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.

Dynamic behaviour of ice streams: the North East Greenland Ice Stream

NASA Astrophysics Data System (ADS)

Bons, Paul D.; Jansen, Daniela; Schaufler, Svenja; de Riese, Tamara; Sachau, Till; Weikusat, Ilka

2017-04-01

The flow of ice towards the margins of ice sheets is far from homogeneous. Ice streams show much higher flow velocities than their surroundings and may extend, for example the North East Greenland Ice Stream (NEGIS), towards the centre of the sheet. The elevated flow velocity inside an ice stream causes marginal shearing and convergent flow, which in turn leads to folding of ice layers. Such folding was documented in the Petermann Glacier in northern Greenland (Bons et al., 2016). 3-dimensional structural modelling using radargrams shows that folding is more intense adjacent to NEGIS than inside it, despite the strong flow perturbation at NEGIS. Analysis of fold amplitude as a function of stratigraphic level indicates that folding adjacent to NEGIS ceased in the early Holocene, while it is currently active inside NEGIS. The presence of folds adjacent of NEGIS, but also at other sites far in the interior of the Greenland Ice Sheet with no direct connection to the present-day surface velocity field, indicates that ice flow is not only heterogeneous in space (as the present-day flow velocity field shows), but also in time. The observations suggest that ice streams are dynamic, ephemeral structures that emerge and die out, and may possibly shift during their existence, but leave traces within the stratigraphic layering of the ice. The dynamic nature of ice streams such as NEGIS speaks against deterministic models for their accelerated flow rates, such as bedrock topography or thermal perturbations at their base. Instead, we suggest that ice streams can also result from strain localisation induced inside the ice sheet by the complex coupling of rheology, anisotropy, grain-size changes and possibly shear heating. Bons, P.D., Jansen, D., Mundel, F., Bauer, C.C., Binder, T., Eisen, O., Jessell, M.W., Llorens, M.-G, Steinbach, F., Steinhage, D. & Weikusat, I. 2016. Converging flow and anisotropy cause large-scale folding in Greenland's ice sheet. Nature Communications 7

Ice Flow in the North East Greenland Ice Stream

NASA Technical Reports Server (NTRS)

Joughin, Ian; Kwok, Ron; Fahnestock, M.; MacAyeal, Doug

1999-01-01

Early observations with ERS-1 SAR image data revealed a large ice stream in North East Greenland (Fahnestock 1993). The ice stream has a number of the characteristics of the more closely studied ice streams in Antarctica, including its large size and gross geometry. The onset of rapid flow close to the ice divide and the evolution of its flow pattern, however, make this ice stream unique. These features can be seen in the balance velocities for the ice stream (Joughin 1997) and its outlets. The ice stream is identifiable for more than 700 km, making it much longer than any other flow feature in Greenland. Our research goals are to gain a greater understanding of the ice flow in the northeast Greenland ice stream and its outlet glaciers in order to assess their impact on the past, present, and future mass balance of the ice sheet. We will accomplish these goals using a combination of remotely sensed data and ice sheet models. We are using satellite radar interferometry data to produce a complete maps of velocity and topography over the entire ice stream. We are in the process of developing methods to use these data in conjunction with existing ice sheet models similar to those that have been used to improve understanding of the mechanics of flow in Antarctic ice streams.

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.

Stochastic ice stream dynamics

PubMed Central

Bertagni, Matteo Bernard; Ridolfi, Luca

2016-01-01

Ice streams are narrow corridors of fast-flowing ice that constitute the arterial drainage network of ice sheets. Therefore, changes in ice stream flow are key to understanding paleoclimate, sea level changes, and rapid disintegration of ice sheets during deglaciation. The dynamics of ice flow are tightly coupled to the climate system through atmospheric temperature and snow recharge, which are known exhibit stochastic variability. Here we focus on the interplay between stochastic climate forcing and ice stream temporal dynamics. Our work demonstrates that realistic climate fluctuations are able to (i) induce the coexistence of dynamic behaviors that would be incompatible in a purely deterministic system and (ii) drive ice stream flow away from the regime expected in a steady climate. We conclude that environmental noise appears to be crucial to interpreting the past behavior of ice sheets, as well as to predicting their future evolution. PMID:27457960

Bedrock structure and the interpretation of palaeo ice stream footprints: examples from the Pleistocene British Ice Sheet

NASA Astrophysics Data System (ADS)

Krabbendam, M.; Bradwell, T.

2009-04-01

To model past and future behaviour of ice sheets, a good understanding of both modern and ancient ice streams is required. The study of present-day ice streams provides detailed data of short-term dynamic changes, whilst the study of Pleistocene palaeo-ice streams can provide crucial constraints on the longer-term evolution of ice sheets. To date, palaeo-ice streams, such as the classical Dubawnt Lake palaeo-ice stream of the former Laurentide Ice Sheet, have been recognised largely on the basis of extremely elongate drumlins and megascale glacial lineations; all soft-sediment features. Whilst it appears that topographically unconstrained ice streams (eg. within the West Antarctic Ice Sheet) are generally underlain by deformable till, topographically constrained ice streams such as Jakobshavn Isbrae do not require deformable sediment and may occur on a bedrock-dominated bed. Analysis of DEM data and geomorphology and structural geology fieldwork in Northern Scotland and Northern England has shown the occurrence of highly streamlined bedforms in bedrock of the former base of topographically controlled palaeo-ice streams, which drained parts of the British Ice Sheet. The bedforms are predominantly bedrock megagrooves with asymmetric cross-profiles. In the Ullapool tributary of the Minch palaeo ice stream, bedrock megagrooves form the dominant evidence for ice streaming. The megagrooves are typically 5-15 m deep, 10-30 m wide and 500 - 3000 m long. Spacing of megagrooves is typically 100 - 200 m. In both study areas, the bedrock is strongly anisotropic, either consisting of thin-bedded strata or strongly foliated metasedimentary rocks, with the strata or foliation having a gentle dip. Megagrooves are best developed where the strike of the anisotropy is sub-parallel (within 10 - 20°) with palaeo ice flow. The bedrock in both areas has a well-developed, relatively densely spaced (< 1m), conjugate joint system. We suggest that asymmetric megagrooves are formed by

Patterned basal seismicity shows sub-ice stream bedforms

NASA Astrophysics Data System (ADS)

Barcheck, C. G.; Tulaczyk, S. M.; Schwartz, S. Y.

2017-12-01

Patterns in seismicity emanating from the bottom of fast-moving ice streams and glaciers may indicate localized patches of higher basal resistance— sometimes called 'sticky spots', or otherwise varying basal properties. These seismogenic basal areas resist an unknown portion of the total driving stress of the Whillans Ice Plain (WIP), in West Antarctica, but may play an important role in the WIP stick-slip cycle and ice stream slowdown. To better understand the mechanism and importance of basal seismicity beneath the WIP, we analyze seismic data collected by a small aperture (< 3km) network of 8 surface and 5 borehole seismometers installed in the main central sticky spot of the WIP. We use a network beamforming technique to detect and roughly locate thousands of small (magnitude < 0), local basal micro-earthquakes in Dec 2014, and we compare the resulting map of seismicity to ice bottom depth measured by airborne radar. The number of basal earthquakes per area within the network is spatially heterogeneous, but a pattern of two 400m wide streaks of high seismicity rates is evident, with >50-500 earthquakes detected per 50x50m grid cell in 2 weeks. These seismically active streaks are elongated approximately in the ice flow direction with a spacing of 750m. Independent airborne radar measurements of ice bottom depth from Jan 2013 show a low-amplitude ( 5m) undulation in the basal topography superposed on a regional gradient in ice bottom depth. The flow-perpendicular wavelength of these low-amplitude undulations is comparable to the spacing of the high seismicity bands, and the streaks of high seismicity intersect local lows in the undulating basal topography. We interpret these seismic and radar observations as showing seismically active sub-ice stream bedforms that are low amplitude and elongated in the direction of ice flow, comparable to the morphology of mega scale glacial lineations (MSGLs), with high basal seismicity rates observed in the MSGL troughs

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.

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.

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.

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.

Interplay of grounding-line dynamics and sub-shelf melting during retreat of the Bjørnøyrenna Ice Stream.

PubMed

Petrini, Michele; Colleoni, Florence; Kirchner, Nina; Hughes, Anna L C; Camerlenghi, Angelo; Rebesco, Michele; Lucchi, Renata G; Forte, Emanuele; Colucci, Renato R; Noormets, Riko

2018-05-08

The Barents Sea Ice Sheet was a marine-based ice sheet, i.e., it rested on the Barents Sea floor during the Last Glacial Maximum (21 ky BP). The Bjørnøyrenna Ice Stream was the largest ice stream draining the Barents Sea Ice Sheet and is regarded as an analogue for contemporary ice streams in West Antarctica. Here, the retreat of the Bjørnøyrenna Ice Stream is simulated by means of two numerical ice sheet models and results assessed against geological data. We investigate the sensitivity of the ice stream to changes in ocean temperature and the impact of grounding-line physics on ice stream retreat. Our results suggest that the role played by sub-shelf melting depends on how the grounding-line physics is represented in the models. When an analytic constraint on the ice flux across the grounding line is applied, the retreat of Bjørnøyrenna Ice Stream is primarily driven by internal ice dynamics rather than by oceanic forcing. This suggests that implementations of grounding-line physics need to be carefully assessed when evaluating and predicting the response of contemporary marine-based ice sheets and individual ice streams to ongoing and future ocean warming.

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.

The dynamics of climate-induced deglacial ice stream acceleration

NASA Astrophysics Data System (ADS)

Robel, A.; Tziperman, E.

2015-12-01

Geological observations indicate that ice streams were a significant contributor to ice flow in the Laurentide Ice Sheet during the Last Glacial Maximum. Conceptual and simple model studies have also argued that the gradual development of ice streams increases the sensitivity of large ice sheets to weak climate forcing. In this study, we use an idealized configuration of the Parallel Ice Sheet Model to explore the role of ice streams in rapid deglaciation. In a growing ice sheet, ice streams develop gradually as the bed warms and the margin expands outward onto the continental shelf. Then, a weak change in equilibrium line altitude commensurate with Milankovitch forcing results in a rapid deglacial response, as ice stream acceleration leads to enhanced calving and surface melting at low elevations. We explain the dynamical mechanism that drives this ice stream acceleration and its broader applicability as a feedback for enhancing ice sheet decay in response to climate forcing. We show how our idealized ice sheet simulations match geomorphological observations of deglacial ice stream variability and previous model-data analyses. We conclude with observations on the potential for interaction between ice streams and other feedback mechanisms within the earth system.

Controls on bedrock bedform development at the base of the Uummannaq Ice Stream System, West Greenland

NASA Astrophysics Data System (ADS)

Lane, Tim; Roberts, David; Rea, Brice; Cofaigh, Colm Ó.

2014-05-01

This research investigates the glacial and non-glacial controls on glacially eroded bedrock bedforms beneath the topographically confined upstream fjord region of the Uummannaq Ice Stream (UIS), West Greenland. The UIS was a cross-shelf ice stream system that operated during the Last Glacial Maximum (LGM), formed of 10 coalescent outlet glaciers. Reconstructions suggest that palaeo-glaciological conditions were similar for all sites in the study, characterised by thick, fast flowing ice moving over a rigid bedrock bed. Areally scoured terrain were mapped using remotely sensed imagery to assess regional-scale patterns of glacial erosion and to select suitable field locations. In the field, bedform measurements were taken from four discrete areas within two neighbouring fjords in the northern Uummannaq region (Rink-Karrat and Ingia). Classic bedrock bedforms indicative of glacially eroded terrain were mapped, including p-forms, roche moutonnées, and whalebacks. Bedform long axes and plucked face orientations display close correlation with palaeo-ice flow directions inferred from striae measurements. Across all sites, elongation ratios (length to width) varied by an order of magnitude between 0.8:1 and 8.4:1. Bedform properties (length, height, width, and long axis orientation) from the four sample areas form individual morphometrically distinct populations. However, bedform populations display high inter-area variability despite their close proximity, and hypothesised similarity in palaeo-glaciological conditions. The relationship of bedforms to palaeo-glaciological conditions in this study is not simple, having been complicated by bedrock properties. Geological structures including: joint frequency; joint dip; joint orientation; bedding plane thickness; and bedding plane dip have provided lines of geological weakness along which glacial erosion has been able to focus, controlling bedform length and width. Lateral plucking, a mechanism previously described for the

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.

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.

Meteorological Drivers of West Antarctic Ice Sheet and Ice Shelf Surface Melt

NASA Astrophysics Data System (ADS)

Scott, R. C.; Nicolas, J. P.; Bromwich, D. H.; Norris, J. R.; Lubin, D.

2017-12-01

We identify synoptic patterns and surface energy balance components driving warming and surface melting on the West Antarctic Ice Sheet (WAIS) and ice shelves using reanalysis and satellite remote sensing data from 1973-present. We have developed a synoptic climatology of atmospheric circulation patterns during the summer melt season using k-means cluster and composite analysis of daily 700-mb geopotential height and near-surface air temperature and wind fields from the ECMWF ERA-Interim reanalysis. Surface melt occurrence is detected in satellite passive microwave brightness temperature observations (K-band, horizontal polarization) beginning with the NASA Nimbus-5 Electrically Scanning Microwave Radiometer (ESMR) and continuing with its more familiar descendants SMMR, SSM/I and SSMIS. To diagnose synoptic precursors and physical processes driving surface melt we combine the circulation climatology and multi-decadal records of cloud cover with surface radiative fluxes from the Extended AVHRR Polar Pathfinder (APP-x) project. We identify three distinct modes of regional summer West Antarctic warming since 1979 involving anomalous ridging over West Antarctica (WA) and the Amundsen Sea (AS). During the 1970s, ESMR data reveal four extensive melt events on the Ross Sea sector of the WAIS also linked to AS blocking. We therefore define an Amundsen Sea Blocking Index (ASBI). The ASBI and synoptic circulation pattern occurrence frequencies are correlated with the tropical Pacific (ENSO) and high latitude Southern Annular Mode (SAM) indices and the West Antarctic melt index. Surface melt in WA is favored by enhanced downwelling infrared and turbulent sensible heat fluxes associated with intrusions of warm, moist marine air. Consistent with recent findings from the Atmospheric Radiation Measurement (ARM) West Antarctic Radiation Experiment (AWARE), marine advection to the Ross sector is favored by El Niño conditions in the tropical Pacific and a negative SAM. We also find

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.

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

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

Using the glacial geomorphology of palaeo-ice streams to understand mechanisms of ice sheet collapse

NASA Astrophysics Data System (ADS)

Stokes, Chris R.; Margold, Martin; Clark, Chris; Tarasov, Lev

2017-04-01

Processes which bring about ice sheet deglaciation are critical to our understanding of glacial-interglacial cycles and ice sheet sensitivity to climate change. The precise mechanisms of deglaciation are also relevant to our understanding of modern-day ice sheet stability and concerns over global sea level rise. Mass loss from ice sheets can be broadly partitioned between melting and a 'dynamic' component whereby rapidly-flowing ice streams/outlet glaciers transfer ice from the interior to the oceans. Surface and basal melting (e.g. of ice shelves) are closely linked to atmospheric and oceanic conditions, but the mechanisms that drive dynamic changes in ice stream discharge are more complex, which generates much larger uncertainties about their future contribution to ice sheet mass loss and sea level rise. A major problem is that observations of modern-day ice streams typically span just a few decades and, at the ice-sheet scale, it is unclear how the entire drainage network of ice streams evolves during deglaciation. A key question is whether ice streams might increase and sustain rates of mass loss over centuries or millennia, beyond those expected for a given ocean-climate forcing. To address this issue, numerous workers have sought to understand ice stream dynamics over longer time-scales using their glacial geomorphology in the palaeo-record. Indeed, 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. Building on this body of work, this paper uses the glacial geomorphology of 117 ice streams in the North American Laurentide Ice Sheet to reconstruct their activity during its deglaciation ( 22,000 to 7,000 years ago). Ice stream activity was characterised by high variability in both time and space, with ice streams switching on and off in different locations. During deglaciation, we find that their overall number decreased, they occupied a

Melting beneath Greenland outlet glaciers and ice streams

NASA Astrophysics Data System (ADS)

Alexander, David; Perrette, Mahé; Beckmann, Johanna

2015-04-01

Basal melting of fast-flowing Greenland outlet glaciers and ice streams due to frictional heating at the ice-bed interface contributes significantly to total glacier mass balance and subglacial meltwater flux, yet modelling this basal melt process in Greenland has received minimal research attention. A one-dimensional dynamic ice-flow model is calibrated to the present day longitudinal profiles of 10 major Greenland outlet glaciers and ice streams (including the Jakobshavn Isbrae, Petermann Glacier and Helheim Glacier) and is validated against published ice flow and surface elevation measurements. Along each longitudinal profile, basal melt is calculated as a function of ice flow velocity and basal shear stress. The basal shear stress is dependent on the effective pressure (difference between ice overburden pressure and water pressure), basal roughness and a sliding parametrization. Model output indicates that where outlet glaciers and ice streams terminate into the ocean with either a small floating ice tongue or no floating tongue whatsoever, the proportion of basal melt to total melt (surface, basal and submarine melt) is 5-10% (e.g. Jakobshavn Isbrae; Daugaard-Jensen Glacier). This proportion is, however, negligible where larger ice tongues lose mass mostly by submarine melt (~1%; e.g. Nioghalvfjerdsfjorden Glacier). Modelled basal melt is highest immediately upvalley of the grounding line, with contributions typically up to 20-40% of the total melt for slippery beds and up to 30-70% for resistant beds. Additionally, modelled grounding line and calving front migration inland for all outlet glaciers and ice streams of hundreds of metres to several kilometres occurs. Including basal melt due to frictional heating in outlet glacier and ice stream models is important for more accurately modelling mass balance and subglacial meltwater flux, and therefore, more accurately modelling outlet glacier and ice stream dynamics and responses to future climate change.

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.

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

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.

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.

Southern Laurentide ice lobes were created by ice streams: Des Moines Lobe in Minnesota, USA

USGS Publications Warehouse

Patterson, C.J.

1997-01-01

Regional mapping in southern Minnesota has illuminated a suite of landforms developed by the Des Moines Lobe that delimit the position of the lobe at its maximum and at lesser readvances. The ice lobe repeatedly advanced, discharged its subglacial water, and subsequently stagnated. Recent glaciological research on Antarctic ice streams has led some glacial geologists to postulate that ice streams drained parts of the marine-based areas of the Laurentide Ice Sheet. I postulate that such ice streams may develop in land-based areas of an ice sheet as well, and that the Des Moines Lobe, 200 km wide and 900 km long, was an outlet glacier of an ice stream. It appears to have been able to advance beyond the Laurentide Ice Sheet as long as adequate water pressure was maintained. However, the outer part of the lobe stagnated because subglacial water that facilitated the flow was able to drain away through tunnel valleys. Stagnation of the lobe is not equivalent to stoppage of the ice stream, because ice repeatedly advanced into and onto the stagnant margins, stacking ice and debris. Similar landforms are also seen in other lobes of the upper midwestern United States.

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

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

Stick-slip Cycles and Tidal Modulation of Ice Stream Flow

NASA Astrophysics Data System (ADS)

Lipovsky, B.; Dunham, E. M.

2016-12-01

The reactivation of a single dormant Antarctic ice stream would double the continent's mass imbalance. Despite importance of understanding the likelihood of such an event, direct observation of the basal processes that lead to the activation and stagnation of streaming ice are minimal. As the only ice stream undergoing stagnation, the Whillans Ice Plain (WIP) occupies a central role in our understanding of these subglacial processes. Complicating matters is the observation, from GPS records, that the WIP experiences most of its motion during episodes of rapid sliding. These sliding events are tidally modulated and separated by 12 hour periods of quiescence. We conduct numerical simulations of ice stream stick-slip cycles. Our simulations include rate- and state-dependent frictional sliding, tidal forcing, inertia, upstream loading in a cross-stream, thickness-averaged formulation. Our principal finding is that ice stream motion may respond to ocean tidal forcing with one of two end member behaviors. In one limit, tidally modulated slip events have rupture velocities that approach the shear wave speed and slip events have a duration that scales with the ice stream width divided by the shear wave speed. In the other limit, tidal modulation results in ice stream sliding velocities with lower amplitude variation but at much longer timescales, i.e. semi-diurnal and longer. This latter behavior more closely mimics the behavior of several active ice streams (Bindschadler, Rutford). We find that WIP slip events exist between these two end member behaviors: rupture velocities are far below the inertial limit yet sliding occurs only episodically. The continuum of sliding behaviors is governed by a critical ice stream width over which slip event nucleate. When the critical width is much longer than the ice stream width, slip events are unable to nucleate. The critical width depends on the subglacial effective pressure, ice thickness, and frictional and elastic constitutive

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.

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.

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.

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.

Holocene Accumulation and Ice Flow near the West Antarctic Ice Sheet Divide Ice Core Site

NASA Technical Reports Server (NTRS)

Koutnik, Michelle R.; Fudge, T.J.; Conway, Howard; Waddington, Edwin D.; Neumann, Thomas A.; Cuffey, Kurt M.; Buizert, Christo; Taylor, Kendrick C.

2016-01-01

The West Antarctic Ice Sheet Divide Core (WDC) provided a high-resolution climate record from near the Ross-Amundsen Divide in Central West Antarctica. In addition, radar-detected internal layers in the vicinity of the WDC site have been dated directly from the ice core to provide spatial variations in the age structure of the region. Using these two data sets together, we first infer a high-resolution Holocene accumulation-rate history from 9.2 thousand years of the ice-core timescale and then confirm that this climate history is consistent with internal layers upstream of the core site. Even though the WDC was drilled only 24 kilometers from the modern ice divide, advection of ice from upstream must be taken into account. We evaluate histories of accumulation rate by using a flowband model to generate internal layers that we compare to observed layers. Results show that the centennially averaged accumulation rate was over 20 percent lower than modern at 9.2 thousand years before present (B.P.), increased by 40 percent from 9.2 to 2.3 thousand years B.P., and decreased by at least 10 percent over the past 2 thousand years B.P. to the modern values; these Holocene accumulation-rate changes in Central West Antarctica are larger than changes inferred from East Antarctic ice-core records. Despite significant changes in accumulation rate, throughout the Holocene the regional accumulation pattern has likely remained similar to today, and the ice-divide position has likely remained on average within 5 kilometers of its modern position. Continent-scale ice-sheet models used for reconstructions of West Antarctic ice volume should incorporate this accumulation history.

Satellite radar interferometry for monitoring ice sheet motion: application to an antarctic ice stream.

PubMed

Goldstein, R M; Engelhardt, H; Kamb, B; Frolich, R M

1993-12-03

Satellite radar interferometry (SRI) provides a sensitive means of monitoring the flow velocities and grounding-line positions of ice streams, which are indicators of response of the ice sheets to climatic change or internal instability. The detection limit is about 1.5 millimeters for vertical motions and about 4 millimeters for horizontal motions in the radar beam direction. The grounding line, detected by tidal motions where the ice goes afloat, can be mapped at a resolution of approximately 0.5 kilometer. The SRI velocities and grounding line of the Rutford Ice Stream, Antarctica, agree fairly well with earlier ground-based data. The combined use of SRI and other satellite methods is expected to provide data that will enhance the understanding of ice stream mechanics and help make possible the prediction of ice sheet behavior.

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

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

Simultaneous teleseismic and geodetic observations of the stick-slip motion of an Antarctic ice stream.

PubMed

Wiens, Douglas A; Anandakrishnan, Sridhar; Winberry, J Paul; King, Matt A

2008-06-05

Long-period seismic sources associated with glacier motion have been recently discovered, and an increase in ice flow over the past decade has been suggested on the basis of secular changes in such measurements. Their significance, however, remains uncertain, as a relationship to ice flow has not been confirmed by direct observation. Here we combine long-period surface-wave observations with simultaneous Global Positioning System measurements of ice displacement to study the tidally modulated stick-slip motion of the Whillans Ice Stream in West Antarctica. The seismic origin time corresponds to slip nucleation at a region of the bed of the Whillans Ice Stream that is likely stronger than in surrounding regions and, thus, acts like an 'asperity' in traditional fault models. In addition to the initial pulse, two seismic arrivals occurring 10-23 minutes later represent stopping phases as the slip terminates at the ice stream edge and the grounding line. Seismic amplitude and average rupture velocity are correlated with tidal amplitude for the different slip events during the spring-to-neap tidal cycle. Although the total seismic moment calculated from ice rigidity, slip displacement, and rupture area is equivalent to an earthquake of moment magnitude seven (M(w) 7), seismic amplitudes are modest (M(s) 3.6-4.2), owing to the source duration of 20-30 minutes. Seismic radiation from ice movement is proportional to the derivative of the moment rate function at periods of 25-100 seconds and very long-period radiation is not detected, owing to the source geometry. Long-period seismic waves are thus useful for detecting and studying sudden ice movements but are insensitive to the total amount of slip.

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.

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

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.

Ice in Channels and Ice-Rock Mixtures in Valleys on Mars: Did They Slide on Deformable Rubble Like Antarctic Ice Streams?

NASA Technical Reports Server (NTRS)

Lucchitta, B. K.

1997-01-01

Recent studies of ice streams in Antarctica reveal a mechanism of basal motion that may apply to channels and valleys on Mars. The mechanism is sliding of the ice on deformable water-saturated till under high pore pressures. It has been suggested by Lucchitta that ice was present in outflow channels on Mars and gave them their distinctive morphology. This ice may have slid like Antarctic ice streams but on rubbly weathering products rather than till. However, to generate water under high pore pressures, elevated heatflow is needed to melt the base of the ice. Either volcanism or higher heatflow more than 2 b.y. ago could have raised the basal temperature. Regarding valley networks, higher heatflow 3 b.y. ago could have allowed sliding of ice-saturated overburden at a few hundred meters depth. If the original, pristine valleys were somewhat deeper than they are now, they could have formed by the same mechanism. Recent sounding of the seafloor in front of the Ross Ice Shelf in Antarctica reveals large persistent patterns of longitudinal megaflutes and drumlinoid forms, which bear remarkable resemblance to longitudinal grooves and highly elongated streamlined islands found on the floors of martian outflow channels. The flutes are interpreted to have formed at the base of ice streams during the last glacial advance. Additional similarities of Antarctic ice streams with martian outflow channels are apparent. Antarctic ice streams are 30 to 80 km wide and hundreds of kilometers long. Martian outflow channels have similar dimensions. Ice stream beds are below sea level. Carr determined that most common floor elevations of 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. Martian channels also have floor gradients that are shallow or go uphill locally and have low surface gradients. The depth to the

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.

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.

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

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.

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

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.

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.

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

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.

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.

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.

Atmospherically-driven collapse of a marine-based ice stream

NASA Astrophysics Data System (ADS)

Greenwood, S. L.; Clason, C. C.

2016-12-01

Marine-terminating glaciers and the sectors of ice sheets that are grounded below sea level are widely considered to be vulnerable to unstable retreat. The southern sector of the retreating Fennoscandian Ice Sheet comprised a large, aqueous-terminating ice sheet catchment grounded well below sea level throughout its deglaciation. However, the behaviour, timing of and controls upon ice sheet retreat through the Baltic and Bothnian basins have thus far been inferred only indirectly from peripheral, terrestrial-based geological archives. Recent acquisition of high-resolution multibeam bathymetry opens these basins up, for the first time, to direct investigation of their glacial footprint and palaeo-ice sheet behaviour. Multibeam data reveal a rich glacial landform legacy of the Bothnian Sea deglaciation. A late-stage palaeo-ice stream formed a narrow corridor of fast flow. Its pathway is overprinted by a vast field of basal crevasse squeeze ridges, while abundant traces of high subglacial meltwater volumes call for considerable input of surface meltwater to the subglacial system. We interpret a short-lived ice stream event under high extension, precipitating large-scale hydrofracture-driven collapse of the ice sheet sector under conditions of high surface melting. Experiments with a physically-based numerical flowline model indicate that the rate and pattern of Bothnian Sea ice stream retreat are most sensitive to surface mass balance change and crevasse propagation, while remarkably insensitive to submarine melting and sea level change. We interpret strongly atmospherically-driven retreat of this marine-based ice sheet sector.

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.

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

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

Islands in the ice stream: were spawning habitats for native salmonids in the Great Lakes created by paleo-ice streams?

USGS Publications Warehouse

Riley, Stephen; Binder, Thomas R.; Tucker, Taaja R.; Menzies, John; Eyles, Nick; Janssen, John; Muir, Andrew M.; Esselman, Peter C.; Wattrus, Nigel J.; Krueger, Charles C.

2016-01-01

Lake trout Salvelinus namaycush, lake whitefish Coregonus clupeaformis and cisco Coregonus artedi are salmonid fishes native to the Laurentian Great Lakes that spawn on rocky substrates in the fall and early winter. After comparing the locations of spawning habitat for these species in the main basin of Lake Huron with surficial substrates and the hypothesized locations of fast-flowing Late Wisconsinan paleo-ice streams, we hypothesize that much of the spawning habitat for these species in Lake Huron is the result of deposition and erosion by paleo-ice streams. This hypothesis may represent a new framework for the identification and protection of spawning habitat for these native species, some of which are currently rare or extirpated in some of the Great Lakes. We further suggest that paleo-ice streams may have been responsible for the creation of native salmonid spawning habitat elsewhere in the Great Lakes and in other glaciated landscapes.

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

West Antarctic Ice Sheet Initiative. Volume 2: Discipline Reviews

NASA Technical Reports Server (NTRS)

Bindschadler, Robert A. (Editor)

1991-01-01

Seven discipline review papers are presented on the state of the knowledge of West Antarctica and opinions on how that knowledge must be increased to predict the future behavior of this ice sheet and to assess its potential to collapse, rapidly raising the global sea level. These are the goals of the West Antarctic Ice Sheet Initiative (WAIS).

