crevasses about 20 km downstream from its present location. At the northern end of the grounding line, the first tensional crevasses are only a few km downstream from its present location. If, as seems likely, ice stream deceleration coincided with the transition from a Mercer/Whillans type grounding zone to a no-slip/slip grounding line, then the oldest tensional crevasses should have advected about 1.5 km downstream (the present speed is ~10 m/a and the stream shut down ~150 years ago). The observed and computed advection distances are similar at the northern end of the Kamb grounding line, but crevasses are an order of magnitude too far downstream at its southern end. Previously measured grounding line retreat of ~30 m/a (Thomas and others,1988) in combination with downstream advection of crevasses still cannot account for the change in crevasse style at the southern edge of the ice stream. The implication is that the grounding line was substantially seaward of its present location several hundred years ago and that it has retreated rapidly since that time.

Greenland Ice Sheet Monitoring Network (GLISN): Contributions to Science and Society

NASA Astrophysics Data System (ADS)

Anderson, K. R.; Bonaime, S.; Clinton, J. F.; Dahl-Jensen, T.; Debski, W. M.; Giardini, D.; Govoni, A.; Kanao, M.; Larsen, T. B.; Lasocki, S.; Lee, W. S.; McCormack, D. A.; Mykkeltveit, S.; Nettles, M.; Stutzmann, E.; Strollo, A.; Sweet, J. R.; Tsuboi, S.; Vallee, M.

2017-12-01

The Greenland Ice Sheet Monitoring Network (GLISN) is a broadband, multi-use seismological network, enhanced by selected geodetic observations, designed with the capability to allow researchers to understand the changes currently occurring in the Arctic, and with the operational characteristics necessary to enable response to those changes as understanding improves. GLISN was established through an international collaboration, with 10 nations coordinating their efforts to develop the current 34-station observing network during the last eight years. All of the data collected are freely and openly available in near-real time. The network was designed to transform the community capability for recording, analysis, and interpretation of seismic signals generated by discrete events in Greenland and the Arctic, as well as those traversing the region. Data from the network support a wide range of uses, including estimation of the properties of the solid Earth that control isostatic adjustment rates and set key boundary conditions for ice-sheet evolution; analysis of tectonic earthquakes throughout Greenland and the Arctic; study of the seismic signals associated with large calving events and changing glacier dynamics; and variations in ice and snow properties within the Greenland Ice Sheet. Recordings from the network have also provided invaluable data for rapid evaluation and understanding of the devastating landslide and tsunami that occurred near Nuugaatsiaq, Greenland, in June, 2017. The GLISN strategy of maximizing data quality from a network of approximately evenly distributed stations, delivering data in near-real time, and archiving a continuous data stream easily accessible to researchers, allows continuous discovery of new uses while also facilitating the generation of data products, such as catalogs of tectonic and glacial earthquakes and GPS-based estimates of snow height, that allow for assessment of change over time.

Greenland ice sheet outlet glacier front changes: comparison of year 2008 with past years

NASA Astrophysics Data System (ADS)

Decker, D. E.; Box, J.; Benson, R.

2008-12-01

NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) imagery are used to calculate inter-annual, end of summer, glacier front area changes at 10 major Greenland ice sheet outlets over the 2000-2008 period. To put the recent 8 end of summer net annual changes into a longer perspective, glacier front position information from the past century are also incorporated. The largest MODIS-era area changes are losses/retreats; found at the relatively large Petermann Gletscher, Zachariae Isstrom, and Jakobshavn Isbrae. The 2007-2008 net ice area losses were 63.4 sq. km, 21.5 sq. km, and 10.9 sq. km, respectively. Of the 10 largest Greenland glaciers surveyed, the total net cumulative area change from end of summer 2000 to 2008 is -536.6 sq km, that is, an area loss equivalent with 6.1 times the area of Manhattan Is. (87.5 sq km) in New York, USA. Ice front advances are evident in 2008; also at relatively large and productive (in terms of ice discharge) glaciers of Helheim (5.7 sq km), Store Gletscher (4.9 sq km), and Kangerdlugssuaq (3.4 sq km). The largest retreat in the 2000-2008 period was 54.2 sq km at Jakobshavn Isbrae between 2002 and 2003; associated with a floating tongue disintegration following a retreat that began in 2001 and has been associated with thinning until floatation is reached; followed by irreversible collapse. The Zachariae Isstrom pro-glacial floating ice shelf loss in 2008 appears to be part of an average ~20 sq km per year disintegration trend; with the exception of the year 2006 (6.2 sq km) advance. If the Zachariae Isstrom retreat continues, we are concerned the largest ice sheet ice stream that empties into Zachariae Isstrom will accelerate, the ice stream front freed of damming back stress, increasing the ice sheet mass budget deficit in ways that are poorly understood and could be surprisingly large. By approximating the width of the surveyed glacier frontal zones, we determine and present effective glacier normalized length (L') changes that also will be presented at the meeting. The narrow Ingia Isbrae advanced in L' the most in 2006-2007 by 9.2 km. Jakobshavn decreased in L' the most in 2002-2003 by 8.0 km. Petermann decreased in length the most in 2000-2001, that is, L' = -5.3 km and again by L' = -3.9 km in 2007-2008. Helheim Gl. retreated in 2004-2005 by L' = -4.6 km and advanced 2005-2006 by L' = 4.4 km. The 10 glacier average L' change from end of summer 2000 end of summer 2008 was 0.6 km. Results from a growing list of glaciers will be presented. We attempt to interpret the observed glacier changes using glaciological theory and regional climate observations.

West Antarctic Balance Fluxes: Impact of Smoothing, Algorithm and Topography.

NASA Astrophysics Data System (ADS)

Le Brocq, A.; Payne, A. J.; Siegert, M. J.; Bamber, J. L.

2004-12-01

Grid-based calculations of balance flux and velocity have been widely used to understand the large-scale dynamics of ice masses and as indicators of their state of balance. This research investigates a number of issues relating to their calculation for the West Antarctic Ice Sheet (see below for further details): 1) different topography smoothing techniques; 2) different grid based flow-apportioning algorithms; 3) the source of the flow direction, whether from smoothed topography, or smoothed gravitational driving stress; 4) different flux routing techniques and 5) the impact of different topographic datasets. The different algorithms described below lead to significant differences in both ice stream margins and values of fluxes within them. This encourages caution in the use of grid-based balance flux/velocity distributions and values, especially when considering the state of balance of individual ice streams. 1) Most previous calculations have used the same numerical scheme (Budd and Warner, 1996) applied to a smoothed topography in order to incorporate the longitudinal stresses that smooth ice flow. There are two options to consider when smoothing the topography, the size of the averaging filter and the shape of the averaging function. However, this is not a physically-based approach to incorporating smoothed ice flow and also introduces significant flow artefacts when using a variable weighting function. 2) Different algorithms to apportion flow are investigated; using 4 or 8 neighbours, and apportioning flow to all down-slope cells or only 2 (based on derived flow direction). 3) A theoretically more acceptable approach of incorporating smoothed ice flow is to use the smoothed gravitational driving stress in x and y components to derive a flow direction. The flux can then be apportioned using the flow direction approach used above. 4) The original scheme (Budd and Warner, 1996) uses an elevation sort technique to calculate the balance flux contribution from all cells to each individual cell. However, elevation sort is only successful when ice cannot flow uphill. Other possible techniques include using a recursive call for each neighbour or using a sparse matrix solution. 5) Two digital elevation models are used as input data, which have significant differences in coastal and mountainous areas and therefore lead to different calculations. Of particular interest is the difference in the Rutford Ice Stream/Carlson Inlet and Kamb Ice Stream (Ice Stream C) fluxes.

Effect of subglacial volcanism on changes in the West Antarctic Ice Sheet

NASA Technical Reports Server (NTRS)

Behrendt, John C.

1993-01-01

Rapid changes in the West Antarctic Ice Sheet (WAIS) may affect future global sea-level changes. Alley and Whillans note that 'the water responsible for separating the glacier from its bed is produced by frictional dissipation and geothermal heat,' but assume that changes in geothermal flux would ordinarily be expected to have slower effects than glaciological parameters. I suggest that episodic subglacial volcanism and geothermal heating may have significantly greater effects on the WAIS than is generally appreciated. The WAIS flows through the active, largely asiesmic West Antarctic rift system (WS), which defines the sub-sea-level bed of the glacier. Various lines of evidence summarized in Behrendt et al. (1991) indicate high heat flow and shallow asthenosphere beneath the extended, weak lithosphere underlying the WS and the WAIS. Behrendt and Cooper suggest a possible synergistic relation between Cenozoic tectonism, episodic mountain uplift and volcanism in the West Antarctic rift system, and the waxing and waning of the Antarctic ice sheet beginning about earliest Oligocene time. A few active volcanoes and late-Cenozoic volcanic rocks are exposed throughout the WS along both flanks, and geophysical data suggest their presence beneath the WAIS. No part of the rift system can be considered inactive. I propose that subglacial volcanic eruptions and ice flow across areas of locally (episodically?) high heat flow--including volcanically active areas--should be considered possibly to have a forcing effect on the thermal regime resulting in increased melting at the base of the ice streams.

Accuracy improvement of the ice flow rate measurements on Antarctic ice sheet by DInSAR method

NASA Astrophysics Data System (ADS)

Shiramizu, Kaoru; Doi, Koichiro; Aoyama, Yuichi

2015-04-01

DInSAR (Differential Interferometric Synthetic Aperture Radar) is an effective tool to measure the flow rate of slow flowing ice streams on Antarctic ice sheet with high resolution. In the flow rate measurement by DInSAR method, we use Digital Elevation Model (DEM) at two times in the estimating process. At first, we use it to remove topographic fringes from InSAR images. And then, it is used to project obtained displacements along Line-Of-Sight (LOS) direction to the actual flow direction. ASTER-GDEM widely-used for InSAR prosessing of the data of polar region has a lot of errors especially in the inland ice sheet area. Thus the errors yield irregular flow rates and directions. Therefore, quality of DEM has a substantial influence on the ice flow rate measurement. In this study, we created a new DEM (resolution 10m; hereinafter referred to as PRISM-DEM) based on ALOS/PRISM images, and compared PRISM-DEM and ASTER-GDEM. The study area is around Skallen, 90km south from Syowa Station, in the southern part of Sôya Coast, East Antarctica. For making DInSAR images, we used ALOS/PALSAR data of 13 pairs (Path633, Row 571-572), observed during the period from November 23, 2007 through January 16, 2011. PRISM-DEM covering the PALSAR scene was created from nadir and backward view images of ALOS/PRISM (Observation date: 2009/1/18) by applying stereo processing with a digital mapping equipment, and then the automatically created a primary DEM was corrected manually to make a final DEM. The number of irregular values of actual ice flow rate was reduced by applying PRISM-DEM compared with that by applying ASTER-GDEM. Additionally, an averaged displacement of approximately 0.5cm was obtained by applying PRISM-DEM over outcrop area, where no crustal displacement considered to occur during the recurrence period of ALOS/PALSAR (46days), while an averaged displacement of approximately 1.65 cm was observed by applying ASTER-GDEM. Since displacements over outcrop area are considered to be apparent ones, the average could be a measure of flow rate estimation accuracy by DInSAR. Therefore, it is concluded that the accuracy of the ice flow rate measurement can be improved by using PRISM-DEM. In this presentation, we will show the results of the estimated flow rate of ice streams in the region of interest, and discuss the additional accuracy improvement of this method.

How might the North American ice sheet influence the northwestern Eurasian climate?

NASA Astrophysics Data System (ADS)

Beghin, P.; Charbit, S.; Dumas, C.; Kageyama, M.; Ritz, C.

2015-10-01

It is now widely acknowledged that past Northern Hemisphere ice sheets covering Canada and northern Europe at the Last Glacial Maximum (LGM) exerted a strong influence on climate by causing changes in atmospheric and oceanic circulations. In turn, these changes may have impacted the development of the ice sheets themselves through a combination of different feedback mechanisms. The present study is designed to investigate the potential impact of the North American ice sheet on the surface mass balance (SMB) of the Eurasian ice sheet driven by simulated changes in the past glacial atmospheric circulation. Using the LMDZ5 atmospheric circulation model, we carried out 12 experiments under constant LGM conditions for insolation, greenhouse gases and ocean. In these experiments, the Eurasian ice sheet is removed. The 12 experiments differ in the North American ice-sheet topography, ranging from a white and flat (present-day topography) ice sheet to a full-size LGM ice sheet. This experimental design allows the albedo and the topographic impacts of the North American ice sheet onto the climate to be disentangled. The results are compared to our baseline experiment where both the North American and the Eurasian ice sheets have been removed. In summer, the sole albedo effect of the American ice sheet modifies the pattern of planetary waves with respect to the no-ice-sheet case, resulting in a cooling of the northwestern Eurasian region. By contrast, the atmospheric circulation changes induced by the topography of the North American ice sheet lead to a strong decrease of this cooling. In winter, the Scandinavian and the Barents-Kara regions respond differently to the American ice-sheet albedo effect: in response to atmospheric circulation changes, Scandinavia becomes warmer and total precipitation is more abundant, whereas the Barents-Kara area becomes cooler with a decrease of convective processes, causing a decrease of total precipitation. The gradual increase of the altitude of the American ice sheet leads to less total precipitation and snowfall and to colder temperatures over both the Scandinavian and the Barents and Kara sea sectors. We then compute the resulting annual surface mass balance over the Fennoscandian region from the simulated temperature and precipitation fields used to force an ice-sheet model. It clearly appears that the SMB is dominated by the ablation signal. In response to the summer cooling induced by the American ice-sheet albedo, high positive SMB values are obtained over the Eurasian region, leading thus to the growth of an ice sheet. On the contrary, the gradual increase of the American ice-sheet altitude induces more ablation over the Eurasian sector, hence limiting the growth of Fennoscandia. To test the robustness of our results with respect to the Eurasian ice sheet state, we carried out two additional LMDZ experiments with new boundary conditions involving both the American (flat or full LGM) and high Eurasian ice sheets. The most striking result is that the Eurasian ice sheet is maintained under full-LGM North American ice-sheet conditions, but loses ~ 10 % of its mass compared to the case in which the North American ice sheet is flat. These new findings qualitatively confirm the conclusions from our first series of experiments and suggest that the development of the Eurasian ice sheet may have been slowed down by the growth of the American ice sheet, offering thereby a new understanding of the evolution of Northern Hemisphere ice sheets throughout glacial-interglacial cycles.

Meltwater pulse recorded in Last Interglacial mollusk shells from Bermuda

NASA Astrophysics Data System (ADS)

Winkelstern, Ian Z.; Rowe, Mark P.; Lohmann, Kyger C.; Defliese, William F.; Petersen, Sierra V.; Brewer, Aaron W.

2017-02-01

The warm climate of Bermuda today is modulated by the nearby presence of the Gulf Stream current. However, iceberg scours in the Florida Strait and the presence of ice-rafted debris in Bermuda Rise sediments indicate that, during the last deglaciation, icebergs discharged from the Laurentide Ice Sheet traveled as far south as subtropical latitudes. We present evidence that an event of similar magnitude affected the subtropics during the Last Interglacial, potentially due to melting of the Greenland Ice Sheet. Using the clumped isotope paleothermometer, we found temperatures 10°C colder and seawater δ18O values 2‰ lower than modern in Last Interglacial Cittarium pica shells from Grape Bay, Bermuda. In contrast, Last Interglacial shells from Rocky Bay, Bermuda, record temperatures only slightly colder and seawater δ18O values similar to modern, likely representing more typical Last Interglacial conditions in Bermuda outside of a meltwater event. The significantly colder ocean temperatures observed in Grape Bay samples illustrate the extreme sensitivity of Bermudian climate to broad-scale ocean circulation changes. They indicate routine meltwater transport in the North Atlantic to near-equatorial latitudes, which would likely have resulted in disruption of the Atlantic Meridional Overturning Circulation. These data demonstrate that future melting of the Greenland Ice Sheet, a potential source of the Last Interglacial meltwater event, could have dramatic climate effects outside of the high latitudes.

Ice streaming in western Scotland and the deglaciation of the Hebrides Shelf and Firth of Lorn

NASA Astrophysics Data System (ADS)

Arosio, Riccardo; Howe, John; O'Cofaigh, Colm; Crocket, Kirsty

2014-05-01

Previously, numerous studies have been undertaken both onshore and offshore to decipher the morphological and sedimentological record in order to better constrain the limits and duration of the British-Irish Ice Sheet (BIIS) (Ballantyne et al. 2009, Bradwell et al. 2008b, Clark et al. 2011, Dunlop et al. 2010, Howe et al. 2012, O'Cofaigh et al., 2012). Late glacial ice sheet dynamics have been revealed to be far more rapid and responsive to climatic amelioration than had previously been considered. Notable in this debate has been the evidence that has been obtained in the inshore and, to a lesser extent, offshore on the UK continental shelf. Here new geomorphological data, principally multibeam echo sounder (MBES) data has provided imagery of previously unseen features interpreted as being glacial in origin. In the wake of these new discoveries this projects aims to investigate the extent, timing, growth and final disintegration of the BIIS across Western Scotland. This area of particular interest for the development of the glaciated North Atlantic margin has been generally neglected in past studies, especially across the mid-outer shelf, which constitutes a missing part in the jigsaw of the reconstructed BIIS during the last ~20.000yrs. We aim to mainly focus on geomorphological analyses of MBES data collected in the Firth of Lorn and Sea of Hebrides; a study of features as moraines, glacial lineations and drumlins will provide important clues on the dynamics and maximum extension of the sheet. Subsequently we will examine the geometry and composition of the shelf sediment infill, aiming to constrain the influence of ice retreat on depositional environments using multi-element geochemical (Pb-isotopes ratios, 14C and OSL dating) and sedimentological techniques. Such an investigation will also give retrospective information on the sources for these sediments, hence more indications on ice configuration. Ultimately we aim to provide a model of deglaciation for the western sector of the BIIS. Keywords: British-Irish Ice Sheet, NW Scotland, glacial bedforms, geochronology References Ballantyne, C.K., Schnabel, C. & Xu, S. 2009. Readvance of the last British Ice Sheet during Greenland Interstade (GI-1): the Wester Ross Readvance, NW Scotland. Quaternary Science Reviews, 28, 783-789 Bradwell, T., Fabel, D., Stoker, M., Mathers, H., McHargue, L., Howe, J., 2008b. Ice caps existed throughout the Late glacial interstadial in northern Scotland. Journal of Quaternary Science 23, 401-407. Clark, C.D., Hughes, A.L.C., Greenwood, S.L., Jordan, C., Sejrup, H.P. 2012. Pattern and timing of retreat of the last British-Irish Ice Sheet. Quaternary Science Reviews. Dunlop, P., Shannon, R., McCabe, M., Quinn, R., Doyle, E. 2010. Marine geophysical evidence for ice sheet extension and recession on the Malin Shelf: New evidence for the western limits of the British-Irish Ice Sheet. Marine Geology, 276: 86-99. Howe, J. A., Dove, D., Bradwell, T. & Gaferia, J. 2012. Submarine geomorphology and glacial history of the Sea of the Hebrides, UK. Marine Geology 315-318, 64-78 O' Cofaigh, C., Dunlop, P. Benetti, S., 2012. Marine geophysical evidence for Late Pleistocene ice sheet extent and recession off northwest Ireland, Quaternary Science Reviews. In press.

Evidence against a late Wisconsinan ice shelf in the Gulf of Maine

USGS Publications Warehouse

Oldale, R.N.; Williams, R.S.; Colman, Steven M.

1990-01-01

Proposals for the formation of a late Wisconsinan ice shelf in the Gulf of Maine during the retreat of the Laurentide Ice Sheet are considered to be inappropriate. An Antarctic-type ice shelf does not fit the field data that indicate temperate glacial, terrestrial, and marine climates for the region between 18 ka and 12 ka. A temperate ice shelf has no modern analogues and may be physically impossible. The preponderance of stratified drift in the Gulf of Maine region supports temperate climates during late Wisconsinan time. It also indicates that glacial meltwater, rather than ice in either an ice sheet or ice shelf, was the primary transport mechanism of glacial sediment and the source for the glaciomarine mud. For these reasons we have proposed glacial analogues for the deglaciation of the Gulf of Maine that consist of temperate or subpolar marine-based glaciers, characterized by depositional environments dominated by meltwater discharge directly to the sea or the sea by way of subaerial meltwater streams. These analogues include Alaskan fjord glaciers, glaciers on the Alaskan continental shelf that discharged meltwater directly into the sea in the not too distant past, and Austfonna (Nordaustandet, Svalbard, Norway) that is presently discharging meltwater in the sea along a grounded ice wall. This last example is the best modern-day analogue for the depositional environment for most of the glaciomarine mud in the Gulf of Maine and deglaciation of the Gulf. 

Shallow-source aeromagnetic anomalies observed over the West Antarctic Ice Sheet compared with coincident bed topography from radar ice sounding - New evidence for glacial "removal" of subglacially erupted late Cenozoic rift-related volcanic edifices

USGS Publications Warehouse

Behrendt, John C.; Blankenship, D.D.; Morse, D.L.; Bell, R.E.

2004-01-01

Aeromagnetic and radar ice sounding results from the 1991-1997 Central West Antarctica (CWA) aerogeophysical survey over part of the West Antarctic Ice Sheet (WAIS) and subglacial area of the volcanically active West Antarctic rift system have enabled detailed examination of specific anomaly sources. These anomalies, previously interpreted as caused by late Cenozoic subglacial volcanic centers, are compared to newly available glacial bed-elevation data from the radar ice sounding compilation of the entire area of the aeromagnetic survey to test this hypothesis in detail. We examined about 1000 shallow-source magnetic anomalies for bedrock topographic expression. Using very conservative criteria, we found over 400 specific anomalies which correlate with bed topography directly beneath each anomaly. We interpret these anomalies as indicative of the relative abundance of volcanic anomalies having shallow magnetic sources. Of course, deeper source magnetic anomalies are present, but these have longer wavelengths, lower gradients and mostly lower amplitudes from those caused by the highly magnetic late Cenozoic volcanic centers. The great bulk of these >400 (40-1200-nT) anomaly sources at the base of the ice have low bed relief (60-600 m, with about 80%10 million years ago. Eighteen of the anomalies examined, about half concentrated in the area of the WAIS divide, have high-topographic expression (as great as 400 m above sea level) and high bed relief (up to 1500 m). All of these high-topography anomaly sources at the base of the ice would isostatically rebound to elevations above sea level were the ice removed. We interpret these 18 anomaly sources as evidence of subaerial eruption of volcanoes whose topography was protected from erosion by competent volcanic flows similar to prominent volcanic peaks that are exposed above the surface of the WAIS. Further, we infer these volcanoes as possibly erupted at a time when the WAIS was absent. In contrast, at the other extreme, there are a number of shallow-source, volcanic appearing magnetic anomalies overlying the very smooth bed topography in the survey area beneath Ice Stream D (Bindshadler Ice Stream); the glacial bed probably comprises a very thin layer of unconsolidated sediments (till). Probably, the volcanic edifices here were removed at a more rapid rate because of fast glacial flow. A few of the very shallow-source "volcanic" anomalies overlie the ice shelf just downstream of the grounding line of Ice Stream D, suggesting a causal relationship, if the volcanism is recent. ?? 2004 Elsevier B.V. All rights reserved.

Arctic Amplification and the Northward shift of a new Greenland melting record

NASA Astrophysics Data System (ADS)

Tedesco, Marco; Mote, Thomas; Fettweis, Xavier; Hanna, Edward; Booth, James; Jeyaratnam, Jeyavinoth; Datta, Rajashree; Briggs, Kate

2016-04-01

Large-scale atmospheric circulation controls the mass and energy balance of the Greenland ice sheet through its impact on radiative budget, runoff and accumulation. Using reanalysis data and the outputs of a regional climate model, here we show that the persistence of an exceptional atmospheric ridge, centred over the Arctic Ocean was responsible for a northward shift of surface melting records over Greenland, and for increased accumulation in the south during the summer of 2015. Concurrently, new records of mean monthly zonal winds at 500 hPa and of the maximum latitude of ridge peaks of the 5700±50 m isohypse over the Arctic were also set. An unprecedented (1948 - 2015) and sustained jet stream easterly flow promoted enhanced runoff, increased surface temperatures and decreased albedo in northern Greenland, while inhibiting melting in the south. The exceptional 2015 summer Arctic atmospheric conditions are consistent with the anticipated effects of Arctic Amplification, including slower zonal winds and increased jet stream wave amplitude. Properly addressing the impact of Arctic Amplification on surface runoff of the Greenland ice sheet is crucial for rigorously quantifying its contribution to current and future sea level rise, and the relative impact of freshwater discharge on the surrounding ocean.

Post-LGM grounding line and calving front translations of the West Antarctic Ice Sheet in the Whales Deep paleo-ice-stream trough, eastern Ross Sea, Antarctica

NASA Astrophysics Data System (ADS)

McGlannan, A. J.; Bart, P. J.; Chow, J.

2016-12-01

A large-area (2500 km2) multibeam survey of the Whales Deep paleo-ice-stream trough, eastern Ross Sea, Antarctica was acquired during NBP1502B. This sector of the continental shelf is important as it was covered by grounded and floating ice, which drained the central part of an expanded West Antarctic Ice Sheet (WAIS) during the last glacial cycle. The seafloor geomorphology shows a well-defined cluster of four back stepping grounding zone wedges (GZWs) that were deposited in a partly overlapping fashion on the middle continental shelf during WAIS retreat. These observations permit two end-member possibilities for how the WAIS grounding line and calving front vacated the trough. In the first scenario, each GZW represents successive landward shifts of the grounding line and calving front. In the second scenario, each GZW represents a large-scale retreat and re-advance of grounded and floating ice. To determine which of these two end-member scenarios most accurately describes WAIS retreat from this sector of Ross Sea, we evaluated a grid of kasten and piston cores. The core stations were selected on the basis of backstepping GZWs along the trough axis. Our core data analyses included an integration of visual core descriptions, x-ray images, grain size, water content, total organic carbon, shear strengths, and diatom assemblage data. Core data reveal a single transgressive succession from proximal diamict overlain by sub-ice-shelf and/or open-marine sediments. These data strongly support the first scenario, suggesting that an ice shelf remained continuously intact during the time that the grounding line successively moved from the shelf edge to the middle shelf by small-scale landward translations until the end of the fourth grounding event. Sedimentologic and diatom-assemblage data from the inner shelf show that only the last middle shelf grounding event ended with a long-distance retreat of grounded and then floating ice to south of the modern calving front.

Modeling the heating and melting of sea ice through light absorption by microalgae

NASA Astrophysics Data System (ADS)

Zeebe, Richard E.; Eicken, Hajo; Robinson, Dale H.; Wolf-Gladrow, Dieter; Dieckmann, Gerhard S.

1996-01-01

In sea ice of polar regions, high concentrations of microalgae are observed during the spring. Algal standing stocks may attain peak values of over 300 mg chl a m-2 in the congelation ice habitat. As of yet, the effect of additional heating of sea ice through conversion of solar radiation into heat by algae has not been investigated in detail. Local effects, such as a decrease in albedo, increasing melt rates, and a decrease of the physical strength of ice sheets may occur. To investigate the effects of microalgae on the thermal regime of sea ice, a time-dependent, one-dimensional thermodynamic model of sea ice was coupled to a bio-optical model. A spectral one-stream model was employed to determine spectral attenuation by snow, sea ice, and microalgae. Beer's law was assumed to hold for every wavelength. Energy absorption was obtained by calculating the divergence of irradiance in every layer of the model (Δz = 1 cm). Changes in sea ice temperature profiles were calculated by solving the heat conduction equation with a finite difference scheme. Model results indicate that when algal biomass is concentrated at the bottom of congelation ice, melting of ice resulting from the additional conversion of solar radiation into heat may effectively destroy the algal habitat, thereby releasing algal biomass into the water column. An algal layer located in the top of the ice sheet induced a significant increase in sea ice temperature (ΔT > 0.3 K) for snow depths less than 5 cm and algal standing stocks higher than 150 mg chl a m-2. Furthermore, under these conditions, brine volume increased by 21% from 181 to 219 parts per thousand, which decreased the physical strength of the ice.

Validation of Modelled Ice Dynamics of the Greenland Ice Sheet using Historical Forcing

NASA Astrophysics Data System (ADS)

Hoffman, M. J.; Price, S. F.; Howat, I. M.; Bonin, J. A.; Chambers, D. P.; Tezaur, I.; Kennedy, J. H.; Lenaerts, J.; Lipscomb, W. H.; Neumann, T.; Nowicki, S.; Perego, M.; Saba, J. L.; Salinger, A.; Guerber, J. R.

2015-12-01

Although ice sheet models are used for sea level rise projections, the degree to which these models have been validated by observations is fairly limited, due in part to the limited duration of the satellite observation era and the long adjustment time scales of ice sheets. Here we describe a validation framework for the Greenland Ice Sheet applied to the Community Ice Sheet Model by forcing the model annually with flux anomalies at the major outlet glaciers (Enderlin et al., 2014, observed from Landsat/ASTER/Operation IceBridge) and surface mass balance (van Angelen et al., 2013, calculated from RACMO2) for the period 1991-2012. The ice sheet model output is compared to ice surface elevation observations from ICESat and ice sheet mass change observations from GRACE. Early results show promise for assessing the performance of different model configurations. Additionally, we explore the effect of ice sheet model resolution on validation skill.

Antarctic Glaciological Data at NSIDC: field data, temperature, and ice velocity

NASA Astrophysics Data System (ADS)

Bauer, R.; Bohlander, J.; Scambos, T.; Berthier, E.; Raup, B.; Scharfen, G.

2003-12-01

An extensive collection of many Antarctic glaciological parameters is available for the polar science community upon request. The National Science Foundation's Office of Polar Programs funds the Antarctic Glaciological Data Center (AGDC) at the National Snow and Ice Data Center (NSIDC) to archive and distribute Antarctic glaciological and cryospheric system data collected by the U.S. Antarctic Program. AGDC facilitates data exchange among Principal Investigators, preserves recently collected data useful to future research, gathers data sets from past research, and compiles continent-wide information useful for modeling and field work planning. Data sets are available via our web site, http://nsidc.org/agdc/. From here, users can access extensive documentation, citation information, locator maps, derived images and references, and the numerical data. More than 50 Antarctic scientists have contributed data to the archive. Among the compiled products distributed by AGDC are VELMAP and THERMAP. THERMAP is a compilation of over 600 shallow firn temperature measurements ('10-meter temperatures') collected since 1950. These data provide a record of mean annual temperature, and potentially hold a record of climate change on the continent. The data are represented with maps showing the traverse route, and include data sources, measurement technique, and additional measurements made at each site, i.e., snow density and accumulation. VELMAP is an archive of surface ice velocity measurements for the Antarctic Ice Sheet. The primary objective of VELMAP is to assemble a historic record of outlet glaciers and ice shelf ice motion over the Antarctic. The collection includes both PI-contributed measurements and data generated at NSIDC using Landsat and SPOT satellite imagery. Tabular data contain position, speed, bearing, and data quality information, and related references. Two new VELMAP data sets are highlighted: the Mertz Glacier and the Institute Ice Stream. Mertz Glacier ice velocity provides an upper limit for change in velocity for this glacier over the past decade. Two pairs of Landsat images were used to compare velocities from 2000-2001 to 1989-2000. No significant change in velocity is observed. A new ice discharge flux of 17.8 km3a-1 was determined, and basal melting at the grounding line was re-calculated at 11 m per year (Berthier et al., 2003, in press). Velocity data for the Institute Ice Stream was compiled at NSIDC using a Landsat images from 1986, 1989, and 1997. The data were recently used in a study outlining the velocity, mass balance, and morphology of the Institute ice stream and nearby Ronne ice shelf area. (Scambos et al., 2003, in review). The study indicates the Institute has regions with flow and morphology characteristics similar to the Ross Embayment ice streams. Ice velocity research contributes to understanding the mass balance and overall stability of the Antarctic Ice Sheet. The archiving of velocity data has proven to be a useful tool to the Antarctic science community, and VELMAP continues to grow as a valuable resource through PI contributions. If you have velocity data that you would like to contribute to the VELMAP archive please contact agdc@nsidc.org. The velocity data used in the two studies presented here can be accessed on the VELMAP web site at http://nsidc.org/data/velmap.

An Imaging System capable of monitoring en-glacial and sub-glacial processes of glaciers, streaming ice and ice margins

NASA Astrophysics Data System (ADS)

Frearson, N.

2012-12-01

Columbia University in New York is developing a geophysical instrumentation package that is capable of monitoring dynamic en-glacial and sub-glacial processes. The instruments include a Riegl Scanning Laser for precise measurements of the ice surface elevation, Stereo photogrammetry from a high sensitivity (~20mK) Infra-Red camera and a high resolution Visible Imaging camera (2456 x 2058 pixels) to document fine scale ice temperature changes and surface features, near surface ice penetrating radar and an ice depth measuring radar that can be used to study interior and basal processes of ice shelves, glaciers, ice streams and ice-sheets. All instrument data sets will be time-tagged and geo-referenced using precision GPS satellite data. Aircraft orientation will be corrected using inertial measurement technology integrated into the pod. This instrumentation will be flown across some of the planets largest outlet glaciers in Antarctica and Greenland. However, a key aspect of the design is that at the conclusion of the program, the Pod, Deployment Arm, Data Acquisition and Power and Environmental Management system will become available for use by the science community at large to install their own instruments onto. It will also be possible to mount the Icepod onto other airframes. The sensor system will become part of a research facility operated for the science community, and data will be maintained at and made available through a Polar Data Center.

Implications of Grain Size Evolution for the Effective Stress Exponent in Ice

NASA Astrophysics Data System (ADS)

Behn, M. D.; Goldsby, D. L.; Hirth, G.

2016-12-01

Viscous flow in ice has typically been described by the Glen law—a non-Newtonian, power-law relationship between stress and strain-rate with a stress exponent n 3. The Glen law is attributed to grain-size-insensitive dislocation creep; however, laboratory and field studies demonstrate that deformation in ice is strongly dependent on grain size. This has led to the hypothesis that at sufficiently low stresses, ice flow is controlled by grain boundary sliding [1], which explicitly incorporates the grain-size dependence of ice rheology. Yet, neither dislocation creep (n 4), nor grain boundary sliding (n 1.8), have stress exponents that match the value of n 3 for the Glen law. Thus, although the Glen law provides an approximate description of ice flow in glaciers and ice sheets, its functional form cannot be explained by a single deformation mechanism. Here we seek to understand the origin of the n 3 dependence of the Glen law through a new model for grain-size evolution in ice. In our model, grain size evolves in response to the balance between dynamic recrystallization and grain growth. To simulate these processes we adapt the "wattmeter" [2], originally developed within the solid-Earth community to quantify grain size in crustal and mantle rocks. The wattmeter posits that grain size is controlled by a balance between the mechanical work required for grain growth and dynamic grain size reduction. The evolution of grain size in turn controls the relative contributions of dislocation creep and grain boundary sliding, and thus the effective stress exponent for ice flow. Using this approach, we first benchmark our grain size evolution model on experimental data and then calculate grain size in two end-member scenarios: (1) as a function of depth within an ice-sheet, and (2) across an ice-stream margin. We show that the calculated grain sizes match ice core observations for the interior of ice sheets. Furthermore, owing to the influence of grain size on strain rate, the variation in grain size with deformation conditions results in an effective stress exponent intermediate between grain boundary sliding and dislocation creep. [1] Goldsby & Kohlstedt, JGR, 2001; [2] Austin & Evans, Geology, 1997

Modes of supraglacial lake drainage and dynamic ice sheet response

NASA Astrophysics Data System (ADS)

Das, S. B.; Behn, M. D.; Joughin, I. R.

2011-12-01

We investigate modes of supraglacial lake drainage using geophysical, ground, and remote sensing observations over the western margin of the Greenland ice sheet. Lakes exhibit a characteristic life cycle defined by a pre-drainage, drainage, and post-drainage phase. In the pre-drainage phase winter snow fills pre-existing cracks and stream channels, efficiently blocking past drainage conduits. As temperatures increase in the spring, surface melting commences, initially saturating the snow pack and subsequently forming a surface network of streams that fills the lake basins. Basins continue to fill until lake drainage commences, which for individual lakes occurs at different times depending on the previous winter snow accumulation and summer temperatures. Three styles of drainage behavior have been observed: (1) no drainage, (2) slow drainage over the side into an adjacent pre-existing crack, and (3) rapid drainage through a new crack formed beneath the lake basin. Moreover, from year-to-year individual lakes exhibit different drainage behaviors. Lakes that drain slowly often utilize the same outflow channel for multiple years, creating dramatic canyons in the ice. Ultimately, these surface channels are advected out of the lake basin and a new channel forms. In the post-drainage phase, melt water continues to access the bed typically through a small conduit (e.g. moulin) formed near a local topographic minimum along the main drainage crack, draining the lake catchment throughout the remainder of the melt season. This melt water input to the bed leads to continued basal lubrication and enhanced ice flow compared to background velocities. Lakes that do not completely drain freeze over to form a surface ice layer that persists into the following year. Our results show that supraglacial lakes show a spectrum of drainage behaviors and that these styles of drainage lead to varying rates and timing of surface meltwater delivery to the bed resulting in different dynamic ice responses.

Last deglaciation of the Svalbard/Barents Sea Ice Sheet - a swath bathymetric and sub-bottom seismic study from the Kveithola Trough

NASA Astrophysics Data System (ADS)

Rebesco, Michele; Liu, Yanguang; Camerlenghi, Angelo; Winsborrow, Monica; Sverre Laberg, Jan; Caburlotto, Andrea; Diviacco, Paolo; Accettella, Daniela; Sauli, Chiara; Wardell, Nigel

2010-05-01

Kveithola Trough, an E-W trending cross-shelf glacial trough in the NW Barents Sea, was surveyed for the first time during the EGLACOM cruise between 8th July and 4th August 2008 on board R/V OGS-Explora. EGLACOM (Evolution of a GLacial Arctic COntinental Margin: the southern Svalbard ice stream-dominated sedimentary system) project is the Italian contribution to the International Polar Year (IPY) Activity 367 (Neogene ice streams and sedimentary processes on high- latitude continental margins - NICE STREAMS). Such IPY activity included as well the Spanish SVAIS 2008 cruise on board BIO Hesperides. EGLACOM data acquisition, focused on the Storfjorden Fan and Kveithola Trough, included a multi-channel seismic (MCS) reflection survey and the simultaneous collection of swath bathymetry and sub-bottom CHIRP profiles. Swath bathymetry in the Kveithola Trough shows that the seafloor is characterized by E-W trending mega-scale glacial lineations (MSGL). These include large-scale ridges about 2 km wide and 15 m high as well as smaller grooves about 100 m wide and a few metres deep. Such MSGL record the fast flow of an ice stream draining the Svalbard/Barents Sea Ice Sheet (SBSIS) during the Last Glacial Maximum (LGM). MSGL are overprinted by transverse sediment ridges about 15 km apart which give rise to a staircase long profile of the trough. Such transverse ridges are interpreted as grounding-zone wedges (GZW) formed by deposition of unconsolidated, saturated subglacial till during ice stream retreat. Sub-bottom (CHIRP) and multi-channel reflection seismic data show that the morphology is controlled by stacked sets of lensoidal transparent units (tills) overlain by a draping glaciomarine unit up to over 15 m thick. Formed during temporary stillstands in grounding-zone position before complete deglaciation, GZW ridges are diagnostic of episodic retreat. Our data allow the reconstruction of deglaciation in the Spitsbergen Bank area, with each stage during deglaciation recorded by deposition of a GZW. Three independent lines of reasoning suggest that an ice cap persisted on Spitsbergen Bank for some thousand years and lasted much longer than those that fed the adjacent glacial troughs: 1) the freshness of the morphology in Kveithola Trough compared to that of adjacent Storfjorden and Bear Island troughs; 2) the volume of sediment in the GZW ridges compared to the small catchment area; 3) preliminary assessment of the stratigraphic position of debris flow deposits on the continental slope. The 15 m of sedimentary drape deposited on top of GZW ridges contains a high-resolution palaeoclimatic record of the last thousand years, which accumulated at a very high average sedimentation rate. Sampling (through drilling) of the thin glaciomarine sediments between the till lenses of the successive GZW ridges may allow the dating of deglaciation phases in the Barents Sea.

The Glacial and Relative Sea Level History of Southern Banks Island, NT, Canada

NASA Astrophysics Data System (ADS)

Vaughan, Jessica Megan

The mapping and dating of surficial glacial landforms and sediments across southern Banks Island document glaciation by the northwest Laurentide Ice Sheet (LIS) during the last glacial maximum. Geomorphic landforms confirm the operation of an ice stream at least 1000 m thick in Amundsen Gulf that was coalescent with thin, cold-based ice crossing the island's interior, both advancing offshore onto the polar continental shelf. Raised marine shorelines across western and southern Banks Island are barren, recording early withdrawal of the Amundsen Gulf Ice Stream prior to the resubmergence of Bering Strait and the re-entry of Pacific molluscs ~13,750 cal yr BP. This withdrawal resulted in a loss of ~60,000 km2 of ice --triggering drawdown from the primary northwest LIS divide and instigating changes in subsequent ice flow. The Jesse moraine belt on eastern Banks Island records a lateglacial stillstand and/or readvance of Laurentide ice in Prince of Wales Strait (13,750 -- 12,750 cal yr BP). Fossiliferous raised marine sediments that onlap the Jesse moraine belt constrain final deglaciation to ~12,600 cal yr BP, a minimum age for the breakup of the Amundsen Gulf Ice Stream. The investigation of a 30 m thick and 6 km wide stratigraphic sequence at Worth Point, southwest Banks Island, identifies an advance of the ancestral LIS during the Mid-Pleistocene (sensu lato), substantially diversifying the glacial record on Banks Island. Glacial ice emplaced during this advance has persisted through at least two glacial-interglacial cycles, demonstrating the resilience of circumpolar permafrost. Pervasive deformation of the stratigraphic sequence also records a detailed history of glaciotectonism in proglacial and subglacial settings that can result from interactions between cold-based ice and permafrost terrain. This newly recognized history rejects the long-established paleoenvironmental model of Worth Point that assumed a simple 'layer-cake' stratigraphy.

Coupled ice sheet-ocean modelling to investigate ocean driven melting of marine ice sheets in Antarctica

NASA Astrophysics Data System (ADS)

Jong, Lenneke; Gladstone, Rupert; Galton-Fenzi, Ben

2017-04-01

Ocean induced melting below the ice shelves of marine ice sheets is a major source of uncertainty for predictions of ice mass loss and Antarctica's resultant contribution to future sea level rise. The floating ice shelves provide a buttressing force against the flow of ice across the grounding line into the ocean. Thinning of these ice shelves due to an increase in melting reduces this force and can lead to an increase in the discharge of grounded ice. Fully coupled modelling of ice sheet-ocean interactions is key to improving understanding the influence of the Southern ocean on the evolution of the Antarctic ice sheet, and to predicting its future behaviour under changing climate conditions. Coupling of ocean and ice sheet models is needed to provide more realistic melt rates at the base of ice shelves and hence make better predictions of the behaviour of the grounding line and the shape of the ice-shelf cavity as the ice sheet evolves. The Framework for Ice Sheet - Ocean Coupling (FISOC) has been developed to provide a flexible platform for performing coupled ice sheet - ocean modelling experiments. We present preliminary results using FISOC to couple the Regional Ocean Modelling System (ROMS) with Elmer/Ice in idealised experiments Marine Ice Sheet-Ocean Model Intercomparison Project (MISOMIP). These experiments use an idealised geometry motivated by that of Pine Island glacier and the adjacent Amundsen Sea in West Antarctica, a region which has shown shown signs of thinning ice and grounding line retreat.

Ice Sheet Model Intercomparison Project (ISMIP6) contribution to CMIP6

PubMed Central

Nowicki, Sophie M.J.; Payne, Tony; Larour, Eric; Seroussi, Helene; Goelzer, Heiko; Lipscomb, William; Gregory, Jonathan; Abe-Ouchi, Ayako; Shepherd, Andrew

2018-01-01

Reducing the uncertainty in the past, present and future contribution of ice sheets to sea-level change requires a coordinated effort between the climate and glaciology communities. The Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) is the primary activity within the Coupled Model Intercomparison Project – phase 6 (CMIP6) focusing on the Greenland and Antarctic Ice Sheets. In this paper, we describe the framework for ISMIP6 and its relationship to other activities within CMIP6. The ISMIP6 experimental design relies on CMIP6 climate models and includes, for the first time within CMIP, coupled ice sheet – climate models as well as standalone ice sheet models. To facilitate analysis of the multi-model ensemble and to generate a set of standard climate inputs for standalone ice sheet models, ISMIP6 defines a protocol for all variables related to ice sheets. ISMIP6 will provide a basis for investigating the feedbacks, impacts, and sea-level changes associated with dynamic ice sheets and for quantifying the uncertainty in ice-sheet-sourced global sea-level change. PMID:29697697

Ice sheet margins and ice shelves

NASA Technical Reports Server (NTRS)

Thomas, R. H.

1984-01-01

The effect of climate warming on the size of ice sheet margins in polar regions is considered. Particular attention is given to the possibility of a rapid response to warming on the order of tens to hundreds of years. It is found that the early response of the polar regions to climate warming would be an increase in the area of summer melt on the ice sheets and ice shelves. For sufficiently large warming (5-10C) the delayed effects would include the breakup of the ice shelves by an increase in ice drainage rates, particularly from the ice sheets. On the basis of published data for periodic changes in the thickness and melting rates of the marine ice sheets and fjord glaciers in Greenland and Antarctica, it is shown that the rate of retreat (or advance) of an ice sheet is primarily determined by: bedrock topography; the basal conditions of the grounded ice sheet; and the ice shelf condition downstream of the grounding line. A program of satellite and ground measurements to monitor the state of ice sheet equilibrium is recommended.

Antarctic ice sheet thickness estimation using the horizontal-to-vertical spectral ratio method with single-station seismic ambient noise

NASA Astrophysics Data System (ADS)

Yan, Peng; Li, Zhiwei; Li, Fei; Yang, Yuande; Hao, Weifeng; Bao, Feng

2018-03-01

We report on a successful application of the horizontal-to-vertical spectral ratio (H / V) method, generally used to investigate the subsurface velocity structures of the shallow crust, to estimate the Antarctic ice sheet thickness for the first time. Using three-component, five-day long, seismic ambient noise records gathered from more than 60 temporary seismic stations located on the Antarctic ice sheet, the ice thickness measured at each station has comparable accuracy to the Bedmap2 database. Preliminary analysis revealed that 60 out of 65 seismic stations on the ice sheet obtained clear peak frequencies (f0) related to the ice sheet thickness in the H / V spectrum. Thus, assuming that the isotropic ice layer lies atop a high velocity half-space bedrock, the ice sheet thickness can be calculated by a simple approximation formula. About half of the calculated ice sheet thicknesses were consistent with the Bedmap2 ice thickness values. To further improve the reliability of ice thickness measurements, two-type models were built to fit the observed H / V spectrum through non-linear inversion. The two-type models represent the isotropic structures of single- and two-layer ice sheets, and the latter depicts the non-uniform, layered characteristics of the ice sheet widely distributed in Antarctica. The inversion results suggest that the ice thicknesses derived from the two-layer ice models were in good concurrence with the Bedmap2 ice thickness database, and that ice thickness differences between the two were within 300 m at almost all stations. Our results support previous finding that the Antarctic ice sheet is stratified. Extensive data processing indicates that the time length of seismic ambient noise records can be shortened to two hours for reliable ice sheet thickness estimation using the H / V method. This study extends the application fields of the H / V method and provides an effective and independent way to measure ice sheet thickness in Antarctica.

Enhanced ice sheet melting driven by volcanic eruptions during the last deglaciation.

PubMed

Muschitiello, Francesco; Pausata, Francesco S R; Lea, James M; Mair, Douglas W F; Wohlfarth, Barbara

2017-10-24

Volcanic eruptions can impact the mass balance of ice sheets through changes in climate and the radiative properties of the ice. Yet, empirical evidence highlighting the sensitivity of ancient ice sheets to volcanism is scarce. Here we present an exceptionally well-dated annual glacial varve chronology recording the melting history of the Fennoscandian Ice Sheet at the end of the last deglaciation (∼13,200-12,000 years ago). Our data indicate that abrupt ice melting events coincide with volcanogenic aerosol emissions recorded in Greenland ice cores. We suggest that enhanced ice sheet runoff is primarily associated with albedo effects due to deposition of ash sourced from high-latitude volcanic eruptions. Climate and snowpack mass-balance simulations show evidence for enhanced ice sheet runoff under volcanically forced conditions despite atmospheric cooling. The sensitivity of past ice sheets to volcanic ashfall highlights the need for an accurate coupling between atmosphere and ice sheet components in climate models.

The Impact of Geothermal Heat on the Scandinavian Ice Sheet's LGM Extent

NASA Astrophysics Data System (ADS)

Szuman, Izabela; Ewertowski, Marek W.; Kalita, Jakub Z.

2016-04-01

The last Scandinavian ice sheet attained its most southern extent over Poland and Germany, protruding c. 200 km south of the main ice sheet mass. There are number of factors that may control ice sheet dynamics and extent. One of the less recognised is geothermal heat, which is heat that is supplied to the base of the ice sheet. A heat at the ice/bed interface plays a crucial role in controlling ice sheet stability, as well as impacting basal temperatures, melting, and ice flow velocities. However, the influence of geothermal heat is still virtually neglected in reconstructions and modelling of paleo-ice sheets behaviour. Only in a few papers is geothermal heat recalled though often in the context of past climatic conditions. Thus, the major question is if and how spatial differences in geothermal heat had influenced paleo-ice sheet dynamics and in consequence their extent. Here, we assumed that the configuration of the ice sheet along its southern margin was moderately to strongly correlated with geothermal heat for Poland and non or negatively correlated for Germany.

Glaciological constraints on current ice mass changes from modelling the ice sheets over the glacial cycles

NASA Astrophysics Data System (ADS)

Huybrechts, P.

2003-04-01

The evolution of continental ice sheets introduces a long time scale in the climate system. Large ice sheets have a memory of millenia, hence the present-day ice sheets of Greenland and Antarctica are still adjusting to climatic variations extending back to the last glacial period. This trend is separate from the direct response to mass-balance changes on decadal time scales and needs to be correctly accounted for when assessing current and future contributions to sea level. One way to obtain estimates of current ice mass changes is to model the past history of the ice sheets and their underlying beds over the glacial cycles. Such calculations assist to distinguish between the longer-term ice-dynamic evolution and short-term mass-balance changes when interpreting altimetry data, and are helpful to isolate the effects of postglacial rebound from gravity and altimetry trends. The presentation will discuss results obtained from 3-D thermomechanical ice-sheet/lithosphere/bedrock models applied to the Antarctic and Greenland ice sheets. The simulations are forced by time-dependent boundary conditions derived from sediment and ice core records and are constrained by geomorphological and glacial-geological data of past ice sheet and sea-level stands. Current simulations suggest that the Greenland ice sheet is close to balance, while the Antarctic ice sheet is still losing mass, mainly due to incomplete grounding-line retreat of the West Antarctic ice sheet since the LGM. The results indicate that altimetry trends are likely dominated by ice thickness changes but that the gravitational signal mainly reflects postglacial rebound.

