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Sample records for age ice sheets

  1. Exposure age and ice-sheet model constraints on Pliocene East Antarctic ice sheet dynamics

    PubMed Central

    Yamane, Masako; Yokoyama, Yusuke; Abe-Ouchi, Ayako; Obrochta, Stephen; Saito, Fuyuki; Moriwaki, Kiichi; Matsuzaki, Hiroyuki

    2015-01-01

    The Late Pliocene epoch is a potential analogue for future climate in a warming world. Here we reconstruct Plio-Pleistocene East Antarctic Ice Sheet (EAIS) variability using cosmogenic nuclide exposure ages and model simulations to better understand ice sheet behaviour under such warm conditions. New and previously published exposure ages indicate interior-thickening during the Pliocene. An ice sheet model with mid-Pliocene boundary conditions also results in interior thickening and suggests that both the Wilkes Subglacial and Aurora Basins largely melted, offsetting increased ice volume. Considering contributions from West Antarctica and Greenland, this is consistent with the most recent IPCC AR5 estimate, which indicates that the Pliocene sea level likely did not exceed +20 m on Milankovitch timescales. The inception of colder climate since ∼3 Myr has increased the sea ice cover and inhibited active moisture transport to Antarctica, resulting in reduced ice sheet thickness, at least in coastal areas. PMID:25908601

  2. Exposure age and ice-sheet model constraints on Pliocene East Antarctic ice sheet dynamics.

    PubMed

    Yamane, Masako; Yokoyama, Yusuke; Abe-Ouchi, Ayako; Obrochta, Stephen; Saito, Fuyuki; Moriwaki, Kiichi; Matsuzaki, Hiroyuki

    2015-04-24

    The Late Pliocene epoch is a potential analogue for future climate in a warming world. Here we reconstruct Plio-Pleistocene East Antarctic Ice Sheet (EAIS) variability using cosmogenic nuclide exposure ages and model simulations to better understand ice sheet behaviour under such warm conditions. New and previously published exposure ages indicate interior-thickening during the Pliocene. An ice sheet model with mid-Pliocene boundary conditions also results in interior thickening and suggests that both the Wilkes Subglacial and Aurora Basins largely melted, offsetting increased ice volume. Considering contributions from West Antarctica and Greenland, this is consistent with the most recent IPCC AR5 estimate, which indicates that the Pliocene sea level likely did not exceed +20 m on Milankovitch timescales. The inception of colder climate since ∼3 Myr has increased the sea ice cover and inhibited active moisture transport to Antarctica, resulting in reduced ice sheet thickness, at least in coastal areas.

  3. Radiostratigraphy and age structure of the Greenland Ice Sheet

    PubMed Central

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

    2015-01-01

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

  4. Ice-age Ice-sheet Rheology: Constraints from the Last Glacial Maximum Form of the Laurentide Ice Sheet

    NASA Technical Reports Server (NTRS)

    Peltier, W. Richard; Goldsby, David L.; Kohlstedt, David L.; Tarasov, Lev

    2000-01-01

    State-ot-the-art thermomechanical models of the modern Greenland ice and the ancient Laurenticle ice sheet that covered Canada at the Last Glacial Maximum (LGM) are not able to explain simultaneously the observed forms of these cryospheric structures when the same, anisotropy-enhanced, version of the conventional Glen flow law is employed to describe their rheology. The LGM Laurenticle ice sheet. predicted to develop in response to orbital climate forcing, is such that the ratio of its thickness to its horizontal extent is extremely large compared to the aspect ratio inferred on the basis of surface-geomorphological and solid-earth-geophysical constraints. We show that if the Glen flow law representation of the rheology is replaced with a new rheology based upon very high quality laboratory measurements of the stress-strain-rate relation, then the aspect ratios of both the modern Greenland ice sheet and the Laurenticle ice sheet, that existed at the LGM, are simultaneously explained with little or no retuning of the flow law.

  5. Ice-age simulations with a caving ice-sheet model

    SciTech Connect

    Pollard, D.

    1982-01-01

    Further extensions and results of a simple northern hemispheric ice-sheet model are described for the Quaternary ice ages. The basic model predicts ice thickness and bedrock deformation in a north-south cross section, with a prescribed snow-budget distribution shifted uniformly in space to represent the orbital perturbations. An ice calving parameterization crudely representing proglacial lakes or marine incursions can attack the ice whenever the tip drops below sea level. As in Pollard (1982a) the basic model produces a large approx. 100,000-yr response and agrees fairly well with the delta/sup 18/O deep-sea core records. Three extensions of the model are described: an alternative treatment of bedrock deformation, a more realistic ice-shelf model of ice calving, and a generalized parameterization for such features as the North Atlantic deglacial meltwater layer. Much the same ice-age simulations and agreement with the delta/sup 18/O records as with the original model are still obtained. The observed phase-correlation between the 100,000-yr cycles and eccentricity is examined. First, the model is shown to give a approx. 100,000-yr response to nearly any kind of higher-frequency forcing. Although over the last two million years the model phase is mainly controlled by the precessional modulation due to eccentricity, over just the last 600,000-yr the observed phase can also be simulated with eccentricity held constant. A definitive conclusion on the phase-control of the Quaternary ice ages is prevented by uncertainty in the deep-sea core time scales before approx. 600,000-yr B.P. In an appendix the model is applied to West Antarctica where unforced internal oscillations occur with periods of about 50,000-yr.

  6. An investigation of the astronomical theory of the ice ages using a simple climate-ice sheet model

    NASA Technical Reports Server (NTRS)

    Pollard, D.

    1978-01-01

    The astronomical theory of the Quaternary ice ages is incorporated into a simple climate model for global weather; important features of the model include the albedo feedback, topography and dynamics of the ice sheets. For various parameterizations of the orbital elements, the model yields realistic assessments of the northern ice sheet. Lack of a land-sea heat capacity contrast represents one of the chief difficulties of the model.

  7. Cosmogenic 10Be Age Constraints on the Holocene Deglaciation of the Scandinavian Ice Sheet

    NASA Astrophysics Data System (ADS)

    Cuzzone, J. K.; Clark, P. U.; Wohlfarth, B.; Lunkka, J.

    2011-12-01

    An important question in climate science is how ice sheets will respond to a climate warmer than present. Because our understanding of how these changes will occur remains limited, reconstructing the deglaciation of former ice sheets allows for a better understanding of how past ice sheets responded to a climate warmer than present along with understanding their contribution to sea-level rise. We will present new cosmogenic 10Be ages from erratic boulders along three transects spanning southern to northern Sweden and Finland that improve our understanding of the deglaciation of the Scandinavian Ice Sheet (SIS) beginning ~ 11.7ka through its final demise during the early Holocene. By constraining the Holocene deglaciation of the SIS and its associated retreat rates, we will establish the SIS contribution to Holocene sea level rise, improving our understanding of ice-sheet response to warming climates.

  8. The last forests in Greenland, and the age of the ice sheet

    NASA Astrophysics Data System (ADS)

    Funder, Svend; Schmidt, Astrid M. Z.; Dahl-Jensen, Dorthe; Steffensen, Jørgen Peder; Willerslev, Eske

    2014-05-01

    Recently ancient DNA (aDNA) studies of the basal ice in the Camp Century ice core, northern Greenland, have shown that mixed coniferous-deciduous forest grew here before the area was invaded and permanently covered by the ice sheet. The coring site is situated only 100 km from the present ice margin and more than 500 km from the ice divide, indicating that since this last inception the northern part of the ice sheet never receded more than 100 km from its present margin. Dating of the basal ice and obtaining an age for the forest and for the beginning of the ice sheet's permanency has been attempted by analyzing for optically stimulated luminescence (OSL), meteoric 10Be/36Cl cosmogenic nuclides, 234U/238U recoil. These methods all provide only minimum ages and show that the forest at Cap Century is older than 500 ka. Comparison with other Pleistocene "forest sites" in Greenland - the Kap København Formation in northernmost Greenland, the DYE-3 ice core in the south, the ODP boring 646 south of Greenland, as well as results from basal ice in the GRIP ice core - extends the minimum age to c. 1 ma. The maximum age is provided by the Kap København Formation, which must be older - or contemporaneous. The formation has recently been confirmed to date within the interval 2-2.5 ma, with a preferred age of 2.3-2.4 ma. Surprisingly, application of the molecular clock of insect COI sequences on the Camp Century aDNA now seem to push the minimum age just as far back - to 2.4 ma, suggesting that the timberline boreal forest at Kap København is contemporaneous with the mixed forest at Camp Century, 600 km to the south. From this we conclude that the northern ice sheet dome, which today contains 85% of the total ice sheet volume, has remained within 100 km of its present margin for at least 1 ma, and possibly may go back as far as 2.4 ma. The ice sheet has therefore survived both interglacials and "super interglacials" that were both warmer and longer than the present. This

  9. Holocene Deglaciation of the Scandinavian Ice Sheet: Preliminary 10Be Ages

    NASA Astrophysics Data System (ADS)

    Cuzzone, J. K.; Clark, P. U.; Marcott, S. A.; Pekka Lunka, J.; Wohlfarth, B.; Carlson, A. E.

    2012-12-01

    The response of ice sheets to a warming climate is not well understood. Because we are limited in our understanding of present dynamics, reconstructing the deglaciation of former ice sheets allows for a better understanding of how past ice sheets responded to a warming climate along with their contribution to sea-level rise. These reconstructions also serve as critical constraints for ice sheet modeling efforts. Here, we present a suite of new 10Be ages from erratic boulders along three transects spanning southern to northern Sweden and Finland, that improve our understanding of the deglaciation of the Scandinavian Ice Sheet (SIS) beginning ~ 11.7ka through its final demise during the early Holocene. Preliminary dates from southern Finland, beginning at the Salpausselka Youngers Dryas moraine (11.5 ± 0.7 ka, n=2), inland southern Finland near Jyvaskyla (11.5 ± 0.5ka, n=2), and coastal Finland (~60km from Gulf of Bothnia) near Vimpeli (11.5 ± 0.4ka, n=1) indicate a rapid retreat following the Younger Dryas for Southern Finland (~500km within uncertainty of ages). Preliminary dates also exist for Northern Finland, near Inari (10.3 ± 0.5ka, n=2). Additional ages now being processed at PRIME Lab, Purdue University, which will establish a basis for SIS retreat from all sampled sites, will also be presented. These new data will help to constrain the Holocene deglaciation of the SIS and its associated retreat rates, and establish the SIS contribution to Holocene sea level rise, which will improve our understanding of ice-sheet response to a warming climate.

  10. Marine ice sheets of Pleistocene age on the East Siberian Continental Margin (Invited)

    NASA Astrophysics Data System (ADS)

    Niessen, F.; Hong, J.; Hegewald, A.; Matthiessen, J. J.; Stein, R. H.; Kim, H.; Kim, S.; Jensen, L.; Jokat, W.; Nam, S.; Kang, S.

    2013-12-01

    Based on swath bathymetry, sediment echosounding, seismic profiling and sediment coring we present results of the RV "Polarstern' cruise ARK-XIII/3 (2008) and RV "Araon" cruise ARA03B (2012), which investigated an area between the Chukchi Borderland and the East Siberian Sea between 165°W and 170°E. At the southern end of the Mendeleev Ridge, close to the Chukchi and East Siberian shelves, evidence is found for the existence of Pleistocene ice sheets/ice shelves, which have grounded several times in up to 1200 m present water depth. We found mega-scale glacial lineations associated with deposition of glaciogenic wedges and debris-flow deposits indicative of sub-glacial erosion and deposition close to the former grounding lines. Glacially lineated areas are associated with large-scale erosion, accentuated by a conspicuous truncation of pre-glacial strata typically capped with mostly thin layers of diamicton draped by pelagic sediments. Our tentative age model suggests that the youngest and shallowest grounding event of an ice sheet should be within Marine Isotope Stage (MIS) 3. The oldest and deepest event predates MIS 6. According to our results, ice sheets of more than one km in thickness continued onto, and likely centered over, the East Siberian Shelf. They were possibly linked to previously suggested ice sheets on the Chukchi Borderland and the New Siberian Islands. We propose that the ice sheets extended northward as thick ice shelves, which grounded on the Mendeleev Ridge to an area up to 78°N within MIS 5 and/or earlier. These results have important implication for the former distribution of thick ice masses in the Arctic Ocean during the Pleistocene. They are relevant for global sea-level variations, albedo, ocean-atmosphere heat exchange, freshwater export from the Arctic Ocean at glacial terminations and the formation of submarine permafrost. The existence of km-thick Pleistocene ice sheets in the western Arctic Ocean during glacial times predating

  11. Holocene Deglaciation of the Scandinavian Ice Sheet: Preliminary 10Be Ages

    NASA Astrophysics Data System (ADS)

    Cuzzone, J. K.; Clark, P. U.; Marcott, S. A.; Lunkka, J.; Wohlfarth, B.; Caffee, M. W.; Carlson, A. E.

    2013-12-01

    The response of ice sheets to a warming climate is not well understood. Because we are limited in our understanding of present dynamics, reconstructing the deglaciation of former ice sheets allows for a better understanding of how past ice sheets responded to a warming climate along with their contribution to sea-level rise. These reconstructions also serve as critical constraints for ice sheet modeling efforts. Here, we present a suite of new 10Be ages from erratic boulders along three transects spanning southern to northern Sweden and Finland, that improve our understanding of the deglaciation of the Scandinavian Ice Sheet (SIS) beginning ~ 11.7ka through its final demise during the early Holocene. Dates from southern Finland, beginning at the Salpausselka Younger Dryas moraine (11.5 × 0.7 ka, n=4), inland southern Finland near Jyvaskyla (11.5 × 0.5ka, n=2), and coastal Finland (~60km from Gulf of Bothnia) near Vimpeli (11.5 × 0.4ka, n=4) indicate a rapid retreat following the Younger Dryas for Southern Finland (~500km within uncertainty of ages). Preliminary dates also exist for Northern Finland, near Inari (10.8 × 0.5ka, n=4) and near Oulu (10.5 × 0.6 ka, n = 4) suggesting a later retreat in the north. Dates from southern Sweden, near Skovde (12.73 × 0.8ka, n=4) to Mora (10.41 × 0.6ka, n=5) suggest a slower retreat (over ~400km). Lastly, dates in Northwestern Sweden suggest a final termination of the SIS around 9.4 × 0.7ka (n = 3). Additional ages are now being processed at PRIME Lab, Purdue University, which will further strengthen our understanding of SIS retreat from all sampled sites. These new data will help to constrain the Holocene deglaciation of the SIS and its associated retreat rates, and establish the SIS contribution to Holocene sea level rise, which will improve our understanding of ice-sheet response to a warming climate.

  12. Climate science: Ice streams waned as ice sheets shrank

    NASA Astrophysics Data System (ADS)

    Briner, Jason P.

    2016-02-01

    It emerges that ice discharge from a major ice sheet did not increase rapidly at the end of the most recent ice age. The finding points to steady, not catastrophic, ice-sheet loss and sea-level rise on millennial timescales. See Letter p.322

  13. Global ice sheet modeling

    SciTech Connect

    Hughes, T.J.; Fastook, J.L.

    1994-05-01

    The University of Maine conducted this study for Pacific Northwest Laboratory (PNL) as part of a global climate modeling task for site characterization of the potential nuclear waste respository site at Yucca Mountain, NV. The purpose of the study was to develop a global ice sheet dynamics model that will forecast the three-dimensional configuration of global ice sheets for specific climate change scenarios. The objective of the third (final) year of the work was to produce ice sheet data for glaciation scenarios covering the next 100,000 years. This was accomplished using both the map-plane and flowband solutions of our time-dependent, finite-element gridpoint model. The theory and equations used to develop the ice sheet models are presented. Three future scenarios were simulated by the model and results are discussed.

  14. Extent of the last ice sheet in northern Scotland tested with cosmogenic 10Be exposure ages

    USGS Publications Warehouse

    Phillips, W.M.; Hall, A.M.; Ballantyne, C.K.; Binnie, S.; Kubik, P.W.; Freeman, S.

    2008-01-01

    The extent of the last British-Irish Ice Sheet (BIIS) in northern Scotland is disputed. A restricted ice sheet model holds that at the global Last Glacial Maximum (LGM; ca. 23-19 ka) the BIIS terminated on land in northern Scotland, leaving Buchan, Caithness and the Orkney Islands ice-free. An alternative model implies that these three areas were ice-covered at the LGM, with the BIIS extending offshore onto the adjacent shelves. We test the two models using cosmogenic 10Be surface exposure dating of erratic boulders and glacially eroded bedrock from the three areas. Our results indicate that the last BIIS covered all of northern Scotland during the LGM, but that widespread deglaciation of Caithness and Orkney occurred prior to rapid warming at ca. 14.5 ka. Copyright ?? 2008 John Wiley & Sons, Ltd.

  15. Ice sheets and nitrogen.

    PubMed

    Wolff, Eric W

    2013-07-01

    Snow and ice play their most important role in the nitrogen cycle as a barrier to land-atmosphere and ocean-atmosphere exchanges that would otherwise occur. The inventory of nitrogen compounds in the polar ice sheets is approximately 260 Tg N, dominated by nitrate in the much larger Antarctic ice sheet. Ice cores help to inform us about the natural variability of the nitrogen cycle at global and regional scale, and about the extent of disturbance in recent decades. Nitrous oxide concentrations have risen about 20 per cent in the last 200 years and are now almost certainly higher than at any time in the last 800 000 years. Nitrate concentrations recorded in Greenland ice rose by a factor of 2-3, particularly between the 1950s and 1980s, reflecting a major change in NOx emissions reaching the background atmosphere. Increases in ice cores drilled at lower latitudes can be used to validate or constrain regional emission inventories. Background ammonium concentrations in Greenland ice show no significant recent trend, although the record is very noisy, being dominated by spikes of input from biomass burning events. Neither nitrate nor ammonium shows significant recent trends in Antarctica, although their natural variations are of biogeochemical and atmospheric chemical interest. Finally, it has been found that photolysis of nitrate in the snowpack leads to significant re-emissions of NOx that can strongly impact the regional atmosphere in snow-covered areas.

  16. Ice sheets and nitrogen

    PubMed Central

    Wolff, Eric W.

    2013-01-01

    Snow and ice play their most important role in the nitrogen cycle as a barrier to land–atmosphere and ocean–atmosphere exchanges that would otherwise occur. The inventory of nitrogen compounds in the polar ice sheets is approximately 260 Tg N, dominated by nitrate in the much larger Antarctic ice sheet. Ice cores help to inform us about the natural variability of the nitrogen cycle at global and regional scale, and about the extent of disturbance in recent decades. Nitrous oxide concentrations have risen about 20 per cent in the last 200 years and are now almost certainly higher than at any time in the last 800 000 years. Nitrate concentrations recorded in Greenland ice rose by a factor of 2–3, particularly between the 1950s and 1980s, reflecting a major change in NOx emissions reaching the background atmosphere. Increases in ice cores drilled at lower latitudes can be used to validate or constrain regional emission inventories. Background ammonium concentrations in Greenland ice show no significant recent trend, although the record is very noisy, being dominated by spikes of input from biomass burning events. Neither nitrate nor ammonium shows significant recent trends in Antarctica, although their natural variations are of biogeochemical and atmospheric chemical interest. Finally, it has been found that photolysis of nitrate in the snowpack leads to significant re-emissions of NOx that can strongly impact the regional atmosphere in snow-covered areas. PMID:23713125

  17. The Physics of Ice Sheets

    ERIC Educational Resources Information Center

    Bassis, J. N.

    2008-01-01

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

  18. Dust Provenance and Radiometric U-Series Ages as Evidence for an Eemian Ice Sheet in Greenland

    NASA Astrophysics Data System (ADS)

    Aciego, S.; Bourdon, B.; Schwander, J.; Stocker, T. F.

    2009-12-01

    The mineralogy and geochemistry of air-transported mineral particles, dust, reflect the prior history of the source material as well as influence the chemistry of the settling locations (rivers, ice sheets and ultimately the oceans). When applied to ice sheets, the atmospheric circulation patterns gleaned from the chemical characteristics of the dust may provide some additional constraints on size and shape of paleo-ice sheets. Furthermore, the ice bound dust grains can be used to determine the age of the ice by using uranium series recoil as a radiometric dating method, provided there is sufficient information about the size and shape of the dust grains and the [U] concentration and isotopic (234U/238U) composition of the ice and dust. The Dye3 ice core is the southern-most deep ice core in Greenland, so should provide a minimum estimate of ice sheet size in the past: the existence of ice is evidence for an ice sheet at any given time. A series of samples from 200 m to 2030 m in depth were analyzed by MC-ICPMS and TIMS for U concentrations and 234U/238U as well as 176Hf/177Hf, 87Sr/86Sr, and 143Nd/144Nd. The radiogenic isotopic compositions of the insoluble dust found in the upper 1800 m falls within the range of previously measured Greenland dust samples: 87Sr/86Sr = 0.7108 - 0.7174, ɛNd = -9.7 - -13.6, and ɛHf = -2 - -5. However, the data trends toward significantly more unradiogenic Nd and Hf and radiogenic Sr values in the lower 100 m: 87Sr/86Sr = 0.7167 - 0.7200, ɛNd = -15.62 - -17.36, and ɛHf = -21 - -25; the deepest sample containing basal sediments having the most extreme values: 87Sr/86Sr = 0.7349 - 0.7785 ɛNd = -37.48 - -41.61, and ɛHf = -24.8 - -39.54. The calculated 234U/238U radiometric age for the deepest ice ranges from 90 to 110 ± 50 ka, in the same range as two possible age models for the Dye3 location, indicating the deepest ice is in the range of 40-60 ka or 85-120 ka. However, based on the radiogenic isotopes, while the dust in the

  19. Future ice ages and the challenges related to final disposal of nuclear waste: The Greenland Ice Sheet Hydrology Project

    NASA Astrophysics Data System (ADS)

    Lehtinen, A.; Claesson-Liljedahl, L.; Näslund, J.-O.; Ruskeeniemi, T.

    2009-04-01

    A deep geological repository for nuclear waste is designed to keep radiotoxic material separated from mankind and the environment for several hundreds of thousands of years. Within this time perspective glacial conditions are expected in high latitudes/Canada and North Europe. Climate induced changes such as the growth of ice sheets and permafrost will influence and alter the ground surface and subsurface environment, which may impact repository safety. In order to understand how climate change, particularly cooling and glaciation, might affect a repository in the long term, the use of present-day analogues helps to reduce the uncertainties and support the assumptions made in safety assessments. There are major uncertainties concerning hydrological processes related to glacial conditions. The impact of glaciations on any planned repository is a key consideration when performing safety assessments as it is one of the strongest perturbations related to climate change in the long term. The main aspects that need to be further investigated include: 1) to what extent does the meltwater produced by an ice sheet penetrates into the bedrock; 2) what is the pressure situation under an ice sheet, driving ground water flow; 3) how much oxygenated water will reach repository depth; 4) to what depth does glacial meltwater penetrate into the bedrock ; 5)what chemical composition does such water has when and if it reaches repository depth; and 6) can taliks (unfrozen ground in a permafrost area) act as concentrated discharge points of deep groundwater potentially transporting radionuclides in case of repository failure? Field data is needed in order to achieve a better and integrated understanding of the problems discussed above. Thus, research in a natural analogue site in Greenland has been planned and initiated by the Finnish (Posiva), Swedish (SKB) and Canadian (NWMO) nuclear waste management companies. The Greenland ice sheet and the Kangerlussuaq area (west Greenland

  20. Clathrates, Ice sheets and Global Climate Change?

    NASA Astrophysics Data System (ADS)

    Weitemeyer, K. A.; Buffett, B. A.