Modulation of Sea Ice Melt Onset and Retreat in the Laptev Sea by the Timing of Snow Retreat in the West Siberian Plain

NASA Astrophysics Data System (ADS)

Crawford, A. D.; Stroeve, J.; Serreze, M. C.; Rajagopalan, B.; Horvath, S.

2017-12-01

As much of the Arctic Ocean transitions to ice-free conditions in summer, efforts have increased to improve seasonal forecasts of not only sea ice extent, but also the timing of melt onset and retreat. This research investigates the potential of regional terrestrial snow retreat in spring as a predictor for subsequent sea ice melt onset and retreat in Arctic seas. One pathway involves earlier snow retreat enhancing atmospheric moisture content, which increases downwelling longwave radiation over sea ice cover downstream. Another pathway involves manipulation of jet stream behavior, which may affect the sea ice pack via both dynamic and thermodynamic processes. Although several possible connections between snow and sea ice regions are identified using a mutual information criterion, the physical mechanisms linking snow retreat and sea ice phenology are most clearly exemplified by variability of snow retreat in the West Siberian Plain impacting melt onset and sea ice retreat in the Laptev Sea. The detrended time series of snow retreat in the West Siberian Plain explains 26% of the detrended variance in Laptev Sea melt onset (29% for sea ice retreat). With modest predictive skill and an average time lag of 53 (88) days between snow retreat and sea ice melt onset (retreat), West Siberian Plains snow retreat is useful for refining seasonal sea ice predictions in the Laptev Sea.

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.

Slow-slip events on the Whillans Ice Plain, Antarctica, described using rate-and-state friction as an ice stream sliding law

NASA Astrophysics Data System (ADS)

Lipovsky, Bradley Paul; Dunham, Eric M.

2017-04-01

The Whillans Ice Plain (WIP), Antarctica, experiences twice daily tidally modulated stick-slip cycles. Slip events last about 30 min, have sliding velocities as high as ˜0.5 mm/s (15 km/yr), and have total slip ˜0.5 m. Slip events tend to occur during falling ocean tide: just after high tide and just before low tide. To reproduce these characteristics, we use rate-and-state friction, which is commonly used to simulate tectonic faulting, as an ice stream sliding law. This framework describes the evolving strength of the ice-bed interface throughout stick-slip cycles. We present simulations that resolve the cross-stream dimension using a depth-integrated treatment of an elastic ice layer loaded by tides and steady ice inflow. Steady sliding with rate-weakening friction is conditionally stable with steady sliding occurring for sufficiently narrow ice streams relative to a nucleation length. Stick-slip cycles occur when the ice stream is wider than the nucleation length or, equivalently, when effective pressures exceed a critical value. Ice streams barely wider than the nucleation length experience slow-slip events, and our simulations suggest that the WIP is in this slow-slip regime. Slip events on the WIP show a sense of propagation, and we reproduce this behavior by introducing a rate-strengthening region in the center of the otherwise rate-weakening ice stream. If pore pressures are raised above a critical value, our simulations predict that the WIP would exhibit quasi-steady tidally modulated sliding as observed on other ice streams. This study validates rate-and-state friction as a sliding law to describe ice stream sliding styles.

Dark ice dynamics of the south-west Greenland Ice Sheet

NASA Astrophysics Data System (ADS)

Tedstone, Andrew J.; Bamber, Jonathan L.; Cook, Joseph M.; Williamson, Christopher J.; Fettweis, Xavier; Hodson, Andrew J.; Tranter, Martyn

2017-11-01

Runoff from the Greenland Ice Sheet (GrIS) has increased in recent years due largely to changes in atmospheric circulation and atmospheric warming. Albedo reductions resulting from these changes have amplified surface melting. Some of the largest declines in GrIS albedo have occurred in the ablation zone of the south-west sector and are associated with the development of dark ice surfaces. Field observations at local scales reveal that a variety of light-absorbing impurities (LAIs) can be present on the surface, ranging from inorganic particulates to cryoconite materials and ice algae. Meanwhile, satellite observations show that the areal extent of dark ice has varied significantly between recent successive melt seasons. However, the processes that drive such large interannual variability in dark ice extent remain essentially unconstrained. At present we are therefore unable to project how the albedo of bare ice sectors of the GrIS will evolve in the future, causing uncertainty in the projected sea level contribution from the GrIS over the coming decades. Here we use MODIS satellite imagery to examine dark ice dynamics on the south-west GrIS each year from 2000 to 2016. We quantify dark ice in terms of its annual extent, duration, intensity and timing of first appearance. Not only does dark ice extent vary significantly between years but so too does its duration (from 0 to > 80 % of June-July-August, JJA), intensity and the timing of its first appearance. Comparison of dark ice dynamics with potential meteorological drivers from the regional climate model MAR reveals that the JJA sensible heat flux, the number of positive minimum-air-temperature days and the timing of bare ice appearance are significant interannual synoptic controls. We use these findings to identify the surface processes which are most likely to explain recent dark ice dynamics. We suggest that whilst the spatial distribution of dark ice is best explained by outcropping of particulates from

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.

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.

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

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

Ice-dammed lateral lake and epishelf lake insights into Holocene dynamics of Marguerite Trough Ice Stream and George VI Ice Shelf, Alexander Island, Antarctic Peninsula

NASA Astrophysics Data System (ADS)

Davies, Bethan J.; Hambrey, Michael J.; Glasser, Neil F.; Holt, Tom; Rodés, Angél; Smellie, John L.; Carrivick, Jonathan L.; Blockley, Simon P. E.

2017-12-01

We present new data regarding the past dynamics of Marguerite Trough Ice Stream, George VI Ice Shelf and valley glaciers from Ablation Point Massif on Alexander Island, Antarctic Peninsula. This ice-free oasis preserves a geological record of ice stream lateral moraines, ice-dammed lakes, ice-shelf moraines and valley glacier moraines, which we dated using cosmogenic nuclide ages. We provide one of the first detailed sediment-landform assemblage descriptions of epishelf lake shorelines. Marguerite Trough Ice Stream imprinted lateral moraines against eastern Alexander Island at 120 m at Ablation Point Massif. During deglaciation, lateral lakes formed in the Ablation and Moutonnée valleys, dammed against the ice stream in George VI Sound. Exposure ages from boulders on these shorelines yielded ages of 13.9 to 9.7 ka. Following recession of the ice stream, George VI Ice Shelf formed in George VI Sound. An epishelf lake formed at 15-20 m asl in Ablation and Moutonnée valleys, dated from 9.4 to 4.6 ka, suggesting that the lake was stable and persistent for some 5000 years. Lake-level lowering occurred after this, with the lake level at 12 m at 3.1 ± 0.4 ka and at 5 m asl today. A readvance of the valley glaciers on Alexander Island at 4.4 ± 0.7 ka is recorded by valley glacier moraines overlying epishelf lake sediments. We speculate that the glacier readvance, which occurred during a period of warmth, may have been caused by a dynamic response of the glaciers to a lowering in surface elevation of George VI Ice Shelf.

Basal hydraulic conditions of Ice Stream B

NASA Technical Reports Server (NTRS)

Engelhardt, Hermann; Kamb, Barclay

1993-01-01

Fifteen boreholes have been drilled to the base of Ice Stream B in the vicinity of UpB Camp. The boreholes are spread over an area of about 500 x 1000 m. Several till cores were retrieved from the bottom of the 1000-m-deep holes. Laboratory tests using a simple shear box revealed a yield strength of basal till of 2 kPa. This agrees well with in-situ measurements using a shear vane. Since the average basal shear stress of Ice Stream B with a surface slope of 0.1 degree is about 20 kPa, the ice stream cannot be supported by till that weak. Additional support for this conclusion comes from the basal water pressure that has been measured in all boreholes as soon as the hot water drill reached bottom. In several boreholes, the water pressure has been continuously monitored; in two of them, over several years. The water pressure varies but stays within 1 bar of flotation where ice overburden pressure and water pressure are equal. The ratio of water and overburden pressure lies between 0.986 and 1.002. This is an extremely high value as compared to other fast-moving ice masses; e.g., Variegated Glacier in surge has a ratio of 0.8, and Columbia Glacier - a fast-moving tidewater glacier - has a ratio of 0.9. It implies that water flow under the glacier occurs in a thin film and not in conduits that would drain away water too rapidly. It also implies that basal sliding must be very effective. Water flow under the glacier was measured in a salt-injection experiment where a salt pulse was released at the bottom of a borehole while 60 m down-glacier, the electrical resistance was measured between two other boreholes. A flow velocity of 7 mm/s was obtained.

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

Increased Water Storage at Ice-stream Onsets: A Critical Mechanism?

NASA Technical Reports Server (NTRS)

Bindschadler, Robert; Choi, Hyeungu

2007-01-01

The interdependence of rapid ice flow, surface topography and the spatial distribution of subglacial water are examined by linking existing theories. The motivation is to investigate whether the acceleration of an ice-stream tributary contains a positive feedback that encourages the retention of subglacial water that leads to faster flow. Periodically varying surface and bed topographies are related through a linear ice-flow perturbation theory for various values of mean surface slope, perturbation amplitude and basal sliding speeds. The topographic variations lead to a periodic variation in hydraulic potential that is used to infer the tendency for subglacial water to be retained in local hydraulic potential minima. If water retention leads to enhanced basal sliding, a positive feedback loop is closed that could explain the transition from slower tributary flow to faster-streaming flow and the sustained downstream acceleration along the tributary-ice-stream system. A sensitivity study illustrates that the same range of topographic wavelengths most effectively transmitted from the bed to the surface also strongly influences the behavior of subglacial water. A lubrication index is defined to qualitatively measure the heterogeneity of the subglacial hydrologic system. Application of this index to field data shows that the transition from tributary to ice stream closely agrees with the location where subglacial water may be first stored.

Fast Recession of a West Antarctic Glacier

NASA Technical Reports Server (NTRS)

Rignot, E. J.

1998-01-01

Satellite radar interferometry observations of Pine Island Glacier, in West Antarctica, reveal that the hinge-line position of this major ice stream retreated 1.2+/-0.2 km per year between 1992 and 1996, which in turn implies ice thinning at 3.5+/-0.6m ice per year.

Geological and paleontological results from the WISSARD (Whillans Ice Stream Subglacial Access Research Drilling) Project

NASA Astrophysics Data System (ADS)

Scherer, R. P.; Powell, R. D.; Coenen, J. J.; Hodson, T. O.; Puttkammer, R.; Tulaczyk, S. M.

2015-12-01

The WISSARD project recovered sediment cores and other geological materials from beneath the Whillans Ice Stream in West Antarctica during two drilling seasons; Subglacial Lake Whillans (SLW) in 2013 and 100km downstream at the ice stream grounding-zone (WGZ) in 2015. SLW is characterized by 2 m of freshwater with a high suspended-sediment load, whereas WGZ has a 10 m column of clear, fully marine water with an active community of marine organisms. Three coring devices were deployed as part of WISSARD, including (1) a multicorer, which recovers 3 unaltered sediment-water interface cores, up to 0.5m, (2) a piston corer, also deployed as a gravity corer, with a 3m core barrel, and (3) a percussion coring system with a 5m core barrel. Sediments recovered from SLW are muddy diamicton with minimal stratification. The sediments are characteristic of active till, not water-column deposition. The till is weak and effective stresses very low, thus till flux from deformation must also be low. Water through flow is sufficient to carry suspended clays and silts, but not transfer large volumes of sediment in the current glaciological regime. Microfossils and geochemical tracers (e.g., biomarkers, 10Be and 14C) in SLW sediments indicate Pleistocene input from open water conditions, plus input and mixing of components derived from older Cenozoic strata. Diatoms and other sedimentary characteristics of SLW are entirely consistent with material previously recovered from upstream sites on the Whillans Ice Stream (UpB), but show evidence of further cumulative subglacial shear strain, suggesting downstream translation as deforming till. Sedimentary components from WGZ indicate significant input from sources other than from the Whillans Ice Stream. Sediment cores include distinct stratigraphic variability, with differing geochemical and sedimentary components indicative of input from changing source beds. Components indicate a mixture of Quaternary and older components. The lower ca

Large subglacial lakes in East Antarctica at the onset of fast-flowing ice streams.

PubMed

Bell, Robin E; Studinger, Michael; Shuman, Christopher A; Fahnestock, Mark A; Joughin, Ian

2007-02-22

Water plays a crucial role in ice-sheet stability and the onset of ice streams. Subglacial lake water moves between lakes and rapidly drains, causing catastrophic floods. The exact mechanisms by which subglacial lakes influence ice-sheet dynamics are unknown, however, and large subglacial lakes have not been closely associated with rapidly flowing ice streams. Here we use satellite imagery and ice-surface elevations to identify a region of subglacial lakes, similar in total area to Lake Vostok, at the onset region of the Recovery Glacier ice stream in East Antarctica and predicted by ice-sheet models. We define four lakes through extensive, flat, featureless regions of ice surface bounded by upstream troughs and downstream ridges. Using ice velocities determined using interferometric synthetic aperture radar (InSAR), we find the onset of rapid flow (moving at 20 to 30 m yr(-1)) of the tributaries to the Recovery Glacier ice stream in a 280-km-wide segment at the downslope margins of these four subglacial lakes. We conclude that the subglacial lakes initiate and maintain rapid ice flow through either active modification of the basal thermal regime of the ice sheet by lake accretion or through scouring bedrock channels in periodic drainage events. We suggest that the role of subglacial lakes needs to be considered in ice-sheet mass balance assessments.

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

Wind-driven Sea-Ice Changes Intensify Subsurface Warm Water Intrusion into the West Antarctic Land Ice Front

NASA Astrophysics Data System (ADS)

Li, X.; Gille, S. T.; shang-Ping, X.; Xie, S. P.; Holland, D. M.; Holland, M. M.

2016-12-01

The climate change observed around Antarctica in recent decades is characterized by distinct zonally asymmetric patterns, with the strongest changes over West Antarctica. These changes are marked by strong land ice melting and sea ice redistribution around West Antarctica. This is associated with temperature and circulation anomalies in the ocean and atmosphere around the same area. In this study, we comprehensively examine the coherency between these changes using a combination of observations and numerical simulations. Results show that the atmospheric circulation changes distinctly drive the changes in ocean circulation and sea ice distribution. In addition, the atmospheric circulation induced sea ice changes play an important role in lifting the subsurface ocean temperature and salinity around the West Antarctica. During recent decades, the Amundsen Sea Low (ASL) has deepened, especially in austral autumn and winter. This deepened ASL has intensified the offshore wind near the coastal regions of the Ross Sea. Driven by these atmospheric changes, more sea ice has formed near West Antarctica in winter. In contrast, more sea ice melts during the summer. This strengthened sea ice seasonality has been observed and successfully reproduced in the model simulation. The wind-driven sea ice changes causes a surface freshening over the Ross and Amundsen Seas, with a subsurface salinity increase over the Ross Sea. The additional fresh/salt water fluxes thus further change the vertical distribution of salinity and strengthen the stratification in the Ross and Amundsen Seas. As a result of the above ice-ocean process, the mixed-layer depth around the Ross and Amundsen Seas shallows. By weakening the vertical heat transport near the surface layer, and inducing an upward movement of the circumpolar deep water (CDW), this process freshened and cooled the surface layer, while the salinity and temperature in the sub-surface ocean are increased, extending from 150 meters to >700

Seismic Excitation of the Ross Ice Shelf by Whillans Ice Stream Stick-Slip Events

NASA Astrophysics Data System (ADS)

Wiens, D.; Pratt, M. J.; Aster, R. C.; Nyblade, A.; Bromirski, P. D.; Stephen, R. A.; Gerstoft, P.; Diez, A.; Cai, C.; Anthony, R. E.; Shore, P.

2015-12-01

Rapid variations in the flow rate of upstream glaciers and ice streams may cause significant deformation of ice shelves. The Whillans Ice Stream (WIS) represents an extreme example of rapid variations in velocity, with motions near the grounding line consisting almost entirely of once or twice-daily stick-slip events with a displacement of up to 0.7 m (Winberry et al, 2014). Here we report observations of compressional waves from the WIS slip events propagating hundreds of kilometers across the Ross Ice Shelf (RIS) detected by broadband seismographs deployed on the ice shelf. The WIS slip events consist of rapid basal slip concentrated at three high friction regions (often termed sticky-spots or asperities) within a period of about 25 minutes (Pratt et al, 2014). Compressional displacement pulses from the second and third sticky spots are detected across the entire RIS up to about 600 km away from the source. The largest pulse results from the third sticky spot, located along the northwestern grounding line of the WIS. Propagation velocities across the ice shelf are significantly slower than the P wave velocity in ice, as the long period displacement pulse is also sensitive to velocities of the water and sediments beneath the ice shelf. Particle motions are, to the limit of resolution, entirely within the horizontal plane and roughly radial with respect to the WIS sticky-spots, but show significant complexity, presumably due to differences in ice velocity, thickness, and the thickness of water and sediment beneath. Study of this phenomenon should lead to greater understanding of how the ice shelf responds to sudden forcing around the periphery.

Seafloor features delineate Late Wisconsinan ice stream configurations in eastern Parry Channel, Canadian Arctic Archipelago

NASA Astrophysics Data System (ADS)

MacLean, B.; Blasco, S.; Bennett, R.; Lakeman, T.; Pieńkowski, A. J.; Furze, M. F. A.; Hughes Clarke, J.; Patton, E.

2017-03-01

Multibeam imagery and 3.5 kHz sub-bottom profiles acquired from CCGS Amundsen between 2003 and 2013 by ArcticNet and the Ocean Mapping Group at the University of New Brunswick provide information on seafloor features, geology, bathymetry and morphology in eastern Parry Channel and the adjoining large channels in the Canadian Arctic Archipelago. Together these include Peel Sound, Barrow Strait, Lancaster Sound, Wellington Channel, Prince Regent Inlet, Admiralty Inlet and Navy Board Inlet. Those data are in part complemented by high resolution single channel seismic reflection profiles acquired by the Geological Survey of Canada in the 1970s and 1980s and by sediment cores that provide chronological and depositional information. The occurrence and pattern of streamlined mega-scale ridge and groove lineations (MSGLs) indicate that these waterways were occupied by glacial ice streams in the past. Chronological information from marine and adjoining terrestrial areas suggests a long history of glacial events ranging in time from Early Pleistocene to Late Wisconsinan. Seafloor morphology and MSGL trends together with terrestrial ice flow patterns indicate that ice streams flowed into Barrow Strait from Peel Sound and Wellington Channel, and ice streams in Prince Regent, Admiralty and Navy Board inlets flowed northward into and eastward along Lancaster Sound. Recession of the ice stream westward along Parry Channel occurred ∼16 cal ka BP to 10.8 cal ka BP. Thick ice-contact sediments deposited by a late ice advance from Prince Regent Inlet constitute the seabed across a large area of western Lancaster Sound. Timing for that late ice advance appears to be bracketed between the 11.5 cal ka BP lift-off of the eastern Parry ice stream north of Prince Leopold Island and the ∼10.0 cal ka BP deglaciation of Prince Regent Inlet. Seafloor morphology and lineation trends suggest that ice delivered by the ice stream in Peel Sound was the westernmost tributary to the ice stream

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

Simulating LGM retreat of the Uummannaq Ice Stream and Rinks Isbrae, Western Greenland using a 1-D ice-stream model constrained by a suite of marine and terrestrial data

NASA Astrophysics Data System (ADS)

Jamieson, Stewart; Roberts, Dave; Rea, Brice; Lane, Timothy; Vieli, Andreas; Cofaigh, Colm Ó.

2014-05-01

We aim to understand what controlled the retreat pattern of the Uummannaq Ice Stream (UIS) during the last deglaciation. Evidence for the pattern of retreat is found in both the marine and terrestrial realms, but because the evidence is temporally and spatially discontinuous, it is challenging to coherently reconstruct both grounding-line retreat and ice-surface thinning such that they are in agreement. Marine stratigraphic and geophysical evidence indicates that the ice stream was grounded close to the continental shelf edge at the Last Glacial Maximum, and retreated rapidly and nonlinearly after 14.8 ka. Cosmogenic nuclide exposure dating on Ubekendt Island at the convergence zone of multiple feeder ice streams show that the ice surface thinned progressively and that the island became ice-free by ca. 12.4 ka. The ice stream then collapsed over the next 1-1.6 kyrs and the ice stream separated into a series of distinct inland arms. In the northernmost Rinks system, there is a 'staircase' of evidence showing ice surface thinning over time, but it is unclear where the grounding line was located during this phase of thinning. Furthermore, it is currently unclear what controlled the nonlinear retreat pattern identified in the Uummannaq system. We develop a numerical model of ice-stream retreat using the marine geophysical data and measurements of sediment strength on the continental shelf to control the boundary conditions. The model has the capability to dynamically and robustly simulate grounding line-retreat behaviour over millennial timescales. We simulate the retreat of the UIS grounding line into the northernmost Rinks system in response to enhanced ocean warming, rising sea level and warming climate. We compare the simulated dynamic behaviour of the UIS against the geomorphological and cosmogenic exposure evidence for ice surface thinning onshore and against dated marine grounding line positions. Our model results enable us to match grounding-line positions in

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

Influence of West Antarctic Ice Sheet collapse on Antarctic surface climate

NASA Astrophysics Data System (ADS)

Steig, Eric J.; Huybers, Kathleen; Singh, Hansi A.; Steiger, Nathan J.; Ding, Qinghua; Frierson, Dargan M. W.; Popp, Trevor; White, James W. C.

2015-06-01

Climate model simulations are used to examine the impact of a collapse of the West Antarctic Ice Sheet (WAIS) on the surface climate of Antarctica. The lowered topography following WAIS collapse produces anomalous cyclonic circulation with increased flow of warm, maritime air toward the South Pole and cold-air advection from the East Antarctic plateau toward the Ross Sea and Marie Byrd Land, West Antarctica. Relative to the background climate, areas in East Antarctica that are adjacent to the WAIS warm, while substantial cooling (several °C) occurs over parts of West Antarctica. Anomalously low isotope-paleotemperature values at Mount Moulton, West Antarctica, compared with ice core records in East Antarctica, are consistent with collapse of the WAIS during the last interglacial period, Marine Isotope Stage 5e. More definitive evidence might be recoverable from an ice core record at Hercules Dome, East Antarctica, which would experience significant warming and positive oxygen isotope anomalies if the WAIS collapsed.

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.

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.

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

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

Using U-Pb Detrital Zircon Geochronology to Study Ice Streams in the Weddell Sea Embayment, Antarctica

NASA Astrophysics Data System (ADS)

Agrios, L.; Licht, K.; Hemming, S. R.; Williams, T.

2016-12-01

Till from major ice streams of the Weddell Sea Embayment contain detrital zircons with distinct U-Pb age populations that can be used as a provenance tool to better understand ice stream dynamics. The ice streams in this study include the Foundation Ice Stream, and Academy, Slessor, and Recovery glaciers, all of which drain ice from the continent's interior into the Weddell Sea. Characterizing the U-Pb detrital zircon ages in till and rocks will (1) provide the zircon provenance signatures of the material carried by the ice stream - when these signatures are found in LGM and older deposits downstream they can enable interpretation of past ice flow history; and (2) constrain ice-covered upstream bedrock geology that supplies the till carried by ice streams and glaciers. U-Pb ages of detrital zircons were measured in 21 samples of onshore till, erratics, and bedrock of potential source rocks. Grains were analyzed by LA-ICPMS at the University of Arizona (n=300). Relative probability U-Pb age density plots of till in moraines along the Foundation Ice Stream and Academy Glacier show prominent peaks at 500-530 and 615-650 Ma, which overlap with the timing of the Ross and Pan-African orogenies. Zircon ages of 1000-1095 Ma are also present. Local bedrock in the Patuxent Range has the most prominent peak at 510 Ma, suggesting the till is predominantly derived from local Patuxent Formation. However, local bedrock also has fewer grains at 1030 Ma which suggests that this age population is carried in the till as well. Prominent peaks in U-Pb ages from till transported by the Recovery Glacier are 530, 635, 1610 and 1770 Ma. Bedrock of this area contains similar age peaks, with the exception of the 635 Ma peak, suggesting that this ice stream is carrying a signature from an unexposed source of this age completely buried by ice. The Slessor Glacier carries zircons with prominent populations at 1710 and 2260-2420 Ma, which overlap with a high-grade metamorphic event in the

3. Ice Plant interior, west section, looking north. Crib wall ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

3. Ice Plant interior, west section, looking north. Crib wall of stacked 4' X 6' 's in foreground. Opening leads to refrigeration equipment area. Freezer room to the rear. - Curtis Wharf, Ice Plant, O & Second Streets, Anacortes, Skagit County, WA

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.

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.

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

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.

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.

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

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.

Provenance of Des Moines lobe till records ice-stream catchment evolution during Laurentide deglaciation

USGS Publications Warehouse

Lusardi, B.A.; Jennings, C.E.; Harris, K.L.

2011-01-01

Mapping and analysis of deposits of the Des Moines lobe of the Laurentide Ice Sheet, active after the Last Glacial Maximum (LGM), reveal several texturally and lithologically distinct tills within what had been considered to be a homogeneous deposit. Although the differences between tills are subtle, minor distinctions are predictable and mappable, and till sheets within the area covered by the lobe can be correlated for hundreds of kilometres parallel to ice flow. Lateral till-sheet contacts are abrupt or overlap in a narrow zone, coincident with a geomorphic discontinuity interpreted to be a shear margin. Till sheets 10 to 20m thick show mixing in their lower 2 to 3m. We suggest that: (i) lithologically distinct till sheets correspond to unique ice-stream source areas; (ii) the sequence of tills deposited by the Des Moines lobe was the result of the evolution and varying dominance of nearby and competing ice streams and their tributaries; and (iii) in at least one instance, more than one ice stream simultaneously contributed to the lobe. Therefore the complex sequence of tills of subtly different provenances, and the unconformities between them record the evolution of an ice-catchment area during Laurentide Ice Sheet drawdown. Till provenance data suggest that, after till is created in the ice-stream source area, the subglacial conditions required for transporting till decline and incorporation of new material is limited. ?? 2011 The Authors. Boreas ?? 2011 The Boreas Collegium.

Understanding Ice Shelf Basal Melting Using Convergent ICEPOD Data Sets: ROSETTA-Ice Study of Ross Ice Shelf

NASA Astrophysics Data System (ADS)

Bell, R. E.; Frearson, N.; Tinto, K. J.; Das, I.; Fricker, H. A.; Siddoway, C. S.; Padman, L.

2017-12-01

The future stability of the ice shelves surrounding Antarctica will be susceptible to increases in both surface and basal melt as the atmosphere and ocean warm. The ROSETTA-Ice program is targeted at using the ICEPOD airborne technology to produce new constraints on Ross Ice Shelf, the underlying ocean, bathymetry, and geologic setting, using radar sounding, gravimetry and laser altimetry. This convergent approach to studying the ice-shelf and basal processes enables us to develop an understanding of the fundamental controls on ice-shelf evolution. This work leverages the stratigraphy of the ice shelf, which is detected as individual reflectors by the shallow-ice radar and is often associated with surface scour, form close to the grounding line or pinning points on the ice shelf. Surface accumulation on the ice shelf buries these reflectors as the ice flows towards the calving front. This distinctive stratigraphy can be traced across the ice shelf for the major East Antarctic outlet glaciers and West Antarctic ice streams. Changes in the ice thickness below these reflectors are a result of strain and basal melting and freezing. Correcting the estimated thickness changes for strain using RIGGS strain measurements, we can develop decadal-resolution flowline distributions of basal melt. Close to East Antarctica elevated melt-rates (>1 m/yr) are found 60-100 km from the calving front. On the West Antarctic side high melt rates primarily develop within 10 km of the calving front. The East Antarctic side of Ross Ice Shelf is dominated by melt driven by saline water masses that develop in Ross Sea polynyas, while the melting on the West Antarctic side next to Hayes Bank is associated with modified Continental Deep Water transported along the continental shelf. The two sides of Ross Ice Shelf experience differing basal melt in part due to the duality in the underlying geologic structure: the East Antarctic side consists of relatively dense crust, with low amplitude

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

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.

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.

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

Timing and east-west correlation of south Swedish ice marginal lines during the Late Weichselian

NASA Astrophysics Data System (ADS)

Lundqvist, Jan; Wohlfarth, Barbara

2000-01-01

The retreat of the Late Weichselian ice sheet over the southern part of Sweden is marked along the southwest coast by distinct marginal moraine ridges. Their timing can directly and indirectly be assessed based on a number of radiocarbon dates and pollen stratigraphic investigations on lake sediment sequences adjacent to the ice marginal lines. Along the southeastern side of the peninsula, the ice recession has been reconstructed based on a combination of clay-varve chronology, pollen and radiocarbon stratigraphy. A morphological correlation of ice marginal lines between the west and east coast is problematic since the distinct west-coast moraines cannot be followed through the central part of the peninsula towards the east coast. This paper is an attempt to reconstruct an age-equivalent west-east extension of the ice-recession lines on the basis of existing data sets. For our correlation we use calibrated radiocarbon ages for ice marginal deposits on the west coast and compare these with a partly radiocarbon-dated clay-varve chronology on the east coast. We conclude that the two oldest moraines on the west coast formed at ˜18,000-16,000 and ˜15,400-14,500 cal yr BP, respectively. During the following rapid deglaciation, which may have coincided with the beginning of the Bølling pollen zone, large parts of southernmost Sweden became ice free, except for higher elevated areas, where stagnant ice remained for another 400-500 yr. A best guess is that the formation of the next younger ice marginal lines may have occurred at ˜14,400-14,200, ˜14,200 and ˜13,400 cal yr BP and during the Younger Dryas cold event.