Transient Conditions at the Ice/bed Interface Under a Palaeo-ice Stream Derived from Numerical Simulation of Groundwater Flow and Sedimentological Observations in a Drumlin Field, NW Poland

NASA Astrophysics Data System (ADS)

Hermanowski, P.; Piotrowski, J. A.

2017-12-01

Evacuation of glacial meltwater through the substratum is an important agent modulating the ice/bed interface processes. The amount of meltwater production, subglacial water pressure, flow patterns and fluxes all affect the strength of basal coupling and thus impact the ice-sheet dynamics. Despite much research into the subglacial processes of past ice sheets which controlled sediment transport and the formation of specific landforms, our understanding of the ice/bed interface remains fragmentary. In this study we numerically simulated, using finite difference and finite element codes, groundwater flow pattern and fluxes during an ice advance in the Stargard Drumlin Field, NW Poland to examine the potential influence of groundwater drainage on the landforming processes. The results are combined with sedimentological observations of the internal composition of the drumlins to validate the outcome of the numerical model. Our numerical experiments of groundwater flow suggest a highly time-dependent response of the subglacial hydrogeological system to the advancing ice margin. This is manifested as diversified areas of downward- and upward-oriented groundwater flows whereby the drumlin field area experienced primarily groundwater discharge towards the ice sole. The investigated drumlins are composed of (i) mainly massive till with thin stringers of meltwater sand, and (ii) sorted sediments carrying ductile deformations. The model results and sedimentological observations suggest a high subglacial pore-water pressure in the drumlin field area, which contributed to sediment deformation intervening with areas of basal decoupling and enhanced basal sliding.

Evolution of ice sheets in the early Quaternary of the central North Sea: 2.58 Ma to 0.78 Ma

NASA Astrophysics Data System (ADS)

Lamb, R.; Huuse, M.; Stewart, M.; Brocklehurst, S. H.

2016-12-01

Integration of chronostratigraphic proxies with 3D seismic and well-log data has allowed for a basin-wide re-interpretation of the onset of glaciation in the central North Sea during the Quaternary. Mapping of seismic geomorphology, calculations of water depth and sediment accumulation rates, and other basin analysis techniques unravel the evolution of the North Sea basin during the early Pleistocene, a period of dramatic global cooling and rapid 41 kyr glacial-interglacial cycles, identifying a system which is increasingly dominated by large, continental-scale ice sheets. Prior to this study continental-scale ice sheets were generally not considered able to grow in a 41 kyr cycle and the earliest date for such an ice sheet in the North Sea was identified at the onset of tunnel valley formation in the Elsterian (0.48 Ma; MIS 12) which forms a large regional-scale glacial unconformity. At the onset of the Pleistocene at 2.58 Ma the North Sea basin was an elongate `mega-fjord' with water depths of up to 350 m, infilling rapidly as the European river systems deposited a large clinoform complex in the southern end of the basin. This period corresponds to the preservation of large scale ice-berg scouring on clinoform topsets, suggesting the presence of marine-terminating ice sheets with repeated calving events. As the Pleistocene progressed and global climate became increasingly colder the North Sea became smaller and shallower due to the continual infill of the basin. At 1.72 Ma the basin reached a critical point between the cold climate and the shallowing of the basin and the first evidence for grounded glaciation in the form of mega-scale glacial lineations is seen at this level. Between 1.72 and 0.48 Ma there is evidence for ice-streaming in the form of multiple MSGL flow sets re-occupying the central North Sea, as well as a large glaciotectonic complex. The glacial geomorphological evidence presented pushes back the date of grounded glaciation in the central North Sea by over one million years relative to the existing models. This raises important questions about the completeness of glaciation histories of Europe and other high and mid latitude land areas that can only be addressed by in depth scrutiny of their adjacent offshore sedimentary records.

ISMIP6 - initMIP: Greenland ice sheet model initialisation experiments

NASA Astrophysics Data System (ADS)

Goelzer, Heiko; Nowicki, Sophie; Payne, Tony; Larour, Eric; Abe Ouchi, Ayako; Gregory, Jonathan; Lipscomb, William; Seroussi, Helene; Shepherd, Andrew; Edwards, Tamsin

2016-04-01

Earlier large-scale Greenland ice sheet sea-level projections e.g. those run during ice2sea and SeaRISE initiatives have shown that ice sheet initialisation can have a large effect on the projections and gives rise to important uncertainties. This intercomparison exercise (initMIP) aims at comparing, evaluating and improving the initialization techniques used in the ice sheet modeling community and to estimate the associated uncertainties. It is the first in a series of ice sheet model intercomparison activities within ISMIP6 (Ice Sheet Model Intercomparison Project for CMIP6). The experiments are conceived for the large-scale Greenland ice sheet and are designed to allow intercomparison between participating models of 1) the initial present-day state of the ice sheet and 2) the response in two schematic forward experiments. The latter experiments serve to evaluate the initialisation in terms of model drift (forward run without any forcing) and response to a large perturbation (prescribed surface mass balance anomaly). We present and discuss first results of the intercomparison and highlight important uncertainties with respect to projections of the Greenland ice sheet sea-level contribution.

Ice Sheet Model Intercomparison Project (ISMIP6) Contribution to CMIP6

NASA Technical Reports Server (NTRS)

Nowicki, Sophie M. J.; Payne, Tony; Larour, Eric; Seroussi, Helene; Goelzer, Heiko; Lipscomb, William; Gregory, Jonathan; Abe-Ouchi, Ayako; Shepherd, Andrew

2016-01-01

Reducing the uncertainty in the past, present, and future contribution of ice sheets to sea-level change requires a coordinated effort between the climate and glaciology communities. The Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) is the primary activity within the Coupled Model Intercomparison Project phase 6 (CMIP6) focusing on the Greenland and Antarctic ice sheets. In this paper, we describe the framework for ISMIP6 and its relationship with other activities within CMIP6. The ISMIP6 experimental design relies on CMIP6 climate models and includes, for the first time within CMIP, coupled ice-sheetclimate models as well as standalone ice-sheet models. To facilitate analysis of the multi-model ensemble and to generate a set of standard climate inputs for standalone ice-sheet models, ISMIP6 defines a protocol for all variables related to ice sheets. ISMIP6 will provide a basis for investigating the feedbacks, impacts, and sea-level changes associated with dynamic ice sheets and for quantifying the uncertainty in ice-sheet-sourced global sea-level change.

Altimetry, gravimetry, GPS and viscoelastic modeling data for the joint inversion for glacial isostatic adjustment in Antarctica (ESA STSE Project REGINA)

NASA Astrophysics Data System (ADS)

Sasgen, Ingo; Martín-Español, Alba; Horvath, Alexander; Klemann, Volker; Petrie, Elizabeth J.; Wouters, Bert; Horwath, Martin; Pail, Roland; Bamber, Jonathan L.; Clarke, Peter J.; Konrad, Hannes; Wilson, Terry; Drinkwater, Mark R.

2018-03-01

The poorly known correction for the ongoing deformation of the solid Earth caused by glacial isostatic adjustment (GIA) is a major uncertainty in determining the mass balance of the Antarctic ice sheet from measurements of satellite gravimetry and to a lesser extent satellite altimetry. In the past decade, much progress has been made in consistently modeling ice sheet and solid Earth interactions; however, forward-modeling solutions of GIA in Antarctica remain uncertain due to the sparsity of constraints on the ice sheet evolution, as well as the Earth's rheological properties. An alternative approach towards estimating GIA is the joint inversion of multiple satellite data - namely, satellite gravimetry, satellite altimetry and GPS, which reflect, with different sensitivities, trends in recent glacial changes and GIA. Crucial to the success of this approach is the accuracy of the space-geodetic data sets. Here, we present reprocessed rates of surface-ice elevation change (Envisat/Ice, Cloud,and land Elevation Satellite, ICESat; 2003-2009), gravity field change (Gravity Recovery and Climate Experiment, GRACE; 2003-2009) and bedrock uplift (GPS; 1995-2013). The data analysis is complemented by the forward modeling of viscoelastic response functions to disc load forcing, allowing us to relate GIA-induced surface displacements with gravity changes for different rheological parameters of the solid Earth. The data and modeling results presented here are available in the PANGAEA database (https://doi.org/10.1594/PANGAEA.875745). The data sets are the input streams for the joint inversion estimate of present-day ice-mass change and GIA, focusing on Antarctica. However, the methods, code and data provided in this paper can be used to solve other problems, such as volume balances of the Antarctic ice sheet, or can be applied to other geographical regions in the case of the viscoelastic response functions. This paper presents the first of two contributions summarizing the work carried out within a European Space Agency funded study: Regional glacial isostatic adjustment and CryoSat elevation rate corrections in Antarctica (REGINA).

Extraction of Ice Sheet Layers from Two Intersected Radar Echograms Near Neem Ice Core in Greenland

NASA Astrophysics Data System (ADS)

Xiong, S.; Muller, J.-P.

2016-06-01

Accumulation of snow and ice over time result in ice sheet layers. These can be remotely sensed where there is a contrast in electromagnetic properties, which reflect variations of the ice density, acidity and fabric orientation. Internal ice layers are assumed to be isochronous, deep beneath the ice surface, and parallel to the direction of ice flow. The distribution of internal layers is related to ice sheet dynamics, such as the basal melt rate, basal elevation variation and changes in ice flow mode, which are important parameters to model the ice sheet. Radar echo sounder is an effective instrument used to study the sedimentology of the Earth and planets. Ice Penetrating Radar (IPR) is specific kind of radar echo sounder, which extends studies of ice sheets from surface to subsurface to deep internal ice sheets depending on the frequency utilised. In this study, we examine a study site where folded ice occurs in the internal ice sheet south of the North Greenland Eemian ice drilling (NEEM) station, where two intersected radar echograms acquired by the Multi-channel Coherent Radar Depth Sounder (MCoRDS) employed in the NASA's Operation IceBridge (OIB) mission imaged this folded ice. We propose a slice processing flow based on a Radon Transform to trace and extract these two sets of curved ice sheet layers, which can then be viewed in 3-D, demonstrating the 3-D structure of the ice folds.

Geological controls on bedrock topography and ice sheet dynamics in the Wilkes Subglacial Basin sector of East Antarctica

NASA Astrophysics Data System (ADS)

Ferraccioli, Fausto; Armadillo, Egidio; Young, Duncan; Blankenship, Donald; Jordan, Tom; Siegert, Martin

2017-04-01

The Wilkes Subglacial Basin extends for 1,400 km into the interior of East Antarctica and hosts several major glaciers that drain a large sector of the East Antarctic Ice Sheet. The deep northern Wilkes Subglacial Basin underlies the catchments of the Matusevich, Cook, Ninnis and Mertz Glaciers, which are largely marine-based and hence potentially particularly sensitive to past and also predicted future ocean and climate warming. Sediment provenance studies suggest that the glaciers flowing in this region may have retreated significantly compared to their modern configuration, as recently as the warm mid-Pliocene interval, potentially contributing several m to global sea level rise (Cook et al.,Nature Geosci., 2013). Here we combine airborne radar, aeromagnetic and airborne gravity observations collected during the international WISE-ISODYN and ICECAP aerogeophysical campaigns with vintage datasets to help unveil subglacial geology and deeper crustal architecture and to assess its influence on bedrock topography and ice sheet dynamics in the northern Wilkes Subglacial Basin. Aeromagnetic images reveal that the Matusevich Glacier is underlain by a ca 480 Ma thrust fault system (the Exiles Thrust), which has also been inferred to have been reactivated in response to intraplate Cenozoic strike-slip faulting. Further to the west, the linear Eastern Basins are controlled by the Prince Albert Fault System. The fault system continues to the south, where it provides structural controls for both the Priestley and Reeves Glaciers. The inland Central Basins continue in the coastal area underlying the fast flowing Cook ice streams, implying that potential ocean-induced changes could propagate further into the interior of the ice sheet. We propose based on an analogy with the Rennick Graben that these deep subglacial basins are controlled by the underlying horst and graben crustal architecture. Given the interpreted subglacial distribution of Beacon sediments and Ferrar tholeiites and uplifted Ross-age basement blocks, we propose that these grabens were reactivated in post-Jurassic times, as observed from geological studies in the Rennick Graben. A remarkable contrast in long-wavelength magnetic anomaly signatures is observed over the coastal and inland segments of the Cook ice stream glacial catchment. We attribute this, to the presence of several km thick early Cambrian to late Neoproterozoic(?) sedimentary basins in the coastal region, in contrast to a prominent Proterozoic basement high at the onset of fast glacial flow further inland. This suggests that there could also be a marked difference in geothermal heat flux at the base of the ice sheet in these two regions, which may in turn exert influences on basal melting and subglacial hydrology networks. Further west, the deep Western Basins provide key topographic controls on the Ninnis Glacier, which is interpreted here, as controlled by a major Paleoproterozoic crustal boundary, separating an inferred linear Archean crustal ribbon from Paleoproterozoic rift basins, which are partially exposed along the coastal segment of the Terre Adelie Craton. The ca 1.7 Ga Mertz Shear Zone flanks the Mertz Glacier, and is interpreted here as a fault splay associated with this major crustal boundary.

Modeling Firn Compaction in Dynamic Regions

NASA Astrophysics Data System (ADS)

Horlings, Annika N.; Christianson, Knut; Waddington, Edwin D.; Stevens, C. Max; Holschuh, Nicholas

2017-04-01

Firn compaction remains the largest source of uncertainty in assessments of ice-sheet mass balance from repeat altimetry measurements due to our limited understanding of the physical processes responsible for the transformation of snow into ice. In addition to the lack of a comprehensive, physically-based constitutive relationship that describes firn compaction, dynamic thinning is an important process in some regions, but is generally neglected in firn-compaction models due to their one-dimensional nature. Here, we report on preliminary results incorporating dynamic strain thinning into firn compaction models. Using a Lagrangian (material-following) reference frame, we first compact each firn element using a standard 1-D firn-compaction model without longitudinal strain. Then, we stretch each firn parcel at each time step by applying a prescribed longitudinal strain rate in the absence of further density changes; this produces additional vertical thinning. To assess variations among firn models, we compare results from eight firn densification models currently included in the UW Community Firn Model. We focus on the Northeast Greenland Ice Stream due to the high extensile strain rates (10-3 yr-1 or higher) in the ice stream's shear margins and the extensive firn-density data in this area from seismic measurements and shallow firn/ice cores. For temperatures and accumulation rates typical for northeast Greenland, our preliminary results indicate up to an 18-meter decrease in bubble close-off depth in the shear margins compared to nearby areas either inside or outside the ice stream, which compares favorably to field data. Further work includes incorporating physically-based constitutive relations and applying these improved models to other dynamic regions, such as the Amundsen Sea Embayment, where dynamic strain thinning has accelerated in recent decades.

History of the Greenland Ice Sheet: paleoclimatic insights

USGS Publications Warehouse

Alley, Richard B.; Andrews, John T.; Brigham-Grette, J.; Clarke, G.K.C.; Cuffey, Kurt M.; Fitzpatrick, J.J.; Funder, S.; Marshall, S.J.; Miller, G.H.; Mitrovica, J.X.; Muhs, D.R.; Otto-Bliesner, B. L.; Polyak, L.; White, J.W.C.

2010-01-01

Paleoclimatic records show that the GreenlandIce Sheet consistently has lost mass in response to warming, and grown in response to cooling. Such changes have occurred even at times of slow or zero sea-level change, so changing sea level cannot have been the cause of at least some of the ice-sheet changes. In contrast, there are no documented major ice-sheet changes that occurred independent of temperature changes. Moreover, snowfall has increased when the climate warmed, but the ice sheet lost mass nonetheless; increased accumulation in the ice sheet's center has not been sufficient to counteract increased melting and flow near the edges. Most documented forcings and ice-sheet responses spanned periods of several thousand years, but limited data also show rapid response to rapid forcings. In particular, regions near the ice margin have responded within decades. However, major changes of central regions of the ice sheet are thought to require centuries to millennia. The paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur. The evidence suggests nearly total ice-sheet loss may result from warming of more than a few degrees above mean 20th century values, but this threshold is poorly defined (perhaps as little as 2 °C or more than 7 °C). Paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming, or changes may have been induced by factors other than temperature, without having been recorded.

Potential Climatic Effects on the Greenland Ice Sheet

NASA Technical Reports Server (NTRS)

Bindschadler, R. A.

1984-01-01

The Greenland Ice Sheet covers an area of 1,720,000 sq. km and contains approximately 2,600,000 cu km of ice. Most of the ice sheet receives an excess of snow accumulation over the amount of ice lost to wind, meltwater run-off or other ablative processes. The majority of mass loss occurs at the margin of the ice sheet as either surface melt, which flows into the sea or calving of icebergs from the tongues of outlet glaciers. Many estimates of these processes were published. An average of five published estimates is summarized. If these estimates are correct, then the Greenland Ice Sheet is in approximate equilibrium and contributes 490 cu km/a of fresh water to the North Atlantic and Arctic Oceans. Climate effects, ice sheet flow, and application of remote sensing to tracking of the ice sheet are discussed.

Current state and future perspectives on coupled ice-sheet - sea-level modelling

NASA Astrophysics Data System (ADS)

de Boer, Bas; Stocchi, Paolo; Whitehouse, Pippa L.; van de Wal, Roderik S. W.

2017-08-01

The interaction between ice-sheet growth and retreat and sea-level change has been an established field of research for many years. However, recent advances in numerical modelling have shed new light on the precise interaction of marine ice sheets with the change in near-field sea level, and the related stability of the grounding line position. Studies using fully coupled ice-sheet - sea-level models have shown that accounting for gravitationally self-consistent sea-level change will act to slow down the retreat and advance of marine ice-sheet grounding lines. Moreover, by simultaneously solving the 'sea-level equation' and modelling ice-sheet flow, coupled models provide a global field of relative sea-level change that is consistent with dynamic changes in ice-sheet extent. In this paper we present an overview of recent advances, possible caveats, methodologies and challenges involved in coupled ice-sheet - sea-level modelling. We conclude by presenting a first-order comparison between a suite of relative sea-level data and output from a coupled ice-sheet - sea-level model.

Interhemispheric ice-sheet synchronicity during the Last Glacial Maximum

NASA Astrophysics Data System (ADS)

Weber, M. E.; Clark, P. U.; Ricken, W.; Mitrovica, J. X.; Hostetler, S. W.; Kuhn, G.

2012-04-01

The timing of the last maximum extent of the Antarctic ice sheets relative to those in the Northern Hemisphere remains poorly understood because only a few findings with robust chronologies exist for Antarctic ice sheets. We developed a chronology for the Weddell Sea sector of the East Antarctic ice sheet that, combined with ages from other Antarctic ice-sheet sectors, indicates the advance to their maximum extent at 29 -28 ka, and retreat from their maximum extent at 19 ka was nearly synchronous with Northern Hemisphere ice sheets (Weber, M.E., Clark, P. U., Ricken, W., Mitrovica, J. X., Hostetler, S. W., and Kuhn, G. (2011): Interhemispheric ice-sheet synchronicity during the Last Glacial Maximum. - Science, 334, 1265-1269, doi: 10.1126:science.1209299). As for the deglaciation, modeling studies suggest a late ice-sheet retreat starting around 14 ka BP and ending around 7 ka BP with a large impact of an unstable West Antarctic Ice Sheet (WAIS) and a small impact of a stable East Antarctic Ice Sheet (EAIS). However, the Weddell Sea sites studied here, as well as sites from the Scotia Sea, provide evidence that specifically the EAIS responded much earlier, possibly provided a significant contribution to the last sea-level rise, and was much more dynamic than previously thought. Using the results of an atmospheric general circulation we conclude that surface climate forcing of Antarctic ice mass balance would likely cause an opposite response, whereby a warming climate would increase accumulation but not surface melting. Furthermore, our new data support teleconnections involving a sea-level fingerprint forced from Northern Hemisphere ice sheets as indicated by gravitational modeling. Also, changes in North Atlantic Deepwater formation and attendant heat flux to Antarctic grounding lines may have contributed to synchronizing the hemispheric ice sheets.

Design and results of the ice sheet model initialisation experiments initMIP-Greenland: an ISMIP6 intercomparison

NASA Astrophysics Data System (ADS)

Goelzer, Heiko; Nowicki, Sophie; Edwards, Tamsin; Beckley, Matthew; Abe-Ouchi, Ayako; Aschwanden, Andy; Calov, Reinhard; Gagliardini, Olivier; Gillet-Chaulet, Fabien; Golledge, Nicholas R.; Gregory, Jonathan; Greve, Ralf; Humbert, Angelika; Huybrechts, Philippe; Kennedy, Joseph H.; Larour, Eric; Lipscomb, William H.; Le clec'h, Sébastien; Lee, Victoria; Morlighem, Mathieu; Pattyn, Frank; Payne, Antony J.; Rodehacke, Christian; Rückamp, Martin; Saito, Fuyuki; Schlegel, Nicole; Seroussi, Helene; Shepherd, Andrew; Sun, Sainan; van de Wal, Roderik; Ziemen, Florian A.

2018-04-01

Earlier large-scale Greenland ice sheet sea-level projections (e.g. those run during the ice2sea and SeaRISE initiatives) have shown that ice sheet initial conditions have a large effect on the projections and give rise to important uncertainties. The goal of this initMIP-Greenland intercomparison exercise is to compare, evaluate, and improve the initialisation techniques used in the ice sheet modelling community and to estimate the associated uncertainties in modelled mass changes. initMIP-Greenland is the first in a series of ice sheet model intercomparison activities within ISMIP6 (the Ice Sheet Model Intercomparison Project for CMIP6), which is the primary activity within the Coupled Model Intercomparison Project Phase 6 (CMIP6) focusing on the ice sheets. Two experiments for the large-scale Greenland ice sheet have been designed to allow intercomparison between participating models of (1) the initial present-day state of the ice sheet and (2) the response in two idealised forward experiments. The forward experiments serve to evaluate the initialisation in terms of model drift (forward run without additional forcing) and in response to a large perturbation (prescribed surface mass balance anomaly); they should not be interpreted as sea-level projections. We present and discuss results that highlight the diversity of data sets, boundary conditions, and initialisation techniques used in the community to generate initial states of the Greenland ice sheet. We find good agreement across the ensemble for the dynamic response to surface mass balance changes in areas where the simulated ice sheets overlap but differences arising from the initial size of the ice sheet. The model drift in the control experiment is reduced for models that participated in earlier intercomparison exercises.

Design and results of the ice sheet model initialisation experiments initMIP-Greenland: an ISMIP6 intercomparison

DOE PAGES

Goelzer, Heiko; Nowicki, Sophie; Edwards, Tamsin; ...

2018-04-19

Earlier large-scale Greenland ice sheet sea-level projections (e.g. those run during the ice2sea and SeaRISE initiatives) have shown that ice sheet initial conditions have a large effect on the projections and give rise to important uncertainties. Here, the goal of this initMIP-Greenland intercomparison exercise is to compare, evaluate, and improve the initialisation techniques used in the ice sheet modelling community and to estimate the associated uncertainties in modelled mass changes. initMIP-Greenland is the first in a series of ice sheet model intercomparison activities within ISMIP6 (the Ice Sheet Model Intercomparison Project for CMIP6), which is the primary activity within themore » Coupled Model Intercomparison Project Phase 6 (CMIP6) focusing on the ice sheets. Two experiments for the large-scale Greenland ice sheet have been designed to allow intercomparison between participating models of (1) the initial present-day state of the ice sheet and (2) the response in two idealised forward experiments. The forward experiments serve to evaluate the initialisation in terms of model drift (forward run without additional forcing) and in response to a large perturbation (prescribed surface mass balance anomaly); they should not be interpreted as sea-level projections. We present and discuss results that highlight the diversity of data sets, boundary conditions, and initialisation techniques used in the community to generate initial states of the Greenland ice sheet. We find good agreement across the ensemble for the dynamic response to surface mass balance changes in areas where the simulated ice sheets overlap but differences arising from the initial size of the ice sheet. The model drift in the control experiment is reduced for models that participated in earlier intercomparison exercises.« less

Design and results of the ice sheet model initialisation experiments initMIP-Greenland: an ISMIP6 intercomparison

DOE Office of Scientific and Technical Information (OSTI.GOV)

Goelzer, Heiko; Nowicki, Sophie; Edwards, Tamsin

Earlier large-scale Greenland ice sheet sea-level projections (e.g. those run during the ice2sea and SeaRISE initiatives) have shown that ice sheet initial conditions have a large effect on the projections and give rise to important uncertainties. Here, the goal of this initMIP-Greenland intercomparison exercise is to compare, evaluate, and improve the initialisation techniques used in the ice sheet modelling community and to estimate the associated uncertainties in modelled mass changes. initMIP-Greenland is the first in a series of ice sheet model intercomparison activities within ISMIP6 (the Ice Sheet Model Intercomparison Project for CMIP6), which is the primary activity within themore » Coupled Model Intercomparison Project Phase 6 (CMIP6) focusing on the ice sheets. Two experiments for the large-scale Greenland ice sheet have been designed to allow intercomparison between participating models of (1) the initial present-day state of the ice sheet and (2) the response in two idealised forward experiments. The forward experiments serve to evaluate the initialisation in terms of model drift (forward run without additional forcing) and in response to a large perturbation (prescribed surface mass balance anomaly); they should not be interpreted as sea-level projections. We present and discuss results that highlight the diversity of data sets, boundary conditions, and initialisation techniques used in the community to generate initial states of the Greenland ice sheet. We find good agreement across the ensemble for the dynamic response to surface mass balance changes in areas where the simulated ice sheets overlap but differences arising from the initial size of the ice sheet. The model drift in the control experiment is reduced for models that participated in earlier intercomparison exercises.« less

Modelling the Climate - Greenland Ice Sheet Interaction in the Coupled Ice-sheet/Climate Model EC-EARTH - PISM

NASA Astrophysics Data System (ADS)

Yang, S.; Madsen, M. S.; Rodehacke, C. B.; Svendsen, S. H.; Adalgeirsdottir, G.

2014-12-01

Recent observations show that the Greenland ice sheet (GrIS) has been losing mass with an increasing speed during the past decades. Predicting the GrIS changes and their climate consequences relies on the understanding of the interaction of the GrIS with the climate system on both global and local scales, and requires climate model systems with an explicit and physically consistent ice sheet module. A fully coupled global climate model with a dynamical ice sheet model for the GrIS has recently been developed. The model system, EC-EARTH - PISM, consists of the EC-EARTH, an atmosphere, ocean and sea ice model system, and the Parallel Ice Sheet Model (PISM). The coupling of PISM includes a modified surface physical parameterization in EC-EARTH adapted to the land ice surface over glaciated regions in Greenland. The PISM ice sheet model is forced with the surface mass balance (SMB) directly computed inside the EC-EARTH atmospheric module and accounting for the precipitation, the surface evaporation, and the melting of snow and ice over land ice. PISM returns the simulated basal melt, ice discharge and ice cover (extent and thickness) as boundary conditions to EC-EARTH. This coupled system is mass and energy conserving without being constrained by any anomaly correction or flux adjustment, and hence is suitable for investigation of ice sheet - climate feedbacks. Three multi-century experiments for warm climate scenarios under (1) the RCP85 climate forcing, (2) an abrupt 4xCO2 and (3) an idealized 1% per year CO2 increase are performed using the coupled model system. The experiments are compared with their counterparts of the standard CMIP5 simulations (without the interactive ice sheet) to evaluate the performance of the coupled system and to quantify the GrIS feedbacks. In particular, the evolution of the Greenland ice sheet under the warm climate and its impacts on the climate system are investigated. Freshwater fluxes from the Greenland ice sheet melt to the Arctic and North Atlantic basin and their influence on the ocean stratification and ocean circulation are analysed. The changes in the surface climate and the atmospheric circulation associated with the impact of the Greenland ice sheet changes are quantified. The interaction between the Greenland ice sheet and Arctic sea ice is also examined.

Results of the Greenland Ice Sheet Model Initialisation Experiments ISMIP6 - initMIP-Greenland

NASA Astrophysics Data System (ADS)

Goelzer, H.; Nowicki, S.; Edwards, T.; Beckley, M.; Abe-Ouchi, A.; Aschwanden, A.; Calov, R.; Gagliardini, O.; Gillet-chaulet, F.; Golledge, N. R.; Gregory, J. M.; Greve, R.; Humbert, A.; Huybrechts, P.; Larour, E. Y.; Lipscomb, W. H.; Le ´h, S.; Lee, V.; Kennedy, J. H.; Pattyn, F.; Payne, A. J.; Rodehacke, C. B.; Rückamp, M.; Saito, F.; Schlegel, N.; Seroussi, H. L.; Shepherd, A.; Sun, S.; Vandewal, R.; Ziemen, F. A.

2016-12-01

Earlier large-scale Greenland ice sheet sea-level projections e.g. those run during ice2sea and SeaRISE initiatives have shown that ice sheet initialisation can have a large effect on the projections and gives rise to important uncertainties. The goal of this intercomparison exercise (initMIP-Greenland) is to compare, evaluate and improve the initialization techniques used in the ice sheet modeling community and to estimate the associated uncertainties. It is the first in a series of ice sheet model intercomparison activities within ISMIP6 (Ice Sheet Model Intercomparison Project for CMIP6). Two experiments for the large-scale Greenland ice sheet have been designed to allow intercomparison between participating models of 1) the initial present-day state of the ice sheet and 2) the response in two schematic forward experiments. The forward experiments serve to evaluate the initialisation in terms of model drift (forward run without any forcing) and response to a large perturbation (prescribed surface mass balance anomaly). We present and discuss final results of the intercomparison and highlight important uncertainties with respect to projections of the Greenland ice sheet sea-level contribution.

Ice core evidence for extensive melting of the greenland ice sheet in the last interglacial.

PubMed

Koerner, R M

1989-05-26

Evidence from ice at the bottom of ice cores from the Canadian Arctic Islands and Camp Century and Dye-3 in Greenland suggests that the Greenland ice sheet melted extensively or completely during the last interglacial period more than 100 ka (thousand years ago), in contrast to earlier interpretations. The presence of dirt particles in the basal ice has previously been thought to indicate that the base of the ice sheets had melted and that the evidence for the time of original growth of these ice masses had been destroyed. However, the particles most likely blew onto the ice when the dimensions of the ice caps and ice sheets were much smaller. Ice texture, gas content, and other evidence also suggest that the basal ice at each drill site is superimposed ice, a type of ice typical of the early growth stages of an ice cap or ice sheet. If the present-day ice masses began their growth during the last interglacial, the ice sheet from the earlier (Illinoian) glacial period must have competely or largely melted during the early part of the same interglacial period. If such melting did occur, the 6-meter higher-than-present sea level during the Sangamon cannot be attributed to disintegration of the West Antarctic ice sheet, as has been suggested.

Low post-glacial rebound rates in the Weddell Sea due to Late Holocene ice-sheet readvance

NASA Astrophysics Data System (ADS)

Bradley, Sarah L.; Hindmarsh, Richard C. A.; Whitehouse, Pippa; Bentley, Michael J.; King, Matt

2014-05-01

The Holocene deglaciation of West Antarctica resulted in widespread ice surface lowering. While many ice-sheet reconstructions generally assume a monotone Holocene retreat for the West Antarctica Ice sheet (WAIS) [Ivins et al., 2013; Peltier, 2004; Whitehouse et al., 2012], an increasing number of glaciological observations infer it is readvancing, following retreat behind the present-day margin[Siegert et al., 2013]. We will show that a readvance in the Weddell Sea region can reconcile two outstanding problems: (i) the present-day widespread occurrence of seemingly stable ice-streams grounded on beds that deepen inland in apparent contradiction to theory [Schoof, 2007]; and (ii) the inability of models of Glacial Isostatic Adjustment (GIA) to match present-day uplift rates [Whitehouse et al., 2012]. Combining a suite of ice loading histories that include a readvance with a model of GIA provides significant improvements to predictions of present-day uplift rates, and we are able to reproduce previously unexplained observations of subsidence in the southern sector of the Weddell Sea. We hypothesize that retreat behind present grounding lines occurred when the bed was lower, and isostatic recovery led to shallowing, ice sheet re-grounding and readvance. We will conclude that some sections of the current WAIS grounding line that are theoretically unstable, may be advancing and that the volume change of the WAIS may have been more complex in the Late Holocene than previously posited. This revised Holocene ice-loading history would have important implications for the GIA correction applied to Gravity Recovery and Climate Experiment (GRACE) data, likely resulting in a reduction in the GIA correction and a smaller estimate of present-day ice mass loss within the Weddell Sea region of the WAIS. Ivins, E. R., T. S. James, J. Wahr, E. J. O. Schrama, F. W. Landerer, and K. M. Simon (2013), Antarctic contribution to sea level rise observed by GRACE with improved GIA correction, Journal of Geophysical Research: Solid Earth, 118(6), 3126-3141. Peltier, W. R. (2004), Global glacial isostasy and the surface of the ice-age earth: The ice-5G (VM2) model and grace, Annu Rev Earth Pl Sc, 32, 111-149. Schoof, C. (2007), Ice sheet grounding line dynamics: Steady states, stability, and hysteresis, Journal of Geophysical Research: Earth Surface, 112(F3). Siegert, M., N. Ross, H. Corr, J. Kingslake, and R. Hindmarsh (2013), Late Holocene ice-flow reconfiguration in the Weddell Sea sector of West Antarctica, Quaternary Sci Rev, 78(0), 98-107. Whitehouse, P. L., M. J. Bentley, G. A. Milne, M. A. King, and I. D. Thomas (2012), A new glacial isostatic adjustment model for Antarctica: calibrated and tested using observations of relative sea-level change and present-day uplift rates, Geophys J Int, 190(3), 1464-1482.

Interhemispheric ice-sheet synchronicity during the last glacial maximum

USGS Publications Warehouse

Weber, Michael E.; Clark, Peter U.; Ricken, Werner; Mitrovica, Jerry X.; Hostetler, Steven W.; Kuhn, Gerhard

2011-01-01

The timing of the last maximum extent of the Antarctic ice sheets relative to those in the Northern Hemisphere remains poorly understood. We develop a chronology for the Weddell Sea sector of the East Antarctic Ice Sheet that, combined with ages from other Antarctic ice-sheet sectors, indicates that the advance to and retreat from their maximum extent was within dating uncertainties synchronous with most sectors of Northern Hemisphere ice sheets. Surface climate forcing of Antarctic mass balance would probably cause an opposite response, whereby a warming climate would increase accumulation but not surface melting. Our new data support teleconnections involving sea-level forcing from Northern Hemisphere ice sheets and changes in North Atlantic deep-water formation and attendant heat flux to Antarctic grounding lines to synchronize the hemispheric ice sheets.

Interhemispheric ice-sheet synchronicity during the Last Glacial Maximum.

PubMed

Weber, Michael E; Clark, Peter U; Ricken, Werner; Mitrovica, Jerry X; Hostetler, Steven W; Kuhn, Gerhard

2011-12-02

The timing of the last maximum extent of the Antarctic ice sheets relative to those in the Northern Hemisphere remains poorly understood. We develop a chronology for the Weddell Sea sector of the East Antarctic Ice Sheet that, combined with ages from other Antarctic ice-sheet sectors, indicates that the advance to and retreat from their maximum extent was within dating uncertainties synchronous with most sectors of Northern Hemisphere ice sheets. Surface climate forcing of Antarctic mass balance would probably cause an opposite response, whereby a warming climate would increase accumulation but not surface melting. Our new data support teleconnections involving sea-level forcing from Northern Hemisphere ice sheets and changes in North Atlantic deep-water formation and attendant heat flux to Antarctic grounding lines to synchronize the hemispheric ice sheets.

Airborne Tomographic Swath Ice Sounding Processing System

NASA Technical Reports Server (NTRS)

Wu, Xiaoqing; Rodriquez, Ernesto; Freeman, Anthony; Jezek, Ken

2013-01-01

Glaciers and ice sheets modulate global sea level by storing water deposited as snow on the surface, and discharging water back into the ocean through melting. Their physical state can be characterized in terms of their mass balance and dynamics. To estimate the current ice mass balance, and to predict future changes in the motion of the Greenland and Antarctic ice sheets, it is necessary to know the ice sheet thickness and the physical conditions of the ice sheet surface and bed. This information is required at fine resolution and over extensive portions of the ice sheets. A tomographic algorithm has been developed to take raw data collected by a multiple-channel synthetic aperture sounding radar system over a polar ice sheet and convert those data into two-dimensional (2D) ice thickness measurements. Prior to this work, conventional processing techniques only provided one-dimensional ice thickness measurements along profiles.

Ice sheet climate modeling: past achievements, ongoing challenges, and future endeavors

NASA Astrophysics Data System (ADS)

Lenaerts, J.

2017-12-01

Fluctuations in surface mass balance (SMB) mask out a substantial portion of contemporary Greenland and Antarctic ice sheet mass loss. That implies that we need accurate, consistent, and long-term SMB time series to isolate the mass loss signal. This in turn requires understanding of the processes driving SMB, and how they interplay. The primary controls on present-day ice sheet SMB are snowfall, which is regulated by large-scale atmospheric variability, and surface meltwater production at the ice sheet's edges, which is a complex result of atmosphere-surface interactions. Additionally, wind-driven snow redistribution and sublimation are large SMB contributors on the downslope areas of the ice sheets. Climate models provide an integrated framework to simulate all these individual ice sheet components. Recent developments in RACMO2, a regional climate model bound by atmospheric reanalyses, have focused on enhancing horizontal resolution, including blowing snow, snow albedo, and meltwater processes. Including these physics not only enhanced our understanding of the ice sheet climate system, but also enabled to obtain increasingly accurate estimates of ice sheet SMB. However, regional models are not suitable to capture the mutual interactions between ice sheet and the remainder of the global climate system in transient climates. To take that next step, global climate models are essential. In this talk, I will highlight our present work on improving ice sheet climate in the Community Earth System Model (CESM). In particular, we focus on an improved representation of polar firn, ice sheet clouds, and precipitation. For this exercise, we extensively use field observations, remote sensing data, as well as RACMO2. Next, I will highlight how CESM is used to enhance our understanding of ice sheet SMB, its drivers, and past and present changes.

An approach to computing discrete adjoints for MPI-parallelized models applied to Ice Sheet System Model 4.11

NASA Astrophysics Data System (ADS)

Larour, Eric; Utke, Jean; Bovin, Anton; Morlighem, Mathieu; Perez, Gilberto

2016-11-01

Within the framework of sea-level rise projections, there is a strong need for hindcast validation of the evolution of polar ice sheets in a way that tightly matches observational records (from radar, gravity, and altimetry observations mainly). However, the computational requirements for making hindcast reconstructions possible are severe and rely mainly on the evaluation of the adjoint state of transient ice-flow models. Here, we look at the computation of adjoints in the context of the NASA/JPL/UCI Ice Sheet System Model (ISSM), written in C++ and designed for parallel execution with MPI. We present the adaptations required in the way the software is designed and written, but also generic adaptations in the tools facilitating the adjoint computations. We concentrate on the use of operator overloading coupled with the AdjoinableMPI library to achieve the adjoint computation of the ISSM. We present a comprehensive approach to (1) carry out type changing through the ISSM, hence facilitating operator overloading, (2) bind to external solvers such as MUMPS and GSL-LU, and (3) handle MPI-based parallelism to scale the capability. We demonstrate the success of the approach by computing sensitivities of hindcast metrics such as the misfit to observed records of surface altimetry on the northeastern Greenland Ice Stream, or the misfit to observed records of surface velocities on Upernavik Glacier, central West Greenland. We also provide metrics for the scalability of the approach, and the expected performance. This approach has the potential to enable a new generation of hindcast-validated projections that make full use of the wealth of datasets currently being collected, or already collected, in Greenland and Antarctica.

An Approach to Computing Discrete Adjoints for MPI-Parallelized Models Applied to the Ice Sheet System Model}

NASA Astrophysics Data System (ADS)

Perez, G. L.; Larour, E. Y.; Morlighem, M.

2016-12-01

Within the framework of sea-level rise projections, there is a strong need for hindcast validation of the evolution of polar ice sheets in a way that tightly matches observational records (from radar and altimetry observations mainly). However, the computational requirements for making hindcast reconstructions possible are severe and rely mainly on the evaluation of the adjoint state of transient ice-flow models. Here, we look at the computation of adjoints in the context of the NASA/JPL/UCI Ice Sheet System Model, written in C++ and designed for parallel execution with MPI. We present the adaptations required in the way the software is designed and written but also generic adaptations in the tools facilitating the adjoint computations. We concentrate on the use of operator overloading coupled with the AdjoinableMPI library to achieve the adjoint computation of ISSM. We present a comprehensive approach to 1) carry out type changing through ISSM, hence facilitating operator overloading, 2) bind to external solvers such as MUMPS and GSL-LU and 3) handle MPI-based parallelism to scale the capability. We demonstrate the success of the approach by computing sensitivities of hindcast metrics such as the misfit to observed records of surface altimetry on the North-East Greenland Ice Stream, or the misfit to observed records of surface velocities on Upernavik Glacier, Central West Greenland. We also provide metrics for the scalability of the approach, and the expected performance. This approach has the potential of enabling a new generation of hindcast-validated projections that make full use of the wealth of datasets currently being collected, or alreay collected in Greenland and Antarctica, such as surface altimetry, surface velocities, and/or gravity measurements.

Mass Balance Changes and Ice Dynamics of Greenland and Antarctic Ice Sheets from Laser Altimetry

NASA Astrophysics Data System (ADS)

Babonis, G. S.; Csatho, B.; Schenk, T.

2016-06-01

During the past few decades the Greenland and Antarctic ice sheets have lost ice at accelerating rates, caused by increasing surface temperature. The melting of the two big ice sheets has a big impact on global sea level rise. If the ice sheets would melt down entirely, the sea level would rise more than 60 m. Even a much smaller rise would cause dramatic damage along coastal regions. In this paper we report about a major upgrade of surface elevation changes derived from laser altimetry data, acquired by NASA's Ice, Cloud and land Elevation Satellite mission (ICESat) and airborne laser campaigns, such as Airborne Topographic Mapper (ATM) and Land, Vegetation and Ice Sensor (LVIS). For detecting changes in ice sheet elevations we have developed the Surface Elevation Reconstruction And Change detection (SERAC) method. It computes elevation changes of small surface patches by keeping the surface shape constant and considering the absolute values as surface elevations. We report about important upgrades of earlier results, for example the inclusion of local ice caps and the temporal extension from 1993 to 2014 for the Greenland Ice Sheet and for a comprehensive reconstruction of ice thickness and mass changes for the Antarctic Ice Sheets.

Preservation of a Preglacial Landscape Under the Center of the Greenland Ice Sheet

NASA Technical Reports Server (NTRS)

Bierman, Paul R.; Corbett, Lee B.; Graly, Joseph A.; Neumann, Thomas Allen; Lini, Andrea; Crosby, Benjamin T.; Rood, Dylan H.

2014-01-01

Continental ice sheets typically sculpt landscapes via erosion; under certain conditions, ancient landscapes can be preserved beneath ice and can survive extensive and repeated glaciation. We used concentrations of atmospherically produced cosmogenic beryllium-10, carbon, and nitrogen to show that ancient soil has been preserved in basal ice for millions of years at the center of the ice sheet at Summit, Greenland. This finding suggests ice sheet stability through the Pleistocene (i.e., the past 2.7 million years). The preservation of this soil implies that the ice has been non-erosive and frozen to the bed for much of that time, that there was no substantial exposure of central Greenland once the ice sheet became fully established, and that preglacial landscapes can remain preserved for long periods under continental ice sheets

Estimating the impact of internal climate variability on ice sheet model simulations

NASA Astrophysics Data System (ADS)

Tsai, C. Y.; Forest, C. E.; Pollard, D.

2016-12-01

Rising sea level threatens human societies and coastal habitats and melting ice sheets are a major contributor to sea level rise (SLR). Thus, understanding uncertainty of both forcing and variability within the climate system is essential for assessing long-term risk of SLR given their impact on ice sheet evolution. The predictability of polar climate is limited by uncertainties from the given forcing, the climate model response to this forcing, and the internal variability from feedbacks within the fully coupled climate system. Among those sources of uncertainty, the impact of internal climate variability on ice sheet changes has not yet been robustly assessed. Here we investigate how internal variability affects ice sheet projections using climate fields from two Community Earth System Model (CESM) large-ensemble (LE) experiments to force a three-dimensional ice sheet model. Each ensemble member in an LE experiment undergoes the same external forcings but with unique initial conditions. We find that for both LEs, 2m air temperature variability over Greenland ice sheet (GrIS) can lead to significantly different ice sheet responses. Our results show that the internal variability from two fully coupled CESM LEs can cause about 25 35 mm differences of GrIS's contribution to SLR in 2100 compared to present day (about 20% of the total change), and 100m differences of SLR in 2300. Moreover, only using ensemble-mean climate fields as the forcing in ice sheet model can significantly underestimate the melt of GrIS. As the Arctic region becomes warmer, the role of internal variability is critical given the complex nonlinear interactions between surface temperature and ice sheet. Our results demonstrate that internal variability from coupled atmosphere-ocean general circulation model can affect ice sheet simulations and the resulting sea-level projections. This study highlights an urgent need to reassess associated uncertainties of projecting ice sheet loss over the next few centuries to obtain robust estimates of the contribution of ice sheet melt to SLR.

Probability based hydrologic catchments of the Greenland Ice Sheet

NASA Astrophysics Data System (ADS)

Hudson, B. D.

2015-12-01

Greenland Ice Sheet melt water impacts ice sheet flow dynamics, fjord and coastal circulation, and sediment and biogeochemical fluxes. Melt water exiting the ice sheet also is a key term in its mass balance. Because of this, knowledge of the area of the ice sheet that contributes melt water to a given outlet (its hydrologic catchment) is important to many ice sheet studies and is especially critical to methods using river runoff to assess ice sheet mass balance. Yet uncertainty in delineating ice sheet hydrologic catchments is a problem that is rarely acknowledged. Ice sheet catchments are delineated as a function of both basal and surface topography. While surface topography is well known, basal topography is less certain because it is dependent on radar surveys. Here, I a present a Monte Carlo based approach to delineating ice sheet catchments that quantifies the impact of uncertain basal topography. In this scheme, over many iterations I randomly vary the ice sheet bed elevation within published error bounds (using Morlighem et al., 2014 bed and bed error datasets). For each iteration of ice sheet bed elevation, I calculate the hydraulic potentiometric surface and route water over its path of 'steepest' descent to delineate the catchment. I then use all realizations of the catchment to arrive at a probability map of all major melt water outlets in Greenland. I often find that catchment size is uncertain, with small, random perturbations in basal topography leading to large variations in catchments size. While some catchments are well defined, others can double or halve in size within published basal topography error bars. While some uncertainty will likely always remain, this work points to locations where studies of ice sheet hydrology would be the most successful, allows reinterpretation of past results, and points to where future radar surveys would be most advantageous.

A Transient Initialization Routine of the Community Ice Sheet Model for the Greenland Ice Sheet

NASA Astrophysics Data System (ADS)

van der Laan, Larissa; van den Broeke, Michiel; Noël, Brice; van de Wal, Roderik

2017-04-01

The Community Ice Sheet Model (CISM) is to be applied in future simulations of the Greenland Ice Sheet under a range of climate change scenarios, determining the sensitivity of the ice sheet to individual climatic forcings. In order to achieve reliable results regarding ice sheet stability and assess the probability of future occurrence of tipping points, a realistic initial ice sheet geometry is essential. The current work describes and evaluates the development of a transient initialization routine, using NGRIP 18O isotope data to create a temperature anomaly field. Based on the latter, surface mass balance components runoff and precipitation are perturbed for the past 125k years. The precipitation and runoff fields originate from a downscaled 1 km resolution version of the regional climate model RACMO2.3 for the period 1961-1990. The result of the initialization routine is a present-day ice sheet with a transient memory of the last glacial-interglacial cycle, which will serve as the future runs' initial condition.

Polar ice-sheet contributions to sea level during past warm periods

NASA Astrophysics Data System (ADS)

Dutton, A.

2015-12-01

Recent sea-level rise has been dominated by thermal expansion and glacier loss, but the contribution from mass loss from the Greenland and Antarctic ice sheets is expected to exceed other contributions under future sustained warming. Due to limitations of existing ice sheet models and the lack of relevant analogues in the historical record, projecting the timing and magnitude of polar ice sheet mass loss in the future remains challenging. One approach to improving our understanding of how polar ice-sheet retreat will unfold is to integrate observations and models of sea level, ice sheets, and climate during past intervals of warmth when the polar ice sheets contributed to higher sea levels. A recent review evaluated the evidence of polar ice sheet mass loss during several warm periods, including interglacials during the mid-Pliocene warm period, Marine Isotope Stage (MIS) 11, 5e (Last Interglacial), and 1 (Holocene). Sea-level benchmarks of ice-sheet retreat during the first of these three periods, when global mean climate was ~1 to 3 deg. C warmer than preindustrial, are useful for understanding the long-term potential for future sea-level rise. Despite existing uncertainties in these reconstructions, it is clear that our present climate is warming to a level associated with significant polar ice-sheet loss in the past, resulting in a conservative estimate for a global mean sea-level rise of 6 meters above present (or more). This presentation will focus on identifying the approaches that have yielded significant advances in terms of past sea level and ice sheet reconstruction as well as outstanding challenges. A key element of recent advances in sea-level reconstructions is the ability to recognize and quantify the imprint of geophysical processes, such as glacial isostatic adjustment (GIA) and dynamic topography, that lead to significant spatial variability in sea level reconstructions. Identifying specific ice-sheet sources that contributed to higher sea levels is a challenge that is currently hindered by limited field evidence at high latitudes. Finally, I will explore the concept of how increasing the quantity and quality of paleo sea level and ice sheet reconstructions can lead to improved quantification of contemporary changes in ice sheets and sea level.

Monitoring southwest Greenland's ice sheet melt with ambient seismic noise.

PubMed

Mordret, Aurélien; Mikesell, T Dylan; Harig, Christopher; Lipovsky, Bradley P; Prieto, Germán A

2016-05-01

The Greenland ice sheet presently accounts for ~70% of global ice sheet mass loss. Because this mass loss is associated with sea-level rise at a rate of 0.7 mm/year, the development of improved monitoring techniques to observe ongoing changes in ice sheet mass balance is of paramount concern. Spaceborne mass balance techniques are commonly used; however, they are inadequate for many purposes because of their low spatial and/or temporal resolution. We demonstrate that small variations in seismic wave speed in Earth's crust, as measured with the correlation of seismic noise, may be used to infer seasonal ice sheet mass balance. Seasonal loading and unloading of glacial mass induces strain in the crust, and these strains then result in seismic velocity changes due to poroelastic processes. Our method provides a new and independent way of monitoring (in near real time) ice sheet mass balance, yielding new constraints on ice sheet evolution and its contribution to global sea-level changes. An increased number of seismic stations in the vicinity of ice sheets will enhance our ability to create detailed space-time records of ice mass variations.