    2002-12-01

    Ice age cycles are associated with large fluctuations in the concentration of atmospheric methane and carbon dioxide. The cause for these fluctuations remains unexplained, although clathrates are often proposed as a potential source of methane. However, the mechanism for methane release from clathrates into the atmosphere has not been established. We examine the possiblity that clathrates accumulate below continential ice sheets during periods of glaciation, permitting substantial release of methane during deglaciation. The source of the methane is due to microbial decomposition of organic material below the ice sheet. We assume that organic material in soils ahead of the ice sheet is frozen in place due to low atmospheric temperatures. Once the ice sheet is present and sufficiently thick, the geothermal gradient adjusts to bring the sediments to the melting point of water. Assuming aneorobic conditions underneath the ice sheet, the presence of methanogens at the basal surface of the ice sheet allow for the conversion of organic carbon to methane. This methane is stored as clathrate when the temperature and pressure conditions at the basal surface permit thermodynamic stability (ice thickness in excess of 250m at 0oC). Subsequent deglaciation destabalizes clathrate causing the release of methane into the atmosphere. We use a numerical model of the Laurentide-Cascade ice sheet (Marshall et. al., 1999) for the areal extent, thickness, and the thermal conditions at the base of the ice sheet as a function of time. In order to bound the available carbon below the ice sheet, we consider two estimates of soil carbon inventory based on tundra and present potential vegetation. Our model quantifies the decrease of carbon in the soil and the accumulation of clathrate as the ice sheet advances. As the ice sheet retreats we track the amplitude and timing of methane released into the atmosphere. The amplitude of predicted fluctuations in atmospheric methane are 80-200ppbv, which

  1. Preliminary Cosmogenic Surface Exposure Ages on Laurentide Ice-sheet Retreat and Opening of the Eastern Lake Agassiz Outlets

    NASA Astrophysics Data System (ADS)

    Leydet, D.; Carlson, A. E.; Sinclair, G.; Teller, J. T.; Breckenridge, A. J.; Caffee, M. W.; Barth, A. M.

    2015-12-01

    The chronology for the eastern outlets of glacial Lake Agassiz holds important consequences for the cause of Younger Dryas cold event during the last deglaciation. Eastward routing of Lake Agassiz runoff was originally hypothesized to have triggered the Younger Dryas. However, currently the chronology of the eastern outlets is only constrained by minimum-limiting radiocarbon ages that could suggest the eastern outlets were still ice covered at the start of the Younger Dryas at ~12.9 ka BP, requiring a different forcing of this abrupt climate event. Nevertheless, the oldest radiocarbon ages are still consistent with an ice-free eastern outlet at the start of the Younger Dryas. Here we will present preliminary 10-Be cosmogenic surface exposure ages from the North Lake, Flat Rock Lake, glacial Lake Kaministiquia, and Lake Nipigon outlets located near Thunder Bay, Ontario. These ages will date the timing of the deglaciation of the Laurentide ice sheet in the eastern outlet region of glacial Lake Agassiz. This will provide an important constraint for the hypothesized freshwater forcing of the cause of Younger Dryas cold event.

  2. Age of the Ørkendalen moraines, Kangerlussuaq, Greenland: constraints on the extent of the southwestern margin of the Greenland Ice Sheet during the Holocene

    NASA Astrophysics Data System (ADS)

    Levy, Laura B.; Kelly, Meredith A.; Howley, Jennifer A.; Virginia, Ross A.

    2012-10-01

    Although Greenland ice core records register relatively stable Holocene climate conditions, the lower elevation margins of the Greenland Ice Sheet (GrIS) experienced significant Holocene fluctuations. These fluctuations include ice sheet recession during the Holocene Thermal Maximum (9-5 ka) and advance during the Little Ice Age (LIA; ˜A.D. 1350-1880). Determining the extent and timing of these fluctuations is important for understanding the response of the GrIS to interglacial climate conditions both warmer and colder than at present and for developing accurate ice sheet models. Sets of moraines marking past extents of the southwestern GrIS margin occur in the Kangerlussuaq region. We focus on the Ørkendalen moraines, a prominent moraine set located within 2 km of the modern ice margin and just outboard of the LIA moraines. We present the first 10Be ages of the Ørkendalen moraines indicating they were deposited at 6.8 ± 0.3 ka. The geomorphic relationship between the Ørkendalen and LIA moraines indicates that the ice sheet margin was inboard of its Ørkendalen extent between ˜6.8 ka and the culmination of the LIA. The age of the Ørkendalen moraines provides an important constraint on the extent of the southwestern GrIS during the middle Holocene.

  3. Fluctuations of the Greenland Ice Sheet since the last ice age: comparisons of the response of marine and land-terminating ice margins to Holocene climate changes

    NASA Astrophysics Data System (ADS)

    Levy, Laura; Larsen, Nicolaj; Kelly, Meredith; Kjær, Kurt; Bjørk, Anders; Kjeldsen, Kristian; Funder, Svend; Applegate, Patrick; Howley, Jennifer; Virginia, Ross

    2016-04-01

    Fluctuations of the margins of the Greenland Ice Sheet (GrIS) in response to Holocene climate change may be used as a proxy for how they may respond to future climate change. Here, we present records of Holocene fluctuations of the margins of the GrIS in southeastern and southwestern Greenland based on geomorphic mapping and 10Be dating of boulders on moraines and boulders on bedrock. We show that in southeastern Greenland the marine-terminating outlet glaciers retreated from the outer coast between 10.4 and 9.4 ka and responded rapidly to early Holocene warming, retreating up-fjord at a rate of ~70-100 m yr-1. These rates are comparable, or higher than, modern retreat rates of 30-100 m yr-1. In contrast, the terrestrial margin of the GrIS in the Kangerlussuaq region of southwestern Greenland retreated only ~25 m yr-1 throughout the early and middle Holocene. These data indicate that forcings such as warm ocean waters, fjord geometry, fjord bathymetry and ice dynamics are potential mechanisms that caused differences in retreat rates between marine and terrestrial-terminating margins of the ice sheet. Additionally, they show that the margins of the GrIS responded sensitively to Holocene climate change.

  4. Ice stream activity scaled to ice sheet volume during Laurentide Ice Sheet deglaciation

    NASA Astrophysics Data System (ADS)

    Stokes, C. R.; Margold, M.; Clark, C. D.; Tarasov, L.

    2016-02-01

    The contribution of the Greenland and West Antarctic ice sheets to sea level has increased in recent decades, largely owing to the thinning and retreat of outlet glaciers and ice streams. This dynamic loss is a serious concern, with some modelling studies suggesting that the collapse of a major ice sheet could be imminent or potentially underway in West Antarctica, but others predicting a more limited response. A major problem is that observations used to initialize and calibrate models typically span only a few decades, and, at the ice-sheet scale, it is unclear how the entire drainage network of ice streams evolves over longer timescales. This represents one of the largest sources of uncertainty when predicting the contributions of ice sheets to sea-level rise. 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. Here we reconstruct the activity of 117 ice streams that operated at various times during deglaciation of the Laurentide Ice Sheet (from about 22,000 to 7,000 years ago) and show that as they activated and deactivated in different locations, their overall number decreased, they occupied a progressively smaller percentage of the ice sheet perimeter and their total discharge decreased. The underlying geology and topography clearly influenced ice stream activity, but—at the ice-sheet scale—their drainage network adjusted and was linked to changes in ice sheet volume. It is unclear whether these findings can be directly translated to modern ice sheets. However, contrary to the view that sees ice streams as unstable entities that can accelerate ice-sheet deglaciation, we conclude that ice streams exerted progressively less influence on ice sheet mass balance during the retreat of the Laurentide Ice Sheet.

  5. Ice stream activity scaled to ice sheet volume during Laurentide Ice Sheet deglaciation.

    PubMed

    Stokes, C R; Margold, M; Clark, C D; Tarasov, L

    2016-02-18

    The contribution of the Greenland and West Antarctic ice sheets to sea level has increased in recent decades, largely owing to the thinning and retreat of outlet glaciers and ice streams. This dynamic loss is a serious concern, with some modelling studies suggesting that the collapse of a major ice sheet could be imminent or potentially underway in West Antarctica, but others predicting a more limited response. A major problem is that observations used to initialize and calibrate models typically span only a few decades, and, at the ice-sheet scale, it is unclear how the entire drainage network of ice streams evolves over longer timescales. This represents one of the largest sources of uncertainty when predicting the contributions of ice sheets to sea-level rise. 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. Here we reconstruct the activity of 117 ice streams that operated at various times during deglaciation of the Laurentide Ice Sheet (from about 22,000 to 7,000 years ago) and show that as they activated and deactivated in different locations, their overall number decreased, they occupied a progressively smaller percentage of the ice sheet perimeter and their total discharge decreased. The underlying geology and topography clearly influenced ice stream activity, but--at the ice-sheet scale--their drainage network adjusted and was linked to changes in ice sheet volume. It is unclear whether these findings can be directly translated to modern ice sheets. However, contrary to the view that sees ice streams as unstable entities that can accelerate ice-sheet deglaciation, we conclude that ice streams exerted progressively less influence on ice sheet mass balance during the retreat of the Laurentide Ice Sheet. PMID:26887494

  6. Interhemispheric Ice-Sheet Synchronicity During the Last Glacial Maximum

    NASA Astrophysics Data System (ADS)

    Weber, M. E.; Clark, P. U.; Kuhn, G.; Ricken, W.; Sprenk, D.

    2011-12-01

    The timing of the last maximum extent of the Antarctic ice sheets relative to those in the Northern Hemisphere remains poorly understood because only a few findings with robust chronologies exist for Antarctic ice sheets. We developed a chronology for the Weddell Sea sector of the East Antarctic ice sheet that, combined with ages from other Antarctic ice-sheet sectors, indicates the advance to and retreat from their maximum extent was nearly synchronous with Northern Hemisphere ice sheets. As for the deglaciation, modeling studies suggest a late ice-sheet retreat starting around 14 ka BP and ending around 7 ka BP with a large impact of an unstable West Antarctic Ice Sheet (WAIS) and a small impact of a stable East Antarctic Ice Sheet (EAIS). However, the Weddell Sea sites studied here, as well as sites from the Scotia Sea, provide evidence that specifically the EAIS responded much earlier, possibly provided a significant contribution to the last sea-level rise, and was much more dynamic than previously thought. Deep-sea sediment sites from the central Scotia Sea "iceberg alley" show four phases of enhanced deposition of ice-rated detritus (IRD) occurred at 19.5, 16.5,14.5, and 12 ka. The first two relate to the two ice-sheet retreat signals documented for the Weddell Sea; the third phase indicates an Antarctic component to meltwater pulse 1a; the fourth phase falls roughly into period of the Younger Dryas. Our modeling studies show that surface climate forcing of Antarctic ice sheets would have likely increased ice mass balance during deglaciation, whereby a warming climate would increase accumulation but not surface melting. We propose that sea-level forcing from Northern Hemisphere ice sheets and changes in North Atlantic deepwater formation and attendant heat flux to Antarctic grounding lines provided the teleconnections to synchronize the hemispheric ice sheets.

  7. Interhemispheric ice-sheet synchronicity during the last glacial maximum

    USGS Publications Warehouse

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

    2011-01-01

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

  8. Interhemispheric ice-sheet synchronicity during the Last Glacial Maximum

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

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

  9. ISSM: Ice Sheet System Model

    NASA Technical Reports Server (NTRS)

    Larour, Eric; Schiermeier, John E.; Seroussi, Helene; Morlinghem, Mathieu

    2013-01-01

    In order to have the capability to use satellite data from its own missions to inform future sea-level rise projections, JPL needed a full-fledged ice-sheet/iceshelf flow model, capable of modeling the mass balance of Antarctica and Greenland into the near future. ISSM was developed with such a goal in mind, as a massively parallelized, multi-purpose finite-element framework dedicated to ice-sheet modeling. ISSM features unstructured meshes (Tria in 2D, and Penta in 3D) along with corresponding finite elements for both types of meshes. Each finite element can carry out diagnostic, prognostic, transient, thermal 3D, surface, and bed slope simulations. Anisotropic meshing enables adaptation of meshes to a certain metric, and the 2D Shelfy-Stream, 3D Blatter/Pattyn, and 3D Full-Stokes formulations capture the bulk of the ice-flow physics. These elements can be coupled together, based on the Arlequin method, so that on a large scale model such as Antarctica, each type of finite element is used in the most efficient manner. For each finite element referenced above, ISSM implements an adjoint. This adjoint can be used to carry out model inversions of unknown model parameters, typically ice rheology and basal drag at the ice/bedrock interface, using a metric such as the observed InSAR surface velocity. This data assimilation capability is crucial to allow spinning up of ice flow models using available satellite data. ISSM relies on the PETSc library for its vectors, matrices, and solvers. This allows ISSM to run efficiently on any parallel platform, whether shared or distrib- ISSM: Ice Sheet System Model NASA's Jet Propulsion Laboratory, Pasadena, California uted. It can run on the largest clusters, and is fully scalable. This allows ISSM to tackle models the size of continents. ISSM is embedded into MATLAB and Python, both open scientific platforms. This improves its outreach within the science community. It is entirely written in C/C++, which gives it flexibility in its

  10. The Elementary Marine Ice Sheet Model (EMISM)

    NASA Astrophysics Data System (ADS)

    Pattyn, Frank

    2015-04-01

    Ice sheet models become more and more components of global climate system modelling instead of stand-alone features to study cryospheric processes. Full coupling of ice sheet models to atmospheric and ocean models requires a standard for ice sheet models, and more precisely for marine ice sheet models, where complex feedbacks between ice and ocean, such as marine ice sheet instability, and the atmosphere, such as the elevation-mass balance feedback, operate at different time scales. Recent model intercomparisons (e.g., SeaRISE, MISMIP) have shown that basic requirements for marine ice sheet models are still lacking and that the complexity of many ice sheet models is focused on processes that are either not well captured numerically (spatial resolution issue) or are of secondary importance compared to the essential features of marine ice sheet dynamics. Here, we propose a new and fast computing ice sheet model, devoid of most complexity, but capturing the essential feedbacks when coupled to ocean or atmospheric models. Its computational efficiency guarantees to easily tests its advantages as well as limits through ensemble modelling. EMISM (Elementary Marine Ice Sheet Model) is a vertically integrated ice sheet model based on the Shallow-Ice Approximation extended a Weertman sliding law. Although vertically integrated, thermomechanical coupling is ensured through a simplified representation of ice sheet thermodynamics based on an analytical solution of the vertical temperature profile, enhanced with strain heating. The marine boundary is represented by a parameterized flux condition similar to Pollard & Deconto (2012), based on Schoof (2007). A simplified ice shelf is added to account for buttressing of ice shelves in this parameterization. The ice sheet model is solved on a finite difference grid and special care is taken to its numerical efficiency and stability. While such model has a series of (known) deficiencies with respect to short time effects, its overall

  11. Collapse of polar ice sheets during the stage 11 interglacial.

    PubMed

    Raymo, Maureen E; Mitrovica, Jerry X

    2012-03-14

    Contentious observations of Pleistocene shoreline features on the tectonically stable islands of Bermuda and the Bahamas have suggested that sea level about 400,000 years ago was more than 20 metres higher than it is today. Geochronologic and geomorphic evidence indicates that these features formed during interglacial marine isotope stage (MIS) 11, an unusually long interval of warmth during the ice age. Previous work has advanced two divergent hypotheses for these shoreline features: first, significant melting of the East Antarctic Ice Sheet, in addition to the collapse of the West Antarctic Ice Sheet and the Greenland Ice Sheet; or second, emplacement by a mega-tsunami during MIS 11 (ref. 4, 5). Here we show that the elevations of these features are corrected downwards by ∼10 metres when we account for post-glacial crustal subsidence of these sites over the course of the anomalously long interglacial. On the basis of this correction, we estimate that eustatic sea level rose to ∼6-13 m above the present-day value in the second half of MIS 11. This suggests that both the Greenland Ice Sheet and the West Antarctic Ice Sheet collapsed during the protracted warm period while changes in the volume of the East Antarctic Ice Sheet were relatively minor, thereby resolving the long-standing controversy over the stability of the East Antarctic Ice Sheet during MIS 11.

  12. Global ice-sheet system interlocked by sea level

    SciTech Connect

    Denton, G.H.; Hughes, T.J.; Karlen, W.

    1986-01-01

    Denton and Hughes postulated that sea level linked a global ice-sheet system with both terrestrial and grounded marine components during later Quaternary ice ages. Summer temperature changes near Northern Hemisphere melting margins initiated sea-level fluctuations that controlled marine components in both polar hemispheres. It was further proposed that variations of this ice-sheet system amplified and transmitted Milankovitch summer half-year insolation changes between 45 and 75/sup 0/N into global climatic changes. New tests of this hypothesis implicate sea level as a major control of the areal extent of grounded portions of the Antarctic Ice Sheet. But factors other than areal changes of the grounded Antarctic Ice Sheet may have strongly influenced Southern Hemisphere climate and terminated the last ice age simultaneously in both polar hemispheres. Atmospheric carbon dioxide linked to high-latitude oceans is the most likely candidate, but another potential influence was high-frequency climatic oscillations. It is postulated that variations in atmospheric carbon dioxide acted through an Antarctic ice shelf linked to the grounded ice sheet to produce and terminate Southern Hemisphere ice-age climate. It is further postulated that Milankovitch summer insolation combined with a warm-high frequency oscillation caused marked recession of Northern Hemisphere ice-sheet melting margins and the North Atlantic polar front about 14,000 /sup 14/C yr B.P. This permitted renewed formation of North Atlantic Deep Water, which could well have controlled atmospheric carbon dioxide. Combined melting and consequent sea-level rise from the three warming factors initiated irreversible collapse of the interlocked global ice-sheet system, which was at its largest but most vulnerable configuration.

  13. Holocene deceleration of the Greenland Ice Sheet.

    PubMed

    MacGregor, Joseph A; Colgan, William T; Fahnestock, Mark A; Morlighem, Mathieu; Catania, Ginny A; Paden, John D; Gogineni, S Prasad

    2016-02-01

    Recent peripheral thinning of the Greenland Ice Sheet is partly offset by interior thickening and is overprinted on its poorly constrained Holocene evolution. On the basis of the ice sheet's radiostratigraphy, ice flow in its interior is slower now than the average speed over the past nine millennia. Generally higher Holocene accumulation rates relative to modern estimates can only partially explain this millennial-scale deceleration. The ice sheet's dynamic response to the decreasing proportion of softer ice from the last glacial period and the deglacial collapse of the ice bridge across Nares Strait also contributed to this pattern. Thus, recent interior thickening of the Greenland Ice Sheet is partly an ongoing dynamic response to the last deglaciation that is large enough to affect interpretation of its mass balance from altimetry. PMID:26912699

  14. Measurements of Past Ice Sheet Elevations in Interior West Antarctica.

    PubMed

    Ackert; Barclay; Borns; Calkin; Kurz; Fastook; Steig

    1999-10-01

    A lateral moraine band on Mount Waesche, a volcanic nunatak in Marie Byrd Land, provides estimates of past ice sheet surface elevations in West Antarctica. Helium-3 and chlorine-36 surface exposure ages indicate that the proximal part of the moraine, up to 45 meters above the present ice surface, was deposited about 10,000 years ago, substantially later than the maximum ice extent in the Ross Embayment. The upper distal part of the moraine may record multiple earlier ice sheet high stands. A nonequilibrium ice sheet model predicts a delay of several thousand years in maximum ice levels at Mount Waesche relative to the maximum ice extent in the Ross Sea. The glacial geologic evidence, coupled with the ice sheet model, indicates that the contribution of the Ross Sea sector of the West Antarctic Ice Sheet to Holocene sea level rise was only about 3 meters. These results eliminate West Antarctic ice as the principle source of the large meltwater pulse during the early Holocene. PMID:10514368

  15. Tropical Pacific response to continental ice sheet topography

    NASA Astrophysics Data System (ADS)

    Lee, Shih-Yu; Chiang, John C. H.; Chang, Ping

    2015-05-01

    The last glacial maximum was marked by maximum land ice extent and lowest greenhouse gases concentration during the last ice age. We explore the impact of glacial continental ice sheet topography on the large-scale tropical ocean-atmosphere climate, in particular the tropical Pacific, in an intermediate complexity coupled model. Increasing the thickness of continental ice sheets causes a southward displaced Pacific Intertropical Convergence Zone (ITCZ) and a strengthening (weakening) of northern (southern) hemisphere winter Hadley cell. The equatorial zonal sea surface temperature gradient weakened with an increased continental ice sheets thickness, the reduction being caused by cooling in the western equatorial Pacific and warming in the eastern equatorial Pacific. The evolution of the tropical climate with changing ice thickness has distinct quasi-linear and nonlinear parts. While the linear part is a direct response to the ice topographic changes, the nonlinear part was a result of the tropical thermocline adjustment. Our analysis of a fully-coupled transient deglacial simulation strongly indicates the dominant role of ice sheet topography in determining the deglacial evolution of the simulated Pacific climate. The thickness of continental ice sheet, separate from ice albedo effect, has significant impact on the tropical ocean-atmosphere climate in particular with the meridional displacement in the Pacific ITCZ. The altered circulation states seen in the model may aid understanding of the relationship between tropical and high-latitude climate records in glacial-interglacial cycles.

  16. Measurements of Past Ice Sheet Elevations in Interior West Antarctica.

    PubMed

    Ackert; Barclay; Borns; Calkin; Kurz; Fastook; Steig

    1999-10-01

    A lateral moraine band on Mount Waesche, a volcanic nunatak in Marie Byrd Land, provides estimates of past ice sheet surface elevations in West Antarctica. Helium-3 and chlorine-36 surface exposure ages indicate that the proximal part of the moraine, up to 45 meters above the present ice surface, was deposited about 10,000 years ago, substantially later than the maximum ice extent in the Ross Embayment. The upper distal part of the moraine may record multiple earlier ice sheet high stands. A nonequilibrium ice sheet model predicts a delay of several thousand years in maximum ice levels at Mount Waesche relative to the maximum ice extent in the Ross Sea. The glacial geologic evidence, coupled with the ice sheet model, indicates that the contribution of the Ross Sea sector of the West Antarctic Ice Sheet to Holocene sea level rise was only about 3 meters. These results eliminate West Antarctic ice as the principle source of the large meltwater pulse during the early Holocene.

  17. Ice-sheet response to oceanic forcing.

    PubMed

    Joughin, Ian; Alley, Richard B; Holland, David M

    2012-11-30

    The ice sheets of Greenland and Antarctica are losing ice at accelerating rates, much of which is a response to oceanic forcing, especially of the floating ice shelves. Recent observations establish a clear correspondence between the increased delivery of oceanic heat to the ice-sheet margin and increased ice loss. In Antarctica, most of these processes are reasonably well understood but have not been rigorously quantified. In Greenland, an understanding of the processes by which warmer ocean temperatures drive the observed retreat remains elusive. Experiments designed to identify the relevant processes are confounded by the logistical difficulties of instrumenting ice-choked fjords with actively calving glaciers. For both ice sheets, multiple challenges remain before the fully coupled ice-ocean-atmosphere models needed for rigorous sea-level projection are available.

  18. Components of the ice age circulation

    NASA Technical Reports Server (NTRS)

    Rind, D.

    1987-01-01

    The effects of ice age boundary conditions on atmospheric dynamics and regional climate patterns are investigated using four GCM simulations. Particular consideration is given to sea surface temperature-sea ice distribution, the appearance of land ice, and the increased elevation of land ice. It is observed that the ice-age sea surface temperature stabilizes the atmosphere over the oceans, increases the frequency of storm tracking through central North America, and amplifies transient eddy energy without increasing baroclinic generation. It is detected that low-elevation ice generates low pressure over eastern North America and southern Europe in winter, while increasing cloud cover and cooling the land in summer. Elevation of the ice sheets cools the land in winter, further intensifies storms off northeastern North America, induces subsidence warming downstream of the European ice sheets in summer, and increases the transient and stationary eddy energy through increased baroclinicity.

  19. 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 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 (???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, ???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 today and atmospheric CO 2 concentration was as high as ???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 CO2. ??2009 Macmillan Publishers Limited. All rights reserved.

  20. Can ice sheets trigger abrupt climatic change?

    SciTech Connect

    Hughes, T.

    1996-11-01

    The discovery in recent years of abrupt climatic changes in climate proxy records from Greenland ice cores and North Atlantic sediment cores, and from other sites around the world, has diverted attention from gradual insolation changes caused by Earth`s orbital variations to more rapid processes on Earth`s surface as forcing Quaternary climatic change. In particular, forcing by ice sheets has been quantified for a major ice stream that drained the Laurentide Ice Sheet along Hudson Strait. The history of these recent discoveries leading to an interest in ice sheets is reviewed, and a case is made that ice sheets may drive abrupt climatic change that is virtually synchronous worldwide. Attention is focused on abrupt inception and termination of a Quaternary glaciation cycle, abrupt changes recorded as stadials and interstadials within the cycle, abrupt changes in ice streams that trigger stadials and interstadials, and abrupt changes in the Laurentide Ice Sheet linked to effectively simultaneous abrupt changes in its ice streams. Remaining work needed to quantify further these changes is discussed. 90 refs., 14 figs.

  1. Microbial abundance in surface ice on the Greenland Ice Sheet

    PubMed Central

    Stibal, Marek; Gözdereliler, Erkin; Cameron, Karen A.; Box, Jason E.; Stevens, Ian T.; Gokul, Jarishma K.; Schostag, Morten; Zarsky, Jakub D.; Edwards, Arwyn; Irvine-Fynn, Tristram D. L.; Jacobsen, Carsten S.