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

Constraining ice sheet history in the Weddell Sea, West Antarctica, using ice fabric at Korff Ice Rise

NASA Astrophysics Data System (ADS)

Brisbourne, A.; Smith, A.; Kendall, J. M.; Baird, A. F.; Martin, C.; Kingslake, J.

2017-12-01

The grounding history of ice rises (grounded area of independent flow regime within a floating ice shelf) can be used to constrain large scale ice sheet history: ice fabric, resulting from the preferred orientation of ice crystals due to the stress regime, can be used to infer this grounding history. With the aim of measuring the present day ice fabric at Korff Ice Rise, West Antarctica, a multi-azimuth wide-angle seismic experiment was undertaken. Three wide-angle common-midpoint gathers were acquired centred on the apex of the ice rise, at azimuths of 60 degrees to one another, to measure variation in seismic properties with offset and azimuth. Both vertical and horizontal receivers were used to record P and S arrivals including converted phases. Measurements of the variation with offset and azimuth of seismic traveltimes, seismic attenuation and shear wave splitting have been used to quantify seismic anisotropy in the ice column. The observations cannot be reproduced using an isotropic ice column model. Anisotropic ray tracing has been used to test likely models of ice fabric by comparison with the data. A model with a weak girdle fabric overlying a strong cluster fabric provides the best fit to the observations. Fabric of this nature is consistent with Korff Ice Rise having been stable for the order of 10,000 years without any ungrounding or significant change in the ice flow configuration across the ice rise for this period. This observation has significant implications for the ice sheet history of the Weddell Sea sector.

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.

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

3. STREAMSIDE PATH, SOUTH OF STREAM, LOOKING WEST AT MIDDLE ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

3. STREAM-SIDE PATH, SOUTH OF STREAM, LOOKING WEST AT MIDDLE FALLS OF THE THREE BRIDGE FALLS NEAR JUNCTION WITH FORSYTHIA WALK Photocopy of photograph, 1930s National Park Service, National Capital Region files - Dumbarton Oaks Park, Thirty-second & R Streets Northwest, Washington, District of Columbia, DC

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.

Evidence for elevated and spatially variable geothermal flux beneath the West Antarctic Ice Sheet

PubMed Central

Schroeder, Dustin M.; Blankenship, Donald D.; Young, Duncan A.; Quartini, Enrica

2014-01-01

Heterogeneous hydrologic, lithologic, and geologic basal boundary conditions can exert strong control on the evolution, stability, and sea level contribution of marine ice sheets. Geothermal flux is one of the most dynamically critical ice sheet boundary conditions but is extremely difficult to constrain at the scale required to understand and predict the behavior of rapidly changing glaciers. This lack of observational constraint on geothermal flux is particularly problematic for the glacier catchments of the West Antarctic Ice Sheet within the low topography of the West Antarctic Rift System where geothermal fluxes are expected to be high, heterogeneous, and possibly transient. We use airborne radar sounding data with a subglacial water routing model to estimate the distribution of basal melting and geothermal flux beneath Thwaites Glacier, West Antarctica. We show that the Thwaites Glacier catchment has a minimum average geothermal flux of ∼114 ± 10 mW/m2 with areas of high flux exceeding 200 mW/m2 consistent with hypothesized rift-associated magmatic migration and volcanism. These areas of highest geothermal flux include the westernmost tributary of Thwaites Glacier adjacent to the subaerial Mount Takahe volcano and the upper reaches of the central tributary near the West Antarctic Ice Sheet Divide ice core drilling site. PMID:24927578

Variations in West Antarctic Ice Front and Passive Microwave Brightness Temperature for 8 Years Duration in 2000s

NASA Astrophysics Data System (ADS)

Kim, J.; Yu, J.; Wang, L.; Liu, H.

2017-12-01

Changes in Antarctic ice sheet are caused by various reasons such as changes in Holocene climate, precipitation, and ocean temperature. Such issues of changes in ice sheet has been mainly focused on the Antarctic peninsula, and it is known that ice retreat of the area is caused by changes in atmospheric and ocean temperatures. For the case of West Antarctica, ice front change research is relatively rarely conducted except the Pine island glacier area. This study has monitored ice front changes of West Antarctica and compared the patterns with the changes in brightness temperature based on remote sensing techniques. We used 2000 Radarsat-1 and 2008 Rasarsat-2 SAR data to delineate coastlines of whole West Antarctica based on the locally thresholding adaptive algorithm. The delineated coast lines are analyzed to figure out ice front change patterns between the duration. The variations in brightness temperature for the same duration are calculated based on Defense Meteorological Satellite Program (DMSP)'s Special Sensor Microwave/Images-Special Sensor Microwave Imager/Sounder (SSM/I-SSMIS) passive microwave data. The results show ice front of West Antarctica shows advancing trend except the pine island glacier area. The brightness temperature had decreasing trend during the study period. It infers that changes in ice front and brightness temperature of West Antarctica have considerable relationships. It is expected that a long term monitoring of the relationship would contribute understanding ice dynamics of West Antarctica significantly.

Effects of acidification on bryophyte communities in West Virginia mountain streams

Treesearch

Steven L. Stephenson; Susan Moyle Studlar; Carolyn J. McQuattie; Pamela J. Edwards

1995-01-01

Bryophytes (mosses and liverworts) are often more responsive to water chemistry changes than are vascular plants. In this study, the relationships of bryophyte communities to stream pH and water chemistry were studied, using six streams on or near the Fernow Experimental Forest in Tucker County, West Virginia. Streams were surveyed with line transects using stratified...

Estimation of traveltime and longitudinal dispersion in streams in West Virginia

USGS Publications Warehouse

Wiley, Jeffrey B.; Messinger, Terence

2013-01-01

Traveltime and dispersion data are important for understanding and responding to spills of contaminants in waterways. The U.S. Geological Survey (USGS), in cooperation with West Virginia Bureau for Public Health, Office of Environmental Health Services, compiled and evaluated traveltime and longitudinal dispersion data representative of many West Virginia waterways. Traveltime and dispersion data were not available for streams in the northwestern part of the State. Compiled data were compared with estimates determined from national equations previously published by the USGS. The evaluation summarized procedures and examples for estimating traveltime and dispersion on streams in West Virginia. National equations developed by the USGS can be used to predict traveltime and dispersion for streams located in West Virginia, but the predictions will be less accurate than those made with graphical interpolation between measurements. National equations for peak concentration, velocity of the peak concentration, and traveltime of the leading edge had root mean square errors (RMSE) of 0.426 log units (127 percent), 0.505 feet per second (ft/s), and 3.78 hours (h). West Virginia data fit the national equations for peak concentration, velocity of the peak concentration, and traveltime of the leading edge with RMSE of 0.139 log units (38 percent), 0.630 ft/s, and 3.38 h, respectively. The national equation for maximum possible velocity of the peak concentration exceeded 99 percent and 100 percent of observed values from the national data set and West Virginia-only data set, respectively. No RMSE was reported for time of passage of a dye cloud, as estimated using the national equation; however, the estimates made using the national equations had a root mean square error of 3.82 h when compared to data gathered for this study. Traveltime and dispersion estimates can be made from the plots of traveltime as a function of streamflow and location for streams with plots available, but

Geoengineering Outlet Glaciers and Ice Streams

NASA Astrophysics Data System (ADS)

Wolovick, Michael

2017-04-01

Mass loss from Greenland and Antarctica is highly sensitive to the presence of warm ocean water that drives melting of ice shelves and marine terminated glaciers. This warm water resides offshore at depth and accesses the grounding line through deep but narrow troughs and fjords. Here, we investigate the possibility of blocking warm water transport through these choke points with an artificial sill. Using a simple width-averaged model of ice stream flow coupled to a buoyant-plume model of submarine melt, we 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 outlet glaciers and ice-shelf cavities. Glaciers with a floating shelf exhibit a strong response to the presence of the artificial sill regardless of our choice of calving law, while tidewater glaciers require a strong linkage between submarine melt and iceberg calving for the artificial sill to have an effect. As a result of this difference and as a result of differing degrees of overdeepening in the basal topography, Antarctica and Greenland present very different societal cost-benefit analyses. Intervention in Greenland would be low-cost and low-reward: the volume of the artificial sill is comparable to existing large public works projects such as the Dubai Islands or the Suez Canal, but the magnitude of averted sea-level rise is small, the success of the intervention depends on the choice of calving law, and the glaciers return to their non-geoengineered trajectories within one to two centuries. Intervention in Antarctica, on the other hand, would be high-cost and high-reward: the volume of the artificial sill is one to two orders of magnitude greater, but the averted sea level rise is much larger, the intervention is successful regardless of the choice of calving law, and the ice streams remain far from their non-geoengineered trajectories throughout the 1000 year duration of our model runs. In both cases, an

Stream water quality in the coal region of West Virginia and Maryland

Treesearch

Kenneth L. Dyer

1982-01-01

This report is a compilation of water quality data for 118 small streams sampled in 27 counties of West Virginia and nine streams in two counties of western Maryland. Forty-eight of these streams drain unmined watersheds; 79 drain areas where coal has been surface mined. Most of these streams were sampled at approximate monthly intervals. The water quality data from...

Rate and style of ice stream retreat constrained by new surface-exposure ages: The Minch, NW Scotland

NASA Astrophysics Data System (ADS)

Bradwell, Tom; Small, David; Fabel, Derek; Dove, Dayton; Cofaigh, Colm O.; Clark, Chris; Consortium, Britice-Chrono

2016-04-01

Chronologically constrained studies of former ice-sheet extents and dynamics are important for understanding past cryospheric responses and modelling future ice-sheet and sea-level change. As part of the BRITICE-CHRONO project, we present new geomorphological and chronological data from a marine-terminating ice stream system in NW Europe that operated during the Late Weichselian Glaciation. A suite of 51 cosmogenic-nuclide exposure ages from ice sheet moraines and glacially transported boulders constrain the maximum extent of the ice sheet on the continental shelf (~28 ka BP) and its subsequent retreat, between ~27 and 16 ka BP, into a large marine embayment (ca. 7000 km2; the Minch, NW Scotland). Recently acquired swath bathymetry and acoustic sub-bottom profiler data reveal several large transverse grounding-zone wedges up to 40 m thick and 5 km wide with diagnostic acoustic-facies architecture. These seabed sediment wedges mark former quasi-stable positions of grounded marine-terminating ice-stream fronts; their size and thickness suggest long-lived stillstands of the order of centuries. Statistically significant clusters of exposure ages from glacial deposits on islands and intervening headlands shed important new light on the age of these marine grounding-zone wedges and, by inference, the rate and timing of Minch palaeo-ice stream retreat. We find strong evidence for episodic ice stream retreat on the continental shelf between ~28-24 ka BP, in the outer Minch between ~24-22 ka BP, and in the central Minch between 22-18.5 ka BP. In contrast, final ice stream deglaciation (<18 ka) across the deepest parts of the inner Minch embayment, was probably rapid and uninterrupted - with the ice sheet margin at or close to the present-day coastline in NW Scotland by 16.1 ka BP. It is hoped that these results will form the empirical basis for future ice-sheet modelling of this dynamically sensitive sector of the British-Irish Ice Sheet.

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.

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.

SPOT satellite mapping of Ice Stream B

NASA Technical Reports Server (NTRS)

Merry, Carolyn J.

1993-01-01

Numerous features of glaciological significance appear on two adjoining SPOT High Resolution Visible (HRV) images that cover the onset region of ice stream B. Many small-scale features, such as crevasses and drift plumes, have been previously observed in aerial photography. Subtle features, such as long flow traces that have not been mapped previously, are also clear in the satellite imagery. Newly discovered features include ladder-like runners and rungs within certain shear margins, flow traces that are parallel to ice flow, unusual crevasse patterns, and flow traces originating within shear margins. An objective of our work is to contribute to an understanding of the genesis of the features observed in satellite imagery. The genetic possibilities for flow traces, other lineations, bands of transverse crevasses, shear margins, mottles, and lumps and warps are described.

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).

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

Recent Climate and Ice-Sheet Changes in West Antarctica Compared with the Past 2,000 Years

NASA Technical Reports Server (NTRS)

Steig, Eric J.; Ding, Qinghua; White, James W.; Kuttel, Marcel; Rupper, Summer B.; Neumann, Thomas Allen; Neff, Peter D.; Gallant, Ailie J. E.; Mayewski, Paul A.; Taylor, Kendrick C.;

Effects of ice and floods on vegetation in streams in cold regions: implications for climate change

PubMed Central

Lind, Lovisa; Nilsson, Christer; Weber, Christine

2014-01-01

Riparian zones support some of the most dynamic and species-rich plant communities in cold regions. A common conception among plant ecologists is that flooding during the season when plants are dormant generally has little effect on the survival and production of riparian vegetation. We show that winter floods may also be of fundamental importance for the composition of riverine vegetation. We investigated the effects of ice formation on riparian and in-stream vegetation in northern Sweden using a combination of experiments and observations in 25 reaches, spanning a gradient from ice-free to ice-rich reaches. The ice-rich reaches were characterized by high production of frazil and anchor ice. In a couple of experiments, we exposed riparian vegetation to experimentally induced winter flooding, which reduced the dominant dwarf-shrub cover and led to colonization of a species-rich forb-dominated vegetation. In another experiment, natural winter floods caused by anchor-ice formation removed plant mimics both in the in-stream and in the riparian zone, further supporting the result that anchor ice maintains dynamic plant communities. With a warmer winter climate, ice-induced winter floods may first increase in frequency because of more frequent shifts between freezing and thawing during winter, but further warming and shortening of the winter might make them less common than today. If ice-induced winter floods become reduced in number because of a warming climate, an important disturbance agent for riparian and in-stream vegetation will be removed, leading to reduced species richness in streams and rivers in cold regions. Given that such regions are expected to have more plant species in the future because of immigration from the south, the distribution of species richness among habitats can be expected to show novel patterns. PMID:25505542

Effects of ice and floods on vegetation in streams in cold regions: implications for climate change.

PubMed

Lind, Lovisa; Nilsson, Christer; Weber, Christine

2014-11-01

Riparian zones support some of the most dynamic and species-rich plant communities in cold regions. A common conception among plant ecologists is that flooding during the season when plants are dormant generally has little effect on the survival and production of riparian vegetation. We show that winter floods may also be of fundamental importance for the composition of riverine vegetation. We investigated the effects of ice formation on riparian and in-stream vegetation in northern Sweden using a combination of experiments and observations in 25 reaches, spanning a gradient from ice-free to ice-rich reaches. The ice-rich reaches were characterized by high production of frazil and anchor ice. In a couple of experiments, we exposed riparian vegetation to experimentally induced winter flooding, which reduced the dominant dwarf-shrub cover and led to colonization of a species-rich forb-dominated vegetation. In another experiment, natural winter floods caused by anchor-ice formation removed plant mimics both in the in-stream and in the riparian zone, further supporting the result that anchor ice maintains dynamic plant communities. With a warmer winter climate, ice-induced winter floods may first increase in frequency because of more frequent shifts between freezing and thawing during winter, but further warming and shortening of the winter might make them less common than today. If ice-induced winter floods become reduced in number because of a warming climate, an important disturbance agent for riparian and in-stream vegetation will be removed, leading to reduced species richness in streams and rivers in cold regions. Given that such regions are expected to have more plant species in the future because of immigration from the south, the distribution of species richness among habitats can be expected to show novel patterns.

Export of Ice-Cavity Water from Pine Island Ice Shelf, West Antarctica

NASA Astrophysics Data System (ADS)

Thurnherr, Andreas; Jacobs, Stanley; Dutrieux, Pierre

2013-04-01

Stability of the West Antarctic Ice Sheet is sensitive to changes in melting at the bottom of floating ice shelves that form the seaward extensions of Antarctic glaciers flowing into the ocean. Not least because observations in the cavities beneath ice shelves are difficult, heat fluxes and melt rates have been inferred from oceanographic measurements obtained near the ice edge (calving fronts). Here, we report on a set of hydrographic and velocity data collected in early 2009 near the calving front of the Amundsen Sea's fast-moving and (until recently) accelerating Pine Island Glacier and its associated ice shelf. CTD profiles collected along the southern half of the meridionally-trending ice front show clear evidence for export of ice-cavity water. That water was carried in the upper ocean along the ice front by a southward current that is possibly related to a striking clockwise gyre that dominated the (summertime) upper-ocean circulation in Pine Island Bay. Signatures of ice-cavity water appear unrelated to current direction along most of the ice front, suggesting that cross-frontal exchange is dominated by temporal variability. However, repeated hydrographic and velocity measurements in a small "ice cove" at the southern end of the calving front show a persistent strong (mean velocity peaking near 0.5 ms-1) outflow of ice-cavity water in the upper 500 m. While surface features (boils) suggested upwelling from deep below the ice shelf, vertical velocity measurements reveal 1) that the mean upwelling within the confines of the cove was too weak to feed the observed outflow, and 2) that large high-frequency internal waves dominated the vertical motion of water inside the cove. These observations indicate that water exchange between the Pine Island Ice Shelf cavity and the Amundsen sea is strongly asymmetric with weak broad inflow at depth and concentrated surface-intensified outflow of melt-laden deep water at the southern edge of the calving front. The lack of

Imaging Basal Crevasses at the Grounding Line of Whillans Ice Stream, West Antarctica

NASA Astrophysics Data System (ADS)

Jacobel, R. W.; Dawson, E. C.; Christianson, K.

2015-12-01

We acquired gridded ground-based radar data at the WIS grounding zone where the transition from limited- or no-slip conditions at the base of grounded ice to free-slip conditions beneath floating ice occurs across a region only a few kilometers wide. This transition is either an elastic-flexural transition from bedrock to hydrostatically-supported elevations (often tidally influenced), a transition from thicker to thinner ice over a flat bed, or some combination of these. In either case, the stress field of the ice changes as it flows across the grounding zone, often resulting in brittle deformation, which is manifested as basal crevassing at the ice-sheet base and sometimes as strand cracks at the surface. The position and morphology of these features reveal important information about the stress state across this transition where ice and ocean interact. Our surveys indicate a complex pattern of basal crevassing with many imaged in two or more profile segments as a linear feature at the bed, usually trending oblique to flow and often extending for several kilometers. Due to the wide beam pattern of our antennas, we image many of the crevasses from off-nadir reflections. Thus their arrival times are later than the primary basal reflection and segments of the crevasse appear "below" the bed, when in fact they are merely trending oblique to the profile. Often these returns have a reversed phase relative to the bed echo because the high dielectric contrast of seawater and a favorable geometry enable reflections with little loss (but a second phase reversal) from the ice-water interface near the crevasse base. In a few cases, these crevasse echoes from targets trending oblique to the profile appear to mimic the geometry of a sub-ice sediment "wedge", while in reality the radar never penetrates below the basal interface. Only about 25% of the crevasses appear to extend any significant distance upward into the basal ice, typically at low angles. A subset of these are

Microbial oxidation as a methane sink beneath the West Antarctic Ice Sheet

NASA Astrophysics Data System (ADS)

Michaud, Alexander B.; Dore, John E.; Achberger, Amanda M.; Christner, Brent C.; Mitchell, Andrew C.; Skidmore, Mark L.; Vick-Majors, Trista J.; Priscu, John C.

2017-08-01

Aquatic habitats beneath ice masses contain active microbial ecosystems capable of cycling important greenhouse gases, such as methane (CH4). A large methane reservoir is thought to exist beneath the West Antarctic Ice Sheet, but its quantity, source and ultimate fate are poorly understood. For instance, O2 supplied by basal melting should result in conditions favourable for aerobic methane oxidation. Here we use measurements of methane concentrations and stable isotope compositions along with genomic analyses to assess the sources and cycling of methane in Subglacial Lake Whillans (SLW) in West Antarctica. We show that sub-ice-sheet methane is produced through the biological reduction of CO2 using H2. This methane pool is subsequently consumed by aerobic, bacterial methane oxidation at the SLW sediment-water interface. Bacterial oxidation consumes >99% of the methane and represents a significant methane sink, and source of biomass carbon and metabolic energy to the surficial SLW sediments. We conclude that aerobic methanotrophy may mitigate the release of methane to the atmosphere upon subglacial water drainage to ice sheet margins and during periods of deglaciation.

Glacial removal of late Cenozoic subglacially emplaced volcanic edifices by the West Antarctic ice sheet

USGS Publications Warehouse

Behrendt, John C.; Blankenship, D.D.; Damaske, D.; Cooper, A. K.

1995-01-01

Local maxima of the horizontal gradient of pseudogravity from closely spaced aeromagnetic surveys over the Ross Sea, northwestern Ross Ice Shelf, and the West Antarctic ice sheet, reveal a linear magnetic rift fabric and numerous subcircular, high-amplitude anomalies. Geophysical data indicate two or three youthful volcanic edifices at widely separated areas beneath the sea and ice cover in the West Antarctic rift system. In contrast, we suggest glacial removal of edifices of volcanic sources of many more anomalies. Magnetic models, controlled by marine seismic reflection and radar ice-sounding data, allow us to infer that glacial removal of the associated late Cenozoic volcanic edifices (probably debris, comprising pillow breccias, and hyaloclastites) has occurred essentially concomitantly with their subglacial eruption. "Removal' of unconsolidated volcanic debris erupted beneath the ice is probably a more appropriate term than "erosion', given its fragmented, ice-contact origin. The exposed volcanoes may have been protected from erosion by the surrounding ice sheet because of more competent rock or high elevation above the ice sheet. -from Authors

Latest Word on Retreat of the West Antarctic Ice Sheet

NASA Technical Reports Server (NTRS)

Bindschadler, R.

2000-01-01

The West Antarctic ice sheet during the Last Glacial Maximum (LGM) is estimated to have been three times its present volume and to have extended close to the edge of the continental shelf Holocene retreat of this ice sheet in the Ross Sea began between 11,000 and 12,000 years ago. This history implies an average contribution of this ice sheet to sea level of 0.9 mm/a. Evidence of dateable past grounding line positions in the Ross sector are broadly consistent with a linear retreat model. However, inferred rates of retreat for some of these grounding line positions are not consistent with a linear retreat model. More rapid retreat approximately 7600 years ago and possible near-stability in the Ross Sea sector at present suggest a slow rate of initial retreat followed by a more rapid-than-average retreat during the late Holocene, returning to a near-zero rate of retreat currently. This model is also consistent with the mid-Holocene high stand observations of eustatic sea level. Recent compilation of Antarctic bed elevations (BEDMAP) illustrates that the LGM and present grounding lines occur in the shallowest waters, further supporting the model of a middle phase of rapid retreat bracketed by an older and a more recent phase of modest retreat. Extension of these hypotheses into the future make subsequent behavior of the West Antarctic ice sheet more difficult to predict but suggest that if it loses its hold on the present shallow bed, the final retreat of the ice sheet could be very rapid.

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.

Phreatomagmatic eruptions under the West Antarctic Ice Sheet: potential hazard for ice sheet stability

NASA Astrophysics Data System (ADS)

Iverson, N. A.; Dunbar, N. W.; Lieb-Lappen, R.; Kim, E. J.; Golden, E. J.; Obbard, R. W.

2014-12-01

Volcanic tephra layers have been seen in most ice cores in Antarctica. These tephra layers are deposited almost instantaneously across wide areas of ice sheets, creating horizons that can provide "pinning points" to adjust ice time scales that may otherwise be lacking detailed chronology. A combination of traditional particle morphology characterization by SEM with new non-destructive X-ray micro-computed tomography (Micro-CT) has been used to analyze selected coarse grained tephra in the West Antarctica Ice Sheet (WAIS) Divide WDC06A ice core. Micro-CT has the ability to image particles as small as 50µm in length (15µm resolution), quantifying both particle shape and size. The WDC06A contains hundreds of dusty layers of which 36 have so far been identified as primary tephra layers. Two of these tephra layers have been characterized as phreatomagmatic eruptions based on SEM imagery and are blocky and platy in nature, with rare magmatic particles. These layers are strikingly different in composition from the typical phonolitic and trachytic tephra produced from West Antarctic volcanoes. These two layers are coarser in grain size, with many particles (including feldspar crystals) exceeding 100µm in length. One tephra layer found at 3149.138m deep in the ice core is a coarse ~1mm thick basanitic tephra layer with a WDC06-7 ice core age of 45,381±2000yrs. The second layer is a ~1.3 cm thick zoned trachyandesite to trachydacite tephra found at 2569.205m deep with an ice core age 22,470±835yrs. Micro-CT analysis shows that WDC06A-3149.138 has normal grading with the largest particles at the bottom of the sample (~160μm). WDC06A-2569.205 has a bimodal distribution of particles with large particles at the top and bottom of the layer. These large particles are more spherical in shape at the base and become more irregular and finer grained higher in the layer, likely showing changes in eruption dynamics. The distinct chemistry as well as the blocky and large grain size

9. INTAKE STREAM ON GROUND LOOKING WEST AS IT FLOWS ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

9. INTAKE STREAM ON GROUND LOOKING WEST AS IT FLOWS DOWNSTREAM TO LAKE MATHEWS, ALL WATER COMING FROM PUMPS. - Colorado River Aqueduct, From Colorado River to Lake Mathews, Parker Dam, San Bernardino County, CA

Implications of basal micro-earthquakes and tremor for ice stream mechanics: Stick-slip basal sliding and till erosion

NASA Astrophysics Data System (ADS)

Barcheck, C. Grace; Tulaczyk, Slawek; Schwartz, Susan Y.; Walter, Jacob I.; Winberry, J. Paul

2018-03-01

The Whillans Ice Plain (WIP) is unique among Antarctic ice streams because it moves by stick-slip. The conditions allowing stick-slip and its importance in controlling ice dynamics remain uncertain. Local basal seismicity previously observed during unstable slip is a clue to the mechanism of ice stream stick-slip and a window into current basal conditions, but the spatial extent and importance of this basal seismicity are unknown. We analyze data from a 2010-2011 ice-plain-wide seismic and GPS network to show that basal micro-seismicity correlates with large-scale patterns in ice stream slip behavior: Basal seismicity is common where the ice moves the least between unstable slip events, with small discrete basal micro-earthquakes happening within 10s of km of the central stick-slip nucleation area and emergent basal tremor occurring downstream of this area. Basal seismicity is largely absent in surrounding areas, where inter-slip creep rates are high. The large seismically active area suggests that a frictional sliding law that can accommodate stick-slip may be appropriate for ice stream beds on regional scales. Variability in seismic behavior over inter-station distances of 1-10 km indicates heterogeneity in local bed conditions and frictional complexity. WIP unstable slips may nucleate when stick-slip basal earthquake patches fail over a large area. We present a conceptual model in which basal seismicity results from slip-weakening frictional failure of over-consolidated till as it is eroded and mobilized into deforming till.

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

Seasonal climate information preserved within West Antarctic ice cores and its relation to large-scale atmospheric circulation and regional sea ice variations

NASA Astrophysics Data System (ADS)

Küttel, M.; Steig, E. J.; Ding, Q.; Battisti, D. S.

2010-12-01

Recent evidence suggests that West Antarctica has been warming since at least the 1950s. With the instrumental record being limited to the mid-20th century, indirect information from stable isotopes (δ18O and δD, hereafter collectively δ) preserved within ice cores have commonly been used to place this warming into a long term context. Here, using a large number of δ records obtained during the International Trans-Antarctic Scientific Expedition (ITASE), past variations in West Antarctic δ are not only investigated over time but also in space. This study therefore provides an important complement to longer records from single locations as e.g. the currently being processed West Antarctic ice sheet (WAIS) Divide ice core. Although snow accumulation rates at the ITASE sites in West Antarctica are variable, they are generally high enough to allow studies on sub-annual scale over the last 50-100 years. Here, we show that variations in δ in this region are strongly related to the state of the large-scale atmospheric circulation as well as sea ice variations in the adjacent Southern Ocean, with important seasonal changes. While a strong relationship to sea ice changes in the Ross and Amundsen Sea as well as to the atmospheric circulation offshore is found during austral fall (MAM) and winter (JJA), only modest correlations are found during spring (SON) and summer (DJF). Interestingly, the correlations with the atmospheric circulation in the latter two seasons have the strongest signal over the Antarctic continent, but not offshore - an important difference to MAM and JJA. These seasonal changes are in good agreement with the seasonally varying predominant circulation: meridional with more frequent storms in the Amundsen Sea during MAM and JJA and more zonal and stable during SON and DJF. The relationship to regional temperature is similarly seasonally variable with highest correlations found during MAM and JJA. Notably, the circulation pattern found to be strongest

Ocean mixing beneath Pine Island Glacier ice shelf, West Antarctica

NASA Astrophysics Data System (ADS)

Kimura, Satoshi; Jenkins, Adrian; Dutrieux, Pierre; Forryan, Alexander; Naveira Garabato, Alberto C.; Firing, Yvonne

2016-12-01

Ice shelves around Antarctica are vulnerable to an increase in ocean-driven melting, with the melt rate depending on ocean temperature and the strength of flow inside the ice-shelf cavities. We present measurements of velocity, temperature, salinity, turbulent kinetic energy dissipation rate, and thermal variance dissipation rate beneath Pine Island Glacier ice shelf, West Antarctica. These measurements were obtained by CTD, ADCP, and turbulence sensors mounted on an Autonomous Underwater Vehicle (AUV). The highest turbulent kinetic energy dissipation rate is found near the grounding line. The thermal variance dissipation rate increases closer to the ice-shelf base, with a maximum value found ˜0.5 m away from the ice. The measurements of turbulent kinetic energy dissipation rate near the ice are used to estimate basal melting of the ice shelf. The dissipation-rate-based melt rate estimates is sensitive to the stability correction parameter in the linear approximation of universal function of the Monin-Obukhov similarity theory for stratified boundary layers. We argue that our estimates of basal melting from dissipation rates are within a range of previous estimates of basal melting.

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

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.