Basal melt beneath whillans ice stream and ice streams A and C

NASA Technical Reports Server (NTRS)

Joughin, I.; Teluezyk, S.; Engelhardt, H.

2002-01-01

We have used a recently derived map of the velocity of Whillans Ice Stream and Ice Streams A and C to help estimate basal melt. Temperature was modeled with a simple vertical advection-diffusion equation, 'tuned' to match temperature profiles. We find that most of the melt occurs beneath the tributaries where larger basal shear stresses and thicker ice favors greater melt (e.g., 10-20 mm/yr). The occurrence of basal freezing is predicted beneath much of the ice plains of Ice Stream C and Whillans Ice Stream. Modelled melt rates for when Ice Stream C was active suggest there was just enough melt water generated in its tributaries to balance basal freezing on its ice plain. Net basal melt for Whillans Ice Stream is positive due to smaller basal temperature gradients. Modelled temperatures on Whillans Ice Stream, however, were constrained by a single temperature profile at UpB. Basal temperature gradients for Whillans B1 and Ice Stream A may have conditions more similar to those beneath Ice Streams C and D, in which case, there may not be sufficient melt to sustain motion. This would be consistent with the steady deceleration of Whillans stream over the last few decades.

Hypsometric amplification and routing moderation of Greenland ice sheet meltwater release

NASA Astrophysics Data System (ADS)

van As, Dirk; Mikkelsen, Andreas Bech; Holtegaard Nielsen, Morten; Box, Jason E.; Claesson Liljedahl, Lillemor; Lindbäck, Katrin; Pitcher, Lincoln; Hasholt, Bent

2017-06-01

Concurrent ice sheet surface runoff and proglacial discharge monitoring are essential for understanding Greenland ice sheet meltwater release. We use an updated, well-constrained river discharge time series from the Watson River in southwest Greenland, with an accurate, observation-based ice sheet surface mass balance model of the ˜ 12 000 km2 ice sheet area feeding the river. For the 2006-2015 decade, we find a large range of a factor of 3 in interannual variability in discharge. The amount of discharge is amplified ˜ 56 % by the ice sheet's hypsometry, i.e., area increase with elevation. A good match between river discharge and ice sheet surface meltwater production is found after introducing elevation-dependent transit delays that moderate diurnal variability in meltwater release by a factor of 10-20. The routing lag time increases with ice sheet elevation and attains values in excess of 1 week for the upper reaches of the runoff area at ˜ 1800 m above sea level. These multi-day routing delays ensure that the highest proglacial discharge levels and thus overbank flooding events are more likely to occur after multi-day melt episodes. Finally, for the Watson River ice sheet catchment, we find no evidence of meltwater storage in or release from the en- and subglacial environments in quantities exceeding our methodological uncertainty, based on the good match between ice sheet runoff and proglacial discharge.

Long-Term Changes In The Behaviour Of Jakobshavns Isbrae, West Greenland During The Late Quaternary-Holocene

NASA Astrophysics Data System (ADS)

O'Cofaigh, C.; Jennings, A.; Moros, M.; Andrews, J. T.; Kilfeather, A.; Dowdeswell, J. A.; Richter, T.

2008-12-01

This poster shows the initial results of a joint scientific project to reconstruct the Late Quaternary-Holocene behavior of Jakobshavns Isbrae in central west Greenland, one of the largest ice streams draining the modern Greenland Ice Sheet. The underlying rationale for this research is to determine if recent observed changes to the mass balance of the Greenland Ice Sheet are part of the natural variability in ice-sheet dynamics, or if they relate to anthropogenically-induced climate warming. Key to resolving this question is an understanding of long-term changes in ice sheet behavior during the Late Quaternary and the Holocene. This research will allow assessment of the links between deglaciation and internal and external environmental controls, such as the influence of inflowing Atlantic Water, and will facilitate modelling of the likely future behavior of the GIS. Currently, four marine sediment cores arrayed along a transect from the Disko Bugt Fan to Disko Bay are providing information on changes in sediment flux and sedimentation style, such as abrupt intervals of iceberg-rafting vs. "normal" hemipelagic sedimentation, as well as the paleoceanographic setting and ice sheet-ocean interactions. The cores are being analysed using a variety of proxies including IRD, mineralogy, oxygen isotopes, foraminiferal assemblages, lithofacies analysis and AMS radiocarbon dating. Data are presented from two piston cores from the continental slope at the trough-mouth fan collected during the HE0006 'shakedown' cruise to Baffin Bay and from two gravity cores recovered in 2007 during MS Merian cruise MSM 05/03 to West Greenland. Slope cores contain sequences of laminated facies interpreted as fine-grained turbidites and intervals of massive, bioturbated, hemipelagic mud. The two Merian cores, contributed to this project by the Baltic Sea Research Institute, were collected from the southern entrance to Disko Bugt and the Vaigat channel north of Disko. Radiocarbon dates from the Disko Bugt core show that it contains a full Holocene record of glacial activity and paleoceanography. The poster will present the initial analyses, including radiocarbon dating, XRF compositional data, magnetic susceptibility, lithofacies and IRD analyses determined from x-radiography, foraminiferal analyses and sediment mineralogy. Additional cores and seismic data for this project will be obtained from a cruise on the Canadian research vessel, CSS Hudson in September 2008, and on the British ship, the RRS James Clark Ross in 2009.

Greenland ice sheet motion insensitive to exceptional meltwater forcing.

PubMed

Tedstone, Andrew J; Nienow, Peter W; Sole, Andrew J; Mair, Douglas W F; Cowton, Thomas R; Bartholomew, Ian D; King, Matt A

2013-12-03

Changes to the dynamics of the Greenland ice sheet can be forced by various mechanisms including surface-melt-induced ice acceleration and oceanic forcing of marine-terminating glaciers. We use observations of ice motion to examine the surface melt-induced dynamic response of a land-terminating outlet glacier in southwest Greenland to the exceptional melting observed in 2012. During summer, meltwater generated on the Greenland ice sheet surface accesses the ice sheet bed, lubricating basal motion and resulting in periods of faster ice flow. However, the net impact of varying meltwater volumes upon seasonal and annual ice flow, and thus sea level rise, remains unclear. We show that two extreme melt events (98.6% of the Greenland ice sheet surface experienced melting on July 12, the most significant melt event since 1889, and 79.2% on July 29) and summer ice sheet runoff ~3.9 σ above the 1958-2011 mean resulted in enhanced summer ice motion relative to the average melt year of 2009. However, despite record summer melting, subsequent reduced winter ice motion resulted in 6% less net annual ice motion in 2012 than in 2009. Our findings suggest that surface melt-induced acceleration of land-terminating regions of the ice sheet will remain insignificant even under extreme melting scenarios.

Greenland ice sheet retreat since the Little Ice Age

NASA Astrophysics Data System (ADS)

Beitch, Marci J.

Late 20th century and 21st century satellite imagery of the perimeter of the Greenland Ice Sheet (GrIS) provide high resolution observations of the ice sheet margins. Examining changes in ice margin positions over time yield measurements of GrIS area change and rates of margin retreat. However, longer records of ice sheet margin change are needed to establish more accurate predictions of the ice sheet's future response to global conditions. In this study, the trimzone, the area of deglaciated terrain along the ice sheet edge that lacks mature vegetation cover, is used as a marker of the maximum extent of the ice from its most recent major advance during the Little Ice Age. We compile recently acquired Landsat ETM+ scenes covering the perimeter of the GrIS on which we map area loss on land-, lake-, and marine-terminating margins. We measure an area loss of 13,327 +/- 830 km2, which corresponds to 0.8% shrinkage of the ice sheet. This equates to an averaged horizontal retreat of 363 +/- 69 m across the entire GrIS margin. Mapping the areas exposed since the Little Ice Age maximum, circa 1900 C.E., yields a century-scale rate of change. On average the ice sheet lost an area of 120 +/- 16 km 2/yr, or retreated at a rate of 3.3 +/- 0.7 m/yr since the LIA maximum.

Laurentide glacial landscapes: the role of ice streams

USGS Publications Warehouse

Patterson, C.J.

1998-01-01

Glacial landforms of the North American prairie can be divided into two suites that result from different styles of ice flow: 1) a lowland suite of level-to-streamlined till consistent with formation beneath ice streams, and 2) an upland and lobe-margin suite of thick, hummocky till and glacial thrust blocks consistent with formation at ice-stream and ice-lobe margins. Southern Laurentide ice lobes hypothetically functioned as outlets of ice streams. Broad branching lowlands bounded by escarpments mark the stable positions of the ice streams that fed the lobes. If the lobes and ice streams were similar to modern ice streams, their fast flow was facilitated by high subglacial water pressure. Favorable geology and topography in the midcontinent encouraged nonuniform ice flow and controlled the location of ice streams and outlet lobes.

Dynamic Antarctic ice sheet during the early to mid-Miocene

PubMed Central

DeConto, Robert M.; Pollard, David; Levy, Richard H.

2016-01-01

Geological data indicate that there were major variations in Antarctic ice sheet volume and extent during the early to mid-Miocene. Simulating such large-scale changes is problematic because of a strong hysteresis effect, which results in stability once the ice sheets have reached continental size. A relatively narrow range of atmospheric CO2 concentrations indicated by proxy records exacerbates this problem. Here, we are able to simulate large-scale variability of the early to mid-Miocene Antarctic ice sheet because of three developments in our modeling approach. (i) We use a climate–ice sheet coupling method utilizing a high-resolution atmospheric component to account for ice sheet–climate feedbacks. (ii) The ice sheet model includes recently proposed mechanisms for retreat into deep subglacial basins caused by ice-cliff failure and ice-shelf hydrofracture. (iii) We account for changes in the oxygen isotopic composition of the ice sheet by using isotope-enabled climate and ice sheet models. We compare our modeling results with ice-proximal records emerging from a sedimentological drill core from the Ross Sea (Andrill-2A) that is presented in a companion article. The variability in Antarctic ice volume that we simulate is equivalent to a seawater oxygen isotope signal of 0.52–0.66‰, or a sea level equivalent change of 30–36 m, for a range of atmospheric CO2 between 280 and 500 ppm and a changing astronomical configuration. This result represents a substantial advance in resolving the long-standing model data conflict of Miocene Antarctic ice sheet and sea level variability. PMID:26903645

Greenland Ice Sheet Surface Temperature, Melt, and Mass Loss: 2000-2006

NASA Technical Reports Server (NTRS)

Hall, Dorothy K.; Williams, Richard S., Jr.; Luthcke, Scott B.; DiGirolamo, Nocolo

2007-01-01

Extensive melt on the Greenland Ice Sheet has been documented by a variety of ground and satellite measurements in recent years. If the well-documented warming continues in the Arctic, melting of the Greenland Ice Sheet will likely accelerate, contributing to sea-level rise. Modeling studies indicate that an annual or summer temperature rise of 1 C on the ice sheet will increase melt by 20-50% therefore, surface temperature is one of the most important ice-sheet parameters to study for analysis of changes in the mass balance of the ice-sheet. The Greenland Ice Sheet contains enough water to produce a rise in eustatic sea level of up to 7.0 m if the ice were to melt completely. However, even small changes (centimeters) in sea level would cause important economic and societal consequences in the world's major coastal cities thus it is extremely important to monitor changes in the ice-sheet surface temperature and to ultimately quantify these changes in terms of amount of sea-level rise. We have compiled a high-resolution, daily time series of surface temperature of the Greenland Ice Sheet, using the I-km resolution, clear-sky land-surface temperature (LST) standard product from the Moderate-Resolution Imaging Spectroradiometer (MODIS), from 2000 - 2006. We also use Gravity Recovery and Climate Experiment (GRACE) data, averaged over 10-day periods, to measure change in mass of the ice sheet as it melt and snow accumulates. Surface temperature can be used to determine frequency of surface melt, timing of the start and the end of the melt season, and duration of melt. In conjunction with GRACE data, it can also be used to analyze timing of ice-sheet mass loss and gain.

Terrestrial Ice Sheets: Studies of Climate History, Internal Structure, Surface, and Bedrock

NASA Astrophysics Data System (ADS)

Thorsteinsson, Th.; Kipfstuhl, J.; Nixdorf, U.; Oerter, H.; Miller, H.; Fritsche, D.; Jung-Rothenhaeusler, F.; Mayer, C.; Schwager, M.; Wilhelms, F.; Steinhage, D.; Goektas, F.

1998-01-01

Recently drilled deep ice cores from Central Greenland (GRIP and GISP2) provide the most detailed results available on climatic variation in the northern hemisphere during the last 100,000 years, a period that includes the Holocene (0-11.5 ka) and most of the Wisconsin glacial period. Summer-winter variation in various physical and chemical properties of polar ice allows dating of ice cores by annual layer counting. Several such methods are currently being employed on an ice core drilled by the new North Greenland Ice Core Project (NGRIP), which is aimed at extending the Greenland ice palaeoclimatic record through the last interglacial, the Eemian. Two examples will be presented: (1) visual and photographic studies of seasonal variation in stratigraphic layering, crystal size, air bubble and clathrate concentration, and (2) studies of electric stratigraphy, using the method of dielectric profiling (DEP). This method records the AC conductivity of ice cores, which is negatively correlated with the concentration of airborne dust in the ice but positively correlated with volcanic and marine aerosols. Comprehensive surface traverse programs, which include shallow coring and ice velocity measurements, have recently been carried out by the Alfred Wegener Institute in previously little-investigated regions of Greenland and Antarctica. Serving partly as reconnaissance prior to deep drilling projects, such studies also help to reduce considerable uncertainties in the mass balance of the two large polar ice sheets and thus in their estimated response to climate change. Main results of a recent traverse in North Greenland include the following: (1) A new map of the accumulation distribution on the ice sheet indicates a large low-accumulation region in Northeast-Greenland; (2) North Greenland records show significantly greater climatic variability during the last 500 yr than corresponding records from the southern part of the ice sheet; and (3) data on variation in accumulation rates do not indicate a definite trend in the region during this century. The Alfred Wegener Institute has in recent years employed both airborne and ground-penetrating ice radar systems to map the bedrock around deep drilling sites in Central and North Greenland, as well as in a planned Antarctic site in Dronning Maud Land. The radar also records shallow and deep internal echoes, caused by rapid variation in density and ice acidity in layers of certain ages, allowing isochrones to be traced over wide reaches of the ice sheet. Disturbances in regular stratigraphic layering, due to ice flow over an irregular bed, were observed in the lowest 200-300 m of the GRIP and GISP2 ice cores. Since the aim of the new NGRIP coring program is to obtain an ice core reaching further back in time than the Central Greenland cores, this site was chosen in a region where the bedrock is relatively flat. Echo-sounding surveys between GRIP and NGREP show that the isochrones lie 100-200 in higher above the bed at NGRIP, indicating that the Eemian layer is unlikely to have been disturbed by ice flow at this location. Due to the flow pattern of ice sheets, layers forming a vertical sequence in the interior regions of an ice sheet can, under favorable conditions, be traced on horizontal profiles at the margins. Some meaningful correlations have already been established between Greenland deep ice core climatic records and corresponding records from ice margins. In these regions, a clear contrast is observed between ice of Holocene origin and significantly darker-looking ice dating from the Wisconsin glacial period, which displays summertime ablation rates 2-4x higher than the Holocene ice. This difference is due to higher concentrations of dust and other impurities in the Wisconsin ice, by 1-2 orders of magnitude, leading to reduced albedo. Furthermore, smaller crystal sizes in the Wisconsin ice lead to a more homogeneous distribution of impurities on the surface, which probably contributes to lowering the albedo. Comprehensive studies of ice crystal size and c-axis orientations on the GRIP and NGRIP deep cores provide detailed information on recrystallization processes in polar ice sheets. Based on the GRIP results, the Central-Greenland ice sheet can be vertically divided into three different recrystallization regimes: (1) normal grain growth regime (0-700 in), in which the average crystal size increases steadily to 4mm diameter; (2) polygonization regime (700-2800m), in which crystals are subdivided due to increasing strain and no further increase in crystal size is observed; and (3) migration recrystallization regime (2800-3050m), where higher temperatures (-10C) cause rapid crystal growth with average diameters increasing to 30 mm in the bottom layers. Higher impurity content in ice dating from glacial periods is seen to exert a strong inhibitive effect on crystal growth. The data on c-axis fabrics demonstrate the development of crystalline anisotropy with depth, leading to significant variation in flow properties. In particular, strong rheological contrasts are observed between glacial and interglacial ice, with fine-grained ice dating from glacial periods deforming more rapidly under conditions of simple shear than more coarse-grained interglacial ice. When the dynamics of ice masses are addressed by modeling, special attention must be given to the transition zone between ice resting on bedrock and floating ice shelves. One application for numerical ice-dynamics models that deal with such transition zones is the investigation of areas with special mass balance characteristics, like ice streams entering ice shelves or ice sheet areas over subglacial lakes. Recent results from a model applied to the ice above Lake Vostok in East Antarctica indicate that comparatively strong basal melting and adjacent refreezing occur close to the western shore of the lake.

Dynamics of the last glacial maximum Antarctic ice-sheet and its response to ocean forcing

PubMed Central

Golledge, Nicholas R.; Fogwill, Christopher J.; Mackintosh, Andrew N.; Buckley, Kevin M.

2012-01-01

Retreat of the Last Glacial Maximum (LGM) Antarctic ice sheet is thought to have been initiated by changes in ocean heat and eustatic sea level propagated from the Northern Hemisphere (NH) as northern ice sheets melted under rising atmospheric temperatures. The extent to which spatial variability in ice dynamics may have modulated the resultant pattern and timing of decay of the Antarctic ice sheet has so far received little attention, however, despite the growing recognition that dynamic effects account for a sizeable proportion of mass-balance changes observed in modern ice sheets. Here we use a 5-km resolution whole-continent numerical ice-sheet model to assess whether differences in the mechanisms governing ice sheet flow could account for discrepancies between geochronological studies in different parts of the continent. We first simulate the geometry and flow characteristics of an equilibrium LGM ice sheet, using pan-Antarctic terrestrial and marine geological data for constraint, then perturb the system with sea level and ocean heat flux increases to investigate ice-sheet vulnerability. Our results identify that fast-flowing glaciers in the eastern Weddell Sea, the Amundsen Sea, central Ross Sea, and in the Amery Trough respond most rapidly to ocean forcings, in agreement with empirical data. Most significantly, we find that although ocean warming and sea-level rise bring about mainly localized glacier acceleration, concomitant drawdown of ice from neighboring areas leads to widespread thinning of entire glacier catchments—a discovery that has important ramifications for the dynamic changes presently being observed in modern ice sheets. PMID:22988078

Dynamics of the last glacial maximum Antarctic ice-sheet and its response to ocean forcing.

PubMed

Golledge, Nicholas R; Fogwill, Christopher J; Mackintosh, Andrew N; Buckley, Kevin M

2012-10-02

Retreat of the Last Glacial Maximum (LGM) Antarctic ice sheet is thought to have been initiated by changes in ocean heat and eustatic sea level propagated from the Northern Hemisphere (NH) as northern ice sheets melted under rising atmospheric temperatures. The extent to which spatial variability in ice dynamics may have modulated the resultant pattern and timing of decay of the Antarctic ice sheet has so far received little attention, however, despite the growing recognition that dynamic effects account for a sizeable proportion of mass-balance changes observed in modern ice sheets. Here we use a 5-km resolution whole-continent numerical ice-sheet model to assess whether differences in the mechanisms governing ice sheet flow could account for discrepancies between geochronological studies in different parts of the continent. We first simulate the geometry and flow characteristics of an equilibrium LGM ice sheet, using pan-Antarctic terrestrial and marine geological data for constraint, then perturb the system with sea level and ocean heat flux increases to investigate ice-sheet vulnerability. Our results identify that fast-flowing glaciers in the eastern Weddell Sea, the Amundsen Sea, central Ross Sea, and in the Amery Trough respond most rapidly to ocean forcings, in agreement with empirical data. Most significantly, we find that although ocean warming and sea-level rise bring about mainly localized glacier acceleration, concomitant drawdown of ice from neighboring areas leads to widespread thinning of entire glacier catchments-a discovery that has important ramifications for the dynamic changes presently being observed in modern ice sheets.

Simulation of the Greenland Ice Sheet over two glacial-interglacial cycles: investigating a sub-ice-shelf melt parameterization and relative sea level forcing in an ice-sheet-ice-shelf model

NASA Astrophysics Data System (ADS)

Bradley, Sarah L.; Reerink, Thomas J.; van de Wal, Roderik S. W.; Helsen, Michiel M.

2018-05-01

Observational evidence, including offshore moraines and sediment cores, confirm that at the Last Glacial Maximum (LGM) the Greenland ice sheet (GrIS) expanded to a significantly larger spatial extent than seen at present, grounding into Baffin Bay and out onto the continental shelf break. Given this larger spatial extent and its close proximity to the neighbouring Laurentide Ice Sheet (LIS) and Innuitian Ice Sheet (IIS), it is likely these ice sheets will have had a strong non-local influence on the spatial and temporal behaviour of the GrIS. Most previous paleo ice-sheet modelling simulations recreated an ice sheet that either did not extend out onto the continental shelf or utilized a simplified marine ice parameterization which did not fully include the effect of ice shelves or neglected the sensitivity of the GrIS to this non-local bedrock signal from the surrounding ice sheets. In this paper, we investigated the evolution of the GrIS over the two most recent glacial-interglacial cycles (240 ka BP to the present day) using the ice-sheet-ice-shelf model IMAU-ICE. We investigated the solid earth influence of the LIS and IIS via an offline relative sea level (RSL) forcing generated by a glacial isostatic adjustment (GIA) model. The RSL forcing governed the spatial and temporal pattern of sub-ice-shelf melting via changes in the water depth below the ice shelves. In the ensemble of simulations, at the glacial maximums, the GrIS coalesced with the IIS to the north and expanded to the continental shelf break to the southwest but remained too restricted to the northeast. In terms of the global mean sea level contribution, at the Last Interglacial (LIG) and LGM the ice sheet added 1.46 and -2.59 m, respectively. This LGM contribution by the GrIS is considerably higher (˜ 1.26 m) than most previous studies whereas the contribution to the LIG highstand is lower (˜ 0.7 m). The spatial and temporal behaviour of the northern margin was highly variable in all simulations, controlled by the sub-ice-shelf melting which was dictated by the RSL forcing and the glacial history of the IIS and LIS. In contrast, the southwestern part of the ice sheet was insensitive to these forcings, with a uniform response in all simulations controlled by the surface air temperature, derived from ice cores.

Final Laurentide ice-sheet deglaciation and Holocene climate-sea level change

USGS Publications Warehouse

Ullman, David J.; Carlson, Anders E.; Hostetler, Steven W.; Clark, Peter U.; Cuzzone, Joshua; Milne, Glenn A.; Winsor, Kelsey; Caffee, Marc A.

2016-01-01

Despite elevated summer insolation forcing during the early Holocene, global ice sheets retained nearly half of their volume from the Last Glacial Maximum, as indicated by deglacial records of global mean sea level (GMSL). Partitioning the GMSL rise among potential sources requires accurate dating of ice-sheet extent to estimate ice-sheet volume. Here, we date the final retreat of the Laurentide Ice Sheet with 10Be surface exposure ages for the Labrador Dome, the largest of the remnant Laurentide ice domes during the Holocene. We show that the Labrador Dome deposited moraines during North Atlantic cold events at ∼10.3 ka, 9.3 ka and 8.2 ka, suggesting that these regional climate events helped stabilize the retreating Labrador Dome in the early Holocene. After Hudson Bay became seasonally ice free at ∼8.2 ka, the majority of Laurentide ice-sheet melted abruptly within a few centuries. We demonstrate through high-resolution regional climate model simulations that the thermal properties of a seasonally ice-free Hudson Bay would have increased Laurentide ice-sheet ablation and thus contributed to the subsequent rapid Labrador Dome retreat. Finally, our new 10Be chronology indicates full Laurentide ice-sheet had completely deglaciated by 6.7 ± 0.4 ka, which re quires that Antarctic ice sheets contributed 3.6–6.5 m to GMSL rise since 6.3–7.1 ka.

The Svalbard-Barents Sea ice-sheet - Historical, current and future perspectives

NASA Astrophysics Data System (ADS)

Ingólfsson, Ólafur; Landvik, Jon Y.

2013-03-01

The history of research on the Late Quaternary Svalbard-Barents Sea ice sheet mirrors the developments of ideas and the shifts of paradigms in glacial theory over the past 150 years. Since the onset of scientific research there in the early 19th Century, Svalbard has been a natural laboratory where ideas and concepts have been tested, and played an important (but rarely acknowledged) role in the break-through of the Ice Age theory in the 1870's. The history of how the scientific perception of the Svalbard-Barents sea ice sheet developed in the mid-20th Century also tells a story of how a combination of fairly scattered and often contradictory observational data, and through both deductive and inductive reasoning, could outline a major ice sheet that had left but few tangible fingerprints. Since the 1980's, with increased terrestrial stratigraphical data, ever more marine geological evidence and better chronological control of glacial events, our perception of the Svalbard-Barents Sea ice sheet has changed. The first reconstructions depicted it as a static, concentric, single-domed ice sheet, with ice flowing from an ice divide over the central northern Barents Sea that expanded and declined in response to large-scale, Late Quaternary climate fluctuations, and which was more or less in tune with other major Northern Hemisphere ice sheets. We now increasingly perceive it as a very dynamic, multidomed ice sheet, controlled by climate fluctuations, relative sea-level change, as well as subglacial topography, substrate properties and basal temperature. In this respect, the Svalbard-Barents Sea ice sheet will increasingly hold the key for understanding the dynamics and processes of how marine-based ice sheets build-up and decay.

How and when to terminate the Pleistocene ice ages?

NASA Astrophysics Data System (ADS)

Abe-Ouchi, A.; Saito, F.; Kawamura, K.; Takahashi, K.; Raymo, M. E.; Okuno, J.; Blatter, H.

2015-12-01

Climate change with wax and wane of large Northern Hemisphere ice sheet occurred in the past 800 thousand years characterized by 100 thousand year cycle with a large amplitude of sawtooth pattern, following a transition from a period of 40 thousand years cycle with small amplitude of ice sheet change at about 1 million years ago. Although the importance of insolation as the ultimate driver is now appreciated, the mechanism what determines timing and strength of terminations are far from clearly understood. Here we show, using comprehensive climate and ice-sheet models, that insolation and internal feedbacks between the climate, the ice sheets and the lithosphere-asthenosphere system explain the 100,000-year periodicity. The responses of equilibrium states of ice sheets to summer insolation show hysteresis, with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles. The hysteresis loop of the North American ice sheet is such that after inception of the ice sheet, its mass balance remains mostly positive through several precession cycles, whose amplitudes decrease towards an eccentricity minimum. The larger the ice sheet grows and extends towards lower latitudes, the smaller is the insolation required to make the mass balance negative. Therefore, once a large ice sheet is established, a moderate increase in insolation is sufficient to trigger a negative mass balance, leading to an almost complete retreat of the ice sheet within several thousand years. We discuss further the mechanism which determine the timing of ice age terminations by examining the role of astronomical forcing and change of atmospheric carbon dioxide contents through sensitivity experiments and comparison of several ice age cycles with different settings of astronomical forcings.

Rapid Holocene thinning of outlet glaciers followed by readvance in the western Ross Embayment, Antarctica

NASA Astrophysics Data System (ADS)

Jones, R. S.; Whitmore, R.; Mackintosh, A.; Norton, K. P.; Eaves, S.; Stutz, J.

2017-12-01

Investigating Antarctic deglaciation following the LGM provides an opportunity to better understand patterns, mechanisms and drivers of ice sheet retreat. In the Ross Sea sector, geomorphic features preserved on the seafloor indicate that streaming East Antarctic outlet glaciers once extended >100 km offshore of South Victoria Land prior to back-stepping towards their modern configurations. In order to adequately interpret the style and causes of this retreat, the timing and magnitude of corresponding ice thickness change is required. We present new constraints on ice surface lowering from Mawson Glacier, an outlet of the East Antarctic Ice Sheet that flows into the western Ross Sea. Surface-exposure (10Be) ages from samples collected in elevation transects above the modern ice surface reveal that rapid thinning occurred at 5-8 ka, broadly coeval with new ages of grounding-line retreat at 6 ka and rapid thinning recorded at nearby Mackay Glacier at 7 ka. Our data also show that a moraine formed near to the modern ice margin of Mawson Glacier at 0.8 ka, which, together with historical observations, indicates that glaciers in this region readvanced during the last thousand years. We argue that 1) the accelerated thinning of outlet glaciers was driven by local grounding-line retreat through overdeepened basins during the early-mid Holocene, and 2) the glaciers subsequently readvanced, possibly linked to late Holocene sea-ice expansion, before retreating to their current positions. Our work demonstrates that these outlet glaciers were closely coupled to environmental and topography-induced perturbations near their termini throughout the Holocene.

Capabilities and performance of Elmer/Ice, a new generation ice-sheet model

NASA Astrophysics Data System (ADS)

Gagliardini, O.; Zwinger, T.; Gillet-Chaulet, F.; Durand, G.; Favier, L.; de Fleurian, B.; Greve, R.; Malinen, M.; Martín, C.; Råback, P.; Ruokolainen, J.; Sacchettini, M.; Schäfer, M.; Seddik, H.; Thies, J.

2013-03-01

The Fourth IPCC Assessment Report concluded that ice-sheet flow models are unable to forecast the current increase of polar ice sheet discharge and the associated contribution to sea-level rise. Since then, the glaciological community has undertaken a huge effort to develop and improve a new generation of ice-flow models, and as a result, a significant number of new ice-sheet models have emerged. Among them is the parallel finite-element model Elmer/Ice, based on the open-source multi-physics code Elmer. It was one of the first full-Stokes models used to make projections for the evolution of the whole Greenland ice sheet for the coming two centuries. Originally developed to solve local ice flow problems of high mechanical and physical complexity, Elmer/Ice has today reached the maturity to solve larger scale problems, earning the status of an ice-sheet model. Here, we summarise almost 10 yr of development performed by different groups. We present the components already included in Elmer/Ice, its numerical performance, selected applications, as well as developments planned for the future.

Ice-sheet thinning and acceleration at Camp Century, Greenlan

NASA Astrophysics Data System (ADS)

Colgan, W. T.

2017-12-01

Camp Century, Greenland (77.18 °N, 61.12 °W, 1900 m), is located approximately 150 km inland from the ice-sheet margin in Northwest Greenland. In-situ and remotely-sensed measurements of ice-sheet elevation at Camp Century exhibit a thinning trend between 1964 and the present. A comparison of 1966 and 2017 firn density profiles indicates that a portion of this ice-sheet thinning is attributable to increased firn compaction rate. In-situ measurements of increasing ice surface velocity over the 1977-2017 period indicate that enhanced horizontal divergence of ice flux is also contributing to ice dynamic thinning at Camp Century. This apparent ice dynamic thinning could potentially result from a migrating local flow divide or decreasing effective ice viscosity. In a shorter-term context, observations of decadal-scale ice-sheet thinning and acceleration at Camp Century highlights underappreciated transience in inland ice form and flow during the satellite era. In a longer-term context, these multi-decadal observations contrast with inferences of millennial-scale ice-sheet thickening and deceleration at Camp Century.

Large Ice Discharge From the Greenland Ice Sheet

NASA Technical Reports Server (NTRS)

Rignot, Eric

1999-01-01

The objectives of this work are to measure the ice discharge of the Greenland Ice Sheet close to the grounding line and/or calving front, and compare the results with mass accumulation and ablation in the interior to estimate the ice sheet mass balance.

Net retreat of Antarctic glacier grounding lines

NASA Astrophysics Data System (ADS)

Konrad, Hannes; Shepherd, Andrew; Gilbert, Lin; Hogg, Anna E.; McMillan, Malcolm; Muir, Alan; Slater, Thomas

2018-04-01

Grounding lines are a key indicator of ice-sheet instability, because changes in their position reflect imbalance with the surrounding ocean and affect the flow of inland ice. Although the grounding lines of several Antarctic glaciers have retreated rapidly due to ocean-driven melting, records are too scarce to assess the scale of the imbalance. Here, we combine satellite altimeter observations of ice-elevation change and measurements of ice geometry to track grounding-line movement around the entire continent, tripling the coverage of previous surveys. Between 2010 and 2016, 22%, 3% and 10% of surveyed grounding lines in West Antarctica, East Antarctica and at the Antarctic Peninsula retreated at rates faster than 25 m yr-1 (the typical pace since the Last Glacial Maximum) and the continent has lost 1,463 km2 ± 791 km2 of grounded-ice area. Although by far the fastest rates of retreat occurred in the Amundsen Sea sector, we show that the Pine Island Glacier grounding line has stabilized, probably as a consequence of abated ocean forcing. On average, Antarctica's fast-flowing ice streams retreat by 110 metres per metre of ice thinning.

ISMIP6: Ice Sheet Model Intercomparison Project for CMIP6

NASA Technical Reports Server (NTRS)

Nowicki, S.

2015-01-01

ISMIP6 (Ice Sheet Model Intercomparison Project for CMIP6) targets the Cryosphere in a Changing Climate and the Future Sea Level Grand Challenges of the WCRP (World Climate Research Program). Primary goal is to provide future sea level contribution from the Greenland and Antarctic ice sheets, along with associated uncertainty. Secondary goal is to investigate feedback due to dynamic ice sheet models. Experiment design uses and augment the existing CMIP6 (Coupled Model Intercomparison Project Phase 6) DECK (Diagnosis, Evaluation, and Characterization of Klima) experiments. Additonal MIP (Model Intercomparison Project)- specific experiments will be designed for ISM (Ice Sheet Model). Effort builds on the Ice2sea, SeaRISE (Sea-level Response to Ice Sheet Evolution) and COMBINE (Comprehensive Modelling of the Earth System for Better Climate Prediction and Projection) efforts.

The Physics of Ice Sheets

ERIC Educational Resources Information Center

Bassis, J. N.

2008-01-01

The great ice sheets in Antarctica and Greenland are vast deposits of frozen freshwater that contain enough to raise sea level by approximately 70 m if they were to completely melt. Because of the potentially catastrophic impact that ice sheets can have, it is important that we understand how ice sheets have responded to past climate changes and…

Minimum and Maximum Potential Contributions to Future Sea Level Rise from Polar Ice Sheets

NASA Astrophysics Data System (ADS)

Deconto, R. M.; Pollard, D.

2017-12-01

New climate and ice-sheet modeling, calibrated to past changes in sea-level, is painting a stark picture of the future fate of the great polar ice sheets if greenhouse gas emissions continue unabated. This is especially true for Antarctica, where a substantial fraction of the ice sheet rests on bedrock more than 500-meters below sea level. Here, we explore the sensitivity of the polar ice sheets to a warming atmosphere and ocean under a range of future greenhouse gas emissions scenarios. The ice sheet-climate-ocean model used here considers time-evolving changes in surface mass balance and sub-ice oceanic melting, ice deformation, grounding line retreat on reverse-sloped bedrock (Marine Ice Sheet Instability), and newly added processes including hydrofracturing of ice shelves in response to surface meltwater and rain, and structural collapse of thick, marine-terminating ice margins with tall ice-cliff faces (Marine Ice Cliff Instability). The simulations improve on previous work by using 1) improved atmospheric forcing from a Regional Climate Model and 2) a much wider range of model physical parameters within the bounds of modern observations of ice dynamical processes (particularly calving rates) and paleo constraints on past ice-sheet response to warming. Approaches to more precisely define the climatic thresholds capable of triggering rapid and potentially irreversible ice-sheet retreat are also discussed, as is the potential for aggressive mitigation strategies like those discussed at the 2015 Paris Climate Conference (COP21) to substantially reduce the risk of extreme sea-level rise. These results, including physics that consider both ice deformation (creep) and calving (mechanical failure of marine terminating ice) expand on previously estimated limits of maximum rates of future sea level rise based solely on kinematic constraints of glacier flow. At the high end, the new results show the potential for more than 2m of global mean sea level rise by 2100, implying that physically plausible upper limits on future sea-level rise might need to be reconsidered.

Results of the Greenland ice sheet model initialisation experiments: ISMIP6 - initMIP-Greenland

NASA Astrophysics Data System (ADS)

Goelzer, Heiko; Nowicki, Sophie; Edwards, Tamsin; Beckley, Matthew

2017-04-01

Ice sheet model initialisation has a large effect on projected future sea-level contributions and gives rise to important uncertainties. The goal of this intercomparison exercise for the continental-scale Greenland ice sheet is therefore to compare, evaluate and improve the initialisation techniques used in the ice sheet modelling community. The initMIP-Greenland project is the first in a series of ice sheet model intercomparison activities within ISMIP6 (Ice Sheet Model Intercomparison Project for CMIP6). The experimental set-up has been designed to allow comparison of the initial present-day state of the Greenland ice sheet between participating models and against observations. Furthermore, the initial states are tested with two schematic forward experiments to evaluate the initialisation in terms of model drift (forward run without any forcing) and response to a large perturbation (prescribed surface mass balance anomaly). We present and discuss results that highlight the wide diversity of data sets, boundary conditions and initialisation techniques used in the community to generate initial states of the Greenland ice sheet.

Data assimilation and prognostic whole ice sheet modelling with the variationally derived, higher order, open source, and fully parallel ice sheet model VarGlaS

NASA Astrophysics Data System (ADS)

Brinkerhoff, D. J.; Johnson, J. V.

2013-07-01

We introduce a novel, higher order, finite element ice sheet model called VarGlaS (Variational Glacier Simulator), which is built on the finite element framework FEniCS. Contrary to standard procedure in ice sheet modelling, VarGlaS formulates ice sheet motion as the minimization of an energy functional, conferring advantages such as a consistent platform for making numerical approximations, a coherent relationship between motion and heat generation, and implicit boundary treatment. VarGlaS also solves the equations of enthalpy rather than temperature, avoiding the solution of a contact problem. Rather than include a lengthy model spin-up procedure, VarGlaS possesses an automated framework for model inversion. These capabilities are brought to bear on several benchmark problems in ice sheet modelling, as well as a 500 yr simulation of the Greenland ice sheet at high resolution. VarGlaS performs well in benchmarking experiments and, given a constant climate and a 100 yr relaxation period, predicts a mass evolution of the Greenland ice sheet that matches present-day observations of mass loss. VarGlaS predicts a thinning in the interior and thickening of the margins of the ice sheet.

Ocean Tide Influences on the Antarctic and Greenland Ice Sheets

NASA Astrophysics Data System (ADS)

Padman, Laurie; Siegfried, Matthew R.; Fricker, Helen A.

2018-03-01

Ocean tides are the main source of high-frequency variability in the vertical and horizontal motion of ice sheets near their marine margins. Floating ice shelves, which occupy about three quarters of the perimeter of Antarctica and the termini of four outlet glaciers in northern Greenland, rise and fall in synchrony with the ocean tide. Lateral motion of floating and grounded portions of ice sheets near their marine margins can also include a tidal component. These tide-induced signals provide insight into the processes by which the oceans can affect ice sheet mass balance and dynamics. In this review, we summarize in situ and satellite-based measurements of the tidal response of ice shelves and grounded ice, and spatial variability of ocean tide heights and currents around the ice sheets. We review sensitivity of tide heights and currents as ocean geometry responds to variations in sea level, ice shelf thickness, and ice sheet mass and extent. We then describe coupled ice-ocean models and analytical glacier models that quantify the effect of ocean tides on lower-frequency ice sheet mass loss and motion. We suggest new observations and model developments to improve the representation of tides in coupled models that are used to predict future ice sheet mass loss and the associated contribution to sea level change. The most critical need is for new data to improve maps of bathymetry, ice shelf draft, spatial variability of the drag coefficient at the ice-ocean interface, and higher-resolution models with improved representation of tidal energy sinks.

Relative sea-level rise around East Antarctica during Oligocene glaciation

NASA Astrophysics Data System (ADS)

Stocchi, Paolo; Escutia, Carlota; Houben, Alexander J. P.; Vermeersen, Bert L. A.; Bijl, Peter K.; Brinkhuis, Henk; Deconto, Robert M.; Galeotti, Simone; Passchier, Sandra; Pollard, David; Brinkhuis, Henk; Escutia, Carlota; Klaus, Adam; Fehr, Annick; Williams, Trevor; Bendle, James A. P.; Bijl, Peter K.; Bohaty, Steven M.; Carr, Stephanie A.; Dunbar, Robert B.; Flores, Jose Abel; Gonzàlez, Jhon J.; Hayden, Travis G.; Iwai, Masao; Jimenez-Espejo, Francisco J.; Katsuki, Kota; Kong, Gee Soo; McKay, Robert M.; Nakai, Mutsumi; Olney, Matthew P.; Passchier, Sandra; Pekar, Stephen F.; Pross, Jörg; Riesselman, Christina; Röhl, Ursula; Sakai, Toyosaburo; Shrivastava, Prakash Kumar; Stickley, Catherine E.; Sugisaki, Saiko; Tauxe, Lisa; Tuo, Shouting; van de Flierdt, Tina; Welsh, Kevin; Yamane, Masako

2013-05-01

During the middle and late Eocene (~ 48-34Myr ago), the Earth's climate cooled and an ice sheet built up on Antarctica. The stepwise expansion of ice on Antarctica induced crustal deformation and gravitational perturbations around the continent. Close to the ice sheet, sea level rose despite an overall reduction in the mass of the ocean caused by the transfer of water to the ice sheet. Here we identify the crustal response to ice-sheet growth by forcing a glacial-hydro isostatic adjustment model with an Antarctic ice-sheet model. We find that the shelf areas around East Antarctica first shoaled as upper mantle material upwelled and a peripheral forebulge developed. The inner shelf subsequently subsided as lithosphere flexure extended outwards from the ice-sheet margins. Consequently the coasts experienced a progressive relative sea-level rise. Our analysis of sediment cores from the vicinity of the Antarctic ice sheet are in agreement with the spatial patterns of relative sea-level change indicated by our simulations. Our results are consistent with the suggestion that near-field processes such as local sea-level change influence the equilibrium state obtained by an ice-sheet grounding line.

Experimental design for three interrelated marine ice sheet and ocean model intercomparison projects: MISMIP v. 3 (MISMIP +), ISOMIP v. 2 (ISOMIP +) and MISOMIP v. 1 (MISOMIP1)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Asay-Davis, Xylar S.; Cornford, Stephen L.; Durand, Gaël

Coupled ice sheet-ocean models capable of simulating moving grounding lines are just becoming available. Such models have a broad range of potential applications in studying the dynamics of marine ice sheets and tidewater glaciers, from process studies to future projections of ice mass loss and sea level rise. The Marine Ice Sheet-Ocean Model Intercomparison Project (MISOMIP) is a community effort aimed at designing and coordinating a series of model intercomparison projects (MIPs) for model evaluation in idealized setups, model verification based on observations, and future projections for key regions of the West Antarctic Ice Sheet (WAIS). Here we describe computationalmore » experiments constituting three interrelated MIPs for marine ice sheet models and regional ocean circulation models incorporating ice shelf cavities. These consist of ice sheet experiments under the Marine Ice Sheet MIP third phase (MISMIP+), ocean experiments under the Ice Shelf-Ocean MIP second phase (ISOMIP+) and coupled ice sheet-ocean experiments under the MISOMIP first phase (MISOMIP1). All three MIPs use a shared domain with idealized bedrock topography and forcing, allowing the coupled simulations (MISOMIP1) to be compared directly to the individual component simulations (MISMIP+ and ISOMIP+). The experiments, which have qualitative similarities to Pine Island Glacier Ice Shelf and the adjacent region of the Amundsen Sea, are designed to explore the effects of changes in ocean conditions, specifically the temperature at depth, on basal melting and ice dynamics. In future work, differences between model results will form the basis for the evaluation of the participating models.« less

Surficial geologic map of the greater Omaha area, Nebraska and Iowa

USGS Publications Warehouse

Shroba, R.R.; Brandt, T.R.; Blossom, J.C.

2001-01-01

Geologic mapping, in support of the USGS Omaha-Kansas City Geologic Mapping Project, shows the spatial distribution of artificial-fill, alluvial, eolian, and glacial deposits and bedrock in and near Omaha, Nebraska. Artificial fill deposits are mapped chiefly beneath commercial structures, segments of interstate highways and other major highways, railroad tracks, airport runways, and military facilities, and in landfills and earth fills. Alluvial deposits are mapped beneath flood plains, in stream terraces, and on hill slopes. They include flood-plain and stream-channel alluvium, sheetwash alluvium, and undivided sheetwash alluvium and stream alluvium. Wind-deposited loess forms sheets that mantle inter-stream areas and late Wisconsin terrace alluvium. Peoria Loess is younger of the two loess sheets and covers much of the inter-stream area in the map area. Loveland Loess is older and is exposed in a few small areas in the eastern part of the map area. Glacial deposits are chiefly heterogeneous, ice-deposited, clayey material (till) and minor interstratified stream-deposited sand and gravel. Except for small outcrops, glacial deposits are covered by eolian and alluvial deposits throughout most of the map area. Bedrock is locally exposed in natural exposures along the major streams and in quarries. It consists of Dakota Sandstone and chiefly limestone and shale of the Lansing and Kansas City Groups. Sand and gravel in flood plain and stream-channel alluvium in the Platte River valley are used mainly for concrete aggregate. Limestone of the Lansing and Kansas City Groups is used for road-surfacing material, rip rap, and fill material.

Modelled Growth and Decay of the Cordilleran Ice Sheet Through the Last Glacial Cycle

NASA Astrophysics Data System (ADS)

Marshall, S. J.; Banwell, A.

2015-12-01

The Cordilleran Ice Sheet in western North America had an enigmatic evolution during the last glacial cycle, developing out of sync with the larger Laurentide and global glaciation. The geological record suggests that the ice sheet emerged late, ca. 45 ka, growing to be a fully-established ice sheet in isotope stages 3 and 2 and deglaciating late in the glacial cycle. This has been a challenge to model, and is a paleoclimatic curiosity, because the western Cordillera of North America is heavily glacierized today, and one would intuitively expect it to act as an inception centre for the Pleistocene ice sheets. The region receives heavy precipitation, and modest cooling should induce large-scale glacier expansion. Indeed, a Cordilleran Ice Sheet quickly nucleates in isotope substage 5d in most ice sheet modeling studies to date, and is a resilient feature throughout the glaciation. The fact that a full-scale Cordilleran Ice Sheet did not develop until relatively late argues for either: (a) ice sheet models that have been inadequate in resolving the process of alpine-style glaciation, i.e., the coalescence of alpine icefields, or (b) a climatic history in western North America that deviated strongly from the hemispheric-scale cooling which drove the growth of the Laurentide and Scandinavian Ice Sheets, as recorded in Greenland. We argue that reasonable reconstructions of Cordilleran Ice Sheet growth and decay implicate a combination of these two considerations. Sufficient model resolution is required to capture the valley-bottom melt that suppresses icefield coalescence, while early-glacial cooling must have been modest in the Pacific sector of North America. We argue for a persistent warm, dry climate relative to that in eastern North America and the Atlantic sector, likely associated with positive feedbacks between atmospheric circulation and the nascent Laurentide Ice Sheet (i.e., peristent circulation patterns similar to those of 2014-2015). This must have been disrupted as the Laurentide thickened and advanced southward, allowing the Cordilleran Ice Sheet to emerge from numerous isolated icefield complexes.

Extensive massive basal-ice structures in West Antarctica relate to ice-sheet anisotropy and ice-flow

NASA Astrophysics Data System (ADS)

Ross, N.; Bingham, R. G.; Corr, H. F. J.; Siegert, M. J.

2016-12-01

Complex structures identified within both the East Antarctic and Greenland ice sheets are thought to be generated by the action of basal water freezing to the ice-sheet base, evolving under ice flow. Here, we use ice-penetrating radar to image an extensive series of similarly complex basal ice facies in West Antarctica, revealing a thick (>500 m) tectonised unit in an area of cold-based and relatively slow-flowing ice. We show that major folding and overturning of the unit perpendicular to ice flow elevates deep, warm ice into the mid ice-sheet column. Fold axes align with present ice flow, and axis amplitudes increase down-ice, suggesting long-term consistency in the direction and convergence of flow. In the absence of basal water, and the draping of the tectonised unit over major subglacial mountain ranges, the formation of the unit must be solely through the deformation of meteoric ice. Internal layer radar reflectivity is consistently greater parallel to flow compared with the perpendicular direction, revealing ice-sheet crystal anisotropy is associated with the folding. By linking layers to the Byrd ice-core site, we show the basal ice dates to at least the last glacial cycle and may be as old as the last interglacial. Deformation of deep-ice in this sector of WAIS, and potentially elsewhere in Antarctica, may be caused by differential shearing at interglacial-glacial boundaries, in a process analogous to that proposed for interior Greenland. The scale and heterogeneity of the englacial structures, and their subsequent impact on ice sheet rheology, means that the nature of ice flow across the bulk of West Antarctica must be far more complex that is currently accounted for by any numerical ice sheet model.

Combustion of available fossil fuel resources sufficient to eliminate the Antarctic Ice Sheet

PubMed Central

Winkelmann, Ricarda; Levermann, Anders; Ridgwell, Andy; Caldeira, Ken

2015-01-01

The Antarctic Ice Sheet stores water equivalent to 58 m in global sea-level rise. We show in simulations using the Parallel Ice Sheet Model that burning the currently attainable fossil fuel resources is sufficient to eliminate the ice sheet. With cumulative fossil fuel emissions of 10,000 gigatonnes of carbon (GtC), Antarctica is projected to become almost ice-free with an average contribution to sea-level rise exceeding 3 m per century during the first millennium. Consistent with recent observations and simulations, the West Antarctic Ice Sheet becomes unstable with 600 to 800 GtC of additional carbon emissions. Beyond this additional carbon release, the destabilization of ice basins in both West and East Antarctica results in a threshold increase in global sea level. Unabated carbon emissions thus threaten the Antarctic Ice Sheet in its entirety with associated sea-level rise that far exceeds that of all other possible sources. PMID:26601273

Combustion of available fossil-fuel resources sufficient to eliminate the Antarctic Ice Sheet

NASA Astrophysics Data System (ADS)

Winkelmann, R.; Levermann, A.; Ridgwell, A.; Caldeira, K.

2015-12-01

The Antarctic Ice Sheet stores water equivalent to 58 meters in global sea-level rise. Here we show in simulations with the Parallel Ice Sheet Model that burning the currently attainable fossil-fuel resources is sufficient to eliminate the ice sheet. With cumulative fossil-fuel emissions of 10 000 GtC, Antarctica is projected to become almost ice-free with an average contribution to sea-level rise exceeding 3 meters per century during the first millennium. Consistent with recent observations and simulations, the West Antarctic Ice Sheet becomes unstable with 600 to 800 GtC of additional carbon emissions. Beyond this additional carbon release, the destabilization of ice basins in both West- and East Antarctica results in a threshold-increase in global sea level. Unabated carbon emissions thus threaten the Antarctic Ice Sheet in its entirety with associated sea-level rise that far exceeds that of all other possible sources.

Combustion of available fossil fuel resources sufficient to eliminate the Antarctic Ice Sheet.