    2015-01-01

    Measuring microbial abundance in glacier ice and identifying its controls is essential for a better understanding and quantification of biogeochemical processes in glacial ecosystems. However, cell enumeration of glacier ice samples is challenging due to typically low cell numbers and the presence of interfering mineral particles. We quantified for the first time the abundance of microbial cells in surface ice from geographically distinct sites on the Greenland Ice Sheet (GrIS), using three enumeration methods: epifluorescence microscopy (EFM), flow cytometry (FCM), and quantitative polymerase chain reaction (qPCR). In addition, we reviewed published data on microbial abundance in glacier ice and tested the three methods on artificial ice samples of realistic cell (102–107 cells ml−1) and mineral particle (0.1–100 mg ml−1) concentrations, simulating a range of glacial ice types, from clean subsurface ice to surface ice to sediment-laden basal ice. We then used multivariate statistical analysis to identify factors responsible for the variation in microbial abundance on the ice sheet. EFM gave the most accurate and reproducible results of the tested methodologies, and was therefore selected as the most suitable technique for cell enumeration of ice containing dust. Cell numbers in surface ice samples, determined by EFM, ranged from ~ 2 × 103 to ~ 2 × 106 cells ml−1 while dust concentrations ranged from 0.01 to 2 mg ml−1. The lowest abundances were found in ice sampled from the accumulation area of the ice sheet and in samples affected by fresh snow; these samples may be considered as a reference point of the cell abundance of precipitants that are deposited on the ice sheet surface. Dust content was the most significant variable to explain the variation in the abundance data, which suggests a direct association between deposited dust particles and cells and/or by their provision of limited nutrients to microbial communities on the GrIS. PMID:25852678

  2. Microbial abundance in surface ice on the Greenland Ice Sheet.

    PubMed

    Stibal, Marek; Gözdereliler, Erkin; Cameron, Karen A; Box, Jason E; Stevens, Ian T; Gokul, Jarishma K; Schostag, Morten; Zarsky, Jakub D; Edwards, Arwyn; Irvine-Fynn, Tristram D L; Jacobsen, Carsten S

    2015-01-01

    Measuring microbial abundance in glacier ice and identifying its controls is essential for a better understanding and quantification of biogeochemical processes in glacial ecosystems. However, cell enumeration of glacier ice samples is challenging due to typically low cell numbers and the presence of interfering mineral particles. We quantified for the first time the abundance of microbial cells in surface ice from geographically distinct sites on the Greenland Ice Sheet (GrIS), using three enumeration methods: epifluorescence microscopy (EFM), flow cytometry (FCM), and quantitative polymerase chain reaction (qPCR). In addition, we reviewed published data on microbial abundance in glacier ice and tested the three methods on artificial ice samples of realistic cell (10(2)-10(7) cells ml(-1)) and mineral particle (0.1-100 mg ml(-1)) concentrations, simulating a range of glacial ice types, from clean subsurface ice to surface ice to sediment-laden basal ice. We then used multivariate statistical analysis to identify factors responsible for the variation in microbial abundance on the ice sheet. EFM gave the most accurate and reproducible results of the tested methodologies, and was therefore selected as the most suitable technique for cell enumeration of ice containing dust. Cell numbers in surface ice samples, determined by EFM, ranged from ~ 2 × 10(3) to ~ 2 × 10(6) cells ml(-1) while dust concentrations ranged from 0.01 to 2 mg ml(-1). The lowest abundances were found in ice sampled from the accumulation area of the ice sheet and in samples affected by fresh snow; these samples may be considered as a reference point of the cell abundance of precipitants that are deposited on the ice sheet surface. Dust content was the most significant variable to explain the variation in the abundance data, which suggests a direct association between deposited dust particles and cells and/or by their provision of limited nutrients to microbial communities on the GrIS. PMID

  3. Evolution of crystal fabric: Ice-Age ice versus Holocene ice

    NASA Astrophysics Data System (ADS)

    Kennedy, J. H.; Pettit, E. C.

    2009-12-01

    Ice-Age ice has smaller crystals and higher concentrations of impurities than Holocene ice; these properties cause it to develop a more strongly-aligned crystal-orientation fabric. In many regions of the Antarctic and Greenland ice sheets, the Ice-Age ice is now at depth and its flow properties may dominate the ice flow patterns, particularly where sliding is minimal. We use a fabric evolution model, based on that developed by Thorsteinsson (2002), to explore the evolution of Ice-Age ice fabric along particle paths for ice within Taylor Glacier, a cold-based outlet glacier of the East Antarctic Ice Sheet. The bulk of the ice within Taylor Glacier consists of Ice-Age and older ice because the Holocene ice has ablated away (there is no Holocene ice remaining within 25km of the terminus, Aciego, 2007). We initialize the evolving fabric based on fabric measurements from Taylor Dome where available (DiPrinzio, 2003) and other ice core records. We compare model results with thin-section data from shallow cores taken near the terminus. As expected, crystal alignment strengthens along the ice particle path. Due to lateral shearing along valley walls and the ice cliffs (terminal ice cliffs are cold in winter and present a resistance to flow), a tilted single maximum is common near the terminus. The highly-aligned fabric of Ice-Age ice is significantly softer than Holocene ice in simple shear parallel to the bed, this softness not only results in faster flow rates for glaciers and ice sheets such as Taylor, but creates a climate-flow-fabric feedback loop through concentrating ice-sheet flow within the Ice-Age ice. Thorsteinsson, T. (2002), Fabric development with nearest-neighbor interaction and dynamic recrystallization, J. Geophys. Res., 107(B1), 2014, doi:10.1029/2001JB000244. S.M. Aciego, K.M. Cuffey, J.L. Kavanaugh, D.L. Morse, J.P. Severinghaus, Pleistocene ice and paleo-strain rates at Taylor Glacier, Antarctica, Quaternary Research, Volume 68, Issue 3, November 2007

  4. Holocene deceleration of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    MacGregor, Joseph A.; Colgan, William T.; Fahnestock, Mark A.; Morlighem, Mathieu; Catania, Ginny A.; Paden, John D.; Gogineni, S. Prasad

    2016-02-01

    Recent peripheral thinning of the Greenland Ice Sheet is partly offset by interior thickening and is overprinted on its poorly constrained Holocene evolution. On the basis of the ice sheet’s radiostratigraphy, ice flow in its interior is slower now than the average speed over the past nine millennia. Generally higher Holocene accumulation rates relative to modern estimates can only partially explain this millennial-scale deceleration. The ice sheet’s dynamic response to the decreasing proportion of softer ice from the last glacial period and the deglacial collapse of the ice bridge across Nares Strait also contributed to this pattern. Thus, recent interior thickening of the Greenland Ice Sheet is partly an ongoing dynamic response to the last deglaciation that is large enough to affect interpretation of its mass balance from altimetry.

  5. On sea level - ice sheet interactions

    NASA Astrophysics Data System (ADS)

    Gomez, Natalya Alissa

    This thesis focuses on the physics of static sea-level changes following variations in the distribution of grounded ice and the influence of these changes on the stability and dynamics of marine ice sheets. Gravitational, deformational and rotational effects associated with changes in grounded ice mass lead to markedly non-uniform spatial patterns of sea-level change. I outline a revised theory for computing post-glacial sea-level predictions and discuss the dominant physical effects that contribute to the patterns of sea-level change associated with surface loading on different timescales. I show, in particular, that a large sea-level fall (rise) occurs in the vicinity of a retreating (advancing) ice sheet on both short and long timescales. I also present an application of the sea-level theory in which I predict the sea-level changes associated with a new model of North American ice sheet evolution and consider the implications of the results for efforts to establish the sources of Meltwater Pulse 1A. These results demonstrate that viscous deformational effects can influence the amplitude of sea-level changes observed at far-field sea-level sites, even when the time window being considered is relatively short (≤ 500 years). Subsequently, I investigate the feedback of sea-level changes on marine ice-sheet stability and dynamics by coupling a global sea-level model to ice-sheet models of increasing complexity. To begin, I incorporate gravitationally self-consistent sea-level changes into an equilibrium marine ice-sheet stability theory to show that the sea-level changes have a stabilizing influence on ice-sheet retreat. Next, I consider the impact of the stabilizing mechanism on the timescale of ice-sheet retreat using a 1D dynamic coupled ice sheet - sea level model. Simulations with the coupled model, which incorporate viscoelastic deformation of the solid Earth, show that local sea-level changes at the grounding line act to slow, and in some cases, halt

  6. Dynamic Antarctic ice-sheet response to deglacial meltwater pulses

    NASA Astrophysics Data System (ADS)

    Weber, Michael; Clark, Peter U.; Timmermann, Axel; Lohmann, Gerrit; Kuhn, Gerhard; Sprenk, Daniela; Gladstone, Rupert

    2013-04-01

    Reconstruction of the last global sea level rise faces uncertainties because only a few robust data evidences are available for Antarctic ice sheets. Deglacial dynamics have mostly been inferred from shallow-water cores on the shelf, where decisive changes are either erased by grounding ice or occur in condensed, lithologically complex successions with partially reversed and generally unreliable 14C ages. Previous modeling studies reconstruct a late ice-sheet retreat starting around 12 ka BP and ending around 7 ka BP with a large impact of an unstable West Antarctic Ice Sheet (WAIS) and a small impact of a stable East Antarctic Ice Sheet (EAIS). However, new findings from two deepwater cores from the Scotia Sea challenge these reconstructions and call for a principal revision of the Antarctic deglacial history. The well-dated sites (Weber et al., 2012, Quaternary Science Reviews) provide the first integrative and representative record of Antarctic Ice Sheet instability. They are located in the central transport route of virtually all Antarctic icebergs, the so-called Iceberg Alley, and demonstrate a highly dynamic Antarctic Ice Sheet during the last deglaciation with eight distinct phases of enhanced iceberg routing, dubbed Antarctic Ice Sheet Events (AIE), in contrast to existing models of a late and monotonous ice-sheet retreat which implied only little contribution to the last, natural, sea-level rise 19,000 to 9,000 years ago. We found the first direct evidence for an Antarctic contribution to Meltwater Pulse 1A in the flux rates of ice-rafted debris. Using an ensemble of transient deglacial model simulations we could show that increased export of warmer Circumpolar Deep Water towards Antarctica contributed to Antarctic Ice Sheet melt by ocean thermal forcing (Weber et al., Science, in review). These new findings hold the potential to substantially revise and improve our understanding of the transient response of the ice sheet to external and internal forcings

  7. 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

  8. Global ice-sheet system interlocked by sea level

    NASA Astrophysics Data System (ADS)

    Denton, George H.; Hughes, Terence J.; Karlén, Wibjörn

    1986-07-01

    Denton and Hughes (1983, Quaternary Research20, 125-144) postulated that sea level linked a global ice-sheet system with both terrestrial and grounded marine components during late Quaternary ice ages. Summer temperature changes near Northern Hemisphere melting margins initiated sea-level fluctuations that controlled marine components in both polar hemispheres. It was further proposed that variations of this ice-sheet system amplified and transmitted Milankovitch summer half-year insolation changes between 45 and 75°N into global climatic changes. New tests of this hypothesis implicate sea level as a major control of the areal extent of grounded portions of the Antarctic Ice Sheet, thus fitting the concept of a globally interlocked ice-sheet system. But recent atmospheric modeling results ( Manabe and Broccoli, 1985, Journal of Geophysical Research90, 2167-2190) suggest that factors other than areal changes of the grounded Antarctic Ice Sheet strongly influenced Southern Hemisphere climate and terminated the last ice age simultaneously in both polar hemispheres. Atmospheric carbon dioxide linked to high-latitude oceans is the most likely candidate ( Shackleton and Pisias, 1985, Atmospheric carbon dioxide, orbital forcing, and climate. In "The Carbon Cycle and Atmospheric CO 2: Natural Variations Archean to Present" (E. T. Sundquest and W. S. Broecker, Eds.), pp. 303-318. Geophysical Monograph 32, American Geophysical Union, Washington, D.C.), but another potential influence was high-frequency climatic oscillations (2500 yr). It is postulated that variations in atmospheric carbon dioxide acted through an Antarctic ice shelf linked to the grounded ice sheet to produce and terminate Southern Hemisphere ice-age climate. It is further postulated that Milankovitch summer insolation combined with a warm high-frequency oscillation caused marked recession of Northern Hemisphere ice-sheet melting margins and the North Atlantic polar front about 14,000 14C yr B.P. This

  9. Clouds enhance Greenland ice sheet meltwater runoff

    NASA Astrophysics Data System (ADS)

    Van Tricht, Kristof; Lhermitte, Stef; Lenaerts, Jan T. M.; Gorodetskaya, Irina V.; L'Ecuyer, Tristan S.; Noël, Brice; van den Broeke, Michiel R.; Turner, David D.; van Lipzig, Nicole P. M.

    2016-04-01

    The Greenland ice sheet has become one of the main contributors to global sea level rise, predominantly through increased meltwater runoff. The main drivers of Greenland ice sheet runoff, however, remain poorly understood. Here we show that clouds enhance meltwater runoff by about one-third relative to clear skies, using a unique combination of active satellite observations, climate model data and snow model simulations. This impact results from a cloud radiative effect of 29.5 (±5.2) W m‑2. Contrary to conventional wisdom, however, the Greenland ice sheet responds to this energy through a new pathway by which clouds reduce meltwater refreezing as opposed to increasing surface melt directly, thereby accelerating bare-ice exposure and enhancing meltwater runoff. The high sensitivity of the Greenland ice sheet to both ice-only and liquid-bearing clouds highlights the need for accurate cloud representations in climate models, to better predict future contributions of the Greenland ice sheet to global sea level rise.

  10. Clouds enhance Greenland ice sheet meltwater runoff.

    PubMed

    Van Tricht, K; Lhermitte, S; Lenaerts, J T M; Gorodetskaya, I V; L'Ecuyer, T S; Noël, B; van den Broeke, M R; Turner, D D; van Lipzig, N P M

    2016-01-12

    The Greenland ice sheet has become one of the main contributors to global sea level rise, predominantly through increased meltwater runoff. The main drivers of Greenland ice sheet runoff, however, remain poorly understood. Here we show that clouds enhance meltwater runoff by about one-third relative to clear skies, using a unique combination of active satellite observations, climate model data and snow model simulations. This impact results from a cloud radiative effect of 29.5 (±5.2) W m(-2). Contrary to conventional wisdom, however, the Greenland ice sheet responds to this energy through a new pathway by which clouds reduce meltwater refreezing as opposed to increasing surface melt directly, thereby accelerating bare-ice exposure and enhancing meltwater runoff. The high sensitivity of the Greenland ice sheet to both ice-only and liquid-bearing clouds highlights the need for accurate cloud representations in climate models, to better predict future contributions of the Greenland ice sheet to global sea level rise.

  11. Clouds enhance Greenland ice sheet meltwater runoff

    NASA Astrophysics Data System (ADS)

    Van Tricht, Kristof; Lhermitte, Stef; Lenaerts, Jan T. M.; Gorodetskaya, Irina V.; L'Ecuyer, Tristan S.; Noël, Brice; van den Broeke, Michiel R.; Turner, David D.; van Lipzig, Nicole P. M.

    2016-04-01

    The Greenland ice sheet has become one of the main contributors to global sea level rise, predominantly through increased meltwater runoff. The main drivers of Greenland ice sheet runoff, however, remain poorly understood. Here we show that clouds enhance meltwater runoff by about one-third relative to clear skies, using a unique combination of active satellite observations, climate model data and snow model simulations. This impact results from a cloud radiative effect of 29.5 (±5.2) W m-2. Contrary to conventional wisdom, however, the Greenland ice sheet responds to this energy through a new pathway by which clouds reduce meltwater refreezing as opposed to increasing surface melt directly, thereby accelerating bare-ice exposure and enhancing meltwater runoff. The high sensitivity of the Greenland ice sheet to both ice-only and liquid-bearing clouds highlights the need for accurate cloud representations in climate models, to better predict future contributions of the Greenland ice sheet to global sea level rise.

  12. Clouds enhance Greenland ice sheet meltwater runoff

    NASA Astrophysics Data System (ADS)

    van Tricht, K.; Lhermitte, S.; Lenaerts, J. T. M.; Gorodetskaya, I. V.; L'Ecuyer, T. S.; Noël, B.; van den Broeke, M. R.; Turner, D. D.; van Lipzig, N. P. M.

    2016-01-01

    The Greenland ice sheet has become one of the main contributors to global sea level rise, predominantly through increased meltwater runoff. The main drivers of Greenland ice sheet runoff, however, remain poorly understood. Here we show that clouds enhance meltwater runoff by about one-third relative to clear skies, using a unique combination of active satellite observations, climate model data and snow model simulations. This impact results from a cloud radiative effect of 29.5 (+/-5.2) W m-2. Contrary to conventional wisdom, however, the Greenland ice sheet responds to this energy through a new pathway by which clouds reduce meltwater refreezing as opposed to increasing surface melt directly, thereby accelerating bare-ice exposure and enhancing meltwater runoff. The high sensitivity of the Greenland ice sheet to both ice-only and liquid-bearing clouds highlights the need for accurate cloud representations in climate models, to better predict future contributions of the Greenland ice sheet to global sea level rise.

  13. Clouds enhance Greenland ice sheet meltwater runoff

    PubMed Central

    Van Tricht, K.; Lhermitte, S.; Lenaerts, J. T. M.; Gorodetskaya, I. V.; L'Ecuyer, T. S.; Noël, B.; van den Broeke, M. R.; Turner, D. D.; van Lipzig, N. P. M.

    2016-01-01

    The Greenland ice sheet has become one of the main contributors to global sea level rise, predominantly through increased meltwater runoff. The main drivers of Greenland ice sheet runoff, however, remain poorly understood. Here we show that clouds enhance meltwater runoff by about one-third relative to clear skies, using a unique combination of active satellite observations, climate model data and snow model simulations. This impact results from a cloud radiative effect of 29.5 (±5.2) W m−2. Contrary to conventional wisdom, however, the Greenland ice sheet responds to this energy through a new pathway by which clouds reduce meltwater refreezing as opposed to increasing surface melt directly, thereby accelerating bare-ice exposure and enhancing meltwater runoff. The high sensitivity of the Greenland ice sheet to both ice-only and liquid-bearing clouds highlights the need for accurate cloud representations in climate models, to better predict future contributions of the Greenland ice sheet to global sea level rise. PMID:26756470

  14. Modelling binge-purge oscillations of the Laurentide ice sheet using a plastic ice sheet

    NASA Astrophysics Data System (ADS)

    Steen-Larsen, H. C.; Dahl-Jensen, D.

    A simple combined heat and ice-sheet model has been used to calculate temperatures at the base of the Laurentide ice sheet. We let the ice sheet surge when the basal temperature reaches the pressure-melting temperature. Driving the system with the observed accumulation and temperature records from the GRIP ice core, Greenland, produces surges corresponding to the observed Heinrich events. This suggests that the mechanism of basal sliding, initiated when the basal temperature reaches the melting point, can explain the surges of the Laurentide ice sheet. This study highlights the importance of the surface temperature and accumulation rate as a means of forcing the timing and strength of the Heinrich events, thus implying important ice-sheet climate feedbacks.

  15. Ice age paleotopography.

    PubMed

    Peltier, W R

    1994-07-01

    A gravitationally self-consistent theory of postglacial relative sea level change is used to infer the variation of surface ice and water cover since the Last Glacial Maximum (LGM). The results show that LGM ice volume was approximately 35 percent lower than suggested by the CLIMAP reconstruction and the maximum heights of the main Laurentian and Fennoscandian ice complexes are inferred to have been commensurately lower with respect to sea level. Use of these Ice Age boundary conditions in atmospheric general circulation models will yield climates that differ significantly from those previously inferred on the basis of the CLIMAP data set.

  16. Ice age paleotopography

    SciTech Connect

    Peltier, W.R. )

    1994-07-08

    A gravitationally self-consistent theory of postglacial relative sea level change is used to infer the variation of surface ice and water cover since the Last Glacial Maximum (LGM). The results show that LGM ice volume was approximately 35 percent lower than suggested by the CLIMAP reconstruction and the maximum heights of the main Laurentian and Fennoscandian ice complexes are inferred to have been commensurately lower with respect to sea level. Use of these Ice Age boundary conditions in atmospheric general circulation models will yield climates that differ significantly from those previously inferred on the basis of the CLIMAP data set.

  17. Conditions for bubble elongation in cold ice-sheet ice

    USGS Publications Warehouse

    Alley, R.B.; Fitzpatrick, J.J.

    1999-01-01

    Highly elongated bubbles are sometimes observed in ice-sheet ice. Elongation is favored by rapid ice deformation, and opposed by diffusive processes. We use simple models to show that vapor transport dominates diffusion except possibly very close to the melting point, and that latent-heat effects are insignificant. Elongation is favored by larger bubbles at pore close-off, but is nearly independent of bubble compression below close-off. The simple presence of highly elongated bubbles indicates only that a critical ice-strain rate has been exceeded for significant time, and provides no information on possible disruption of stratigraphic continuity by ice deformation.

  18. Ice Sheet Stratigraphy Can Constrain Basal Slip

    NASA Astrophysics Data System (ADS)

    Wolovick, M.; Creyts, T. T.; Buck, W. R.; Bell, R. E.

    2014-12-01

    Basal slip is an important component of ice sheet mass flux and dynamics. Basal slip varies over time due to variations in basal temperature, water pressure, and sediment cover. All of these factors can create coherent patterns of basal slip that migrate over time. Our knowledge of the spatial variability in basal slip comes from inversions of driving stress, ice thickness, and surface velocity, but these inversions contain no information about temporal variability. We do not know if the patterns in slip revealed by those inversions move over time. While englacial stratigraphy has classically been used to constrain surface accumulation and geothermal flux, it is also sensitive to horizontal gradients in basal slip. Here we show that englacial stratigraphy can constrain the velocity of basal slip patterns. Englacial stratigraphy responds strongly to patterns of basal slip that move downstream over time close to the ice sheet velocity. In previous work, we used a thermomechanical model to discover that thermally controlled slip patterns migrate downstream and create stratigraphic structures, but we were unable to directly control the pattern velocity, as that arose naturally out of the model physics. Here, we use a kinematic flowline model that allows us to directly control pattern velocity, and thus is applicable to a wide variety of slip mechanisms in addition to basal temperature. We find that the largest and most intricate stratigraphic structures develop when the pattern moves at the column-average ice velocity. Patterns that move slower than the column-average ice velocity produce overturned stratigraphy in the lower part of the ice sheet, while patterns moving at the column-average eventually cause the entire ice sheet to overturn if they persist long enough. Based on these forward models, we develop an interpretive guide for deducing moving patterns in basal slip from ice sheet internal layers. Ice sheet internal stratigraphy represents a potentially vast

  19. Models for polythermal ice sheets and glaciers

    NASA Astrophysics Data System (ADS)

    Hewitt, Ian; Schoof, Christian

    2016-04-01

    The dynamics of ice-sheets and glaciers depend sensitively on their thermal structure. Many ice masses are polythermal, containing both cold ice, with temperature below the melting point, and temperate ice, with temperature at the melting point. The temperate ice is really an ice-water mixture, with water produced at grain boundaries by dissipative heating. Although the water content is typically small, it can have an important effect on ice dynamics; water content controls ice viscosity, and internal meltwater percolation affects hydrology. Locations where this may be important are in the enhanced shear layer at the base of fast-flowing outlet glaciers, and in the shear margins of ice streams. In this study, we present a simplified model to describe the temperature and water-content of polythermal ice masses, accounting for the possibility of gravity- and pressure-driven water drainage according to Darcy's law. The model is based on the principle of energy conservation and the theory of viscous compaction. Numerical solutions are described and a number of illustrative test problems presented. The model is compared with existing methods in the literature, including enthalpy gradient methods, to which it reduces under certain conditions. Based on the results of our analysis, we suggest a modified enthalpy method that allows for drainage under gravity but that can be relatively easily implemented in ice-sheet models.

  20. 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.