Earthquake-induced deformations on ice-stream landforms in Kuusamo, eastern Finnish Lapland

NASA Astrophysics Data System (ADS)

Sutinen, Raimo; Hyvönen, Eija; Middleton, Maarit; Airo, Meri-Liisa

2018-01-01

Kuusamo in eastern Finnish Lapland is characterized by ice-streamlined landforms as well as clusters of historical and recent earthquakes (Mw < 4). Since recent earthquakes are often found to be located on the traces of postglacial faults (PGFs) within the Fennoscandian shield we postulate that some part of the ice-stream landforms have been deformed by the past earthquakes in Kuusamo. Airborne LiDAR (Light Detection And Ranging) DEMs (digital elevation models) revealed significant numbers of postglacial deformations, such as liquefaction deformations, rotational landslides, earth flows as well as kettle holes (craters), on the fluted surfaces within the Kuusamo ice-stream fan. We found these deformations to be a common feature on the Archean granitoid gneisses and within a 20 km wide and NW-SE oriented corridor between the major intrusives, the Iivaara nepheline syenite and the Näränkävaara gabbro. Of the paleolandslides, liquefaction morphologies were generally developed on the distal slopes (1.3-2.8%; 0.75-1.6°) of the streamlined forms. Sedimentary anisotropy, obtained with azimuthal electrical conductivity (σa; skin depth down to 3-6 m), of the deformed flutes significantly deviated from the non-deformed (clean) ones. The fields of the Pulju moraine, a subglacial landform, formed a grounding zone for the ice-streaming SW of the paleolandslide cluster. We therefore propose that both subglacial and postglacial earthquake-induced landforms are present in Kuusamo. No PGFs could be verified in the Kuusamo area, yet gravity, airborne magnetic, and LiDAR morphological lineaments suggest that the old Paleoproterozoic structures have been reactivated as strike-slip faults, due to the lithospheric plate stresses and glacio-isostatic adjustment (GIA).

Subglacial Volcanism in West-Antarctica - A Geologic and Ice Dynamical Perspective

NASA Astrophysics Data System (ADS)

Vogel, S. W.; Tulaczyk, S.; Carter, S.; Renne, P.; Turrin, B. D.; Joughin, I.

2004-12-01

Subglacial volcanic eruptions may increase the contribution of the West-Antarctic Ice-Sheet (WAIS) to global sea-level rise in the near-future by enhancing basal melt water production and ice flow lubrication. Geophysical data have led scientists to believe that the ice sheet may be located over an extensive, young volcanic province containing ~1 million cubic kilometers of basalts (Behrendt, 1964; Behrendt et. al., 1991; 1995; 1998). While not all scientists may recognize this theory of widespread subglacial volcanism, so far no scientific paper has challenged its existence. Here we present the first geologic constraints on the presence/absence of widespread Late Cenozoic subglacial volcanism beneath the WAIS and investigate the potential influence of an individual subglacial volcano (Blankenship et. al., 1993) on the flow dynamic of WAIS. Properties of subglacial sediments indicate limited presence of subglacial volcanic rocks. Moreover, the only two basaltic pebbles, recovered from the region, are of Mesozoic-Paleozoic age (~100 to ~500 million years). While these findings reduce the potential for widespread near-future increases in ice discharge from WAIS due to eruptions of subglacial volcanoes, they do not rule out the presence of individual hot spots associated with volcanic centers beneath the WAIS. Fuel for the existence of a proposed volcano (Mt. Casertz) on the Whitmore Mountain Ross Sea Transitional Crust (WRT; Blankenship et. al., 1993), in the southern part of the WAIS, comes from thermo-dynamical modeling in comparison with observed ice velocities. Ice velocities (Joughin et. al., 1999; 2002) downstream of Mt. Casertz indicate significant basal sliding, where thermo-dynamical models suggest that the ice sheet is frozen to its base. Routing of basal melt water, produced in the vicinity of Mt. Casertz, may lubricate the ice base in parts of the WRT, thus enabling basal sliding and enhancing the discharge of ice in this sector of the WAIS. The only

Combined ice core and climate-model evidence for the collapse of the West Antarctic Ice Sheet during Marine Isotope Stage 5e.

NASA Astrophysics Data System (ADS)

Steig, Eric J.; Huybers, Kathleen; Singh, Hansi A.; Steiger, Nathan J.; Frierson, Dargan M. W.; Popp, Trevor; White, James W. C.

2015-04-01

It has been speculated that collapse of the West Antarctic Ice Sheet explains the very high eustatic sea level rise during the last interglacial period, marine isotope stage (MIS) 5e, but the evidence remains equivocal. Changes in atmospheric circulation resulting from a collapse of the West Antarctic Ice Sheet (WAIS) would have significant regional impacts that should be detectable in ice core records. We conducted simulations using general circulation models (GCMs) at varying levels of complexity: a gray-radiation aquaplanet moist GCM (GRaM), the slab ocean version of GFDL-AM2 (also as an aquaplanet), and the fully-coupled version of NCAR's CESM with realistic topography. In all the experiments, decreased elevation from the removal of the WAIS leads to greater cyclonic circulation over the West Antarctic region. This creates increased advection of relatively warm marine air from the Amundsen-Bellingshausen Seas towards the South Pole, and increased cold-air advection from the East Antarctic plateau towards the Ross Sea and coastal Marie Byrd Land. The result is anomalous warming in some areas of the East Antarctic interior, and significant cooling in Marie Byrd Land. Comparison of ice core records shows good agreement with the model predictions. In particular, isotope-paleotemperature records from ice cores in East Antarctica warmed more between the previous glacial period (MIS 6) and MIS 5e than coastal Marie Byrd Land. These results add substantial support to other evidence for WAIS collapse during the last interglacial period.

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

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

Increased West Antarctic and unchanged East Antarctic ice discharge over the last 7 years

NASA Astrophysics Data System (ADS)

Gardner, Alex S.; Moholdt, Geir; Scambos, Ted; Fahnstock, Mark; Ligtenberg, Stefan; van den Broeke, Michiel; Nilsson, Johan

2018-02-01

Ice discharge from large ice sheets plays a direct role in determining rates of sea-level rise. We map present-day Antarctic-wide surface velocities using Landsat 7 and 8 imagery spanning 2013-2015 and compare to earlier estimates derived from synthetic aperture radar, revealing heterogeneous changes in ice flow since ˜ 2008. The new mapping provides complete coastal and inland coverage of ice velocity north of 82.4° S with a mean error of < 10 m yr-1, resulting from multiple overlapping image pairs acquired during the daylight period. Using an optimized flux gate, ice discharge from Antarctica is 1929 ± 40 Gigatons per year (Gt yr-1) in 2015, an increase of 36 ± 15 Gt yr-1 from the time of the radar mapping. Flow accelerations across the grounding lines of West Antarctica's Amundsen Sea Embayment, Getz Ice Shelf and Marguerite Bay on the western Antarctic Peninsula, account for 88 % of this increase. In contrast, glaciers draining the East Antarctic Ice Sheet have been remarkably constant over the period of observation. Including modeled rates of snow accumulation and basal melt, the Antarctic ice sheet lost ice at an average rate of 183 ± 94 Gt yr-1 between 2008 and 2015. The modest increase in ice discharge over the past 7 years is contrasted by high rates of ice sheet mass loss and distinct spatial patters of elevation lowering. The West Antarctic Ice Sheet is experiencing high rates of mass loss and displays distinct patterns of elevation lowering that point to a dynamic imbalance. We find modest increase in ice discharge over the past 7 years, which suggests that the recent pattern of mass loss in Antarctica is part of a longer-term phase of enhanced glacier flow initiated in the decades leading up to the first continent-wide radar mapping of ice flow.

Role of stream ice on fall and winter movements and habitat use by bull trout and cutthroat trout in Montana headwater streams

Treesearch

Michael J. Jakober; Thomas E. McMahon; Russell F. Thurow; Christopher G. Clancy

1998-01-01

We used radiotelemetry and underwater observation to assess fall and winter movements and habitat use by bull trout Salvelinus confluentus and westslope cutthroat trout Oncorhynchus clarki lewisi in two headwater streams in the Bitterroot River drainage, Montana, that varied markedly in habitat availability and stream ice conditions. Bull trout and cutthroat trout made...

The extent and timing of the last British-Irish Ice Sheet offshore of west Ireland-preliminary findings

NASA Astrophysics Data System (ADS)

Peters, Jared; Benetti, Sara; Dunlop, Paul; Cofaigh, Colm Ó.

2014-05-01

Recently interpreted marine geophysical data from the western Irish shelf has provided the first direct evidence that the last British-Irish Ice Sheet (BIIS) extended westwards onto the Irish continental shelf as a grounded ice mass composed of several lobes with marine-terminating margins. Marine terminating ice margins are known to be sensitive to external forcing mechanisms and currently there is concern regarding the future stability of marine based ice sheets, such as the West Antarctic Ice Sheet, in a warming world. Given its position, the glaciated western Irish continental shelf is a prime location to investigate the processes of how marine-based ice sheets responded to past climatic and oceanic events, which may in turn help us better predict the future trajectory of the marine sectors of modern Ice Sheets. However, despite the potential importance of the former Irish ice margin to our understanding of ice sheet behaviour, the timing and nature of its advance and retreat is currently poorly understood. This study aims to describe the depositional history of the last BIIS on the continental shelf west of Ireland and age-constrain the rate of retreat of two ice lobes that extended from Galway Bay and Clew Bay. This is being accomplished through a multifaceted analysis of at least 29 sediment cores gathered across the continental shelf offshore of counties Galway and Mayo, Ireland. This poster shows results from initial sedimentological descriptions of cores from the mid to outer shelf, which support previous geomorphic interpretations of BIIS history. Preliminary palaeoenvironmental results from ongoing micropaleontological analyses are also discussed and provide new data that verifies sedimentary interpretations on ice proximity. Finally, results from several radiocarbon dates are discussed, which limit these deposits to the last glacial maximum and constrain the timings of ice advance and retreat on the continental shelf west of Ireland.

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.

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 diatom record from beneath the West Antarctic Ice Sheet and the global proxy perspective

NASA Technical Reports Server (NTRS)

Scherer, Reed P.

1993-01-01

Recent glaciological evaluation and modeling of the marine-based West Antarctic Ice Sheet (WAIS) support the possibility that the WAIS disintegrated during one or more Pleistocene interglacial period(s). The magnitude of sea level and oxygen isotope variation during certain late-Pleistocene interglacial periods is also consistent with the possibility of major retreat of the WAIS. Although oxygen isotopes from deep-sea sediments provide the best available proxy record for global ice volume (despite the ambiguities in the record), the source of ice volume changes must be hypothesized. Based on the intensity of interglacial isotopic shifts recorded in Southern Ocean marine sedimentary records, stage 11 (400,000 years ago) is the strongest candidate for WAIS collapse, but the records for stages 9, 7, and 5.5 are all consistent with the possibility of multiple late-Pleistocene collapses. Seismic reflection studies through the WAIS have revealed thick successions of strata with seismic characteristics comparable to upper Tertiary marine sediments. Small samples of glacial diamictons from beneath the ice sheet have been collected via hot-water drilled access holes. These sediments include mixed diatom assemblages of varying ages. Late-Miocene diatoms dominate many samples, probably reflecting marine deposition in West Antarctic basins prior to development of a dominantly glacial phase in West Antarctica. In addition to late-Miocene diatoms, samples from Upstream B (1988/89) contain rare post-Miocene diatoms, many of which imply deposition in the West Antarctic interior during one or more Pleistocene deglaciation periods. Age-diagnostic fossils in glacial sediments beneath ice sheets provide relatively coarse chronostratigraphic control, but they do contain direct evidence of regional deglaciation. Thus, sub-glacial till samples provide the evidence regarding the source of ice sheet variability seen in well-dated proxy records. Combined, these independent data sets can

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.

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

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.

Accelerated West Antarctic ice mass loss continues to outpace East Antarctic gains

NASA Astrophysics Data System (ADS)

Harig, Christopher; Simons, Frederik J.

2015-04-01

While multiple data sources have confirmed that Antarctica is losing ice at an accelerating rate, different measurement techniques estimate the details of its geographically highly variable mass balance with different levels of accuracy, spatio-temporal resolution, and coverage. Some scope remains for methodological improvements using a single data type. In this study we report our progress in increasing the accuracy and spatial resolution of time-variable gravimetry from the Gravity Recovery and Climate Experiment (GRACE). We determine the geographic pattern of ice mass change in Antarctica between January 2003 and June 2014, accounting for glacio-isostatic adjustment (GIA) using the IJ05_R2 model. Expressing the unknown signal in a sparse Slepian basis constructed by optimization to prevent leakage out of the regions of interest, we use robust signal processing and statistical estimation methods. Applying those to the latest time series of monthly GRACE solutions we map Antarctica's mass loss in space and time as well as can be recovered from satellite gravity alone. Ignoring GIA model uncertainty, over the period 2003-2014, West Antarctica has been losing ice mass at a rate of - 121 ± 8 Gt /yr and has experienced large acceleration of ice mass losses along the Amundsen Sea coast of - 18 ± 5 Gt /yr2, doubling the mass loss rate in the past six years. The Antarctic Peninsula shows slightly accelerating ice mass loss, with larger accelerated losses in the southern half of the Peninsula. Ice mass gains due to snowfall in Dronning Maud Land have continued to add about half the amount of West Antarctica's loss back onto the continent over the last decade. We estimate the overall mass losses from Antarctica since January 2003 at - 92 ± 10 Gt /yr.

Estimation of Flood-Frequency Discharges for Rural, Unregulated Streams in West Virginia

USGS Publications Warehouse

Wiley, Jeffrey B.; Atkins, John T.

2010-01-01

Flood-frequency discharges were determined for 290 streamgage stations having a minimum of 9 years of record in West Virginia and surrounding states through the 2006 or 2007 water year. No trend was determined in the annual peaks used to calculate the flood-frequency discharges. Multiple and simple least-squares regression equations for the 100-year (1-percent annual-occurrence probability) flood discharge with independent variables that describe the basin characteristics were developed for 290 streamgage stations in West Virginia and adjacent states. The regression residuals for the models were evaluated and used to define three regions of the State, designated as Eastern Panhandle, Central Mountains, and Western Plateaus. Exploratory data analysis procedures identified 44 streamgage stations that were excluded from the development of regression equations representative of rural, unregulated streams in West Virginia. Regional equations for the 1.1-, 1.5-, 2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500-year flood discharges were determined by generalized least-squares regression using data from the remaining 246 streamgage stations. Drainage area was the only significant independent variable determined for all equations in all regions. Procedures developed to estimate flood-frequency discharges on ungaged streams were based on (1) regional equations and (2) drainage-area ratios between gaged and ungaged locations on the same stream. The procedures are applicable only to rural, unregulated streams within the boundaries of West Virginia that have drainage areas within the limits of the stations used to develop the regional equations (from 0.21 to 1,461 square miles in the Eastern Panhandle, from 0.10 to 1,619 square miles in the Central Mountains, and from 0.13 to 1,516 square miles in the Western Plateaus). The accuracy of the equations is quantified by measuring the average prediction error (from 21.7 to 56.3 percent) and equivalent years of record (from 2.0 to 70

Evidence for heterogeneous (and possibly transient) geothermal flux beneath the Ross-Amundsen ice divide of the West Antarctic ice sheet

NASA Astrophysics Data System (ADS)

Blankenship, D. D.; Danque, H. A.; Quartini, E.; Young, D. A.

2012-12-01

It is well established that the geological framework for the evolution of the marine-based West Antarctic ice sheet (WAIS) is the Cretaceous through Cenozoic rifting of the underlying lithosphere. The southern flank of this rift along the Whitmore Mountains underlies the upper reaches of the Ross Sea catchment of the WAIS and has been identified as a site of active subglacial volcanism. Interestingly, the northern flank of this rift represented by the upward doming of the Marie Byrd Land volcanic province has not yet been associated with active subglacial volcanism. Similarly, it is not known whether the heterogeneity of geothermal flux associated with these existing and potential rift flank volcanic provinces extends across the floor of the rift between the rift flanks. Here we present geophysical evidence for heterogeneous geothermal flux associated with active subglacial volcanism along the northern rift flank adjacent to Marie Byrd Land where it intersects the ice divide for the Ross and Amundsen Sea sectors for the WAIS. We further evaluate the evidence for the continuity of heterogeneous geothermal flux along this ice divide and across the rift floor between the two flanks of the West Antarctic rift system.

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.

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.

West Antarctic Ice Sheet Initiative. Volume 1: Science and Implementation Plan

NASA Technical Reports Server (NTRS)

Bindschadler, Robert A. (Editor)

1990-01-01

The Science and Implementation Plan of the West Antarctic Ice Sheet Initiative (WAIS) is described. The goal of this initiative is the prediction of the future behavior of this ice sheet and an assessment of its potential to collapse, rapidly raising global sea level. The multidisciplinary nature of WAIS reflects the complexity of the polar ice sheet environment. The project builds upon past and current polar studies in many fields and meshes with future programs of both the U.S. and other countries. Important tasks in each discipline are described and a coordinated schedule by which the majority of these tasks can be accomplished in 5 years is presented. The companion report (Volume 2) contains seven discipline review papers on the state of knowledge of Antarctica and opinions on how that knowledge must be increased to attain the WAIS goal.

Ice-dammed lake drainage evolution at Russell Glacier, west Greenland

NASA Astrophysics Data System (ADS)

Carrivick, Jonathan L.; Tweed, Fiona S.; Ng, Felix; Quincey, Duncan J.; Mallalieu, Joseph; Ingeman-Nielsen, Thomas; Mikkelsen, Andreas B.; Palmer, Steven J.; Yde, Jacob C.; Homer, Rachel; Russell, Andrew J.; Hubbard, Alun

2017-11-01

Glaciological and hydraulic factors that control the timing and mechanisms of glacier lake outburst floods (GLOFs) remain poorly understood. This study used measurements of lake level at fifteen minute intervals and known lake bathymetry to calculate lake outflow during two GLOF events from the northern margin of Russell Glacier, west Greenland. We used measured ice surface elevation, interpolated subglacial topography and likely conduit geometry to inform a melt enlargement model of the outburst evolution. The model was tuned to best-fit the hydrograph’s rising limb and timing of peak discharge in both events; it achieved Mean Absolute Errors of < 5 %. About one third of the way through the rising limb, conduit melt enlargement became the dominant drainage mechanism. Lake water temperature, which strongly governed the enlargement rate, preconditioned the high peak discharge and short duration of these floods. We hypothesize that both GLOFs were triggered by ice dam flotation, and localised hydraulic jacking sustained most of their early-stage outflow, explaining the particularly rapid water egress in comparison to that recorded at other ice-marginal lakes. As ice overburden pressure relative to lake water hydraulic head diminished, flow became confined to a subglacial conduit. This study has emphasised the inter-play between ice dam thickness and lake level, drainage timing, lake water temperature and consequently rising stage lake outflow and flood evolution.

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.

Variability of Basal Melt Beneath the Pine Island Glacier Ice Shelf, West Antarctica

NASA Technical Reports Server (NTRS)

Bindschadler, Robert; Vaughan, David G.; Vornberger, Patricia

2011-01-01

Observations from satellite and airborne platforms are combined with model calculations to infer the nature and efficiency of basal melting of the Pine Island Glacier ice shelf, West Antarctica, by ocean waters. Satellite imagery shows surface features that suggest ice-shelf-wide changes to the ocean s influence on the ice shelf as the grounding line retreated. Longitudinal profiles of ice surface and bottom elevations are analyzed to reveal a spatially dependent pattern of basal melt with an annual melt flux of 40.5 Gt/a. One profile captures a persistent set of surface waves that correlates with quasi-annual variations of atmospheric forcing of Amundsen Sea circulation patterns, establishing a direct connection between atmospheric variability and sub-ice-shelf melting. Ice surface troughs are hydrostatically compensated by ice-bottom voids up to 150m deep. Voids form dynamically at the grounding line, triggered by enhanced melting when warmer-than-average water arrives. Subsequent enlargement of the voids is thermally inefficient (4% or less) compared with an overall melting efficiency beneath the ice shelf of 22%. Residual warm water is believed to cause three persistent polynyas at the ice-shelf front seen in Landsat imagery. Landsat thermal imagery confirms the occurrence of warm water at the same locations.

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.

Late Oligocene glacimarine sedimentation of the central Ross Sea and implications for the evolution of the West Antarctic Ice Sheet

NASA Astrophysics Data System (ADS)

Kraus, C.; Mckay, R. M.; Naish, T.; Levy, R. H.; Kulhanek, D. K.

2015-12-01

Today the West Antarctic Ice Sheet (WAIS) is grounded mostly below sea level, making it sensitive to oceanic temperature and circulation changes. However, recent reconstructions of the Cenozoic bedrock topographic evolution of West Antarctica have suggested that the West Antarctic Ice Sheet (WAIS) may have first formed as a terrestrial ice sheet at the Eocene-Oligocene boundary (33 Ma), when there was up to 20% more land area in West Antarctica. At some point during the Oligocene or Miocene (23 - 5 Ma) vast areas of West Antarctica became an over-deepened marine-based continental shelf, as is observed today. The evolution of the WAIS through this transition is largely unconstrained, but as atmospheric CO2 fluctuated between 600 and 200 ppm over the past 34 Ma, determining the development of a marine-based WAIS is critical in the context of understanding the sensitivity of ice sheet systems to environmental change. Our research re-examines the sediment cores recovered from the central Ross Sea, a principal drainage area of the WAIS, at Deep Sea Drilling Project Site 270 (77°26.48'S, 178°30.19'W). These cores contain a glacimarine sequence of late Oligocene age (28 - 23.1 Ma). Sedimentological (visual core description, facies, grain size analysis), geochemical (x-ray fluorescence), geophysical (seismic) techniques, and physical properties (magnetic susceptibility) are used to construct a sedimentation model of this sequence, in order to track the late Oligocene evolution of the WAIS. The late Oligocene warming (25 - 23 Ma) is examined in detail because proximal Antarctic geological records of ice sheet extent, proxy environmental data, and atmospheric CO2 appear to be at odds with the composite δ18O record of global temperature and ice volume at this time. Moreover, our research provides insights into the sensitivity of marine-based ice sheets, and supports the hypothesis that they are unstable above a CO2 threshold of 400 ppm. Our preliminary results also

Ice processes affect habitat use and movements of adult cutthroat trout and brook trout in a Wyoming foothills stream

USGS Publications Warehouse

Lindstrom, J.W.; Hubert, W.A.

2004-01-01

Habitat use and movements of 25 adult cutthroat trout Oncorhynchus clarkii and 25 adult brook trout Salvelinus fontinalis from fall through winter 2002-2003 were assessed by means of radiotelemetry in a 7-km reach of a Rocky Mountains foothills stream. Temporal dynamics of winter habitat conditions were evaluated by regularly measuring the features of 30 pools and 5 beaver Castor canadensis ponds in the study reach. Groundwater inputs at three locations raised mean daily water temperatures in the stream channel during winter to 0.2-0.6??C and kept at least 250 m of the downstream channel free of ice, but the lack of surface ice further downstream led to the occurrence of frazil ice and anchor ice in pools and unstable habitat conditions for trout. Pools in segments that were not affected by groundwater inputs and beaver ponds tended to be stable and snow accumulated on the surface ice. Pools throughout the study reach tended to become more stable as snow accumulated. Both cutthroat trout and brook trout selected beaver ponds as winter progressed but tended to use lateral scour pools in proportion to their availability. Tagged fish not in beaver ponds selected lateral scour pools that were deeper than average and stable during winter. Movement frequencies by tagged fish decreased from fall through winter, but some individuals of both species moved during winter. Ice processes affected both the habitat use and movement patterns of cutthroat trout and brook trout in this foothills stream.

Recent Ice Loss from the Fleming and Other Glaciers, Wordie Bay, West Antarctic Peninsula

NASA Technical Reports Server (NTRS)

Rignot, E.; Casassa, G.; Gogineni, S.; Kanagaratnam, P.; Krabill, W.; Pritchard, H.; Rivera, A.; Thomas, R.; Turner, J.; Vaughan, D.

2005-01-01

Satellite radar interferometry data from 1995 to 2004, and airborne ice thickness data from 2002, reveal that the glaciers flowing into former Wordie Ice Shelf, West Antarctic Peninsula, discharge 6.8 +/- 0.3 km(exp 3)/yr of ice, which is 84 +/- 30 percent larger than a snow accumulation of 3.7 +/- 0.8 km(exp 3)/yr over a 6,300 km(exp 2) drainage basin. Airborne and ICESat laser altimetry elevation data reveal glacier thinning at rates up to 2 m/yr. Fifty km from its ice front, Fleming Glacier flows 50 percent faster than it did in 1974 prior to the main collapse of Wordie Ice Shelf. We conclude that the glaciers accelerated following ice shelf removal, and have been thinning and losing mass to the ocean over the last decade. This and other observations suggest that the mass loss from the northern part of the Peninsula is not negligible at present.

Ice Bridge Antarctic Sea Ice

NASA Image and Video Library

2009-10-21

Sea ice is seen out the window of NASA's DC-8 research aircraft as it flies 2,000 feet above the Bellingshausen Sea in West Antarctica on Wednesday, Oct., 21, 2009. This was the fourth science flight of NASA’s Operation Ice Bridge airborne Earth science mission to study Antarctic ice sheets, sea ice, and ice shelves. Photo Credit: (NASA/Jane Peterson)

Ice Stream Slowdown Will Drive Long-Term Thinning of the Ross Ice Shelf, With or Without Ocean Warming

NASA Astrophysics Data System (ADS)

Campbell, Adam J.; Hulbe, Christina L.; Lee, Choon-Ki

2018-01-01

As time series observations of Antarctic change proliferate, it is imperative that mathematical frameworks through which they are understood keep pace. Here we present a new method of interpreting remotely sensed change using spatial statistics and apply it to the specific case of thickness change on the Ross Ice Shelf. First, a numerical model of ice shelf flow is used together with empirical orthogonal function analysis to generate characteristic patterns of response to specific forcings. Because they are continuous and scalable in space and time, the patterns allow short duration observations to be placed in a longer time series context. Second, focusing only on changes that are statistically significant, the synthetic response surfaces are used to extract magnitude and timing of past events from the observational data. Slowdown of Kamb and Whillans Ice Streams is clearly detectable in remotely sensed thickness change. Moreover, those past events will continue to drive thinning into the future.

Techniques for estimating flood-depth frequency relations for streams in West Virginia

USGS Publications Warehouse

Wiley, J.B.

1987-01-01

Multiple regression analyses are applied to data from 119 U.S. Geological Survey streamflow stations to develop equations that estimate baseline depth (depth of 50% flow duration) and 100-yr flood depth on unregulated streams in West Virginia. Drainage basin characteristics determined from the 100-yr flood depth analysis were used to develop 2-, 10-, 25-, 50-, and 500-yr regional flood depth equations. Two regions with distinct baseline depth equations and three regions with distinct flood depth equations are delineated. Drainage area is the most significant independent variable found in the central and northern areas of the state where mean basin elevation also is significant. The equations are applicable to any unregulated site in West Virginia where values of independent variables are within the range evaluated for the region. Examples of inapplicable sites include those in reaches below dams, within and directly upstream from bridge or culvert constrictions, within encroached reaches, in karst areas, and where streams flow through lakes or swamps. (Author 's abstract)

The First Annual West Antarctic Ice Sheet (WAIS) Science Workshop

NASA Technical Reports Server (NTRS)

Bindschadler, Robert A. (Editor)

1993-01-01

A compilation of abstracts presented at the workshop are presented. The goal was to answer the question, what is the future behavior and potential for rapid collapse of the West Antarctic Ice Sheet (WAIS)? The workshop was organized into four sessions corresponding to the four objectives identified as necessary to reach the WAIS workshop goal: history, current behavior, internal dynamics, and environmental interactions. Presentations were organized by their relevance to each objective, rather than by discipline.

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

Biomarker-based reconstruction of late Holocene sea-ice variability: East versus West Greenland continental shelf.

NASA Astrophysics Data System (ADS)

Kolling, H. M.; Stein, R. H.; Fahl, K.

2016-12-01

Sea is a critical component of the climate system and its role is not yet fully understood e.g. the recent rapid decrease in sea ice is not clearly reflected in climate models. This illustrates the need for high-resolution proxy-based sea-ice reconstructions going beyond the time scale of direct measurements in order to understand the processes controlling present and past natural variability of sea ice on short time scales. Here we present the first comparison of two high-resolution biomarker records from the East and West Greenland Shelf for the late Holocene. Both areas are highly sensitive to sea-ice changes as they are influenced by the East Greenland Current, the main exporter of Arctic freshwater and sea ice. On the East Greenland Shelf, we do not find any clear evidence for a long-term increase of sea ice during the late Holocene Neoglacial. This sea-ice record seems to be more sensitive to short-term climate events, such as the Roman Warm Period, the Dark Ages, the Medieval Warm Period and the Little Ice Age. In contrary, the West Greenland Shelf record shows a strong and gradual increase in sea ice concentration and a reduction in marine productivity markers starting near 1.6 ka. In general, the increase in sea ice seems to follow the decreasing solar insolation trend. Short-term events are not as clearly pronounced as on the East Greenland Shelf. A comparison to recently published foraminiferal records from the same cores (Perner et al., 2011, 2015) illuminates the differences of biomarker and micropaleontoligical proxies. It seems that the general trend is reflected in both proxies but the signal of small-scale events is preserved rather differently, pointing towards different environmental requirements of the species behind both proxies. References: Perner, K., et al., 2011. Quat. Sci. Revs. 30, 2815-2826 Perner, K., et al., 2015. Quat. Sci. Revs. 129, 296-307

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.