PubMed

Winkelmann, Ricarda; Levermann, Anders; Ridgwell, Andy; Caldeira, Ken

2015-09-01

The Antarctic Ice Sheet stores water equivalent to 58 m in global sea-level rise. We show in simulations using the Parallel Ice Sheet Model that burning the currently attainable fossil fuel resources is sufficient to eliminate the ice sheet. With cumulative fossil fuel emissions of 10,000 gigatonnes of carbon (GtC), Antarctica is projected to become almost ice-free with an average contribution to sea-level rise exceeding 3 m per century during the first millennium. Consistent with recent observations and simulations, the West Antarctic Ice Sheet becomes unstable with 600 to 800 GtC of additional carbon emissions. Beyond this additional carbon release, the destabilization of ice basins in both West and East Antarctica results in a threshold increase in global sea level. Unabated carbon emissions thus threaten the Antarctic Ice Sheet in its entirety with associated sea-level rise that far exceeds that of all other possible sources.

Capabilities and performance of the new generation ice-sheet model Elmer/Ice

NASA Astrophysics Data System (ADS)

Gagliardini, O.; Zwinger, T.; Durand, G.; Favier, L.; de Fleurian, B.; Gillet-chaulet, F.; Seddik, H.; Greve, R.; Mallinen, M.; Martin, C.; Raback, P.; Ruokolainen, J.; Schäfer, M.; Thies, J.

2012-12-01

Since the Fourth IPCC Assessment Report, and its conclusion about the inability of ice-sheet flow models to forecast the current increase of polar ice sheet discharge and associated contribution to sea-level rise, a huge development effort has been undertaken by the glaciological community. All around the world, models have been improved and, interestingly, a significant number of new ice-sheet models have emerged. Among them, the parallel finite-element model Elmer/Ice (based on the open-source multi-physics code Elmer) was one of the first full-Stokes models used to make projections of the future of the whole Greenland ice sheet for the coming two centuries. Originally developed to solve dedicated local ice flow problems of high mechanical and physical complexity, Elmer/Ice has today reached the maturity to solve larger scale problems, earning the status of an ice-sheet model. In this presentation, we summarise the almost 10 years of development performed by different groups. We present the components already included in Elmer/Ice, its numerical performance, selected applications, as well as developments planed for the future.

Discharge of New Subglacial Lake on Whillians Ice Stream: Implication for Ice Stream Flow Dynamics.

NASA Astrophysics Data System (ADS)

Sergienko, O. V.; Fricker, H. A.; Bindschadler, R. A.; Vornberger, P. L.; Macayeal, D. R.

2006-12-01

One of the surprise discoveries made possible by the ICESat laser altimeter mission of 2004-2006 is the presence of a large subglacial lake below the grounding zone of Whillians Ice Stream (dubbed here `Lake Helen' after the discoverer, Helen Fricker). What is even more surprising is the fact that this lake discharged a substantial portion of its volume during the ICESat mission, and changes in lake volume and surface elevation of the ice stream are documented in exquisite detail [Fricker et al., in press]. The presence and apparent dynamism of large subglacial lakes in the grounding zone of a major ice stream raises questions about their effects on ice-stream dynamics. Being liquid and movable, water modifies basal friction spatially and temporally. Melting due to shear heating and geothermal flux reduces basal traction, making the ice stream move fast. However, when water collects in a depression to form a lake, it potentially deprives the surrounding bed of lubricating water, and additionally makes the ice surface flat, thereby locally decreasing the ice stream driving stress. We study the effect of formation and discharge of a subglacial lake at the mouth of and ice stream using a two dimensional, vertically integrated, ice-stream model. The model is forced by the basal friction, ice thickness and surface elevation. The basal friction is obtained by inversion of the ice surface velocity, ice thickness and surface elevation come from observations. To simulate the lake formation we introduce zero basal friction and "inflate" the basal elevation of the ice stream at the site of the lake. Sensitivity studies of the response of the surrounding ice stream and ice shelf flow are performed to delineate the influence of near-grounding-line subglacial water storage for ice streams in general.

Seafloor glacial geomorphology in a cross shelf trough: insights into the deglaciation of the Melville Bay Ice Stream

NASA Astrophysics Data System (ADS)

Newton, Andrew; Huuse, Mads

2016-04-01

Compared to other glaciated margins such as offshore mid-Norway and Svalbard, the Greenland continental shelf has, until recently, been the subject of only a limited amount of academic and industry research. This has been mainly due to the difficulty and expense of obtaining data in such harsh and operationally complex settings. Climate amelioration and technological advance has, particularly in recent years, allowed both academics and industry to substantially increase data collection across the many glaciated continental shelves in the Northern Hemisphere. Baffin Bay has been one of the primary regions of interest for the hydrocarbon industry which has sought to operate in the frontier basins offshore Greenland. As a result of these industry operations, a large database of geophysical and geological data has been collected. Some of this data has been made available to glacial scientists and provides a unique opportunity to investigate the seafloor geomorphology for regions where the majority of previous work has been hypothetical rather than grounded in geological evidence. In the work presented here we present a landform record offshore NW Greenland in the Melville Bay cross-shelf trough. This is one of the largest troughs on the entire Greenland shelf and measures up to 140 km in width. Shallow-marine cores collected in the coastal part of the trough show bedrock of Miocene age and indicate that a significant cover has likely been removed from the shelf by ice streams operating through the Late Cenozoic. This material has then been deposited at the shelf edge as a trough mouth fan. Using multibeam and seismic reflection data a large number of glacial landforms are observed and mapped in the trough. These include mega-scale glacial lineations, grounding-zone wedges, iceberg scours, and iceberg grounding pits. These landforms are used to reconstruct the ice dynamics of the Melville Bugt Ice Stream at the last glacial maximum and during its deglaciation. The observed grounding-zone wedges suggest that initial retreat was punctuated with two still-stands of long enough duration to accumulate material at the grounding zone. As the ice sheet decayed further, a rapid retreat of over 30 km occurred before the grounding line margin was stabilised again. Understanding the nonlinear rate of grounding-line retreat such as that presented here is crucial for the future modelling of not just the evolution of the Greenland Ice Sheet but also those elsewhere.

Deglaciation of Fennoscandia

NASA Astrophysics Data System (ADS)

Stroeven, Arjen P.; Hättestrand, Clas; Kleman, Johan; Heyman, Jakob; Fabel, Derek; Fredin, Ola; Goodfellow, Bradley W.; Harbor, Jonathan M.; Jansen, John D.; Olsen, Lars; Caffee, Marc W.; Fink, David; Lundqvist, Jan; Rosqvist, Gunhild C.; Strömberg, Bo; Jansson, Krister N.

2016-09-01

To provide a new reconstruction of the deglaciation of the Fennoscandian Ice Sheet, in the form of calendar-year time-slices, which are particularly useful for ice sheet modelling, we have compiled and synthesized published geomorphological data for eskers, ice-marginal formations, lineations, marginal meltwater channels, striae, ice-dammed lakes, and geochronological data from radiocarbon, varve, optically-stimulated luminescence, and cosmogenic nuclide dating. This is summarized as a deglaciation map of the Fennoscandian Ice Sheet with isochrons marking every 1000 years between 22 and 13 cal kyr BP and every hundred years between 11.6 and final ice decay after 9.7 cal kyr BP. Deglaciation patterns vary across the Fennoscandian Ice Sheet domain, reflecting differences in climatic and geomorphic settings as well as ice sheet basal thermal conditions and terrestrial versus marine margins. For example, the ice sheet margin in the high-precipitation coastal setting of the western sector responded sensitively to climatic variations leaving a detailed record of prominent moraines and other ice-marginal deposits in many fjords and coastal valleys. Retreat rates across the southern sector differed between slow retreat of the terrestrial margin in western and southern Sweden and rapid retreat of the calving ice margin in the Baltic Basin. Our reconstruction is consistent with much of the published research. However, the synthesis of a large amount of existing and new data support refined reconstructions in some areas. For example, the LGM extent of the ice sheet in northwestern Russia was located far east and it occurred at a later time than the rest of the ice sheet, at around 17-15 cal kyr BP. We also propose a slightly different chronology of moraine formation over southern Sweden based on improved correlations of moraine segments using new LiDAR data and tying the timing of moraine formation to Greenland ice core cold stages. Retreat rates vary by as much as an order of magnitude in different sectors of the ice sheet, with the lowest rates on the high-elevation and maritime Norwegian margin. Retreat rates compared to the climatic information provided by the Greenland ice core record show a general correspondence between retreat rate and climatic forcing, although a close match between retreat rate and climate is unlikely because of other controls, such as topography and marine versus terrestrial margins. Overall, the time slice reconstructions of Fennoscandian Ice Sheet deglaciation from 22 to 9.7 cal kyr BP provide an important dataset for understanding the contexts that underpin spatial and temporal patterns in retreat of the Fennoscandian Ice Sheet, and are an important resource for testing and refining ice sheet models.

High-resolution record of the deglaciation of the British-Irish Ice Sheet from North Atlantic deep-sea sediments.

NASA Astrophysics Data System (ADS)

Tarlati, S.; Benetti, S.; Callard, L.; O'Cofaigh, C.; Dunlop, P.; Chiverrell, R. C.; Fabel, D.; Moreton, S.; Clark, C.

2016-12-01

During the last glacial maximum the British-Irish Ice Sheet (BIIS) covered the majority of Ireland and Britain. Recent studies have described the BIIS as largely marine-based and highly dynamic with several advances and retreats recorded on the continental shelf. The focus of this study is the more recent sediment record from the Donegal Barra Fan (DBF), the largest sediment depocentre formed by the ice streaming of the western BIIS onto the North Atlantic continental margin. In this project, well-preserved, glacially-derived, deep-water sediments from 3 cores, up to 6.7 m long and retrieved from the DBF, are used to investigate and chronologically constrain the pattern of deglaciation of the BIIS. Deep-water sediments can record continuous sedimentation through time, avoiding hiatuses and erosional surfaces characteristic of a glacial environment and allow a detailed reconstruction of deglacial processes. Five lithofacies have been identified using sedimentology, x-rays, physical properties and grain size analysis. They include bioturbated foraminifera-bearing muds, interpreted as hemipelagic and contouritic deposits from interglacial periods. Chaotic and laminated muds, ice-rafted debris (IRD)-rich layers and laminated mud to sand couplets are characteristic of the glacial period including ice-sheet maximum extent and the beginning of retreat. These represent downslope mass movements, plumites from meltwater alongside melting icebergs and turbidites. Radiocarbon dates from foraminifera suggest that the deglacial sedimentary sequence is up to 5m thick. The IRD concentration and abundance of the foraminifera Neogloboquadrina pachyderma sinistral indicate a minimum of 3 different calving events during deglaciation and a marked Younger Dryas cooling and ice calving period. Additionally the δ 18O record will be used to investigate the record of climatic changes in the region and x-ray fluorescence will be used to assess sediment provenance during deglaciation.

Greenland deep boreholes inform on sliding and deformation of the basal ice

NASA Astrophysics Data System (ADS)

Dahl-Jensen, D.

2017-12-01

Repeated measurements of the deformation of the deep boreholes on the Greenland ice sheet informs on the basal sliding, near basal deformation and in general on the horizontal velocity through the ice. Results of the logging of the boreholes at Dye3, GRIP, NGRIP, NEEM and Camp Century through the last 40 years by the Danish Ice and Climate group will be presented and discussed. The results on the flow will be compared with the information on ice properties, impurity load and bedrock entrained material from the deep ice cores and the radio echo sounding images near the drill sites.The results show that the basal movement often happens in an impurity rich zone above the bedrock while pure basal sliding is limited even in the presence of basal water and significant basal melt.Most of the deep ice core sites are located close to ice divides where the surface velocity is limited so significant basal sliding is not expected. Exceptions are the surface velocities at Camp Century and Dye 3, both being 13 m/yr.Finally, the ongoing deep drilling at EGRIP will shortly be presented where we are drilling in the center of the North East Greenland Ice Stream (NEGIS).

Ice sheets play important role in climate change

NASA Astrophysics Data System (ADS)

Clark, Peter U.; MacAyeal, Douglas R.; Andrews, John T.; Bartlein, Patrick J.

Ice sheets once were viewed as passive elements in the climate system enslaved to orbitally generated variations in solar radiation. Today, modeling results and new geologic records suggest that ice sheets actively participated in late-Pleistocene climate change, amplifying or driving significant variability at millennial as well as orbital timescales. Although large changes in global ice volume were ultimately caused by orbital variations (the Milankovitch hypothesis), once in existence, the former ice sheets behaved dynamically and strongly influenced regional and perhaps even global climate by altering atmospheric and oceanic circulation and temperature.Experiments with General Circulation Models (GCMs) yielded the first inklings of ice sheets' climatic significance. Manabe and Broccoli [1985], for example, found that the topographic and albedo effects of ice sheets alone explain much of the Northern Hemisphere cooling identified in paleoclimatic records of the last glacial maximum (˜21 ka).

Tidal influences on a future evolution of the Filchner-Ronne Ice Shelf cavity in the Weddell Sea, Antarctica

NASA Astrophysics Data System (ADS)

Mueller, Rachael D.; Hattermann, Tore; Howard, Susan L.; Padman, Laurie

2018-02-01

Recent modeling studies of ocean circulation in the southern Weddell Sea, Antarctica, project an increase over this century of ocean heat into the cavity beneath Filchner-Ronne Ice Shelf (FRIS). This increase in ocean heat would lead to more basal melting and a modification of the FRIS ice draft. The corresponding change in cavity shape will affect advective pathways and the spatial distribution of tidal currents, which play important roles in basal melting under FRIS. These feedbacks between heat flux, basal melting, and tides will affect the evolution of FRIS under the influence of a changing climate. We explore these feedbacks with a three-dimensional ocean model of the southern Weddell Sea that is forced by thermodynamic exchange beneath the ice shelf and tides along the open boundaries. Our results show regionally dependent feedbacks that, in some areas, substantially modify the melt rates near the grounding lines of buttressed ice streams that flow into FRIS. These feedbacks are introduced by variations in meltwater production as well as the circulation of this meltwater within the FRIS cavity; they are influenced locally by sensitivity of tidal currents to water column thickness (wct) and non-locally by changes in circulation pathways that transport an integrated history of mixing and meltwater entrainment along flow paths. Our results highlight the importance of including explicit tidal forcing in models of future mass loss from FRIS and from the adjacent grounded ice sheet as individual ice-stream grounding zones experience different responses to warming of the ocean inflow.

Past ice-sheet behaviour: retreat scenarios and changing controls in the Ross Sea, Antarctica

NASA Astrophysics Data System (ADS)

Halberstadt, Anna Ruth W.; Simkins, Lauren M.; Greenwood, Sarah L.; Anderson, John B.

2016-05-01

Studying the history of ice-sheet behaviour in the Ross Sea, Antarctica's largest drainage basin can improve our understanding of patterns and controls on marine-based ice-sheet dynamics and provide constraints for numerical ice-sheet models. Newly collected high-resolution multibeam bathymetry data, combined with two decades of legacy multibeam and seismic data, are used to map glacial landforms and reconstruct palaeo ice-sheet drainage. During the Last Glacial Maximum, grounded ice reached the continental shelf edge in the eastern but not western Ross Sea. Recessional geomorphic features in the western Ross Sea indicate virtually continuous back-stepping of the ice-sheet grounding line. In the eastern Ross Sea, well-preserved linear features and a lack of small-scale recessional landforms signify rapid lift-off of grounded ice from the bed. Physiography exerted a first-order control on regional ice behaviour, while sea floor geology played an important subsidiary role. Previously published deglacial scenarios for Ross Sea are based on low-spatial-resolution marine data or terrestrial observations; however, this study uses high-resolution basin-wide geomorphology to constrain grounding-line retreat on the continental shelf. Our analysis of retreat patterns suggests that (1) retreat from the western Ross Sea was complex due to strong physiographic controls on ice-sheet drainage; (2) retreat was asynchronous across the Ross Sea and between troughs; (3) the eastern Ross Sea largely deglaciated prior to the western Ross Sea following the formation of a large grounding-line embayment over Whales Deep; and (4) our glacial geomorphic reconstruction converges with recent numerical models that call for significant and complex East Antarctic ice sheet and West Antarctic ice sheet contributions to the ice flow in the Ross Sea.

Seafloor Morphology And Sediment Discharge Of The Storfjorden And Kveithola Palaeo-Ice Streams (NW Barents Sea) During The Last Deglaciation

NASA Astrophysics Data System (ADS)

Camerlenghi, Angelo; Rebesco, Michele; Pedrosa, Mayte; Demol, Ben; Giulia Lucchi, Renata; Urgeles, Roger; Colmenero-Hidalgo, Elena; Andreassen, Karin; Sverre Laberg, Jan; Winsborrow, Monica

2010-05-01

IPY Activity N. 367 focusing on Neogene ice streams and sedimentary processes on high- latitude continental margins (NICE-STREAMS) resulted in two coordinated cruises on the adjacent Storfjorden and Kveithola trough-mouth fans in the NW Barents Sea: SVAIS Cruise of BIO Hespérides, summer 2007, and EGLACOM Cruise of Cruise R/V OGS-Explora, summer 2008. The objectives were to acquire a high-resolution set of bathymetric, seismic and sediment core data in order to decipher the Neogene architectural development of the glacially-dominated NW Barents Sea continental margin in response to natural climate change. The paleo-ice streams drained ice from southern Spitsbergen, Spitsbergen Bank, and Bear Island. The short distance from the ice source to the calving front produced a short residence time of ice, and therefore a rapid response to climatic changes. In the outer trough of southern Storfjorden, lobate moraines superimpose and are cut by very large linear features attributed to mega-iceberg scours. In the adjacent Kveithola trough, a fresh morphology includes mega-scale glacial lineations overprinted by transverse grounding-zone wedges, diagnostic of episodic ice stream retreat. A 15 m thick glacimarine drape suggests an high post-deglaciation sedimentation rate. Preliminary interpretation suggests that the retreat of the Svalbard/Barents Sea Ice Sheet was highly dynamic and that grounded ice persisted on Spitsbergen Bank for some thousands years after the main Barents Sea deglaciation.The Storfjorden continental slope is divided into three wide lobes. Opposite the two northernmost lobes the slope is dominated by straight gullies in the upper part, and deposition of debris lobes on the mid and lower parts. In contrast, the southernmost lobe is characterized by widespread occurrence of submarine landslides. Sediment failure has accompanied the evolution of the southern Storfjorden and Kveithola margin throughout the Late Neogene, with very large mass transport deposits up to 200 m thick in the early phases of the development of the glacially influenced margin. Conversely, the central and northern parts of the Storfjorden margin have prograded without appreciable episodes of mass failure. Sedimentation has occurred through alternate layering of decimeter-thick glacial debris flows deposits, with laminated and acoustically transparent interglacial sediment drape. Gullies and paleo-gullies incise the glacial debris flows and are covered by the interglacial drape. They are formed early during each deglaciation phase, most likely by the erosive action of short-lived hyperpycnal flows generated by sediment-laden subglacial meltwater discharge. In sediment cores thick finely-laminated sedimentary beds on the upper continental slope of the southern part of the margin indicate preferential deposition by settlement of meltwater sediment plumes. High sedimentation rates of plumites may contribute to the slope instability and suggest that meltwater discharge was focused on the southern Storfjorden and Kveithola paleo-ice streams.

Duality of Ross Ice Shelf systems: crustal boundary, ice sheet processes and ocean circulation from ROSETTA-Ice surveys

NASA Astrophysics Data System (ADS)

Tinto, K. J.; Siddoway, C. S.; Padman, L.; Fricker, H. A.; Das, I.; Porter, D. F.; Springer, S. R.; Siegfried, M. R.; Caratori Tontini, F.; Bell, R. E.

2017-12-01

Bathymetry beneath Antarctic ice shelves controls sub-ice-shelf ocean circulation and has a major influence on the stability and dynamics of the ice sheets. Beneath the Ross Ice Shelf, the sea-floor bathymetry is a product of both tectonics and glacial processes, and is influenced by the processes it controls. New aerogeophysical surveys have revealed a fundamental crustal boundary bisecting the Ross Ice Shelf and imparting a duality to the Ross Ice Shelf systems, encompassing bathymetry, ocean circulation and ice flow history. The ROSETTA-Ice surveys were designed to increase the resolution of Ross Ice Shelf mapping from the 55 km RIGGS survey of the 1970s to a 10 km survey grid, flown over three years from New York Air National Guard LC130s. Radar, LiDAR, gravity and magnetic instruments provide a top to bottom profile of the ice shelf and the underlying seafloor, with 20 km resolution achieved in the first two survey seasons (2015 and 2016). ALAMO ocean-profiling floats deployed in the 2016 season are measuring the temperature and salinity of water entering and exiting the sub-ice water cavity. A significant east-west contrast in the character of the magnetic and gravity fields reveals that the lithospheric boundary between East and West Antarctica exists not at the base of the Transantarctic Mountains (TAM), as previously thought, but 300 km further east. The newly-identified boundary spatially coincides with the southward extension of the Central High, a rib of shallow basement identified in the Ross Sea. The East Antarctic side is characterized by lower amplitude magnetic anomalies and denser TAM-type lithosphere compared to the West Antarctic side. The crustal structure imparts a fundamental duality on the overlying ice and ocean, with deeper bathymetry and thinner ice on the East Antarctic side creating a larger sub-ice cavity for ocean circulation. The West Antarctic side has a shallower seabed, more restricted ocean access and a more complex history of ice stream behavior. The crustal boundary governs the interaction between these systems exerts a fundamental control on the stability of the Ross Ice Shelf.

Dynamic Inland Propagation of Thinning Due to Ice Loss at the Margins of the Greenland Ice Sheet

NASA Technical Reports Server (NTRS)

Wang, Wei Li; Li, Jun J.; Zwally, H. Jay

2012-01-01

Mass-balance analysis of the Greenland ice sheet based on surface elevation changes observed by the European Remote-sensing Satellite (ERS) (1992-2002) and Ice, Cloud and land Elevation Satellite (ICESat) (2003-07) indicates that the strongly increased mass loss at lower elevations (<2000 m) of the ice sheet, as observed during 2003-07, appears to induce interior ice thinning at higher elevations. In this paper, we perform a perturbation experiment with a three-dimensional anisotropic ice-flow model (AIF model) to investigate this upstream propagation. Observed thinning rates in the regions below 2000m elevation are used as perturbation inputs. The model runs with perturbation for 10 years show that the extensive mass loss at the ice-sheet margins does in fact cause interior thinning on short timescales (i.e. decadal). The modeled pattern of thinning over the ice sheet agrees with the observations, which implies that the strong mass loss since the early 2000s at low elevations has had a dynamic impact on the entire ice sheet. The modeling results also suggest that even if the large mass loss at the margins stopped, the interior ice sheet would continue thinning for 300 years and would take thousands of years for full dynamic recovery.

A Synthesis of the Basal Thermal State of the Greenland Ice Sheet

NASA Technical Reports Server (NTRS)

Macgregor, J. A.; Fahnestock, M. A.; Catania, G. A.; Aschwanden, A.; Clow, G. D.; Colgan, W. T.; Gogineni, S. P.; Morlighem, M.; Nowicki, S. M. J.; Paden, J. D.;

Determining Greenland Ice Sheet Accumulation Rates from Radar Remote Sensing

NASA Technical Reports Server (NTRS)

Jezek, Kenneth C.

2001-01-01

An important component of NASA's Program for Arctic Regional Climate Assessment (PARCA) is a mass balance investigation of the Greenland Ice Sheet. The mass balance is calculated by taking the difference between the snow accumulation and the ice discharge of the ice sheet. Uncertainties in this calculation include the snow accumulation rate, which has traditionally been determined by interpolating data from ice core samples taken throughout the ice sheet. The sparse data associated with ice cores, coupled with the high spatial and temporal resolution provided by remote sensing, have motivated scientists to investigate relationships between accumulation rate and microwave observations.

Iceberg Ploughmarks Indicate Past Rapid Iceberg Calving and Retreat of Pine Island-Thwaites Ice Stream due to Marine Ice-Cliff Instability Processes

NASA Astrophysics Data System (ADS)

Wise, M.; Dowdeswell, J. A.; Larter, R. D.; Jakobsson, M.

2016-12-01

Seafloor ploughmarks provide evidence of past and present iceberg dimensions and drift direction. Today, Pine Island and Thwaites glaciers, which account for 35% of mass loss from the West Antarctic Ice Sheet (WAIS), calve mainly large, tabular icebergs, which, when grounded, produce `toothcomb-like' multi-keeled ploughmarks. High-resolution multi-beam swath bathymetry of the mid-shelf Pine Island Trough and adjacent banks, reveals many linear-curvilinear depressions interpreted as iceberg-keel ploughmarks, the majority of which are single-keeled in form. From measurements of ploughmark planform and cross-sections, we find iceberg calving from the palaeo-Pine Island-Thwaites Ice Stream was not characterised by small numbers of large, tabular icebergs, but instead, by a large number of `smaller' icebergs with v-shaped keels. Geological evidence of ploughmark form and water-depth distribution indicates calving-margin thicknesses ( 950 m) and subaerial ice-cliff elevations ( 100 m) equivalent to the theoretical threshold recently predicted to trigger ice-cliff structural collapse through Marine Ice Cliff Instability (MICI) processes. Significantly, our proposed period of iceberg ploughing predates the early Holocene climate optimum, and likely occurred in an absence of widespread surface melt. We therefore provide the first observational evidence of rapid retreat of the Palaeo-Pine Island-Thwaites ice stream from the crest of a large, mid-shelf sedimentary depocentre or grounding-zone wedge, to a restabilising position 112 km offshore of the December 2013 calving line, driven by MICI processes commencing 12.3 cal. ka BP. We emphasise the effective operation of MICI processes without extensive surface melt and induced hydrofracture, and conclude that such processes are unlikely to be confined to the past, given the steep, retrograde bed-slope which the modern grounding lines of Pine Island and Thwaites Glaciers are approaching, and the absence of any discernible restabilising features upstream of the modern grounding-zone. We expect MICI to contribute significantly to future ice retreat and sea-level rise under a warming climate, and emphasise the importance of its inclusion in future modelling studies.

Centennial-scale Holocene climate variations amplified by Antarctic Ice Sheet discharge

NASA Astrophysics Data System (ADS)

Bakker, Pepijn; Clark, Peter U.; Golledge, Nicholas R.; Schmittner, Andreas; Weber, Michael E.

2017-01-01

Proxy-based indicators of past climate change show that current global climate models systematically underestimate Holocene-epoch climate variability on centennial to multi-millennial timescales, with the mismatch increasing for longer periods. Proposed explanations for the discrepancy include ocean-atmosphere coupling that is too weak in models, insufficient energy cascades from smaller to larger spatial and temporal scales, or that global climate models do not consider slow climate feedbacks related to the carbon cycle or interactions between ice sheets and climate. Such interactions, however, are known to have strongly affected centennial- to orbital-scale climate variability during past glaciations, and are likely to be important in future climate change. Here we show that fluctuations in Antarctic Ice Sheet discharge caused by relatively small changes in subsurface ocean temperature can amplify multi-centennial climate variability regionally and globally, suggesting that a dynamic Antarctic Ice Sheet may have driven climate fluctuations during the Holocene. We analysed high-temporal-resolution records of iceberg-rafted debris derived from the Antarctic Ice Sheet, and performed both high-spatial-resolution ice-sheet modelling of the Antarctic Ice Sheet and multi-millennial global climate model simulations. Ice-sheet responses to decadal-scale ocean forcing appear to be less important, possibly indicating that the future response of the Antarctic Ice Sheet will be governed more by long-term anthropogenic warming combined with multi-centennial natural variability than by annual or decadal climate oscillations.

Surface Energy and Mass Balance Model for Greenland Ice Sheet and Future Projections

NASA Astrophysics Data System (ADS)

Liu, Xiaojian

The Greenland Ice Sheet contains nearly 3 million cubic kilometers of glacial ice. If the entire ice sheet completely melted, sea level would raise by nearly 7 meters. There is thus considerable interest in monitoring the mass balance of the Greenland Ice Sheet. Each year, the ice sheet gains ice from snowfall and loses ice through iceberg calving and surface melting. In this thesis, we develop, validate and apply a physics based numerical model to estimate current and future surface mass balance of the Greenland Ice Sheet. The numerical model consists of a coupled surface energy balance and englacial model that is simple enough that it can be used for long time scale model runs, but unlike previous empirical parameterizations, has a physical basis. The surface energy balance model predicts ice sheet surface temperature and melt production. The englacial model predicts the evolution of temperature and meltwater within the ice sheet. These two models can be combined with estimates of precipitation (snowfall) to estimate the mass balance over the Greenland Ice Sheet. We first compare model performance with in-situ observations to demonstrate that the model works well. We next evaluate how predictions are degraded when we statistically downscale global climate data. We find that a simple, nearest neighbor interpolation scheme with a lapse rate correction is able to adequately reproduce melt patterns on the Greenland Ice Sheet. These results are comparable to those obtained using empirical Positive Degree Day (PDD) methods. Having validated the model, we next drove the ice sheet model using the suite of atmospheric model runs available through the CMIP5 atmospheric model inter-comparison, which in turn built upon the RCP 8.5 (business as usual) scenarios. From this exercise we predict how much surface melt production will increase in the coming century. This results in 4-10 cm sea level equivalent, depending on the CMIP5 models. Finally, we try to bound melt water production from CMIP5 data with the model by assuming that the Greenland Ice Sheet is covered in black carbon (lowering the albedo) and perpetually covered by optically thick clouds (increasing long wave radiation). This upper bound roughly triples surface meltwater production, resulting in 30 cm of sea level rise by 2100. These model estimates, combined with prior research suggesting an additional 40-100 cm of sea level rise associated with dynamical discharge, suggest that the Greenland Ice Sheet is poised to contribute significantly to sea level rise in the coming century.

Recent Changes in the Greenland Ice Sheet as Seen from Space

NASA Technical Reports Server (NTRS)

Hall, Dorothy K.

2011-01-01

Many changes in the Greenland Ice Sheet have been reported in the recent scientific literature and have been attributed to various responses of the ice sheet due to regional (and global) warming. Because melting of the ice sheet would contribute approximately 7 m to sea-level rise, the lives and habitat of hundreds of millions of people worldwide would be directly and indirectly affected if continued ice-sheet melting occurs. As mean-annual global temperatures have increased, there has been an increasing focus on studying the Greenland Ice Sheet using available satellite data, and numerous expeditions have been undertaken. Regional "clear-sky" surface temperature increases since the early 1980s in the Arctic, measured using Advanced Very High Resolution Radiometer (AVHRR) infrared data, range from 0.57+/-0.02 C to 0.72+/-0.10 C per decade. Arctic warming has important implications for ice-sheet mass balance because much of the periphery of the Greenland Ice Sheet is already near O C during the melt season, and is thus vulnerable to more extensive melting if temperatures continue to increase. An increase in melting of the ice sheet would accelerate sea-level rise, an issue of increasing concern to billions of people worldwide. The surface temperature of the ice sheet has been studied in even greater detail using Moderate-Resolution Imaging Spectroradiometer (MODIS) data in the six individual drainage basins as well as for the ice sheet as a whole. Surface temperature trends in the decade of the 2000s have not been strong, according to the MODIS measurements. In addition to surface-temperature increases over the last few decades as measured by AVHRR, other changes have been observed such as accelerated movement of many of Greenland's outlet glaciers and sudden draining of supraglacial lakes. Decreasing mass of the ice sheet since (at least) 2002 has been measured using Gravity Recovery and Climate Experiment (GRACE) data, along with an build-up of ice at the higher elevations and a decrease of ice at the lower elevations as measured using airborne Lidar and Ice, Cloud and Land Elevation Satellite (ICESat) data. The seminar will address the above issues using a variety of NASA satellite data and ground observations.

LIVVkit: An extensible, python-based, land ice verification and validation toolkit for ice sheet models

NASA Astrophysics Data System (ADS)

Kennedy, Joseph H.; Bennett, Andrew R.; Evans, Katherine J.; Price, Stephen; Hoffman, Matthew; Lipscomb, William H.; Fyke, Jeremy; Vargo, Lauren; Boghozian, Adrianna; Norman, Matthew; Worley, Patrick H.

2017-06-01

To address the pressing need to better understand the behavior and complex interaction of ice sheets within the global Earth system, significant development of continental-scale, dynamical ice sheet models is underway. Concurrent to the development of the Community Ice Sheet Model (CISM), the corresponding verification and validation (V&V) process is being coordinated through a new, robust, Python-based extensible software package, the Land Ice Verification and Validation toolkit (LIVVkit). Incorporated into the typical ice sheet model development cycle, it provides robust and automated numerical verification, software verification, performance validation, and physical validation analyses on a variety of platforms, from personal laptops to the largest supercomputers. LIVVkit operates on sets of regression test and reference data sets, and provides comparisons for a suite of community prioritized tests, including configuration and parameter variations, bit-for-bit evaluation, and plots of model variables to indicate where differences occur. LIVVkit also provides an easily extensible framework to incorporate and analyze results of new intercomparison projects, new observation data, and new computing platforms. LIVVkit is designed for quick adaptation to additional ice sheet models via abstraction of model specific code, functions, and configurations into an ice sheet model description bundle outside the main LIVVkit structure. Ultimately, through shareable and accessible analysis output, LIVVkit is intended to help developers build confidence in their models and enhance the credibility of ice sheet models overall.

Algae Drive Enhanced Darkening of Bare Ice on the Greenland Ice Sheet

NASA Astrophysics Data System (ADS)

Stibal, Marek; Box, Jason E.; Cameron, Karen A.; Langen, Peter L.; Yallop, Marian L.; Mottram, Ruth H.; Khan, Alia L.; Molotch, Noah P.; Chrismas, Nathan A. M.; Calı Quaglia, Filippo; Remias, Daniel; Smeets, C. J. P. Paul; van den Broeke, Michiel R.; Ryan, Jonathan C.; Hubbard, Alun; Tranter, Martyn; van As, Dirk; Ahlstrøm, Andreas P.

2017-11-01

Surface ablation of the Greenland ice sheet is amplified by surface darkening caused by light-absorbing impurities such as mineral dust, black carbon, and pigmented microbial cells. We present the first quantitative assessment of the microbial contribution to the ice sheet surface darkening, based on field measurements of surface reflectance and concentrations of light-absorbing impurities, including pigmented algae, during the 2014 melt season in the southwestern part of the ice sheet. The impact of algae on bare ice darkening in the study area was greater than that of nonalgal impurities and yielded a net albedo reduction of 0.038 ± 0.0035 for each algal population doubling. We argue that algal growth is a crucial control of bare ice darkening, and incorporating the algal darkening effect will improve mass balance and sea level projections of the Greenland ice sheet and ice masses elsewhere.

Landsat TM image maps of the Shirase and Siple Coast ice streams, West Antarctica

USGS Publications Warehouse

Ferrigno, Jane G.; Mullins, Jerry L.; Stapleton, Jo Anne; Bindschadler, Robert; Scambos, Ted A.; Bellisime, Lynda B.; Bowell, Jo-Ann; Acosta, Alex V.

1994-01-01

Fifteen 1: 250000 and one 1: 1000 000 scale Landsat Thematic Mapper (TM) image mosaic maps are currently being produced of the West Antarctic ice streams on the Shirase and Siple Coasts. Landsat TM images were acquired between 1984 and 1990 in an area bounded approximately by 78°-82.5°S and 120°- 160° W. Landsat TM bands 2, 3 and 4 were combined to produce a single band, thereby maximizing data content and improving the signal-to-noise ratio. The summed single band was processed with a combination of high- and low-pass filters to remove longitudinal striping and normalize solar elevation-angle effects. The images were mosaicked and transformed to a Lambert conformal conic projection using a cubic-convolution algorithm. The projection transformation was controled with ten weighted geodetic ground-control points and internal image-to-image pass points with annotation of major glaciological features. The image maps are being published in two formats: conventional printed map sheets and on a CD-ROM.

The Distribution of Basal Water Beneath the Greenland Ice Sheet from Radio-Echo Sounding

NASA Astrophysics Data System (ADS)

Jordan, T.; Williams, C.; Schroeder, D. M.; Martos, Y. M.; Cooper, M.; Siegert, M. J.; Paden, J. D.; Huybrechts, P.; Bamber, J. L.

2017-12-01

There is widespread, but often indirect, evidence that a significant fraction of the Greenland Ice Sheet is thawed at the bed. This includes major outlet glaciers and around the NorthGRIP ice-core in the interior. However, the ice-sheet-wide distribution of basal water is poorly constrained by existing observations, and the spatial relationship between basal water and other ice-sheet and subglacial properties is therefore largely unexplored. In principle, airborne radio-echo sounding (RES) surveys provide the necessary information and spatial coverage to infer the presence of basal water at the ice-sheet scale. However, due to uncertainty and spatial variation in radar signal attenuation, the commonly used water diagnostic, bed-echo reflectivity, is highly ambiguous and prone to spatial bias. Here we introduce a new RES diagnostic for the presence of basal water which incorporates both sharp step-transitions and rapid fluctuations in bed-echo reflectivity. This has the advantage of being (near) independent of attenuation model, and enables a decade of recent Operation Ice Bride RES survey data to be combined in a single map for basal water. The ice-sheet-wide water predictions are compared with: bed topography and drainage network structure, existing knowledge of the thermal state and geothermal heat flux, and ice velocity. In addition to the fast flowing ice-sheet margins, we also demonstrate widespread water routing and storage in parts of the slow-flowing northern interior. Notably, this includes a quasi-linear `corridor' of basal water, extending from NorthGRIP to Petermann glacier, which spatially correlates with a region of locally high (magnetic-derived) geothermal heat flux. The predicted water distribution places a new constraint upon the basal thermal state of the Greenland Ice Sheet, and could be used as an input for ice-sheet model simulations.

Marine evidence of a deconvolving Antarctic Ice Sheet during post-LGM retreat of the Ross Sea sector

NASA Astrophysics Data System (ADS)

Prothro, L. O.; Yokoyama, Y.; Simkins, L. M.; Anderson, J. B.; Majewski, W.; Yamane, M.; Ohkouchi, N.

2017-12-01

Predictive models of ice sheet and sea level change are dependent on observational data of ice-sheet behavior for model testing and tuning. The geologic record contains a wealth of information about ice-sheet dynamics, with fewer logistical, spatial, and temporal limitations than are involved in data acquisition along contemporary ice margins. However, past ice-sheet behavior is still largely uncertain or contested due to issues with obtaining meaningful radiocarbon dates. We minimize bias from glacially-reworked carbon and limitations from unknown geomorphic context and uncertainty in sediment facies identification by using careful sedimentary analyses within a geomorphic framework, as well as selection of appropriate dating methods. Our study area, the Ross Sea sector of Antarctica, is the primary drainage outlet for 25% of the continent's grounded ice. During the Last Glacial Maximum, the low-profile, marine-based West Antarctic Ice Sheet (WAIS) and the steeper profile, largely land-based East Antarctic Ice Sheet (EAIS) converged in the Ross Sea to flow out to or near the continental shelf edge. Geomorphic and sedimentary data reveal that during their subsequent retreat to form the Ross Sea Embayment, the two ice sheets behaved differently, with the WAIS rapidly retreating tens of kilometers followed by extended pauses, while the EAIS retreated steadily, with shorter (decadal- to century-long) pauses. This behavior leads us to believe that the two ice sheets may have contributed diachronously to sea level. By acquiring accurate timing of grounding line retreat, we are able to calculate volumes of ice lost throughout deglaciation, as well as associated sea level contributions. In addition, we attempt to rectify the contradicting marine and terrestrial interpretations of retreat patterns from the Ross Sea continental shelf.

Limited Impact of Subglacial Supercooling Freeze-on for Greenland Ice Sheet Stratigraphy

NASA Astrophysics Data System (ADS)

Dow, Christine F.; Karlsson, Nanna B.; Werder, Mauro A.

2018-02-01

Large units of disrupted radiostratigraphy (UDR) are visible in many radio-echo sounding data sets from the Greenland Ice Sheet. This study investigates whether supercooling freeze-on rates at the bed can cause the observed UDR. We use a subglacial hydrology model to calculate both freezing and melting rates at the base of the ice sheet in a distributed sheet and within basal channels. We find that while supercooling freeze-on is a phenomenon that occurs in many areas of the ice sheet, there is no discernible correlation with the occurrence of UDR. The supercooling freeze-on rates are so low that it would require tens of thousands of years with minimal downstream ice motion to form the hundreds of meters of disrupted radiostratigraphy. Overall, the melt rates at the base of the ice sheet greatly overwhelm the freeze-on rates, which has implications for mass balance calculations of Greenland ice.

Relationship between dissolved organic matter quality and microbial community composition across polar glacial environments.

PubMed

Smith, H J; Dieser, M; McKnight, D M; SanClements, M D; Foreman, C M

2018-05-14

Vast expanses of Earth's surface are covered by ice, with microorganisms in these systems affecting local and global biogeochemical cycles. We examined microbial assemblages from habitats fed by glacial meltwater within the McMurdo Dry Valleys, Antarctica, and on the west Greenland Ice Sheet, (GrIS) evaluating potential physicochemical factors explaining trends in community structure. Microbial assemblages present in the different Antarctic dry valley habitats were dominated by Sphingobacteria and Flavobacteria, while Gammaproteobacteria and Sphingobacteria prevailed in west GrIS supraglacial environments. Microbial assemblages clustered by location (Canada Glacier, Cotton Glacier, west GrIS) and were separated by habitat type (i.e. ice, cryoconite holes, supraglacial lakes, sediment, and stream water). Community dissimilarities were strongly correlated with dissolved organic matter (DOM) quality. Microbial meltwater assemblages were most closely associated with different protein-like components of the DOM pool. Microbes in environments with mineral particles (i.e. stream sediments, cryoconite holes) were linked to DOM containing more humic-like fluorescence. Our results demonstrate the establishment of distinct microbial communities within ephemeral glacial meltwater habitats, with DOM-microbe interactions playing an integral role in shaping communities on local and polar spatial scales.

Enhanced ice sheet growth in Eurasia owing to adjacent ice-dammed lakes.

PubMed

Krinner, G; Mangerud, J; Jakobsson, M; Crucifix, M; Ritz, C; Svendsen, J I

2004-01-29

Large proglacial lakes cool regional summer climate because of their large heat capacity, and have been shown to modify precipitation through mesoscale atmospheric feedbacks, as in the case of Lake Agassiz. Several large ice-dammed lakes, with a combined area twice that of the Caspian Sea, were formed in northern Eurasia about 90,000 years ago, during the last glacial period when an ice sheet centred over the Barents and Kara seas blocked the large northbound Russian rivers. Here we present high-resolution simulations with an atmospheric general circulation model that explicitly simulates the surface mass balance of the ice sheet. We show that the main influence of the Eurasian proglacial lakes was a significant reduction of ice sheet melting at the southern margin of the Barents-Kara ice sheet through strong regional summer cooling over large parts of Russia. In our simulations, the summer melt reduction clearly outweighs lake-induced decreases in moisture and hence snowfall, such as has been reported earlier for Lake Agassiz. We conclude that the summer cooling mechanism from proglacial lakes accelerated ice sheet growth and delayed ice sheet decay in Eurasia and probably also in North America.

Interaction of ice sheets and climate during the past 800 000 years

NASA Astrophysics Data System (ADS)

Stap, L. B.; van de Wal, R. S. W.; de Boer, B.; Bintanja, R.; Lourens, L. J.

2014-12-01

During the Cenozoic, land ice and climate interacted on many different timescales. On long timescales, the effect of land ice on global climate and sea level is mainly set by large ice sheets in North America, Eurasia, Greenland and Antarctica. The climatic forcing of these ice sheets is largely determined by the meridional temperature profile resulting from radiation and greenhouse gas (GHG) forcing. As a response, the ice sheets cause an increase in albedo and surface elevation, which operates as a feedback in the climate system. To quantify the importance of these climate-land ice processes, a zonally averaged energy balance climate model is coupled to five one-dimensional ice sheet models, representing the major ice sheets. In this study, we focus on the transient simulation of the past 800 000 years, where a high-confidence CO2 record from ice core samples is used as input in combination with Milankovitch radiation changes. We obtain simulations of atmospheric temperature, ice volume and sea level that are in good agreement with recent proxy-data reconstructions. We examine long-term climate-ice-sheet interactions by a comparison of simulations with uncoupled and coupled ice sheets. We show that these interactions amplify global temperature anomalies by up to a factor of 2.6, and that they increase polar amplification by 94%. We demonstrate that, on these long timescales, the ice-albedo feedback has a larger and more global influence on the meridional atmospheric temperature profile than the surface-height-temperature feedback. Furthermore, we assess the influence of CO2 and insolation by performing runs with one or both of these variables held constant. We find that atmospheric temperature is controlled by a complex interaction of CO2 and insolation, and both variables serve as thresholds for northern hemispheric glaciation.

Ice sheet radar altimetry

NASA Technical Reports Server (NTRS)

Zwally, J.

1988-01-01

The surface topography of the Greenland and Antarctic ice sheets between 72 degrees north and south was mapped using radar altimetry data from the U.S. Navy GEOSAT. The glaciological objectives of this activity were to study the dynamics of the ice flow, changes in the position of floating ice-shelf fronts, and ultimately to measure temporal changes in ice surface elevation indicative of ice sheet mass balance.

Ice shelf fracture parameterization in an ice sheet model

NASA Astrophysics Data System (ADS)

Sun, Sainan; Cornford, Stephen L.; Moore, John C.; Gladstone, Rupert; Zhao, Liyun

2017-11-01

Floating ice shelves exert a stabilizing force onto the inland ice sheet. However, this buttressing effect is diminished by the fracture process, which on large scales effectively softens the ice, accelerating its flow, increasing calving, and potentially leading to ice shelf breakup. We add a continuum damage model (CDM) to the BISICLES ice sheet model, which is intended to model the localized opening of crevasses under stress, the transport of those crevasses through the ice sheet, and the coupling between crevasse depth and the ice flow field and to carry out idealized numerical experiments examining the broad impact on large-scale ice sheet and shelf dynamics. In each case we see a complex pattern of damage evolve over time, with an eventual loss of buttressing approximately equivalent to halving the thickness of the ice shelf. We find that it is possible to achieve a similar ice flow pattern using a simple rule of thumb: introducing an enhancement factor ˜ 10 everywhere in the model domain. However, spatially varying damage (or equivalently, enhancement factor) fields set at the start of prognostic calculations to match velocity observations, as is widely done in ice sheet simulations, ought to evolve in time, or grounding line retreat can be slowed by an order of magnitude.

Holocene deceleration of the Greenland Ice Sheet.

PubMed

MacGregor, Joseph A; Colgan, William T; Fahnestock, Mark A; Morlighem, Mathieu; Catania, Ginny A; Paden, John D; Gogineni, S Prasad

2016-02-05

Recent peripheral thinning of the Greenland Ice Sheet is partly offset by interior thickening and is overprinted on its poorly constrained Holocene evolution. On the basis of the ice sheet's radiostratigraphy, ice flow in its interior is slower now than the average speed over the past nine millennia. Generally higher Holocene accumulation rates relative to modern estimates can only partially explain this millennial-scale deceleration. The ice sheet's dynamic response to the decreasing proportion of softer ice from the last glacial period and the deglacial collapse of the ice bridge across Nares Strait also contributed to this pattern. Thus, recent interior thickening of the Greenland Ice Sheet is partly an ongoing dynamic response to the last deglaciation that is large enough to affect interpretation of its mass balance from altimetry. Copyright © 2016, American Association for the Advancement of Science.

Long term ice sheet mass change rates and inter-annual variability from GRACE gravimetry.

NASA Astrophysics Data System (ADS)

Harig, C.

2017-12-01

The GRACE time series of gravimetry now stretches 15 years since its launch in 2002. Here we use Slepian functions to estimate the long term ice mass trends of Greenland, Antarctica, and several glaciated regions. The spatial representation shows multi-year to decadal regional shifts in accelerations, in agreement with increases in radar derived ice velocity. Interannual variations in ice mass are of particular interest since they can directly link changes in ice sheets to the drivers of change in the polar ocean and atmosphere. The spatial information retained in Slepian functions provides a tool to determine how this link varies in different regions within an ice sheet. We present GRACE observations of the 2013-2014 slowdown in mass loss of the Greenland ice sheet, which was concentrated in specific parts of the ice sheet and in certain months of the year. We also discuss estimating the relative importance of climate factors that control ice mass balance, as a function of location of the glacier/ice cap as well as the spatial variation within an ice sheet by comparing gravimetry with observations of surface air temperature, ocean temperature, etc. as well as model data from climate reanalysis products.

A Late-Glacial sedimentary sequence at KIlkeel, Northern Ireland: implications for the glaciation of the Irish Sea Basin

NASA Astrophysics Data System (ADS)

Merritt, Jon; Roberson, Sam; Cooper, Mark

2017-04-01

This paper re-evaluates the nature and timing of a Late-Glacial ice sheet re-advance in the north western sector of the Irish Sea basin. The sedimentary archive in the region records the collapse of the Irish Sea Ice Stream, a major outlet glacier of the British-Irish Ice Sheet. The region documents the interplay between southerly flowing Scottish ice, ice flowing southeast from Lough Neagh and locally sourced Mournes ice. We present the results of sedimentological analysis of a glacigenic sequence exposed in a modern cliff section 3 km long between Derryoge and Kilkeel, Co. Down, Northern Ireland. The interaction between an advancing ice-sheet outlet lobe and rapidly changing sea levels are examined using facies analysis and micromorphology. The section is composed of four lithofacies associations (LAs). These are, from the base, a laminated, fossiliferous and deformed silt (LA1) at least 4.5 m thick that contains lenses of diamicton and discontinuous rafts of sandy gravel. Marine shells form the axis of a fold hinge, part of a lightly tectonised channel fill within the raft. LA1 is overlain by a sandy diamict (LA2) up to 14 m thick containing mainly local clasts with some of northern provenance. Within LA2 are wide channel structures infilled by laminated clayey silts (LA2b). These form deposits up to 14 m thick and contain small-scale folds, discrete shear zones and ball-and-pillow structures. LA2b forms a lithofacies association with LA2, consisting of a lower subfacies of sheared and deformed silts, overlain by sandy diamicton, capped by a striated boulder pavement. These are interpreted to represent retreat/advance cycles of a marine terminating ice margin. Up to five such cycles are identified. LA2 is widely punctuated by fissures and conduits infilled by loose sands and gravels. These are inferred to be emplaced by subglacial meltwater during the final stages of ice sheet advance. Covering both LA2 and LA2b, LA3 is a unit of glaciofluvial outwash, composed of cross-trough stratified sandy gravels, with flame structures indicative of syn-depositional loading. The entire sequence is capped by loose interbedded sands and gravels (LA4) representing a Late-Glacial raised beach. Evidence of a marine terminating ice margin provides support for high relative sea levels in the north western sector of the Irish Sea during deglaciation. Forthcoming dates from shells with the rafted subaqueous fan deposits underlying LF2 provide the opportunity to constrain either: a) sea-level rise prior to the onset of Irish Sea Basin glaciation, or, b) Late-Glacial sea level rise following deglaciation of the Irish Sea and prior to the re-advance of local ice masses.