  1. Records of past ice sheet fluctuations in interior East Antarctica

    USGS Publications Warehouse

    Liu, Xiaohan; Huang, Feixin; Kong, Ping; Fang, Aimin; Li, Xiaoli

    2007-01-01

    The results of a land-based multi-disciplinary study of the past ice surface elevation in the Grove Mountains of interior East Antarctica support a dynamic evolution of the East Antarctic Ice Sheet (EAIS). Moraine boulders of sedimentary rocks and spore pollen assemblage imply a significant shrinkage of the EAIS, with its margin retreating south of the Grove Mountains (~450 km south of recent coast line) before the middle Pliocene. The exposure ages indicate that the ice sheet subsequently re-advanced, with the ice surface rising locally at least 450 m higher than today. It then went back down constantly from before 2.3 Ma to 1.6 Ma. The glacial topography and existence of soil show that the ice surface fluctuation continued since the early Quaternary, but with highest levels never exceeding ~100 m higher than today.

  2. Entrainment, transport and concentration of meteorites in polar ice sheets

    NASA Technical Reports Server (NTRS)

    Drewry, D. J.

    1986-01-01

    Glaciers and ice sheets act as slow-moving conveyancing systems for material added to both their upper and lower surfaces. Because the transit time for most materials is extremely long the ice acts as a major global storage facility. The effects of horizontal and vertical motions on the flow patterns of Antarctic ice sheets are summarized. The determination of the source areas of meteorites and their transport paths is a problem of central importance since it relates not only directly to concentration mechanisms but also to the wider issues in glaciology and meteorites. The ice and snow into which a meteorite falls, and which moves with it to the concentration area, encodes information about the infall area. The principle environmental conditions being former elevation, temperature (also related to elevation), and age of the ice. This encoded information could be used to identify the infall area.

  3. Holocene accumulation and ice flow near the West Antarctic Ice Sheet Divide ice core site

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

    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 kyr 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 km 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% lower than modern at 9.2 kyr before present (B.P.), increased by 40% from 9.2 to 2.3 kyr B.P., and decreased by at least 10% over the past 2 kyr 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 km of its modern position. Continent-scale ice-sheet models used for reconstructions of West Antarctic ice volume should incorporate this accumulation history.

  4. Leakage of the Greenland Ice Sheet through accelerated ice flow

    NASA Astrophysics Data System (ADS)

    Rignot, E.

    2005-12-01

    A map of coastal velocities of the Greenland ice sheet was produced from Radarsat-1 acquired during the background mission of 2000 and combined with radio echo sounding data to estimate the ice discharge from the ice sheet. On individual glaciers, ice discharge was compared with snow input from the interior and melt above the flux gate to determine the glacier mass balance. Time series of velocities on several glaciers at different latitudes reveal seasonal fluctuations of only 7-8 percent so that winter velocities are only 2 percent less than the yearly mean. The results show the northern Greenland glaciers to be close to balance yet losing mass. No change in ice flow is detected on Petermann, 79north and Zachariae Isstrom in 2000-2004. East Greenland glaciers are in balance and flowing steadily north of Kangerdlussuaq, but Kangerdlussuaq, Helheim and all the southeastern glaciers are thinning dramatically. All these glaciers accelerated, Kangerdlussuaq in 2000, Helheim prior to 2004, and southeast Greenland glaciers accelerated 10 to 50 percent in 2000-2004. Glacier acceleration is generally brutal, probably once the glacier reached a threshold, and sustained. In the northwest, most glaciers are largely out of balance. Jakobshavn accelerated significantly in 2002, and glaciers in its immediate vicinity accelerated more than 50 percent in 2000-2004. Less is known about southwest Greenland glaciers due to a lack of ice thickness data but the glaciers have accelerated there as well and are likely to be strongly out of balance despite thickening of the interior. Overall, I estimate the mass balance of the Greenland ice sheet to be about -80 +/-10 cubic km of ice per year in 2000 and -110 +/-15 cubic km of ice per year in 2004, i.e. more negative than based on partial altimetry surveys of the outlet glaciers. As climate continues to warm, more glaciers will accelerate, and the mass balance will become increasingly negative, regardless of the evolution of the ice sheet

  5. Active volcanism beneath the West Antarctic ice sheet and implications for ice-sheet stability

    USGS Publications Warehouse

    Blankenship, D.D.; Bell, R.E.; Hodge, S.M.; Brozena, J.M.; Behrendt, John C.; Finn, C.A.

    1993-01-01

    IT is widely understood that the collapse of the West Antarctic ice sheet (WAIS) would cause a global sea level rise of 6 m, yet there continues to be considerable debate about the detailed response of this ice sheet to climate change1-3. Because its bed is grounded well below sea level, the stability of the WAIS may depend on geologically controlled conditions at the base which are independent of climate. In particular, heat supplied to the base of the ice sheet could increase basal melting and thereby trigger ice streaming, by providing the water for a lubricating basal layer of till on which ice streams are thought to slide4,5. Ice streams act to protect the reservoir of slowly moving inland ice from exposure to oceanic degradation, thus enhancing ice-sheet stability. Here we present aerogeophysical evidence for active volcanism and associated elevated heat flow beneath the WAIS near the critical region where ice streaming begins. If this heat flow is indeed controlling ice-stream formation, then penetration of ocean waters inland of the thin hot crust of the active portion of the West Antarctic rift system could lead to the disappearance of ice streams, and possibly trigger a collapse of the inland ice reservoir.

  6. How ice age climate got the shakes

    SciTech Connect

    Kerr, R.A.

    1993-05-14

    Records in Greenland ice, ocean mud, and ancient corals are revealing abrupt climate shifts during the last ice age. The climate at the end of the last ice age apparently jumped from cold to warmer conditions, jumped back to cold, and then jumped into the present warm weather conditions. The mechanism for this erratic behavior is unknown, but appears to be an interaction of North Atlantic ocean currents and the ice sheets themselves. Warm water from the tropics would evaporate and become more saline and dense as it moved north. The colder, denser water would then sink and flow back to the tropics. The melting of ice caused by the warm water would decrease the salinity of the North Atlantic current, the water would not sink, the return current would be shut down, and the waters surrounding the ice sheets would become colder, slowing melting of the sheets. The cycle could be started again by collapse of the ice sheets from their internal heat. There may be other switches that could cause sudden climate change, as may be evidenced by links between changes in the Pacific and a decade of erratic weather in North America. Researcher would like to identify these switches to prevent them from being activated by human activity.

  7. 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

  8. Measuring Ice Sheet Height with ICESat-2

    NASA Astrophysics Data System (ADS)

    Walsh, K.; Smith, B.; Neumann, T.; Hancock, D.

    2015-12-01

    ICESat-2 is NASA's next-generation laser altimeter, designed to measure changes in ice sheet height and sea ice freeboard. Over the ice sheets, it will use a continuous repeat-track pointing strategy to ensure that it accurately measures elevation changes along a set of reference tracks. Over most of the area of Earth's ice sheets, ICESat-2 will provide coverage with a track-to-track spacing better than ~3 km. The onboard ATLAS instrument will use a photon-counting approach to provide a global geolocated photon point cloud, which is then converted into surface-specific elevation data sets. In this presentation, we will outline our strategy for taking the low-level photon point cloud and turning it into measurements posted at 20 m along-track for a set of pre-defined reference points by (1) selecting groups of photon events (PEs) around each along-track point, (2) refining the initial PE selection by fitting selected PEs with an along-track segment model and eliminating outliers to the model, (3) applying histogram-based corrections to the surface height based on the residuals to the along-track segment model, (4) calculate error estimates based on estimates of relative contributions of signal and noise PEs to the observed PE count, and (5) determining the final location and surface height of the along-track segment. These measurements are then corrected for short-scale (100-200 m) across-track surface topography around the reference points to develop a time series of land ice heights. The resulting data products will allow us to measure ice sheet elevation change with a point-for-point accuracy of a few centimeters over Earth's ice sheets.

  9. New reconstructions of Eurasian Ice Sheet build up and deglaciation

    NASA Astrophysics Data System (ADS)

    Hughes, A. L.; Gyllencreutz, R.; Mangerud, J.; Svendsen, J. I.; Lohne, O. S.

    2012-12-01

    . In the database, each date is classified on the basis of stratigraphic information to facilitate interpretation of the ice sheet evolution, attributed to the source publication, fully documented with information relevant to its interpretation and searchable by: location, dated material, dating technique, stratigraphic position or setting, derived age and associated errors, pertinent comments from the source publication and sample elevation or depth, core name, laboratory id and/or sample name as applicable. For internal consistency all radiocarbon ages have been recalibrated using the most recent calibration curve (INTCAL09) and all terrestrial cosmogenic nuclide (TCN) exposure ages are reported using the same production rate and scaling model. The uncalibrated 14C and TCN ages as reported in the source publications are also given. TCN results are additionally reported with all the necessary details required to re-calculate the ages with different production rate and scaling models.

  10. Studies of ice sheet hydrology using SAR

    NASA Technical Reports Server (NTRS)

    Bindschadler, R. A.; Vornberger, P. L.

    1989-01-01

    Analysis of SAR data of the Greenland ice sheet in summer and winter suggest the use of SAR to monitor the temporal hydrology of ice sheets. Comparisons of each SAR data set with summer Landsat TM imagery show an areal-positive correlation with summer SAR data and a negative correlation with winter SAR data. It is proposed that the summer SAR data are most sensitive to the variable concentrations of free water in the surface snow and that the winter SAR data indicate variations in snow grain size.

  11. Rewriting Ice Sheet "Glacier-ology"

    NASA Astrophysics Data System (ADS)

    Bindschadler, R.

    2006-12-01

    The revolution in glaciology driven by the suite of increasingly sophisticated satellite instruments has been no more extreme than in the area of ice dynamics. Years ago, glaciologists were (probably unwittingly) selective in what properties of mountain glaciers were also applied to ice sheets. This reinforced the view that they responded slowly to their environment. Notions of rapid response driven by the ideas of John Mercer, Bill Budd and Terry Hughes were politely rejected by the centrists of mainstream glaciological thought. How the tables have turned--and by the ice sheets themselves, captured in the act of rapidly changing by modern remote sensors! The saw-toothed record of sea-level change over past glacial-interglacial cycles required the existence of rapid ice loss processes. Satellite based observations, supported by hard-earned field observations have extended the time scale over which ice sheets can suddenly change to ever shorter intervals: from centuries, to decades, to years to even minutes. As changes continue to be observed, the scientific community is forced to consider new or previously ignored processes to explain these observations. The penultimate goal of ice-sheet dynamics is to credibly predict the future of both the Greenland and Antarctic ice sheets. In this important endeavor, there is no substitute for our ability to observe. Without the extensive data sets provided by remote sensing, numerical models can be neither tested nor improved. The impact of remote sensing on our existing ability to predict the future must be compared to our probable state of knowledge and ability were these data never collected. Among many satellite observed phenomena we would be largely or wholly ignorant of are the recent acceleration of ice throughout much of coastal Greenland; the sudden disintegration of multiple ice shelves along the Antarctic Peninsula; and the dramatic thinning and acceleration of the Amundsen Sea sector of West Antarctica. These

  12. Characteristics of basal ice and subglacial water at Dome Fuji, Antarctica ice sheet

    NASA Astrophysics Data System (ADS)

    Motoyama, H.; Uemura, R.; Hirabayashi, M.; Miyake, T.; Kuramoto, T.; Tanaka, Y.; Dome Fuji Ice Core Project, M.

    2008-12-01

    (Introduction): The second deep ice coring project at Dome Fuji, Antarctica reached a depth of 3035.22 m during the austral summer season in 2006/2007. The recovered ice cores contain records of global environmental changes going back about 720,000 years. (Estimation of basal ice melt): The borehole measurement was carried out on January 2nd in 2007 when the temperature disturbance in the borehole calmed down by the rest of drilling for 2 days. Temperature measurement was performed after 0 C thermometer test was done in the ground. The temperature sensor of pt100 installed in the skate-like anti-torque was used. We did not have the enough time until the temperature of thermometer was matched with the temperature of ice sheet. Some error was included in ice temperature data. The resistance of pt100 sensor was converted to temperature in the borehole measurement machine. But we used only two electrical lines for pt100 sensor. Rate of heat flow in the ice sheet was calculated using the vertical temperature gradient of the ice sheet and rate of heat conductivity of ice. The deepest part of heat flux using temperatures at 3000m and 3030m was about 45mW/m2. We assumed that this value was the heat flux from the bedrock in the ice sheet. Heat flux to the bedrock surface in the ground was assumed 54.6mW/m2 adopted by ice sheet model (P. Huybrechts, 2006). Then the heat flux for basal ice melt was about 10mW/m2. This value was equaled to melting of 1.1mm of ice thickness per year. On the other hand, the annual layer thickness under 2500m was not changed so much and its average was 1.3mm of ice thickness. So the annual layer thickness and melting rate of basal ice was the same in ordering way. Or ice equivalent in annual layer is melting every year. The age of the deepest part of ice core is guessed at 720,000 years old and the ice older than basal ice has melted away. (The state of basal ice): When the ice core drilling depth passed 3031.44m, amount of ice chip more abundant

  13. Clouds enhance Greenland ice sheet mass loss

    NASA Astrophysics Data System (ADS)

    Van Tricht, Kristof; Gorodetskaya, Irina V.; L'Ecuyer, Tristan; Lenaerts, Jan T. M.; Lhermitte, Stef; Noel, Brice; Turner, David D.; van den Broeke, Michiel R.; van Lipzig, Nicole P. M.

    2015-04-01

    Clouds have a profound influence on both the Arctic and global climate, while they still represent one of the key uncertainties in climate models, limiting the fidelity of future climate projections. The potentially important role of thin liquid-containing clouds over Greenland in enhancing ice sheet melt has recently gained interest, yet current research is spatially and temporally limited, focusing on particular events, and their large scale impact on the surface mass balance remains unknown. We used a combination of satellite remote sensing (CloudSat - CALIPSO), ground-based observations and climate model (RACMO) data to show that liquid-containing clouds warm the Greenland ice sheet 94% of the time. High surface reflectivity (albedo) for shortwave radiation reduces the cloud shortwave cooling effect on the absorbed fluxes, while not influencing the absorption of longwave radiation. Cloud warming over the ice sheet therefore dominates year-round. Only when albedo values drop below ~0.6 in the coastal areas during summer, the cooling effect starts to overcome the warming effect. The year-round excess of energy due to the presence of liquid-containing clouds has an extensive influence on the mass balance of the ice sheet. Simulations using the SNOWPACK snow model showed not only a strong influence of these liquid-containing clouds on melt increase, but also on the increased sublimation mass loss. Simulations with the Community Earth System Climate Model for the end of the 21st century (2080-2099) show that Greenland clouds contain more liquid water path and less ice water path. This implies that cloud radiative forcing will be further enhanced in the future. Our results therefore urge the need for improving cloud microphysics in climate models, to improve future projections of ice sheet mass balance and global sea level rise.

  14. The Rapid Ice Sheet Change Observatory (RISCO)

    NASA Astrophysics Data System (ADS)

    Morin, P.; Howat, I. M.; Ahn, Y.; Porter, C.; McFadden, E. M.

    2010-12-01

    The recent expansion of observational capacity from space has revealed dramatic, rapid changes in the Earth’s ice cover. These discoveries have fundamentally altered how scientists view ice-sheet change. Instead of just slow changes in snow accumulation and melting over centuries or millennia, important changes can occur in sudden events lasting only months, weeks, or even a single day. Our understanding of these short time- and space-scale processes, which hold important implications for future global sea level rise, has been impeded by the low temporal and spatial resolution, delayed sensor tasking, incomplete coverage, inaccessibility and/or high cost of data available to investigators. New cross-agency partnerships and data access policies provide the opportunity to dramatically improve the resolution of ice sheet observations by an order of magnitude, from timescales of months and distances of 10’s of meters, to days and meters or less. Advances in image processing technology also enable application of currently under-utilized datasets. The infrastructure for systematically gathering, processing, analyzing and distributing these data does not currently exist. Here we present the development of a multi-institutional, multi-platform observatory for rapid ice change with the ultimate objective of helping to elucidate the relevant timescales and processes of ice sheet dynamics and response to climate change. The Rapid Ice Sheet Observatory (RISCO) gathers observations of short time- and space-scale Cryosphere events and makes them easily accessible to investigators, media and general public. As opposed to existing data centers, which are structured to archive and distribute diverse types of raw data to end users with the specialized software and skills to analyze them, RISCO focuses on three types of geo-referenced raster (image) data products in a format immediately viewable with commonly available software. These three products are (1) sequences of images

  15. connecting the dots between Greenland ice sheet surface melting and ice flow dynamics (Invited)

    NASA Astrophysics Data System (ADS)

    Box, J. E.; Colgan, W. T.; Fettweis, X.; Phillips, T. P.; Stober, M.

    2013-12-01

    This presentation is of a 'unified theory' in glaciology that first identifies surface albedo as a key factor explaining total ice sheet mass balance and then surveys a mechanistic self-reinforcing interaction between melt water and ice flow dynamics. The theory is applied in a near-real time total Greenland mass balance retrieval based on surface albedo, a powerful integrator of the competing effects of accumulation and ablation. New snowfall reduces sunlight absorption and increases meltwater retention. Melting amplifies absorbed sunlight through thermal metamorphism and bare ice expansion in space and time. By ';following the melt'; we reveal mechanisms linking existing science into a unified theory. Increasing meltwater softens the ice sheet in three ways: 1.) sensible heating given the water temperature exceeds that of the ice sheet interior; 2.) Some infiltrating water refreezes, transferring latent heat to the ice; 3.) Friction from water turbulence heats the ice. It has been shown that for a point on the ice sheet, basal lubrication increases ice flow speed to a time when an efficient sub-glacial drainage network develops that reduces this effect. Yet, with an increasing melt duration the point where the ice sheet glides on a wet bed increases inland to a larger area. This effect draws down the ice surface elevation, contributing to the ';elevation feedback'. In a perpetual warming scenario, the elevation feedback ultimately leads to ice sheet loss reversible only through much slower ice sheet growth in an ice age environment. As the inland ice sheet accelerates, the horizontal extension pulls cracks and crevasses open, trapping more sunlight, amplifying the effect of melt accelerated ice. As the bare ice area increases, the direct sun-exposed crevassed and infiltration area increases further allowing the ice warming process to occur more broadly. Considering hydrofracture [a.k.a. hydrofracking]; surface meltwater fills cracks, attacking the ice integrity

  16. Satellite remote sensing for ice sheet research

    NASA Technical Reports Server (NTRS)

    Thomas, R. H.; omplexity of the land cover and land use p; omplexity of the land cover and land use p

    1985-01-01

    Potential research applications of satellite data over the terrestrial ice sheets of Greenland and Antarctica are assessed and actions required to ensure acquisition of relevant data and appropriate processing to a form suitable for research purposes are recommended. Relevant data include high-resolution visible and SAR imagery, infrared, passive-microwave and scatterometer measurements, and surface topography information from laser and radar altimeters.

  17. Is the Antarctic ice sheet growing?

    NASA Technical Reports Server (NTRS)

    Jacobs, S. S.

    1992-01-01

    A brief review is presented of recent observations of surface accumulation on Antarctica. It is concluded that it is as yet too early to say with confidence whether the ice sheet has recently been growing or shrinking, given the variability in accumulation pattern and the larger uncertainties in melting and calving.

  18. Rapid Access Ice Drill: A New Tool for Exploration of the Deep Antarctic Ice Sheets and Subglacial Geology

    NASA Astrophysics Data System (ADS)

    Goodge, J. W.; Severinghaus, J. P.

    2014-12-01

    The Rapid Access Ice Drill (RAID) will penetrate the Antarctic ice sheets in order to core through deep ice, the glacial bed, and into bedrock below. This new technology will provide a critical first look at the interface between major ice caps and their subglacial geology. Currently in construction, RAID is a mobile drilling system capable of making several long boreholes in a single field season in Antarctica. RAID is interdisciplinary and will allow access to polar paleoclimate records in ice >1 Ma, direct observation at the base of the ice sheets, and recovery of rock cores from the ice-covered East Antarctic craton. RAID uses a diamond rock-coring system as in mineral exploration. Threaded drill-pipe with hardened metal bits will cut through ice using reverse circulation of Estisol for pressure-compensation, maintenance of temperature, and removal of ice cuttings. Near the bottom of the ice sheet, a wireline bottom-hole assembly will enable diamond coring of ice, the glacial bed, and bedrock below. Once complete, boreholes will be kept open with fluid, capped, and made available for future down-hole measurement of thermal gradient, heat flow, ice chronology, and ice deformation. RAID will also sample for extremophile microorganisms. RAID is designed to penetrate up to 3,300 meters of ice and take sample cores in less than 200 hours. This rapid performance will allow completion of a borehole in about 10 days before moving to the next drilling site. RAID is unique because it can provide fast borehole access through thick ice; take short ice cores for paleoclimate study; sample the glacial bed to determine ice-flow conditions; take cores of subglacial bedrock for age dating and crustal history; and create boreholes for use as an observatory in the ice sheets. Together, the rapid drilling capability and mobility of the drilling system, along with ice-penetrating imaging methods, will provide a unique 3D picture of the interior Antarctic ice sheets.

  19. The little ice age

    SciTech Connect

    Grove, J.M.

    1988-01-01

    The Little Ice Age, a period of glacier expansion in alpine regions that began sometime between the twelfth and sixteenth centuries and lasted until late in the nineteenth century, was recorded not only in glacial features dated by geologic techniques but also in historical documents such as field sketches, land values, and weather records, especially in the Alps. Indirect evidence of its impact in other parts of the world includes the records of sea-ice extent near Iceland and Greenland, the fate of the Viking settlements in Greenland, and many other suggestions that the climate was colder in the recent past than it is today. Jean Grove's book is an authoritative, superbly documented, and excellently written summary of the abundant evidence of climatic change during the last few centuries in the context of broader climatic variations of the last 10,000 years. This summary provides a much-needed perspective for considering the magnitude and frequency of natural climatic variations in the past, given predictions for the future. In the final chapter, Grove notes that natural climatic variations, including another minor ice age, might be expected in the future but at the end of the Little Ice Age coincided with the increased burning of fossil fuels during the industralization of Europe and North America. This coincidence does indeed suggest that modern scientists already have had a significant impact on the global climate.

  20. Volcano-ice age link discounted

    SciTech Connect

    Kerr, R.A.

    1996-05-10

    Speculation that huge volcanic eruptions may have caused an immediate `volcanic winter` that devastated early humans and accelerated a slide into the Ice Age. However, further information from the Greenland ice sheet about the Toba errumption on the island of Sumatra 70,000 years ago, seems to indicate that such volcanic actions wasn`t a major climatic catalyst. This article discusses the evidence and further possibilities.

  1. Ice-sheet driven enhancement of geothermal flux: preliminary model results

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    Previous observations in parts of West Antarctica and Greenland, including near the head of the Northeast Greenland Ice Stream, have indicated rapid basal melting, suggesting higher geothermal flux than typical for the expected geological setting. Growth and decay of ice sheets over ice-age cycles cause large and geologically rapid changes in loading and flexure beneath and nearby. Oscillating load will cause oscillating melt volume in deep rocks, and because melt extraction increases with melt volume more rapidly than linearly, ice-age cycling will tend to move melt upward. Melt motion may be greatly aided by fracturing promoted by flexural stresses from the varying ice sheets. Preliminary results from ice-sheet models coupled to lithosphere and asthenosphere will be presented, suggesting that ice-sheet changes may be affecting their basal heat flux.

  2. Flow variability in the Scandinavian ice sheet: modelling the coupling between ice sheet flow and hydrology

    NASA Astrophysics Data System (ADS)

    Arnold, Neil; Sharp, Martin

    2002-02-01

    There is increasing geologic evidence for periodic flow variability within large ice sheets, manifested as spatially and temporally variable areas of fast ice flow, and resulting in the very complex patterns of lineations observed in formerly glaciated areas. However, many ice sheet models do not replicate this behaviour. A possible reason for this is that such models do not include a detailed treatment of basal hydrology. Changes in the character of sub-glacial drainage systems are believed to cause surges in valley glaciers. Recent ice sheet models, which have included basal hydrology or at least a link between basal velocity and the presence of water at the bed, often show flow variability. However, these models have typically assumed a deformable bed, or have made no assumptions about the nature of the bed. Whilst these assumptions seem applicable to areas close to the former margins of Quaternary ice sheets, they are less applicable to interior areas. These areas typically show thin or scanty till cover over eroded bedrock, and the presence of eskers, which are indicative of drainage in sub-glacial tunnels. We have developed a two-dimensional time-dependent ice sheet model that includes hard-bed basal hydrology. This allows calculation of sub-glacial water pressures and the use of a water pressure dependent sliding law to calculate ice sheet velocities. When used to simulate the Weichselian Scandinavian ice sheet, with late Quaternary climate and sea level as forcing functions, this model develops localised areas of fast-flowing ice, which vary in extent and in distance of penetration into the interior of the ice sheet both spatially and temporally. The behaviour of these lobes depends crucially on the influence of the evolving ice sheet topography on the routing of subglacial water flow, due to the resulting variations in the subglacial hydraulic potential which drive the water flow. Bedrock topography also has some influence, but fast flow areas are not

  3. The Greenland Ice Sheet Monitoring Network (GLISN)

    NASA Astrophysics Data System (ADS)

    Anderson, K. R.; Beaudoin, B. C.; Butler, R.; Clinton, J. F.; Dahl-Jensen, T.; Ekstrom, G.; Giardini, D.; Govoni, A.; Hanka, W.; Kanao, M.; Larsen, T.; Lasocki, S.; McCormack, D. A.; Mykkeltveit, S.; Nettles, M.; Agostinetti, N. P.; Stutzmann, E.; Tsuboi, S.; Voss, P.