Towards multi-decadal to multi-millennial ice core records from coastal west Greenland ice caps

NASA Astrophysics Data System (ADS)

Das, Sarah B.; Osman, Matthew B.; Trusel, Luke D.; McConnell, Joseph R.; Smith, Ben E.; Evans, Matthew J.; Frey, Karen E.; Arienzo, Monica; Chellman, Nathan

2017-04-01

The Arctic region, and Greenland in particular, is undergoing dramatic change as characterized by atmospheric warming, decreasing sea ice, shifting ocean circulation patterns, and rapid ice sheet mass loss, but longer records are needed to put these changes into context. Ice core records from the Greenland ice sheet have yielded invaluable insight into past climate change both regionally and globally, and provided important constraints on past surface mass balance more directly, but these ice cores are most often from the interior ice sheet accumulation zone, at high altitude and hundreds of kilometers from the coast. Coastal ice caps, situated around the margins of Greenland, have the potential to provide novel high-resolution records of local and regional maritime climate and sea surface conditions, as well as contemporaneous glaciological changes (such as accumulation and surface melt history). But obtaining these records is extremely challenging. Most of these ice caps are unexplored, and thus their thickness, age, stratigraphy, and utility as sites of new and unique paleoclimate records is largely unknown. Access is severely limited due to their high altitude, steep relief, small surface area, and inclement weather. Furthermore, their relatively low elevation and marine moderated climate can contribute to significant surface melting and degradation of the ice stratigraphy. We recently targeted areas near the Disko Bay region of central west Greenland where maritime ice caps are prevalent but unsampled, as potential sites for new multi-decadal to multi-millennial ice core records. In 2014 & 2015 we identified two promising ice caps, one on Disko Island (1250 m. asl) and one on Nuussuaq Peninsula (1980 m. asl) based on airborne and ground-based geophysical observations and physical and glaciochemical stratigraphy from shallow firn cores. In spring 2015 we collected ice cores at both sites using the Badger-Eclipse electromechanical drill, transported by a medley

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.

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.

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

Ice Front at Venable Ice Shelf

NASA Image and Video Library

2013-06-13

This photo, taken onboard the Chilean Navy P3 aircraft, shows the ice front of Venable Ice Shelf, West Antarctica, in October 2008. It is an example of a small-size ice shelf that is a large melt water producer.

Reconstruction of the Final Phases of Activity and Retreat of the North Sea Lobe Ice Stream during the Late Devensian

NASA Astrophysics Data System (ADS)

Grimoldi, E.; Roberts, D. H.; Evans, D. J. A.; Stewart, H. A.; Sejrup, H. P.; Hjelstuen, B. O. B.; Haflidason, H.; Clark, C.

2016-12-01

The deglacial history of the former eastern margin of the last British and Irish Ice Sheet (BIIS) is still poorly understood, particularly in the western North Sea basin. The North Sea Lobe (NSL) affected the area, although gaps remain in our knowledge of the geomorphological and sedimentary imprint that the ice stream left on the seafloor and, more importantly, of its way of final retreat. In this work we analyse new high-resolution multibeam bathymetry, 2D seismic profiles and five vibro-cores, collected in the western North Sea in collaboration with the Britice-Chrono project, and provide new insights on the seafloor geomorphology and acoustic and lithological facies that characterize the Quaternary sediments of the area. The presence of bedrock-cored lineations orientated WNW-ESE to NW-SE indicates that the NSL was fed by the Forth ice stream which moved offshore from southern Scotland. Moraine ridges and two grounding zone wedges, perpendicular to the lineations, suggest that the NSL underwent different phases of stillstand/readvance and retreated towards the north-west. Five acoustic facies (AF) were identified, four of which are found on top of pre-Quaternary strata (AF 1), though their lateral extension is discontinuous. They are interpreted to represent glacigenic diamicts (AF 2 and 3), that are overlain by glacimarine (AF 4) and by Holocene deposits (AF 5). The vibro-cores penetrate in depth until reaching the top of AF 3. This facies correlates to the diamictic sediments observed in the cores, which are characterized by soft silts and clays and abundant clasts. The glacimarine sediments generally appear as highly laminated silts and clays with dropstones that usually become less frequent going upwards in the cores. These sediments are also characterized by foraminifera species associated with glacial environments. Foraminifera tests were dated within the galcimarine sequences in two cores and will help constrain the timing of ice retreat. By compiling

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;

Assessing the extent of nitrogen saturation in northern West Virginia forested watersheds: a survey of stream nitrate concentrations

Treesearch

Karl W. J. Williard; David R. DeWalle; Pamela J. Edwards

2003-01-01

Twenty-seven forested watersheds in northern West Virginia were sampled for stream nitrate concentrations during summer 1997 and fall 1998 baseflow periods to determine if Fernow watershed 4, an often-cited and studied nitrogen saturated basin, was anomalous or regionally representative in terms of stream nitrate levels. Baseflow stream NO3-N...

Validation of the Antarctic Snow Accumulation and Ice Discharge Basal Stress Boundary in the South Eastern Region of the Ross Ice Shelf, Antarctica

NASA Astrophysics Data System (ADS)

Nelson, C. B.; King, K.

2015-12-01

The largest ice shelf in Antarctic, Ross Ice Shelf, was investigated over the years of (1970-2015). Near the basal stress boundary between the ice shelf and the West Antarctic ice sheet, ice velocity ranges from a few meters per year to several hundred meters per year in ice streams. Most of the drainage from West Antarctica into the Ross Ice Shelf flows down two major ice streams, each of which discharges more than 20 km3 of ice each year. Along with velocity changes, the warmest water below parts of the Ross Ice Shelf resides in the lowest portion of the water column because of its high salinity. Vertical mixing caused by tidal stirring can thus induce ablation by lifting the warm water into contact with the ice shelf. This process can cause melting over a period of time and eventually cause breakup of ice shelf. With changes occurring over many years a validation is needed for the Antarctic Snow Accumulation and Ice Discharge (ASAID) basal stress boundary created in 2003. After the 2002 Larsen B Ice Shelf disintegration, nearby glaciers in the Antarctic Peninsula accelerated up to eight times their original speed over the next 18 months. Similar losses of ice tongues in Greenland have caused speed-ups of two to three times the flow rates in just one year. Rapid changes occurring in regions surrounding Antarctica are causing concern in the polar science community to research changes occurring in coastal zones over time. During the research, the team completed study on the Ross Ice Shelf located on the south western coast of the Antarctic. The study included a validation of the ABSB vs. the natural basal stress boundary (NBSB) along the Ross Ice Shelf. The ASAID BSB was created in 2003 by a team of researchers headed by National Aeronautics and Space Administration Goddard Space Flight Center (NASA GSFC), with an aim of studying coastal deviations as it pertains to the mass balance of the entire continent. The point data file was aimed at creating a replica of the

Ice Bridge Antarctic Sea Ice

NASA Image and Video Library

2009-10-21

An iceberg is seen out the window of NASA's DC-8 research aircraft as it flies 2,000 feet above the Amundsen Sea in West Antarctica on Wednesday, Oct., 21, 2009. This was the fourth science flight of NASA’s Operation Ice Bridge airborne Earth science mission to study Antarctic ice sheets, sea ice, and ice shelves. Photo Credit: (NASA/Jane Peterson)

Englacial layer mapping correlation and consistency techniques: an example from airborne ice penetrating radar profiles in West Antarctica

NASA Astrophysics Data System (ADS)

Sudunagunta, V.; Ballal, S.; Albach, R.; Muldoon, G.; Quartini, E.; Cavitte, M. G.; Young, D. A.; Blankenship, D. D.

2016-12-01

Ice sheets are important considerations in projections of sea level rise and studies of climate history. Satellite imagery, ice-penetrating radar, and ground penetrating radar are commonly used to understand the dynamics and health of ice sheets. We focus on how to accurately interpret ice-penetrating radar data by tracing isochrones dated by comparison to deep ice cores so that an extensive knowledge of the West Antarctic Ice Sheet's internal stratigraphy is obtained. The radar data shows englacial reflectors and isochrones are interpreted englacial reflectors. We analyze these features and attempt to understand their possible origins. Accurate interpretation of radar data is crucial because the data will be used to evaluate ice flow evolution and boundary conditions. It can also be used to validate simulated ice sheet models. However to do so, isochrones must be traced and connected correctly. Our approach accomplishes this and has the potential to be applied to other ice sheets on Earth and in extraterrestrial systems. We discuss the methodology utilized by our team to interpret data from the West Antarctic Ice Sheet collected using airborne ice-penetrating radar. The seismic reflection interpretation environment Landmark DecisionSpace was adapted to display and interpret the radar returns. When tracing isochrones, a group approach is used to maximize accuracy. In gridded surveys, loops are used to continuously check isochrones, in addition to retracing of isochrones by different group members to check for errors in interpretation. As loops are made, areas of possible scientific merit, such as isochrone drawdowns or Raymond bumps, are documented. A key aspect of this approach is the ability to work in a shared environment with a collaborative database like DecisionSpace. We will explore how we identified these features, their root causes, and subsequent implications for understanding ice sheet dynamics.

Abbot Ice Shelf, structure of the Amundsen Sea continental margin and the southern boundary of the Bellingshausen Plate seaward of West Antarctica.

PubMed

Cochran, James R; Tinto, Kirsty J; Bell, Robin E

2015-05-01

Inversion of NASA Operation IceBridge airborne gravity over the Abbot Ice Shelf in West Antarctica for subice bathymetry defines an extensional terrain made up of east-west trending rift basins formed during the early stages of Antarctica/Zealandia rifting. Extension is minor, as rifting jumped north of Thurston Island early in the rifting process. The Amundsen Sea Embayment continental shelf west of the rifted terrain is underlain by a deeper, more extensive sedimentary basin also formed during rifting between Antarctica and Zealandia. A well-defined boundary zone separates the mildly extended Abbot extensional terrain from the deeper Amundsen Embayment shelf basin. The shelf basin has an extension factor, β , of 1.5-1.7 with 80-100 km of extension occurring across an area now 250 km wide. Following this extension, rifting centered north of the present shelf edge and proceeded to continental rupture. Since then, the Amundsen Embayment continental shelf appears to have been tectonically quiescent and shaped by subsidence, sedimentation, and the advance and retreat of the West Antarctic Ice Sheet. The Bellingshausen Plate was located seaward of the Amundsen Sea margin prior to incorporation into the Antarctic Plate at about 62 Ma. During the latter part of its independent existence, Bellingshausen plate motion had a clockwise rotational component relative to Antarctica producing convergence across the north-south trending Bellingshausen Gravity Anomaly structure at 94°W and compressive deformation on the continental slope between 94°W and 102°W. Farther west, the relative motion was extensional along an east-west trending zone occupied by the Marie Byrd Seamounts. Abbot Ice Shelf is underlain by E-W rift basins created at ∼90 Ma Amundsen shelf shaped by subsidence, sedimentation, and passage of the ice sheet Bellingshausen plate boundary is located near the base of continental slope and rise.

Abbot Ice Shelf, structure of the Amundsen Sea continental margin and the southern boundary of the Bellingshausen Plate seaward of West Antarctica

PubMed Central

Cochran, James R; Tinto, Kirsty J; Bell, Robin E

2015-01-01

Inversion of NASA Operation IceBridge airborne gravity over the Abbot Ice Shelf in West Antarctica for subice bathymetry defines an extensional terrain made up of east-west trending rift basins formed during the early stages of Antarctica/Zealandia rifting. Extension is minor, as rifting jumped north of Thurston Island early in the rifting process. The Amundsen Sea Embayment continental shelf west of the rifted terrain is underlain by a deeper, more extensive sedimentary basin also formed during rifting between Antarctica and Zealandia. A well-defined boundary zone separates the mildly extended Abbot extensional terrain from the deeper Amundsen Embayment shelf basin. The shelf basin has an extension factor, β, of 1.5–1.7 with 80–100 km of extension occurring across an area now 250 km wide. Following this extension, rifting centered north of the present shelf edge and proceeded to continental rupture. Since then, the Amundsen Embayment continental shelf appears to have been tectonically quiescent and shaped by subsidence, sedimentation, and the advance and retreat of the West Antarctic Ice Sheet. The Bellingshausen Plate was located seaward of the Amundsen Sea margin prior to incorporation into the Antarctic Plate at about 62 Ma. During the latter part of its independent existence, Bellingshausen plate motion had a clockwise rotational component relative to Antarctica producing convergence across the north-south trending Bellingshausen Gravity Anomaly structure at 94°W and compressive deformation on the continental slope between 94°W and 102°W. Farther west, the relative motion was extensional along an east-west trending zone occupied by the Marie Byrd Seamounts. Key Points: Abbot Ice Shelf is underlain by E-W rift basins created at ∼90 Ma Amundsen shelf shaped by subsidence, sedimentation, and passage of the ice sheet Bellingshausen plate boundary is located near the base of continental slope and rise PMID:26709352

Ice-flow reorganization in West Antarctica 2.5 kyr ago dated using radar-derived englacial flow velocities

NASA Astrophysics Data System (ADS)

Kingslake, Jonathan; Martín, Carlos; Arthern, Robert J.; Corr, Hugh F. J.; King, Edward C.

2016-09-01

We date a recent ice-flow reorganization of an ice divide in the Weddell Sea Sector, West Antarctica, using a novel combination of inverse methods and ice-penetrating radars. We invert for two-dimensional ice flow within an ice divide from data collected with a phase-sensitive ice-penetrating radar while accounting for the effect of firn on radar propagation and ice flow. By comparing isochronal layers simulated using radar-derived flow velocities with internal layers observed with an impulse radar, we show that the divide's internal structure is not in a steady state but underwent a disturbance, potentially implying a regional ice-flow reorganization, 2.5 (1.8-2.9) kyr B.P. Our data are consistent with slow ice flow in this location before the reorganization and the ice divide subsequently remaining stationary. These findings increase our knowledge of the glacial history of a region that lacks dated constraints on late-Holocene ice-sheet retreat and provides a key target for models that reconstruct and predict ice-sheet behavior.

Modeling of Ice Flow and Internal Layers Along a Flow Line Through Swiss Camp in West Greenland

NASA Technical Reports Server (NTRS)

Wang, W. L.; Zwally, H. Jay; Abdalati, W.; Luo, S.; Koblinsky, Chester J. (Technical Monitor)

2001-01-01

An anisotropic ice flow line model is applied to a flow line through Swiss Camp (69.57 N, 49.28 W) in West Greenland to estimate the dates of internal layers detected by Radio-Echo Sounding measurements. The effect of an anisotropic ice fabric on ice flow is incorporated into the steady state flow line model. The stress-strain rate relationship for anisotropic ice is characterized by an enhancement factor based on the laboratory observations of ice deformation under combined compression and shear stresses. By using present-day data of accumulation rate, surface temperature, surface elevation and ice thickness along the flow line as model inputs, a very close agreement is found between the isochrones generated from the model and the observed internal layers with confirmed dates. The results indicate that this part of Greenland ice sheet is primarily in steady state.

A 19-year radar altimeter elevation change time-series of the East and West Antarctic ice sheets

NASA Astrophysics Data System (ADS)

Sundal, A. V.; Shepherd, A.; Wingham, D.; Muir, A.; Mcmillan, M.; Galin, N.

2012-12-01

We present 19 years of continuous radar altimeter observations of the East and West Antarctic ice sheets acquired by the ERS-1, ERS-2, and ENVISAT satellites between May 1992 and September 2010. Time-series of surface elevation change were developed at 39,375 crossing points of the satellite orbit ground tracks using the method of dual cycle crossovers (Zwally et al., 1989; Wingham et al., 1998). In total, 46.5 million individual measurements were included in the analysis, encompassing 74 and 76 % of the East and West Antarctic ice sheet, respectively. The satellites were cross-calibrated by calculating differences between elevation changes occurring during periods of mission overlap. We use the merged time-series to explore spatial and temporal patterns of elevation change and to characterise and quantify the signals of Antarctic ice sheet imbalance. References: Wingham, D., Ridout, A., Scharroo, R., Arthern, R. & Shum, C.K. (1998): Antarctic elevation change from 1992 to 1996. Science, 282, 456-458. Zwally, H. J., Brenner, A. C., Major, J. A., Bindschadler, R. A. & Marsh, J. G. (1989): Growth of Greenland ice-sheet - measurements. Science, 246, 1587-1589.

Marine geological and geophysical records of the last British-Irish Ice Sheet on the continental shelf west of Ireland

NASA Astrophysics Data System (ADS)

O'Cofaigh, Colm; Callard, S. Louise; Benetti, Sara; Chiverell, Richard C.; Saher, Margot; van Landeghem, Katrien; Livingstone, Stephen J.; Scourse, James; Clark, Chris D.

2015-04-01

The record of glaciation on the continental shelf west of Ireland has, until recently, been relatively poorly studied. The UK NERC funded project BRITICE-CHRONO collected marine geophysical data in the form of multibeam swath bathymetry and sub-bottom profiles supplemented by over 50 vibro- and piston cores across the continental shelf west of Ireland during cruise JC106 of the RRS James Cook in 2014. Across the western Irish shelf, offshore of counties Galway and Clare, a series of large arcuate moraines record the former presence of a grounded ice sheet on the shelf. However, geophysical data from further to the west across the Porcupine Bank show a series of ridges and wedge-shaped sedimentary features whose form is consistent with an origin as moraines and/or grounding-zone wedges. Sediment cores from several of these landforms recovered stiff, massive diamictons containing reworked shells that are interpreted as subglacial tills. Cores from the eastern Porcupine Bank recovered laminated muds with cold-water glacimarine foraminifera, in some cases overlying till. Collectively the geophysical and sedimentary data imply the presence of grounded ice across the northern Porcupine Bank and thus much further west on the Irish margin than has previously been considered. This ice underwent retreat in a glacimarine setting. The large 'Olex Moraine' on the western Irish shelf is thus interpreted as recessional feature. Work is currently underway to dates these features and to obtain a retreat chronology for this sector of the last British-Irish Ice Sheet.

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.

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

Ice Core Records of West Greenland Melt and Climate Forcing

NASA Astrophysics Data System (ADS)

Graeter, K. A.; Osterberg, E. C.; Ferris, D. G.; Hawley, R. L.; Marshall, H. P.; Lewis, G.; Meehan, T.; McCarthy, F.; Overly, T.; Birkel, S. D.

2018-04-01

Remote sensing observations and climate models indicate that the Greenland Ice Sheet (GrIS) has been losing mass since the late 1990s, mostly due to enhanced surface melting from rising summer temperatures. However, in situ observational records of GrIS melt rates over recent decades are rare. Here we develop a record of frozen meltwater in the west GrIS percolation zone preserved in seven firn cores. Quantifying ice layer distribution as a melt feature percentage (MFP), we find significant increases in MFP in the southernmost five cores over the past 50 years to unprecedented modern levels (since 1550 CE). Annual to decadal changes in summer temperatures and MFP are closely tied to changes in Greenland summer blocking activity and North Atlantic sea surface temperatures since 1870. However, summer warming of 1.2°C since 1870-1900, in addition to warming attributable to recent sea surface temperature and blocking variability, is a critical driver of high modern MFP levels.

Receiving and use of streams of monodisperse ice granules for cleaning and deactivation of surfaces

NASA Astrophysics Data System (ADS)

Boukharov, A.; Balashov, A.; Timohin, A.; Ivanov, A.; Holin, B.

2017-11-01

The most generally useful methods for cleaning and processing of surfaces are the sand-jets and shot blasting jets. Installations of this kind are used for cleaning of corrosion surfaces, the oil-dirt deposits, paint coatings. However the use of these installations follows to high investment and operational expenditure, larger risk of operators disease, the negative affect for a environment. These problems can be solved with the use of new cleaning method through application of mono-disperse (identical by the size and the form) ice granules of 300 - 1000 microns, accelerated by air stream in the nozzle device to the speed of 10 - 100 m/s. In view of the extreme complexity of the receiving such particles by means of cooling and the subsequent freezing of water drops are necessary additional experimental researches. For study of thermal processes of receiving mono-disperse ice granules the experimental installation was created and experiments on deactivation and cleaning of surfaces with pollution of various types are made. Experiments showed that by means of a stream of the accelerated ice granules it is rather successfully possible to delete oil-dirt deposits, outdated paint coats and rust. Besides, efficient deactivation of radioactive surfaces is possible. The coefficient deactivation of γ activity is highest.

Geoengineering Marine Ice Sheets

NASA Astrophysics Data System (ADS)

Wolovick, M.

2017-12-01

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.

Radar Interferometry Detection of Hinge Line Migration on Rutford Ice Stream and Carlson Inlet, Antarctica

NASA Technical Reports Server (NTRS)

Rignot, Eric

1997-01-01

Satellite synthetic-aperture radar (SAR) Interferometry is employed to map the hinge line, or limit of tidal flexing, of Rutford Ice Stream and Carlson Inlet, Antarctica, and detect its migration between 1992 and 1996. The hinge line is mapped using a model fit from an elastic beam theory.

RICE ice core: Black Carbon reflects climate variability at Roosevelt Island, West Antarctica

NASA Astrophysics Data System (ADS)

Ellis, Aja; Edwards, Ross; Bertler, Nancy; Winton, Holly; Goodwin, Ian; Neff, Peter; Tuohy, Andrea; Proemse, Bernadette; Hogan, Chad; Feiteng, Wang

2015-04-01

The Roosevelt Island Climate Evolution (RICE) project successfully drilled a deep ice core from Roosevelt Island during the 2011/2012 and 2012/2013 seasons. Located in the Ross Ice Shelf in West Antarctica, the site is an ideal location for investigating climate variability and the past stability of the Ross Ice Shelf. Black carbon (BC) aerosols are emitted by both biomass burning and fossil fuels, and BC particles emitted in the southern hemisphere are transported in the atmosphere and preserved in Antarctic ice. The past record of BC is expected to be sensitive to climate variability, as it is modulated by both emissions and transport. To investigate BC variability over the past 200 years, we developed a BC record from two overlapping ice cores (~1850-2012) and a high-resolution snow pit spanning 2010-2012 (cal. yr). Consistent results are found between the snow pit profiles and ice core records. Distinct decadal trends are found with respect to BC particle size, and the record indicates a steady rise in BC particle size over the last 100 years. Differences in emission sources and conditions may be a possible explanation for changes in BC size. These records also show a significant increase in BC concentration over the past decade with concentrations rising over 1.5 ppb (1.5*10^-9 ng/g), suggesting a fundamental shift in BC deposition to the site.

Negative magnetic anomaly over Mt. Resnik, a subaerially erupted volcanic peak beneath the West Antarctic Ice Sheet

USGS Publications Warehouse

Behrendt, John C.; Finn, C.; Morse, D.L.; Blankenship, D.D.

2006-01-01

Mt. Resnik is one of the previously reported 18 subaerially erupted volcanoes (in the West Antarctic rift system), which have high elevation and high bed relief beneath the WAIS in the Central West Antarctica (CWA) aerogeophysical survey. Mt. Resnik lies 300 m below the surface of the West Antarctic Ice Sheet (WAIS); it has 1.6 km topographic relief, and a conical form defined by radar ice-sounding of bed topography. It has an associated complex negative magnetic anomaly revealed by the CWA survey. We calculated and interpreted magnetic models fit to the Mt. Resnik anomaly as a volcanic source comprising both reversely and normally magnetized (in the present field direction) volcanic flows, 0.5-2.5-km thick, erupted subaerially during a time of magnetic field reversal. The Mt. Resnik 305-nT anomaly is part of an approximately 50- by 40-km positive anomaly complex extending about 30 km to the west of the Mt. Resnik peak, associated with an underlying source complex of about the same area, whose top is at the bed of the WAIS. The bed relief of this shallow source complex has a maximum of only about 400 m, whereas the modeled source is >3 km thick. From the spatial relationship we interpret that this source and Mt Resnik are approximately contemporaneous. Any subglacially (older?) erupted edifices comprising hyaloclastite or other volcanic debris, which formerly overlaid the source to the west, were removed by the moving WAIS into which they were injected as is the general case for the ???1000 volcanic centers at the base of the WAIS. The presence of the magnetic field reversal modeled for Mt. Resnik may represent the Bruhnes-Matayama reversal at 780 ka (or an earlier reversal). There are ???100 short-wavelength, steep-gradient, negative magnetic anomalies observed over the West Antarctic Ice Sheet (WAIS), or about 10% of the approximately 1000 short-wavelength, shallow-source, high-amplitude (50- >1000 nT) "volcanic" magnetic anomalies in the CWA survey. These

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

Physical characteristics of stream subbasins in the Pomme de Terre River Basin, west-central Minnesota

USGS Publications Warehouse

Lorenz, D.L.; Payne, G.A.

1994-01-01

Data describing the physical characteristics of stream subbasins upstream from selected points on streams in the Pomme de Terre River Basin, located in west-central Minnesota, are presented in this report. The physical characteristics are the drainage area of the subbasin, the percentage area of the subbasin covered only by lakes, the percentage area of the subbasin covered by both lakes and wetlands, the main-channel length, and the main-channel slope. The points on the stream include outlets of subbasins of at least 5 square miles, outfalls of sewage treatment plants, and locations of U.S. Geological Survey low-flow, high-flow, and continuous-record gaging stations.

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.

Ice-Ocean Interactions to the North-West of Greenland: Glaciers, Straits, Ice Bridges, and the Rossby Radius (Invited)

NASA Astrophysics Data System (ADS)

Muenchow, A.; Falkner, K. K.; Melling, H.; Johnson, H. L.; Huntley, H. S.; Ryan, P.; Friends Of Petermann

2010-12-01

Petermann Glacier at 81 N latitude is a major outlet glacier adjacent to Nares Strait. It terminates in a long (70 km), narrow (16 km) and thin (50 m) floating tongue and has a grounding line more than 500 m below sea level. A calving event in 2010 reduced the floating area by 25% and produced a single 240 km2 ice island currently moving south in Nares Strait where it will likely interact with island to potentially create a temporary polynya in Nares Strait. The 2010 calving from Petermann Glacier contributes <10% to its mass balance as more than 80% is lost due to basal melting by the ocean. Hence the largely unexplored physics at the ice-ocean interface determine how a changing climate impacts this outlet glacier. Conducting exploratory surveys inside Petermann Fjord in 2003, 2007, and 2009, we find a 1100 m deep fjord connected to Nares Strait via a sill at 350-450 m depth. The fjord receives about 3 times the amount of heat required for the basal melt rates. Furthermore, limited data and analytical modeling suggests a 3-dimensional circulation over the upper 300-m of the water column with a coastally trapped buoyant outflow. We integrate these findings with more complete oceanic time series data from an array moored in Nares Strait from 2003 through 2009 near 80.5 N. In the past Nares Strait and Petermann Fjord were covered by land fast sea ice during the 9-10 month long winter season. Archeological and remotely sensed records indicate that an ice bridge formed regularly at the southern end of Nares Strait creating the North-Water polynya near 79 N latitude. Since 2006 this ice bridge has largely failed to form, leading, perhaps, to the occasional formation of a secondary ice bridge 300 km to the north where Nares Strait connects to the Arctic Ocean. However, this ice bridge appears to form for shorter periods only. Consequently Arctic sea ice can now exit the Arctic in winter via pathways to the west of Greenland all year. We speculate that this changed ocean

Abbot Ice Shelf, the Amundsen Sea Continental Margin and the Southern Boundary of the Bellingshausen Plate Seaward of West Antarctica

NASA Astrophysics Data System (ADS)

Cochran, J. R.; Tinto, K. J.; Bell, R. E.

2014-12-01

The Abbot Ice Shelf extends 450 km along the coast of West Antarctica between 103°W and 89°W and straddles the boundary between the Bellingshausen Sea continental margin, which overlies a former subduction zone, and Amundsen Sea rifted continental margin. Inversion of NASA Operation IceBridge airborne gravity data for sub-ice bathymetry shows that the western part of the ice shelf, as well as Cosgrove Ice Shelf to the south, are underlain by a series of east-west trending rift basins. The eastern boundary of the rifted terrain coincides with the eastern boundary of rifting between Antarctica and Zealandia and the rifts formed during the early stages of this rifting. Extension in these rifts is minor as rifting quickly jumped north of Thurston Island. The southern boundary of the Cosgrove Rift is aligned with the southern boundary of a sedimentary basin under the Amundsen Embayment continental shelf to the west, also formed by Antarctica-Zealandia rifting. The shelf basin has an extension factor, β, of 1.5 - 1.7 with 80 -100 km of extension occurring in an area now ~250 km wide. Following this extension early in the rifting process, rifting centered to the north of the present shelf edge and proceeded to continental rupture. Since then, the Amundsen Embayment continental shelf has been tectonically quiescent and has primarily been shaped though subsidence, sedimentation and the passage of the West Antarctic Ice Sheet back and forth across it. The former Bellingshausen Plate was located seaward of the Amundsen Sea margin prior to its incorporation into the Antarctic Plate at ~62 Ma. During the latter part of its existence, Bellingshausen plate motion had a clockwise rotational component relative to Antarctica producing convergence between the Bellingshausen and Antarctic plates east of 102°W. Seismic reflection and gravity data show that this convergence is expressed by an area of intensely deformed sediments beneath the continental slope from 102°W to 95°W and

DEVELOPMENT OF A FISH INDEX OF BIOTIC INTEGRITY TO ASSESS THE CONDITION OF WEST VIRGINIA STREAMS: TECHNICAL SUPPORT DOCUMENT

EPA Science Inventory

This report documents the development of a fish index of biotic integrity (IBI) for wadeable streams in West Virginia and to determine the applicability of the IBI in streams with different thermal regimes. The IBI will be developed using fish data collected at EMAP sites from 1...

Geostatistical Methods For Determination of Roughness, Topography, And Changes of Antarctic Ice Streams From SAR And Radar Altimeter Data

NASA Technical Reports Server (NTRS)

Herzfeld, Ute C.

2002-01-01

The central objective of this project has been the development of geostatistical methods fro mapping elevation and ice surface characteristics from satellite radar altimeter (RA) and Syntheitc Aperture Radar (SAR) data. The main results are an Atlas of elevation maps of Antarctica, from GEOSAT RA data and an Atlas from ERS-1 RA data, including a total of about 200 maps with 3 km grid resolution. Maps and digital terrain models are applied to monitor and study changes in Antarctic ice streams and glaciers, including Lambert Glacier/Amery Ice Shelf, Mertz and Ninnis Glaciers, Jutulstraumen Glacier, Fimbul Ice Shelf, Slessor Glacier, Williamson Glacier and others.