Exposure age and ice-sheet model constraints on Pliocene East Antarctic ice sheet dynamics.

PubMed

Yamane, Masako; Yokoyama, Yusuke; Abe-Ouchi, Ayako; Obrochta, Stephen; Saito, Fuyuki; Moriwaki, Kiichi; Matsuzaki, Hiroyuki

2015-04-24

The Late Pliocene epoch is a potential analogue for future climate in a warming world. Here we reconstruct Plio-Pleistocene East Antarctic Ice Sheet (EAIS) variability using cosmogenic nuclide exposure ages and model simulations to better understand ice sheet behaviour under such warm conditions. New and previously published exposure ages indicate interior-thickening during the Pliocene. An ice sheet model with mid-Pliocene boundary conditions also results in interior thickening and suggests that both the Wilkes Subglacial and Aurora Basins largely melted, offsetting increased ice volume. Considering contributions from West Antarctica and Greenland, this is consistent with the most recent IPCC AR5 estimate, which indicates that the Pliocene sea level likely did not exceed +20 m on Milankovitch timescales. The inception of colder climate since ∼3 Myr has increased the sea ice cover and inhibited active moisture transport to Antarctica, resulting in reduced ice sheet thickness, at least in coastal areas.

Clouds enhance Greenland ice sheet meltwater runoff.

PubMed

Van Tricht, K; Lhermitte, S; Lenaerts, J T M; Gorodetskaya, I V; L'Ecuyer, T S; Noël, B; van den Broeke, M R; Turner, D D; van Lipzig, N P M

2016-01-12

The Greenland ice sheet has become one of the main contributors to global sea level rise, predominantly through increased meltwater runoff. The main drivers of Greenland ice sheet runoff, however, remain poorly understood. Here we show that clouds enhance meltwater runoff by about one-third relative to clear skies, using a unique combination of active satellite observations, climate model data and snow model simulations. This impact results from a cloud radiative effect of 29.5 (±5.2) W m(-2). Contrary to conventional wisdom, however, the Greenland ice sheet responds to this energy through a new pathway by which clouds reduce meltwater refreezing as opposed to increasing surface melt directly, thereby accelerating bare-ice exposure and enhancing meltwater runoff. The high sensitivity of the Greenland ice sheet to both ice-only and liquid-bearing clouds highlights the need for accurate cloud representations in climate models, to better predict future contributions of the Greenland ice sheet to global sea level rise.

Clouds enhance Greenland ice sheet meltwater runoff

PubMed Central

Van Tricht, K.; Lhermitte, S.; Lenaerts, J. T. M.; Gorodetskaya, I. V.; L'Ecuyer, T. S.; Noël, B.; van den Broeke, M. R.; Turner, D. D.; van Lipzig, N. P. M.

2016-01-01

The Greenland ice sheet has become one of the main contributors to global sea level rise, predominantly through increased meltwater runoff. The main drivers of Greenland ice sheet runoff, however, remain poorly understood. Here we show that clouds enhance meltwater runoff by about one-third relative to clear skies, using a unique combination of active satellite observations, climate model data and snow model simulations. This impact results from a cloud radiative effect of 29.5 (±5.2) W m−2. Contrary to conventional wisdom, however, the Greenland ice sheet responds to this energy through a new pathway by which clouds reduce meltwater refreezing as opposed to increasing surface melt directly, thereby accelerating bare-ice exposure and enhancing meltwater runoff. The high sensitivity of the Greenland ice sheet to both ice-only and liquid-bearing clouds highlights the need for accurate cloud representations in climate models, to better predict future contributions of the Greenland ice sheet to global sea level rise. PMID:26756470

Landforms, sediments and dates to constrain rates and style of marine-influenced ice sheet decay; the BRITICE-CHRONO project.

NASA Astrophysics Data System (ADS)

Clark, Chris

2014-05-01

Uncertainty exists regarding the future mass of the Antarctic and Greenland ice sheets and how they will respond to forcings from sea level, and atmospheric and ocean temperatures. If we want to know more about the mechanisms and rate of change of shrinking ice sheets, then why not examine an ice sheet that has fully disappeared and track its retreat through time? If achieved in enough detail such information on ice retreat could be a data-rich playground for improving the next breed of numerical ice sheet models to be used in ice and sea level forecasting. We regard that the last British-Irish Ice Sheet is a good target for this work, on account of its small size, density of information and with its numerous researchers already investigating it. Geomorphological mapping across the British Isles and the surrounding continental shelf has revealed the nature and distribution of glacial landforms. Here we demonstrate how such data have been used to build a pattern of ice margin retreat. The BRITICE-CHRONO consortium of Quaternary scientists and glaciologists, are now working on a project running from 2012 - 2017 to produce an ice sheet wide database of geochronometric dates to constrain and then understand ice margin retreat. This is being achieved by focusing on 8 transects running from the continental shelf edge to a short distance (10s km) onshore and acquiring marine and terrestrial samples for geochronometric dating. The project includes funding for 587 radiocarbon, 140 OSL and 158 TCN samples for surface exposure dating; with sampling accomplished by two research cruises and 16 fieldwork campaigns. Results will reveal the timing and rate of change of ice margin recession for each transect, and combined with existing landform and dating databases, will be used to build an ice sheet-wide empirical reconstruction of retreat. Simulations using two numerical ice sheet models, fitted against the margin data, will help us understand the nature and significance of sea-level rise and ocean/atmosphere forcing on influencing the rate of retreat and ice sheet demise and the effect that bed topography has in controlling this.

Improving Surface Mass Balance Over Ice Sheets and Snow Depth on Sea Ice

NASA Technical Reports Server (NTRS)

Koenig, Lora Suzanne; Box, Jason; Kurtz, Nathan

2013-01-01

Surface mass balance (SMB) over ice sheets and snow on sea ice (SOSI) are important components of the cryosphere. Large knowledge gaps remain in scientists' abilities to monitor SMB and SOSI, including insufficient measurements and difficulties with satellite retrievals. On ice sheets, snow accumulation is the sole mass gain to SMB, and meltwater runoff can be the dominant single loss factor in extremely warm years such as 2012. SOSI affects the growth and melt cycle of the Earth's polar sea ice cover. The summer of 2012 saw the largest satellite-recorded melt area over the Greenland ice sheet and the smallest satellite-recorded Arctic sea ice extent, making this meeting both timely and relevant.

Ice-sheet response to oceanic forcing.

PubMed

Joughin, Ian; Alley, Richard B; Holland, David M

2012-11-30

The ice sheets of Greenland and Antarctica are losing ice at accelerating rates, much of which is a response to oceanic forcing, especially of the floating ice shelves. Recent observations establish a clear correspondence between the increased delivery of oceanic heat to the ice-sheet margin and increased ice loss. In Antarctica, most of these processes are reasonably well understood but have not been rigorously quantified. In Greenland, an understanding of the processes by which warmer ocean temperatures drive the observed retreat remains elusive. Experiments designed to identify the relevant processes are confounded by the logistical difficulties of instrumenting ice-choked fjords with actively calving glaciers. For both ice sheets, multiple challenges remain before the fully coupled ice-ocean-atmosphere models needed for rigorous sea-level projection are available.

Modeling North American Ice Sheet Response to Changes in Precession and Obliquity

NASA Astrophysics Data System (ADS)

Tabor, C.; Poulsen, C. J.; Pollard, D.

2012-12-01

Milankovitch theory proposes that changes in insolation due to orbital perturbations dictate the waxing and waning of the ice sheets (Hays et al., 1976). However, variations in solar forcing alone are insufficient to produce the glacial oscillations observed in the climate record. Non-linear feedbacks in the Earth system likely work in concert with the orbital cycles to produce a modified signal (e.g. Berger and Loutre, 1996), but the nature of these feedbacks remain poorly understood. To gain a better understand of the ice dynamics and climate feedbacks associated with changes in orbital configuration, we use a complex Earth system model consisting of the GENESIS GCM and land surface model (Pollard and Thompson, 1997), the Pennsylvania State University ice sheet model (Pollard and DeConto, 2009), and the BIOME vegetation model (Kaplan et al., 2001). We began this study by investigating ice sheet sensitivity to a range of commonly used ice sheet model parameters, including mass balance and albedo, to optimize simulations for Pleistocene orbital cycles. Our tests indicate that choice of mass balance and albedo parameterizations can lead to significant differences in ice sheet behavior and volume. For instance, use of an insolation-temperature mass balance scheme (van den Berg, 2008) allows for a larger ice sheet response to orbital changes than the commonly employed positive degree-day method. Inclusion of a large temperature dependent ice albedo, representing phenomena such as melt ponds and dirty ice, also enhances ice sheet sensitivity. Careful tuning of mass balance and albedo parameterizations can help alleviate the problem of insufficient ice sheet retreat during periods of high summer insolation (Horton and Poulsen, 2007) while still accurately replicating the modern climate. Using our optimized configuration, we conducted a series of experiments with idealized transient orbits in an asynchronous coupling scheme to investigate the influence of obliquity and precession on the Laurentide and Cordillera ice sheets of North America. Preliminary model results show that the ice sheet response to changes in obliquity are larger than for precession despite providing a smaller direct insolation variation in the Northern Hemisphere high latitudes. A combination of enhanced Northern Hemisphere mid-latitude temperature gradient and longer cycle duration allow for a larger ice sheet response to obliquity than would be expected from insolation forcing alone. Conversely, a shorter duration dampens the ice sheet response to precession. Nevertheless, the precession cycle does cause significant changes in ice volume, a feature not observed in the Early Pleistocene δ18O records (Raymo and Nisancioglu, 2003). Future work will examine the climate response to an idealized transient orbit that includes concurrent variations in obliquity, precession, and eccentricity.

Looking Into and Through the Ross Ice Shelf - ROSETTA-ICE

NASA Astrophysics Data System (ADS)

Bell, R. E.

2015-12-01

Our current understanding of the structure and stability of the Ross Ice Shelf is based on satellite studies of the ice surface and the 1970's RIGGS program. The study of the flowlines evident in the MODIS imagery combined with surface geophysics has revealed a complex history with ice streams Mercer, Whillans and Kamb changing velocity over the past 1000 years. Here, we present preliminary IcePod and IceBridge radar data acquired in December 2014 and November 2013 across the Ross Ice Shelf that show clearly, for the first time, the structure of the ice shelf and provide insights into ice-ocean interaction. The three major layers of the ice shelf are (1) the continental meteoric ice layer), ice formed on the grounded ice sheet that entered the ice shelf where ice streams and outlet glaciers crossed the grounding line (2) the locally accumulating meteoric ice layer, ice and snow that forms from snowfall on the floating ice shelf and (3) a basal marine ice layer. The locally accumulating meteoric ice layer contains well-defined internal layers that are generally parallel to the ice surface and thickens away from the grounding line and reaches a maximum thickness of 220m along the line crossing Roosevelt Island. The continental meteoric layer is located below a broad irregular internal reflector, and is characterized by irregular internal layers. These internal layers are often folded, likely a result of deformation as the ice flowed across the grounding line. The basal marine ice layer, up to 50m thick, is best resolved in locations where basal crevasses are present, and appears to thicken along the flow at rates of decimeters per year. Each individual flowband of the ice shelf contains layers that are distinct in their structure. For example, the thickness of the locally accumulated layer is a function of both the time since crossing the grounding line and the thickness of the incoming ice. Features in the meteoric ice, such as distinct folds, can be traced between the two IceBridge lines located 47 km apart. The ROSETTA-ICE program will begin a systematic mapping of the Ross Ice Shelf and sub-ice topography using the IcePod system beginning in 2015. Together the new gravity-derived bathymetry and the mapping of the ice shelf structure will provide key insights into the stability of the ice shelf.

Antarctic climate and ice-sheet configuration during the early Pliocene interglacial at 4.23 Ma

NASA Astrophysics Data System (ADS)

Golledge, Nicholas R.; Thomas, Zoë A.; Levy, Richard H.; Gasson, Edward G. W.; Naish, Timothy R.; McKay, Robert M.; Kowalewski, Douglas E.; Fogwill, Christopher J.

2017-07-01

The geometry of Antarctic ice sheets during warm periods of the geological past is difficult to determine from geological evidence, but is important to know because such reconstructions enable a more complete understanding of how the ice-sheet system responds to changes in climate. Here we investigate how Antarctica evolved under orbital and greenhouse gas conditions representative of an interglacial in the early Pliocene at 4.23 Ma, when Southern Hemisphere insolation reached a maximum. Using offline-coupled climate and ice-sheet models, together with a new synthesis of high-latitude palaeoenvironmental proxy data to define a likely climate envelope, we simulate a range of ice-sheet geometries and calculate their likely contribution to sea level. In addition, we use these simulations to investigate the processes by which the West and East Antarctic ice sheets respond to environmental forcings and the timescales over which these behaviours manifest. We conclude that the Antarctic ice sheet contributed 8.6 ± 2.8 m to global sea level at this time, under an atmospheric CO2 concentration identical to present (400 ppm). Warmer-than-present ocean temperatures led to the collapse of West Antarctica over centuries, whereas higher air temperatures initiated surface melting in parts of East Antarctica that over one to two millennia led to lowering of the ice-sheet surface, flotation of grounded margins in some areas, and retreat of the ice sheet into the Wilkes Subglacial Basin. The results show that regional variations in climate, ice-sheet geometry, and topography produce long-term sea-level contributions that are non-linear with respect to the applied forcings, and which under certain conditions exhibit threshold behaviour associated with behavioural tipping points.

IGLOO: an Intermediate Complexity Framework to Simulate Greenland Ice-Ocean Interactions

NASA Astrophysics Data System (ADS)

Perrette, M.; Calov, R.; Beckmann, J.; Alexander, D.; Beyer, S.; Ganopolski, A.

2017-12-01

The Greenland ice-sheet is a major contributor to current and future sea level rise associated to climate warming. It is widely believed that over a century time scale, surface melting is the main driver of Greenland ice volume change, in contrast to melting by the ocean. It is due to relatively warmer air and less ice area exposed to melting by ocean water compared to Antarctica, its southern, larger twin. Yet most modeling studies do not have adequate grid resolution to represent fine-scale outlet glaciers and fjords at the margin of the ice sheet, where ice-ocean interaction occurs, and must use rather crude parameterizations to represent this process. Additionally, the ice-sheet area grounded below sea level has been reassessed upwards in the most recent estimates of bedrock elevation under the Greenland ice sheet, revealing a larger potential for marine-mediated melting than previously thought. In this work, we develop an original approach to estimate potential Greenland ice sheet contribution to sea level rise from ocean melting, in an intermediate complexity framework, IGLOO. We use a medium-resolution (5km) ice-sheet model coupled interactively to a number of 1-D flowline models for the individual outlet glaciers. We propose a semi-objective methodology to derive 1-D glacier geometries from 2-D Greenland datasets, as well as preliminary results of coupled ice-sheet-glaciers simulations with IGLOO.

Influence of temperature fluctuations on equilibrium
ice sheet volume

NASA Astrophysics Data System (ADS)

Bøgeholm Mikkelsen, Troels; Grinsted, Aslak; Ditlevsen, Peter

2018-01-01

Forecasting the future sea level relies on accurate modeling of the response of the Greenland and Antarctic ice sheets to changing temperatures. The surface mass balance (SMB) of the Greenland Ice Sheet (GrIS) has a nonlinear response to warming. Cold and warm anomalies of equal size do not cancel out and it is therefore important to consider the effect of interannual fluctuations in temperature. We find that the steady-state volume of an ice sheet is biased toward larger size if interannual temperature fluctuations are not taken into account in numerical modeling of the ice sheet. We illustrate this in a simple ice sheet model and find that the equilibrium ice volume is approximately 1 m SLE (meters sea level equivalent) smaller when the simple model is forced with fluctuating temperatures as opposed to a stable climate. It is therefore important to consider the effect of interannual temperature fluctuations when designing long experiments such as paleo-spin-ups. We show how the magnitude of the potential bias can be quantified statistically. For recent simulations of the Greenland Ice Sheet, we estimate the bias to be 30 Gt yr-1 (24-59 Gt yr-1, 95 % credibility) for a warming of 3 °C above preindustrial values, or 13 % (10-25, 95 % credibility) of the present-day rate of ice loss. Models of the Greenland Ice Sheet show a collapse threshold beyond which the ice sheet becomes unsustainable. The proximity of the threshold will be underestimated if temperature fluctuations are not taken into account. We estimate the bias to be 0.12 °C (0.10-0.18 °C, 95 % credibility) for a recent estimate of the threshold. In light of our findings it is important to gauge the extent to which this increased variability will influence the mass balance of the ice sheets.

Coupled ice sheet - climate simulations of the last glacial inception and last glacial maximum with a model of intermediate complexity that includes a dynamical downscaling of heat and moisture

NASA Astrophysics Data System (ADS)

Quiquet, Aurélien; Roche, Didier M.

2017-04-01

Comprehensive fully coupled ice sheet - climate models allowing for multi-millenia transient simulations are becoming available. They represent powerful tools to investigate ice sheet - climate interactions during the repeated retreats and advances of continental ice sheets of the Pleistocene. However, in such models, most of the time, the spatial resolution of the ice sheet model is one order of magnitude lower than the one of the atmospheric model. As such, orography-induced precipitation is only poorly represented. In this work, we briefly present the most recent improvements of the ice sheet - climate coupling within the model of intermediate complexity iLOVECLIM. On the one hand, from the native atmospheric resolution (T21), we have included a dynamical downscaling of heat and moisture at the ice sheet model resolution (40 km x 40 km). This downscaling accounts for feedbacks of sub-grid precipitation on large scale energy and water budgets. From the sub-grid atmospheric variables, we compute an ice sheet surface mass balance required by the ice sheet model. On the other hand, we also explicitly use oceanic temperatures to compute sub-shelf melting at a given depth. Based on palaeo evidences for rate of change of eustatic sea level, we discuss the capability of our new model to correctly simulate the last glacial inception ( 116 kaBP) and the ice volume of the last glacial maximum ( 21 kaBP). We show that the model performs well in certain areas (e.g. Canadian archipelago) but some model biases are consistent over time periods (e.g. Kara-Barents sector). We explore various model sensitivities (e.g. initial state, vegetation, albedo) and we discuss the importance of the downscaling of precipitation for ice nucleation over elevated area and for the surface mass balance of larger ice sheets.

Using paleoclimate data to improve models of the Antarctic Ice Sheet

NASA Astrophysics Data System (ADS)

King, M. A.; Phipps, S. J.; Roberts, J. L.; White, D.

2016-12-01

Ice sheet models are the most descriptive tools available to simulate the future evolution of the Antarctic Ice Sheet (AIS), including its contribution towards changes in global sea level. However, our knowledge of the dynamics of the coupled ice-ocean-lithosphere system is inevitably limited, in part due to a lack of observations. Furthemore, to build computationally efficient models that can be run for multiple millennia, it is necessary to use simplified descriptions of ice dynamics. Ice sheet modeling is therefore an inherently uncertain exercise. The past evolution of the AIS provides an opportunity to constrain the description of physical processes within ice sheet models and, therefore, to constrain our understanding of the role of the AIS in driving changes in global sea level. We use the Parallel Ice Sheet Model (PISM) to demonstrate how paleoclimate data can improve our ability to predict the future evolution of the AIS. A large, perturbed-physics ensemble is generated, spanning uncertainty in the parameterizations of four key physical processes within ice sheet models: ice rheology, ice shelf calving, and the stress balances within ice sheets and ice shelves. A Latin hypercube approach is used to optimally sample the range of uncertainty in parameter values. This perturbed-physics ensemble is used to simulate the evolution of the AIS from the Last Glacial Maximum ( 21,000 years ago) to present. Paleoclimate records are then used to determine which ensemble members are the most realistic. This allows us to use data on past climates to directly constrain our understanding of the past contribution of the AIS towards changes in global sea level. Critically, it also allows us to determine which ensemble members are likely to generate the most realistic projections of the future evolution of the AIS.

Modeling the evolution of the Laurentide Ice Sheet from MIS 3 to the Last Glacial Maximum: an approach using sea level modeling and ice flow dynamics

NASA Astrophysics Data System (ADS)

Weisenberg, J.; Pico, T.; Birch, L.; Mitrovica, J. X.

2017-12-01

The history of the Laurentide Ice Sheet since the Last Glacial Maximum ( 26 ka; LGM) is constrained by geological evidence of ice margin retreat in addition to relative sea-level (RSL) records in both the near and far field. Nonetheless, few observations exist constraining the ice sheet's extent across the glacial build-up phase preceding the LGM. Recent work correcting RSL records along the U.S. mid-Atlantic dated to mid-MIS 3 (50-35 ka) for glacial-isostatic adjustment (GIA) infer that the Laurentide Ice Sheet grew by more than three-fold in the 15 ky leading into the LGM. Here we test the plausibility of a late and extremely rapid glaciation by driving a high-resolution ice sheet model, based on a nonlinear diffusion equation for the ice thickness. We initialize this model at 44 ka with the mid-MIS 3 ice sheet configuration proposed by Pico et al. (2017), GIA-corrected basal topography, and mass balance representative of mid-MIS 3 conditions. These simulations predict rapid growth of the eastern Laurentide Ice Sheet, with rates consistent with achieving LGM ice volumes within 15 ky. We use these simulations to refine the initial ice configuration and present an improved and higher resolution model for North American ice cover during mid-MIS 3. In addition we show that assumptions of ice loads during the glacial phase, and the associated reconstructions of GIA-corrected basal topography, produce a bias that can underpredict ice growth rates in the late stages of the glaciation, which has important consequences for our understanding of the speed limit for ice growth on glacial timescales.

Holocene temperature history at the west Greenland Ice Sheet margin reconstructed from lake sediments

NASA Astrophysics Data System (ADS)

Axford, Y.; Losee, S.; Briner, J. P.; Francis, D.; Langdon, P. G.; Walker, I.

2011-12-01

Paleoclimate proxy data can help reduce uncertainties regarding how the Greenland Ice Sheet, and thus global sea level, will respond to future climate change. Studies of terrestrial deposits along Greenland's margins offer opportunities to reconstruct both past temperature changes and the associated changes in Greenland Ice Sheet extent, thus empirically characterizing the ice sheet's response to temperature change. Here we present Holocene paleoclimate reconstructions developed from sediment records of five lakes along the western ice sheet margin, near Jakobshavn Isbræ and Disko Bugt. Insect (Chironomidae, or non-biting midge) remains from North Lake provide quantitative estimates of summer temperatures over the past ca. 7500 years at multi-centennial resolution, and changes in sediment composition at all five lakes offer evidence for glacier fluctuations, changes in lake productivity, and other environmental changes throughout the Holocene. Aims of this study include quantification of warmth in the early to mid Holocene, when summer solar insolation forcing exceeded present-day values at northern latitudes and the local Greenland Ice Sheet margin receded inboard of its present position, and the magnitude of subsequent Neoglacial and Little Ice Age cooling that drove ice sheet expansion. We find that the Jakobshavn Isbrae region experienced the warmest temperatures of the Holocene (with summers 2 to 3.5 degrees C warmer than present) between ~6000 and 4000 years ago. Neoglacial cooling began rather abruptly ~4000 years ago and intensified 3000 years ago. Our proxy data suggest that the coldest summers of the Holocene occurred during the 18th and 19th centuries in the Jakobshavn region. These results agree well with previous glacial geologic studies reconstructing local ice margin positions through the Holocene. Such reconstructions of paleoclimate and past ice sheet extent provide targets for testing and improving ice sheet models.

Large-scale evolution of the central-east Greenland margin: New insights to the North Atlantic glaciation history

NASA Astrophysics Data System (ADS)

Pérez, Lara F.; Nielsen, Tove; Knutz, Paul C.; Kuijpers, Antoon; Damm, Volkmar

2018-04-01

The continental shelf of central-east Greenland is shaped by several glacially carved transverse troughs that form the oceanward extension of the major fjord systems. The evolution of these troughs through time, and their relation with the large-scale glaciation of the Northern Hemisphere, is poorly understood. In this study seismostratigraphic analyses have been carried out to determine the morphological and structural development of this important sector of the East Greenland glaciated margin. The age of major stratigraphic discontinuities has been constrained by a direct tie to ODP site 987 drilled in the Greenland Sea basin plain off Scoresby Sund fan system. The areal distribution and internal facies of the identified seismic units reveal the large-scale depositional pattern formed by ice-streams draining a major part of the central-east Greenland ice sheet. Initial sedimentation along the margin was, however, mainly controlled by tectonic processes related to the margin construction, continental uplift, and fluvial processes. From late Miocene to present, progradational and erosional patterns point to repeated glacial advances across the shelf. The evolution of depo-centres suggests that ice sheet advances over the continental shelf have occurred since late Miocene, about 2 Myr earlier than previously assumed. This cross-shelf glaciation is more pronounced during late Miocene and early Pliocene along Blosseville Kyst and around the Pliocene/Pleistocene boundary off Scoresby Sund; indicating a northward migration of the glacial advance. The two main periods of glaciation were separated by a major retreat of the ice sheet to an inland position during middle Pliocene. Mounded-wavy deposits interpreted as current-related deposits suggest the presence of changing along-slope current dynamics in concert with the development of the modern North Atlantic oceanographic pattern.

Role of ice-ocean interaction on glacier instability: Results from numerical modeling applied to Petermann Glacier (Invited)

NASA Astrophysics Data System (ADS)

Nick, F.; Hubbard, A.; Vieli, A.; van der Veen, C. J.; Box, J. E.; Bates, R.; Luckman, A. J.

2009-12-01

Calving of icebergs and bottom melting from ice shelves accounts for roughly half the ice transferred from the Greenland Ice Sheet into the surrounding ocean, and virtually all of the ice loss from the Antarctic Ice Sheet. Petermann Glacier (north Greenland) with its 16 km wide and 80 km long floating tongue, experiences massive bottom melting. We apply a numerical ice flow model using a physically-based calving criterion based on crevasse depth to investigate the contribution of processes such as bottom melting, sea ice or sikkusak disintegration, surface run off and iceberg calving to the mass balance and instability of Petermann Glacier and its ice shelf. Our modeling study provides insights into the role of ice-ocean interaction, and on how to incorporate calving in ice sheet models, improving our ability to predict future ice sheet change.

Role of ice-ocean interaction on glacier instability: Results from numerical modelling applied to Petermann Glacier

NASA Astrophysics Data System (ADS)

Nick, Faezeh M.; Hubbard, Alun; van der Veen, Kees; Vieli, Andreas

2010-05-01

Calving of icebergs and bottom melting from ice shelves accounts for roughly half the ice transferred from the Greenland Ice Sheet into the surrounding ocean, and virtually all of the ice loss from the Antarctic Ice Sheet. Petermann Glacier (north Greenland) with its 16 km wide and 80 km long floating tongue, experiences massive bottom melting. We apply a numerical ice flow model using a physically-based calving criterion based on crevasse depth to investigate the contribution of processes such as bottom melting, sea ice or sikkusak disintegration, surface run off and iceberg calving to the mass balance and instability of Petermann Glacier and its ice shelf. Our modelling study provides insights into the role of ice-ocean interaction, and on how to incorporate calving in ice sheet models, improving our ability to predict future ice sheet change.

The Gamburtsev mountains and the origin and early evolution of the Antarctic Ice Sheet.

PubMed

Bo, Sun; Siegert, Martin J; Mudd, Simon M; Sugden, David; Fujita, Shuji; Xiangbin, Cui; Yunyun, Jiang; Xueyuan, Tang; Yuansheng, Li

2009-06-04

Ice-sheet development in Antarctica was a result of significant and rapid global climate change about 34 million years ago. Ice-sheet and climate modelling suggest reductions in atmospheric carbon dioxide (less than three times the pre-industrial level of 280 parts per million by volume) that, in conjunction with the development of the Antarctic Circumpolar Current, led to cooling and glaciation paced by changes in Earth's orbit. Based on the present subglacial topography, numerical models point to ice-sheet genesis on mountain massifs of Antarctica, including the Gamburtsev mountains at Dome A, the centre of the present ice sheet. Our lack of knowledge of the present-day topography of the Gamburtsev mountains means, however, that the nature of early glaciation and subsequent development of a continental-sized ice sheet are uncertain. Here we present radar information about the base of the ice at Dome A, revealing classic Alpine topography with pre-existing river valleys overdeepened by valley glaciers formed when the mean summer surface temperature was around 3 degrees C. This landscape is likely to have developed during the initial phases of Antarctic glaciation. According to Antarctic climate history (estimated from offshore sediment records) the Gamburtsev mountains are probably older than 34 million years and were the main centre for ice-sheet growth. Moreover, the landscape has most probably been preserved beneath the present ice sheet for around 14 million years.

Collapse of polar ice sheets during the stage 11 interglacial.

PubMed

Raymo, Maureen E; Mitrovica, Jerry X

2012-03-14

Contentious observations of Pleistocene shoreline features on the tectonically stable islands of Bermuda and the Bahamas have suggested that sea level about 400,000 years ago was more than 20 metres higher than it is today. Geochronologic and geomorphic evidence indicates that these features formed during interglacial marine isotope stage (MIS) 11, an unusually long interval of warmth during the ice age. Previous work has advanced two divergent hypotheses for these shoreline features: first, significant melting of the East Antarctic Ice Sheet, in addition to the collapse of the West Antarctic Ice Sheet and the Greenland Ice Sheet; or second, emplacement by a mega-tsunami during MIS 11 (ref. 4, 5). Here we show that the elevations of these features are corrected downwards by ∼10 metres when we account for post-glacial crustal subsidence of these sites over the course of the anomalously long interglacial. On the basis of this correction, we estimate that eustatic sea level rose to ∼6-13 m above the present-day value in the second half of MIS 11. This suggests that both the Greenland Ice Sheet and the West Antarctic Ice Sheet collapsed during the protracted warm period while changes in the volume of the East Antarctic Ice Sheet were relatively minor, thereby resolving the long-standing controversy over the stability of the East Antarctic Ice Sheet during MIS 11.

Airborne Laser Altimetry Mapping of the Greenland Ice Sheet: Application to Mass Balance Assessment

NASA Technical Reports Server (NTRS)

Abdalati, W.; Krabill, W.; Frederick, E.; Manizade, S.; Martin, C.; Sonntag, J.; Swift, R.; Thomas, R.; Wright, W.; Yungel, J.

2000-01-01

In 1998 and '99, the Arctic Ice Mapping (AIM) program completed resurveys of lines occupied 5 years earlier revealing elevation changes of the Greenland ice sheet and identifying areas of significant thinning, thickening and balance. In planning these surveys, consideration had to be given to the spatial constraints associated with aircraft operation, the spatial nature of ice sheet behavior, and limited resources, as well as temporal issues, such as seasonal and interannual variability in the context of measurement accuracy. This paper examines the extent to which the sampling and survey strategy is valid for drawing conclusions on the current state of balance of the Greenland ice sheet. The surveys covered the entire ice sheet with an average distance of 21.4 km between each location on the ice sheet and the nearest flight line. For most of the ice sheet, the elevation changes show relatively little spatial variability, and their magnitudes are significantly smaller than the observed elevation change signal. As a result, we conclude that the density of the sampling and the accuracy of the measurements are sufficient to draw meaningful conclusions on the state of balance of the entire ice sheet over the five-year survey period. Outlet glaciers, however, show far more spatial and temporal variability, and each of the major ones is likely to require individual surveys in order to determine its balance.

Hydrogeology of the Waverly-Sayre area in Tioga and Chemung ounties, New York and Bradford County, Pennsylvania

USGS Publications Warehouse

Reynolds, Richard J.

2002-01-01

The hydrogeology of a 135-square-mile area centered at Waverly, N.Y. and Sayre, Pa. is summarized in a set of five maps and a sheet of geologic sections, all at 1:24,000 scale, that depict locations of wells and test holes (sheet 1), surficial geology (sheet 2), altitude of the water table (sheet 3), saturated thickness of the surficial aquifer (sheet 4), thickness of the lacustrine confining unit (sheet 5), and geologic sections (sheet 6). The valley-fill deposits that form the aquifer system in the Waverly-Sayre area occupy an area of approximately 30 square miles, within the valleys of the Susquehanna River, Chemung River, and Cayuta Creek.The saturated thickness of the surficial aquifer, which consists of alluvium, valley-train outwash, and underlying ice-contact deposits, ranges from zero to 90 feet and is greatest in areas where (1) the outwash is underlain by ice-contact sand and gravel or (2) the outwash is overlain by alluvium and alluvial fans. Estimated transmissivity of the surficial aquifer ranges from 5,600 to 100,270 feet squared per day, and estimated hydraulic conductivity ranges from 50 feet per day for ice-contact deposits to 1,300 feet per day for well-sorted, valley-train outwash.The surficial aquifer is underlain by deposits of lacustrine sand, silt, and clay in the main valleys; these deposits reach thicknesses of as much as 150 ft and form a thick confining unit. Beneath the lacustrine silt and clay confining unit is a thin, discontinuous sand and gravel aquifer whose thickness averages 5 feet but may be as much as 30 feet locally. This confined aquifer supplies many domestic well in the area; yields average about 22 gallons per minute for 6-inch-diameter, open-ended wells. Average annual recharge to the aquifer system is estimated to be approximately 52.5 Mgal/d (million gallons per day), of which 29.7 Mgal/d is from direct precipitation, 7.6 Mgal/d is from unchanneled upland runoff that infiltrates the stratified drift along the valley wall, and 15.2 Mgal/d is from infiltration from tributary streams on the valley floor.

LIVVkit: An extensible, python-based, land ice verification and validation toolkit for ice sheet models

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kennedy, Joseph H.; Bennett, Andrew R.; Evans, Katherine J.

To address the pressing need to better understand the behavior and complex interaction of ice sheets within the global Earth system, significant development of continental-scale, dynamical ice sheet models is underway. Concurrent to the development of the Community Ice Sheet Model (CISM), the corresponding verification and validation (V&V) process is being coordinated through a new, robust, Python-based extensible software package, the Land Ice Verification and Validation toolkit (LIVVkit). Incorporated into the typical ice sheet model development cycle, it provides robust and automated numerical verification, software verification, performance validation, and physical validation analyses on a variety of platforms, from personal laptopsmore » to the largest supercomputers. LIVVkit operates on sets of regression test and reference data sets, and provides comparisons for a suite of community prioritized tests, including configuration and parameter variations, bit-for-bit evaluation, and plots of model variables to indicate where differences occur. LIVVkit also provides an easily extensible framework to incorporate and analyze results of new intercomparison projects, new observation data, and new computing platforms. LIVVkit is designed for quick adaptation to additional ice sheet models via abstraction of model specific code, functions, and configurations into an ice sheet model description bundle outside the main LIVVkit structure. Furthermore, through shareable and accessible analysis output, LIVVkit is intended to help developers build confidence in their models and enhance the credibility of ice sheet models overall.« less

LIVVkit: An extensible, python-based, land ice verification and validation toolkit for ice sheet models

DOE PAGES

Kennedy, Joseph H.; Bennett, Andrew R.; Evans, Katherine J.; ...

2017-03-23

To address the pressing need to better understand the behavior and complex interaction of ice sheets within the global Earth system, significant development of continental-scale, dynamical ice sheet models is underway. Concurrent to the development of the Community Ice Sheet Model (CISM), the corresponding verification and validation (V&V) process is being coordinated through a new, robust, Python-based extensible software package, the Land Ice Verification and Validation toolkit (LIVVkit). Incorporated into the typical ice sheet model development cycle, it provides robust and automated numerical verification, software verification, performance validation, and physical validation analyses on a variety of platforms, from personal laptopsmore » to the largest supercomputers. LIVVkit operates on sets of regression test and reference data sets, and provides comparisons for a suite of community prioritized tests, including configuration and parameter variations, bit-for-bit evaluation, and plots of model variables to indicate where differences occur. LIVVkit also provides an easily extensible framework to incorporate and analyze results of new intercomparison projects, new observation data, and new computing platforms. LIVVkit is designed for quick adaptation to additional ice sheet models via abstraction of model specific code, functions, and configurations into an ice sheet model description bundle outside the main LIVVkit structure. Furthermore, through shareable and accessible analysis output, LIVVkit is intended to help developers build confidence in their models and enhance the credibility of ice sheet models overall.« less

Dynamic influence of pinning points on marine ice-sheet stability: a numerical study in Dronning Maud Land, East Antarctica

DOE PAGES

Favier, Lionel; Pattyn, Frank; Berger, Sophie; ...

2016-11-09

The East Antarctic ice sheet is likely more stable than its West Antarctic counterpart because its bed is largely lying above sea level. However, the ice sheet in Dronning Maud Land, East Antarctica, contains marine sectors that are in contact with the ocean through overdeepened marine basins interspersed by grounded ice promontories and ice rises, pinning and stabilising the ice shelves. In this paper, we use the ice-sheet model BISICLES to investigate the effect of sub-ice-shelf melting, using a series of scenarios compliant with current values, on the ice-dynamic stability of the outlet glaciers between the Lazarev and Roi Baudouinmore » ice shelves over the next millennium. Overall, the sub-ice-shelf melting substantially impacts the sea-level contribution. Locally, we predict a short-term rapid grounding-line retreat of the overdeepened outlet glacier Hansenbreen, which further induces the transition of the bordering ice promontories into ice rises. Furthermore, our analysis demonstrated that the onset of the marine ice-sheet retreat and subsequent promontory transition into ice rise is controlled by small pinning points, mostly uncharted in pan-Antarctic datasets. Pinning points have a twofold impact on marine ice sheets. They decrease the ice discharge by buttressing effect, and they play a crucial role in initialising marine ice sheets through data assimilation, leading to errors in ice-shelf rheology when omitted. Our results show that unpinning increases the sea-level rise by 10%, while omitting the same pinning point in data assimilation decreases it by 10%, but the more striking effect is in the promontory transition time, advanced by two centuries for unpinning and delayed by almost half a millennium when the pinning point is missing in data assimilation. As a result, pinning points exert a subtle influence on ice dynamics at the kilometre scale, which calls for a better knowledge of the Antarctic margins.« less

Dynamic influence of pinning points on marine ice-sheet stability: a numerical study in Dronning Maud Land, East Antarctica

DOE Office of Scientific and Technical Information (OSTI.GOV)

Favier, Lionel; Pattyn, Frank; Berger, Sophie

The East Antarctic ice sheet is likely more stable than its West Antarctic counterpart because its bed is largely lying above sea level. However, the ice sheet in Dronning Maud Land, East Antarctica, contains marine sectors that are in contact with the ocean through overdeepened marine basins interspersed by grounded ice promontories and ice rises, pinning and stabilising the ice shelves. In this paper, we use the ice-sheet model BISICLES to investigate the effect of sub-ice-shelf melting, using a series of scenarios compliant with current values, on the ice-dynamic stability of the outlet glaciers between the Lazarev and Roi Baudouinmore » ice shelves over the next millennium. Overall, the sub-ice-shelf melting substantially impacts the sea-level contribution. Locally, we predict a short-term rapid grounding-line retreat of the overdeepened outlet glacier Hansenbreen, which further induces the transition of the bordering ice promontories into ice rises. Furthermore, our analysis demonstrated that the onset of the marine ice-sheet retreat and subsequent promontory transition into ice rise is controlled by small pinning points, mostly uncharted in pan-Antarctic datasets. Pinning points have a twofold impact on marine ice sheets. They decrease the ice discharge by buttressing effect, and they play a crucial role in initialising marine ice sheets through data assimilation, leading to errors in ice-shelf rheology when omitted. Our results show that unpinning increases the sea-level rise by 10%, while omitting the same pinning point in data assimilation decreases it by 10%, but the more striking effect is in the promontory transition time, advanced by two centuries for unpinning and delayed by almost half a millennium when the pinning point is missing in data assimilation. As a result, pinning points exert a subtle influence on ice dynamics at the kilometre scale, which calls for a better knowledge of the Antarctic margins.« less

Response of the Antarctic ice sheet to ocean forcing using the POPSICLES coupled ice sheet-ocean model

NASA Astrophysics Data System (ADS)

Martin, D. F.; Asay-Davis, X.; Price, S. F.; Cornford, S. L.; Maltrud, M. E.; Ng, E. G.; Collins, W.

2014-12-01

We present the response of the continental Antarctic ice sheet to sub-shelf-melt forcing derived from POPSICLES simulation results covering the full Antarctic Ice Sheet and the Southern Ocean spanning the period 1990 to 2010. Simulations are performed at 0.1 degree (~5 km) ocean resolution and ice sheet resolution as fine as 500 m using adaptive mesh refinement. A comparison of fully-coupled and comparable standalone ice-sheet model results demonstrates the importance of two-way coupling between the ice sheet and the ocean. The POPSICLES model couples the POP2x ocean model, a modified version of the Parallel Ocean Program (Smith and Gent, 2002), and the BISICLES ice-sheet model (Cornford et al., 2012). BISICLES makes use of adaptive mesh refinement to fully resolve dynamically-important regions like grounding lines and employs a momentum balance similar to the vertically-integrated formulation of Schoof and Hindmarsh (2009). Results of BISICLES simulations have compared favorably to comparable simulations with a Stokes momentum balance in both idealized tests like MISMIP3D (Pattyn et al., 2013) and realistic configurations (Favier et al. 2014). POP2x includes sub-ice-shelf circulation using partial top cells (Losch, 2008) and boundary layer physics following Holland and Jenkins (1999), Jenkins (2001), and Jenkins et al. (2010). Standalone POP2x output compares well with standard ice-ocean test cases (e.g., ISOMIP; Losch, 2008) and other continental-scale simulations and melt-rate observations (Kimura et al., 2013; Rignot et al., 2013). A companion presentation, "Present-day circum-Antarctic simulations using the POPSICLES coupled land ice-ocean model" in session C027 describes the ocean-model perspective of this work, while we focus on the response of the ice sheet and on details of the model. The figure shows the BISICLES-computed vertically-integrated ice velocity field about 1 month into a 20-year coupled Antarctic run. Groundling lines are shown in green.

ISSM: Ice Sheet System Model

NASA Technical Reports Server (NTRS)

Larour, Eric; Schiermeier, John E.; Seroussi, Helene; Morlinghem, Mathieu

2013-01-01

In order to have the capability to use satellite data from its own missions to inform future sea-level rise projections, JPL needed a full-fledged ice-sheet/iceshelf flow model, capable of modeling the mass balance of Antarctica and Greenland into the near future. ISSM was developed with such a goal in mind, as a massively parallelized, multi-purpose finite-element framework dedicated to ice-sheet modeling. ISSM features unstructured meshes (Tria in 2D, and Penta in 3D) along with corresponding finite elements for both types of meshes. Each finite element can carry out diagnostic, prognostic, transient, thermal 3D, surface, and bed slope simulations. Anisotropic meshing enables adaptation of meshes to a certain metric, and the 2D Shelfy-Stream, 3D Blatter/Pattyn, and 3D Full-Stokes formulations capture the bulk of the ice-flow physics. These elements can be coupled together, based on the Arlequin method, so that on a large scale model such as Antarctica, each type of finite element is used in the most efficient manner. For each finite element referenced above, ISSM implements an adjoint. This adjoint can be used to carry out model inversions of unknown model parameters, typically ice rheology and basal drag at the ice/bedrock interface, using a metric such as the observed InSAR surface velocity. This data assimilation capability is crucial to allow spinning up of ice flow models using available satellite data. ISSM relies on the PETSc library for its vectors, matrices, and solvers. This allows ISSM to run efficiently on any parallel platform, whether shared or distrib- ISSM: Ice Sheet System Model NASA's Jet Propulsion Laboratory, Pasadena, California uted. It can run on the largest clusters, and is fully scalable. This allows ISSM to tackle models the size of continents. ISSM is embedded into MATLAB and Python, both open scientific platforms. This improves its outreach within the science community. It is entirely written in C/C++, which gives it flexibility in its design, and the power/speed that C/C++ allows. ISSM is svn (subversion) hosted, on a JPL repository, to facilitate its development and maintenance. ISSM can also model propagation of rifts using contact mechanics and mesh splitting, and can interface to the Dakota software. To carry out sensitivity analysis, mesh partitioning algorithms are available, based on the Scotch, Chaco, and Metis partitioners that ensure equal area mesh partitions can be done, which are then usable for sampling and local reliability methods.

Poro-elastic Properties of Whillan's Ice Stream Till: Implications for Basal Stick-Slip

NASA Astrophysics Data System (ADS)

Leeman, J.; Valdez, R. D.; Alley, R. B.; Anandakrishnan, S.; Saffer, D. M.

2016-12-01

Whillans ice stream, West Antarctica, flows rapidly from the West Antarctic ice sheet into the Ross Ice Shelf. Regions of highly compacted till, termed sticky-spots, pin the ice in place. Upstream ice flow increases driving stress, until minor changes in buttressing stresses from tides affecting the ice shelf cause the main sticky-spot to fail, triggering diurnal or semidiurnal stick-slip events. The mechanical and hydrological properties of the till partially control the basal conditions, generation and persistence of the sticky spots, and thus the dynamics of the rupture and healing processes. Here we present laboratory tests on core samples of the till beneath Whillan's Ice Stream collected in the 1989-1993 field seasons. Two types of tests were performed on till cores: stepped loading and cyclic loading. In the stepped loading test, the effective stress was increased from 0.1 to 10 MPa in a series of steps, and the permeability measured at each step. Cyclic loading tests consisted of a series of effective stress oscillations with 24 h period lasting 5-10 d each, increasing in amplitude from 20-150 kPa. The permeability was measured after each set of oscillations to investigate the role of cyclic loading in driving enhanced compaction. Compressional wave velocity (Vp) was also measured during both test sequences. We observe sample initial porosities of 30% and permeabilities of 3x10-17 m2. During stepped loading tests, porosity is reduced to 20% and permeability to 8x10-18 m2. Vp ranged from 2.2-2.95 km s-1 and was well fit by an effective medium model. Application of this model to Vp obtained by field seismic surveys is consistent with low ( 50 kPa) effective vertical stresses in the uppermost till. Cyclic loading sequences reduced porosity by 4% and permeability by an order of magnitude. A transient numerical model based on our data shows that over the tidal timescale, a layer of stiffened till 10 cm thick should develop. We suggest that this provides one mechanism to generate and maintain sticky spots and modify the stiffness of the system.

Study of elevation changes along a profile crossing the Greenland Ice Sheet

NASA Astrophysics Data System (ADS)

Hvidegaard, S. M.; Sandberg, L.

2009-04-01

In recent years much research has focused on determining how the Greenland Ice Sheet is responding to the observed climate changes. There is wide agreement on the fact that the Ice Sheet is currently loosing mass, and studies have shown that the mass loss is found near the ice edge and that no significant changes are found in the central part of the Ice Sheet. As a part of European Space Agency's CryoSat Validation Experiment (CryoVEx) running from 2004 to 2008, the National Space Institute (DTU Space) measured the elevations along a profile crossing the Greenland Ice Sheet. The elevation observations were carried out in 2004, 2006 and 2008 using airborne laser altimetry from a Twin Otter aircraft. The observed profile follows the old EGIG line (Expédition Glaciologique au Groenland, measured in the 1950's) situated between 69-71N, heading nearly east-west. This unique dataset gives the opportunity to study elevation changes along the profile crossing the ice sheet. With this work, we outline the observed elevation changes from the different zones of the ice sheet. We furthermore compare elevation changes based on coincident ICESat and airborne laser altimeter data.

A 25-year Record of Antarctic Ice Sheet Elevation and Mass Change

NASA Astrophysics Data System (ADS)

Shepherd, A.; Muir, A. S.; Sundal, A.; McMillan, M.; Briggs, K.; Hogg, A.; Engdahl, M.; Gilbert, L.

2017-12-01

Since 1992, the European Remote-Sensing (ERS-1 and ERS-2), ENVISAT, and CryoSat-2 satellite radar altimeters have measured the Antarctic ice sheet surface elevation, repeatedly, at approximately monthly intervals. These data constitute the longest continuous record of ice sheet wide change. In this paper, we use these observations to determine changes in the elevation, volume and mass of the East Antarctic and West Antarctic ice sheets, and of parts of the Antarctic Peninsula ice sheet, over a 25-year period. The root mean square difference between elevation rates computed from our survey and 257,296 estimates determined from airborne laser measurements is 54 cm/yr. The longevity of the satellite altimeter data record allows to identify and chart the evolution of changes associated with meteorology and ice flow, and we estimate that 3.6 % of the continental ice sheet, and 21.7 % of West Antarctica, is in a state of dynamical imbalance. Based on this partitioning, we estimate the mass balance of the East and West Antarctic ice sheet drainage basins and the root mean square difference between these and independent estimates derived from satellite gravimetry is less than 5 Gt yr-1.

Implementation of higher-order vertical finite elements in ISSM v4.13 for improved ice sheet flow modeling over paleoclimate timescales

NASA Astrophysics Data System (ADS)

Cuzzone, Joshua K.; Morlighem, Mathieu; Larour, Eric; Schlegel, Nicole; Seroussi, Helene

2018-05-01

Paleoclimate proxies are being used in conjunction with ice sheet modeling experiments to determine how the Greenland ice sheet responded to past changes, particularly during the last deglaciation. Although these comparisons have been a critical component in our understanding of the Greenland ice sheet sensitivity to past warming, they often rely on modeling experiments that favor minimizing computational expense over increased model physics. Over Paleoclimate timescales, simulating the thermal structure of the ice sheet has large implications on the modeled ice viscosity, which can feedback onto the basal sliding and ice flow. To accurately capture the thermal field, models often require a high number of vertical layers. This is not the case for the stress balance computation, however, where a high vertical resolution is not necessary. Consequently, since stress balance and thermal equations are generally performed on the same mesh, more time is spent on the stress balance computation than is otherwise necessary. For these reasons, running a higher-order ice sheet model (e.g., Blatter-Pattyn) over timescales equivalent to the paleoclimate record has not been possible without incurring a large computational expense. To mitigate this issue, we propose a method that can be implemented within ice sheet models, whereby the vertical interpolation along the z axis relies on higher-order polynomials, rather than the traditional linear interpolation. This method is tested within the Ice Sheet System Model (ISSM) using quadratic and cubic finite elements for the vertical interpolation on an idealized case and a realistic Greenland configuration. A transient experiment for the ice thickness evolution of a single-dome ice sheet demonstrates improved accuracy using the higher-order vertical interpolation compared to models using the linear vertical interpolation, despite having fewer degrees of freedom. This method is also shown to improve a model's ability to capture sharp thermal gradients in an ice sheet particularly close to the bed, when compared to models using a linear vertical interpolation. This is corroborated in a thermal steady-state simulation of the Greenland ice sheet using a higher-order model. In general, we find that using a higher-order vertical interpolation decreases the need for a high number of vertical layers, while dramatically reducing model runtime for transient simulations. Results indicate that when using a higher-order vertical interpolation, runtimes for a transient ice sheet relaxation are upwards of 5 to 7 times faster than using a model which has a linear vertical interpolation, and this thus requires a higher number of vertical layers to achieve a similar result in simulated ice volume, basal temperature, and ice divide thickness. The findings suggest that this method will allow higher-order models to be used in studies investigating ice sheet behavior over paleoclimate timescales at a fraction of the computational cost than would otherwise be needed for a model using a linear vertical interpolation.