    2010-12-01

    The GreenLand Ice Sheet monitoring Network (GLISN) is an international, broadband seismic capability for Greenland, being installed and implemented through the collaboration of Denmark, Canada, Germany, Italy, Japan, Norway, Poland, Switzerland, and USA. GLISN is a real-time sensor array of seismic stations to enhance and upgrade the performance of the sparse Greenland seismic infrastructure for detecting, locating, and characterizing glacial earthquakes and other cryo-seismic phenomena, and contributing to our understanding of Ice Sheet dynamics. Complementing data from satellites, geodesy, and other sources, and in concert with these technologies, GLISN will provide a powerful tool for detecting change, and will advance new frontiers of research in the glacial systems; the underlying geological and geophysical processes affecting the Greenland Ice Sheet; interactions between oceans, climate, and the cryosphere; and other multidisciplinary areas of interest to geoscience and climate dynamics. The glacial processes that induce seismic events (internal deformation, sliding at the base, disintegration at the calving front, drainage of supra-glacial lakes) are all integral to the overall dynamics of glaciers, and seismic observations of glaciers therefore provide a quantitative means for monitoring changes in their behavior over time. Long-term seismic monitoring of the Greenland Ice Sheet will contribute to identifying possible unsuspected mechanisms and metrics relevant to ice sheet collapse, and will provide new constraints on Ice Sheet dynamic processes and their potential roles in sea-level rise during the coming decades. GLISN will provide a new, fiducial reference network in and around Greenland for monitoring these phenomena in real-time, and for the broad seismological study of Earth and earthquakes. The 2010 summer field season saw the installation or upgrade of 9 stations in the GLISN network. Sites visited under the GLISN project include Station Nord (NOR

  4. Ice Age Geomorphology of North America

    NASA Astrophysics Data System (ADS)

    Wickert, A. D.; Anderson, R. S.; Mitrovica, J. X.; Picard, K.

    2012-12-01

    The Last Glacial Cycle in North America dramatically modified drainage patterns and geomorphology on a continental scale. As a consequence, the evolution of river systems holds information on the patterns of glaciation and isostatic response. This information can, in principle, be used to reconstruct the volumes of ice sheet sectors and eroded material by connecting the upstream ice sheets with stable isotope and other sedimentary records in offshore basins. Here we integrate this coupled geomorphic-hydrologic-glacial-sedimentary-paleoceanographic system to solve both the forward problem, how rivers evolve in response to Ice Age forcing, as well as the inverse problem, how fluvial systems record Quaternary history. The connections that define this system provide a link between climate and geomorphology that extends beyond the traditionally considered watershed-to-landscape scale by incorporating solid Earth deformations, large-scale shoreline migration, and the high amplitude changes in climate that drive the growth and decay of major ice sheets and water delivery to the bounding river systems. We address this continental scale problem using a valley-resolving drainage reconstruction that incorporates a realistic ice sheet history, a gravitationally self-consistent treatment of ice-age sea-level changes that includes shoreline migration, and precipitation and evapotranspiration retrodicted using general circulation model (GCM) runs. Drainage divides over the flat-lying North American interior migrate hundreds to thousands of kilometers in response to dynamic interactions between ice sheets and solid Earth response, and these changes coupled with post last glacial maximum (LGM) ice sheet melting drive high-amplitude variability in water and sediment discharge to the oceans. The Mackenzie River Delta records a sedimentary record produced by a highly non-eustatic sea level history and massive glacial sediment inputs routed along the axis that divided the Cordilleran

  5. Uncertainty in Ice Crystal Orientation Distributions in Ice Sheets

    NASA Astrophysics Data System (ADS)

    Hay, Michael; Waddington, Edwin

    2016-04-01

    Crystal-orientation fabrics in polar ice sheets have a strong influence on ice flow due to the plastic anisotropy of ice. Crystal orientations evolve primarily in response to applied strain, but are also affected by temperature, impurities, interactions with neighbors, and other factors. While the evolution of each ice crystal is physically deterministic, in limited samples, such as those from ice-core thin sections, measured samples are stochastic due to sampling error. Even in continuum representations from models, crystal orientation distribution functions (ODFs) can be treated as stochastic due to uncertainties in how they developed. Here, we present results on the statistics of crystal orientation fabrics. We show a first-order estimate of the sampling distribution of fabric eigenvalues and fabric eigenvectors from ice-core thin sections. We also analyze uncertainty in electron backscatter diffraction measurements. In addition to sampling error, the strain histories of fabrics are generally poorly constrained, and may have varied in unknown ways through time. Nearby layers in ice sheets can also experience different strain histories due to inherent variabilities such as transient flow, or differences in impurities. This means that the continuum ODF itself can be treated as stochastic, because it depends on an effectively-stochastic unknown strain-history. To explore this, we analyze the effects of strain and vorticity variability on the evolution of the continuum ice-crystal ODF. We recast Jeffery's equation for the evolution of the ODF as a stochastic differential equation, with vorticity and strain perturbed by Gaussian processes. From this, we run a Monte-Carlo ensemble to determine likely bounds of true continuum ODF variability in response to random perturbations of strain and vorticity.

  6. Meteorites and the Antarctic ice sheet

    NASA Technical Reports Server (NTRS)

    Cassidy, W. A.

    1986-01-01

    The majority of the meteorite finds were located in the Allan Hills site. All the expected goals involving the recovery of rare or previously unknown types of meteorites, and even the recovery of lunar ejecta, were realized. The relationship between these remarkable concentrations of meteorites and the Antarctic ice sheet itself were less well documented. Ice flow vector studies were made and concentration models were proposed. Earlier estimates of the abundances of meteorite types were based on the number of falls in the world collections. The accumulated data and the future collected data will allow more reliable estimates of the source region of most meteorites.

  7. Greenland Ice Sheet glacier motion and ice loss: New understanding of ice sheet behavior through remote sensing

    NASA Astrophysics Data System (ADS)

    Moon, T. A.; Fahnestock, M. A.; Scambos, T.; Joughin, I.

    2015-12-01

    Ice loss from the Greenland Ice Sheet makes up roughly a third of current sea level rise, also generating substantial local and regional freshwater fluxes. Containing more than 6 meters of sea level rise equivalent in ice, Greenland has the potential to contribute much more to rising ocean levels and freshening water in the future. Understanding the dynamics of the ice sheet, particularly the behavior of fast flowing coastal outlet glaciers, is critical to improving predictions of future ice sheet change and associated impacts. Combining velocity, glacier ice front, sea ice, and ice sheet surface melt data, we made several important advances in characterizing and understanding seasonal glacier behavior and the processes driving change: 1) seasonal velocity patterns fall into at least 3 distinct patterns, 2) these seasonal velocity patterns likely indicate differences in glacier responsiveness to ocean versus subglacial hydrologic processes, and 3) in some regions seasonal versus multi-year velocity changes appear most strongly influenced by different environmental factors. Further progress was previously hampered by limits in measurement resolution across space and time. To address this challenge, we are creating a new - and continuously growing - ice velocity dataset from Landsat 8 imagery. This data stream supports comprehensive global measurements of ice flow, providing a leap in our understanding of ice sheet motion across space and time. We offer a high-level discussion of our research findings and an introduction to the new Landsat 8-enabled data stream. Our results and measurement capabilities deliver critical new knowledge about ice sheet behavior and interaction with ocean and climate factors. These advances, in turn, have important implications for other elements of Earth system research, including climate, oceanography, and biology.

  8. How and when to terminate the Pleistocene ice ages?

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  9. Flow-pattern evolution of the last British Ice Sheet

    NASA Astrophysics Data System (ADS)

    Hughes, Anna L. C.; Clark, Chris D.; Jordan, Colm J.

    2014-04-01

    We present a 10-stage reconstruction of the evolution in ice-flow patterns of the last British Ice Sheet from build-up to demise derived from geomorphological evidence. 100 flowsets identified in the subglacial bedform record (drumlins, mega-scale glacial lineations, and ribbed moraine) are combined with ancillary evidence (erratic-transport paths, absolute dates and a semi-independently reconstructed retreat pattern) to define flow patterns, ice divides and ice-sheet margins during build-up, maximum glaciation and retreat. Overprinting and cross-cutting of landform assemblages are used to define the relative chronology of flow patterns and a tentative absolute chronology is presented based on a collation of available dates for ice advance and retreat. The ice-flow configuration of the last British Ice Sheet was not static. Some ice divides were remarkably stable, persisting through multiple stages of the ice-sheet evolution, whereas others were transient features existing for a short time and/or shifting in position 10s km. The 10 reconstructed stages of ice-sheet geometry capture two main modes of operation; first as an integrated ice sheet with a broadly N-S orientated ice divide, and second as a multi-domed ice sheet orientated parallel with the shelf edge. A thick integrated ice sheet developed as ice expanded out of source areas in Scotland to envelop southerly ice caps in northern England and Wales, and connect with the Irish Ice Sheet to the west and the Scandinavian Ice Sheet across the North Sea. Following break-up of ice over the North Sea, ice streaming probably drove mass loss and ice-sheet thinning to create a more complex divide structure, where ice-flow patterns were largely controlled by the form of the underlying topography. Ice surface lowering occurred before separation of, and retreat to, multiple ice centres centred over high ground. We consider this 10-stage reconstruction of the evolution in ice-sheet configuration to be the simplest palaeo

  10. Ice Sheet System Model as Educational Entertainment

    NASA Astrophysics Data System (ADS)

    Perez, G.

    2013-12-01

    Understanding the importance of polar ice sheets and their role in the evolution of Sea Level Rise (SLR), as well as Climate Change, is of paramount importance for policy makers as well as the public and schools at large. For example, polar ice sheets and glaciers currently account for 1/3 of the SLR signal, a ratio that will increase in the near to long-term future, which has tremendous societal ramifications. Consequently, it is important to increase awareness about our changing planet. In our increasingly digital society, mobile and web applications are burgeoning venues for such outreach. The Ice Sheet System Model (ISSM) is a software that was developed at the Jet Propulsion Laboratory/CalTech/NASA, in collaboration with University of California Irvine (UCI), with the goal of better understanding the evolution of polar ice sheets. It is a state-of-the-art framework, which relies on higher-end cluster-computing to address some of the aforementioned challenges. In addition, it is a flexible framework that can be deployed on any hardware; in particular, on mobile platforms such as Android or iOS smart phones. Here, we look at how the ISSM development team managed to port their model to these platforms, what the implications are for improving how scientists disseminate their results, and how a broader audience may familiarize themselves with running complex climate models in simplified scenarios which are highly educational and entertaining in content. We also look at the future plans toward a web portal fully integrated with mobile technologies to deliver the best content to the public, and to provide educational plans/lessons that can be used in grades K-12 as well as collegiate under-graduate and graduate programs.

  11. ISMIP6: Ice Sheet Model Intercomparison Project for CMIP6

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    The sea level projections made by the glaciological community as part of the Intergovernmental Panel on Climate Change (IPCC) process have often been out of phase with the projections considered by the wider Coupled Model Intercomparison Project (CMIP) community. For instance in AR5, the ice2sea and SeaRISE (Sea-level Response to Ice Sheet Evolution) ice sheet projects predominantly worked with AR4 scenarios, while the CMIP5 community used new future scenarios. As the next phase of CMIP is being designed (CMIP6), an effort for ice sheet models to be better integrated in the CMIP6 initiative has been proposed to the CMIP panel. We present the framework for the new effort, ISMIP6, the Ice Sheet Model Intercomparison Project for CMIP6. The primary goal of ISMIP6 is to improve projections of sea level rise via improved projections of the evolution of the Greenland and Antarctic ice sheets under a changing climate, along with a quantification of associated uncertainties (including uncertainty in both climate forcing and ice-sheet response). This goal requires an evaluation of AOGCM climate over and surrounding the ice sheets; analysis of simulated ice-sheet response from standalone models forced "offline" with CMIP AOGCM outputs and, where possible, with coupled ice sheet-AOGCM models; and experiments with standalone ice sheet models targeted at exploring the uncertainty associated with ice sheets physics, dynamics and numerical implementation. A secondary goal is to investigate the role of feedbacks between ice sheets and climate in order to gain insight into the impact of increased mass loss from the ice sheets on regional and global sea level, and of the implied ocean freshening on the coupled ocean-atmosphere circulation. These goals map into both Cryosphere and Sea-Level Rise Grand Challenges relevant to Climate and Cryosphere (CliC) and the World Climate Research Program (WCRP).

  12. Color of Greenland: Tracing the Dark Ice Exposed at the Ice Sheet Margin

    NASA Astrophysics Data System (ADS)

    Starke, S. E.; Bell, R. E.; Tinto, K. J.; Das, I.; Winckler, G.

    2014-12-01

    The color and albedo of the surface of a large ice sheet is critical to its response to a changing climate. Decreasing the ice surface albedo enhances surface melt and has been suggested as a mechanism to trigger rapid collapse. Each summer, dark bands of ice 20-40 km wide are exposed along the margin of the Greenland ice sheet. These dark bands are clearly visible in satellite imagery and best developed along the west coast. We use airborne radar data in both northeast and western Greenland to demonstrate that the dark bands are the result of outcropping stratigraphy. Where these dark bands are exposed at the ice surface correlate with locations where the well defined stratigraphy imaged with airborne radar is truncated at the surface. Surface work in the northeast by Boogild and coworkers (2010) has constrained the age of the three major intervals of exposed strata. Pink or brown Pleistocene ice lies closest to the ice margin, and is overlain by white pre-Boreal ice. The impurity-rich dark strata, are dated as early Holocene. The dark strata are likely a result of either periods of elevated dust during the Holocene or excess melt during the Holocene Climatic Optimum. We use satellite data to map the extent of the exposed dark ice in Greenland using imagery from Landsat 8, Landsat 7, ASTER VNIR, EO1 Ali, and Quickbird with spatial resolutions ranging from 0.65m to 30m. Image acquisition focused on the months of July and August when the stratigraphy is best exposed. Little dark ice is presently exposed in the southeastern margin of the Greenland ice sheet as this region experiences higher surface accumulation. By examining satellite images from multiple years we have identified areas where the patterns of the dark ice are changing. Both movement of the strata towards the margin due to ice flow and inland retreat due to increased erosion are documented. An outstanding question is what will be color of the strata exposed as the bare ice region expands in Greenland

  13. Continuous broadband seismic observation on the Greenland Ice Sheet under Greenland Ice Sheet monitoring Network

    NASA Astrophysics Data System (ADS)

    Tsuboi, Seiji; Kanao, Masaki; Tono, Yoko; Himeno, Tetsuto; Toyokuni, Genti; Childs, Dean; Dahl-Jensen, Trine; anderson, Kent

    2013-04-01

    We have installed the ice sheet broadband seismograph station, called ICE-S (DK.ICESG) in June 2011, in collaboration with IRIS Polar Services under the GreenLand Ice Sheet monitoring Network (GLISN), which is a new, international, broadband seismic capability for Greenland being implemented through the collaboration between Denmark, Canada, France, Germany, Italy, Japan, Norway, Poland, Switzerland, and the USA. The primary purpose of GLISN project is to define the fine structure and detailed mechanisms of glacial earthquakes within the Greenland Ice Sheet. These glacial earthquakes in the magnitude range 4.6-5.1 may be modeled as a large glacial ice mass sliding downhill several meters on its basal surface over duration of 30 to 60 seconds. Glacial earthquakes have been observed at seismic stations within Greenland (Larsen et al, 2006), but the coverage was very sparse and a broadband, real-time seismic network was needed to be installed throughout Greenland's Ice Sheet and perimeter. The National Institute for Polar Research and Japan Agency for Marine-Earth Science and Technology are members of GLISN project and we have started to operate ICESG station since 2011. The station is equipped with a CMG-3T broadband seismometer and a Quanterra Q330 data logger. We have visited the station again in May, 2012 and successfully retrieved one year of continuous records from the broadband seismometer and updated the telemetry system to eventually allow real time monitoring of the station. ICESG station is now daily sending 1 Hz continuous data over the iridium satellite system using RUDICS. The observed three component seismograms demonstrate that the quality of this ice sheet station is good enough to record not only local earthquakes around Greeland but also teleseismic earthquakes. We could record three component broadband seismograms for April 11, 2012 Off the west coast of Northern Sumatra earthquake (Mw8.6). These seismograms show high signal to noise ratio

  14. West antarctic ice sheet collapse: Chimera or clear danger

    SciTech Connect

    Alley, R.B. ); MacAyeal, D.R. )

    1993-01-01

    The specter of a west antarctic collapse has been with us for 25 years. Recently, certain official assessments concerned primarily with the future response to projected global warming have concluded that Antarctica will not cause much sea-level rise within the planning horizon of a century or so. At the same time startling new results on ice sheet (in)stability have been emerging, pointing to less stability then previously believed. Some recent results are reviewed briefly: Heinrich layers in the North Atlantic show basally lubricated surges of the Laurentide ice sheet; the west antarctic ice sheet collapsed recently; the modern west antarctic ice sheet is changing rapidly locally; the bed of ice stream B is exceptionally well lubricated by water and water-saturated soft sediments; the modern ice sheet is thinning slowly on average; a model west antarctic ice sheet undergoes rapid collapses long after forcing and probably related to penetration of warmth to the bed. 23 refs.

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

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

  16. Mars Ice Age, Simulated

    NASA Technical Reports Server (NTRS)

    2003-01-01

    December 17, 2003

    This simulated view shows Mars as it might have appeared during the height of a possible ice age in geologically recent time.

    Of all Solar System planets, Mars has the climate most like that of Earth. Both are sensitive to small changes in orbit and tilt. During a period about 2.1 million to 400,000 years ago, increased tilt of Mars' rotational axis caused increased solar heating at the poles. A new study using observations from NASA's Mars Global Surveyor and Mars Odyssey orbiters concludes that this polar warming caused mobilization of water vapor and dust into the atmosphere, and buildup of a surface deposit of ice and dust down to about 30 degrees latitude in both hemispheres. That is the equivalent of the southern Unites States or Saudi Arabia on Earth. Mars has been in an interglacial period characterized by less axial tilt for about the last 300,000 years. The ice-rich surface deposit has been degrading in the latitude zone of 30 degrees to 60 degrees as water-ice returns to the poles.

    In this illustration prepared for the December 18, 2003, cover of the journal Nature, the simulated surface deposit is superposed on a topography map based on altitude measurements by Global Surveyor and images from NASA's Viking orbiters of the 1970s.

    Mars Global Surveyor and Mars Odyssey are managed by NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, for the NASA Office of Space Science, Washington.

  17. Determining Greenland Ice Sheet Accumulation Rates from Radar Remote Sensing

    NASA Technical Reports Server (NTRS)

    Jezek, Kenneth C.

    2001-01-01

    An important component of NASA's Program for Arctic Regional Climate Assessment (PARCA) is a mass balance investigation of the Greenland Ice Sheet. The mass balance is calculated by taking the difference between the snow accumulation and the ice discharge of the ice sheet. Uncertainties in this calculation include the snow accumulation rate, which has traditionally been determined by interpolating data from ice core samples taken throughout the ice sheet. The sparse data associated with ice cores, coupled with the high spatial and temporal resolution provided by remote sensing, have motivated scientists to investigate relationships between accumulation rate and microwave observations.

  18. Ice streaming and the demise of the Last British Ice Sheet: geomorphological evidence, modelling experiments, and cosmogenic nuclide chronology

    NASA Astrophysics Data System (ADS)

    Bradwell, T.; Hubbard, A.; Fabel, D.; Golledge, N.; Stoker, M.; Everest, J.; Finlayson, A.; Howe, J.

    2010-12-01

    We synthesise recent work on the palaeoglaciology of the British-Irish Ice Sheet, focusing on the glacial geomorphology preserved on the seabed around the northern UK [Bradwell et al., 2008]; and a suite of numerical modelling experiments spanning the last ~40 ka [Hubbard et al., 2009]. In addition, we present a new temporal dataset to better constrain the decay of ice stream sectors within the last British Ice Sheet. Our suite of cosmogenic exposure ages dovetails well with the emerging view recorded in other environmental proxies. Modelling experiments, forced by NGRIP ice-core data, show an extremely dynamic ice sheet drained by transient but recurrent ice streams which dynamically switch and fluctuate in extent and intensity on a centennial time-scale. Our Be-10 cosmogenic chronology sheds new light on the timing of ice stream activity and cessation, placing the separation of the British and Fennoscandian ice sheets at ~24 ka BP - closely associated with the iceberg discharge event Heinrich-2. In the NW sector, the Minch Ice Stream, a quasi-stable feature of the last British Ice Sheet, probably scavenged ice from an adjacent catchment shortly before its demise c. 18 ka BP. In the NE sector, a large ice stream in the Moray Firth continued to operate until c. 15 ka BP - whereby model simulations show a rapid collapse, within the space of 100 yrs. Ultimately, the resolution of such short-lived events lies within the uncertainties of currently available dating techniques. Hence, further high-temporal resolution studies are required to explore the role of internal (glaciological) vs external (eustatic and climatic) forcing on the stability of ice streams within marine-terminating ice sheets.

  19. 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

  20. Late Pleistocene ice age scenarios based on observational evidence

    NASA Astrophysics Data System (ADS)

    Deblonde, G.; Peltier, W. R.

    1993-04-01

    Ice age scenarios for the last glacial-interglacial cycle, based on observations of Boyle and Keigwin (1982) concerning the North Atlantic thermohaline circulation and of Barnola et al. (1987) concerning atmospheric CO2 variations derived from the Vostok ice cores, are analyzed. Northern Hemisphere continental ice sheets are simulated with an energy balance model (EBM) that is asynchronously coupled to vertically integrated ice sheet models based on the Glen flow law. The EBM includes both a realistic land-sea distribution and temperature-albedo feedback and is driven with orbital variations of effective solar insolation. With the addition of atmospheric CO2 and ocean heat flux variations, but not in their absence, a complete collapse is obtained for the Eurasian ice sheet but not for the North American ice sheet. Further feedback mechanisms, perhaps involving more accurate modeling of the dynamics of the mostly marine-based Laurentide complex, appear necessary to explain termination I.

  1. Ice-Sheet Enhancement of Volcanism and Geothermal Heat Flux: a Stress Modeling Approach

    NASA Astrophysics Data System (ADS)

    Stevens, N. T.; Parizek, B. R.; Alley, R. B.

    2015-12-01

    Bore-hole and geophysically inferred geothermal heat fluxes beneath the Greenland Ice Sheet, particularly at the head of the Northeast Greenland Ice Stream, are in some places higher than suggested by the underlying geology. Geologically rapid changes in lithospheric loading during ice-sheet growth and decay produce large changes in the effective stress state beneath and nearby. Oscillating loads will cause oscillating melt volume in deep rocks, and the nonlinear increase of melt migration velocity with melt fraction means that extended ice-age cycling will enhance upward melt migration. Our numerically efficient simulations of ice-sheet/lithosphere interactions produce crustal stresses similar to values estimated to allow dike emplacement and vug-wave migration. Maximum tensile and shear stresses shift both horizontally and vertically during ice sheet growth and decay, suggesting multi-step transport of melt upwards to or near the base of the ice sheet. We thus suggest that regions of high geothermal heat flux arose from cyclic ice-sheet loading, which enhanced melt extraction from a deep source (possibly linked to passage of the Iceland hot spot). We further suggest that similar processes may have been important elsewhere beneath or near present or former ice sheets, potentially enhancing volcanism as well as geothermal flux.

  2. Uncertainty quantification for ice sheet inverse problems

    NASA Astrophysics Data System (ADS)

    Petra, N.; Ghattas, O.; Stadler, G.; Zhu, H.