Application of Control Method on a West Antarctic Glacier

NASA Astrophysics Data System (ADS)

Schmeltz, M.; Rignot, E. J.; Macayeal, D. R.

2002-12-01

We use surface velocity inferred with Interferometric synthetic-aperture radar and a control method to estimate unknown basal characteristics of a fast-moving glacier in West Antarctica, Pine Island Glacier. Previous modelling experiments on Pine Island Glacier have shown that using a coupled ice-stream/ice-shelf flow model in a forward approach (trial and error method) we were able to reproduce fairly well the surface velocity. Some discrepancies remained, however, that are partly due to uncertainties in the thickness map and incertainty in our chosen basal stress distribution (because of the non-unicity of the solution). The control method allow us to take the basal stress (or basal friction, since they are related through the velocity), as an unknown parameter. Results given by the control method should provide better reliable inputs for further modelling experiments. We investigate the results' sensitivity to the initial value of the basal stress. The inferred ratio basal drag/driving stress seems to be always low upstream, 60 to 80 km upstream of the grounding line, as if the ice stream was behaving like an ice shelf, and also reveals the presence of a snake shape channel of low ratio basal drag/driving stress, surrounded by a higher ratio, in the main flow of increasing velocity, from 20 to 40 km upstream of the grounding line.

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

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.

Modeling englacial radar attenuation at Siple Dome, West Antarctica, using ice chemistry and temperature data

USGS Publications Warehouse

MacGregor, J.A.; Winebrenner, D.P.; Conway, H.; Matsuoka, K.; Mayewski, P.A.; Clow, G.D.

2007-01-01

The radar reflectivity of an ice-sheet bed is a primary measurement for discriminating between thawed and frozen beds. Uncertainty in englacial radar attenuation and its spatial variation introduces corresponding uncertainty in estimates of basal reflectivity. Radar attenuation is proportional to ice conductivity, which depends on the concentrations of acid and sea-salt chloride and the temperature of the ice. We synthesize published conductivity measurements to specify an ice-conductivity model and find that some of the dielectric properties of ice at radar frequencies are not yet well constrained. Using depth profiles of ice-core chemistry and borehole temperature and an average of the experimental values for the dielectric properties, we calculate an attenuation rate profile for Siple Dome, West Antarctica. The depth-averaged modeled attenuation rate at Siple Dome (20.0 ?? 5.7 dB km-1) is somewhat lower than the value derived from radar profiles (25.3 ?? 1.1 dB km-1). Pending more experimental data on the dielectric properties of ice, we can match the modeled and radar-derived attenuation rates by an adjustment to the value for the pure ice conductivity that is within the range of reported values. Alternatively, using the pure ice dielectric properties derived from the most extensive single data set, the modeled depth-averaged attenuation rate is 24.0 ?? 2.2 dB km-1. This work shows how to calculate englacial radar attenuation using ice chemistry and temperature data and establishes a basis for mapping spatial variations in radar attenuation across an ice sheet. Copyright 2007 by the American Geophysical Union.

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.

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

Summer Drivers of Atmospheric Variability Affecting Ice Shelf Thinning in the Amundsen Sea Embayment, West Antarctica

NASA Astrophysics Data System (ADS)

Deb, Pranab; Orr, Andrew; Bromwich, David H.; Nicolas, Julien P.; Turner, John; Hosking, J. Scott

2018-05-01

Satellite data and a 35-year hindcast of the Amundsen Sea Embayment summer climate using the Weather Research and Forecasting model are used to understand how regional and large-scale atmospheric variability affects thinning of ice shelves in this sector of West Antarctica by melting from above and below (linked to intrusions of warm water caused by anomalous westerlies over the continental shelf edge). El Niño episodes are associated with an increase in surface melt but do not have a statistically significant impact on westerly winds over the continental shelf edge. The location of the Amundsen Sea Low and the polarity of the Southern Annular Mode (SAM) have negligible impact on surface melting, although a positive SAM and eastward shift of the Amundsen Sea Low cause anomalous westerlies over the continental shelf edge. The projected future increase in El Niño episodes and positive SAM could therefore increase the risk of disintegration of West Antarctic ice shelves.

Forced Climate Changes in West Antarctica and the Indo-Pacific by Northern Hemisphere Ice Sheet Topography

NASA Astrophysics Data System (ADS)

Jones, T. R.; Roberts, W. H. G.; Steig, E. J.; Cuffey, K. M.; Markle, B. R.; White, J. W. C.

2017-12-01

The behavior of the Indo-Pacific climate system across the last deglaciation is widely debated. Resolving these debates requires long term and continuous climate proxy records. Here, we use an ultra-high resolution and continuous water isotope record from an ice core in the Pacific sector of West Antarctica. In conjunction with the HadCM3 coupled ocean-atmosphere GCM, we demonstrate that the climate of both West Antarctica and the Indo-Pacific were substantially altered during the last deglaciation by the same forcing mechanism. Critically, these changes are not dependent on ENSO strength, but rather the location of deep tropical convection, which shifts at 16 ka in response to climate perturbations induced by the Laurentide Ice Sheet. The changed rainfall patterns in the tropics explain the deglacial shift from expanded-grasslands to rainforest-dominated ecosystems in Indonesia. High-frequency climate variability in the Southern Hemisphere is also changed, through a tropical Pacific teleconnection link dependent on the propogration of Rossby Waves.

Evidence for the stability of the West Antarctic Ice Sheet divide for 1.4 million years

NASA Astrophysics Data System (ADS)

Hein, Andrew S.; Woodward, John; Marrero, Shasta M.; Dunning, Stuart A.; Steig, Eric J.; Freeman, Stewart P. H. T.; Stuart, Finlay M.; Winter, Kate; Westoby, Matthew J.; Sugden, David E.

2016-02-01

Past fluctuations of the West Antarctic Ice Sheet (WAIS) are of fundamental interest because of the possibility of WAIS collapse in the future and a consequent rise in global sea level. However, the configuration and stability of the ice sheet during past interglacial periods remains uncertain. Here we present geomorphological evidence and multiple cosmogenic nuclide data from the southern Ellsworth Mountains to suggest that the divide of the WAIS has fluctuated only modestly in location and thickness for at least the last 1.4 million years. Fluctuations during glacial-interglacial cycles appear superimposed on a long-term trajectory of ice-surface lowering relative to the mountains. This implies that as a minimum, a regional ice sheet centred on the Ellsworth-Whitmore uplands may have survived Pleistocene warm periods. If so, it constrains the WAIS contribution to global sea level rise during interglacials to about 3.3 m above present.

Seismic Stratigraphic Evidence From SE Ross Sea for Late Oligocene Glaciers and ice Streams Issuing From Marie Byrd Land

NASA Astrophysics Data System (ADS)

Sorlien, C. C.; Luyendyk, B. P.; Wilson, D. S.; Decesari, R. C.; Bartek, L. R.; Diebold, J. B.

2006-12-01

The extent of the West Antarctic ice sheet during mid-Cenozoic time is controversial and important to climate models. High-resolution multichannel seismic reflection data were acquired using the RVIB Palmer along the edge of the Ross Ice Shelf across the Eastern Basin of Ross Sea, in an area where calving of the ice shelf has exposed seafloor that has not been accessible to marine geophysics in several decades. A sub-basin in the far southeast corner of Ross Sea contains a succession of sediment-filled troughs, each capped by an unconformity. These troughs range between 2 and 20 km across, are 100 to 150 m-deep, with the narrower ones bounded by flat-topped ridges interpreted as moraines. We interpret the troughs interval to slightly predate 25 Ma. Reflections just 100 m below the troughs interval can be directly correlated to near DSDP270 where they underlie strata dated at ~25 Ma. A deeper stack of prograding sequences associated with a flat- topped ridge are interpreted as pre-25 Ma, possibly early Oligocene, deltas formed adjacent to the grounding line of a glacier, and the flat-topped ridge to be a moraine. The shallowest of the stack of unconformities capping the broad troughs can be projected across a basement ridge on trend with Roosevelt Island to a regional angular unconformity ("Red"), present across 70 km to deep sedimentary Eastern Basin. This unconformity represents about 1 km of missing stratigraphic section, is smooth and level, and splits into several major sequence boundaries within deep Eastern Basin. The second shallowest of these boundaries is dated about 14 Ma at DSDP-270. We interpret this unconformity to be cut by regional thick, grounded ice at depths several hundred meters below sea level. Pre-25 Ma strata show evidence of narrow erosional troughs and reflective mounds or ridges on the west flank of the basement ridge, but such features are not present in southern deep Eastern Basin near the ice shelf edge. This is evidence that the

Impact of sea-ice melt on dimethyl sulfide (sulfoniopropionate) inventories in surface waters of Marguerite Bay, West Antarctic Peninsula.

PubMed

Stefels, Jacqueline; van Leeuwe, Maria A; Jones, Elizabeth M; Meredith, Michael P; Venables, Hugh J; Webb, Alison L; Henley, Sian F

2018-06-28

The Southern Ocean is a hotspot of the climate-relevant organic sulfur compound dimethyl sulfide (DMS). Spatial and temporal variability in DMS concentration is higher than in any other oceanic region, especially in the marginal ice zone. During a one-week expedition across the continental shelf of the West Antarctic Peninsula (WAP), from the shelf break into Marguerite Bay, in January 2015, spatial heterogeneity of DMS and its precursor dimethyl sulfoniopropionate (DMSP) was studied and linked with environmental conditions, including sea-ice melt events. Concentrations of sulfur compounds, particulate organic carbon (POC) and chlorophyll a in the surface waters varied by a factor of 5-6 over the entire transect. DMS and DMSP concentrations were an order of magnitude higher than currently inferred in climatologies for the WAP region. Particulate DMSP concentrations were correlated most strongly with POC and the abundance of haptophyte algae within the phytoplankton community, which, in turn, was linked with sea-ice melt. The strong sea-ice signal in the distribution of DMS(P) implies that DMS(P) production is likely to decrease with ongoing reductions in sea-ice cover along the WAP. This has implications for feedback processes on the region's climate system.This article is part of the theme issue 'The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change'. © 2018 The Author(s).

Regional Relations in Bankfull Channel Characteristics determined from flow measurements at selected stream-gaging stations in West Virginia, 1911-2002

USGS Publications Warehouse

Messinger, Terence; Wiley, Jeffrey B.

2004-01-01

Three bankfull channel characteristics?cross-sectional area, width, and depth?were significantly correlated with drainage area in regression equations developed for two regions in West Virginia. Channel characteristics were determined from analysis of flow measurements made at 74 U.S. Geological Survey stream-gaging stations at flows between 0.5 and 5.0 times bankfull flow between 1911 and 2002. Graphical and regression analysis were used to delineate an 'Eastern Region' and a 'Western Region,' which were separated by the boundary between the Appalachian Plateaus and Valley and Ridge Physiographic Provinces. Streams that drained parts of both provinces had channel characteristics typical of the Eastern Region, and were grouped with it. Standard error for the six regression equations, three for each region, ranged between 8.7 and 16 percent. Cross-sectional area and depth were greater relative to drainage area for the Western Region than they were for the Eastern Region. Regression equations were defined for streams draining between 46.5 and 1,619 square miles for the Eastern Region, and between 2.78 and 1,354 square miles for the Western Region. Stream-gaging stations with two or more cross sections where flow had been measured at flows between 0.5 and 5.0 times the 1.5-year flow showed poor replication of channel characteristics compared to the 95-percent confidence intervals of the regression, suggesting that within-reach variability for the stream-gaging stations may be substantial. A disproportionate number of the selected stream-gaging stations were on large (drainage area greater than 100 square miles) streams in the central highlands of West Virginia, and only one stream-gaging station that met data-quality criteria was available to represent the region within about 50 miles of the Ohio River north of Parkersburg, West Virginia. Many of the cross sections were at bridges, which can change channel shape. Although the data discussed in this report may not be

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.

Evidence for the stability of the West Antarctic Ice Sheet divide for 1.4 million years

PubMed Central

Hein, Andrew S.; Woodward, John; Marrero, Shasta M.; Dunning, Stuart A.; Steig, Eric J.; Freeman, Stewart P. H. T.; Stuart, Finlay M.; Winter, Kate; Westoby, Matthew J.; Sugden, David E.

2016-01-01

Past fluctuations of the West Antarctic Ice Sheet (WAIS) are of fundamental interest because of the possibility of WAIS collapse in the future and a consequent rise in global sea level. However, the configuration and stability of the ice sheet during past interglacial periods remains uncertain. Here we present geomorphological evidence and multiple cosmogenic nuclide data from the southern Ellsworth Mountains to suggest that the divide of the WAIS has fluctuated only modestly in location and thickness for at least the last 1.4 million years. Fluctuations during glacial–interglacial cycles appear superimposed on a long-term trajectory of ice-surface lowering relative to the mountains. This implies that as a minimum, a regional ice sheet centred on the Ellsworth-Whitmore uplands may have survived Pleistocene warm periods. If so, it constrains the WAIS contribution to global sea level rise during interglacials to about 3.3 m above present. PMID:26838462

Mass balance reassessment of glaciers draining into the Abbot and Getz Ice Shelves of West Antarctica

NASA Astrophysics Data System (ADS)

Chuter, S. J.; Martín-Español, A.; Wouters, B.; Bamber, J. L.

2017-07-01

We present a reassessment of input-output method ice mass budget estimates for the Abbot and Getz regions of West Antarctica using CryoSat-2-derived ice thickness estimates. The mass budget is 8 ± 6 Gt yr-1 and 5 ± 17 Gt yr-1 for the Abbot and Getz sectors, respectively, for the period 2006-2008. Over the Abbot region, our results resolve a previous discrepancy with elevation rates from altimetry, due to a previous 30% overestimation of ice thickness. For the Getz sector, our results are at the more positive bound of estimates from other techniques. Grounding line velocity increases up to 20% between 2007 and 2014 alongside mean elevation rates of -0.67 ± 0.13 m yr-1 between 2010 and 2013 indicate the onset of a dynamic thinning signal. Mean snowfall trends of -0.33 m yr-1 water equivalent since 2006 indicate recent mass trends are driven by both ice dynamics and surface processes.

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.

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.

West Antarctic Ice Sheet cloud cover and surface radiation budget from NASA A-Train satellites

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

Scott, Ryan C.; Lubin, Dan; Vogelmann, Andrew M.

Clouds are an essential parameter of the surface energy budget influencing the West Antarctic Ice Sheet (WAIS) response to atmospheric warming and net contribution to global sea-level rise. A four-year record of NASA A-Train cloud observations is combined with surface radiation measurements to quantify the WAIS radiation budget and constrain the three-dimensional occurrence frequency, thermodynamic phase partitioning, and surface radiative effect of clouds over West Antarctica (WA). The skill of satellite-modeled radiative fluxes is confirmed through evaluation against measurements at four Antarctic sites (WAIS Divide Ice Camp, Neumayer, Syowa, and Concordia Stations). And due to perennial high-albedo snow and icemore » cover, cloud infrared emission dominates over cloud solar reflection/absorption leading to a positive net all-wave cloud radiative effect (CRE) at the surface, with all monthly means and 99.15% of instantaneous CRE values exceeding zero. The annual-mean CRE at theWAIS surface is 34 W m -2, representing a significant cloud-induced warming of the ice sheet. Low-level liquid-containing clouds, including thin liquid water clouds implicated in radiative contributions to surface melting, are widespread and most frequent in WA during the austral summer. Clouds warm the WAIS by 26 W m -2, in summer, on average, despite maximum offsetting shortwave CRE. Glaciated cloud systems are strongly linked to orographic forcing, with maximum incidence on the WAIS continuing downstream along the Transantarctic Mountains.« less

West Antarctic Ice Sheet cloud cover and surface radiation budget from NASA A-Train satellites

DOE PAGES

Scott, Ryan C.; Lubin, Dan; Vogelmann, Andrew M.; ...

2017-04-26

Clouds are an essential parameter of the surface energy budget influencing the West Antarctic Ice Sheet (WAIS) response to atmospheric warming and net contribution to global sea-level rise. A four-year record of NASA A-Train cloud observations is combined with surface radiation measurements to quantify the WAIS radiation budget and constrain the three-dimensional occurrence frequency, thermodynamic phase partitioning, and surface radiative effect of clouds over West Antarctica (WA). The skill of satellite-modeled radiative fluxes is confirmed through evaluation against measurements at four Antarctic sites (WAIS Divide Ice Camp, Neumayer, Syowa, and Concordia Stations). And due to perennial high-albedo snow and icemore » cover, cloud infrared emission dominates over cloud solar reflection/absorption leading to a positive net all-wave cloud radiative effect (CRE) at the surface, with all monthly means and 99.15% of instantaneous CRE values exceeding zero. The annual-mean CRE at theWAIS surface is 34 W m -2, representing a significant cloud-induced warming of the ice sheet. Low-level liquid-containing clouds, including thin liquid water clouds implicated in radiative contributions to surface melting, are widespread and most frequent in WA during the austral summer. Clouds warm the WAIS by 26 W m -2, in summer, on average, despite maximum offsetting shortwave CRE. Glaciated cloud systems are strongly linked to orographic forcing, with maximum incidence on the WAIS continuing downstream along the Transantarctic Mountains.« less

The Patterned Topography of Ice Stream Beds; Insight from the Spatial Frequency of Mega-Scale Glacial Lineations

NASA Astrophysics Data System (ADS)

Spagnolo, M.; Bartholomaus, T. C.; Clark, C.; Stokes, C.; Atkinson, N.; Dowdeswell, J. A.; Ely, J.; Graham, A. G. C.; Hogan, K.; King, E. C.; Livingstone, S. J.; Pritchard, H. D.

2016-12-01

The formation of Mega-Scale Glacial Lineations (MSGLs), key to the understanding of how fast flowing ice streams interact with, and are controlled by, their beds is unresolved. Here we present a contribution to this debate based on a technique applied for the first time to these subglacial landscapes. 2D Fourier spectra were obtained from 22 datasets extracted from various offshore and terrestrial settings in Antarctica and Canada, including 11 samples from ice stream beds rich in MSGLs, both palaeo and extant, as well as terrains characterised by iceberg furrows, shelf-break canyons, streamlined bedrock, crag-and-tails and fluvial landforms. The Fourier analyses produces amplitude vs. wavelength plots for all possible orientations across all sampled datasets, thus allowing us to quantify which wavelengths are dominant and how strong their Fourier signal is. Uniquely amongst all other analysed terrains, MSGLs are characterised by amplitudes that are generally low along most orientations, but much higher than average along the consistent orientation of the landform's long axis. This is especially evident within a range of wavelengths between 300 and 1100 m, where a few dominant wavelengths show much higher amplitudes than all others. This distinct spectral signature could serve as a guide for models of ice stream flow and landscape evolution and allow for the automatic identification of MSGLs. The small number of dominant wavelengths also indicates that MSGLs represent a patterned topography in the sense that they are characterised by a regular lateral spacing and should be considered as a spatially self-organised phenomenon. Taken together, these results support the idea that some form(s) of instability is a key ingredient in the formation of the MSGLs.

Geoologic controls on the architecture of the Antarctic Ice Sheet's basal interface: New results from West and East Antarctica from long range geophysics (Invited)

NASA Astrophysics Data System (ADS)

Young, D. A.; Blankenship, D. D.; Greenbaum, J. S.; Richter, T.; Aitken, A.; Siegert, M. J.; Roberts, J. L.

2013-12-01

The ice-rock interface underlying the Antarctic Ice Sheet was shaped by interactions between underlying gondwanan geology and the overlying ice sheet. The ice sheet now preserves from sedimentary infill an incredibly rugged terrain which now plays a critical role in shaping subglacial hydrology, and thus shape ice sheet behavior. This terrain can by imaged through aerogeophysical means, in particular through ice penetrating radar, while airborne potential fields measurements provide insight into the geological framework that controlled erosion. Over the post IPY era, the density of airborne coverage is only now reaching the point where small scale structure can be identified and placed in context. Of particular importance is understanding the formation of focused erosional valleys, 30-50 km wide, representing now buried subglacial fjords. After initial data from the GIMBLE project in West Antarctica, and five years of sustained long range ICECAP surveys over East Antarctica , we now have a better view of the diversity of these features. The local erosion of these valleys, often cutting through significant topographic barriers, irregularly samples the underlying geology, provided a complex story in the sediment to the Antarctic margin. These valleys now provide the subglacial conduits for significant ice sheet catchments, in particular for subglacial water, including the inland catchments of DeVicq, Thwaites, and Pine Island Glaciers in West Antarctica, and Denman Glacier, Totten Glacier, Byrd Glacier and Cook Ice Shelf in East Antarctica. We find that these features, now sometimes hundreds of kilometers inland of the modern grounding line, often nucleate on or are aligned with structure inherited from the assembly of the Antarctic continent. While many of these features currently host active outlet glaciers or their tributaries, some do not, implying avenues for ice sheet change. In West Antarctica, we find a new deep connection between the coast and interior basin

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.

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

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.

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.

Abbot Ice Shelf, structure of the Amundsen Sea continental margin and the southern boundary of the Bellingshausen Plate seaward of West Antarctica

NASA Astrophysics Data System (ADS)

Cochran, James R.; Tinto, Kirsty J.; Bell, Robin E.

2015-05-01

Inversion of NASA Operation IceBridge airborne gravity over the Abbot Ice Shelf in West Antarctica for subice bathymetry defines an extensional terrain made up of east-west trending rift basins formed during the early stages of Antarctica/Zealandia rifting. Extension is minor, as rifting jumped north of Thurston Island early in the rifting process. The Amundsen Sea Embayment continental shelf west of the rifted terrain is underlain by a deeper, more extensive sedimentary basin also formed during rifting between Antarctica and Zealandia. A well-defined boundary zone separates the mildly extended Abbot extensional terrain from the deeper Amundsen Embayment shelf basin. The shelf basin has an extension factor, β, of 1.5-1.7 with 80-100 km of extension occurring across an area now 250 km wide. Following this extension, rifting centered north of the present shelf edge and proceeded to continental rupture. Since then, the Amundsen Embayment continental shelf appears to have been tectonically quiescent and shaped by subsidence, sedimentation, and the advance and retreat of the West Antarctic Ice Sheet. The Bellingshausen Plate was located seaward of the Amundsen Sea margin prior to incorporation into the Antarctic Plate at about 62 Ma. During the latter part of its independent existence, Bellingshausen plate motion had a clockwise rotational component relative to Antarctica producing convergence across the north-south trending Bellingshausen Gravity Anomaly structure at 94°W and compressive deformation on the continental slope between 94°W and 102°W. Farther west, the relative motion was extensional along an east-west trending zone occupied by the Marie Byrd Seamounts. The copyright line for this article was changed on 5 JUN 2015 after original online publication.

Comparison of glacial and non-glacial-fed streams to evaluate the loading of persistent organic pollutants through seasonal snow/ice melt.

PubMed

Bizzotto, E C; Villa, S; Vaj, C; Vighi, M

2009-02-01

The release of persistent organic pollutants (PCBs, HCB, HCHs and DDTs) accumulated in Alpine glaciers, was studied during spring-summer 2006 on the Frodolfo glacial-fed stream (Italian Alps). Samples were also taken on a non-glacial stream in the same valley, to compare POP contribution from different water sources (glacier ice, recent snow and spring). In late spring and early summer (May, June) recent snow melting is the most important process. POP contamination is more affected by local emissions and transport, and comparable levels have been measured in both streams for all studied compounds. In late summer and autumn (July-October), the contribution of ice melting strongly increases. In the glacial-fed stream the concentration of chlorinated pesticides (HCHs and DDTs) is about one order of magnitude higher than in the non-glacial-fed. A different behaviour was observed for PCBs, characterised by a peak in June showing, in both streams, concentrations three orders of magnitude higher than the background levels measured in May and in October. This result should be attributed to local emissions rather than long range atmospheric transport (LRAT). This hypothesis is supported by the PCB congener profile in June strictly comparable to the most commonly used Aroclor technical mixtures. The different seasonal behaviour observed for the different groups of chemicals indicates the POP loading in glacial streams is a combined role of long range atmospheric transport and local emission.

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

Aerodynamic heating and the deflection of drops by an obstacle in an air stream in relation to aircraft icing

NASA Technical Reports Server (NTRS)

Kantrowitz, Arthur

1940-01-01

Two topics of interest to persons attempting to apply the heat method of preventing ice formation on aircraft are considered. Surfaces moving through air at high speed are shown, both theoretically and experimentally, to be subject to important aerodynamic heating effects that will materially reduce the heat required to prevent ice. Numerical calculations of the path of water drops in an air stream around a circular cylinder are given. From these calculations, information is obtained on the percentage of the swept area cleared of drops.

The aeromagnetic method as a tool to identify Cenozoic magmatism in the West Antarctic Rift System beneath the West Antarctic Ice Sheet: a review; Thiel subglacial volcano as possible source of the ash layer in the WAISCOR

USGS Publications Warehouse

Behrendt, John C.

2013-01-01

The West Antarctic Ice Sheet (WAIS) flows through the volcanically active West Antarctic Rift System (WARS). The aeromagnetic method has been the most useful geophysical tool for identification of subglacial volcanic rocks, since 1959–64 surveys, particularly combined with 1978 radar ice-sounding. The unique 1991–97 Central West Antarctica (CWA) aerogeophysical survey covering 354,000 km2 over the WAIS, (5-km line-spaced, orthogonal lines of aeromagnetic, radar ice-sounding, and aerogravity measurements), still provides invaluable information on subglacial volcanic rocks, particularly combined with the older aeromagnetic profiles. These data indicate numerous 100–>1000 nT, 5–50-km width, shallow-source, magnetic anomalies over an area greater than 1.2 × 106 km2, mostly from subglacial volcanic sources. I interpreted the CWA anomalies as defining about 1000 “volcanic centers” requiring high remanent normal magnetizations in the present field direction. About 400 anomaly sources correlate with bed topography. At least 80% of these sources have less than 200 m relief at the WAIS bed. They appear modified by moving ice, requiring a younger age than the WAIS (about 25 Ma). Exposed volcanoes in the WARS are The present rapid changes resulting from global warming, could be accelerated by subglacial volcanism.

An unusual early Holocene diatom event north of the Getz Ice Shelf (Amundsen Sea): Implications for West Antarctic Ice Sheet development

NASA Astrophysics Data System (ADS)

Esper, O.; Gersonde, R.; Hillenbrand, C.; Kuhn, G.; Smith, J.

2011-12-01

Modern global change affects not only the polar north but also, and to increasing extent, the southern high latitudes, especially the Antarctic regions covered by the West Antarctic Ice Sheet (WAIS). Consequently, knowledge of the mechanisms controlling past WAIS dynamics and WAIS behaviour at the last deglaciation is critical to predict its development in a future warming world. Geological and palaeobiological information from major drainage areas of the WAIS, like the Amundsen Sea Embayment, shed light on the history of the WAIS glaciers. Sediment records obtained from a deep inner shelf basin north of Getz Ice Shelf document a deglacial warming in three phases. Above a glacial diamicton and a sediment package barren of microfossils that document sediment deposition by grounded ice and below an ice shelf or perennial sea ice cover (possibly fast ice), respectively, a sediment section with diatom assemblages dominated by sea ice taxa indicates ice shelf retreat and seasonal ice-free conditions. This conclusion is supported by diatom-based summer temperature reconstructions. The early retreat was followed by a phase, when exceptional diatom ooze was deposited around 12,500 cal. years B.P. [1]. Microscopical inspection of this ooze revealed excellent preservation of diatom frustules of the species Corethron pennatum together with vegetative Chaetoceros, thus an assemblage usually not preserved in the sedimentary record. Sediments succeeding this section contain diatom assemblages indicating rather constant Holocene cold water conditions with seasonal sea ice. The deposition of the diatom ooze can be related to changes in hydrographic conditions including strong advection of nutrients. However, sediment focussing in the partly steep inner shelf basins cannot be excluded as a factor enhancing the thickness of the ooze deposits. It is not only the presence of the diatom ooze but also the exceptional preservation and the species composition of the diatom assemblage

Performance of an airborne imaging 92/183 GHz radiometer during the Bering Sea Marginal Ice Zone Experiment (MIZEX-WEST)

NASA Technical Reports Server (NTRS)

Gagliano, J. A.; Mcsheehy, J. J.; Cavalieri, D. J.

1983-01-01

An airborne imaging 92/183 GHz radiometer was recently flown onboard NASA's Convair 990 research aircraft during the February 1983 Bering Sea Marginal Ice Zone Experiment (MIZEX-WEST). The 92 GHz portion of the radiometer was used to gather ice signature data and to generate real-time millimeter wave images of the marginal ice zone. Dry atmospheric conditions in the Arctic resulted in good surface ice signature data for the 183 GHz double sideband (DSB) channel situated + or - 8.75 GHz away from the water vapor absorption line. The radiometer's beam scanner imaged the marginal ice zone over a + or - 45 degrees swath angle about the aircraft nadir position. The aircraft altitude was 30,000 feet (9.20 km) maximum and 3,000 feet (0.92 km) minimum during the various data runs. Calculations of the minimum detectable target (ice) size for the radiometer as a function of aircraft altitude were performed. In addition, the change in the atmospheric attenuation at 92 GHz under varying weather conditions was incorporated into the target size calculations. A radiometric image of surface ice at 92 GHz in the marginal ice zone is included.

Water-quality, stream-habitat, and biological data for West Fork Double Bayou, Cotton Bayou, and Hackberry Gully, Chambers County, Texas, 2006-07

USGS Publications Warehouse

Brown, Dexter W.; Turco, Michael J.