Evolution of Meltwater on the McMurdo Ice Shelf, Antarctica During Two Summer Melt Seasons

NASA Astrophysics Data System (ADS)

Macdonald, G. J.; Banwell, A. F.; Willis, I.; Mayer, D. P.; Hansen, E. K.; MacAyeal, D. R.

2017-12-01

Ice shelves surround > 50% of Antarctica's coast and their response to climate change is key to the ice sheet's future and global sea-level rise. Observations of the development and drainage of 2750 lakes prior to the collapse of the Larsen B Ice Shelf, combined with our understanding of ice-shelf flexure/fracture, suggest that surface meltwater plays a key role in ice-shelf stability, although the present state of knowledge remains limited. Here, we report results of an investigation into the seasonal evolution of meltwater on the McMurdo Ice Shelf (MIS) during the 2015/16 and 2016/17 austral summers using satellite remote sensing, complemented by ground survey. Although the MIS is relatively far south (78° S), it experiences relatively high ablation rates in the west due to adiabatically warmed winds, making it a useful example of how meltwater could evolve on more southerly ice shelves in a warming climate. We calculate the areas and depths of ponded surface meltwater on the ice shelf at different stages of the two melt seasons using a modified NDWI approach and water-depth algorithm applied to both Landsat 8 and Worldview imagery. Data from two automatic weather stations on the ice shelf are used to drive a positive degree-day model to compare our observations of surface water volumes with modelled meltwater production. Results suggest that the spatial and temporal variations in surface meltwater coverage on the ice shelf vary not only with climatic conditions but also in response to other important processes. First, a rift that widens and propagates between the two melt seasons intercepts meltwater streams, redirecting flow and facilitating ponding elsewhere. Second, some lakes from previous years remain frozen over and become pedestalled, causing streams to divert around their perimeter. Third, surface debris conditions also cause large-scale spatial variation in melt rates and the flow and storage of water.

Sea-level response to ice sheet evolution: An ocean perspective

NASA Technical Reports Server (NTRS)

Jacobs, Stanley S.

1991-01-01

The ocean's influence upon and response to Antarctic ice sheet changes is considered in relation to sea level rise over recent and future decades. Assuming present day ice fronts are in approximate equilibrium, a preliminary budget for the ice sheet is estimated from accumulation vs. iceberg calving and the basal melting that occurs beneath floating ice shelves. Iceberg calving is derived from the volume of large bergs identified and tracked by the Navy/NOAA Joint Ice Center and from shipboard observations. Basal melting exceeds 600 cu km/yr and is concentrated near the ice fronts and ice shelf grounding lines. An apparent negative mass balance for the Antarctic ice sheet may result from an anomalous calving rate during the past decade, but there are large uncertainties associated with all components of the ice budget. The results from general circulation models are noted in the context of projected precipitation increases and ocean temperature changes on and near the continent. An ocean research program that could help refine budget estimates is consistent with goals of the West Antarctic Ice Sheet Initiative.

Ice cores and SeaRISE: What we do (and don't) know

NASA Technical Reports Server (NTRS)

Alley, Richard B.

1991-01-01

Ice core analyses are needed in SeaRISE to learn what the West Antarctic ice sheet and other marine ice sheets were like in the past, what climate changes led to their present states, and how they behave. The major results of interest to SeaRISE from previous ice core analyses in West Antarctic are that the end of the last ice age caused temperature and accumulation rate increases in inland regions, leading to ice sheet thickening followed by thinning to the present.

Sensitivity of Pliocene ice sheets to orbital forcing

USGS Publications Warehouse

Dolan, A.M.; Haywood, A.M.; Hill, D.J.; Dowsett, H.J.; Hunter, S.J.; Lunt, D.J.; Pickering, S.J.

2011-01-01

The stability of the Earth's major ice sheets is a critical uncertainty in predictions of future climate and sea level change. One method of investigating the behaviour of the Greenland and the Antarctic ice sheets in a warmer-than-modern climate is to look back at past warm periods of Earth history, for example the Pliocene. This paper presents climate and ice sheet modelling results for the mid-Pliocene warm period (mPWP; 3.3 to 3.0 million years ago), which has been identified as a key interval for understanding warmer-than-modern climates (Jansen et al., 2007). Using boundary conditions supplied by the United States Geological Survey PRISM Group (Pliocene Research, Interpretation and Synoptic Mapping), the Hadley Centre coupled ocean–atmosphere climate model (HadCM3) and the British Antarctic Survey Ice Sheet Model (BASISM), we show large reductions in the Greenland and East Antarctic Ice Sheets (GrIS and EAIS) compared to modern in standard mPWP experiments. We also present the first results illustrating the variability of the ice sheets due to realistic orbital forcing during the mid-Pliocene. While GrIS volumes are lower than modern under even the most extreme (cold) mid-Pliocene orbit (losing at least 35% of its ice mass), the EAIS can both grow and shrink, losing up to 20% or gaining up to 10% of its present-day volume. The changes in ice sheet volume incurred by altering orbital forcing alone means that global sea level can vary by more than 25 m during the mid-Pliocene. However, we have also shown that the response of the ice sheets to mPWP orbital hemispheric forcing can be in anti-phase, whereby the greatest reductions in EAIS volume are concurrent with the smallest reductions of the GrIS. If this anti-phase relationship is in operation throughout the mPWP, then the total eustatic sea level response would be dampened compared to the ice sheet fluctuations that are theoretically possible. This suggests that maximum eustatic sea level rise does not correspond to orbital maxima, but occurs at times where the anti-phasing of Northern and Southern Hemisphere ice sheet retreat is minimised.

Sea-level and solid-Earth deformation feedbacks in ice sheet modelling

NASA Astrophysics Data System (ADS)

Konrad, Hannes; Sasgen, Ingo; Klemann, Volker; Thoma, Malte; Grosfeld, Klaus; Martinec, Zdeněk

2014-05-01

The interactions of ice sheets with the sea level and the solid Earth are important factors for the stability of the ice shelves and the tributary inland ice (e.g. Thomas and Bentley, 1978; Gomez et al, 2012). First, changes in ice extent and ice thickness induce viscoelastic deformation of the Earth surface and Earth's gravity field. In turn, global and local changes in sea level and bathymetry affect the grounding line and, subsequently, alter the ice dynamic behaviour. Here, we investigate these feedbacks for a synthetic ice sheet configuration as well as for the Antarctic ice sheet using a three-dimensional thermomechanical ice sheet and shelf model, coupled to a viscoelastic solid-Earth and gravitationally self-consistent sea-level model. The respective ice sheet undergoes a forcing from rising sea level, warming ocean, and/or changing surface mass balance. The coupling is realized by exchanging ice thickness, Earth surface deformation and sea level periodically. We apply several sets of viscoelastic Earth parameters to our coupled model, e.g. simulating a low-viscous upper mantle present at the Antarctic Peninsula (Ivins et al., 2011). Special focus of our study lies on the evolution of Earth surface deformation and local sea level changes, as well as on the accompanying grounding line evolution. N. Gomez, D. Pollard, J. X. Mitrovica, P. Huybers, and P. U. Clark 2012. Evolution of a coupled marine ice sheet-sea level model, J. Geophys. Res., 117, F01013, doi:10.1029/2011JF002128. E. R. Ivins, M. M. Watkins, D.-N. Yuan, R. Dietrich, G. Casassa, and A. Rülke 2011. On-land ice loss and glacial isostatic adjustment at the Drake Passage: 2003-2009, J. Geophys. Res. 116, B02403, doi: 10.1029/2010JB007607 R. H. Thomas and C. R. Bentley 1978. A model for Holocene retreat of the West Antarctic Ice Sheet, Quaternary Research, 10 (2), pages 150-170, doi: 10.1016/0033-5894(78)90098-4.

Future Antarctic bed topography and its implications for ice sheet dynamics

NASA Astrophysics Data System (ADS)

Adhikari, S.; Ivins, E. R.; Larour, E.; Seroussi, H.; Morlighem, M.; Nowicki, S.

2014-06-01

The Antarctic bedrock is evolving as the solid Earth responds to the past and ongoing evolution of the ice sheet. A recently improved ice loading history suggests that the Antarctic Ice Sheet (AIS) has generally been losing its mass since the Last Glacial Maximum. In a sustained warming climate, the AIS is predicted to retreat at a greater pace, primarily via melting beneath the ice shelves. We employ the glacial isostatic adjustment (GIA) capability of the Ice Sheet System Model (ISSM) to combine these past and future ice loadings and provide the new solid Earth computations for the AIS. We find that past loading is relatively less important than future loading for the evolution of the future bed topography. Our computations predict that the West Antarctic Ice Sheet (WAIS) may uplift by a few meters and a few tens of meters at years AD 2100 and 2500, respectively, and that the East Antarctic Ice Sheet is likely to remain unchanged or subside minimally except around the Amery Ice Shelf. The Amundsen Sea Sector in particular is predicted to rise at the greatest rate; one hundred years of ice evolution in this region, for example, predicts that the coastline of Pine Island Bay will approach roughly 45 mm yr-1 in viscoelastic vertical motion. Of particular importance, we systematically demonstrate that the effect of a pervasive and large GIA uplift in the WAIS is generally associated with the flattening of reverse bed slope, reduction of local sea depth, and thus the extension of grounding line (GL) towards the continental shelf. Using the 3-D higher-order ice flow capability of ISSM, such a migration of GL is shown to inhibit the ice flow. This negative feedback between the ice sheet and the solid Earth may promote stability in marine portions of the ice sheet in the future.

Future Antarctic bed topography and its implications for ice sheet dynamics

NASA Astrophysics Data System (ADS)

Adhikari, S.; Ivins, E.; Larour, E.; Seroussi, H.; Morlighem, M.; Nowicki, S.

2014-01-01

The Antarctic bedrock is evolving as the solid Earth responds to the past and ongoing evolution of the ice sheet. A~recently improved ice loading history suggests that the Antarctic Ice Sheet (AIS) is generally losing its mass since the last glacial maximum (LGM). In a sustained warming climate, the AIS is predicted to retreat at a greater pace primarily via melting beneath the ice shelves. We employ the glacial isostatic adjustment (GIA) capability of the Ice Sheet System Model (ISSM) to combine these past and future ice loadings and provide the new solid Earth computations for the AIS. We find that the past loading is relatively less important than future loading on the evolution of the future bed topography. Our computations predict that the West Antarctic Ice Sheet (WAIS) may uplift by a few meters and a few tens of meters at years 2100 and 2500 AD, respectively, and that the East Antarctic Ice Sheet (EAIS) is likely to remain unchanged or subside minimally except around the Amery Ice Shelf. The Amundsen Sea Sector in particular is predicted to rise at the greatest rate; one hundred years of ice evolution in this region, for example, predicts that the coastline of Pine Island Bay approaches roughly 45 mm yr-1 in viscoelastic vertical motion. Of particular importance, we systematically demonstrate that the effect of a pervasive and large GIA uplift in the WAIS is associated with the flattening of reverse bed, reduction of local sea depth, and thus the extension of grounding line (GL) towards the continental shelf. Using the 3-D higher-order ice flow capability of ISSM, such a migration of GL is shown to inhibit the ice flow. This negative feedback between the ice sheet and the solid Earth may promote the stability to marine portions of the ice sheet in future.

Future Antarctic Bed Topography and Its Implications for Ice Sheet Dynamics

NASA Technical Reports Server (NTRS)

Adhikari, Surendra; Ivins, Erik R.; Larour, Eric Y.; Seroussi, Helene L.; Morlighem, Mathieu; Nowicki, S.

2014-01-01

The Antarctic bedrock is evolving as the solid Earth responds to the past and ongoing evolution of the ice sheet. A recently improved ice loading history suggests that the Antarctic Ice Sheet (AIS) has generally been losing its mass since the Last Glacial Maximum. In a sustained warming climate, the AIS is predicted to retreat at a greater pace, primarily via melting beneath the ice shelves.We employ the glacial isostatic adjustment (GIA) capability of the Ice Sheet System Model (ISSM) to combine these past and future ice loadings and provide the new solid Earth computations for the AIS.We find that past loading is relatively less important than future loading for the evolution of the future bed topography. Our computations predict that the West Antarctic Ice Sheet (WAIS) may uplift by a few meters and a few tens of meters at years AD 2100 and 2500, respectively, and that the East Antarctic Ice Sheet is likely to remain unchanged or subside minimally except around the Amery Ice Shelf. The Amundsen Sea Sector in particular is predicted to rise at the greatest rate; one hundred years of ice evolution in this region, for example, predicts that the coastline of Pine Island Bay will approach roughly 45mmyr-1 in viscoelastic vertical motion. Of particular importance, we systematically demonstrate that the effect of a pervasive and large GIA uplift in the WAIS is generally associated with the flattening of reverse bed slope, reduction of local sea depth, and thus the extension of grounding line (GL) towards the continental shelf. Using the 3-D higher-order ice flow capability of ISSM, such a migration of GL is shown to inhibit the ice flow. This negative feedback between the ice sheet and the solid Earth may promote stability in marine portions of the ice sheet in the future.

BRITICE-CHRONO: Constraining rates and style of marine-influenced ice sheet decay to provide a data-rich playground for ice sheet modellers

NASA Astrophysics Data System (ADS)

Clark, Chris

2014-05-01

Uncertainty exists regarding the fate of the Antarctic and Greenland ice sheets and how they will respond to forcings from sea level and atmospheric and ocean temperatures. If we want to know more about the mechanisms and rate of change of shrinking ice sheets, then why not examine an ice sheet that has fully disappeared and track its retreat through time? If achieved in enough detail such information could become a data-rich playground for improving the next breed of numerical ice sheet models to be used in ice and sea level forecasting. We regard that the last British-Irish Ice Sheet is a good target for this work, on account of its small size, density of information and with its numerous researchers already investigating it. BRITICE-CHRONO is a large (>45 researchers) NERC-funded consortium project comprising Quaternary scientists and glaciologists who will search the seafloor around Britain and Ireland and parts of the landmass in order to find and extract samples of sand, rock and organic matter that can be dated (OSL; Cosmogenic; 14C) to reveal the timing and rate of change of the collapsing British-Irish Ice Sheet. The purpose is to produce a high resolution dataset on the demise on an ice sheet - from the continental shelf edge and across the marine to terrestrial transition. Some 800 new date assessments will be added to those that already exist. This poster reports on the hypotheses that underpin the work. Data on retreat will be collected by focusing on 8 transects running from the continental shelf edge to a short distance (10s km) onshore and acquiring marine and terrestrial samples for geochronometric dating. The project includes funding for 587 radiocarbon, 140 OSL and 158 TCN samples for surface exposure dating; with sampling accomplished by two research cruises and 16 fieldwork campaigns. Results will reveal the timing and rate of change of ice margin recession for each transect, and combined with existing landform and dating databases, will be used to build an ice sheet-wide empirical reconstruction of retreat incorporating Bayesian analysis to assess uncertainty. We invite and encourage ice sheet modellers to use our data for modelling experiments and in particular to explore the role of bed topography in modulating ice retreat.

A calving law for ice sheet models; Investigating the role of surface melt on dynamics of Greenland outlet glaciers

NASA Astrophysics Data System (ADS)

Nick, F. M.; van der Veen, C. J.; Vieli, A.

2008-12-01

alving of icebergs accounts for perhaps as much as half the ice transferred from the Greenland Ice Sheet into the surrounding ocean, and virtually all of the ice loss from the Antarctic Ice Sheet. We have formulated a calving model that can be readily incorporated into time-evolving numerical ice-flow models. Our model is based on downward penetration of water-filled surface crevasses and upward propagation of basal crevasses. A calving event occurs when the depth of the surface crevasse (which increases as melting progresses through the summer) reaches the height of the basal crevasse. Our numerical ice sheet model is able to reproduce observed seasonal changes of Greenland outlet glaciers, such as fluctuations in flow speed and terminus positions. We have applied the model to Helheim Glacier on the east coast, and Petermann Glacier in the northwest. Our model suggests that rapid retreat of the claving front is highly affected by the amplified calving rate due to increasing water level in surface crevasses during warmer summers. Our results show little response to seasonally enhanced basal lubrication from surface melt. This modeling study provides insights into the role of surface and basal hydrology to ice sheet dynamics and on how to incorporate calving in ice sheet models and therefore advances our ability to predict future ice sheet change.

Greenland Regional and Ice Sheet-wide Geometry Sensitivity to Boundary and Initial conditions

NASA Astrophysics Data System (ADS)

Logan, L. C.; Narayanan, S. H. K.; Greve, R.; Heimbach, P.

2017-12-01

Ice sheet and glacier model outputs require inputs from uncertainly known initial and boundary conditions, and other parameters. Conservation and constitutive equations formalize the relationship between model inputs and outputs, and the sensitivity of model-derived quantities of interest (e.g., ice sheet volume above floatation) to model variables can be obtained via the adjoint model of an ice sheet. We show how one particular ice sheet model, SICOPOLIS (SImulation COde for POLythermal Ice Sheets), depends on these inputs through comprehensive adjoint-based sensitivity analyses. SICOPOLIS discretizes the shallow-ice and shallow-shelf approximations for ice flow, and is well-suited for paleo-studies of Greenland and Antarctica, among other computational domains. The adjoint model of SICOPOLIS was developed via algorithmic differentiation, facilitated by the source transformation tool OpenAD (developed at Argonne National Lab). While model sensitivity to various inputs can be computed by costly methods involving input perturbation simulations, the time-dependent adjoint model of SICOPOLIS delivers model sensitivities to initial and boundary conditions throughout time at lower cost. Here, we explore both the sensitivities of the Greenland Ice Sheet's entire and regional volumes to: initial ice thickness, precipitation, basal sliding, and geothermal flux over the Holocene epoch. Sensitivity studies such as described here are now accessible to the modeling community, based on the latest version of SICOPOLIS that has been adapted for OpenAD to generate correct and efficient adjoint code.

How Fast? How Far? How Much? What We Need to Learn from the Past Behavior of Ice Sheets, and What They Might Not Tell Us

NASA Astrophysics Data System (ADS)

Alley, R. B.

2016-12-01

Paleoclimatic data support physical understanding that changes in ice sheets are primarily caused by changes in ocean temperature and in melting from above. With interesting qualifications, ice sheets tend to grow as accumulation rate in central regions drops into an ice age, and to shrink as accumulation rate rises. Changes in sea level may be influential but generally are too small and slow to be of primary importance. Thus, future atmospheric warming, oceanic warming and changes in oceanic circulation are especially important to future ice-sheet behavior. Paleoclimatic data support models and physical understanding that sustained warming beyond thresholds will cause progressively larger sea-level rise, up to quite high values, although the thresholds remain poorly quantified. Several indirect lines of evidence indicate great shrinkage or loss of parts or all of the Greenland ice sheet and marine sectors of the Antarctic ice sheet under warmth corresponding to CO2 levels similar to the modern or committed level. Despite this evidence, the state of the ice sheets during the most recent times warmer than today, including MIS 5e, remains unclear. The Greenland ice sheet did survive MIS 5e, but that may reflect warmth sufficient to remove the ice sheet but not sustained long enough to do so; greater warming in the future could cause much faster sea-level rise than generated in the past. Several indirect lines of evidence indicate that the marine basins of the West Antarctic Ice Sheet deglaciated during MIS 5e, and targeted field data could clarify this greatly. Physical understanding suggests, however, that even if this deglaciation did occur, it may have been slower than is possible in an even warmer future world; past rates of sea-level rise may define minimum rather than likely future rates.

The influence of atmospheric grid resolution in a climate model-forced ice sheet simulation

NASA Astrophysics Data System (ADS)

Lofverstrom, Marcus; Liakka, Johan

2018-04-01

Coupled climate-ice sheet simulations have been growing in popularity in recent years. Experiments of this type are however challenging as ice sheets evolve over multi-millennial timescales, which is beyond the practical integration limit of most Earth system models. A common method to increase model throughput is to trade resolution for computational efficiency (compromise accuracy for speed). Here we analyze how the resolution of an atmospheric general circulation model (AGCM) influences the simulation quality in a stand-alone ice sheet model. Four identical AGCM simulations of the Last Glacial Maximum (LGM) were run at different horizontal resolutions: T85 (1.4°), T42 (2.8°), T31 (3.8°), and T21 (5.6°). These simulations were subsequently used as forcing of an ice sheet model. While the T85 climate forcing reproduces the LGM ice sheets to a high accuracy, the intermediate resolution cases (T42 and T31) fail to build the Eurasian ice sheet. The T21 case fails in both Eurasia and North America. Sensitivity experiments using different surface mass balance parameterizations improve the simulations of the Eurasian ice sheet in the T42 case, but the compromise is a substantial ice buildup in Siberia. The T31 and T21 cases do not improve in the same way in Eurasia, though the latter simulates the continent-wide Laurentide ice sheet in North America. The difficulty to reproduce the LGM ice sheets in the T21 case is in broad agreement with previous studies using low-resolution atmospheric models, and is caused by a substantial deterioration of the model climate between the T31 and T21 resolutions. It is speculated that this deficiency may demonstrate a fundamental problem with using low-resolution atmospheric models in these types of experiments.

Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume.

PubMed

Abe-Ouchi, Ayako; Saito, Fuyuki; Kawamura, Kenji; Raymo, Maureen E; Okuno, Jun'ichi; Takahashi, Kunio; Blatter, Heinz

2013-08-08

The growth and reduction of Northern Hemisphere ice sheets over the past million years is dominated by an approximately 100,000-year periodicity and a sawtooth pattern (gradual growth and fast termination). Milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles, and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity, obliquity and precession cycles. Yet insolation alone cannot explain the strong 100,000-year cycle, suggesting that internal climatic feedbacks may also be at work. Earlier conceptual models, for example, showed that glacial terminations are associated with the build-up of Northern Hemisphere 'excess ice', but the physical mechanisms underpinning the 100,000-year cycle remain unclear. Here we show, using comprehensive climate and ice-sheet models, that insolation and internal feedbacks between the climate, the ice sheets and the lithosphere-asthenosphere system explain the 100,000-year periodicity. The responses of equilibrium states of ice sheets to summer insolation show hysteresis, with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles. The hysteresis loop of the North American ice sheet is such that after inception of the ice sheet, its mass balance remains mostly positive through several precession cycles, whose amplitudes decrease towards an eccentricity minimum. The larger the ice sheet grows and extends towards lower latitudes, the smaller is the insolation required to make the mass balance negative. Therefore, once a large ice sheet is established, a moderate increase in insolation is sufficient to trigger a negative mass balance, leading to an almost complete retreat of the ice sheet within several thousand years. This fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound, which is the lithosphere-asthenosphere response. Carbon dioxide is involved, but is not determinative, in the evolution of the 100,000-year glacial cycles.

Ice sheet altimetry

NASA Technical Reports Server (NTRS)

Brooks, R. L.

1981-01-01

Generalized surface slopes were computed for the Antarctic and Greenland ice sheets by differencing plotted contour levels and dividing them by the distance between the contours. It was observed that more than 90% of the ice sheets have surface slopes less than 1%. Seasat test mode-1 Seasat altimeter measurements over Greenland were analyzed by comparisons with collinear and intersecting normal mode Seasat altimeter passes. Over the ice sheet, the computed surface elevations from test mode-1 measurements were consistently lower by about 45 m and the AGC levels were down by approximately 6 dB. No test mode-1 data were acquired over Antarctica. It is concluded that analysis of the existing altimeter data base over the two ice sheets is crucial in designing a future improved altimeter tracking capability. It is recommended that additional waveform retracking be performed to characterize ice sheet topography as a function of geographic area and elevation.

Sensitivity of an Antarctic Ice Sheet Model to Sub-Ice-Shelf Melting

NASA Astrophysics Data System (ADS)

Lipscomb, W. H.; Leguy, G.; Urban, N. M.; Berdahl, M.

2017-12-01

Theory and observations suggest that marine-based sectors of the Antarctic ice sheet could retreat rapidly under ocean warming and increased melting beneath ice shelves. Numerical models of marine ice sheets vary widely in sensitivity, depending on grid resolution and the parameterization of key processes (e.g., calving and hydrofracture). Here we study the sensitivity of the Antarctic ice sheet to ocean warming and sub-shelf melting in standalone simulations of the Community Ice Sheet Model (CISM). Melt rates either are prescribed based on observations and high-resolution ocean model output, or are derived from a plume model forced by idealized ocean temperature profiles. In CISM, we vary the model resolution (between 1 and 8 km), Stokes approximation (shallow-shelf, depth-integrated higher-order, or 3D higher-order) and calving scheme to create an ensemble of plausible responses to sub-shelf melting. This work supports a broader goal of building statistical and reduced models that can translate large-scale Earth-system model projections to changes in Antarctic ocean temperatures and ice sheet discharge, thus better quantifying uncertainty in Antarctic-sourced sea-level rise.

Oceanic Forcing of Ice-Sheet Retreat: West Antarctica and More

NASA Astrophysics Data System (ADS)

Alley, Richard B.; Anandakrishnan, Sridhar; Christianson, Knut; Horgan, Huw J.; Muto, Atsu; Parizek, Byron R.; Pollard, David; Walker, Ryan T.

2015-05-01

Ocean-ice interactions have exerted primary control on the Antarctic Ice Sheet and parts of the Greenland Ice Sheet, and will continue to do so in the near future, especially through melting of ice shelves and calving cliffs. Retreat in response to increasing marine melting typically exhibits threshold behavior, with little change for forcing below the threshold but a rapid, possibly delayed shift to a reduced state once the threshold is exceeded. For Thwaites Glacier, West Antarctica, the threshold may already have been exceeded, although rapid change may be delayed by centuries, and the reduced state will likely involve loss of most of the West Antarctic Ice Sheet, causing >3 m of sea-level rise. Because of shortcomings in physical understanding and available data, uncertainty persists about this threshold and the subsequent rate of change. Although sea-level histories and physical understanding allow the possibility that ice-sheet response could be quite fast, no strong constraints are yet available on the worst-case scenario. Recent work also suggests that the Greenland and East Antarctic Ice Sheets share some of the same vulnerabilities to shrinkage from marine influence.

The role of ice shelves in the Holocene evolution of the Antarctic ice sheet

NASA Astrophysics Data System (ADS)

Bernales, Jorge; Rogozhina, Irina; Thomas, Maik

2014-05-01

Using the continental-scale ice sheet-shelf model SICOPOLIS (Greve, 1997 [1]; Sato and Greve, 2012 [2]), we assess the influence of ice shelves on the Holocene evolution and present-day geometry of the Antarctic ice sheet. We have designed a series of paleoclimate simulations driven by a time-evolved climate forcing that couples the surface temperature record from the Vostok ice core with precipitation pattern using an empirical relation of Dahl-Jensen et al., (1998) [3]. Our numerical experiments show that the geometry of ice shelves is determined by the evolution of climate and ocean conditions over time scales of 15 to 25 kyr. This implies that the initial configuration of ice shelves at the Last Glacial Maximum (LGM, about 21 kyr before present) has a significant effect on the modelled Early Holocene volume of ice shelves (up to 20%) that gradually diminishes to a negligible level for the present-day ice shelf configuration. Thus, the present-day geometry of the Antarctic ice shelves can be attained even if an ice-shelf-free initial condition is chosen at the LGM. However, the grounded ice volume, thickness and dynamic states are found to be sensitive to the ice shelf dynamics over a longer history spanning several tens of thousands of years. A presence of extensive marine ice at the LGM, supported by sediment core reconstructions (e.g. Naish et al., 2009 [4]), has a clear buttressing effect on the grounded ice that remains significant over a period of 30 to 50 kyr. If ice-shelf-free conditions are prescribed at the LGM, the modelled Early Holocene and present-day grounded ice volumes are underestimated by up to 10%, as opposed to simulations incorporating ice shelf dynamics over longer periods. The use of ice-shelf-free LGM conditions thus results in 50 to over 200 meters thinner ice sheet across much of East Antarctica. References [1] Greve, R. (1997). Application of a polythermal three-dimensional ice sheet model to the Greenland ice sheet: response to steady-state and transient climate scenarios. Journal of Climate, 10(5), 901-918. [2] Sato, T., and Greve, R. (2012). Sensitivity experiments for the Antarctic ice sheet with varied sub-ice-shelf melting rates. Annals of Glaciology, 53(60), 221-228. [3] Dahl-Jensen, D., Mosegaard, K., Gundestrup, N., Clow, G. D., Johnsen, S. J., Hansen, A. W., and Balling, N. (1998). Past temperatures directly from the Greenland ice sheet. Science, 282(5387), 268-271. [4] Naish, T., Powell, R., Levy, R., Wilson, G., Scherer, R., Talarico, F., ... and Schmitt, D. (2009). Obliquity-paced Pliocene West Antarctic ice sheet oscillations. Nature, 458(7236), 322-328.

Deformation, warming and softening of Greenland’s ice by refreezing meltwater

NASA Astrophysics Data System (ADS)

Bell, Robin E.; Tinto, Kirsteen; Das, Indrani; Wolovick, Michael; Chu, Winnie; Creyts, Timothy T.; Frearson, Nicholas; Abdi, Abdulhakim; Paden, John D.

2014-07-01

Meltwater beneath the large ice sheets can influence ice flow by lubrication at the base or by softening when meltwater refreezes to form relatively warm ice. Refreezing has produced large basal ice units in East Antarctica. Bubble-free basal ice units also outcrop at the edge of the Greenland ice sheet, but the extent of refreezing and its influence on Greenland’s ice flow dynamics are unknown. Here we demonstrate that refreezing of meltwater produces distinct basal ice units throughout northern Greenland with thicknesses of up to 1,100 m. We compare airborne gravity data with modelled gravity anomalies to show that these basal units are ice. Using radar data we determine the extent of the units, which significantly disrupt the overlying ice sheet stratigraphy. The units consist of refrozen basal water commonly surrounded by heavily deformed meteoric ice derived from snowfall. We map these units along the ice sheet margins where surface melt is the largest source of water, as well as in the interior where basal melting is the only source of water. Beneath Petermann Glacier, basal units coincide with the onset of fast flow and channels in the floating ice tongue. We suggest that refreezing of meltwater and the resulting deformation of the surrounding basal ice warms the Greenland ice sheet, modifying the temperature structure of the ice column and influencing ice flow and grounding line melting.

Subglacial Lake CECs: Discovery and in situ survey of a privileged research site in West Antarctica

NASA Astrophysics Data System (ADS)

Rivera, Andrés.; Uribe, José; Zamora, Rodrigo; Oberreuter, Jonathan

2015-05-01

We report the discovery and on-the-ground radar mapping of a subglacial lake in Antarctica, that we have named Lake CECs (Centro de Estudios Científicos) in honor of the institute we belong to. It is located in the central part of the West Antarctic Ice Sheet, right underneath the Institute Ice Stream and Minnesota Glacier ice divide, and has not experienced surface elevation changes during the last 10 years. The ratio between the area of the subglacial lake and that of its feeding basin is larger than those for either subglacial lakes Ellsworth or Whillans, and it has a depth comparable to that of Ellsworth and greater than that of Whillans. Its ice thickness is ˜600 m less than that over Ellsworth. The lake is very likely a system with long water residence time. The recent finding of microbial life in Lake Whillans emphasizes the potential of Subglacial Lake CECs for biological exploration.

NASA: First Map Of Thawed Areas Under Greenland Ice Sheet

NASA Image and Video Library

2017-12-08

NASA researchers have helped produce the first map showing what parts of the bottom of the massive Greenland Ice Sheet are thawed – key information in better predicting how the ice sheet will react to a warming climate. Greenland’s thick ice sheet insulates the bedrock below from the cold temperatures at the surface, so the bottom of the ice is often tens of degrees warmer than at the top, because the ice bottom is slowly warmed by heat coming from the Earth’s depths. Knowing whether Greenland’s ice lies on wet, slippery ground or is anchored to dry, frozen bedrock is essential for predicting how this ice will flow in the future, But scientists have very few direct observations of the thermal conditions beneath the ice sheet, obtained through fewer than two dozen boreholes that have reached the bottom. Now, a new study synthesizes several methods to infer the Greenland Ice Sheet’s basal thermal state –whether the bottom of the ice is melted or not– leading to the first map that identifies frozen and thawed areas across the whole ice sheet. Map caption: This first-of-a-kind map, showing which parts of the bottom of the Greenland Ice Sheet are likely thawed (red), frozen (blue) or still uncertain (gray), will help scientists better predict how the ice will flow in a warming climate. Credit: NASA Earth Observatory/Jesse Allen Read more: go.nasa.gov/2avKgl2 NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

Future Antarctic bed topography and its implications for ice sheet dynamics

NASA Astrophysics Data System (ADS)

Adhikari, Surendra; Ivins, Erik; Larour, Eric; Seroussi, Helene; Morlighem, Mathieu; Nowicki, Sophie

2014-05-01

A recently improved ice loading history suggests that the Antarctic Ice Sheet (AIS) has been generally losing its mass since the last glacial maximum. In a sustained warming climate, the AIS is predicted to retreat at a greater pace primarily via melting beneath the ice shelves. We employ the glacial isostatic adjustment (GIA) capability of the Ice Sheet System Model (ISSM) to combine these past and future ice loadings and provide the new solid Earth computations for the AIS. We find that the past loading is relatively less important than future loading on the evolution of the future bed topography. Our computations predict that the West Antarctic Ice Sheet (WAIS) may uplift by a few meters and a few tens of meters at years 2100 and 2500 AD, respectively, and that the East Antarctic Ice Sheet (EAIS) is likely to remain unchanged or subside minimally except around the Amery Ice Shelf. The Amundsen Sea Sector of WAIS in particular is predicted to rise at the greatest rate; one hundred years of ice evolution in this region, for example, predicts that the coastline of Pine Island Bay approaches roughly 45 mm/yr in viscoelastic vertical motion. Of particular importance, we systematically demonstrate that the effect of a pervasive and large GIA uplift in the WAIS is associated with the flattening of reverse bed, reduction of local sea depth, and thus the extension of grounding line (GL) towards the continental shelf. Using the 3-D higher-order ice flow capability of ISSM, such a migration of GL is shown to inhibit the ice flow. This negative feedback between the ice sheet and the solid Earth may promote the stability to marine portions of the ice sheet in the future.

A Detailed Geophysical Investigation of the Grounding of Henry Ice Rise, with Implications for Holocene Ice-Sheet Extent.

NASA Astrophysics Data System (ADS)

Wearing, M.; Kingslake, J.

2017-12-01

It is generally assumed that since the Last Glacial Maximum the West Antarctic Ice Sheet (WAIS) has experienced monotonic retreat of the grounding line (GL). However, recent studies have cast doubt on this assumption, suggesting that the retreat of the WAIS grounding line may have been followed by a significant advance during the Holocene in the Weddell and Ross Sea sectors. Constraining this evolution is important as reconstructions of past ice-sheet extent are used to spin-up predictive ice-sheet models and correct mass-balance observations for glacial isostatic adjustment. Here we examine in detail the formation of the Henry Ice Rise (HIR), which ice-sheet model simulations suggest played a key role in Holocene ice-mass changes in the Weddell Sea sector. Observations from a high-resolution ground-based, ice-penetrating radar survey are best explained if the ice rise formed when the Ronne Ice Shelf grounded on a submarine high, underwent a period of ice-rumple flow, before the GL migrated outwards to form the present-day ice rise. We constrain the relative chronology of this evolution by comparing the alignment and intersection of isochronal internal layers, relic crevasses, surface features and investigating the dynamic processes leading to their complex structure. We also draw analogies between HIR and the neighbouring Doake Ice Rumples. The date of formation is estimated using vertical velocities derived with a phase-sensitive radio-echo sounder (pRES). Ice-sheet models suggest that the formation of the HIR and other ice rises may have halted and reversed large-scale GL retreat. Hence the small-scale dynamics of these crucial regions could have wide-reaching consequences for future ice-sheet mass changes and constraining their formation and evolution further would be beneficial. One stringent test of our geophysics-based conclusions would be to drill to the bed of HIR to sample the ice for isotopic analysis and the bed for radiocarbon analysis.

Bathymetric and oceanic controls on Abbot Ice Shelf thickness and stability

NASA Astrophysics Data System (ADS)

Cochran, J. R.; Jacobs, S. S.; Tinto, K. J.; Bell, R. E.

2014-05-01

Ice shelves play key roles in stabilizing Antarctica's ice sheets, maintaining its high albedo and returning freshwater to the Southern Ocean. Improved data sets of ice shelf draft and underlying bathymetry are important for assessing ocean-ice interactions and modeling ice response to climate change. The long, narrow Abbot Ice Shelf south of Thurston Island produces a large volume of meltwater, but is close to being in overall mass balance. Here we invert NASA Operation IceBridge (OIB) airborne gravity data over the Abbot region to obtain sub-ice bathymetry, and combine OIB elevation and ice thickness measurements to estimate ice draft. A series of asymmetric fault-bounded basins formed during rifting of Zealandia from Antarctica underlie the Abbot Ice Shelf west of 94° W and the Cosgrove Ice Shelf to the south. Sub-ice water column depths along OIB flight lines are sufficiently deep to allow warm deep and thermocline waters observed near the western Abbot ice front to circulate through much of the ice shelf cavity. An average ice shelf draft of ~200 m, 15% less than the Bedmap2 compilation, coincides with the summer transition between the ocean surface mixed layer and upper thermocline. Thick ice streams feeding the Abbot cross relatively stable grounding lines and are rapidly thinned by the warmest inflow. While the ice shelf is presently in equilibrium, the overall correspondence between draft distribution and thermocline depth indicates sensitivity to changes in characteristics of the ocean surface and deep waters.

Reconstructions of the Weichselian ice sheet, a comparative study of a thermo-mechanical approach to GIA driven models.

NASA Astrophysics Data System (ADS)

Schmidt, Peter; Lund, Björn; Näslund, Jens-Ove; Fastook, James

2014-05-01

Observations of glacial isostatic adjustment (GIA) have been used both to study the mechanical properties of the Earth and to invert for Northern Hemisphere palaeo-ice-sheets. This is typically done by solving the sea-level equation using simplified scaling laws to control ice-sheet thickness. However, past ice-sheets can also be reconstructed based on thermo-mechanical modelling driven by palaeo-climate data, invoking simple analytical models to account for the Earth's response. Commonly, both approaches use dated geological markers to constrain the ice-sheet margin location. Irrespective of the approach, the resulting ice-sheet reconstruction depends on the earth response, although the interdependence between the ice model and the earth model differs and therefore the two types of reconstructions could provide complementary information on Earth properties. We compare a thermo-mechanical reconstruction of the Weichselian ice-sheet using the UMISM model (Näslund, 2010) to two GIA driven reconstructions, ANU (Lambeck et al., 2010) and ICE-5G (Peltier & Fairbanks, 2006), commonly used in GIA modelling. We evaluate the three reconstructions both in terms of ice-sheet configurations and predicted Fennoscandian surface deformation ICE-5G comprise the largest reconstructed ice-sheet whereas ANU and UMISM are more similar in volume and areal extent. Significant differences still exists between ANU and UMISM, especially during the final deglaciation phase. Prior to the final retreat of the ice-sheet, ICE-5G is displays a massive and more or less constant ice-sheet configuration, while both ANU and UMISM fluctuates with at times almost ice-free conditions, such as during MIS3. This results in ICE-5G being close to isostatic equilibrium at LGM, whereas ANU and UMISM are not. Hence, the pre-LGM evolution of the Weichselian ice-sheet needs to be considered in GIA studies. For example, perturbing the ANU or UMISM reconstructions we find that changes more recent than 36 kyr BP may change the predicted uplift velocities by more than 0.1 mm/yr, while changes more recent than 55 kyr BP may change the predicted uplift 10 kyr ago by more than 5 m. Despite their differences we find that all three reconstructions can equally well fit observations of the present day uplift in Fennoscandia, as well as the observed sea-level curve along the Ångerman river, Sweden, albeit with different optimal earth models. However, only for ANU can a single optimal earth model be determined as a bifurcation in the optimal viscosity arises from the generally faster present day rebound rates in ICE-5G and UMISM, resulting in a range of well-fitting earth models for the latter reconstructions. Studying models with a reasonable fit to observed present day uplift velocities we find general trends of over- and under-prediction, indicating that all three ice-sheet reconstructions need improvement. In general, all three reconstructions tend to over-predict the uplift rates in southwestern Fennoscandia, whereas over Finland ICE-5G generally over-predicts and ANU generally under-predicts the uplift rates. UMISM tend to under-predict the velocities over central to northern Sweden and similar trends can also be seen in ANU and ICE-5G.

Capturing total chronological and spatial uncertainties in palaeo-ice sheet reconstructions: the DATED example

NASA Astrophysics Data System (ADS)

Hughes, Anna; Gyllencreutz, Richard; Mangerud, Jan; Svendsen, John Inge

2017-04-01

Glacial geologists generate empirical reconstructions of former ice-sheet dynamics by combining evidence from the preserved record of glacial landforms (e.g. end moraines, lineations) and sediments with chronological evidence (mainly numerical dates derived predominantly from radiocarbon, exposure and luminescence techniques). However the geomorphological and sedimentological footprints and chronological data are both incomplete records in both space and time, and all have multiple types of uncertainty associated with them. To understand ice sheets' response to climate we need numerical models of ice-sheet dynamics based on physical principles. To test and/or constrain such models, empirical reconstructions of past ice sheets that capture and acknowledge all uncertainties are required. In 2005 we started a project (Database of the Eurasian Deglaciation, DATED) to produce an empirical reconstruction of the evolution of the last Eurasian ice sheets, (including the British-Irish, Scandinavian and Svalbard-Barents-Kara Seas ice sheets) that is fully documented, specified in time, and includes uncertainty estimates. Over 5000 dates relevant to constraining ice build-up and retreat were assessed for reliability and used together with published ice-sheet margin positions based on glacial geomorphology to reconstruct time-slice maps of the ice sheets' extent. The DATED maps show synchronous ice margins with maximum-minimum uncertainty bounds for every 1000 years between 25-10 kyr ago. In the first version of results (DATED-1; Hughes et al. 2016) all uncertainties (both quantitative and qualitative, e.g. precision and accuracy of numerical dates, correlation of moraines, stratigraphic interpretations) were combined based on our best glaciological-geological assessment and expressed in terms of distance as a 'fuzzy' margin. Large uncertainties (>100 km) exist; predominantly across marine sectors and other locations where there are spatial gaps in the dating record (e.g. the timing of coalescence and separation of the Scandinavian and Svalbard-Barents-Kara ice sheets) but also in well-studied areas due to conflicting yet apparently equally robust data. In the four years since the DATED-1 census (1 January 2013), the volume of new information (from both dates and mapped glacial geomorphology) has grown significantly ( 1000 new dates). Here, we present work towards the updated version of results, DATED-2, that attempts to further reduce and explicitly report all uncertainties inherent in ice sheet reconstructions. Hughes, A. L. C., Gyllencreutz, R., Lohne, Ø. S., Mangerud, J., Svendsen, J. I. 2016: The last Eurasian ice sheets - a chronological database and time-slice reconstruction, DATED-1. Boreas, 45, 1-45. 10.1111/bor.12142

Sea-level response to abrupt ocean warming of Antarctic ice shelves

NASA Astrophysics Data System (ADS)

Pattyn, Frank

2016-04-01

Antarctica's contribution to global sea-level rise increases steadily. A fundamental question remains whether the ice discharge will lead to marine ice sheet instability (MISI) and collapse of certain sectors of the ice sheet or whether ice loss will increase linearly with the warming trends. Therefore, we employ a newly developed ice sheet model of the Antarctic ice sheet, called f.ETISh (fast Elementary Thermomechanical Ice Sheet model) to simulate ice sheet response to abrupt perturbations in ocean and atmospheric temperature. The f.ETISh model is a vertically integrated hybrid (SSA/SIA) ice sheet model including ice shelves. Although vertically integrated, thermomechanical coupling is ensured through a simplified representation of ice sheet thermodynamics based on an analytical solution of the vertical temperature profile, including strain heating and horizontal advection. The marine boundary is represented by a flux condition either coherent with power-law basal sliding (Pollard & Deconto (2012) based on Schoof (2007)) or according to Coulomb basal friction (Tsai et al., 2015), both taking into account ice-shelf buttressing. Model initialization is based on optimization of the basal friction field. Besides the traditional MISMIP tests, new tests with respect to MISI in plan-view models have been devised. The model is forced with stepwise ocean and atmosphere temperature perturbations. The former is based on a parametrised sub-shelf melt (limited to ice shelves), while the latter is based on present-day mass balance/surface temperature and corrected for elevation changes. Surface melting is introduced using a PDD model. Results show a general linear response in mass loss to ocean warming. Nonlinear response due to MISI occurs under specific conditions and is highly sensitive to the basal conditions near the grounding line, governed by both the initial conditions and the basal sliding/deformation model. The Coulomb friction model leads to significantly higher sensitivity compared to power-law sliding. On longer time scales, West-antarctic inter-basin connections favor nonlinear response.

The Research on Elevation Change of Antarctic Ice Sheet Based on CRYOSAT-2 Alimeter

NASA Astrophysics Data System (ADS)

Sun, Q.; Wan, J.; Liu, S.; Li, Y.

2018-04-01

In this paper, the Cryosat-2 altimeter data distributed by the ESA, and these data are processed to extract the information of the elevation change of the Antarctic ice sheet from 2010 to 2017. Firstly, the main pretreatment preprocessing for Cryosat-2 altimetry data is crossover adjustment and elimination of rough difference. Then the grid DEM of the Antarctic ice sheet was constructed by using the kriging interpolation method,and analyzed the spatial characteristic time characteristics of the Antarctic ice sheet. The latitude-weighted elevation can be obtained by using the elevation data of each cycle, and then the general trend of the Antarctic ice sheet elevation variation can be seen roughly.

First Younger Dryas moraines in Greenland

NASA Astrophysics Data System (ADS)

Funder, Svend; Larsen, Nicolaj K.; Linge, Henriette; Möller, Per; Schomacker, Anders; Fabel, Derek; Kjær, Kurt H.; Xu, Sheng

2016-04-01

Over the Greenland ice sheet the Younger Dryas (YD) cold climate oscillation (12.9-11.7 kaBP) began with up to 10°C drop in temperatures and ended with up to 12°C abrupt warming. In the light of the present warming and melting of the ice sheet, and its importance for future climate change, the ice sheet's response to these dramatic changes in the past is of great interest. However, even though much effort has gone into charting YD ice margin behaviour around Greenland in recent years, no clear-cut signal of response to the oscillation has been uncovered. Here we show evidence to suggest that three major outlets from a local ice cap at Greenland's north coast advanced and retreated synchronously during YD. The evidence comprises OSL (optically stimulated luminescence) dates from a marine transgression of the coastal valleys that preceded the advance, and exposure ages from boulders on the moraines, formed by glaciers that overrode the marine sediment. The OSL ages suggest a maximum age of 12.4 ±0.6 kaBP for the marine incursion, and 10 exposure ages on boulders from the three moraines provide an average minimum age of 12.5 ±0.7 kaBP for the moraines, implying that the moraines were formed within the interval 11.8-13.0 kaBP. Elsewhere in Greenland evidence for readvance has been recorded in two areas. Most notably, in the East Greenland fjord zone outlet glaciers over a stretch of 800 km coast advanced through the fjords. In Scoresby Sund, where the moraines form a wide belt, an extensive 14C and exposure dating programme has shown that the readvance here probably culminated before YD, while cessation of moraine formation and rapid retreat from the moraine belt did not commence until c. 11.5 kaBP, but no moraines have so far been dated to YD. Readvance is also seen in Disko Bugt, the largest ice sheet outlet in West Greenland. However, here the advance and retreat of the ice stream took place in mid YD times, and lasted only a few hundred years, while YD in general was characterised by large scale, more than 200 km, retreat on the shelf. Therefore, although readvance and retreat occurred in both areas, the readvance was apparently not triggered by the initial YD cooling nor was the retreat caused by the abrupt warming at the end. At all other sites with a record that run through or into YD - Southeast Greenland, South Greenland, northern West Greenland - the ice margins were apparently retreating through YD, leaving the north coast as the only area with evidence for a climatically conditioned YD readvance/retreat. The apparent mismatch between ice core temperatures and ice margin behaviour is generally seen as a function of reduced AMOC (Atlantic Meridional Overturning Circulation), inducing both higher seasonality with very cold winters and warm summers, and also occurrence of warm subsurface water to melt the ice sheet margin along some coasts. Therefore the ice margin response to the cold oscillation was to some extent determined by the nearness to the North Atlantic - with North Greenland being the farthest away. Although this may explain why glaciers advanced in North Greenland, while they melted in more southerly parts, it still leaves the question with a bearing on the future: why don't we see any ice margin response neither to the initial YD cooling, nor to the abrupt warming at the end?

State of balance of the cryosphere

NASA Technical Reports Server (NTRS)

Van Der Veen, C. J.

1991-01-01

Available observations and mass balance estimates of the cryosphere are summarized. Problems discussed include mountain glaciers, the Greenland ice sheet, the Antarctic ice sheet, conventional glacier measurement techniques, and satellite applications in glacier mass balance studies. It is concluded that the interior part of the Greenland ice sheet is thickening or in near equilibrium. Estimates of the mass balance of the Antarctic ice sheet suggest that it is positive, although the error limits allow for a slightly negative balance.

Why Europa's icy shell may convect, but ice sheets do not: a glaciological perspective

NASA Astrophysics Data System (ADS)

Bassis, J. N.

2016-12-01

Jupiter's moon Europa is covered in an icy shell that lies over a liquid ocean. Geological evidence and numerical models suggest that Europa's icy shell convects, providing the possibility that Europa may experience a form of plate tectonics and could even harbor life in its subsurface ocean. The hypothesis that Europa convects is supported by both models and geological evidence. Surprisingly, when we apply similar calculations and (assumptions) used by planetary scientists to infer convection in icy moons like Europa we find that these models also predict that vigorous convection should also occur in portions of our own terrestrial ice sheets and ice shelves where we have firm evidence to the contrary. We can explain the lack of convection within our own ice sheets by recognizing that instead of the diffusion creep limited rheology frequently invoked by planetary scientists, terrestrial ice undergoes power-law creep down to very low strain rates. Glaciological studies find that power-law creep is required to explain the structure of vertical strain rate near ice sheet divides and shape of the ice sheets near an ice divide. However, when we now apply a rheology that is consistent with terrestrial ice sheet dynamics to icy moon conditions, we find conditions are far less favorable for convection in icy moons, with only a very limited parameter regime where convection can occur. Given the many unknowns (grain size, impurities etc.) it is challenging to draw strong conclusions about the behavior of icy moons . Nonetheless, the lack of convection in terrestrial ice sheets provides an important constraint on the dynamics of icy moons and models that explain convection of icy moons should also explain the lack of convection on terrestrial ice sheets.

The East Antarctic Ice Sheet and the Gamburtsev Subglacial Mountains (Invited)

NASA Astrophysics Data System (ADS)

Bell, R. E.; Studinger, M.; Ferraccioli, F.; Damaske, D.; Finn, C.; Braaten, D. A.; Fahnestock, M. A.; Jordan, T. A.; Corr, H.; Elieff, S.; Frearson, N.; Block, A. E.; Rose, K.