    2011-12-01

    Modeling the dynamics of polar ice sheets is critical for projections of future sea level rise. Yet, there remain large uncertainties in the basal boundary conditions and in the non-Newtonian constitutive relations employed within ice sheet models. In this presentation, we consider the problem of estimating uncertainty in the solution of (large-scale) ice sheet inverse problems within the framework of Bayesian inference. Computing the general solution of the inverse problem-i.e., the posterior probability density-is intractable with current methods on today's computers, due to the expense of solving the forward model (3D full Stokes flow with nonlinear rheology) and the high dimensionality of the uncertain parameters (which are discretizations of the basal slipperiness field and the Glen's law exponent field). However, under the assumption of Gaussian noise and prior probability densities, and after linearizing the parameter-to-observable map, the posterior density becomes Gaussian, and can therefore be characterized by its mean and covariance. The mean is given by the solution of a nonlinear least squares optimization problem, which is equivalent to a deterministic inverse problem with appropriate interpretation and weighting of the data misfit and regularization terms. To obtain this mean, we solve a deterministic ice sheet inverse problem; here, we infer parameters arising from discretizations of basal slipperiness and rheological exponent fields. For this purpose, we minimize a regularized misfit functional between observed and modeled surface flow velocities. The resulting least squares minimization problem is solved using an adjoint-based inexact Newton method, which uses first and second derivative information. The posterior covariance matrix is given (in the linear-Gaussian case) by the inverse of the Hessian of the least squares cost functional of the deterministic inverse problem. Direct computation of the Hessian matrix is prohibitive, since it would

  3. Probability based hydrologic catchments of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Hudson, B. D.

    2015-12-01

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

  4. Reconstructing the last Newfoundland Ice Sheet,Canada.

    NASA Astrophysics Data System (ADS)

    McHenry, Maureen; Dunlop, Paul

    2015-04-01

    The Newfoundland Ice Sheet which formed part of the North American Ice Sheet Complex was situated on the margins of the northwest Atlantic Ocean during the Wisconsinan glaciation (~80ka BP to 10ka BP). This complex consisted of the Laurentide, the Cordilleran and Innuitian Ice Sheets, the Canadian Maritime Provinces Ice Cover and the Newfoundland Ice Sheet (NIS). Although all were confluent at the last glacial maximum, the NIS is known to have supported independent ice centres with advances from the Laurentide Ice Sheet being restricted to Newfoundland's northern and western margins. Given its distinctive position, it is likely the evolution of the NIS through the last glacial cycle was influenced by a number of external and internal drivers including configuration changes in the Laurentide Ice Sheet, ice stream initiation and shutdown, changes in oceanic circulation and fluctuating sea levels and climate signals from the wider Amphi-North Atlantic. As such Newfoundland is a key location for investigating ice sheet response to a number of internal and external forcing mechanisms during glacial cycles. An established technique for reconstructing former ice sheet behaviour is the mapping and spatial analysis of glacial landforms. This provides a valuable record of former ice sheet extent and behaviour through time as well as ice sheet retreat during deglaciation. Here we present new mapping based on our interpretation of SPOT satellite imagery and Digital Elevation Models of the entire Island of Newfoundland as well as swath bathymetric imagery from several locations offshore. Our new database consisting of ~150,000 individually mapped subglacial bedforms that includes drumlins, crag and tails, glacially moulded bedrock lineations and ribbed moraines significantly increases the known landform record in this region. The new database shows Newfoundland has a complex palimpsest landscape that records multiple ice sheet events across the island. Here we report our

  5. Greenland ice sheet motion insensitive to exceptional meltwater forcing

    PubMed Central

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

    2013-01-01

    Changes to the dynamics of the Greenland ice sheet can be forced by various mechanisms including surface-melt–induced ice acceleration and oceanic forcing of marine-terminating glaciers. We use observations of ice motion to examine the surface melt–induced dynamic response of a land-terminating outlet glacier in southwest Greenland to the exceptional melting observed in 2012. During summer, meltwater generated on the Greenland ice sheet surface accesses the ice sheet bed, lubricating basal motion and resulting in periods of faster ice flow. However, the net impact of varying meltwater volumes upon seasonal and annual ice flow, and thus sea level rise, remains unclear. We show that two extreme melt events (98.6% of the Greenland ice sheet surface experienced melting on July 12, the most significant melt event since 1889, and 79.2% on July 29) and summer ice sheet runoff ∼3.9σ above the 1958–2011 mean resulted in enhanced summer ice motion relative to the average melt year of 2009. However, despite record summer melting, subsequent reduced winter ice motion resulted in 6% less net annual ice motion in 2012 than in 2009. Our findings suggest that surface melt–induced acceleration of land-terminating regions of the ice sheet will remain insignificant even under extreme melting scenarios. PMID:24248343

  6. Greenland ice sheet motion insensitive to exceptional meltwater forcing.

    PubMed

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

    2013-12-01

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

  7. Question of Ages of Cenozoic Volcanic Centers Inferred Beneath the West Antarctic Ice Sheet (WAIS) in the West Antarctic Rift System (WR) from Coincident Aeromagnetic and Radar Ice Sounding Surveys

    NASA Astrophysics Data System (ADS)

    Behrendt, J. C.; Finn, C. A.; Blankenship, D. D.

    2007-12-01

    The recently acquired radar ice sounding surveys (Holt, et al., 2006) extending the 1990s Central West Antarctica (CWA) aerogeophysical survey to the Amundsen and Bellingshausen sea coasts allows us to revise a thought experiment reported by Behrendt et al., 1991 from very limited bed elevation data. Were the ice of the WAIS flowing through the WR to be compressed to the density of crustal rock, almost all of the area beneath the WAIS would be at or above sea level, much >1 km elevation. There are only about 10-20% of the very deep areas (such as the Bentley subglacial trench and the Byrd Subglacial Basin) filled with 3-4-km thick ice that would be well below sea level. The age of the 5-7-km high rift shoulder bounding the asymmetric WR from northern Victoria Land through the Horlick Mountains (where it diverges from the Transantarctic Mountains) to the Ellsworth Mountains has been reported as old as Cretaceous. Volcanic exposures associated with the West Antarctic rift system in the present WAIS area extend at least to 34 Ma and the West Antarctic ice sheet has flowed through the rift possibly as far back in time as 25 Ma. Active volcanism has been reported for the WR at only a few widely scattered locations, so speculations about present volcanic activity beneath the WAIS are quite uncertain, and it is probably quite rare. The Central West Antarctic aeromagnetic and radar ice sounding survey carried out in the 1990s revealed about 1000 "volcanic centers" characterized by 100-1000 nT shallow source magnetic anomalies, at least 400 of which have associated bed topography. About 80% of these show relief <200 m and have been interpreted as smoothed off as they were erupted (injected) into the moving WAIS. Several kilometer-thick highly magnetic sources are required to fit these anomalies requiring high remanent magnetizations in the present field direction. We interpreted these sources as subvolcanic intrusions which must be younger than about 100 Ma because the

  8. Stationary Waves of the Ice Age Climate.

    NASA Astrophysics Data System (ADS)

    Cook, Kerry H.; Held, Isaac M.

    1988-08-01

    A linearized, steady state, primitive equation model is used to simulate the climatological zonal asymmetries (stationary eddies) in the wind and temperature fields of the 18 000 YBP climate during winter. We compare these results with the eddies simulated in the ice age experiments of Broccoli and Manabe, who used CLIMAP boundary conditions and reduced atmospheric CO2 in an atmospheric general circulation model (GCM) coupled with a static mixed layer ocean model. The agreement between the models is good, indicating that the linear model can be used to evaluate the relative influences of orography, diabatic heating, and transient eddy heat and momentum transports in generating stationary waves. We find that orographic forcing dominates in the ice age climate. The mechanical influence of the continental ice sheets on the atmosphere is responsible for most of the changes between the present day and ice age stationary eddies. This concept of the ice age climate is complicated by the sensitivity of the stationary eddies to the large increase in the magnitude of the zonal mean meridional temperature gradient simulated in the ice age GCM.

  9. Growth of Greenland ice sheet - Measurement

    NASA Technical Reports Server (NTRS)

    Zwally, H. Jay; Bindschadler, Robert A.; Marsh, James G.; Brenner, Anita C.; Major, Judy A.

    1989-01-01

    Measurements of ice-sheet elevation change by satellite altimetry show that the Greenland surface elevation south of 72 deg north latitude is increasing. The vertical velocity of the surface is 0.20 + or - 0.06 meters/year from measured changes in surface elevations at 5906 intersections between Geosat paths in 1985 and Seasat in 1978, and 0.28 + or - 0.02 meters/year from 256,694 intersections of Geosat paths during a 548-day period of 1985 to 1986.

  10. Past temperatures directly from the greenland ice sheet

    PubMed

    Dahl-Jensen; Mosegaard; Gundestrup; Clow; Johnsen; Hansen; Balling

    1998-10-01

    A Monte Carlo inverse method has been used on the temperature profiles measured down through the Greenland Ice Core Project (GRIP) borehole, at the summit of the Greenland Ice Sheet, and the Dye 3 borehole 865 kilometers farther south. The result is a 50, 000-year-long temperature history at GRIP and a 7000-year history at Dye 3. The Last Glacial Maximum, the Climatic Optimum, the Medieval Warmth, the Little Ice Age, and a warm period at 1930 A.D. are resolved from the GRIP reconstruction with the amplitudes -23 kelvin, +2.5 kelvin, +1 kelvin, -1 kelvin, and +0.5 kelvin, respectively. The Dye 3 temperature is similar to the GRIP history but has an amplitude 1.5 times larger, indicating higher climatic variability there. The calculated terrestrial heat flow density from the GRIP inversion is 51.3 milliwatts per square meter. PMID:9765146

  11. Climate investigations using ice sheet and mass balance models with emphasis on North American glaciation

    NASA Astrophysics Data System (ADS)

    Birkel, Sean David

    This dissertation describes the application of the University of Maine Ice Sheet Model (UM-ISM) and Environmental Change Model (UM-ECM) to understanding mechanisms of ice-sheet/climate integration during ice ages. The UM-ECM, written by the author for this research, calculates equilibrium biome and snow/ice mass balance solutions for the globe based on modern input climatology and user-defined parameter values. The program was produced in conjunction with a National Science Foundation ITEST grant meant to seed inquiry-based classroom study of Earth systems using computer models. To that end, the UM-ECM serves as both a research and teaching tool. The model has a web-based interface, which has been tested with a group of middle school science teachers with a focus on local to global-scale climate learning. Initially, the UM-ISM and UM-ECM are used to reconstruct the former ice cap of the Wind River Mountains, Wyoming, in a companion study to a UMaine field research effort to document worldwide glacier recession during the last termination. It is found that the ice cap likely formed in response to a 5--6 °C cooling in conjunction with a precipitation doubling relative to modern conditions. Moreover, the maximum ice cap could have disappeared within 90 years if subjected to modem climate conditions. These results support hypotheses that the western U.S. became wetter during glacial stadials due to a southward-shifted North American storm track in response to Laurentide Ice Sheet orography, and that ice caps of the western U.S. are exceptionally sensitivity to climatic perturbation. The UMaine ice sheet and climate models are then used to assess the coupling between the Laurentide Ice Sheet and climate during ice-age cycles. It is shown that the classic "sawtooth" pattern of global sea-level change can be reproduced in the model by linking size of the polar atmospheric cell over eastern Canada to size of the Laurentide Ice Sheet and the magnitude of insolation forcing

  12. ISMIP6: Ice Sheet Model Intercomparison Project for CMIP6

    NASA Technical Reports Server (NTRS)

    Nowicki, S.

    2015-01-01

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

  13. Ice sheets play important role in climate change

    NASA Astrophysics Data System (ADS)

    Clark, Peter U.; MacAyeal, Douglas R.; Andrews, John T.; Bartlein, Patrick J.

    Ice sheets once were viewed as passive elements in the climate system enslaved to orbitally generated variations in solar radiation. Today, modeling results and new geologic records suggest that ice sheets actively participated in late-Pleistocene climate change, amplifying or driving significant variability at millennial as well as orbital timescales. Although large changes in global ice volume were ultimately caused by orbital variations (the Milankovitch hypothesis), once in existence, the former ice sheets behaved dynamically and strongly influenced regional and perhaps even global climate by altering atmospheric and oceanic circulation and temperature.Experiments with General Circulation Models (GCMs) yielded the first inklings of ice sheets' climatic significance. Manabe and Broccoli [1985], for example, found that the topographic and albedo effects of ice sheets alone explain much of the Northern Hemisphere cooling identified in paleoclimatic records of the last glacial maximum (˜21 ka).

  14. Importance of Nisar Mission for Ice Sheet Studies

    NASA Astrophysics Data System (ADS)

    Rignot, E. J.; Scheuchl, B.; Mouginot, J.; Morlighem, M.

    2014-12-01

    This presentation addresses how the synthetic-aperture radar (SAR) satellite mission under discussion between NASA and ISRO - entitled NISAR - will help us better understand and project the evolution of ice sheets and glaciers in a changing climate. NISAR is a dedicated L-band interferometry mission that will document changes in ice flow dynamics, grounding line positions and other critical boundaries over the lifetime of its mission. Changes in ice sheet dynamics represent by far the largest uncertainty in sea level projections. NISAR will better constrain critical boundaries of ice sheets at the base (basal friction) and at the seaward margins (ice melt rate) by providing the first set of continuous, systematic and comprehensive observations of ice sheet dynamics that will help us better understand ice sheets and glaciers and enable massive data assimilation in numerical ice sheet models. NISAR will contribute observations of areas of irreversible retreat taking place in Greenland and Antarctica, provide detailed time series of glacier velocities throughout entire seasonal cycles, document grounding line dynamics on weekly time scales, enable estimations of temporal and spatial changes in basal friction during glacial retreat; it will also in combination with other data help us map the bed topography of entire ice sheets at a high spatial resolution, document changes in ice shelf melt rate around the periphery of the continents, and provide a first systematic 3D vector mapping of ice velocity. NISAR will constitute a much needed warning system for ice sheet and ice shelf changes, it will document fundamental processes poorly observed in the past (e.g. calving, ice shelf melt, grounding line dynamics) and enable robust data assimilation to play a critical role in reducing uncertainties of coupled numerical models of ocean-ice-atmosphere interactions. This work was performed at UCI and JPL under a contract with NASA.

  15. The last deglaciation of the southeastern sector of the Scandinavian ice sheet.

    PubMed

    Rinterknecht, V R; Clark, P U; Raisbeck, G M; Yiou, F; Bitinas, A; Brook, E J; Marks, L; Zelcs, V; Lunkka, J-P; Pavlovskaya, I E; Piotrowski, J A; Raukas, A

    2006-03-10

    The Scandinavian Ice Sheet (SIS) was an important component of the global ice sheet system during the last glaciation, but the timing of its growth to or retreat from its maximum extent remains poorly known. We used 115 cosmogenic beryllium-10 ages and 70 radiocarbon ages to constrain the timing of three substantial ice-margin fluctuations of the SIS between 25,000 and 12,000 years before the present. The age of initial deglaciation indicates that the SIS may have contributed to an abrupt rise in global sea level. Subsequent ice-margin fluctuations identify opposite mass-balance responses to North Atlantic climate change, indicating differing ice-sheet sensitivities to mean climate state.

  16. Luminescence Chronology for the Formation of Glacial Lake Calgary, Southern Alberta, Canada: Age Constraints for the Initiation of the Late Pleistocene Retreat of the Laurentide Ice Sheet from its Western Margin

    NASA Astrophysics Data System (ADS)

    Munyikwa, K.; Rittenour, T. M.

    2014-12-01

    Glacial Lake Calgary in southern Alberta, Canada, was a Late Pleistocene proglacial lake that formed along the southwest margin of the Laurentide Ice Sheet (LIS), dammed by the retreating ice sheet margin. Attempts to constrain the age of the lake using radiocarbon methods have been hampered by the lack of datable organic material. In an effort to apply an alternative chronometer, this study uses two optically stimulated luminescence (OSL) dating approaches to date fine grained sand and silt that were deposited in the lake during its existence. OSL dating determines the depositional ages of sediments by measuring the energy from ionizing radiation that is stored in mineral grains such as quartz and feldspar. Dividing the stored energy, also referred to as the paleodose, by the rate at which the dose accumulated, allows an age to be ascertained. In one method applied in this study, the paleodose stored in the feldspar component of the sediment is determined using normalized infrared stimulated luminescence signals acquired using a portable OSL reader. In the second method, blue optically stimulated luminescence signals obtained from quartz separates from the sediment by employing a regular OSL reader and standard protocols are used to determine the paleodose. After correcting the feldspar data for anomalous fading, the age results from the two dating approaches are compared. The ages signify a time period by which the LIS had retreated from the study area and, hence, serve as constraints for the initiation of the retreat of the ice sheet from its western limit. Advantages and limitations of the dating methods are briefly discussed. Constraining the chronology of the retreat of the LIS from western Canada allows for a better understanding of the driving forces behind ice sheet retreat. Secondly, assigning a temporal scale to the postglacial evolution of the environment of the region permits a better insight into the dynamics of the physical and biological

  17. Modeling the fracture of ice sheets on parallel computers.

    SciTech Connect

    Waisman, Haim; Bell, Robin; Keyes, David; Boman, Erik Gunnar; Tuminaro, Raymond Stephen

    2010-03-01

    The objective of this project is to investigate the complex fracture of ice and understand its role within larger ice sheet simulations and global climate change. At the present time, ice fracture is not explicitly considered within ice sheet models due in part to large computational costs associated with the accurate modeling of this complex phenomena. However, fracture not only plays an extremely important role in regional behavior but also influences ice dynamics over much larger zones in ways that are currently not well understood. Dramatic illustrations of fracture-induced phenomena most notably include the recent collapse of ice shelves in Antarctica (e.g. partial collapse of the Wilkins shelf in March of 2008 and the diminishing extent of the Larsen B shelf from 1998 to 2002). Other fracture examples include ice calving (fracture of icebergs) which is presently approximated in simplistic ways within ice sheet models, and the draining of supraglacial lakes through a complex network of cracks, a so called ice sheet plumbing system, that is believed to cause accelerated ice sheet flows due essentially to lubrication of the contact surface with the ground. These dramatic changes are emblematic of the ongoing change in the Earth's polar regions and highlight the important role of fracturing ice. To model ice fracture, a simulation capability will be designed centered around extended finite elements and solved by specialized multigrid methods on parallel computers. In addition, appropriate dynamic load balancing techniques will be employed to ensure an approximate equal amount of work for each processor.

  18. History of the Greenland Ice Sheet: paleoclimatic insights

    USGS Publications Warehouse

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

    2010-01-01

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

  19. A Community Ice Sheet Model for Sea Level Prediction

    NASA Astrophysics Data System (ADS)

    Lipscomb, William; Bindschadler, Robert; Bueler, Ed; Holland, David; Johnson, Jesse; Price, Stephen

    2009-01-01

    Building a Next-Generation Community Ice Sheet Model; Los Alamos, New Mexico, 18-20 August 2008; Recent observations show that ice sheets can respond to climate change on annual to decadal timescales and that the Greenland and West Antarctic ice sheets are losing mass at an increasing rate. The current generation of ice sheet models cannot provide credible predictions of ice sheet retreat, as underscored by the Intergovernmental Panel on Climate Change (IPCC) in its Fourth Assessment Report (2007). The IPCC provided neither a best estimate nor an upper bound for 21st-century sea level rise because of uncertainties in the dynamic response of ice sheets. In response to this need, a workshop was held at Los Alamos National Laboratory (LANL). The workshop was sponsored by the LANL Institute for Geophysics and Planetary Physics, with additional support from the U.S. Department of Energy and National Science Foundation. The workshop's goal was to create a detailed plan (including commitments from individual researchers) for developing, testing, and implementing a Community Ice Sheet Model (CISM) to aid in predicting sea level rise. This model will be freely available to the glaciology and climate modeling communities and will be the ice sheet component of the Community Climate System Model (CCSM), a major contributor to IPCC assessments.

  20. 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).

  1. Rapid Holocene thinning of an East Antarctic outlet glacier driven by marine ice sheet instability

    PubMed Central

    Jones, R. S.; Mackintosh, A. N.; Norton, K. P.; Golledge, N. R.; Fogwill, C. J.; Kubik, P. W.; Christl, M.; Greenwood, S. L.

    2015-01-01

    Outlet glaciers grounded on a bed that deepens inland and extends below sea level are potentially vulnerable to ‘marine ice sheet instability'. This instability, which may lead to runaway ice loss, has been simulated in models, but its consequences have not been directly observed in geological records. Here we provide new surface-exposure ages from an outlet of the East Antarctic Ice Sheet that reveal rapid glacier thinning occurred approximately 7,000 years ago, in the absence of large environmental changes. Glacier thinning persisted for more than two and a half centuries, resulting in hundreds of metres of ice loss. Numerical simulations indicate that ice surface drawdown accelerated when the otherwise steadily retreating glacier encountered a bedrock trough. Together, the geological reconstruction and numerical simulations suggest that centennial-scale glacier thinning arose from unstable grounding line retreat. Capturing these instability processes in ice sheet models is important for predicting Antarctica's future contribution to sea level change. PMID:26608558

  2. Understanding Recent Mass Balance Changes of the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    vanderVeen, Cornelius

    2003-01-01

    The ultimate goal of this project is to better understand the current transfer of mass between the Greenland Ice Sheet, the world's oceans and the atmosphere, and to identify processes controlling the rate of this transfer, to be able to predict with greater confidence future contributions to global sea level rise. During the first year of this project, we focused on establishing longer-term records of change of selected outlet glaciers, reevaluation of mass input to the ice sheet and analysis of climate records derived from ice cores, and modeling meltwater production and runoff from the margins of the ice sheet.

  3. Response of the ice sheets to fluctuating temperatures

    NASA Astrophysics Data System (ADS)

    Bøgeholm Mikkelsen, Troels; Grinsted, Aslak; Ditlevsen, Peter

    2016-04-01

    Forecasting the future sea level relies on accurate modeling of the response of the Greenland and Antarctic ice sheets to changing tempera- tures. Using coupled climate and ice sheet models long time forecasting is often made computationally feasible by running the ice sheet model in off-line mode, such that the temperature and precipitation fields govern- ing the mass balance of the ice sheets are taken to be constant over time. As the temperature and precipitation fluctuates, the asymmetry in the typical time scales for accumulation and ablation would result in a bias in the resulting mass balance of the ice sheet. We show that the steady state of the ice sheet is biased toward larger size of the ice sheet, if the short time scale fluctuations in temperature are not taken into account. This could potentially imply that the critical global temperature increase for ice sheet collapse is overestimated, thus the risk of collapse in a given climate change scenario underestimated. Our results highlight the need to consider the variability and not only the mean of the forcing of the mass balance of the ice sheet. We estimate that the effect of temperature variability on surface mass balance of the Greenland Ice Sheet in recent ensemble forecasting should be adjusted downward by as much as 10 percent of the present day observed value, if assuming a 2 degree warming. We are thus closer to a potential tipping point, than previously anticipated. Many predicted scenarios of the future climate show an increased variability in temperature over much of the Earth. In light of the findings presented here, it is important to gauge the extent to which this increased variability will further influence climate change.

  4. Marine Ice Sheet Instability in the Former British-Irish Ice Sheet Linked to Rising Boreal Summer Insolation From 23 ka.

    NASA Astrophysics Data System (ADS)

    Small, D.

    2015-12-01

    Recent work has shown that marine sectors of the remaining ice sheets could exhibit rapid deglaciation linked to potential changes in oceanic forcing factors. Our ability to identify potential causes of marine ice sheet instability is limited by the scarcity of suitable analogues from the palaeo-record where evidence of rapid deglaciation can be linked to changes in the various potential forcing factors. Of the former Pleistocene ice sheets, the British-Irish Ice Sheet (BIIS) provides a useful analogue with large amounts of previous work providing detailed contextual information. Its western margin was marine terminating and drained by numerous fast flowing ice streams while its position next to a major surficial artery of the Atlantic Meridionial Overturning Circulation (AMOC) rendered it potentially sensitive to small climatic perturbations. Here we present new cosmogenic exposure ages that constrain the collapse of a marine sector of the former British and Irish Ice Sheet to ca. 20 ka. By comparing our new terrestrial dating constraints to proximal marine sediment records, including a new δ18ONps record, we are able to untangle the possible forcing mechanisms of this instability. The collapse occurs during an interval of generally cold surface conditions without significant large-scale oceanic reorganization. Increased calving driven by an increase in surface melt provides an alternative potential mechanism to link marine ice sheet instability to an increase in summer air temperature driven by rising boreal insolation after 23 ka suggesting potential hypersensitivity to small forcings with implications for understanding the vulnerability of marine sectors of remaining ice sheets.