2009-01-01

The U.S. Geological Survey (USGS), in cooperation with the Houston-Galveston Area Council and the Texas Commission on Environmental Quality, collected water-quality, stream-habitat, and biological data from two sites at West Fork Double Bayou, two sites at Cotton Bayou, and one site at Hackberry Gully in Chambers County, Texas, during July 2006-August 2007. Water-quality data-collection surveys consisted of synoptic 24-hour continuous measurements of water temperature, pH, specific conductance, and dissolved oxygen at the five sites and periodically collected samples at four sites analyzed for several properties and constituents of interest. Stream-habitat data were collected at each of four sites three times during the study. At each site, a representative stream reach was selected and within this reach, five evenly spaced stream transects were determined. At each transect, stream attributes (wetted channel width, water depth, bottom material, instream cover) and riparian attributes (bank slope and erosion potential, width of natural vegetation, type of vegetation, percentage tree canopy) were measured. Benthic macroinvertebrate and fish data were collected from the same reaches identified for habitat evaluation. A total of 2,572 macroinvertebrate individuals were identified from the four reaches; insect taxa were more abundant than non-insect taxa at all reaches. A total of 1,082 fish, representing 30 species and 13 families, were collected across all reaches. Stream-habitat and aquatic biota (benthic macroinvertebrates and fish) were assessed at the four sites to evaluate aquatic life use. Habitat quality index scores generally indicated 'intermediate' aquatic life use at most reaches. Benthic macroinvertebrate metrics scores indicated generally 'intermediate' aquatic life use for the West Fork Double Bayou reaches and generally 'high' aquatic life use for the Cotton Bayou and Hackberry Gully reaches. Index of biotic integrity scores for fish indicated generally

Biochemical evidence for minimal vegetation change in peatlands of the West Siberian Lowland during the Medieval Climate Anomaly and Little Ice Age

NASA Astrophysics Data System (ADS)

Philben, Michael; Kaiser, Karl; Benner, Ronald

2014-05-01

Peatland vegetation is controlled primarily by the depth of the water table, making peat paleovegetation a useful climate archive. We applied a biochemical approach to quantitatively estimate the plant sources of peat carbon based on (1) neutral sugar compositions of Sphagnum, vascular plants, and lichens and (2) lignin phenol compositions of vascular plants. We used these biochemical indices to characterize vegetation change over the last 2000 years in four peat cores from the West Siberian Lowland (Russia) to investigate climate change during the Medieval Climate Anomaly and Little Ice Age. The vegetation was dominated by Sphagnum in all four cores, but was punctuated by several rapid but transient transitions to vascular plant dominance in the two cores from the southern West Siberian Lowland (<60°N latitude). Lichen contributions were evident at the end of the Medieval Climate Anomaly and during the Little Ice Age in the two cores from northern West Siberian Lowland (>60°N), possibly indicating permafrost development. However, there was no evidence for sustained vegetation change in response to either climatic event in cores from southern West Siberian Lowland. This suggests that these climatic events were relatively mild in the southern West Siberian Lowland, although the sensitivity of bog plant communities to climate change remains poorly understood.

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.

Glacier and Ice Shelves Studies Using Satellite SAR Interferometry

NASA Technical Reports Server (NTRS)

Rignot, Eric

1999-01-01

Satellite radar interferometry is a powerful technique to measure the surface velocity and topography of glacier ice. On ice shelves, a quadruple difference technique separates tidal motion from the steady creep flow deformation of ice. The results provide a wealth of information about glacier grounding lines , mass fluxes, stability, elastic properties of ice, and tidal regime. The grounding line, which is where the glacier detaches from its bed and becomes afloat, is detected with a precision of a few tens of meters. Combining this information with satellite radar altimetry makes it possible to measure glacier discharge into the ocean and state of mass balance with greater precision than ever before, and in turn provide a significant revision of past estimates of mass balance of the Greenland and Antarctic Ice Sheets. Analysis of creep rates on floating ice permits an estimation of basal melting at the ice shelf underside. The results reveal that the action of ocean water in sub-ice-shelf cavities has been largely underestimated by oceanographic models and is the dominant mode of mass release to the ocean from an ice shelf. Precise mapping of grounding line positions also permits the detection of grounding line migration, which is a fine indicator of glacier change, independent of our knowledge of snow accumulation and ice melting. This technique has been successfully used to detect the rapid retreat of Pine Island Glacier, the largest ice stream in West Antarctica. Finally, tidal motion of ice shelves measured interferometrically provides a modern, synoptic view of the physical processes which govern the formation of tabular icebergs in the Antarctic.

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

Snow Accumulation Variability Over the West Antarctic Ice Sheet Since 1900: A Comparison of Ice Core Records With ERA-20C Reanalysis

NASA Astrophysics Data System (ADS)

Wang, Yetang; Thomas, Elizabeth R.; Hou, Shugui; Huai, Baojuan; Wu, Shuangye; Sun, Weijun; Qi, Shanzhong; Ding, Minghu; Zhang, Yulun

2017-11-01

This study uses a set of 37 firn core records over the West Antarctic Ice Sheet (WAIS) to test the performance of the twentieth century from the European Centre for Medium-Range Weather Forecasts (ERA-20C) reanalysis for snow accumulation and quantify temporal variability in snow accumulation since 1900. The firn cores are allocated to four geographical areas demarcated by drainage divides (i.e., Antarctic Peninsula (AP), western WAIS, central WAIS, and eastern WAIS) to calculate stacked records of regional snow accumulation. Our results show that the interannual variability in ERA-20C precipitation minus evaporation (P - E) agrees well with the corresponding ice core snow accumulation composites in each of the four geographical regions, suggesting its skill for simulating snow accumulation changes before the modern satellite era (pre-1979). Snow accumulation experiences significantly positive trends for the AP and eastern WAIS, a negative trend for the western WAIS, and no significant trend for the central WAIS from 1900 to 2010. The contrasting trends are associated with changes in the large-scale moisture transport driven by a deepening of the low-pressure systems and anomalies of sea ice in the Amundsen Sea Low region.

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

Analysis of a jet stream induced gravity wave associated with an observed ice cloud over Greenland

NASA Astrophysics Data System (ADS)

Buss, S.; Hertzog, A.; Hostettler, C.; Bui, T. P.; Lüthi, T.; Wernli, H.

2003-11-01

A polar stratospheric ice cloud (PSC type II) was observed by airborne lidar above Greenland on 14 January 2000. Is was the unique observation of an ice cloud over Greenland during the SOLVE/THESEO 2000 campaign. Mesoscale simulations with the hydrostatic HRM model are presented which, in contrast to global analyses, are capable to produce a vertically propagating gravity wave that induces the low temperatures at the level of the PSC afforded for the ice formation. The simulated minimum temperature is ~8 K below the driving analyses and ~3 K below the frost point, exactly coinciding with the location of the observed ice cloud. Despite the high elevations of the Greenland orography the simulated gravity wave is not a mountain wave. Analyses of the horizontal wind divergence, of the background wind profiles, of backward gravity wave ray-tracing trajectories, of HRM experiments with reduced Greenland topography and of several instability diagnostics near the tropopause level provide consistent evidence that the wave is emitted by the geostrophic adjustment of a jet instability associated with an intense, rapidly evolving, anticyclonically curved jet stream. In order to evaluate the potential frequency of such non-orographic polar stratospheric cloud events, an approximate jet instability diagnostic is performed for the winter 1999/2000. It indicates that ice-PSCs are only occasionally generated by gravity waves emanating from an unstable jet.

Uneven onset and pace of ice-dynamical imbalance in the Amundsen Sea Embayment, West Antarctica

NASA Astrophysics Data System (ADS)

Konrad, Hannes; Gilbert, Lin; Cornford, Stephen L.; Payne, Antony; Hogg, Anna; Muir, Alan; Shepherd, Andrew

2017-01-01

We combine measurements acquired by five satellite altimeter missions to obtain an uninterrupted record of ice sheet elevation change over the Amundsen Sea Embayment, West Antarctica, since 1992. Using these data, we examine the onset of surface lowering arising through ice-dynamical imbalance, and the pace at which it has propagated inland, by tracking elevation changes along glacier flow lines. Surface lowering has spread slowest (<6 km/yr) along the Pope, Smith, and Kohler (PSK) Glaciers, due to their small extent. Pine Island Glacier (PIG) is characterized by a continuous inland spreading of surface lowering, notably fast at rates of 13 to 15 km/yr along tributaries draining the southeastern lobe, possibly due to basal conditions or tributary geometry. Surface lowering on Thwaites Glacier (THG) has been episodic and has spread inland fastest (10 to 12 km/yr) along its central flow lines. The current episodes of surface lowering started approximately 10 years before the first measurements on PSK, around 1990 on PIG, and around 2000 on THG. Ice-dynamical imbalance across the sector has therefore been uneven during the satellite record.

The Global and Local Climatic Response to the Collapse of the West Antarctic Ice Sheet

NASA Astrophysics Data System (ADS)

Huybers, K. M.; Singh, H.; Steiger, N. J.; Frierson, D. M.; Steig, E. J.; Bitz, C. M.

2014-12-01

Glaciologists have suggested that a relatively small external forcing may compromise the stability of the West Antarctic Ice Sheet (WAIS). Further, there is compelling physical evidence that the WAIS has collapsed in the past, at times when the mean global temperature was only a few degrees warmer than it is today. In addition to a rapid increase in global sea level, the collapse of the WAIS could also affect the global circulation of the atmosphere. Ice sheets are some of the largest topographic features on Earth, causing large regional anomalies in albedo and radiative balance. Our work uses idealized aquaplanet models in tandem with a fully coupled ocean/atmosphere/sea-ice model (CCSM4) to compare the atmospheric, radiative, and oceanic response to a complete loss of the WAIS. Initial findings indicate that the loss of the WAIS leads to a weakening and equator-ward shift of the zonal winds, a development of strong zonal asymmetries in the meridional wind, and a northward migration of the Intertropical Convergence Zone. We aim to characterize how the local and global climate is affected by the presence of the WAIS, and how changes in the distribution of Southern Hemisphere ice may be represented in the proxy record.

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.

The Antartic Ice Borehole Probe

NASA Technical Reports Server (NTRS)

Behar, A.; Carsey, F.; Lane, A.; Engelhardt, H.

2000-01-01

The Antartic Ice Borehole Probe mission is a glaciological investigation, scheduled for November 2000-2001, that will place a probe in a hot-water drilled hole in the West Antartic ice sheet. The objectives of the probe are to observe ice-bed interactions with a downward looking camera, and ice inclusions and structure, including hypothesized ice accretion, with a side-looking camera.

Firn-air Properties and Influences at the West Antarctic Ice Sheet Divide

NASA Astrophysics Data System (ADS)

Battle, M. O.; Severinghaus, J. P.; Montzka, S. A.; Sofen, E. D.; Tans, P. P.

2007-12-01

In December 2005, we collected samples of firn air from a pair of dedicated boreholes drilled at the West Antarctic Ice Sheet Divide (WAIS-D), immediately adjacent to the WAIS-D deep ice coring effort currently underway at 79° 28'S, 112° 7'W at an elevation of ~1800m. The site is characterized by moderate temperatures (annual mean of -31°C) and moderate accumulation (24 cm/yr ice-equivalent). These samples were analyzed for a wide variety of atmospheric species by laboratories at the Scripps Institution of Oceanography, NOAA-ESRL, University of Colorado/INSTAAR, UC Irvine and Penn. State University. In this presentation, we focus on general properties of the firn air at this site and the influences on its composition, as inferred from concentration data for CO2, CH4, and a range of halogenated species, as well as the stable isotope ratios of N2 and several noble gases. Preliminary analyses indicate the presence of a shallow convective zone (a few meters or less), a diffusive region extending down to roughly 65m and a lock-in zone from 65m to the firn-ice transition at 76.5m. There is also evidence of a thermally-driven seasonal cycle in composition in the upper 25m of the firn. Modeling studies indicate that the accumulation rate at this site is low enough that the downward advection of air accompanying firn compression has a very small influence on the firn air profile. Air at the bottom of the diffusive column has a CO2-based age of 10-15 years (depending on the definition of "mean age"), while the air at the firn-ice transition is ~38 years old. Concentrations of halogenated species in the samples collected imply atmospheric histories that are generally consistent with those derived from direct atmospheric measurements and from firn air collected at other sites. Additional properties of the air, and their controlling processes will also be presented.

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.

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

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.

The Subglacial Access and Fast Ice Research Experiment - SAFIRE - on Store Glacier, West Greenland

NASA Astrophysics Data System (ADS)

Christoffersen, P.; Hubbard, B. P.; Doyle, S. H.; Young, T. J.; Hofstede, C. M.; Bougamont, M. H.; Todd, J.; Toberg, N.; Nicholls, K. W.; Box, J.; Walter, J. I.; Hubbard, A.

2015-12-01

Marine-terminating outlet glaciers drain 90 percent of the Greenland Ice Sheet and are responsible for about half of the ice sheet's net annual mass loss, which currently raises global sea level by 1 mm per year. The basal controls on these fast-flowing glaciers are, however, poorly understood, with the implication that numerical ice sheet models needed to predict future dynamic ice loss from Greenland relies on uncertain and often untested basal parameterizations. The Subglacial Access and Fast Ice Research Experiment - SAFIRE - is addressing this paucity of observational constraints by drilling to the bed of Store Glacier, a fast-flowing outlet glacier terminating in Uummannaq Fjord, West Greenland. In 2014, we gained access to the bed in four boreholes drilled to depths of 603-616 m near the center of the glacier, 30 km inland from the calving terminus where ice flows at a rate of 700 m/year. A seismic survey showed the glacier bed to consist of water-saturated, soft sediment. The water level in all four boreholes nevertheless dropped rapidly to 80 m below the ice surface when the drill connected with a basal water system, indicating effective drainage over a sedimentary bed. We were able to install wired sensor strings at the bed (water pressure, temperature, electrical conductivity and turbidity) and within the glacier (temperature and tilt) in three boreholes. The sensors operated for up to 80+ days before cables stretched and ultimately snapped due to high internal strain. The data collected during this sensor deployment show ice as cold as -21 degrees Celcius; yet, temperature of water in the basal water system was persistently above the local freezing point. With diurnal variations detected in several sensor records, we hypothesise that surface water lubricates the ice flow while also warming basal ice. The fast basal motion of Store Glacier not only occurs by basal sliding, but from high rates of concentrated strain in the bottom third of the glacier

Microbial Community Structure of Subglacial Lake Whillans, West Antarctica

PubMed Central

Achberger, Amanda M.; Christner, Brent C.; Michaud, Alexander B.; Priscu, John C.; Skidmore, Mark L.; Vick-Majors, Trista J.; Adkins, W.

2016-01-01

Subglacial Lake Whillans (SLW) is located beneath ∼800 m of ice on the Whillans Ice Stream in West Antarctica and was sampled in January of 2013, providing the first opportunity to directly examine water and sediments from an Antarctic subglacial lake. To minimize the introduction of surface contaminants to SLW during its exploration, an access borehole was created using a microbiologically clean hot water drill designed to reduce the number and viability of microorganisms in the drilling water. Analysis of 16S rRNA genes (rDNA) amplified from samples of the drilling and borehole water allowed an evaluation of the efficacy of this approach and enabled a confident assessment of the SLW ecosystem inhabitants. Based on an analysis of 16S rDNA and rRNA (i.e., reverse-transcribed rRNA molecules) data, the SLW community was found to be bacterially dominated and compositionally distinct from the assemblages identified in the drill system. The abundance of bacteria (e.g., Candidatus Nitrotoga, Sideroxydans, Thiobacillus, and Albidiferax) and archaea (Candidatus Nitrosoarchaeum) related to chemolithoautotrophs was consistent with the oxidation of reduced iron, sulfur, and nitrogen compounds having important roles as pathways for primary production in this permanently dark ecosystem. Further, the prevalence of Methylobacter in surficial lake sediments combined with the detection of methanogenic taxa in the deepest sediment horizons analyzed (34–36 cm) supported the hypothesis that methane cycling occurs beneath the West Antarctic Ice Sheet. Large ratios of rRNA to rDNA were observed for several operational taxonomic units abundant in the water column and sediments (e.g., Albidiferax, Methylobacter, Candidatus Nitrotoga, Sideroxydans, and Smithella), suggesting a potentially active role for these taxa in the SLW ecosystem. Our findings are consistent with chemosynthetic microorganisms serving as the ecological foundation in this dark subsurface environment, providing new

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.

Roosevelt Island Climate Evolution Project (RICE): A 65 Kyr ice core record of black carbon aerosol deposition to the Ross Ice Shelf, West Antarctica.

NASA Astrophysics Data System (ADS)

Edwards, Ross; Bertler, Nancy; Tuohy, Andrea; Neff, Peter; Proemse, Bernedette; Feiteng, Wang; Goodwin, Ian; Hogan, Chad

2015-04-01

Emitted by fires, black carbon aerosols (rBC) perturb the atmosphere's physical and chemical properties and are climatically active. Sedimentary charcoal and other paleo-fire records suggest that rBC emissions have varied significantly in the past due to human activity and climate variability. However, few paleo rBC records exist to constrain reconstructions of the past rBC atmospheric distribution and its climate interaction. As part of the international Roosevelt Island Climate Evolution (RICE) project, we have developed an Antarctic rBC ice core record spanning the past ~65 Kyr. The RICE deep ice core was drilled from the Roosevelt Island ice dome in West Antarctica from 2011 to 2013. The high depth resolution (~ 1 cm) record was developed using a single particle intracavity laser-induced incandescence soot photometer (SP2) coupled to an ice core melter system. The rBC record displays sub-annual variability consistent with both austral dry-season and summer biomass burning. The record exhibits significant decadal to millennial-scale variability consistent with known changes in climate. Glacial rBC concentrations were much lower than Holocene concentrations with the exception of several periods of abrupt increases in rBC. The transition from glacial to interglacial rBC concentrations occurred over a much longer time relative to other ice core climate proxies such as water isotopes and suggests . The protracted increase in rBC during the transition may reflected Southern hemisphere ecosystem / fire regime changes in response to hydroclimate and human activity.

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

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.

Changes in ice dynamics and mass balance of the Antarctic ice sheet.

PubMed

Rignot, Eric

2006-07-15

The concept that the Antarctic ice sheet changes with eternal slowness has been challenged by recent observations from satellites. Pronounced regional warming in the Antarctic Peninsula triggered ice shelf collapse, which led to a 10-fold increase in glacier flow and rapid ice sheet retreat. This chain of events illustrated the vulnerability of ice shelves to climate warming and their buffering role on the mass balance of Antarctica. In West Antarctica, the Pine Island Bay sector is draining far more ice into the ocean than is stored upstream from snow accumulation. This sector could raise sea level by 1m and trigger widespread retreat of ice in West Antarctica. Pine Island Glacier accelerated 38% since 1975, and most of the speed up took place over the last decade. Its neighbour Thwaites Glacier is widening up and may double its width when its weakened eastern ice shelf breaks up. Widespread acceleration in this sector may be caused by glacier ungrounding from ice shelf melting by an ocean that has recently warmed by 0.3 degrees C. In contrast, glaciers buffered from oceanic change by large ice shelves have only small contributions to sea level. In East Antarctica, many glaciers are close to a state of mass balance, but sectors grounded well below sea level, such as Cook Ice Shelf, Ninnis/Mertz, Frost and Totten glaciers, are thinning and losing mass. Hence, East Antarctica is not immune to changes.

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

New Constraints on Post-LGM WAIS Retreat from the Whales Deep Paleo-ice-stream Trough in Eastern Ross Sea

NASA Astrophysics Data System (ADS)

DeCesare, M.; Bart, P. J.; Rosenheim, B. E.

2016-02-01

New multibeam and seismic data acquired during NBP1502 show that a back-stepping cluster containing at least four grounding zone wedges (GZWs) define a bathymetric saddle on the middle shelf of the Whales Deep paleo-ice-stream trough in eastern Ross Sea. Our synthesis of geophysical data with jumbo piston/kasten cores show that we penetrated diamict, sub-ice shelf and open marine sediments associated with four temporally distinct grounding events. A high number of well-preserved benthic and planktonic foraminifera were found in sediments we interpret to have been deposited in sub-ice shelf and open marine environments. A low number of similarly well-preserved benthic foraminifera were recovered from the underlying ice proximal diamict that was deposited on the GZW foreset. We tentatively propose that the pristine foraminifera are in situ and that these specimens provide a unique opportunity to constrain the retreat of grounded and floating ice from the eastern Ross Sea outer continental shelf. Our ongoing synthesis of new radiocarbon dates, stable isotope (δ18O and δ13C) and element/calcium ratios (e.g., Mg/Ca, B/Ca) will be presented.

Climate hazards, adaptation and "resilience" of societies (early Little Ice Age, west of France).

NASA Astrophysics Data System (ADS)

Athimon, Emmanuelle; Maanan, Mohamed

2016-04-01

Over the past ten to fifteen years, climate hazards and adaptation have received more attention due to the current climate change. Climate historians have gathered strong evidence that the world's climate has evolved over the past millennium and one of the most significant changes took place during the Little Ice Age. Recently, a set of questions has emerged: what were the effects of the Little Ice Age on human's societies? How did humans adapt to these climate changes? How did they react to extreme weather-related events? Using examples of climate hazards from the West of France during the beginning of the Little Ice Age (xivth-xviith centuries) such as storms, flooding, drought, harsh winters, the poster aims at showing how the past societies can constitute a source of inspiration for present ones. Through schemas, this research exposes the system's rebound capacity, points out the importance of the historical depth in research on human's adaptation and resilience and shows the value of integrating a historical approach. It reveals that History contributes to the knowledge of the relationship between societies and climate hazards. Data on climate hazards and adaptation of societies stem from historical sources such as chronicles, diaries, books of accounts, records of cities repairs. To protect themselves and their goods, medieval and modern societies had developed specific skills, practices and strategies. From the xivth to the xviiith century, there is an increase of defense by dikes in the low Loire, as for example the construction of those amongst Longué and Ponts-de-Cé between the early xivth century and 1407. The French kingdom's authorities also tried increasingly to provide technical, material, logistical and fiscal support: for instance, during the winter 1564-1565, several bridges have been destroyed by a river flooding in Nantes. The King Charles IX then offered to people of Nantes part of the funds from taxes on the main activities such as the

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

Ecological Status of Wyoming Streams, 2000-2003

USGS Publications Warehouse

Peterson, David A.; Hargett, Eric G.; Wright, Peter R.; Zumberge, Jeremy R.

2007-01-01

The ecological status of perennial streams in Wyoming was determined and compared with the status of perennial streams throughout 12 States in the western United States, using data collected as part of the Western Pilot Environmental Monitoring and Assessment Program (EMAP-West). Results for Wyoming are compared and contrasted in the context of the entire EMAP-West study area (west-wide) and climatic regions (based on aggregated ecoregions) within Wyoming. In Wyoming, ecological status, estimated as the proportion of the perennial stream length in least disturbed, most disturbed, and intermediate disturbance condition, based on ecological indicators of vertebrate and invertebrate assemblages was similar, in many cases, to the status of those assemblages determined for EMAP-West. Ecological status based on chemical and physical habitat stressors also was similar in Wyoming to west-wide proportions in many cases. Riparian disturbance was one of the most common physical stressors west-wide and in Wyoming. The estimates of riparian disturbance indicated about 90 percent of the stream length in the xeric climatic region in Wyoming was rated most disturbed, compared to about 30 percent rated most disturbed in the mountain climatic region in Wyoming. Results from analyses using a macroinvertebrate multi-metric index (MMI) and macroinvertebrate ratio of observed to expected taxa (O/E) developed specifically for the west-wide EMAP study were compared to results using a macroinvertebrate MMI and O/E developed for Wyoming. Proportions of perennial stream length in various condition categories determined from macroinvertebrate MMIs often were similar in Wyoming to proportions observed west-wide. Differences were larger, but not extreme, between west-wide and Wyoming O/E models. An aquatic life use support decision matrix developed for interpreting the Wyoming MMI and O/E model data indicated about one-half of the stream length statewide achieves the State's narrative aquatic

Tidal Modulation of Ice-shelf Flow: a Viscous Model of the Ross Ice Shelf

NASA Technical Reports Server (NTRS)

Brunt, Kelly M.; MacAyeal, Douglas R.

2014-01-01

Three stations near the calving front of the Ross Ice Shelf, Antarctica, recorded GPS data through a full spring-neap tidal cycle in November 2005. The data revealed a diurnal horizontal motion that varied both along and transverse to the long-term average velocity direction, similar to tidal signals observed in other ice shelves and ice streams. Based on its periodicity, it was hypothesized that the signal represents a flow response of the Ross Ice Shelf to the diurnal tides of the Ross Sea. To assess the influence of the tide on the ice-shelf motion, two hypotheses were developed. The first addressed the direct response of the ice shelf to tidal forcing, such as forces due to sea-surface slopes or forces due to sub-ice-shelf currents. The second involved the indirect response of ice-shelf flow to the tidal signals observed in the ice streams that source the ice shelf. A finite-element model, based on viscous creep flow, was developed to test these hypotheses, but succeeded only in falsifying both hypotheses, i.e. showing that direct tidal effects produce too small a response, and indirect tidal effects produce a response that is not smooth in time. This nullification suggests that a combination of viscous and elastic deformation is required to explain the observations.

High-resolution sub-ice-shelf seafloor records of twentieth century ungrounding and retreat of Pine Island Glacier, West Antarctica

NASA Astrophysics Data System (ADS)

Davies, D.; Bingham, R. G.; Graham, A. G. C.; Spagnolo, M.; Dutrieux, P.; Vaughan, D. G.; Jenkins, A.; Nitsche, F. O.

2017-09-01

Pine Island Glacier Ice Shelf (PIGIS) has been thinning rapidly over recent decades, resulting in a progressive drawdown of the inland ice and an upstream migration of the grounding line. The resultant ice loss from Pine Island Glacier (PIG) and its neighboring ice streams presently contributes an estimated ˜10% to global sea level rise, motivating efforts to constrain better the rate of future ice retreat. One route toward gaining a better understanding of the processes required to underpin physically based projections is provided by examining assemblages of landforms and sediment exposed over recent decades by the ongoing ungrounding of PIG. Here we present high-resolution bathymetry and sub-bottom-profiler data acquired by autonomous underwater vehicle (AUV) surveys beneath PIGIS in 2009 and 2014, respectively. We identify landforms and sediments associated with grounded ice flow, proglacial and subglacial sediment transport, overprinting of lightly grounded ice-shelf keels, and stepwise grounding line retreat. The location of a submarine ridge (Jenkins Ridge) coincides with a transition from exposed crystalline bedrock to abundant sediment cover potentially linked to a thick sedimentary basin extending upstream of the modern grounding line. The capability of acquiring high-resolution data from AUV platforms enables observations of landforms and understanding of processes on a scale that is not possible in standard offshore geophysical surveys.

Mounting evidence for intense ocean interaction with the Pine Island Glacier Ice Shelf

NASA Astrophysics Data System (ADS)

Bindschadler, R.; Holland, D.; Vaughan, D.; Vornberger, P.

2008-12-01

The spatial signature of thinning and acceleration of the Pine Island Glacier has led to the inference that these changes originate at the seaward end of the glacier, possibly within or under the ice shelf (Payne et al., 2004; Shepherd et al., 2004). We present new analyses resulting from both new and archived satellite imagery of the ice shelf that supports this inference and provides new insights into strong seasonal and intra- annual characters of ocean-ice shelf interaction. Strong longitudinal variations in both thickness and surface elevation measured by British Antarctic Survey airborne radars (Vaughan et al., 2006) have wavelengths that correspond roughly to the annual motion of the ice shelf. These could be caused by seasonal variations in flow speed, but such variations of flow speed have never been reported and are not seen in the most recent continuous GPS observations of the ice shelf. We suggest that these strong variations in ice thickness, as large as 200 meters in an average thickness of 600 meters, are caused by seasonal variations in the properties of the water circulating underneath the ice shelf. One likely explanation is that the dominant water mass reaching the deepest parts of the ice shelf alternates between cold High Salinity Shelf Water in the winter and warm Circumpolar Deep Water in the summer. Evidence for recent strengthening of the sub- shelf circulation is the sudden occurrence of three persistent polynyas immediately adjacent to the ice front. These are located in precisely the locations expected from modeled sub-shelf circulation (Payne et al., 2007). This mode was never observed in any satellite imagery prior to the 1999-2000 austral summer (data of 7 summers since 1973 were available), but has occurred in 7 of the 9 summers since and persists throughout the summer. Payne, A.J., A. Vieli, A.P. Shepherd, D.J. Wingham and E. Rignot, 2004. Recent dramatic thinning of largest West Antarctic ice stream triggered by oceans, Geophysical

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.

Analysis of a jet stream induced gravity wave associated with an observed stratospheric ice cloud over Greenland

NASA Astrophysics Data System (ADS)

Buss, S.; Hertzog, A.; Hostettler, C.; Bui, T. B.; Lüthi, D.; Wernli, H.

2004-08-01

A polar stratospheric ice cloud (PSC type II) was observed by airborne lidar above Greenland on 14 January 2000. It was the unique observation of an ice cloud over Greenland during the SOLVE/THESEO 2000 campaign. Mesoscale simulations with the hydrostatic HRM model are presented which, in contrast to global analyses, are capable to produce a vertically propagating gravity wave that induces the low temperatures at the level of the PSC afforded for the ice formation. The simulated minimum temperature is ~8 K below the driving analyses and ~4.5 K below the frost point, exactly coinciding with the location of the observed ice cloud. Despite the high elevations of the Greenland orography the simulated gravity wave is not a mountain wave. Analyses of the horizontal wind divergence, of the background wind profiles, of backward gravity wave ray-tracing trajectories, of HRM experiments with reduced Greenland topography and of several diagnostics near the tropopause level provide evidence that the wave is emitted from an intense, rapidly evolving, anticyclonically curved jet stream. The precise physical process responsible for the wave emission could not be identified definitely, but geostrophic adjustment and shear instability are likely candidates.