2009-12-01

Models of the onset of glaciation in Antarctica routinely document the early growth of the ice sheet on the summit of the Gamburtsev Subglacial Mountains in the center of the East Antarctic Craton. While ice sheet models replicate the formation of the East Antarctic ice sheet 35 million years ago, the age, evolution and structure of the Gamburtsev Mountains remain completely unresolved. During the International Polar Year scientists from seven nations have launched a major collaborative program (AGAP) to explore the Gamburtsev Subglacial Mountains buried by the East Antarctic ice sheet and bounded by numerous subglacial lakes. The AGAP umbrella is a multi-national, multi-disciplinary effort and includes aerogeophysics, passive seismology, traverse programs and will be complimented by future ice core and bedrock drilling. A major new airborne data set including gravity; magnetics; ice thickness; SAR images of the ice-bed interface; near-surface and deep internal layers; and ice surface elevation is providing insights into a more dynamic East Antarctica. More than 120,000 km of aerogeophysical data have been acquired from two remote field camps during the 2008/09 field season. AGAP effort was designed to address several fundamental questions including: 1) What role does topography play in the nucleation of continental ice sheets? 2) How do tectonic processes control the formation, distribution, and stability of subglacial lakes? The preliminary analysis of this major new data set indicated these 3000m high mountains are deeply dissected by a dendritic system. The northern margin of the mountain range terminates against the inland extent of the Lambert Graben. Evidence of the onset of glaciation is preserved as cirques and U shaped valleys along the axis of the uplifted massifs. The geomorphology reflects the interaction between the ice sheet and the Gamburtsev Mountains. Bright reflectors in the radar data in the deep valleys indicate the presence of water that has the potential to influence ice sheet flow. Crevassing and disrupted internal layers are present in the deep ice found in the inland extent of the Lambert Graben. Preliminary analysis indicates both a more dynamic East Antarctic ice sheet and a more complex tectonic evolution for East Antarctica.

Towards Greenland Glaciation: cumulative or abrupt transition?

NASA Astrophysics Data System (ADS)

Ramstein, Gilles; Tan, Ning; Ladant, Jean-baptiste; Dumas, Christophe; Contoux, Camille

2017-04-01

During the mid-Pliocene warming period (3-3.3 Ma BP), the global annual mean temperatures inferred by data and model studies were 2-3° warmer than pre-industrial values. Accordingly, Greenland ice sheet volume is supposed to reach at the most, only half of that of present-day [Haywood et al. 2010]. Around 2.7-2.6 Ma BP, just ˜ 500 kyr after the warming peak of mid-Pliocene, the Greenland ice sheet has reached its full size [Lunt et al. 2008]. A crucial question concerns the evolution of the Greenland ice sheet from half to full size during the 3 - 2.5 Ma period. Data show a decreasing trend of atmospheric CO2 concentration from 3 Ma to 2.5 Ma [Seki et al.2010; Bartoli et al. 2011; Martinez et al. 2015]. However, a recent study [Contoux et al. 2015] suggests that a lowering of CO2 is not sufficient to initiate a perennial glaciation on Greenland and must be combined with low summer insolation to preserve the ice sheet during insolation maxima. This suggests rather a cumulative process than an abrupt event. In order to diagnose the evolution of the ice sheet build-up, we carry on, for the first time, a transient simulation of climate and ice sheet evolutions from 3 Ma to 2.5 Ma. This strategy enables us to investigate the waxing and waning of the ice sheet during several orbital cycles. We use a tri-dimensional interpolation method designed by Ladant et al. (2014), which allows the evolution of CO2 concentration and of orbital parameters, and the evolution of the Greenland ice sheet size to be taken into account. By interpolating climatic snapshot simulations ran with various possible combinations of CO2, orbits and ice sheet sizes, we can build a continuous climatic forcing that is then used to provide 500 kyrs-long ice sheet simulations. With such a tool, we may offer a physically based answer to different CO2 reconstructions scenarios and analyse which one is the most consistent with Greenland ice sheet buildup.

New deglacial and Holocene micropaleontological and geochemical records from the southern margin of the Svalbard Archipelago (Arctic Ocean)

NASA Astrophysics Data System (ADS)

Rigual-Hernández, Andrés.

2010-05-01

This study is presented in the context of the Spanish research project "The development of an Arctic ice stream-dominated sedimentary system: The southern Svalbard continental margin" (SVAIS), developed within the framework of the International Polar Year (IPY) Activity N. 367 (NICE STREAMS). Its main goal is to understand the evolution of glacial continental margins and their relationship with the changes in ice sheet dynamics induced by natural climatic changes, combining the geophysical data with the sediment record both collected during an oceanographic cruise in the Storfjorden area (SW Svalbard margin) in August 2007. This marine depositional system, dominated by an ice stream during the last glacial period, was selected due to its small size inducing a rapid response to climatic changes, and for the oceanographic relevance of the area for global ocean circulation. The results obtained aim to define the sedimentary architecture and morphology, and to provide more insight into the paleoceanographic and paleoclimatic evolution of the region. We specifically report on new micropaleontological and geochemical data obtained from the sediment cores. A preliminary age model indicates that the sediment sequences cover approximately the Last Deglaciation and the Holocene. Microfossils are generally well preserved, although the abundances of the different groups show marked shifts along the record. Low concentrations of coccolithophores, diatoms, planktic foraminifers and cysts of organic-walled dinoflagellates (dinocysts) are found at the lower half of the sequence (IRD-rich, coarser-grained sediments), and increase towards the Late Holocene (fine-grained bioturbated sediments). The Climatic Optimum is characterized by the warmest sea surface temperatures as estimated from the fossil assemblage, diverse transfer functions and biogeochemical proxies, and by high nutrient contents in the bottom waters shown by light carbon isotope values and high Cd/Ca ratios in benthic foraminifers. Dilution by terrigenous material, related to the retreat of the Barents Sea Ice Sheet in response to changes in the strength of the Atlantic-sourced, warm Western Spitsbergen Current, seems to be important in driving the abundances of microfossils and of organic compounds. The different stages of the Deglaciation and the Holocene and the associated modifications in the surface oceanic environment are documented by changes in the fossil assemblage composition of the different microfossil groups, while synchronous changes in the bottom water masses are registered by stable isotope and trace element analyses of benthic foraminifers.

A Warmer Atmosphere on Mars near the Noachian-Hesperian Boundary: Evidence from Basal Melting of the South Polar Ice Cap (Dorsa Argentea Formation)

NASA Astrophysics Data System (ADS)

Fastook, J. L.; Head, J. W.; Marchant, D. R.; Forget, F.; Madeleine, J.-B.

2012-05-01

Eskers in the Dorsa Argentea Formation imply the presence of an ice sheet with a wet bed. With an ice sheet model, we examine a range of geothermal heat fluxes and warmer climates to determine what conditions could produce such an ice sheet.

Antarctic ice-sheet loss driven by basal melting of ice shelves.

PubMed

Pritchard, H D; Ligtenberg, S R M; Fricker, H A; Vaughan, D G; van den Broeke, M R; Padman, L

2012-04-25

Accurate prediction of global sea-level rise requires that we understand the cause of recent, widespread and intensifying glacier acceleration along Antarctic ice-sheet coastal margins. Atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves, potentially limiting their ability to buttress the flow of grounded tributary glaciers. Indeed, recent ice-shelf collapse led to retreat and acceleration of several glaciers on the Antarctic Peninsula. But the extent and magnitude of ice-shelf thickness change, the underlying causes of such change, and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted. Here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum-Antarctic pattern of ice-shelf thinning through increased basal melt. We deduce that this increased melt is the primary control of Antarctic ice-sheet loss, through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow. The highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf. Wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of Antarctic ice shelves, through ocean upwelling in the Amundsen and Bellingshausen seas, and atmospheric warming on the Antarctic Peninsula. This implies that climate forcing through changing winds influences Antarctic ice-sheet mass balance, and hence global sea level, on annual to decadal timescales.

On the pattern of WAIS retreat in eastern Ross Sea based on a regional synthesis of new geophysical and geological data acquired during NBP1502

NASA Astrophysics Data System (ADS)

McGlannan, A. J.; Bart, P. J.; Anderson, J. B.

2016-02-01

New multibeam and seismic data acquired during NBP1502 reveal that a series of backstepping grounding zone wedges (GZWs) were constructed on the middle shelf as the West Antarctic Ice Sheet (WAIS) retreated from the Whales Deep paleo-ice stream trough. The geomorphological information provided by these geophysical data were used to acquire a regional grid of jumbo-piston and kasten cores. Here, we present our regional synthesis of the new geophysical and geological data. The distributions of upcore transitions from diamict to sub-ice-shelf facies on the outer-most shelf demonstrate that as the grounded ice retreated in four discrete backsteps, the calving front remained in the vicinity of the shelf edge, approximately 50 kilometers to the north. In contrast, the upcore transition at the fourth backstep shows GZW diamict directly overlain by open-marine facies. We interpret this to indicate that a major retreat of both grounded and floating ice was associated with the termination of the middle-shelf grounding event. The minimum retreat distance was greater than 100 kilometers.

Ice sheet topography by satellite altimetry

USGS Publications Warehouse

Brooks, R.L.; Campbell, W.J.; Ramseier, R.O.; Stanley, H.R.; Zwally, H.J.

1978-01-01

The surface elevation of the southern Greenland ice sheet and surface features of the ice flow are obtained from the radar altimeter on the GEOS 3 satellite. The achieved accuracy in surface elevation is ???2 m. As changes in surface elevation are indicative of changes in ice volume, the mass balance of the present ice sheets could be determined by repetitive mapping of the surface elevation and the surface could be monitored to detect surging or significant changes in ice flow. ?? 1978 Nature Publishing Group.

West Antarctic Ice Sheet retreat driven by Holocene warm water incursions

PubMed Central

Hillenbrand, Claus-Dieter; Smith, James A.; Hodell, David A.; Greaves, Mervyn; Poole, Christopher R.; Kender, Sev; Williams, Mark; Andersen, Thorbjørn Joest; Jernas, Patrycja E.; Klages, Johann P.; Roberts, Stephen J.; Gohl, Karsten; Larter, Robert D.; Kuhn, Gerhard

2017-01-01

Glaciological and oceanographic observations coupled with numerical models show that warm Circumpolar Deep Water (CDW) upwelling onto the West Antarctic continental shelf causes melting of the undersides of floating ice shelves. Because these ice shelves buttress glaciers feeding into them, their ocean-induced thinning is driving Antarctic ice-sheet loss today. Here we present the first multi-proxy data based reconstruction of variability in CDW inflow to the Amundsen Sea sector, the most vulnerable part of the West Antarctic Ice Sheet, during the last 11,000 years. The chemical composition of foraminifer shells and benthic foraminifer assemblages in marine sediments indicate that enhanced CDW upwelling, controlled by the latitudinal position of the Southern Hemisphere westerly winds, forced deglaciation of this sector both until 7,500 years ago, when an ice-shelf collapse may have caused rapid ice-sheet thinning further upstream, and since the 1940s. These results increase confidence in the predictive capability of current ice-sheet models. PMID:28682333

West Antarctic Ice Sheet retreat driven by Holocene warm water incursions.

PubMed

Hillenbrand, Claus-Dieter; Smith, James A; Hodell, David A; Greaves, Mervyn; Poole, Christopher R; Kender, Sev; Williams, Mark; Andersen, Thorbjørn Joest; Jernas, Patrycja E; Elderfield, Henry; Klages, Johann P; Roberts, Stephen J; Gohl, Karsten; Larter, Robert D; Kuhn, Gerhard

2017-07-05

Glaciological and oceanographic observations coupled with numerical models show that warm Circumpolar Deep Water (CDW) incursions onto the West Antarctic continental shelf cause melting of the undersides of floating ice shelves. Because these ice shelves buttress glaciers feeding into them, their ocean-induced thinning is driving Antarctic ice-sheet retreat today. Here we present a multi-proxy data based reconstruction of variability in CDW inflow to the Amundsen Sea sector, the most vulnerable part of the West Antarctic Ice Sheet, during the Holocene epoch (from 11.7 thousand years ago to the present). The chemical compositions of foraminifer shells and benthic foraminifer assemblages in marine sediments indicate that enhanced CDW upwelling, controlled by the latitudinal position of the Southern Hemisphere westerly winds, forced deglaciation of this sector from at least 10,400 years ago until 7,500 years ago-when an ice-shelf collapse may have caused rapid ice-sheet thinning further upstream-and since the 1940s. These results increase confidence in the predictive capability of current ice-sheet models.

Radiocarbon chronology of the last deglaciation in the Baffin Bay reveals asynchronous melting of Arctic and Laurentide ice sheets

NASA Astrophysics Data System (ADS)

Jackson, Rebecca; Kucera, Michal; Vogt, Christoph; Wacker, Lukas

2016-04-01

The transition from the last ice age into the Holocene interglacial was characterised by rapid retreat of North American ice sheets, discharging large quantities of meltwater into the Labrador Sea. Whereas the meltwater chronology of the Laurentide Ice Sheet is well documented, the deglacial history of the American Arctic ice sheets (Inuit Ice sheet and northern Greenland Ice Sheet) draining into the Labrador Sea via the Baffin Bay is less well constrained. Here we present the first high-resolution radiocarbon-dated deglacial records from the Canadian and Greenland margins of the central Baffin Bay. Sedimentological and geochemical data confirm the presence during Termination I of two events of enhanced delivery of detrital carbonate (Baffin Bay Detrital Carbonate Events) dated to 14.2-13.7 ka BP and 12.7-11 ka BP. The events are synchronous across the Baffin Bay and their mineralogical signature indicates a common source of detrital carbonate from the Canadian Arctic, with a synchronous clastic source proximal to Greenland. The events postdate Heinrich layers and their onset is not linked to Greenland temperature change. This indicates that the deglaciation of American Arctic ice sheets and associated meltwater discharge were decoupled from the dominant North Atlantic climate mode.

Pleistocene hydrology of North America: The role of ice sheets in reorganizing groundwater flow systems

NASA Astrophysics Data System (ADS)

Person, Mark; McIntosh, Jennifer; Bense, Victor; Remenda, V. H.

2007-09-01

While the geomorphic consequences of Pleistocene megafloods have been known for some time, it has been only in the past 2 decades that hydrogeologists and glaciologists alike have begun to appreciate the important impact that ice sheet-aquifer interactions have had in controlling subsurface flow patterns, recharge rates, and the distribution of fresh water in confined aquifer systems across North America. In this paper, we document the numerous lines of geochemical, isotopic, and geomechanical evidence of ice sheet hydrogeology across North America. We also review the mechanical, thermal, and hydrologic processes that control subsurface fluid migration beneath ice sheets. Finite element models of subsurface fluid flow, permafrost formation, and ice sheet loading are presented to investigate the coupled nature of transport processes during glaciation/deglaciation. These indicate that recharge rates as high as 10 times modern values occurred as the Laurentide Ice Sheet overran the margins of sedimentary basins. The effects of ice sheet loading and permafrost formation result in complex transient flow patterns within aquifers and confining units alike. Using geochemical and environmental isotopic data, we estimate that the volume of glacial meltwater emplaced at the margins of sedimentary basins overrun by the Laurentide Ice Sheet totals about 3.7 × 104 km3, which is about 0.2% of the volume of the Laurentide Ice Sheet. Subglacial infiltration estimates based on continental-scale hydrologic models are even higher (5-10% of meltwater generated). These studies in sum call into question the widely held notion that groundwater flow patterns within confined aquifer systems are controlled primarily by the water table configuration during the Pleistocene. Rather, groundwater flow patterns were likely much more complex and transient in nature than has previously been thought. Because Pleistocene recharge rates are believed to be highly variable, these studies have profound implications for water resource managers charged with determining sustainable pumping rates from confined aquifers that host ice sheet meltwater.

An ice sheet model validation framework for the Greenland ice sheet.

PubMed

Price, Stephen F; Hoffman, Matthew J; Bonin, Jennifer A; Howat, Ian M; Neumann, Thomas; Saba, Jack; Tezaur, Irina; Guerber, Jeffrey; Chambers, Don P; Evans, Katherine J; Kennedy, Joseph H; Lenaerts, Jan; Lipscomb, William H; Perego, Mauro; Salinger, Andrew G; Tuminaro, Raymond S; van den Broeke, Michiel R; Nowicki, Sophie M J

2017-01-01

We propose a new ice sheet model validation framework - the Cryospheric Model Comparison Tool (CmCt) - that takes advantage of ice sheet altimetry and gravimetry observations collected over the past several decades and is applied here to modeling of the Greenland ice sheet. We use realistic simulations performed with the Community Ice Sheet Model (CISM) along with two idealized, non-dynamic models to demonstrate the framework and its use. Dynamic simulations with CISM are forced from 1991 to 2013 using combinations of reanalysis-based surface mass balance and observations of outlet glacier flux change. We propose and demonstrate qualitative and quantitative metrics for use in evaluating the different model simulations against the observations. We find that the altimetry observations used here are largely ambiguous in terms of their ability to distinguish one simulation from another. Based on basin- and whole-ice-sheet scale metrics, we find that simulations using both idealized conceptual models and dynamic, numerical models provide an equally reasonable representation of the ice sheet surface (mean elevation differences of <1 m). This is likely due to their short period of record, biases inherent to digital elevation models used for model initial conditions, and biases resulting from firn dynamics, which are not explicitly accounted for in the models or observations. On the other hand, we find that the gravimetry observations used here are able to unambiguously distinguish between simulations of varying complexity, and along with the CmCt, can provide a quantitative score for assessing a particular model and/or simulation. The new framework demonstrates that our proposed metrics can distinguish relatively better from relatively worse simulations and that dynamic ice sheet models, when appropriately initialized and forced with the right boundary conditions, demonstrate predictive skill with respect to observed dynamic changes occurring on Greenland over the past few decades. An extensible design will allow for continued use of the CmCt as future altimetry, gravimetry, and other remotely sensed data become available for use in ice sheet model validation.

Ice-sheet contributions to future sea-level change.

PubMed

Gregory, J M; Huybrechts, P

2006-07-15

Accurate simulation of ice-sheet surface mass balance requires higher spatial resolution than is afforded by typical atmosphere-ocean general circulation models (AOGCMs), owing, in particular, to the need to resolve the narrow and steep margins where the majority of precipitation and ablation occurs. We have developed a method for calculating mass-balance changes by combining ice-sheet average time-series from AOGCM projections for future centuries, both with information from high-resolution climate models run for short periods and with a 20km ice-sheet mass-balance model. Antarctica contributes negatively to sea level on account of increased accumulation, while Greenland contributes positively because ablation increases more rapidly. The uncertainty in the results is about 20% for Antarctica and 35% for Greenland. Changes in ice-sheet topography and dynamics are not included, but we discuss their possible effects. For an annual- and area-average warming exceeding 4.5+/-0.9K in Greenland and 3.1+/-0.8K in the global average, the net surface mass balance of the Greenland ice sheet becomes negative, in which case it is likely that the ice sheet would eventually be eliminated, raising global-average sea level by 7m.

Mapping Solid and Liquid Meltwater Retention on the Greenland and Antarctic Ice Sheets from Space

NASA Astrophysics Data System (ADS)

Miller, J.; Bringer, A.; Jezek, K. C.; Johnson, J. T.; Scambos, T.; Forster, R. R.; Long, D. G.

2017-12-01

We use satellite and airborne microwave radiometry to explore the potential for mapping both solid (infiltration ice) and liquid (firn aquifers) meltwater retention on ice sheets. Meltwater retention in firn is currently poorly understood, especially on an ice sheet-scale, however, critical to understanding the ultimate fate of liquid meltwater produced at the surface of ice sheets. Is it contributing to sea level? Or, is it being buffered prior to escaping into the ocean? We previously developed a simple satellite retrieval technique to map firn aquifers on the Greenland ice sheet using distinct L-band brightness temperature signatures that decrease on timescales of months following surface freeze-up, however, similar L-band brightness temperature signatures that decrease on timescales ranging from weeks to days are also present throughout the percolation facies of both the Greenland and Antarctic ice sheets. We hypothesize this characteristic family of temporal signatures represents meltwater retention within firn, where slowly decreasing signatures are characteristic of meltwater retention within perennial firn aquifers, and rapidly decreasing signatures are characteristic of meltwater retention as superimposed ice. Decreasing signatures on timescales between likely represent a continuum of firn characteristics, such as transient firn aquifers, perched firn aquifers, ice layers, ice pipes and lenses, and iced firn. To investigate these temporal signatures, we use L-band (1.4 GHz) brightness temperature observations collected over the Greenland and Antarctic ice sheets by the interferometric MIRAS instrument aboard ESA's Soil Moisture and Ocean Salinity (SMOS) satellite, and the radiometer aboard NASA's Soil Moisture Active Passive (SMAP) satellite. We will also investigate spectral signatures using multi-frequency L-band brightness temperature data (0.5-2 GHz) to be collected over several firn aquifer areas on the Greenland ice sheet by the Ohio State University developed Ultra-Wideband Software-Defined Microwave Radiometer (UWBRAD) as part of our airborne field campaign to be conducted in September 2017.

Modelling large-scale ice-sheet-climate interactions at the last glacial inception

NASA Astrophysics Data System (ADS)

Browne, O. J. H.; Gregory, J. M.; Payne, A. J.; Ridley, J. K.; Rutt, I. C.

2010-05-01

In order to investigate the interactions between coevolving climate and ice-sheets on multimillenial timescales, a low-resolution atmosphere-ocean general circulation model (AOGCM) has been coupled to a three-dimensional thermomechanical ice-sheet model. We use the FAMOUS AOGCM, which is almost identical in formulation to the widely used HadCM3 AOGCM, but on account of its lower resolution (7.5° longitude × 5° latitude in the atmosphere, 3.75°× 2.5° in the ocean) it runs about ten times faster. We use the community ice-sheet model Glimmer at 20 km resolution, with the shallow ice approximation and an annual degree-day scheme for surface mass balance. With the FAMOUS-Glimmer coupled model, we have simulated the growth of the Laurentide and Fennoscandian ice sheets at the last glacial inception, under constant orbital forcing and atmospheric composition for 116 ka BP. Ice grows in both regions, totalling 5.8 m of sea-level equivalent in 10 ka, slower than proxy records suggest. Positive climate feedbacks reinforce this growth at local scales (order hundreds of kilometres), where changes are an order of magnitude larger than on the global average. The albedo feedback (higher local albedo means a cooler climate) is important in the initial expansion of the ice-sheet area. The topography feedback (higher surface means a cooler climate) affects ice-sheet thickness and is not noticeable for the first 1 ka. These two feedbacks reinforce each other. Without them, the ice volume is ~90% less after 10 ka. In Laurentia, ice expands initially on the Canadian Arctic islands. The glaciation of the islands eventually cools the nearby mainland climate sufficiently to produce a positive mass balance there. Adjacent to the ice-sheets, cloud feedbacks tend to reduce the surface mass balance and restrain ice growth; this is an example of a local feedback whose simulation requires a model that includes detailed atmospheric physics.

Understanding Recent Mass Balance Changes of the Greenland Ice Sheet

NASA Technical Reports Server (NTRS)

vanderVeen, Cornelius

2003-01-01

The ultimate goal of this project is to better understand the current transfer of mass between the Greenland Ice Sheet, the world's oceans and the atmosphere, and to identify processes controlling the rate of this transfer, to be able to predict with greater confidence future contributions to global sea level rise. During the first year of this project, we focused on establishing longer-term records of change of selected outlet glaciers, reevaluation of mass input to the ice sheet and analysis of climate records derived from ice cores, and modeling meltwater production and runoff from the margins of the ice sheet.

Ice-Sheet Dynamics and Millennial-Scale Climate Variability in the North Atlantic across the Middle Pleistocene Transition (Invited)

NASA Astrophysics Data System (ADS)

Hodell, D. A.; Nicholl, J.

2013-12-01

During the Middle Pleistocene Transition (MPT), the climate system evolved from a more linear response to insolation forcing in the '41-kyr world' to one that was decidedly non-linear in the '100-kyr world'. Smaller ice sheets in the early Pleistocene gave way to larger ice sheets in the late Pleistocene with an accompanying change in ice sheet dynamics. We studied Sites U1308 (49° 52.7'N, 24° 14.3'W; 3871 m) and U1304 (53° 3.4'N, 33° 31.8'W; 3024 m) in the North Atlantic to determine how ice sheet dynamics and millennial-scale climate variability evolved as glacial boundary conditions changed across the MPT. The frequency of ice-rafted detritus (IRD) in the North Atlantic was greater during glacial stages prior to 650 ka (MIS 16), reflecting more frequent crossing of an ice volume threshold when the climate system spent more time in the 'intermediate ice volume' window, resulting in persistent millennial scale variability. The rarity of Heinrich Events containing detrital carbonate and more frequent occurrence of IRD events prior to 650 ka may indicate the presence of 'low-slung, slippery ice sheets' that flowed more readily than their post-MPT counterparts (Bailey et al., 2010). Ice volume surpassed a critical threshold across the MPT that permitted ice sheets to survive boreal summer insolation maxima, thereby increasing ice volume and thickness, lengthening glacial cycles, and activating the dynamical processes responsible for Laurentide Ice Sheet instability in the region of Hudson Strait (i.e., Heinrich events). The excess ice volume during post-MPT glacial maxima provided a large, unstable reservoir of freshwater to be released to the North Atlantic during glacial terminations with the potential to perturb Atlantic Meridional Overtunring Circulation. We speculate that orbital- and millennial-scale variability co-evolved across the MPT and the interaction of processes on orbital and suborbital time scales gave rise to the changing patterns of glacial-interglacial cycles through the Quaternary. Bailey, I., Bolton, C.T., DeConto, R.M., Pollard, D., Schiebel, R. and Wilson, P.A. (2010) A low threshold for North Atlantic ice rafting from "low-slung slippery" late Pliocene ice sheets. Paleoceanography, 25, PA1212-[14pp]. (doi:10.1029/2009PA001736).

Last Interglacial climate and sea-level evolution from a coupled ice sheet-climate model

NASA Astrophysics Data System (ADS)

Goelzer, Heiko; Huybrechts, Philippe; Loutre, Marie-France; Fichefet, Thierry

2016-12-01

As the most recent warm period in Earth's history with a sea-level stand higher than present, the Last Interglacial (LIG, ˜ 130 to 115 kyr BP) is often considered a prime example to study the impact of a warmer climate on the two polar ice sheets remaining today. Here we simulate the Last Interglacial climate, ice sheet, and sea-level evolution with the Earth system model of intermediate complexity LOVECLIM v.1.3, which includes dynamic and fully coupled components representing the atmosphere, the ocean and sea ice, the terrestrial biosphere, and the Greenland and Antarctic ice sheets. In this setup, sea-level evolution and climate-ice sheet interactions are modelled in a consistent framework.Surface mass balance change governed by changes in surface meltwater runoff is the dominant forcing for the Greenland ice sheet, which shows a peak sea-level contribution of 1.4 m at 123 kyr BP in the reference experiment. Our results indicate that ice sheet-climate feedbacks play an important role to amplify climate and sea-level changes in the Northern Hemisphere. The sensitivity of the Greenland ice sheet to surface temperature changes considerably increases when interactive albedo changes are considered. Southern Hemisphere polar and sub-polar ocean warming is limited throughout the Last Interglacial, and surface and sub-shelf melting exerts only a minor control on the Antarctic sea-level contribution with a peak of 4.4 m at 125 kyr BP. Retreat of the Antarctic ice sheet at the onset of the LIG is mainly forced by rising sea level and to a lesser extent by reduced ice shelf viscosity as the surface temperature increases. Global sea level shows a peak of 5.3 m at 124.5 kyr BP, which includes a minor contribution of 0.35 m from oceanic thermal expansion. Neither the individual contributions nor the total modelled sea-level stand show fast multi-millennial timescale variations as indicated by some reconstructions.

Geological and geomorphological insights into Antarctic ice sheet evolution.

PubMed

Sugden, David E; Bentley, Michael J; O Cofaigh, Colm

2006-07-15

Technical advances in the study of ice-free parts of Antarctica can provide quantitative records that are useful for constraining and refining models of ice sheet evolution and behaviour. Such records improve our understanding of system trajectory, influence the questions we ask about system stability and help to define the ice-sheet processes that are relevant on different time-scales. Here, we illustrate the contribution of cosmogenic isotope analysis of exposed bedrock surfaces and marine geophysical surveying to the understanding of Antarctic ice sheet evolution on a range of time-scales. In the Dry Valleys of East Antarctica, 3He dating of subglacial flood deposits that are now exposed on mountain summits provide evidence of an expanded and thicker Mid-Miocene ice sheet. The survival of surface boulders for approximately 14Myr, the oldest yet measured, demonstrates exceptionally low rates of subsequent erosion and points to the persistence and stability of the dry polar desert climate since that time. Increasingly, there are constraints on West Antarctic ice sheet fluctuations during Quaternary glacial cycles. In the Sarnoff Mountains of Marie Byrd Land in West Antarctica, 10Be and 26Al cosmogenic isotope analysis of glacial erratics and bedrock reveal steady thinning of the ice sheet from 10400 years ago to the present, probably as a result of grounding line retreat. In the Antarctic Peninsula, offshore analysis reveals an extensive ice sheet at the last glacial maximum. Based on radiocarbon dating, deglaciation began by 17000cal yr BP and was complete by 9500cal yr BP. Deglaciation of the west and east sides of the Antarctic Peninsula ice sheet occurred at different times and rates, but was largely complete by the Early Holocene. At that time ice shelves were less extensive on the west side of the Antarctic Peninsula than they are today. The message from the past is that individual glacier drainage basins in Antarctica respond in different and distinctive ways to global climate change, depending on the link between regional topography and climate setting.

Icy Layers and Climate Fluctuations near the Martian North Pole

NASA Image and Video Library

2010-03-31

The Martian north polar layered deposits are an ice sheet much like the Greenland ice sheet on the Earth in this image from NASA Mars Reconnaissance Orbiter. This Martian ice sheet contains many layers that record variations in the Martian climate.

Rapid Access Ice Drill: A New Tool for Exploration of the Deep Antarctic Ice Sheets and Subglacial Geology

NASA Astrophysics Data System (ADS)

Goodge, J. W.; Severinghaus, J. P.

2014-12-01

The Rapid Access Ice Drill (RAID) will penetrate the Antarctic ice sheets in order to core through deep ice, the glacial bed, and into bedrock below. This new technology will provide a critical first look at the interface between major ice caps and their subglacial geology. Currently in construction, RAID is a mobile drilling system capable of making several long boreholes in a single field season in Antarctica. RAID is interdisciplinary and will allow access to polar paleoclimate records in ice >1 Ma, direct observation at the base of the ice sheets, and recovery of rock cores from the ice-covered East Antarctic craton. RAID uses a diamond rock-coring system as in mineral exploration. Threaded drill-pipe with hardened metal bits will cut through ice using reverse circulation of Estisol for pressure-compensation, maintenance of temperature, and removal of ice cuttings. Near the bottom of the ice sheet, a wireline bottom-hole assembly will enable diamond coring of ice, the glacial bed, and bedrock below. Once complete, boreholes will be kept open with fluid, capped, and made available for future down-hole measurement of thermal gradient, heat flow, ice chronology, and ice deformation. RAID will also sample for extremophile microorganisms. RAID is designed to penetrate up to 3,300 meters of ice and take sample cores in less than 200 hours. This rapid performance will allow completion of a borehole in about 10 days before moving to the next drilling site. RAID is unique because it can provide fast borehole access through thick ice; take short ice cores for paleoclimate study; sample the glacial bed to determine ice-flow conditions; take cores of subglacial bedrock for age dating and crustal history; and create boreholes for use as an observatory in the ice sheets. Together, the rapid drilling capability and mobility of the drilling system, along with ice-penetrating imaging methods, will provide a unique 3D picture of the interior Antarctic ice sheets.

Glacial Lake Musselshell: Late Wisconsin slackwater on the Laurentide ice margin in central Montana, USA

USGS Publications Warehouse

Davis, N.K.; Locke, W. W.; Pierce, K.L.; Finkel, R.C.

2006-01-01

Cosmogenic surface exposure ages of glacial boulders deposited in ice-marginal Lake Musselshell suggest that the lake existed between 20 and 11.5 ka during the Late Wisconsin glacial stage (MIS 2), rather than during the Late Illinoian stage (MIS 6) as traditionally thought. The altitude of the highest ice-rafted boulders and the lowest passes on the modern divide indicate that glacial lake water in the Musselshell River basin reached at least 920-930 m above sea level and generally remained below 940 m. Exposures of rhythmically bedded silt and fine sand indicate that Lake Musselshell is best described as a slackwater system, in which the ice-dammed Missouri and Musselshell Rivers rose and fell progressively throughout the existence of the lake rather than establishing a lake surface with a stable elevation. The absence of varves, deltas and shorelines also implies an unstable lake. The changing volume of the lake implies that the Laurentide ice sheet was not stable at its southernmost position in central Montana. A continuous sequence of alternating slackwater lake sediment and lacustrine sheetflood deposits indicates that at least three advances of the Laurentide ice sheet occurred in central Montana between 20 and 11.5 ka. Between each advance, it appears that Lake Musselshell drained to the north and formed two outlet channels that are now occupied by extremely underfit streams. A third outlet formed when the water in Lake Musselshell fully breached the Larb Hills, resulting in the final drainage of the lake. The channel through the Larb Hills is now occupied by the Missouri River, implying that the present Missouri River channel east of the Musselshell River confluence was not created until the Late Wisconsin, possibly as late as 11.5 ka. ?? 2005 Elsevier B.V. All rights reserved.

Abrupt Bølling warming and ice saddle collapse contributions to the Meltwater Pulse 1a rapid sea level rise: North American MWP1a Contribution

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gregoire, Lauren J.; Otto-Bliesner, Bette; Valdes, Paul J.

Elucidating the source(s) of Meltwater Pulse 1a, the largest rapid sea level rise caused by ice melt (14-18 m in less than 340 years, 14,600 years ago), is important for understanding mechanisms of rapid ice melt and the links with abrupt climate change. Here we quantify how much and by what mechanisms the North American ice sheet could have contributed to Meltwater Pulse 1a, by driving an ice sheet model with two transient climate simulations of the last 21,000 years. Ice sheet perturbed physics ensembles were run to account for model uncertainties, constraining ice extent and volume with reconstructions ofmore » 21,000 years ago to present. We determine that the North American ice sheet produced 3-4 m global mean sea level rise in 340 years due to the abrupt Bølling warming, but this response is amplified to 5-6 m when it triggers the ice sheet saddle collapse.« less

Abrupt Bølling warming and ice saddle collapse contributions to the Meltwater Pulse 1a rapid sea level rise: North American MWP1a Contribution

DOE PAGES

Gregoire, Lauren J.; Otto-Bliesner, Bette; Valdes, Paul J.; ...

2016-08-23

Elucidating the source(s) of Meltwater Pulse 1a, the largest rapid sea level rise caused by ice melt (14-18 m in less than 340 years, 14,600 years ago), is important for understanding mechanisms of rapid ice melt and the links with abrupt climate change. Here we quantify how much and by what mechanisms the North American ice sheet could have contributed to Meltwater Pulse 1a, by driving an ice sheet model with two transient climate simulations of the last 21,000 years. Ice sheet perturbed physics ensembles were run to account for model uncertainties, constraining ice extent and volume with reconstructions ofmore » 21,000 years ago to present. We determine that the North American ice sheet produced 3-4 m global mean sea level rise in 340 years due to the abrupt Bølling warming, but this response is amplified to 5-6 m when it triggers the ice sheet saddle collapse.« less

The impact of dynamic topography on the bedrock elevation and volume of the Pliocene Antarctic Ice Sheet

NASA Astrophysics Data System (ADS)

Austermann, Jacqueline; Pollard, David; Mitrovica, Jerry X.; Moucha, Robert; Forte, Alessandro M.; DeConto, Robert M.

2015-04-01

Reconstructions of the Antarctic ice sheet over long timescales (i.e. Myrs) require estimates of bedrock elevation through time. Ice sheet models have accounted, with varying levels of sophistication, for changes in the bedrock elevation due to glacial isostatic adjustment (GIA), but they have neglected other processes that may perturb topography. One notable example is dynamic topography, the deflection of the solid surface of the Earth due to convective flow within the mantle. Numerically predicted changes in dynamic topography have been used to correct paleo shorelines for this departure from eustasy, but the effect of such changes on ice sheet stability is unknown. In this study we use numerical predictions of time-varying dynamic topography to reconstruct bedrock elevation below the Antarctic ice sheet during the mid Pliocene warm period (~3 Ma). Moreover, we couple this reconstruction to a three-dimensional ice sheet model to explore the impact of dynamic topography on the evolution of the Antarctic ice sheet since the Pliocene. Our modeling indicates significant uplift in the area of the Transantarctic Mountains (TAM) and the adjacent Wilkes basin. This predicted uplift, which is at the lower end of geological inferences of uplift of the TAM, implies a lower elevation of the basin in the Pliocene. Relative to simulations that do not include dynamic topography, the lower elevation leads to a smaller Antarctic Ice Sheet volume and a more significant retreat of the grounding line in the Wilkes basin, both of which are consistent with offshore sediment core data. We conclude that reconstructions of the Antarctic Ice Sheet during the mid-Pliocene warm period should be based on bedrock elevation models that include the impact of both GIA and dynamic topography.

Repeated large-scale retreat and advance of Totten Glacier indicated by inland bed erosion.

PubMed

Aitken, A R A; Roberts, J L; van Ommen, T D; Young, D A; Golledge, N R; Greenbaum, J S; Blankenship, D D; Siegert, M J

2016-05-19

Climate variations cause ice sheets to retreat and advance, raising or lowering sea level by metres to decametres. The basic relationship is unambiguous, but the timing, magnitude and sources of sea-level change remain unclear; in particular, the contribution of the East Antarctic Ice Sheet (EAIS) is ill defined, restricting our appreciation of potential future change. Several lines of evidence suggest possible collapse of the Totten Glacier into interior basins during past warm periods, most notably the Pliocene epoch, causing several metres of sea-level rise. However, the structure and long-term evolution of the ice sheet in this region have been understood insufficiently to constrain past ice-sheet extents. Here we show that deep ice-sheet erosion-enough to expose basement rocks-has occurred in two regions: the head of the Totten Glacier, within 150 kilometres of today's grounding line; and deep within the Sabrina Subglacial Basin, 350-550 kilometres from this grounding line. Our results, based on ICECAP aerogeophysical data, demarcate the marginal zones of two distinct quasi-stable EAIS configurations, corresponding to the 'modern-scale' ice sheet (with a marginal zone near the present ice-sheet margin) and the retreated ice sheet (with the marginal zone located far inland). The transitional region of 200-250 kilometres in width is less eroded, suggesting shorter-lived exposure to eroding conditions during repeated retreat-advance events, which are probably driven by ocean-forced instabilities. Representative ice-sheet models indicate that the global sea-level increase resulting from retreat in this sector can be up to 0.9 metres in the modern-scale configuration, and exceeds 2 metres in the retreated configuration.

Antarctic Ice Sheet Discharge Driven by Atmosphere-Ocean Feedbacks Across the Last Glacial Termination

NASA Astrophysics Data System (ADS)

Fogwill, C. J.; Turney, C. S.; Golledge, N. R.; Etheridge, D. M.; Rubino, M.; Thornton, D.; Baker, A.; Weber, M. E.; Woodward, J.; van Ommen, T. D.; Moy, A. D.; Davies, S. M.; Bird, M. I.; Winter, K.; Munksgaard, N.; Menviel, L.; Rootes, C.; Vohra, J.; Rivera, A.; Cooper, A.

2016-12-01

Reconstructing the dynamic response of the Antarctic ice sheets to warming during the Last Glacial Termination (LGT; 18,000-11,650 yrs ago) allows us to identify ice-climate feedbacks that could improve future projections1,2. Whilst the sequence of events during this period are reasonably well-known, relatively poor chronological control has precluded precise alignment of ice, atmospheric and marine records2, making it difficult to assess relationships between Antarctic ice-sheet dynamics, climate change and sea-level rise3-5. Here we present results from a highly-resolved `horizontal ice core'6,7 from the Weddell Sea Embayment, which records millennial-scale ice-sheet dynamics across this extensive sector of Antarctica. Counterintuitively, we find ice-sheet surface drawdown of 600 m across the Antarctic Cold Reversal (ACR; 14,600-12,700 yrs ago)5, with stabilisation during the subsequent millennia of atmospheric warming. Earth system and ice-sheet modelling highlights that this response was likely sustained by strong ocean-ice feedbacks4,8; however, the drivers remain uncertain. Given the coincidence of the ice-sheet changes recorded with marked shifts in atmospheric circulation9,10,11we suggest that millennial-scale Antarctic ice-sheet behaviour was initiated and sustained by global atmospheric teleconnections across the LGT. This has important ramifications ice-sheet stability under contemporary climate change, with changing atmospheric and oceanic circulation patterns. 1 Collins, M. et al. in Climate Change 2013: The Physical Science Basis. 2 Weber, M. E. et al. Nature 510, 134-138, (2014). 3 Weaver, A. J., et al., Science 299, 1709-1713, (2003). 4 Golledge, N. R. et al. Nat Commun 5, (2014). 5 Pedro, J. B. et al. Nature Geosci9. 51-55 (2015). 6 Turney, C. S. M. et al. Journal of Quaternary Science 28, 697-704 (2013). 7 Winter, K. et al. Geophys. Res. Lett.43. 5. 2019-2026 (2016). 8 Menviel, L., A. et al., Quaternary Science Reviews 30, 1155-1172 (2011). 9 Hogg, A. et al. Scientific Reports 6(2016). 10 Hughen, K. A., et al., Radiocarbon 46, 1161-1187 (2004). 11 Anderson, R. F. et al. Science 323, 1443-1448, doi:10.1126/science.1167441 (2009).

Global ice-sheet system interlocked by sea level

NASA Astrophysics Data System (ADS)

Denton, George H.; Hughes, Terence J.; Karlén, Wibjörn

1986-07-01

Denton and Hughes (1983, Quaternary Research20, 125-144) postulated that sea level linked a global ice-sheet system with both terrestrial and grounded marine components during late Quaternary ice ages. Summer temperature changes near Northern Hemisphere melting margins initiated sea-level fluctuations that controlled marine components in both polar hemispheres. It was further proposed that variations of this ice-sheet system amplified and transmitted Milankovitch summer half-year insolation changes between 45 and 75°N into global climatic changes. New tests of this hypothesis implicate sea level as a major control of the areal extent of grounded portions of the Antarctic Ice Sheet, thus fitting the concept of a globally interlocked ice-sheet system. But recent atmospheric modeling results ( Manabe and Broccoli, 1985, Journal of Geophysical Research90, 2167-2190) suggest that factors other than areal changes of the grounded Antarctic Ice Sheet strongly influenced Southern Hemisphere climate and terminated the last ice age simultaneously in both polar hemispheres. Atmospheric carbon dioxide linked to high-latitude oceans is the most likely candidate ( Shackleton and Pisias, 1985, Atmospheric carbon dioxide, orbital forcing, and climate. In "The Carbon Cycle and Atmospheric CO 2: Natural Variations Archean to Present" (E. T. Sundquest and W. S. Broecker, Eds.), pp. 303-318. Geophysical Monograph 32, American Geophysical Union, Washington, D.C.), but another potential influence was high-frequency climatic oscillations (2500 yr). It is postulated that variations in atmospheric carbon dioxide acted through an Antarctic ice shelf linked to the grounded ice sheet to produce and terminate Southern Hemisphere ice-age climate. It is further postulated that Milankovitch summer insolation combined with a warm high-frequency oscillation caused marked recession of Northern Hemisphere ice-sheet melting margins and the North Atlantic polar front about 14,000 14C yr B.P. This permitted renewed formation of North Atlantic Deep Water, which could well have controlled atmospheric carbon dioxide ( W. S. Broecker, D. M. Peteet, and D. Rind, 1985, Nature ( London) 315, 21-26). Combined melting and consequent sea-level rise from the three warming factors initiated irreversible collapse of the interlocked global ice-sheet system, which was at its largest but most vulnerable configuration.

Greenland-Wide Seasonal Temperatures During the Last Deglaciation

NASA Astrophysics Data System (ADS)

Buizert, C.; Keisling, B. A.; Box, J. E.; He, F.; Carlson, A. E.; Sinclair, G.; DeConto, R. M.

2018-02-01

The sensitivity of the Greenland ice sheet to climate forcing is of key importance in assessing its contribution to past and future sea level rise. Surface mass loss occurs during summer, and accounting for temperature seasonality is critical in simulating ice sheet evolution and in interpreting glacial landforms and chronologies. Ice core records constrain the timing and magnitude of climate change but are largely limited to annual mean estimates from the ice sheet interior. Here we merge ice core reconstructions with transient climate model simulations to generate Greenland-wide and seasonally resolved surface air temperature fields during the last deglaciation. Greenland summer temperatures peak in the early Holocene, consistent with records of ice core melt layers. We perform deglacial Greenland ice sheet model simulations to demonstrate that accounting for realistic temperature seasonality decreases simulated glacial ice volume, expedites the deglacial margin retreat, mutes the impact of abrupt climate warming, and gives rise to a clear Holocene ice volume minimum.

The Pliocene-Pleistocene transition and the onset of the Northern Hemisphere glacial inception

NASA Astrophysics Data System (ADS)

Robinson, A.; Calov, R.; Ganopolski, A.

2011-12-01

The Pliocene-Pleistocene transition (PPT, ca. 3.3-2.4 Ma BP) marks a shift in the Earth's climate and is believed to coincide with the inception of the Northern Hemisphere (NH) ice sheets. This transition is not only characterized by a gradual reduction in atmospheric CO2 concentration, paleo records also show a strengthening in the amplitude of δ18O data and intensified ice rafted debris deposition in the North Atlantic. Previous modeling studies have demonstrated that the drop in atmospheric CO2 plays an important role in the glaciation of the NH ice sheets, and more specifically, it is considered to be the primary cause of the glaciation of Greenland. Here we apply a novel approach to produce transient simulations of the entire PPT, in order to study the glaciation of Greenland and the NH ice sheets and additionally, to investigate which conditions are necessary for full-scale glaciation. The fully-coupled Earth system model of intermediate complexity CLIMBER-2 is used to explore the effects of a suite of orbital and CO2 forcing scenarios on total NH glaciation. CLIMBER-2 includes low-resolution sub-models of the atmosphere, vegetation, ocean and ice sheets - the latter is designed to simulate the big NH ice sheets with a rather low resolution (and high computational efficiency). As a refinement, the results of the global simulations are then used to force regional simulations of the Greenland Ice Sheet (GIS) using the higher resolution (20 km) regional climate-ice sheet model, REMBO-SICOPOLIS. We present results of transient simulations driven by orbital forcing and several CO2 reduction scenarios that are consistent with best estimates from data for this time period. We discuss the growth and persistence of the NH ice sheets in terms of the forcing and feedbacks involved. Additionally, we present a set of simulations with the growth of the NH ice sheets disabled, in order to quantify the effect the large ice sheets have on global and regional temperature anomalies. By simulating the Greenland Ice Sheet (GIS) in our high resolution coupled global-regional approach, we identify with greater precision, the conditions neccesary for inception of the GIS and link these to global climatic changes.

Variability of Surface Temperature and Melt on the Greenland Ice Sheet, 2000-2011

NASA Technical Reports Server (NTRS)

Hall, Dorothy K.; Comiso, Josefino, C.; Shuman, Christopher A.; Koenig, Lora S.; DiGirolamo, Nicolo E.

2012-01-01

Enhanced melting along with surface-temperature increases measured using infrared satellite data, have been documented for the Greenland Ice Sheet. Recently we developed a climate-quality data record of ice-surface temperature (IST) of the Greenland Ice Sheet using the Moderate-Resolution Imaging Spectroradiometer (MODIS) 1ST product -- http://modis-snow-ice.gsfc.nasa.gov. Using daily and mean monthly MODIS 1ST maps from the data record we show maximum extent of melt for the ice sheet and its six major drainage basins for a 12-year period extending from March of 2000 through December of 2011. The duration of the melt season on the ice sheet varies in different drainage basins with some basins melting progressively earlier over the study period. Some (but not all) of the basins also show a progressively-longer duration of melt. The short time of the study period (approximately 12 years) precludes an evaluation of statistically-significant trends. However the dataset provides valuable information on natural variability of IST, and on the ability of the MODIS instrument to capture changes in IST and melt conditions indifferent drainage basins of the ice sheet.

Simulating a Dynamic Antarctic Ice Sheet in the Early to Middle Miocene

NASA Astrophysics Data System (ADS)

Gasson, E.; DeConto, R.; Pollard, D.; Levy, R. H.

2015-12-01

There are a variety of sources of geological data that suggest major variations in the volume and extent of the Antarctic ice sheet during the early to middle Miocene. Simulating such variability using coupled climate-ice sheet models is problematic due to a strong hysteresis effect caused by height-mass balance feedback and albedo feedback. This results in limited retreat of the ice sheet once it has reached the continental size, as likely occurred prior to the Miocene. Proxy records suggest a relatively narrow range of atmospheric CO2 during the early to middle Miocene, which exacerbates this problem. We use a new climate forcing which accounts for ice sheet-climate feedbacks through an asynchronous GCM-RCM coupling, which is able to better resolve the narrow Antarctic ablation zone in warm climate simulations. When combined with recently suggested mechanisms for retreat into subglacial basins due to ice shelf hydrofracture and ice cliff failure, we are able to simulate large-scale variability of the Antarctic ice sheet in the Miocene. This variability is equivalent to a seawater oxygen isotope signal of ~0.5 ‰, or a sea level equivalent change of ~35 m, for a range of atmospheric CO2 between 280 - 500 ppm.

Melt-induced speed-up of Greenland ice sheet offset by efficient subglacial drainage.

PubMed

Sundal, Aud Venke; Shepherd, Andrew; Nienow, Peter; Hanna, Edward; Palmer, Steven; Huybrechts, Philippe

2011-01-27

Fluctuations in surface melting are known to affect the speed of glaciers and ice sheets, but their impact on the Greenland ice sheet in a warming climate remains uncertain. Although some studies suggest that greater melting produces greater ice-sheet acceleration, others have identified a long-term decrease in Greenland's flow despite increased melting. Here we use satellite observations of ice motion recorded in a land-terminating sector of southwest Greenland to investigate the manner in which ice flow develops during years of markedly different melting. Although peak rates of ice speed-up are positively correlated with the degree of melting, mean summer flow rates are not, because glacier slowdown occurs, on average, when a critical run-off threshold of about 1.4 centimetres a day is exceeded. In contrast to the first half of summer, when flow is similar in all years, speed-up during the latter half is 62 ± 16 per cent less in warmer years. Consequently, in warmer years, the period of fast ice flow is three times shorter and, overall, summer ice flow is slower. This behaviour is at odds with that expected from basal lubrication alone. Instead, it mirrors that of mountain glaciers, where melt-induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient. A model of ice-sheet flow that captures switching between cavity and channel drainage modes is consistent with the run-off threshold, fast-flow periods, and later-summer speeds we have observed. Simulations of the Greenland ice-sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage; a simple model of basal lubrication alone misses key aspects of the ice sheet's response to climate warming.