  5. Antarctic marine ice sheet retreat in the Ross Sea during the early Holocene

    NASA Astrophysics Data System (ADS)

    Mckay, R. M.; Golledge, N.; Naish, T.; Maas, S.; Levy, R. H.; Kuhn, G.; Lee, J. I.; Dunbar, G. B.

    2015-12-01

    Geological constraints on the timing of the retreat of the Last Glacial Maximum (LGM) Antarctic Ice Sheets provide critical insights into the processes controlling marine-based ice sheet stability. The over-deepened, seaward shallowing bathymetry of Antarctica's continental shelves is ideally configured to promote past, and potentially future, marine ice-sheet instability. The retreat history of the LGM ice sheet in the Ross Sea region is primarily constrained by C-14 ages on coastal beach ridges and relict penguin colonies along the Transantarctic Mountain front in the Western Ross Sea. Although these terrestrial sites offer more reliable dates than imprecise C-14 chronologies derived from bulk marine sediments, they may reflect retreat of local piedmont glaciers derived from East Antarctic outlet glaciers rather than representing the timing of retreat of the ice sheet in the central Ross Embayment. We present a sedimentary facies succession and foraminifera-based C-14 chronology from a core collected beneath the Ross Ice Shelf via a hot water drill access hole used for the ANDRILL Coulman High site survey. The site is to the east of Ross Island and distal from the coast, and yields a minimum age for glacial retreat that is approximately 1000 yrs earlier than suggested by coastal records along the nearby Victoria Land coast. We examine the implications of this constraint on the timing of ice sheet retreat in the context of model simulations and new multi-beam bathymetry data acquired in the Western Ross Sea. On the basis of these data we hypothesize that marine-based ice sheet retreat was triggered by oceanic forcings along most of the Pacific Ocean coastline of Antarctica simultaneously, but continued retreat in the Ross Sea occurred primarily as a consequence of marine ice sheet instability.

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

    PubMed

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

    2016-05-01

    The Greenland ice sheet presently accounts for ~70% of global ice sheet mass loss. Because this mass loss is associated with sea-level rise at a rate of 0.7 mm/year, the development of improved monitoring techniques to observe ongoing changes in ice sheet mass balance is of paramount concern. Spaceborne mass balance techniques are commonly used; however, they are inadequate for many purposes because of their low spatial and/or temporal resolution. We demonstrate that small variations in seismic wave speed in Earth's crust, as measured with the correlation of seismic noise, may be used to infer seasonal ice sheet mass balance. Seasonal loading and unloading of glacial mass induces strain in the crust, and these strains then result in seismic velocity changes due to poroelastic processes. Our method provides a new and independent way of monitoring (in near real time) ice sheet mass balance, yielding new constraints on ice sheet evolution and its contribution to global sea-level changes. An increased number of seismic stations in the vicinity of ice sheets will enhance our ability to create detailed space-time records of ice mass variations. PMID:27386524

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

    PubMed

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

    2016-05-01

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

  8. Antarctic Peninsula Ice Sheet evolution during the Cenozoic Era

    NASA Astrophysics Data System (ADS)

    Davies, Bethan J.; Hambrey, Michael J.; Smellie, John L.; Carrivick, Jonathan L.; Glasser, Neil F.

    2012-01-01

    The Antarctic Peninsula region is currently undergoing rapid environmental change, resulting in the thinning, acceleration and recession of glaciers and the sequential collapse of ice shelves. It is important to view these changes in the context of long-term palaeoenvironmental complexity and to understand the key processes controlling ice sheet growth and recession. In addition, numerical ice sheet models require detailed geological data for tuning and testing. Therefore, this paper systematically and holistically reviews published geological evidence for Antarctic Peninsula Ice Sheet variability for each key locality throughout the Cenozoic, and brings together the prevailing consensus of the extent, character and behaviour of the glaciations of the Antarctic Peninsula region. Major contributions include a downloadable database of 186 terrestrial and marine calibrated dates; an original reconstruction of the LGM ice sheet; and a new series of isochrones detailing ice sheet retreat following the LGM. Glaciation of Antarctica was initiated around the Eocene/Oligocene transition in East Antarctica. Palaeogene records of Antarctic Peninsula glaciation are primarily restricted to King George Island, where glacigenic sediments provide a record of early East Antarctic glaciations, but with modification of far-travelled erratics by local South Shetland Island ice caps. Evidence for Neogene glaciation is derived primarily from King George Island and James Ross Island, where glaciovolcanic strata indicate that ice thicknesses reached 500-850 m during glacials. This suggests that the Antarctic Peninsula Ice Sheet draped, rather than drowned, the topography. Marine geophysical investigations indicate multiple ice sheet advances during this time. Seismic profiling of continental shelf-slope deposits indicates up to ten large advances of the Antarctic Peninsula Ice Sheet during the Early Pleistocene, when the ice sheet was dominated by 40 kyr cycles. Glacials became more

  9. Timing of deglaciation of the Late Wisconsin Cordilleran ice sheet in Yukon Territory

    NASA Astrophysics Data System (ADS)

    Bond, J. D.; Ward, B. C.; Gosse, J. C.

    2012-12-01

    Deglaciation of the northern margin of the Cordilleran ice sheet and its rate of recession from the last glacial maximum (LGM) is poorly understood. The northern Cordilleran ice sheet consisted of a series of quasi-independent ice lobes that coalesced during the LGM to form a continuous carapace of ice over southern Yukon. To reconstruct the recession history of the ice sheet we focused on the the Cassiar lobe, which covered the south-central part of the Yukon, including Whitehorse. The main accumulation area for Cassiar lobe was in north-central British Columbia and included ice originating from the eastern Coast Mountains and the Cassiar Mountains. We used cosmogenic nuclide exposure dating (mainly 10Be) on glacial erratics to reconstruct timing and rate of deglaciation. Glacial erratics were sampled in three different locations including near the glacial limit, a mid-deglaciation setting at Whitehorse and in the eastern Coast Mountain accumulation zone at White Pass. Deglacial ages from different source areas will also be reported. This reconstruction provides a useful chronology for deglacial modeling of the ice sheet, ages that can be applied to deglacial landforms in the Whitehorse area and the timing of landscape change with respect to new opportunities for human migration following the last ice age.

  10. Quaternary and Tertiary microfossils from beneath Ice Stream B: Evidence for a dynamic West Antarctic Ice Sheet history

    NASA Astrophysics Data System (ADS)

    Scherer, Reed P.

    1991-10-01

    Some glaciologists have suggested that the West Antarctic Ice Sheet, which is grounded well below sea level, may be susceptible to rapid grounding-line retreat and disintegration. However, until now, geologic evidence of previous ice sheet "collapses" has been lacking. Sediments that have recently been collected from beneath the West Antarctic Ice Sheet at Ice Stream B contain direct evidence of ice-free conditions in the West Antarctic interior during certain Cenozoic intervals, both prior to and subsequent to the development of grounded ice sheets in West Antarctica. The sediments contain rare but diverse microfossils that represent a wide variety of biostratigraphic ages and depositional environments. Microfossils present include relatively common marine and non-marine diatoms and sponge spicules, plus rare foraminifera, nannofossils, radiolarians, silicoflagellates, chrysophyte cysts and palynomorphs. Clasts of Neogene freshwater diatomite demonstrate the former presence of large lake systems in West Antarctica, possibly as part the Cenozoic West Antarctica rift system. Age-diagnostic marine fossils in the sediment include Late Paleogene calcareous nannofossils and planktonic foraminifera, Miocene marine planktonic diatoms and, significantly, late Pleistocene marine diatoms. Relatively common late Miocene diatoms probably reflect marine deposition prior to initiation of a dominantly glacial phase in West Antarctica. It is likely that Pliocene and early Pleistocene diatoms were deposited in the West Antarctic interior during certain warm interglacials, but these have been eroded and transported toward the continental shelf edge during repeated ice sheet expansions. The late Pleistocene diatoms from Upstream B were deposited in the West Antarctic interior basins during a marine phase, subsequent to an ice sheet collapse, during at least one late Pleistocene interglacial. This discovery provides an indication of the complex history of the West Antarctic Ice Sheet

  11. Modeling the Fracture of Ice Sheets on Parallel Computers

    SciTech Connect

    Waisman, Haim; Tuminaro, Ray

    2013-10-10

    The objective of this project was to investigate the complex fracture of ice and understand its role within larger ice sheet simulations and global climate change. This objective was achieved by developing novel physics based models for ice, novel numerical tools to enable the modeling of the physics and by collaboration with the ice community experts. At the present time, ice fracture is not explicitly considered within ice sheet models due in part to large computational costs associated with the accurate modeling of this complex phenomena. However, fracture not only plays an extremely important role in regional behavior but also influences ice dynamics over much larger zones in ways that are currently not well understood. To this end, our research findings through this project offers significant advancement to the field and closes a large gap of knowledge in understanding and modeling the fracture of ice sheets in the polar regions. Thus, we believe that our objective has been achieved and our research accomplishments are significant. This is corroborated through a set of published papers, posters and presentations at technical conferences in the field. In particular significant progress has been made in the mechanics of ice, fracture of ice sheets and ice shelves in polar regions and sophisticated numerical methods that enable the solution of the physics in an efficient way.

  12. Evidence for a former large ice sheet in the Orville Coast- Ronne Ice Shelf area, Antarctica.

    USGS Publications Warehouse

    Carrara, P.

    1981-01-01

    The Orville Coast area of the Antarctic Peninsula was extensively glacierized in the past. Striations, polished rock surfaces, and erratics on nunatak summits indicate that this area was covered by a broad regional ice sheet whose grounded ice margin was on the continental shelf, in the present-day Ronne Ice Shelf area. If the glacial history of Antarctica has been controlled by eustatic sea-level changes, the destruction of this ice sheet would have been contemporaneous with that of the Ross Sea ice sheet due to the world-wide rise of eustatic sea-level at the end of the Wisconsin glaciation. -Author

  13. How can blue-ice moraines constrain elevation changes of the West Antarctic Ice Sheet in the southern Ellsworth Mountains?

    NASA Astrophysics Data System (ADS)

    Sugden, D.; Woodward, J.; Marrero, S.; Hein, A.; Dunning, S.

    2013-12-01

    Observational data in the Weddell Sea sector of the Antarctic Ice Sheet has not yet allowed the dating of elevated glacier trimlines and associated deposits in the Ellsworth Mountains. This uncertainty limits the value of models of changing ice-sheet configuration, volume and, by extension, sea level during glacial cycles and earlier. Here we present the first results of a study into the origin and evolution of blue-ice moraines in the Heritage Range, southern Ellsworth Mountains and begin to unravel the long record of ice-sheet history they hold. Our findings so far are: (a) ground penetrating radar shows boulders and finer debris in the glacier ice and that they are related to ice structures and to basal debris emerging at the glacier surface, (b) exposure ages of surface debris reveals boulders both recently exposed and those pre-dating the Last Glacial Maximum (LGM), (c) during the LGM the ice was thicker and the limit is marked by a zone of perched boulders and, in topographic hollows, by buried glacier ice, (d) weathered high elevation deposits with exposure ages in excess of 400 ka have been overridden by glacier ice and thus may represent deposits of much greater age, (e) the relationship between blue-ice moraines and deposits from local wind-drift glaciers makes it possible to constrain the date and duration of Holocene thinning. A forthcoming field season in 2013-2014 to re-measure 90 stakes for horizontal movement and ablation should help quantify the rate of blue-ice moraine formation. We will also obtain rock cores to establish rates of weathering of the older deposits and thus constrain their age more tightly.

  14. Paleofluvial mega-canyon beneath the central Greenland ice sheet.

    PubMed

    Bamber, Jonathan L; Siegert, Martin J; Griggs, Jennifer A; Marshall, Shawn J; Spada, Giorgio

    2013-08-30

    Subglacial topography plays an important role in modulating the distribution and flow of basal water. Where topography predates ice sheet inception, it can also reveal insights into former tectonic and geomorphological processes. Although such associations are known in Antarctica, little consideration has been given to them in Greenland, partly because much of the ice sheet bed is thought to be relatively flat and smooth. Here, we present evidence from ice-penetrating radar data for a 750-km-long subglacial canyon in northern Greenland that is likely to have influenced basal water flow from the ice sheet interior to the margin. We suggest that the mega-canyon predates ice sheet inception and will have influenced basal hydrology in Greenland over past glacial cycles. PMID:23990558

  15. Paleofluvial mega-canyon beneath the central Greenland ice sheet.

    PubMed

    Bamber, Jonathan L; Siegert, Martin J; Griggs, Jennifer A; Marshall, Shawn J; Spada, Giorgio

    2013-08-30

    Subglacial topography plays an important role in modulating the distribution and flow of basal water. Where topography predates ice sheet inception, it can also reveal insights into former tectonic and geomorphological processes. Although such associations are known in Antarctica, little consideration has been given to them in Greenland, partly because much of the ice sheet bed is thought to be relatively flat and smooth. Here, we present evidence from ice-penetrating radar data for a 750-km-long subglacial canyon in northern Greenland that is likely to have influenced basal water flow from the ice sheet interior to the margin. We suggest that the mega-canyon predates ice sheet inception and will have influenced basal hydrology in Greenland over past glacial cycles.

  16. Using blue-ice moraines to constrain elevation changes of the West Antarctic Ice Sheet in the southern Ellsworth Mountains

    NASA Astrophysics Data System (ADS)

    Sugden, David; Woodward, John; Dunning, Stuart; Hein, Andy; Marrero, Shasta; Le-Brocq, Anne

    2014-05-01

    Observations in the Weddell Sea sector of the Antarctic Ice Sheet have not yet allowed the dating of elevated glacier trimlines and associated deposits in the Ellsworth Mountains. This uncertainty limits the value of models of changing ice-sheet configuration, volume and, by extension, sea level during glacial cycles and earlier. Here we present the emerging results of a study into the origin and evolution of blue-ice moraines in the Heritage Range, southern Ellsworth Mountains, and begin to unravel the long record of ice-sheet history they hold. Our findings so far are: (a) Ground Penetrating Radar shows that the blue-ice moraines are equilibrium forms bringing basal debris to the ice surface; the compressive ice flow is caused by enhanced ablation at the mountain foot. (b) Moraines are concentrated in embayments that focus katabatic winds and their location is largely controlled by topography. (c) The elevated blue-ice moraines in the southern Ellsworth Mountains hold a continuous record of West Antarctic Ice Sheet history going back 600,000 years; so far we have not found evidence of de-glacial intervals. (d) Thinning since the LGM (~40 ka?) is < 450 m and agrees with views of modest changes in the Weddell Sea sector during glacial cycles; most thinning occurred in the Holocene (6-3 ka). (e) Downslope flow of debris-covered ice in embayments follows ice surface lowering; it transports old clasts downslope and exposes fresh clasts, thus complicating the interpretation of exposure ages. We hope that a second field season in 2014 to re-measure 90 stakes for horizontal movement and ablation will help quantify the rate of blue-ice moraine formation.

  17. Climatic warming and basal melting of large ice sheets: possible implications for East Antarctica

    SciTech Connect

    Saari, M.R.; Yuen, D.A.; Schubert, G.

    1987-01-01

    Climatic warming is shown to be capable of inducing shear heating instability and basal melting in a model ice sheet that is creeping slowly downslope. Growth times of the instability are calculated from a nonlinear analysis of temperature and flow in the model ice sheet whose surface undergoes a prescribed increase of temperature. The source of instability lies in the decrease of maximum ice thickness for steady downslope creep with increasing surface temperature. A surface temperature increase of 5 to 10 k can cause instability on a 10/sup 4/ year time scale for realistic ice rheology. The instability occurs suddenly after a prolonged period of dormancy. The instability might be relevant to the East Antarctic ice sheet. Warming associated with the Holocene interglacial epoch that heralded the end of the last ice age may have set the East Antarctic ice sheet on a course toward wide-spread instability some 10/sup 4/ years later. The present CO/sub 2/-induced climate warming is also a potential trigger for instability and basal melting of the East Antarctic ice sheet.

  18. Reconstructing the dynamics of the Greenland ice sheet during the last deglaciation

    NASA Astrophysics Data System (ADS)

    Keisling, Benjamin; DeConto, Robert

    2016-04-01

    Today, some outlet glaciers of the Greenland ice sheet (GrIS) are rapidly retreating and may mobilize large volumes of interior ice in the coming centuries. The last period that saw such dramatic, sustained retreat of the GrIS was the last deglaciation, when the ice sheet retreated from its Last Glacial Maximum (LGM) extent. Previous studies have used relative sea level observations to constrain changes in ice thickness and retreat timing during the deglaciation (e.g. Fleming and Lambert 2004, Simpson et al. 2009, Lecavalier et al. 2014). Here we build on these studies by isolating the drivers of ice-sheet retreat, and their spatial and temporal dynamics, during this period. Inclusion of ice-cliff failure and hydrofracturing parameterizations in our model has resulted in a better fit to paleodata for the Antarctic ice sheet, but this modeling approach has not been applied to the GrIS. Here we use a three-dimensional hybrid SSA/SIA ice-sheet model (Pollard et al. 2015) at 10km resolution over Greenland to simulate the last deglaciation. Boundary conditions for the last glacial maximum produce an LGM ice sheet with 3.81 meters sea level equivalent (m s.l.e.) of additional ice. The LGM ice sheet advances to the shelf-break in west, south, and east Greenland with an expansive ice shelf extending across Davis Strait. Applying modern atmospheric and oceanic forcing to the LGM ice sheet yields 1.25 and 1.09 m s.l.e. of melt, respectively, and 1.72 m s.l.e. for both. Ocean warming initially results in a higher rate and magnitude of retreat, but increased surface evaporation over open water results in additional accumulation that offsets losses in 10 kyr simulations. Here, we test the sensitivity of the magnitude of deglacial ice-sheet retreat to uncertainty in bedrock elevation and basal slding coefficients, the applied climate forcing, and the mass balance scheme (positive degree-day or energy balance). We also implement a deglacial climate forcing based on recently

  19. Laurentide ice-sheet instability during the last deglaciation

    NASA Astrophysics Data System (ADS)

    Ullman, David J.; Carlson, Anders E.; Anslow, Faron S.; Legrande, Allegra N.; Licciardi, Joseph M.

    2015-07-01

    Changes in the amount of summer incoming solar radiation (insolation) reaching the Northern Hemisphere are the underlying pacemaker of glacial cycles. However, not all rises in boreal summer insolation over the past 800,000 years resulted in deglaciation to present-day ice volumes, suggesting that there may be a climatic threshold for the disappearance of land-based ice. Here we assess the surface mass balance stability of the Laurentide ice sheet--the largest glacial ice mass in the Northern Hemisphere--during the last deglaciation (24,000 to 9,000 years ago). We run a surface energy balance model with climate data from simulations with a fully coupled atmosphere-ocean general circulation model for key time slices during the last deglaciation. We find that the surface mass balance of the Laurentide ice sheet was positive throughout much of the deglaciation, and suggest that dynamic discharge was mainly responsible for mass loss during this time. Total surface mass balance became negative only in the early Holocene, indicating the transition to a new state where ice loss occurred primarily by surface ablation. We conclude that the Laurentide ice sheet remained a viable ice sheet before the Holocene and began to fully deglaciate only once summer temperatures and radiative forcing over the ice sheet increased by 6-7 °C and 16-20 W m-2, respectively, relative to full glacial conditions.

  20. A time-dependent ice sheet model - Preliminary results

    NASA Technical Reports Server (NTRS)

    Bindschadler, R. A.; Gore, R.

    1982-01-01

    A numerical model of ice sheet flow is developed, and preliminary results are described. This model includes vertical resolution of temperature, stress, and strain rate which represents a considerable improvement over previous vertically averaged ice sheet models. The model follows the flow of ice along a flow line within an ice sheet drainage basin. Longitudinal stresses and basal sliding are included. Basal sliding is dependent on the base shear stress and a specified distribution of basal water pressure. The numerical methods used to solve the coupled set of stress and velocity equations for the static and time-evolutionary cases are discussed. A steady state profile simulating an ice stream is calculated for a particular set of input parameters, and changes in the profile are examined for different choices of parameters. Preliminary studies of response behavior are completed using a simplified ice sheet geometry with a fixed terminus or grounding line. The results of these studies illustrate ice sheet thinning in response to a lowered sea level or to a reduction in the extent of ice rises (or pinning points) within ice shelves.

  1. Permafrost-ice-sheet interactions during the Quaternary

    NASA Astrophysics Data System (ADS)

    Willeit, Matteo; Ganopolski, Andrey

    2016-04-01

    Permafrost influences a number of processes which are relevant for local and global climate. For example, it is well known that permafrost plays an important role in global carbon and methane cycles. Less is known about the interaction between permafrost and ice sheets. We recently included a permafrost module in the Earth system model CLIMBER-2 to explore the coupled Northern Hemisphere (NH) permafrost-ice-sheet evolution during the Quaternary. The model has been shown to perform generally well at reproducing present-day permafrost extent and thickness. Modelled permafrost extent at the Last Glacial Maximum (LGM) agrees well with reconstructions and previous modeling estimates. In a previous study we showed that over the last glacial cycle permafrost has a relatively modest impact on simulated NH ice sheet volume except at LGM, when including permafrost increases ice volume by about 15 m sea level equivalent in our model. This is explained by a delayed melting of the ice base from below by the geothermal heat flux when the ice sheet sits on a porous sediment layer and permafrost has to be melted first. Permafrost affects ice sheet dynamics only when ice extends over areas covered by thick sediments, which is the case at LGM. In transient model simulations of the "40 kyr world" of the early Pleistocene we show that when all continents are covered by a thick sediment layer the response of ice volume to the obliquity component of orbital forcing is enhanced while the response to precession is dampened. We therefore argue that permafrost could have played a role for ice sheet evolution when all continents were covered by a thick sediment layer, as was likely the case in the early Pleistocene before the sediment layer was gradually eroded by expanding ice sheets over parts of northern Canada and Scandinavia.

  2. Recent ice ages on Mars.

    PubMed

    Head, James W; Mustard, John F; Kreslavsky, Mikhail A; Milliken, Ralph E; Marchant, David R

    2003-12-18

    A key pacemaker of ice ages on the Earth is climatic forcing due to variations in planetary orbital parameters. Recent Mars exploration has revealed dusty, water-ice-rich mantling deposits that are layered, metres thick and latitude dependent, occurring in both hemispheres from mid-latitudes to the poles. Here we show evidence that these deposits formed during a geologically recent ice age that occurred from about 2.1 to 0.4 Myr ago. The deposits were emplaced symmetrically down to latitudes of approximately 30 degrees--equivalent to Saudi Arabia and the southern United States on the Earth--in response to the changing stability of water ice and dust during variations in obliquity (the angle between Mars' pole of rotation and the ecliptic plane) reaching 30-35 degrees. Mars is at present in an 'interglacial' period, and the ice-rich deposits are undergoing reworking, degradation and retreat in response to the current instability of near-surface ice. Unlike the Earth, martian ice ages are characterized by warmer polar climates and enhanced equatorward transport of atmospheric water and dust to produce widespread smooth deposits down to mid-latitudes.

  3. Improved Timing of Deglaciation of the Southwestern Scandinavian Ice Sheet Using 10Be Dating

    NASA Astrophysics Data System (ADS)

    Gump, D.; Briner, J. P.; Svendsen, J. I.; Mangerud, J.

    2015-12-01

    We present 28 new 10Be ages from glacial erratic boulders to constrain Scandinavian Ice Sheet deglaciation along the major fjord system of Boknafjorden in southwest Norway. Results indicate ages in the range 20-14 ka and complement our previous findings that the Norwegian Channel Ice Stream (NCIS) had retreated some 400 km as early as ~20 ka (Svendsen et al., 2015) and further corroborate that this was followed by a second pulse of deglaciation at ~16 ka. After the immediate coast was rendered an ice-free corridor at ~20 ka, our new suite of ages identifies ~16 ka as a period of a possible culmination of re-advance, and almost certainly the onset of a subsequent period of retreat. These findings are promising for the possibility of long lake sediment archives from areas around the mouth of Boknafjorden. Additionally, by coupling our new 10Be ages of erratic boulders from sea level and from summits bordering Boknafjorden with topographic profiles and rudimentary ice-sheet profile calculations (Benn and Hulton, 2010), we are able to estimate spatial and temporal Scandinavian Ice Sheet history along both vertical and horizontal transects. Our results not only fill chronological gaps and add to a growing database of ages of deglaciation from the southwest Norway, but also provide new constraints for a three-dimensional reconstruction of the Scandinavian Ice Sheet during deglaciation.