In order to evaluate the potential frequency of such non-orographic polar stratospheric cloud events, the non-linear balance equation diagnostic is performed for the winter 1999/2000. It indicates that ice-PSCs are only occasionally generated by gravity waves emanating from spontaneous adjustment.

Ice electrode electrolytic cell

DOEpatents

Glenn, D.F.; Suciu, D.F.; Harris, T.L.; Ingram, J.C.

1993-04-06

This invention relates to a method and apparatus for removing heavy metals from waste water, soils, or process streams by electrolytic cell means. The method includes cooling a cell cathode to form an ice layer over the cathode and then applying an electric current to deposit a layer of the heavy metal over the ice. The metal is then easily removed after melting the ice. In a second embodiment, the same ice-covered electrode can be employed to form powdered metals.

Ice electrode electrolytic cell

DOEpatents

Glenn, David F.; Suciu, Dan F.; Harris, Taryl L.; Ingram, Jani C.

1993-01-01

This invention relates to a method and apparatus for removing heavy metals from waste water, soils, or process streams by electrolytic cell means. The method includes cooling a cell cathode to form an ice layer over the cathode and then applying an electric current to deposit a layer of the heavy metal over the ice. The metal is then easily removed after melting the ice. In a second embodiment, the same ice-covered electrode can be employed to form powdered metals.

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

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.

Investigation of land ice-ocean interaction with a fully coupled ice-ocean model: 1. Model description and behavior

NASA Astrophysics Data System (ADS)

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

2012-06-01

Antarctic ice shelves interact closely with the ocean cavities beneath them, with ice shelf geometry influencing ocean cavity circulation, and heat from the ocean driving changes in the ice shelves, as well as the grounded ice streams that feed them. We present a new coupled model of an ice stream-ice shelf-ocean system that is used to study this interaction. The model is capable of representing a moving grounding line and dynamically responding ocean circulation within the ice shelf cavity. Idealized experiments designed to investigate the response of the coupled system to instantaneous increases in ocean temperature show ice-ocean system responses on multiple timescales. Melt rates and ice shelf basal slopes near the grounding line adjust in 1-2 years, and downstream advection of the resulting ice shelf thinning takes place on decadal timescales. Retreat of the grounding line and adjustment of grounded ice takes place on a much longer timescale, and the system takes several centuries to reach a new steady state. During this slow retreat, and in the absence of either an upward-or downward-sloping bed or long-term trends in ocean heat content, the ice shelf and melt rates maintain a characteristic pattern relative to the grounding line.

Water Ice on Pluto

NASA Image and Video Library

2015-10-16

The Ralph instrument on NASA's New Horizons spacecraft detected water ice on Pluto's surface, picking up on the ice's near-infrared spectral characteristics. (See featured image from Oct. 8, 2015.) The middle panel shows a region west of Pluto's "heart" feature -- which the mission team calls Tombaugh Regio -- about 280 miles (450 kilometers) across. It combines visible imagery from Ralph's Multispectral Visible Imaging Camera (MVIC) with infrared spectroscopy from the Linear Etalon Imaging Spectral Array (LEISA). Areas with the strongest water ice spectral signature are highlighted in blue. Major outcrops of water ice occur in regions informally called Viking Terra, along Virgil Fossa west of Elliot crater, and in Baré Montes. Numerous smaller outcrops are associated with impact craters and valleys between mountains. In the lower left panel, LEISA spectra are shown for two regions indicated by cyan and magenta boxes. The white curve is a water ice model spectrum, showing similar features to the cyan spectrum. The magenta spectrum is dominated by methane ice absorptions. The lower right panel shows an MVIC enhanced color view of the region in the white box, with MVIC's blue, red and near-infrared filters displayed in blue, green and red channels, respectively. The regions showing the strongest water ice signature are associated with terrains that are actually a lighter shade of red. http://photojournal.jpl.nasa.gov/catalog/PIA20030

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

Teleseismic Earthquake Signals Observed on an Ice Shelf

NASA Astrophysics Data System (ADS)

Baker, M. G.; Aster, R. C.; Anthony, R. E.; Wiens, D.; Nyblade, A.; Bromirski, P. D.; Stephen, R. A.; Gerstoft, P.

2015-12-01

The West Antarctic Rift System (WARS) is one of Earth's largest continental extension zones. Study of the WARS is complicated by the presence of the West Antarctic Ice Sheet, the Ross Ice Shelf, and the Ross Sea. Recent deployments of broadband seismographs in the POLENET project have allowed passive seismic techniques, such as receiver function analysis and surface wave dispersion, to be widely utilized to infer crustal and mantle velocity structure across much of the WARS and West Antarctica. However, a large sector of the WARS lies beneath the Ross Ice Shelf. In late 2014, 34 broadband seismographs were deployed atop the ice shelf to jointly study deep Earth structure and the dynamics of the ice shelf. Ice shelf conditions present strong challenges to broadband teleseismic imaging: 1) The presence of complicating signals in the microseism through long-period bands due to the influence of ocean gravity waves; 2) The strong velocity contrasts at the ice-water and water-sediment interfaces on either side of the water layer give rise to large amplitude reverberations; 3) The water layer screens S-waves or P-to-S phases originating from below the water layer. We present an initial analysis of the first teleseismic earthquake arrivals collected on the ice shelf at the end of the 2014 field season from a limited subset of these stations.

Stream water quality in coal mined areas of the lower Cheat River Basin, West Virginia and Pennsylvania, during low-flow conditions, July 1997

USGS Publications Warehouse

Williams, Donald R.; Clark, Mary E.; Brown, Juliane B.

1999-01-01

IntroductionThe Cheat River Basin is in the Allegheny Plateau and Allegheny Mountain Sections of the Appalachian Plateau Physiographic Province (Fenneman, 1946) and is almost entirely within the state of West Virginia. The Cheat River drains an area of 1,422 square miles in Randolph, Tucker, Preston, and Monongalia Counties in West Virginia and Fayette County in Pennsylvania. From its headwaters in Randolph County, W.Va., the Cheat River flows 157 miles north to the Pennsylvania state line, where it enters the Monongahela River. The Cheat River drainage comprises approximately 19 percent of the total Monongahela River Basin. The Cheat River and streams within the Cheat River Basin are characterized by steep gradients, rock channels, and high flow velocities that have created a thriving white-water rafting industry for the area. The headwaters of the Cheat River contain some of the most pristine and aesthetic streams in West Virginia. The attraction to the area, particularly the lower part of the Cheat River Basin (the lower 412 square miles of the basin), has been suppressed because of poor water quality. The economy of the Lower Cheat River Basin has been dominated by coal mining over many decades. As a result, many abandoned deep and surface mines discharge untreated acid mine drainage (AMD), which degrades water quality, into the Cheat River and many of its tributary streams. Approximately 60 regulated mine-related discharges (West Virginia Department of Environmental Protection, 1996) and 185 abandoned mine sites (U.S. Office of Surface Mining, 1998) discharge treated and untreated AMD into the Cheat River and its tributaries.The West Virginia Department of Environmental Protection (WVDEP) Office of Abandoned Mine Lands and Reclamation (AML&R) has recently completed several AMD reclamation projects throughout the Cheat River Basin that have collectively improved the mainstem water quality. The AML&R office is currently involved in acquiring grant funds and

Ice-Shelf Tidal Flexure and Subglacial Pressure Variations

NASA Technical Reports Server (NTRS)

Walker, Ryan T.; Parizek, Byron R.; Alley, Richard B.; Anandakrishnan, Sridhar; Riverman, Kiya L.; Christianson, Knut

2013-01-01

We develop a model of an ice shelf-ice stream system as a viscoelastic beam partially supported by an elastic foundation. When bed rock near the grounding line acts as a fulcrum, leverage from the ice shelf dropping at low tide can cause significant (approx 1 cm) uplift in the first few kilometers of grounded ice.This uplift and the corresponding depression at high tide lead to basal pressure variations of sufficient magnitude to influence subglacial hydrology.Tidal flexure may thus affect basal lubrication, sediment flow, and till strength, all of which are significant factors in ice-stream dynamics and grounding-line stability. Under certain circumstances, our results suggest the possibility of seawater being drawn into the subglacial water system. The presence of sea water beneath grounded ice would significantly change the radar reflectivity of the grounding zone and complicate the interpretation of grounded versus floating ice based on ice-penetrating radar observations.

Channelized melting drives thinning under Dotson ice shelf, Western Antarctic Ice Sheet

NASA Astrophysics Data System (ADS)

Gourmelen, N.; Goldberg, D.; Snow, K.; Henley, S. F.; Bingham, R. G.; Kimura, S.; Hogg, A.; Shepherd, A.; Mouginot, J.; Lenaerts, J.; Ligtenberg, S.; Van De Berg, W. J.

2017-12-01

The majority of meteoric ice that forms in West Antarctica leaves the ice sheet through floating ice shelves, many of which have been thinning substantially over the last 25 years. A significant proportion of ice-shelf thinning has been driven by submarine melting facilitated by increased access of relatively warm (>0.6oC) modified Circumpolar Deep Water to sub-shelf cavities. Ice shelves play a significant role in stabilising the ice sheet from runaway retreat and regulating its contribution to sea level change. Ice-shelf melting has also been implicated in sustaining high primary productivity in Antarctica's coastal seas. However, these processes vary regionally and are not fully understood. Under some ice shelves, concentrated melting leads to the formation of inverted channels. These channels guide buoyant melt-laden outflow, which can lead to localised melting of the sea ice cover. The channels may also potentially lead to heightened crevassing, which in turn affects ice-shelf stability. Meanwhile, numerical studies suggest that buttressing loss is sensitive to the location of ice removal within an ice-shelf. Thus it is important that we observe spatial patterns, as well as magnitudes, of ice-shelf thinning, in order to improve understanding of the ocean drivers of thinning and of their impacts on ice-shelf stability. Here we show from high-resolution altimetry measurements acquired between 2010 to 2016 that Dotson Ice Shelf, West Antarctica, thins in response to basal melting focussed along a single 5 km-wide and 60 km-long channel extending from the ice shelf's grounding zone to its calving front. The coupled effect of geostrophic circulation and ice-shelf topography leads to the observed concentration of basal melting. Analysis of previous datasets suggests that this process has been ongoing for at least the last 25 years. If focused thinning continues at present rates, the channel would melt through within 40-50 years, almost two centuries before it is

Volcanic rocks and subglacial volcanism beneath the West Antarctic Ice Sheet in the West Antarctic Rift System, (WAIS) from aeromagnetic and radar ice sounding - Thiel Subglacial Volcano as possible source of the ash layer in the WAISCORE

NASA Astrophysics Data System (ADS)

Behrendt, J. C.

2012-12-01

Radar ice sounding and aeromagnetic surveys reported over the West Antarctic Ice Sheet (WAIS) have been interpreted as evidence of subglacial volcanic eruptions over a very extensive area (>500,000 km2 ) of the volcanically active West Antarctic rift system interpreted as caused by subglacial volcanic rocks. Several active volcanoes have shown evidence of eruption through the WAIS and several other active volcanoes are present beneath the WAIS reported from radar and aeromagnetic data. Five-kilometer spaced coincident aeromagnetic and radar ice sounding surveys since 1990 provide three dimensional characterization of the magnetic field and bed topography beneath the ice sheet. These 5-50-km-width, semicircular magnetic anomalies range from 100->1000 nT as observed ~1 km over the 2-3 km thick ice have been interpreted as evidence of subglacial eruptions. Comparison of a carefully selected subset of ~400 of the >1000 high-amplitude anomalies in the CWA survey having topographic expression at the glacier bed, showed >80% had less than 200-m relief. About 18 high-amplitude subglacial magnetic sources also have high topography and bed relief (>600 m) interpreted as subaerially erupted volcanic peaks when the WAIS was absent, whose competent lava flows protected their edifices from erosion. All of these would have high elevation above sea-level, were the ice removed and glacial rebound to have occurred. Nine of these subaerially erupted volcanoes are concentrated in the WAIS divide area. Behrendt et al., 1998 interpreted a circular ring of positive magnetic anomalies overlying the WAIS divide as caused by a volcanic caldera. The area is characterized by high elevation bed topography. The negative regional magnetic anomaly surrounding the caldera anomalies was interpreted as the result of a shallow Curie isotherm. High heat flow inferred from temperature logging in the WAISCORE (G. Clow 2012, personal communication; Conway, 2011) and a prominent volcanic ash layer in the

Long-term Glacial History of the West Antarctic Ice Sheet from Cosmogenic Nuclides in a Subglacial Bedrock Core

NASA Astrophysics Data System (ADS)

Spector, P. E.; Stone, J.; Hillebrand, T.; Gombiner, J. H.

2017-12-01

To investigate the response of the West Antarctic Ice Sheet (WAIS) to climatic conditions warmer than present, we are analyzing cosmogenic nuclides in a bedrock core from beneath 150 m of ice at a site near the Pirrit Hills. Our aim is to determine whether the WAIS has thinned in the past, exposing bedrock at this site, and if so, when. This will help to determine the vulnerability of the ice sheet to future warming, and identify climatic thresholds capable of inducing WAIS collapse. We selected a site where the ice-sheet surface lies at 1300 m, approximately halfway from the ice-sheet divide to the grounding line. We expect ice thickness at the site to reflect WAIS dynamics, rather than local meteorology or topography. Ice flow speeds are moderate and ice above the core site is thin enough to remain cold-based, limiting the possibility of subglacial erosion which would compromise the cosmogenic nuclide record. We targeted a subglacial ridge adjacent to an exposed granite nunatak. This lithology provides minerals suitable for analysis of multiple cosmogenic nuclides with different half-lives. Although we aimed to collect two cores from different depths to compare exposure histories, hydrofracture of the basal ice prevented us from reaching the bed at the first drill site. The second hole produced 5.5 m of discontinuous ice core above 8 m of bedrock core. Initial analyses of quartz from the bedrock show low levels of Be-10. Further analyses of Be-10, Al-26, Cl-36 and Ne-21 from the full length of the core will be required to determine whether this is because the surface has never been exposed, or because the cosmogenic nuclide profile has been truncated by glacial erosion. We will present comprehensive cosmogenic nuclide data, and discuss implications for WAIS deglaciation history, at the meeting. Supported by US National Science Foundation awards ANT-1142162 and PLR-1341728.

The geomorphic signature of past ice sheets in the marine record

NASA Astrophysics Data System (ADS)

Dowdeswell, J. A.

2016-12-01

The deglaciation of high-latitude continental shelves since the Last Glacial Maximum has revealed suites of subglacial and ice-contact landforms that have remained well-preserved beneath tens to hundreds of metres of water. Once ice has retreated, sedimentation is generally low on polar shelves during interglacials and the submarine landforms have not, therefore, been buried by subsequent sedimentation. By contrast, the beds of modern ice sheets are hidden by several thousand metres of ice, which is much more difficult than water to penetrate using geophysical methods. These submarine glacial landforms provide insights into past ice-sheet form and flow, and information on the processes that have taken place beneath former ice sheets. Examples will be shown of streamlined subglacial landforms that indicate the distribution and dimensions of former ice streams on high-latitde continental margins. Distinctive landform assemblages characterise ice stream and inter-ice stream areas. Landforms, including subglacially formed channel systems in inner- and mid-shelf areas, and the lack of them on sedimentary outer shelves, allow inferences to be made about subglacial hydrology. The distribution of grounding-zone wedges and other transverse moraine ridges also provides evidence on the nature of ice-sheet retreat - whether by rapid collapse, episodic retreat or by the slow retreat of grounded ice. Such information can be used to test the predictive capability of ice-sheet numerical models. These marine geophysical and geological observations of submarine glacial landforms enhance our understanding of the form and flow of past ice masses at scales ranging from ice sheets (1000s of km in flow-line and margin length), through ice streams (100s of km long), to surge-type glaciers (10s of km long).

Integrating terrestrial and marine records of the LGM in McMurdo Sound, Antarctica: implications for grounded ice expansion, ice flow, and deglaciation of the Ross Sea Embayment

NASA Astrophysics Data System (ADS)

Christ, A. J.; Marchant, D. R.

2017-12-01

During the LGM, grounded glacier ice filled the Ross Embayment and deposited glacial drift on volcanic islands and peninsulas in McMurdo Sound, as well as along coastal regions of the Transantarctic Mountains (TAM), including the McMurdo Dry Valleys and Royal Society Range. The flow geometry and retreat history of this ice remains debated, with contrasting views yielding divergent implications for both the fundamental cause of Antarctic ice expansion as well as the interaction and behavior of ice derived from East and West Antarctica during late Quaternary time. We present terrestrial geomorphologic evidence that enables the reconstruction of former ice elevations, ice-flow paths, and ice-marginal environments in McMurdo Sound. Radiocarbon dates of fossil algae interbedded with ice-marginal sediments provide a coherent timeline for local ice retreat. These data are integrated with marine-sediment records and multi-beam data to reconstruct late glacial dynamics of grounded ice in McMurdo Sound and the western Ross Sea. The combined dataset suggest a dominance of ice flow toward the TAM in McMurdo Sound during all phases of glaciation, with thick, grounded ice at or near its maximum extent between 19.6 and 12.3 calibrated thousands of years before present (cal. ka). Our data show no significant advance of locally derived ice from the TAM into McMurdo Sound, consistent with the assertion that Late Pleistocene expansion of grounded ice in McMurdo Sound, and throughout the wider Ross Embayment, occurs in response to lower eustatic sea level and the resulting advance of marine-based outlet glaciers and ice streams (and perhaps also reduced oceanic heat flux), rather than local increases in precipitation and ice accumulation. Finally, when combined with allied data across the wider Ross Embayment, which show that widespread deglaciation outside McMurdo Sound did not commence until 13.1 ka, the implication is that retreat of grounded glacier ice in the Ross Embayment did

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.

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

Estimating magnitude and frequency of peak discharges for rural, unregulated, streams in West Virginia

USGS Publications Warehouse

Wiley, J.B.; Atkins, John T.; Tasker, Gary D.

2000-01-01

Multiple and simple least-squares regression models for the log10-transformed 100-year discharge with independent variables describing the basin characteristics (log10-transformed and untransformed) for 267 streamflow-gaging stations were evaluated, and the regression residuals were plotted as areal distributions that defined three regions of the State, designated East, North, and South. Exploratory data analysis procedures identified 31 gaging stations at which discharges are different than would be expected for West Virginia. Regional equations for the 2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500-year peak discharges were determined by generalized least-squares regression using data from 236 gaging stations. Log10-transformed drainage area was the most significant independent variable for all regions.Equations developed in this study are applicable only to rural, unregulated, streams within the boundaries of West Virginia. The accuracy of estimating equations is quantified by measuring the average prediction error (from 27.7 to 44.7 percent) and equivalent years of record (from 1.6 to 20.0 years).

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

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

Drainage areas of the Twelvepole Creek basin, West Virginia; Big Sandy River basin, West Virginia; Tug Fork basin, Virginia, Kentucky, West Virginia

USGS Publications Warehouse

Wilson, M.W.

1979-01-01

Drainage areas were determined for 61 basins in the Twelvepole Creek basin, West Virginia; 11 basins of the Big Sandy River Basin, West Virginia; and 210 basins in the Tug Fork basin of Virginia, Kentucky, and West Virginia. Most basins with areas greater than 5 square miles were included. Drainage areas were measured with electronic digitizing equipment, and supplementary measurements were made with a hand planimeter. Stream mileages were determined by measuring, with a graduated plastic strip, distances from the mouth of each stream to the measuring point on that stream. Mileages were reported to the nearest one-hundredth of a mile in all cases. The latitude and longitude of each measuring point was determined with electronic digitizing equipment and is reported to the nearest second. The information is listed in tabular form in downstream order. Measuring points for the basins are located in the tables by intersecting tributaries, by counties, by map quadrangles, or by latitude and longitude. (Woodard-USGS)

Using aerogravity and seismic data to model the bathymetry and upper crustal structure beneath the Pine Island Glacier ice shelf, West Antarctica

NASA Astrophysics Data System (ADS)

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

2013-12-01

Recent estimates indicate that ice shelves along the Amundsen Sea coast in West Antarctica are losing substantial mass through sub-ice-shelf melting and contributing to the accelerating mass loss of the grounded ice buttressed by them. For Pine Island Glacier (PIG), relatively warm Circumpolar Deep Water has been identified as the key driver of the sub-ice-shelf melting although poor constraints on PIG sub-ice shelf have restricted thorough understanding of these ice-ocean interactions. Aerogravity data from NASA's Operation IceBridge (OIB) have been useful in identifying large-scale (on the order of ten kilometers) features but the results have relatively large uncertainties due to the inherent non-uniqueness of the gravity inversion. Seismic methods offer the most direct means of providing water thickness and upper crustal geological constraints, but availability of such data sets over the PIG ice shelf has been limited due to logistical constraints. Here we present a comparative analysis of the bathymetry and upper crustal structure beneath the ice shelf of PIG through joint inversion of OIB aerogravity data and in situ active-source seismic measurements collected in the 2012-13 austral summer. Preliminary results indicate improved resolution of the ocean cavity, particularly in the interior and sides of the PIG ice shelf, and sedimentary drape across the region. Seismically derived variations in ice and ocean water densities are also applied to the gravity inversion to produce a more robust model of PIG sub-ice shelf structure, as opposed to commonly used single ice and water densities across the entire study region. Misfits between the seismically-constrained gravity inversion and that estimated previously from aerogravity alone provide insights on the sensitivity of gravity measurements to model perturbations and highlight the limitations of employing gravity data to model ice shelf environments when no other sub-ice constraints are available.

Digital outlines and topography of the glaciers of the American West

USGS Publications Warehouse

Fountain, Andrew G.; Hoffman, Matthew; Jackson, Keith; Basagic, Hassan; Nylen, Thomas; Percy, David

2007-01-01

Alpine glaciers have generally receded during the past century (post-“Little Ice Age”) because of climate warming (Oerlemans and others, 1998; Mann and others, 1999; Dyurgerov and Meier, 2000; Grove, 2001). This general retreat has accelerated since the mid 1970s, when a shift in atmospheric circulation occurred (McCabe and Fountain, 1995; Dyurgerov and Meier, 2000). The loss in glacier cover has had several profound effects. First, the shrinkage of glaciers results in a net increase in stream flow, typically in late summer when water supplies are at the lowest levels (Fountain and Tangborn, 1985). This additional water is important to ecosystems (Hall and Fagre, 2003) and to human water needs (Tangborn, 1980). However, if shrinkage continues, the net contribution to stream flow will diminish, and the effect upon these benefactors will be adverse. Glacier shrinkage is also a significant factor in current sea level rise (Meier, 1984; Dyurgerov and Meier, 2000). Second, many of the glaciers in the West Coast States are located on stratovolcanoes, and continued recession will leave oversteepened river valleys. These valleys, once buttressed by ice are now subject to failure, creating conditions for lahars (Walder and Driedger, 1994; O’Connor and others, 2001). Finally, reduction or loss of glaciers reduce or eliminate glacial activity as an important geomorphic process on landscape evolution and alters erosion rates in high alpine areas (Hallet and others, 1996). Because of the importance of glaciers to studies of climate change, hazards, and landscape modification, glacier inventories have been published for Alaska (Manley, in press), China (http://wdcdgg.westgis.ac.cn/DATABASE/Glacier/Glacier.asp), Nepal (Mool and others, 2001), Switzerland (Paul and others, 2002), and the Tyrolian Alps of Austria (Paul, 2002), among other locales. To provide the necessary data for assessing the magnitude and rate of glacier change in the American West, exclusive of Alaska

Changes in flow of Crosson and Dotson ice shelves, West Antarctica, in response to elevated melt

NASA Astrophysics Data System (ADS)

Lilien, David A.; Joughin, Ian; Smith, Benjamin; Shean, David E.

2018-04-01

Crosson and Dotson ice shelves are two of the most rapidly changing outlets in West Antarctica, displaying both significant thinning and grounding-line retreat in recent decades. We used remotely sensed measurements of velocity and ice geometry to investigate the processes controlling their changes in speed and grounding-line position over the past 20 years. We combined these observations with inverse modeling of the viscosity of the ice shelves to understand how weakening of the shelves affected this speedup. These ice shelves have lost mass continuously since the 1990s, and we find that this loss results from increasing melt beneath both shelves and the increasing speed of Crosson. High melt rates persisted over the period covered by our observations (1996-2014), with the highest rates beneath areas that ungrounded during this time. Grounding-line flux exceeded basin-wide accumulation by about a factor of 2 throughout the study period, consistent with earlier studies, resulting in significant loss of grounded as well as floating ice. The near doubling of Crosson's speed in some areas during this time is likely the result of weakening of its margins and retreat of its grounding line. This speedup contrasts with Dotson, which has maintained its speed despite increasingly high melt rates near its grounding line, likely a result of the sustained competency of the shelf. Our results indicate that changes to melt rates began before 1996 and suggest that observed increases in melt in the 2000s compounded an ongoing retreat of this system. Advection of a channel along Dotson, as well as the grounding-line position of Kohler Glacier, suggests that Dotson experienced a change in flow around the 1970s, which may be the initial cause of its continuing retreat.

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

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

A varied subglacial landscape under Thwaites Glacier, West Antarctica

NASA Astrophysics Data System (ADS)

Christianson, K. A.; Holschuh, N.; Paden, J. D.; Sprick, J.; Peters, L. E.; Anandakrishnan, S.; Alley, R. B.

2017-12-01

Deglaciated landscapes, whether subaerial or submarine, are often host to a rich panoply of subglacial landforms, such as drumlims, crags, megascale glacial lineations, grounding-line wedges, deep meltwater channels, and more. These landforms are formed and shaped by interactions between the ice and underlying substrate, and thus have implications for the flow of the overlying ice. Robust interpretations of the relationship between the ice and its substrate based on subglacial landforms that remain after deglaciation have been inhibited by a dearth of high-resolution observations of currently glaciated subglacial landscapes, where ice flow speed is known and where subglacial conditions can be ascertained using geophysical methods. Past direct observations of landforms under currently fast-flowing ice have been limited to a few ice streams, where relatively homogeneous, thick dilatant till layers may favor formation of specific subglacial features, i.e., megascale glacial lineations and grounding-zone wedges. Here we present two detailed gridded subglacial topographies, obtained from ice-penetrating radar measurements, from Thwaites Glacier, West Antarctica, where ice flows over a highly variable bed (in both topography and model-inferred basal shear stress). One grid is located ˜170 km downstream from the ice divide where ice is moving ˜100 m/yr. Here the ice advects over a broad basin and then flows into a subglacial ridge (of several hundred meters amplitude) oriented orthogonally to flow. A deep canyon ( 400 m) that cuts through this ridge in roughly the ice-flow direction and relatively soft sediments on the downstream side of the basin (immediately upstream of the canyon) suggest that a large subglacial lake may have formed in this location and drained catastrophically, as has been hypothesized as the formation mechanism for the deep canyons observed on the Amundsen Sea continental shelf. Numerous multiscale glacial lineations are also observed in the

Estimation of composite hydraulic resistance in ice-covered alluvial streams

NASA Astrophysics Data System (ADS)

Ghareh Aghaji Zare, Soheil; Moore, Stephanie A.; Rennie, Colin D.; Seidou, Ousmane; Ahmari, Habib; Malenchak, Jarrod

2016-02-01

Formation, propagation, and recession of ice cover introduce a dynamic boundary layer to the top of rivers during northern winters. Ice cover affects water velocity magnitude and distribution, water level and consequently conveyance capacity of the river. In this research, total resistance, i.e., "composite resistance," is studied for a 4 month period including stable ice cover, breakup, and open water stages in Lower Nelson River (LNR), northern Manitoba, Canada. Flow and ice characteristics such as water velocity and depth and ice thickness and condition were measured continuously using acoustic techniques. An Acoustic Doppler Current Profiler (ADCP) and Shallow Water Ice Profiling Sonar (SWIPS) were installed simultaneously on a bottom mount and deployed for this purpose. Total resistance to the flow and boundary roughness are estimated using measured bulk hydraulic parameters. A novel method is developed to calculate composite resistance directly from measured under ice velocity profiles. The results of this method are compared to the measured total resistance and to the calculated composite resistance using formulae available in literature. The new technique is demonstrated to compare favorably to measured total resistance and to outperform previously available methods.

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

Anomalously high arsenic concentration in a West Antarctic ice core and its relationship to copper mining in Chile

NASA Astrophysics Data System (ADS)

Schwanck, Franciele; Simões, Jefferson C.; Handley, Michael; Mayewski, Paul A.; Bernardo, Ronaldo T.; Aquino, Francisco E.

2016-01-01

Arsenic variability records are preserved in snow and ice cores and can be utilized to reconstruct air pollution history. The Mount Johns ice core (79°55‧S; 94°23‧W and 91.2 m depth) was collected from the West Antarctic Ice Sheet in the 2008/09 austral summer. Here, we report the As concentration variability as determined by 2137 samples from the upper 45 m of this core using ICP-SFMS (CCI, University of Maine, USA). The record covers approximately 125 years (1883-2008) showing a mean concentration of 4.32 pg g-1. The arsenic concentration in the core follows global copper mining evolution, particularly in Chile (the largest producer of Cu). From 1940 to 1990, copper-mining production increased along with arsenic concentrations in the MJ core, from 1.92 pg g-1 (before 1900) to 7.94 pg g-1 (1950). In the last two decades, environmental regulations for As emissions have been implemented, forcing smelters to treat their gases to conform to national and international environmental standards. In Chile, decontamination plants required by the government started operating from 1993 to 2000. Thereafter, Chilean copper production more than doubled while As emission levels declined, and the same reduction was observed in the Mount Johns ice core. After 1999, arsenic concentrations in our samples decreased to levels comparable to the period before 1900.