Exploring changes in vertical ice extent along the margin of the East Antarctic Ice Sheet in western Dronning Maud Land - initial results of the MAGIC-DML collaboration

NASA Astrophysics Data System (ADS)

Lifton, N. A.; Newall, J. C.; Fredin, O.; Glasser, N. F.; Fabel, D.; Rogozhina, I.; Bernales, J.; Prange, M.; Sams, S.; Eisen, O.; Hättestrand, C.; Harbor, J.; Stroeven, A. P.

2017-12-01

Numerical ice sheet models constrained by theory and refined by comparisons with observational data are a central component of work to address the interactions between the cryosphere and changing climate, at a wide range of scales. Such models are tested and refined by comparing model predictions of past ice geometries with field-based reconstructions from geological, geomorphological, and ice core data. However, on the East Antarctic Ice sheet, there are few empirical data with which to reconstruct changes in ice sheet geometry in the Dronning Maud Land (DML) region. In addition, there is poor control on the regional climate history of the ice sheet margin, because ice core locations, where detailed reconstructions of climate history exist, are located on high inland domes. This leaves numerical models of regional glaciation history in this near-coastal area largely unconstrained. MAGIC-DML is an ongoing Swedish-US-Norwegian-German-UK collaboration with a focus on improving ice sheet models by combining advances in numerical modeling with filling critical data gaps that exist in our knowledge of the timing and pattern of ice surface changes on the western Dronning Maud Land margin. A combination of geomorphological mapping using remote sensing data, field investigations, cosmogenic nuclide surface exposure dating, and numerical ice-sheet modeling are being used in an iterative manner to produce a comprehensive reconstruction of the glacial history of western Dronning Maud Land. We will present an overview of the project, as well as field observations and preliminary in situ cosmogenic nuclide measurements from the 2016/17 expedition.

Modern foraminifera assemblages in the Amundsen Sea Embayment

NASA Astrophysics Data System (ADS)

Ewa Jernas, Patrycja; Kuhn, Gerhard; Hillenbrand, Claus-Dieter; Lander Rasmussen, Tine; Forwick, Matthias; Mackensen, Andreas; Schröder, Michael; Smith, James; Klages, Johann Philipp

2015-04-01

The West Antarctic Ice Sheet (WAIS) is considered the most unstable part of the Antarctic Ice Sheet. As the WAIS is mostly grounded below sea level, its stability is of great concern. A collapse of large parts of the WAIS would result in a significant global sea-level rise. At present, the WAIS shows dramatic ice loss in its Amundsen Sea sector, especially in Pine Island Bay. Pine Island Glacier (PIG) is characterised by fast flow, major thinning and rapid grounding-line retreat. Its mass los over recent decades is generally attributed to melting caused by the inflow of warm Circumpolar Deep Water (CDW). Future melting of PIG may result in a sea level tipping point, because it could trigger widespread collapse of the WAIS, especially when considering ongoing climate change. Our research project aims to establish proxies (integration of foraminifera, sediment properties and oceanographic data) for modern environmental conditions by analysing seafloor surface sediments along a transect from the glacier proximal settings to the middle-outer shelf in the eastern Amundsen Sea Embayment. These proxies will then be applied on sediment records spanning the Holocene back to the Last Glacial Maximum for reconstructing spatial and temporal variations of CDW upwelling and ice-ocean interactions during the past c. 23,000 years. We will present preliminary results from the analyses of ten short marine sediment cores (multi and box cores) collected during expeditions JR179 (2008) and ANT-XXVI/3 (2010) along a transect from inner Pine Island Bay to the middle-outer shelf part of the Abbot Palaeo-Ice Stream Trough at water depths ranging from 458 m (middle shelf) to 1444 m (inner shelf). The sediment cores are currently investigated for distribution patterns of planktonic and benthic foraminifera and grain-size distribution at 1 cm resolution. Core tops (0-10 cm) were stained with Rose Bengal for living benthic foraminifera investigations. The chronology of the cores will be based on 210Pb and calibrated 14C dates. First results reveal the presence of living benthic foraminifera in surface sediments of all investigated cores suggesting that modern seabed surfaces were recovered. Moreover, a core retrieved from a water depth of 793 m in the Abbot Palaeo-Ice Stream Trough shows particularly high abundances of planktonic foraminifera Neogloboquadrina pachyderma.

Sensitivity of grounding line dynamics to viscoelastic deformation of the solid Earth: Inferences from a fully coupled ice sheet - solid Earth model

NASA Astrophysics Data System (ADS)

Konrad, H.; Sasgen, I.; Thoma, M.; Klemann, V.; Grosfeld, K.; Martinec, Z.

2013-12-01

The interactions of ice sheets with the sea level and the solid Earth are important factors for the stability of the ice shelves and the tributary inland ice (e.g. Thomas and Bentley, 1978; Gomez et al, 2012). First, changes in ice extent and ice thickness induce viscoelastic deformation of the Earth surface and Earth's gravity field. In turn, global and local changes in sea level and bathymetry affect the grounding line and, subsequently, alter the ice dynamic behaviour. Here, we investigate these feedbacks for a synthetic ice sheet configuration as well as for the Antarctic ice sheet using a three-dimensional thermomechanical ice sheet and shelf model, coupled to a viscoelastic solid-Earth and gravitationally self-consistent sea-level model. The respective ice sheet undergoes a forcing from rising sea level, warming ocean, and/or changing surface mass balance. The coupling is realized by exchanging ice thickness, Earth surface deformation, and sea level periodically. We apply several sets of viscoelastic Earth parameters to our coupled model, e.g. simulating a low-viscous upper mantle present at the Antarctic Peninsula (Ivins et al., 2011). Special focus of our study lies on the evolution of Earth surface deformation and local sea level changes, as well as on the accompanying grounding line evolution. N. Gomez, D. Pollard, J. X. Mitrovica, P. Huybers, and P. U. Clark 2012. Evolution of a coupled marine ice sheet-sea level model, J. Geophys. Res., 117, F01013, doi:10.1029/2011JF002128. E. R. Ivins, M. M. Watkins, D.-N. Yuan, R. Dietrich, G. Casassa, and A. Rülke 2011. On-land ice loss and glacial isostatic adjustment at the Drake Passage: 2003-2009, J. Geophys. Res. 116, B02403, doi: 10.1029/2010JB007607 R. H. Thomas and C. R. Bentley 1978. A model for Holocene retreat of the West Antarctic Ice Sheet, Quaternary Research, 10 (2), pages 150-170, doi: 10.1016/0033-5894(78)90098-4.

Evaluation of Ice sheet evolution and coastline changes from 1960s in Amery Ice Shelf using multi-source remote sensing images

NASA Astrophysics Data System (ADS)

Qiao, G.; Ye, W.; Scaioni, M.; Liu, S.; Feng, T.; Liu, Y.; Tong, X.; Li, R.

2013-12-01

Global change is one of the major challenges that all the nations are commonly facing, and the Antarctica ice sheet changes have been playing a critical role in the global change research field during the past years. Long time-series of ice sheet observations in Antarctica would contribute to the quantitative evaluation and precise prediction of the effects on global change induced by the ice sheet, of which the remote sensing technology would make critical contributions. As the biggest ice shelf and one of the dominant drainage systems in East Antarctic, the Amery Ice Shelf has been making significant contributions to the mass balance of the Antarctic. Study of Amery Ice shelf changes would advance the understanding of Antarctic ice shelf evolution as well as the overall mass balance. At the same time, as one of the important indicators of Antarctica ice sheet characteristics, coastlines that can be detected from remote sensing imagery can help reveal the nature of the changes of ice sheet evolution. Most of the scientific research on Antarctica with satellite remote sensing dated from 1970s after LANDSAT satellite was brought into operation. It was the declassification of the cold war satellite reconnaissance photographs in 1995, known as Declassified Intelligence Satellite Photograph (DISP) that provided a direct overall view of the Antarctica ice-sheet's configuration in 1960s, greatly extending the time span of Antarctica surface observations. This paper will present the evaluation of ice-sheet evolution and coastline changes in Amery Ice Shelf from 1960s, by using multi-source remote sensing images including the DISP images and the modern optical satellite images. The DISP images scanned from negatives were first interior-oriented with the associated parameters, and then bundle block adjustment technology was employed based on the tie points and control points, to derive the mosaic image of the research region. Experimental results of coastlines generated from DISP images and that from ASTER images were analyzed, and the changes and evolution of Amery ice shelf were then evaluated, following by the discussion of the possible drives.

The Antarctic Ice Sheet during the last Interglaciation: Insights from my Thesis

NASA Astrophysics Data System (ADS)

Whipple, Matthew; Lunt, Dan; Singarayer, Joy; Bradley, Sarah; Milne, Glenn; Wolff, Eric; Siddall, Mark

2015-04-01

The last interglaciation represents a period of warmer climates and higher sea levels, and a useful analogue to future climate. While many studies have focussed on the response of the Greenland Ice sheet, far less is known about the response of the Antarctic ice sheet. Here, I present the summarised results of my PhD thesis "Constraints on the minimum extent of the Antarctic ice sheet during the last interglaciation". Firstly, I cover the timings of interglaciation in Antarctica, and their differences with respect to the Northern Hemisphere timings, based on paleo sea level indicators, and oceanic temperature records. I move on to cover climate forcings, and how they influence the ice sheet, relative to present, and early Holocene. Secondly, I present thesis results, from looking at ice core stable water isotopes. These are compared with Isostatic and Climatic modelling results, for various different Ice sheet scenarios, as to the resulting Climate, from changes in Elevation, Temperature, Precipitation, and Sublimation, all contributing to the recorded stable water isotope record. Thirdly, I move on to looking at the mid-field relative sea level records, from Australia and Argentina. Using isostatic modelling, these are used to assess the relative contribution of the Eastern and Western Antarctic Ice sheets. Although data uncertainties result in us being to identify the contribution from West Antarctica. Overall, using model-data comparison, we find a lack of evidence for a substantial retreat of the Wilkes Subglacial basin. No data location is close enough to determine the existence of the marine based West Antarctic Ice sheet. Model uncertainty is unable to constrain evidence of variations in ice thickness in East Antarctica.

Ice Sheet Roughness Estimation Based on Impulse Responses Acquired in the Global Ice Sheet Mapping Orbiter Mission

NASA Astrophysics Data System (ADS)

Niamsuwan, N.; Johnson, J. T.; Jezek, K. C.; Gogineni, P.

2008-12-01

The Global Ice Sheet Mapping Orbiter (GISMO) mission was developed to address scientific needs to understand the polar ice subsurface structure. This NASA Instrument Incubator Program project is a collaboration between Ohio State University, the University of Kansas, Vexcel Corporation and NASA. The GISMO design utilizes an interferometric SAR (InSAR) strategy in which ice sheet reflected signals received by a dual-antenna system are used to produce an interference pattern. The resulting interferogram can be used to filter out surface clutter so as to reveal the signals scattered from the base of the ice sheet. These signals are further processed to produce 3D-images representing basal topography of the ice sheet. In the past three years, the GISMO airborne field campaigns that have been conducted provide a set of useful data for studying geophysical properties of the Greenland ice sheet. While topography information can be obtained using interferometric SAR processing techniques, ice sheet roughness statistics can also be derived by a relatively simple procedure that involves analyzing power levels and the shape of the radar impulse response waveforms. An electromagnetic scattering model describing GISMO impulse responses has previously been proposed and validated. This model suggested that rms-heights and correlation lengths of the upper surface profile can be determined from the peak power and the decay rate of the pulse return waveform, respectively. This presentation will demonstrate a procedure for estimating the roughness of ice surfaces by fitting the GISMO impulse response model to retrieved waveforms from selected GISMO flights. Furthermore, an extension of this procedure to estimate the scattering coefficient of the glacier bed will be addressed as well. Planned future applications involving the classification of glacier bed conditions based on the derived scattering coefficients will also be described.

Surface water hydrology and the Greenland Ice Sheet

NASA Astrophysics Data System (ADS)

Smith, L. C.; Yang, K.; Pitcher, L. H.; Overstreet, B. T.; Chu, V. W.; Rennermalm, A. K.; Cooper, M. G.; Gleason, C. J.; Ryan, J.; Hubbard, A.; Tedesco, M.; Behar, A.

2016-12-01

Mass loss from the Greenland Ice Sheet now exceeds 260 Gt/year, raising global sea level by >0.7 mm annually. Approximately two-thirds of this total mass loss is now driven by negative ice sheet surface mass balance (SMB), attributed mainly to production and runoff of meltwater from the ice sheet surface. This new dominance of runoff as a driver of GrIS total mass loss will likely persist owing to anticipated further increases in surface melting, reduced meltwater storage in firn, and the waning importance of dynamical mass losses (ice calving) as the ice sheets retreat from their marine-terminating margins. It also creates the need and opportunity for integrative research pairing traditional surface water hydrology approaches with glaciology. As one example, we present a way to measure supraglacial "runoff" (i.e. specific discharge) at the supraglacial catchment scale ( 101-102 km2), using in situ measurements of supraglacial river discharge and high-resolution satellite/drone mapping of upstream catchment area. This approach, which is standard in terrestrial hydrology but novel for ice sheet science, enables independent verification and improvement of modeled SMB runoff estimates used to project sea level rise. Furthermore, because current SMB models do not consider the role of fluvial watershed processes operating on the ice surface, inclusion of even a simple surface routing model materially improves simulations of runoff delivered to moulins, the critical pathways for meltwater entry into the ice sheet. Incorporating principles of surface water hydrology and fluvial geomorphology and into glaciological models will thus aid estimates of Greenland meltwater runoff to the global ocean as well as connections to subglacial hydrology and ice sheet dynamics.

Impacts of polar ice sheets on the East Asian monsoon during the MIS-13 interglacial

NASA Astrophysics Data System (ADS)

Shi, Feng; Yin, Qiuzhen; Nikolova, Irina; Guo, Zhengtang; Berger, Andre

2017-04-01

Among all the interglacials of the last one million years, Marine Isotope Stage (MIS) 13 has the highest δ18O value over the past 800 ka in the deep-sea sediments. This would indicate that MIS-13 is the coolest interglacial if assuming δ18O mainly represents global ice volume. The Antarctic ice core records show also that MIS-13 is the coolest interglacial over Antarctica with almost the lowest greenhouse gases concentrations (GHG). However, many proxy records from the northern hemisphere (NH) indicate that MIS-13 is at least as warm as or even warmer than the recent interglacials, with extremely strong summer monsoon and a possible melting of Greenland ice sheet. In this study, based on proxy reconstructions, different scenarios regarding the size of the Greenland and Antarctic ice sheets are made, and the response of the East Asian summer monsoon to these scenarios are tested by using the models HadCM3 and LOVECLIM as well as factor separation analysis and under the astronomical and GHG configurations of MIS-13. The results show that the influence of the disappearance of Greenland ice sheet on the surface temperature is quite localized, mainly over the northern high latitudinal regions, however, the influence of the bigger southern Hemisphere (SH) ice sheet on the surface temperature is very global, especially in the southern hemisphere. This ice sheet condition has an impact on the precipitation pattern over tropical-subtropical regions. It causes much more summer precipitation over all the East Asian monsoon region, in consistent with the paleosol record from southern China. The scenario of melted Greenland ice sheet and of larger SH ice sheets provides one of the explanations of the strong monsoon rainfall documented by the proxy data.

Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets.

PubMed

Pritchard, Hamish D; Arthern, Robert J; Vaughan, David G; Edwards, Laura A

2009-10-15

Many glaciers along the margins of the Greenland and Antarctic ice sheets are accelerating and, for this reason, contribute increasingly to global sea-level rise. Globally, ice losses contribute approximately 1.8 mm yr(-1) (ref. 8), but this could increase if the retreat of ice shelves and tidewater glaciers further enhances the loss of grounded ice or initiates the large-scale collapse of vulnerable parts of the ice sheets. Ice loss as a result of accelerated flow, known as dynamic thinning, is so poorly understood that its potential contribution to sea level over the twenty-first century remains unpredictable. Thinning on the ice-sheet scale has been monitored by using repeat satellite altimetry observations to track small changes in surface elevation, but previous sensors could not resolve most fast-flowing coastal glaciers. Here we report the use of high-resolution ICESat (Ice, Cloud and land Elevation Satellite) laser altimetry to map change along the entire grounded margins of the Greenland and Antarctic ice sheets. To isolate the dynamic signal, we compare rates of elevation change from both fast-flowing and slow-flowing ice with those expected from surface mass-balance fluctuations. We find that dynamic thinning of glaciers now reaches all latitudes in Greenland, has intensified on key Antarctic grounding lines, has endured for decades after ice-shelf collapse, penetrates far into the interior of each ice sheet and is spreading as ice shelves thin by ocean-driven melt. In Greenland, glaciers flowing faster than 100 m yr(-1) thinned at an average rate of 0.84 m yr(-1), and in the Amundsen Sea embayment of Antarctica, thinning exceeded 9.0 m yr(-1) for some glaciers. Our results show that the most profound changes in the ice sheets currently result from glacier dynamics at ocean margins.

Programme for Monitoring of the Greenland Ice Sheet - Ice Surface Velocities

NASA Astrophysics Data System (ADS)

Andersen, S. B.; Ahlstrom, A. P.; Boncori, J. M.; Dall, J.

2011-12-01

In 2007, the Danish Ministry of Climate and Energy launched the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) as an ongoing effort to assess changes in the mass budget of the Greenland Ice Sheet. Iceberg calving from the outlet glaciers of the Greenland Ice Sheet, often termed the ice-dynamic mass loss, is responsible for an important part of the mass loss during the last decade. To quantify this part of the mass loss, we combine airborne surveys yielding ice-sheet thickness along the entire margin, with surface velocities derived from satellite synthetic-aperture radar (SAR). In order to derive ice sheet surface velocities from SAR a processing chain has been developed for GEUS by DTU Space based on a commercial software package distributed by GAMMA Remote Sensing. The processor, named SUSIE (Scripts and Utilities for SAR Ice-motion Estimation), can use both differential SAR interferometry and offset-tracking techniques to measure the horizontal velocity components, providing also an estimate of the corresponding measurement error. So far surface velocities have been derived for a number of sites including Nioghalvfjerdsfjord Glacier, the Kangerlussuaq region, the Nuuk region, Helheim Glacier and Daugaard-Jensen Glacier using data from ERS-1/ERS-2, ENVISAT ASAR and ALOS Palsar. Here we will present these first results.

Radar attenuation and temperature within the Greenland Ice Sheet

USGS Publications Warehouse

MacGregor, Joseph A; Li, Jilu; Paden, John D; Catania, Ginny A; Clow, Gary D.; Fahnestock, Mark A; Gogineni, Prasad S.; Grimm, Robert E.; Morlighem, Mathieu; Nandi, Soumyaroop; Seroussi, Helene; Stillman, David E

2015-01-01

The flow of ice is temperature-dependent, but direct measurements of englacial temperature are sparse. The dielectric attenuation of radio waves through ice is also temperature-dependent, and radar sounding of ice sheets is sensitive to this attenuation. Here we estimate depth-averaged radar-attenuation rates within the Greenland Ice Sheet from airborne radar-sounding data and its associated radiostratigraphy. Using existing empirical relationships between temperature, chemistry, and radar attenuation, we then infer the depth-averaged englacial temperature. The dated radiostratigraphy permits a correction for the confounding effect of spatially varying ice chemistry. Where radar transects intersect boreholes, radar-inferred temperature is consistently higher than that measured directly. We attribute this discrepancy to the poorly recognized frequency dependence of the radar-attenuation rate and correct for this effect empirically, resulting in a robust relationship between radar-inferred and borehole-measured depth-averaged temperature. Radar-inferred englacial temperature is often lower than modern surface temperature and that of a steady state ice-sheet model, particularly in southern Greenland. This pattern suggests that past changes in surface boundary conditions (temperature and accumulation rate) affect the ice sheet's present temperature structure over a much larger area than previously recognized. This radar-inferred temperature structure provides a new constraint for thermomechanical models of the Greenland Ice Sheet.

Evidence of unfrozen liquids and seismic anisotropy at the base of the polar ice sheets

NASA Astrophysics Data System (ADS)

Wittlinger, Gérard; Farra, Véronique

2015-03-01

We analyze seismic data from broadband stations located on the Antarctic and Greenland ice sheets to determine polar ice seismic velocities. P-to-S converted waves at the ice/rock interface and inside the ice sheets and their multiples (the P-receiver functions) are used to estimate in-situ P-wave velocity (Vp) and P-to-S velocity ratio (Vp/Vs) of polar ice. We find that the polar ice sheets have a two-layer structure; an upper layer of variable thickness (about 2/3 of the total thickness) with seismic velocities close to the standard ice values, and a lower layer of approximately constant thickness with standard Vp but ∼25% smaller Vs. The lower layer ceiling corresponds approximately to the -30 °C isotherm. Synthetic modeling of P-receiver functions shows that strong seismic anisotropy and low vertical S velocity are needed in the lower layer. The seismic anisotropy results from the preferred orientation of ice crystal c-axes toward the vertical. The low vertical S velocity may be due to the presence of unfrozen liquids resulting from premelting at grain joints and/or melting of chemical solutions buried in the ice. The strongly preferred ice crystal orientation fabric and the unfrozen fluids may facilitate polar ice sheet basal flow.

How much can Greenland melt? An upper bound on mass loss from the Greenland Ice Sheet through surface melting

NASA Astrophysics Data System (ADS)

Liu, X.; Bassis, J. N.

2015-12-01

With observations showing accelerated mass loss from the Greenland Ice Sheet due to surface melt, the Greenland Ice Sheet is becoming one of the most significant contributors to sea level rise. The contribution of the Greenland Ice Sheet o sea level rise is likely to accelerate in the coming decade and centuries as atmospheric temperatures continue to rise, potentially triggering ever larger surface melt rates. However, at present considerable uncertainty remains in projecting the contribution to sea level of the Greenland Ice Sheet both due to uncertainty in atmospheric forcing and the ice sheet response to climate forcing. Here we seek an upper bound on the contribution of surface melt from the Greenland to sea level rise in the coming century using a surface energy balance model coupled to an englacial model. We use IPCC Representative Concentration Pathways (RCP8.5, RCP6, RCP4.5, RCP2.6) climate scenarios from an ensemble of global climate models in our simulations to project the maximum rate of ice volume loss and related sea-level rise associated with surface melting. To estimate the upper bound, we assume the Greenland Ice Sheet is perpetually covered in thick clouds, which maximize longwave radiation to the ice sheet. We further assume that deposition of black carbon darkens the ice substantially turning it nearly black, substantially reducing its albedo. Although assuming that all melt water not stored in the snow/firn is instantaneously transported off the ice sheet increases mass loss in the short term, refreezing of retained water warms the ice and may lead to more melt in the long term. Hence we examine both assumptions and use the scenario that leads to the most surface melt by 2100. Preliminary models results suggest that under the most aggressive climate forcing, surface melt from the Greenland Ice Sheet contributes ~1 m to sea level by the year 2100. This is a significant contribution and ignores dynamic effects. We also examined a lower bound, assuming negligible longwave radiation and albedo near the maximum observed for freshly fallen snow. Even under this scenarios preliminary estimates suggest tens of centimeters of sea level rise by 2100.

Insolation-driven 100 kyr glacial cycles and millennial climate change

NASA Astrophysics Data System (ADS)

Abe-Ouchi, A.; Saito, F.; Kawamura, K.; Raymo, M. E.; Okuno, J.; Takahashi, K.; Blatter, H.

2013-12-01

The waxing and waning of Northern Hemisphere ice sheets over the past one million years is dominated by an approximately 100-kyr periodicity and a sawtooth pattern (gradual growth and fast termination). Milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles, and statistical tests demonstrated that the glacial cycles are indeed linked to eccentricity, obliquity and precession cycles. However, insolation alone cannot explain the strong 100 kyr cycle which presumably arises through internal climatic feedbacks. Prior work with conceptual models, for example, showed that glacial terminations are associated with the build-up of Northern Hemisphere 'excess ice', but the physical mechanisms of 100-kyr cycle at work remain unclear. Here, using comprehensive climate and ice sheet models, we show that the ~100-kyr periodicity is explained by insolation and internal feedback amongst the climate, ice sheet and lithosphere/asthenosphere system (reference). We found that equilibrium states of ice sheets exhibit hysteresis responses to summer insolation, and that the shape and position of the hysteresis loop play a key role in determining the periodicities of glacial cycles. The hysteresis loop of the North American ice sheet is such that, after its inception, the ice sheet mass balance remains mostly positive or neutral through several precession cycles whose amplitude decreases towards an eccentricity minimum. The larger the ice sheet grows and extends towards lower latitudes, the smaller is the insolation required to turn the mass balance to negative. Therefore, once the large ice sheet is established, only a moderate increase in insolation can trigger a negative mass balance, leading to a complete retreat within several thousand years, due to the delayed isostatic rebound. The effect of ocean circulation and millennial scale climate change are not playing the dominant role for determing the 100kyr cycle, but are effective for modifying the speed and geographical pattern of the waxing and waning of the Northern Hemisphere ice sheets and their melt water. (reference of the basic results: Abe-Ouchi et al, 2013, Insolation-driven 100,000 year glacial cycles and hysteresis of ice-sheet volume, Nature, 500, 190-193.)

Understanding ice sheet evolution to avoid massive sea level rise instead of experiencing it (Louis Agassiz Medal Lecture)

NASA Astrophysics Data System (ADS)

Rignot, Eric

2017-04-01

With unabated climate warming, massive sea level rise from the melting of ice sheets in Greenland and Antarctica looms at the horizon. This is unfortunately an experiment that we can afford to run only once. Satellite and airborne sensors have significantly helped reveal the magnitude of the mass balance of the ice sheets, where the changes take place, when they started, how they change with time and the nature of the physical processes controlling them. These observations have constrained the maturation of numerical modeling techniques for projecting changes in these ice sheets, including the coupling of ocean and ice sheet models, yet significant uncertainties remain to make these projections directly policy relevant and many challenges remain. I will review the state of balance of the ice sheets as we know it today and the fundamental processes that will drive fast ice sheet retreat and sea level change: ice-ocean interaction and iceberg calving. Ice-ocean interaction are dominated by the wind-forced intrusion of warm, salty, subsurface waters toward the ice sheet periphery to melt ice from below at rates orders of magnitude greater than at the surface. In Greenland, these rates are difficult to observe, but model simulations indicate rates of ice melt along vertical calving faces of meters per day, along with undercutting of the ice faces. Constraining the temperature of the ocean waters from high resolution models and observations, however, remains a significant challenge. I will describe the progress we have made in addressing one major issue which is the mapping of fjord bathymetry around Greenland to define the pathways for warm waters. In Antarctica, the rates of melt are measured from remote sensing data but averaged over long periods, so that we are dependent on in-situ observations to understand the interaction of ocean waters with ice within the sub-ice-shelf cavities. I will describe progress made in mapping the bathymetry of the ice shelves and how the results have impacted our understanding of these interactions. In terms of calving, there is a range of processes acting upon the glacier and ice shelf faces, proceeding from the surface and mostly from below, that are still not sufficiently well explored. I will discuss processes elucidated in Greenland (undercutting and rotation of ice blocks near floatation) and those that are not well known in Antarctica.

Outreach/education interface for Cryosphere models using the Virtual Ice Sheet Laboratory

NASA Astrophysics Data System (ADS)

Larour, E. Y.; Halkides, D. J.; Romero, V.; Cheng, D. L.; Perez, G.

2014-12-01

In the past decade, great strides have been made in the development of models capable of projecting the future evolution of glaciers and the polar ice sheets in a changing climate. These models are now capable of replicating some of the trends apparent in satellite observations. However, because this field is just now maturing, very few efforts have been dedicated to adapting these capabilities to education. Technologies that have been used in outreach efforts in Atmospheric and Oceanic sciences still have not been extended to Cryospheric Science. We present a cutting-edge, technologically driven virtual laboratory, geared towards outreach and k-12 education, dedicated to the polar ice sheets on Antarctica and Greenland, and their role as major contributors to sea level rise in coming decades. VISL (Virtual Ice Sheet Laboratory) relies on state-of-the art Web GL rendering of polar ice sheets, Android/iPhone and web portability using Javascript, as well as C++ simulations (back-end) based on the Ice Sheet System Model, the NASA model for simulating the evolution of polar ice sheets. Using VISL, educators and students can have an immersive experience into the world of polar ice sheets, while at the same exercising the capabilities of a state-of-the-art climate model, all of it embedded into an education experience that follows the new STEM standards for education.This work was performed at the California Institute of Technology's Jet Propulsion Laboratory under a contract with the National Aeronautics and Space Administration's Cryosphere Science Program.

Capabilities and performance of Elmer/Ice, a new-generation ice sheet model

NASA Astrophysics Data System (ADS)

Gagliardini, O.; Zwinger, T.; Gillet-Chaulet, F.; Durand, G.; Favier, L.; de Fleurian, B.; Greve, R.; Malinen, M.; Martín, C.; Råback, P.; Ruokolainen, J.; Sacchettini, M.; Schäfer, M.; Seddik, H.; Thies, J.

2013-08-01

The Fourth IPCC Assessment Report concluded that ice sheet flow models, in their current state, were unable to provide accurate forecast for the increase of polar ice sheet discharge and the associated contribution to sea level rise. Since then, the glaciological community has undertaken a huge effort to develop and improve a new generation of ice flow models, and as a result a significant number of new ice sheet models have emerged. Among them is the parallel finite-element model Elmer/Ice, based on the open-source multi-physics code Elmer. It was one of the first full-Stokes models used to make projections for the evolution of the whole Greenland ice sheet for the coming two centuries. Originally developed to solve local ice flow problems of high mechanical and physical complexity, Elmer/Ice has today reached the maturity to solve larger-scale problems, earning the status of an ice sheet model. Here, we summarise almost 10 yr of development performed by different groups. Elmer/Ice solves the full-Stokes equations, for isotropic but also anisotropic ice rheology, resolves the grounding line dynamics as a contact problem, and contains various basal friction laws. Derived fields, like the age of the ice, the strain rate or stress, can also be computed. Elmer/Ice includes two recently proposed inverse methods to infer badly known parameters. Elmer is a highly parallelised code thanks to recent developments and the implementation of a block preconditioned solver for the Stokes system. In this paper, all these components are presented in detail, as well as the numerical performance of the Stokes solver and developments planned for the future.

An investigation of the astronomical theory of the ice ages using a simple climate-ice sheet model

NASA Technical Reports Server (NTRS)

Pollard, D.

1978-01-01

The astronomical theory of the Quaternary ice ages is incorporated into a simple climate model for global weather; important features of the model include the albedo feedback, topography and dynamics of the ice sheets. For various parameterizations of the orbital elements, the model yields realistic assessments of the northern ice sheet. Lack of a land-sea heat capacity contrast represents one of the chief difficulties of the model.

Laurentide ice-sheet instability during the last deglaciation

NASA Astrophysics Data System (ADS)

Ullman, David J.; Carlson, Anders E.; Anslow, Faron S.; Legrande, Allegra N.; Licciardi, Joseph M.

2015-07-01

Changes in the amount of summer incoming solar radiation (insolation) reaching the Northern Hemisphere are the underlying pacemaker of glacial cycles. However, not all rises in boreal summer insolation over the past 800,000 years resulted in deglaciation to present-day ice volumes, suggesting that there may be a climatic threshold for the disappearance of land-based ice. Here we assess the surface mass balance stability of the Laurentide ice sheet--the largest glacial ice mass in the Northern Hemisphere--during the last deglaciation (24,000 to 9,000 years ago). We run a surface energy balance model with climate data from simulations with a fully coupled atmosphere-ocean general circulation model for key time slices during the last deglaciation. We find that the surface mass balance of the Laurentide ice sheet was positive throughout much of the deglaciation, and suggest that dynamic discharge was mainly responsible for mass loss during this time. Total surface mass balance became negative only in the early Holocene, indicating the transition to a new state where ice loss occurred primarily by surface ablation. We conclude that the Laurentide ice sheet remained a viable ice sheet before the Holocene and began to fully deglaciate only once summer temperatures and radiative forcing over the ice sheet increased by 6-7 °C and 16-20 W m-2, respectively, relative to full glacial conditions.

Modeling the fracture of ice sheets on parallel computers.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Waisman, Haim; Bell, Robin; Keyes, David

2010-03-01

The objective of this project is to investigate the complex fracture of ice and understand its role within larger ice sheet simulations and global climate change. At the present time, ice fracture is not explicitly considered within ice sheet models due in part to large computational costs associated with the accurate modeling of this complex phenomena. However, fracture not only plays an extremely important role in regional behavior but also influences ice dynamics over much larger zones in ways that are currently not well understood. Dramatic illustrations of fracture-induced phenomena most notably include the recent collapse of ice shelves inmore » Antarctica (e.g. partial collapse of the Wilkins shelf in March of 2008 and the diminishing extent of the Larsen B shelf from 1998 to 2002). Other fracture examples include ice calving (fracture of icebergs) which is presently approximated in simplistic ways within ice sheet models, and the draining of supraglacial lakes through a complex network of cracks, a so called ice sheet plumbing system, that is believed to cause accelerated ice sheet flows due essentially to lubrication of the contact surface with the ground. These dramatic changes are emblematic of the ongoing change in the Earth's polar regions and highlight the important role of fracturing ice. To model ice fracture, a simulation capability will be designed centered around extended finite elements and solved by specialized multigrid methods on parallel computers. In addition, appropriate dynamic load balancing techniques will be employed to ensure an approximate equal amount of work for each processor.« less

New details about the LGM extent and subsequent retreat of the West Antarctic Ice Sheet from the easternmost Amundsen Sea Embayment shelf

NASA Astrophysics Data System (ADS)

Klages, J. P.; Hillenbrand, C. D.; Kuhn, G.; Smith, J. A.; Graham, A. G. C.; Nitsche, F. O.; Frederichs, T.; Arndt, J. E.; Gebhardt, C.; Robin, Z.; Uenzelmann-Neben, G.; Gohl, K.; Jernas, P.; Wacker, L.

2017-12-01

In recent years several previously undiscovered grounding-zone wedges (GZWs) have been described within the Abbot-Cosgrove palaeo-ice stream trough on the easternmost Amundsen Sea Embayment shelf. These GZWs document both the Last Glacial Maximum (LGM; 26.5-19 cal. ka BP) grounding-line extent and the subsequent episodic retreat within this trough that neighbors the larger Pine Island-Thwaites trough to the west. Here we combine bathymetric, seismic, and geologic data showing that 1) the grounding line in Abbot Trough did not reach the continental shelf break at any time during the last glacial period, and 2) a prominent stacked GZW constructed from six individual wedges lying upon another was deposited 100 km upstream from the LGM grounding-line position. The available data allow for calculating volumes for most of these individual GZWs and for the entire stack. Sediment cores were recovered seawards from the outermost GZW in the trough, and from the individual wedges of the stacked GZW in order to define the LGM grounding-line extent, and provide minimum grounding-line retreat ages for the respective positions on the stacked GZW. We present implications of a grounded-ice free outer shelf throughout the last glacial period. Furthermore, we assess the significance of the grounding-line stillstand period recorded by the stacked GZW in Abbot Trough for the timing of post-LGM retreat of the West Antarctic Ice Sheet from the Amundsen Sea Embayment shelf.

The Connemara Fan: a major glacial grounding line fan west of Ireland

NASA Astrophysics Data System (ADS)

McCarron, Stephen; Praeg, Daniel; Monteys, Xavier; Scott, Gill

2014-05-01

Glacigenic topography on the mid-shelf (~130-350 m water depth) west of Galway, Ireland appears to have the morphological form, internal architecture and sediments associated with a large glacial grounding-line fan. Seismic data collected in 2009 and 2012 (during the GLAMAR and GATEWAYS 1 campaigns) reveal that the broad, arcuate ridges of the 'Olex moraine' form the landward part of a fan system which prograded beyond the mid-shelf break (defining the outer margin of the 'Clare Platform') westwards into the Porcupine Seabight. The topography is comparable to larger shelf-edge trough-mouth fans found further north along the same margin, however no discernible 'trough' has been identified on the Clare Platform. The ridge and fan topographic assemblage is renamed the 'Connemara Fan' in its entirety, based on its genetic relations and geographic location due west of Connemara, western Ireland. A macrofossil recovered from within a debris flow on the outer fan slope comprised of remobilised plumites dates to ~ 20 ka Cal B.P., indicating sediment reworking downslope following deglacial sediment input to at least that time. The Connemara Fan is the most southerly glacigenic fan identified along the north-east Atlantic margin. Its identification also adds to our knowledge of possibly multiple generations of ice sheets feeding onto the Irish shelf from west-central Ireland and the occurrence of ice sheet geometries and dynamics that evacuated ice, melt-water and sediment (ice streams?) westwards across the Clare Platform during past glaciations.

Abrupt shift in the observed runoff from the southwestern Greenland ice sheet

PubMed Central

Ahlstrøm, Andreas P.; Petersen, Dorthe; Langen, Peter L.; Citterio, Michele; Box, Jason E.

2017-01-01

The recent decades of accelerating mass loss of the Greenland ice sheet have arisen from an increase in both surface meltwater runoff and ice flow discharge from tidewater glaciers. Despite the role of the Greenland ice sheet as the dominant individual cryospheric contributor to sea level rise in recent decades, no observational record of its mass loss spans the 30-year period needed to assess its climatological state. We present for the first time a 40-year (1975–2014) time series of observed meltwater discharge from a >6500-km2 catchment of the southwestern Greenland ice sheet. We find that an abrupt 80% increase in runoff occurring between the 1976–2002 and 2003–2014 periods is due to a shift in atmospheric circulation, with meridional exchange events occurring more frequently over Greenland, establishing the first observation-based connection between ice sheet runoff and climate change. PMID:29242827

Uncertainty quantification of Antarctic contribution to sea-level rise using the fast Elementary Thermomechanical Ice Sheet (f.ETISh) model

NASA Astrophysics Data System (ADS)

Bulthuis, Kevin; Arnst, Maarten; Pattyn, Frank; Favier, Lionel

2017-04-01

Uncertainties in sea-level rise projections are mostly due to uncertainties in Antarctic ice-sheet predictions (IPCC AR5 report, 2013), because key parameters related to the current state of the Antarctic ice sheet (e.g. sub-ice-shelf melting) and future climate forcing are poorly constrained. Here, we propose to improve the predictions of Antarctic ice-sheet behaviour using new uncertainty quantification methods. As opposed to ensemble modelling (Bindschadler et al., 2013) which provides a rather limited view on input and output dispersion, new stochastic methods (Le Maître and Knio, 2010) can provide deeper insight into the impact of uncertainties on complex system behaviour. Such stochastic methods usually begin with deducing a probabilistic description of input parameter uncertainties from the available data. Then, the impact of these input parameter uncertainties on output quantities is assessed by estimating the probability distribution of the outputs by means of uncertainty propagation methods such as Monte Carlo methods or stochastic expansion methods. The use of such uncertainty propagation methods in glaciology may be computationally costly because of the high computational complexity of ice-sheet models. This challenge emphasises the importance of developing reliable and computationally efficient ice-sheet models such as the f.ETISh ice-sheet model (Pattyn, 2015), a new fast thermomechanical coupled ice sheet/ice shelf model capable of handling complex and critical processes such as the marine ice-sheet instability mechanism. Here, we apply these methods to investigate the role of uncertainties in sub-ice-shelf melting, calving rates and climate projections in assessing Antarctic contribution to sea-level rise for the next centuries using the f.ETISh model. We detail the methods and show results that provide nominal values and uncertainty bounds for future sea-level rise as a reflection of the impact of the input parameter uncertainties under consideration, as well as a ranking of the input parameter uncertainties in the order of the significance of their contribution to uncertainty in future sea-level rise. In addition, we discuss how limitations posed by the available information (poorly constrained data) pose challenges that motivate our current research.

The Ice Sheet Mass Balance Inter-comparison Exercise

NASA Astrophysics Data System (ADS)

Shepherd, A.; Ivins, E. R.

2015-12-01

Fluctuations in the mass of ice stored in Antarctica and Greenland are of considerable societal importance. The Ice Sheet Mass Balance Inter-Comparison Exercise (IMBIE) is a joint-initiative of ESA and NASA aimed at producing a single estimate of the global sea level contribution to polar ice sheet losses. Within IMBIE, estimates of ice sheet mass balance are developed from a variety of satellite geodetic techniques using a common spatial and temporal reference frame and a common appreciation of the contributions due to external signals. The project brings together the laboratories and space agencies that have been instrumental in developing independent estimates of ice sheet mass balance to date. In its first phase, IMBIE involved 27 science teams, and delivered a first community assessment of ice sheet mass imbalance to replace 40 individual estimates. The project established that (i) there is good agreement between the three main satellite-based techniques for estimating ice sheet mass balance, (ii) combining satellite data sets leads to significant improvement in certainty, (iii) the polar ice sheets contributed 11 ± 4 mm to global sea levels between 1992 and 2012, and (iv) that combined ice losses from Antarctica and Greenland have increased over time, rising from 10% of the global trend in the early 1990's to 30% in the late 2000's. Demand for an updated assessment has grown, and there are now new satellite missions, new geophysical corrections, new techniques, and new teams producing data. The period of overlap between independent satellite techniques has increased from 5 to 12 years, and the full period of satellite data over which an assessment can be performed has increased from 19 to 40 years. It is also clear that multiple satellite techniques are required to confidently separate mass changes associated with snowfall and ice dynamical imbalance - information that is of critical importance for climate modelling. This presentation outlines the approach for the second phase of IMBIE, including the project organisation, the work programme and schedule, the main science goals, and its current status, and reviews the recent and historical contributions that the Antarctic and Greenland ice sheets have made to global sea level rise.

Sea-level feedback lowers projections of future Antarctic Ice-Sheet mass loss

PubMed Central

Gomez, Natalya; Pollard, David; Holland, David

2015-01-01

The stability of marine sectors of the Antarctic Ice Sheet (AIS) in a warming climate has been identified as the largest source of uncertainty in projections of future sea-level rise. Sea-level fall near the grounding line of a retreating marine ice sheet has a stabilizing influence on the ice sheets, and previous studies have established the importance of this feedback on ice age AIS evolution. Here we use a coupled ice sheet–sea-level model to investigate the impact of the feedback mechanism on future AIS retreat over centennial and millennial timescales for a range of emission scenarios. We show that the combination of bedrock uplift and sea-surface drop associated with ice-sheet retreat significantly reduces AIS mass loss relative to a simulation without these effects included. Sensitivity analyses show that the stabilization tends to be greatest for lower emission scenarios and Earth models characterized by a thin elastic lithosphere and low-viscosity upper mantle, as is the case for West Antarctica. PMID:26554381

Annual accumulation over the Greenland ice sheet interpolated from historical and newly compiled observation data

USGS Publications Warehouse

Shen, Dayong; Liu, Yuling; Huang, Shengli

2012-01-01

The estimation of ice/snow accumulation is of great significance in quantifying the mass balance of ice sheets and variation in water resources. Improving the accuracy and reducing uncertainty has been a challenge for the estimation of annual accumulation over the Greenland ice sheet. In this study, we kriged and analyzed the spatial pattern of accumulation based on an observation data series including 315 points used in a recent research, plus 101 ice cores and snow pits and newly compiled 23 coastal weather station data. The estimated annual accumulation over the Greenland ice sheet is 31.2 g cm−2 yr−1, with a standard error of 0.9 g cm−2 yr−1. The main differences between the improved map developed in this study and the recently published accumulation maps are in the coastal areas, especially southeast and southwest regions. The analysis of accumulations versus elevation reveals the distribution patterns of accumulation over the Greenland ice sheet.

An Antarctic stratigraphic record of step-wise ice-sheet growth through the Eocene-Oligocene transition

NASA Astrophysics Data System (ADS)

Passchier, S.; Ciarletta, D. J.; Miriagos, T.; Bijl, P.; Bohaty, S. M.

2016-12-01

The Antarctic cryosphere plays a critical role in the ocean-atmosphere system, but its early evolution is still poorly known. With a near-field record from Prydz Bay, Antarctica, we conclude that Antarctic continental ice-sheet growth commenced with the EOT-1 "precursor" glaciation, during a time of Subantarctic surface ocean cooling and a decline in atmospheric pCO2. Prydz Bay lies downstream of a major East Antarctic ice-sheet drainage system and the Gamburtsev Mountains, a likely nucleation point for the first ice sheets. Its sedimentary records uniquely constrain the timing of ice-sheet advance onto the continental shelf. We investigate a detrital record extracted from three Ocean Drilling Program drill holes in Prydz Bay within a new depositional and chronological framework spanning the late Eocene to early Oligocene ( 36-33 Ma). The chemical index of alteration (CIA) and the S-index, calculated from the major element geochemistry of bulk samples, yield estimates of chemical weathering intensities and mean annual temperature (MAT) on the East Antarctic continent. We document evidence for late Eocene mountain glaciation along with transient warm events at 35.8-34.8 Ma. These data and our sedimentological analyses confirm the presence of ephemeral mountain glaciers on East Antarctica during the late Eocene between 35.9 and 34.4 Ma. Furthermore, we document the stepwise climate cooling of the Antarctic hinterland from 34.4 Ma as the ice sheet advanced towards the edges of the continent during EOT-1. The youngest part of our data set correlates to the time interval of the Oi-1 glaciation, when the ice-sheet in Prydz Bay extended to the outer shelf. Cooling and ice growth on Antarctica was spatially variable and ice sheets formed under declining pCO2. These results point to complex ice sheet - atmosphere - ocean - solid-earth feedbacks.

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

USGS Publications Warehouse

Naish, T.; Powell, R.; Levy, R.; Wilson, G.; Scherer, R.; Talarico, F.; Krissek, L.; Niessen, F.; Pompilio, M.; Wilson, T.; Carter, L.; DeConto, R.; Huybers, P.; McKay, R.; Pollard, D.; Ross, J.; Winter, D.; Barrett, P.; Browne, G.; Cody, R.; Cowan, E.; Crampton, J.; Dunbar, G.; Dunbar, N.; Florindo, F.; Gebhardt, C.; Graham, I.; Hannah, M.; Hansaraj, D.; Harwood, D.; Helling, D.; Henrys, S.; Hinnov, L.; Kuhn, G.; Kyle, P.; Laufer, A.; Maffioli, P.; Magens, D.; Mandernack, K.; McIntosh, W.; Millan, C.; Morin, R.; Ohneiser, C.; Paulsen, T.; Persico, D.; Raine, I.; Reed, J.; Riesselman, C.; Sagnotti, L.; Schmitt, D.; Sjunneskog, C.; Strong, P.; Taviani, M.; Vogel, S.; Wilch, T.; Williams, T.

2009-01-01

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth's orbital geometry control the ice ages1, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles2. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the 'warmer-than-present' early-Pliocene epoch (5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming3. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, 40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth's axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to 3 °C warmer than today4 and atmospheric CO2 concentration was as high as 400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model7 that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt8 under conditions of elevated CO2.

Obliquity-paced Pliocene West Antarctic ice sheet oscillations.

PubMed

Naish, T; Powell, R; Levy, R; Wilson, G; Scherer, R; Talarico, F; Krissek, L; Niessen, F; Pompilio, M; Wilson, T; Carter, L; DeConto, R; Huybers, P; McKay, R; Pollard, D; Ross, J; Winter, D; Barrett, P; Browne, G; Cody, R; Cowan, E; Crampton, J; Dunbar, G; Dunbar, N; Florindo, F; Gebhardt, C; Graham, I; Hannah, M; Hansaraj, D; Harwood, D; Helling, D; Henrys, S; Hinnov, L; Kuhn, G; Kyle, P; Läufer, A; Maffioli, P; Magens, D; Mandernack, K; McIntosh, W; Millan, C; Morin, R; Ohneiser, C; Paulsen, T; Persico, D; Raine, I; Reed, J; Riesselman, C; Sagnotti, L; Schmitt, D; Sjunneskog, C; Strong, P; Taviani, M; Vogel, S; Wilch, T; Williams, T

2009-03-19

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth's orbital geometry control the ice ages, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the 'warmer-than-present' early-Pliocene epoch ( approximately 5-3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, approximately 40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth's axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to approximately 3 degrees C warmer than today and atmospheric CO(2) concentration was as high as approximately 400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt under conditions of elevated CO(2).

Diverse landscapes beneath Pine Island Glacier influence ice flow.

PubMed

Bingham, Robert G; Vaughan, David G; King, Edward C; Davies, Damon; Cornford, Stephen L; Smith, Andrew M; Arthern, Robert J; Brisbourne, Alex M; De Rydt, Jan; Graham, Alastair G C; Spagnolo, Matteo; Marsh, Oliver J; Shean, David E

2017-11-20

The retreating Pine Island Glacier (PIG), West Antarctica, presently contributes ~5-10% of global sea-level rise. PIG's retreat rate has increased in recent decades with associated thinning migrating upstream into tributaries feeding the main glacier trunk. To project future change requires modelling that includes robust parameterisation of basal traction, the resistance to ice flow at the bed. However, most ice-sheet models estimate basal traction from satellite-derived surface velocity, without a priori knowledge of the key processes from which it is derived, namely friction at the ice-bed interface and form drag, and the resistance to ice flow that arises as ice deforms to negotiate bed topography. Here, we present high-resolution maps, acquired using ice-penetrating radar, of the bed topography across parts of PIG. Contrary to lower-resolution data currently used for ice-sheet models, these data show a contrasting topography across the ice-bed interface. We show that these diverse subglacial landscapes have an impact on ice flow, and present a challenge for modelling ice-sheet evolution and projecting global sea-level rise from ice-sheet loss.

Collapse of the West Antarctic Ice Sheet after local destabilization of the Amundsen Basin

PubMed Central

Feldmann, Johannes; Levermann, Anders

2015-01-01

The future evolution of the Antarctic Ice Sheet represents the largest uncertainty in sea-level projections of this and upcoming centuries. Recently, satellite observations and high-resolution simulations have suggested the initiation of an ice-sheet instability in the Amundsen Sea sector of West Antarctica, caused by the last decades’ enhanced basal ice-shelf melting. Whether this localized destabilization will yield a full discharge of marine ice from West Antarctica, associated with a global sea-level rise of more than 3 m, or whether the ice loss is limited by ice dynamics and topographic features, is unclear. Here we show that in the Parallel Ice Sheet Model, a local destabilization causes a complete disintegration of the marine ice in West Antarctica. In our simulations, at 5-km horizontal resolution, the region disequilibrates after 60 y of currently observed melt rates. Thereafter, the marine ice-sheet instability fully unfolds and is not halted by topographic features. In fact, the ice loss in Amundsen Sea sector shifts the catchment's ice divide toward the Filchner–Ronne and Ross ice shelves, which initiates grounding-line retreat there. Our simulations suggest that if a destabilization of Amundsen Sea sector has indeed been initiated, Antarctica will irrevocably contribute at least 3 m to global sea-level rise during the coming centuries to millennia. PMID:26578762