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

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

  5. Basal Dynamics and Internal Structure of Ice Sheets

    NASA Astrophysics Data System (ADS)

    Wolovick, Michael J.

    The internal structure of ice sheets reflects the history of flow and deformation experienced by the ice mass. Flow and deformation are controlled by processes occurring within the ice mass and at its boundaries, including surface accumulation or ablation, ice rheology, basal topography, basal sliding, and basal melting or freezing. The internal structure and basal environment of ice sheets is studied with ice-penetrating radar. Recently, radar observations in Greenland and Antarctica have imaged large englacial structures rising from near the bed that deform the overlying stratigraphy into anticlines, synclines, and overturned folds. The mechanisms that may produce these structures include basal freeze-on, travelling slippery patches at the ice base, and rheological contrasts within the ice column. In this thesis, I explore the setting and mechanisms that produce large basal stratigraphic structures inside ice sheets. First, I use radar data to map subglacial hydrologic networks that deliver meltwater uphill towards freeze-on structures in East Antarctica. Next, I use a thermomechanical flowline model to demonstrate that trains of alternating slippery and sticky patches can form underneath ice sheets and travel downstream over time. The disturbances to the ice flow field produced by these travelling patches produce stratigraphic folds resembling the observations. I then examine the overturned folds produced by a single travelling sticky patch using a kinematic flowline model. This model is used to interpret stratigraphic measurements in terms of the dynamic properties of basal slip. Finally, I use a simple local one-dimensional model to estimate the thickness of basal freeze-on that can be produced based on the supply of available meltwater, the thermal boundary conditions, ice sheet geometry, and the ice flow regime.

  6. Deglaciation of a Soft-Bedded Laurentide Ice Sheet

    NASA Astrophysics Data System (ADS)

    Licciardi, Joseph M.; Clark, Peter U.; Jenson, John W.; Macayeal, Douglas R.

    We present a series of numerical reconstructions of the Laurentide Ice Sheet during the last deglaciation (18-7 14C ka) that evaluates the sensitivity of ice-sheet geometry to subglacial sediment deformation. These reconstructions assume that the Laurentide Ice Sheet flowed over extensive areas of water-saturated, deforming sediment (soft beds) corresponding to the St. Lawrence lowland, the Great Lakes region, the western prairies of the U.S. and Canada, and the Hudson Bay and Hudson Strait regions. Sediment rheology is based on a constitutive law that incorporates experimental results from late Wisconsin till deposited by the Laurentide Ice Sheet which suggest only mildly nonlinear viscoplastic behavior. By varying the effective viscosity of till, we produced four reconstructions for the ice sheet during the last glacial maximum 18 14C ka, and two reconstructions each of the ice sheet at 14, 13, 12, 11 and 10 14C ka. We also produced one reconstruction for 9, 8.4, 8, and 7 14C ka. Reconstructions that assume a low effective viscosity for all areas of deforming sediment show a multidomed ice sheet with a large bowl-shaped depression over Hudson Bay and thin ice (<1000 m above modern sea-level) over the western and southern margins. Those reconstructions that assume a higher effective viscosity of till in the Hudson Bay region than for the western and southern margins also show a multidomed ice sheet but with considerably thicker ice over Hudson Bay and a more northerly position of the central ice divide. These two different geometries may represent ice-sheet orographic changes associated with a Heinrich event. Further increases in effective viscosity of till, approaching the effective viscosity of ice, would result in a high, monolithic ice dome centered over Hudson Bay, reinforcing the notion that a multidomed ice sheet reflects the distribution of substrate geology. Modeled ice-surface geometry at the last glacial maximum shows many of the same general features

  7. Ice sheets viewed from the ocean: the contribution of marine science to understanding modern and past ice sheets.

    PubMed

    Ó Cofaigh, Colm

    2012-12-13

    Over the last two decades, marine science, aided by technological advances in sediment coring, geophysical imaging and remotely operated submersibles, has played a major role in the investigation of contemporary and former ice sheets. Notable advances have been achieved with respect to reconstructing the extent and flow dynamics of the large polar ice sheets and their mid-latitude counterparts during the Quaternary from marine geophysical and geological records of landforms and sediments on glacier-influenced continental margins. Investigations of the deep-sea ice-rafted debris record have demonstrated that catastrophic collapse of large (10(5)-10(6) km(2)) ice-sheet drainage basins occurred on millennial and shorter time scales and had a major influence on oceanography. In the last few years, increasing emphasis has been placed on understanding physical processes at the ice-ocean interface, particularly at the grounding line, and on determining how these processes affect ice-sheet stability. This remains a major challenge, however, owing to the logistical constraints imposed by working in ice-infested polar waters and ice-shelf cavities. Furthermore, despite advances in reconstructing the Quaternary history of mid- and high-latitude ice sheets, major unanswered questions remain regarding West Antarctic ice-sheet stability, and the long-term offshore history of the East Antarctic and Greenland ice sheets remains poorly constrained. While these are major research frontiers in glaciology, and ones in which marine science has a pivotal role to play, realizing such future advances will require an integrated collaborative approach between oceanographers, glaciologists, marine geologists and numerical modellers. PMID:23129711

  8. Radar attenuation and temperature within the Greenland Ice Sheet

    USGS Publications Warehouse

    MacGregor, Joseph A; Li, Jilu; Paden, John D; Catania, Ginny A; Clow, Gary D.; Fahnestock, Mark A; Gogineni, Prasad S.; Grimm, Robert E.; Morlighem, Mathieu; Nandi, Soumyaroop; Seroussi, Helene; Stillman, David E

    2015-01-01

    The flow of ice is temperature-dependent, but direct measurements of englacial temperature are sparse. The dielectric attenuation of radio waves through ice is also temperature-dependent, and radar sounding of ice sheets is sensitive to this attenuation. Here we estimate depth-averaged radar-attenuation rates within the Greenland Ice Sheet from airborne radar-sounding data and its associated radiostratigraphy. Using existing empirical relationships between temperature, chemistry, and radar attenuation, we then infer the depth-averaged englacial temperature. The dated radiostratigraphy permits a correction for the confounding effect of spatially varying ice chemistry. Where radar transects intersect boreholes, radar-inferred temperature is consistently higher than that measured directly. We attribute this discrepancy to the poorly recognized frequency dependence of the radar-attenuation rate and correct for this effect empirically, resulting in a robust relationship between radar-inferred and borehole-measured depth-averaged temperature. Radar-inferred englacial temperature is often lower than modern surface temperature and that of a steady state ice-sheet model, particularly in southern Greenland. This pattern suggests that past changes in surface boundary conditions (temperature and accumulation rate) affect the ice sheet's present temperature structure over a much larger area than previously recognized. This radar-inferred temperature structure provides a new constraint for thermomechanical models of the Greenland Ice Sheet.

  9. Seasonal Greenland Ice Sheet ice flow variations in regions of differing bed and surface topography

    NASA Astrophysics Data System (ADS)

    Sole, A. J.; Livingstone, S. J.; Rippin, D. M.; Hill, J.; McMillan, M.; Quincey, D. J.

    2015-12-01

    The contribution of the Greenland Ice Sheet (GrIS) to future sea-level rise is uncertain. Observations reveal the important role of basal water in controlling ice-flow to the ice sheet margin. In Greenland, drainage of large volumes of surface meltwater to the ice sheet bed through moulins and hydrofracture beneath surface lakes dominates the subglacial hydrological system and provides an efficient means of moving mass and heat through the ice sheet. Ice surface and bed topography influence where meltwater can access the bed, and the nature of its subsequent flow beneath the ice. However, no systematic investigation into the influence of topographic variability on Greenland hydrology and dynamics exists. Thus, physical processes controlling storage and drainage of surface and basal meltwater, and the way these affect ice flow are not comprehensively understood. This presents a critical obstacle in efforts to predict the future evolution of the GrIS. Here we present high-resolution satellite mapping of the ice-surface drainage network (e.g. lakes, channels and moulins) and measurements of seasonal variations in ice flow in south west Greenland. The region is comprised of three distinct subglacial terrains which vary in terms of the amplitude and wavelength and thus the degree to which basal topography is reflected in the ice sheet surface. We find that the distribution of surface hydrological features is related to the transfer of bed topography to the ice sheet surface. For example, in areas of thinner ice and high bed relief, moulins occur more frequently and are more uniformly dispersed, indicating a more distributed influx of surface-derived meltwater to the ice sheet bed. We investigate the implications of such spatial variations in surface hydrology on seasonal ice flow rates.

  10. Sturgis and Tekonsha ice advances: Evidence for thin ice sheets in southern Michigan

    SciTech Connect

    Straw, W.T.; Kehew, A.E.; Passero, R.N. . Dept. of Geology)

    1993-03-01

    Advance of an ice sheet through the Saginaw Lowland to the position of Sturgis, in St. Joseph County, Michigan initially produced the Sturgis Moraine and adjacent outwash apron. As this ice sheet down wasted, meltwater impounded by it overtopped ice divides to form meltwater streams that cut canyons in the ice and underlying glacial sediments. Continued wasting was attended by formation of supraglacial streams that developed waterfalls as they eroded headward. Complete melting of the ice revealed a subglacial topography marked by drumlins, many of which are irregular, attesting to formation near the attenuated margin of the ice sheet. Prior to complete wasting of this stagnant ice mass, ice of the Lake Michigan Lobe advanced from the west-northwest to form the Tekonsha Moraine. Exposed only where this relatively thin ice sheet advanced onto a highland in east-central Kalamazoo County, the southwestern extension of this moraine was buried by outwash from ice that formed the very prominent Kalamazoo Moraine. That the Tekonsha ice sheet stagnated and wasted in place is revealed by two prominent elongate kames, several escarpments, lake and wetland basins produced by melting of ice masses buried by glacial outwash, and sequential filling of the lowland produced by melting of this ice sheet by a series of alluvial fans. The sequence of fan emplacement is indicated by marginal relationships of the fans and the number and size of depressions formed by melting of relic blocks of ice. The first-formed Dry Prairie fan is marked by numerous large depressions while the Prairie Ronde, the last formed fan exhibits only small widely spaced depressions formed by melting of the last vestiges of this ice sheet.

  11. Evidence for an extensive Antarctic Ice Sheet by 37 Ma

    NASA Astrophysics Data System (ADS)

    Carter, Andrew; Riley, Teal; Hillenbrand, Claus-Dieter; Rittner, Martin

    2016-04-01

    We present observational evidence that both the East and West Antarctic ice sheets had expanded to the coast by 37 Ma, predating, by at least 3 Myr, a major drop in atmospheric CO2 at the Eocene-Oligocene boundary widely considered responsible for Antarctic Ice Sheet expansion. Our evidence comes from the provenance (geochronology, thermochronometry, mineralogy) of iceberg-rafted debris identified in Late Eocene marine sediments from (ODP) Leg 113 Site 696 in the NW Weddell Sea. The existence of an significant Antarctic Ice Sheet in a Late Eocene high pCO2 world calls into question the role of atmospheric CO2 concentrations as the dominant mechanism for ice sheet expansion and whether topography and ocean circulation only play a secondary role.

  12. Correlating Cordilleran Ice Sheet Collapse with North Atlantic Heinrich Events using Global Radiocarbon Plateaus.

    NASA Astrophysics Data System (ADS)

    Hendy, I. L.; Cosma, T.

    2006-12-01

    The small, ephemeral Cordilleran Ice Sheet, present in Alaska, British Columbia and northern Washington during the last glacial cycle is believed to have behaved differently than the larger Laurentice Ice Sheet to climate forcing. High quality chronology is required to understand the relationship between this ice sheet and global climate change. Presently MD02-2496 (48°58.47N: 127°02.14W; 1190m water depth) is the highest resolution paleoclimate record available for the last 50 Ka in the Pacific northwest and contains intervals of glacial-marine sedimentation. High resolution dating based on 36 radiocarbon dates provide a chronology that includes radiocarbon age plateaus, while planktonic foraminiferal stable isotopes offer a continuous record of climate change. Glacial-marine sediments collected from this site on the continental slope west of Vancouver Island, British Columbia, document three intervals of iceberg discharge during the last ~50 Ka. Gradually increasing quantities of ice rafted detritus (grains >250μm, g-1; IRD) followed by abrupt cessation within ~500 years is strongly suggestive of catastrophic iceberg discharge. The penultimate event is correlated to marine invasion of the Juan de Fuca Strait, and Puget Sound, while the final IRD event with that of Georgia Strait. We posit that these previously unknown IRD events represent repeated rapid iceberg discharge related to Cordilleran Ice Sheet collapse. The events occur near the end of radiocarbon plateaus at 13.35 ±90 and 14.05 ±70 14C Kyr BP (not reservoir corrected). If these plateaus correlate with the 12.2 and 13.3 14C Kyr BP plateaus recorded in Carriaco Basin and elsewhere, local reservoir ages can be calculated and vary between 1,150 and 1,550 years similar to those derived locally from glacial wood-shell pairs. Furthermore, if the plateaus result from reduced North Atlantic Deep Water export and consequently Heinrich Events, the Cordilleran IRD events are related to North Atlantic iceberg

  13. The application of in situ 14C to Holocene terrestrial Antarctic ice-sheet reconstruction

    NASA Astrophysics Data System (ADS)

    Fogwill, Christopher; Turney, Christian; Hippe, Kristina; Rood, Dylan; Golledge, Nick; Wacker, Lukas; Wieler, Rainer

    2013-04-01

    Determining detailed Holocene ice-sheet behaviour in Antarctica is critical for understanding and predicting likely dynamic responses of ice sheets to future climate change. However, to date terrestrial studies using in situ cosmogenic isotopes have struggled to overcome inheritance issues in this cold-based ice setting. Here we present new terrestrial geological constraints that take advantage of recent technological developments in the extraction and measurement of in situ cosmogenic radiocarbon (14C)- a cosmogenic nuclide with a considerably shorter half-life than that of 10Be (10Be: 1.36x103 kyr; 14C: 5.73 kyr) - to assess the potential influence of prior exposure or recycling of glacial erratics and therefore improve reconstructions of past ice-sheet surface profile changes. Glacial erratics were sampled from steep exposed bedrock surfaces in the Ellsworth Mountains overlooking the Weddell Sea, serving as 'dipsticks' that allow us to reconstruct past surface elevation changes in the Rutford Ice Stream as it decayed through the Holocene. Our in situ 14C analysis reveals a complex relationship, reflecting inheritance and burial, in samples that record anomalously 'old' apparent 10Be exposure ages from previous exposure. Our results enable us to test and refine previous interpretations and thereby reduce chronological uncertainties in Holocene ice-sheet change in this sector of Antarctica, demonstrating the exiting potential of in situ 14C in ice-sheet reconstruction.

  14. Subglacial lake drainage detected beneath the Greenland ice sheet

    PubMed Central

    Palmer, Steven; McMillan, Malcolm; Morlighem, Mathieu

    2015-01-01

    The contribution of the Greenland ice sheet to sea-level rise has accelerated in recent decades. Subglacial lake drainage events can induce an ice sheet dynamic response—a process that has been observed in Antarctica, but not yet in Greenland, where the presence of subglacial lakes has only recently been discovered. Here we investigate the water flow paths from a subglacial lake, which drained beneath the Greenland ice sheet in 2011. Our observations suggest that the lake was fed by surface meltwater flowing down a nearby moulin, and that the draining water reached the ice margin via a subglacial tunnel. Interferometric synthetic aperture radar-derived measurements of ice surface motion acquired in 1995 suggest that a similar event may have occurred 16 years earlier, and we propose that, as the climate warms, increasing volumes of surface meltwater routed to the bed will cause such events to become more common in the future. PMID:26450175

  15. Spatial patterns in backscatter strength across the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    Jezek, K. C.

    1993-01-01

    The relationship between the physical properties of the Greenland ice sheet and Synthetic Aperture Radar (SAR) data collected from aircraft and from ERS-1 is addressed. Limited aircraft data are combined with a description of the glacier surface to predict qualitatively the spatial and seasonal variation in backscatter strength across the ice sheet. In particular the model predicts relatively low backscatter near the ice edge where scattering is dominated by rough surface effects. Backscatter increases through the lake zone as volume scattering becomes important. Strongest backscatter is found in the percolation facies where volume scatter from snow grains and volume scatter from large, buried ice bodies becomes important. Backscatter weakens in the interior ice sheet where fine grained snow is the only mechanism producing backscatter.

  16. An MIS 3 age organic deposit from Balglass Burn, central Scotland: palaeoenvironmental significance and implications for the timing of the onset of the LGM ice sheet in the vicinity of the British Isles

    NASA Astrophysics Data System (ADS)

    Brown, Eleanor J.; Rose, James; Coope, Russell G.; Lowe, John J.

    2007-03-01

    This paper reports the analysis of a glaciotectonised organic deposit located between a lower weathered till and an upper unweathered till at Balglass Burn, north of the Campsie Fells in central Scotland, UK, close to the centre of ice accumulation in Scotland. Sedimentology, pollen, macroscopic plant remains and Coleoptera indicate accumulation in a small pond, as part of an open, tundra landscape with low floral diversity. MCR palaeotemperature reconstructions for the Coleoptera give a mean temperature for the warmest and the coldest months of 8 to 10°C and -26 to -10°C respectively, indicating the presence of at least discontinuous permafrost. Six AMS dates on Carex fruit and Coleoptera fragments give ages between 34 480 and 28 050 14C yr BP (ca. 39.8 to ca. 32.8 cal. yr BP; Fairbanks et al. ([2005])). The upper till and the glaciotectonism are attributed to glaciation during the LGM. Glaciotectonic deformation means that the ages do not indicate sequential development and it is not possible to relate this palaeo-evidence to the fine resolution palaeoclimatic signature for MIS 3. However, the fact that this part of central Scotland was ice-free at this time means that some recent proposals suggesting that the British ice sheet began to accumulate around the middle of MIS 3 are unlikely to be correct. Copyright

  17. Evidence of Meltwater Retention within the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Rennermalm, A. K.; Smith, L. C.; Chu, V. W.; Box, J. E.; Forster, R. R.; van den Broeke, M. R.

    2012-12-01

    Greenland ice sheet water release, and the magnitude of sub- and englacial storage, firn densification, internal refreezing and other hydrologic processes that delay or reduce true water export to the global ocean remain poorly understood. This problem is compounded by scant hydrometerological measurements. Here, ice sheet surface meltwater runoff and proglacial river discharge determined between 2008 and 2010 for three sites near Kangerlussuaq, western Greenland were used to establish the water budget for a small ice sheet watershed. The water budget could not be closed in the three years, even when uncertainty ranges were considered. Instead, between 12% and 53% of ice sheet surface runoff is retained within the catchment each melt year (time between onset of ice sheet runoff in two consecutive years) most likely in en- and subglacial storages. Evidence suggests that while some holdover summer meltwater may escape during the cold-season, this water volume is too small to close the budget. Thus, this study indicates that the Greenland ice sheet cryo-hydrologic system may remain active year round, and that meltwater may be retained in the pro glacial area, internally, or in firn layers for prolonged time periods before release to the ocean.

  18. Staggering through the ice ages

    SciTech Connect

    Monastersky, R.

    1994-07-30

    Because the steady orbital cycles of earth, thought to control the ice ages, cannot easily account for the evidence of repeated rapid climatic shifts during the last ice age. Without knowing what made the iceage climate so in temperate, scientists cannot tell whether today's interglacial period is immune to the sudden swings. Information about climate instability has emerged from two drilling projects in the middle of Greenland where crews bored through the 3-kilometer think glacial cap. This paper describes the discoveries and the evidence for rapid climatic shifts, including conflicting results from different sites. The concerns about global warming are making questions about these shifts of increased interest. The possibility exists that modern climate harbors an inherently unstable element that could trigger wild wings in response to the atmospheric buildup of greenhouse gases. On the other hand sudden climatic shifts might have been unique to the ice-age Earth.

  19. Sensitivity of Pliocene ice sheets to orbital forcing

    USGS Publications Warehouse

    Dolan, A.M.; Haywood, A.M.; Hill, D.J.; Dowsett, H.J.; Hunter, S.J.; Lunt, D.J.; Pickering, S.J.

    2011-01-01

    The stability of the Earth's major ice sheets is a critical uncertainty in predictions of future climate and sea level change. One method of investigating the behaviour of the Greenland and the Antarctic ice sheets in a warmer-than-modern climate is to look back at past warm periods of Earth history, for example the Pliocene. This paper presents climate and ice sheet modelling results for the mid-Pliocene warm period (mPWP; 3.3 to 3.0 million years ago), which has been identified as a key interval for understanding warmer-than-modern climates (Jansen et al., 2007). Using boundary conditions supplied by the United States Geological Survey PRISM Group (Pliocene Research, Interpretation and Synoptic Mapping), the Hadley Centre coupled ocean–atmosphere climate model (HadCM3) and the British Antarctic Survey Ice Sheet Model (BASISM), we show large reductions in the Greenland and East Antarctic Ice Sheets (GrIS and EAIS) compared to modern in standard mPWP experiments. We also present the first results illustrating the variability of the ice sheets due to realistic orbital forcing during the mid-Pliocene. While GrIS volumes are lower than modern under even the most extreme (cold) mid-Pliocene orbit (losing at least 35% of its ice mass), the EAIS can both grow and shrink, losing up to 20% or gaining up to 10% of its present-day volume. The changes in ice sheet volume incurred by altering orbital forcing alone means that global sea level can vary by more than 25 m during the mid-Pliocene. However, we have also shown that the response of the ice sheets to mPWP orbital hemispheric forcing can be in anti-phase, whereby the greatest reductions in EAIS volume are concurrent with the smallest reductions of the GrIS. If this anti-phase relationship is in operation throughout the mPWP, then the total eustatic sea level response would be dampened compared to the ice sheet fluctuations that are theoretically possible. This suggests that maximum eustatic sea level rise does not

  20. Climate-forcing & Feedbacks of the Late Paleozoic Ice Age

    NASA Astrophysics Data System (ADS)

    Montanez, I. P.; Brand, U.; Poulsen, C. J.; Horton, D. E.

    2011-12-01

    Evaluating climate-forcing and feedbacks during pre-Cenozoic ice ages requires reconstructing marine-terrestrial linkages between atmospheric composition, the regional hydroclimate expression of mean climate change, ice sheets, and sea-level. Here we evaluate the role of different climate parameters and their linkages during the Carboniferous icehouse through integration of a recently developed ID-TIMS U-Pb constrained sea-level history, brachiopod stable isotope time-series from shallow marine regions of paleotropical Pangaea, atmospheric pCO2 inferred from paleosol minerals and fossil leaf stomatal indices, ice sheet variations constrained by the distribution of high-latitude Gondwanan glacial deposits, and paleoclimate simulations. Within chronostratigraphic uncertainty, long-term sea-level lowstands coincide with glacial maxima defined from high-latitude Gondwanan basins, whereas long-term highstands are coeval with glacial minima suggesting a dynamic late Paleozoic icehouse. Superimposed shorter-term sea-level events define a stepwise onset (late Mississippian) and contraction of Carboniferous ice sheets prior to the initiation of Early Permian ice sheets. Sea level fluctuations, at different temporal scales parallel trends defined by brachiopod oxygen and carbon isotope compositions and paleo-atmospheric pCO2 estimates inferred using mineral and biologic proxies. A protracted (~9 my) stepwise sea level rise beginning in the middle Pennsylvanian and culminating in an earliest Gzhelian peak is coincident with overall increasing CO2 levels throughout this interval and substantially decreased effective moisture in paleotropical Pangaea. This possibly CO2-forced period of waning continental ice sheets and sea-level highstand encompassed a large-scale floral turnover across the mid-to-late Pennyslvanian boundary and the onset of the demise of paleotropical rainforests across much of Pangaea. Ocean-atmosphere-ice sheet climate simulations for this period reveal a

  1. SPICE: Sentinel-3 Performance Improvement for Ice Sheets

    NASA Astrophysics Data System (ADS)

    McMillan, Malcolm; Shepherd, Andrew; Roca, Monica; Escorihuela, Maria Jose; Thibaut, Pierre; Remy, Frederique; Escola, Roger; Benveniste, Jerome; Ambrozio, Americo; Restano, Marco

    2016-04-01

    Since the launch of ERS-1 in 1991, polar-orbiting satellite radar altimeters have provided a near continuous record of ice sheet elevation change, yielding estimates of ice sheet mass imbalance at the scale of individual ice sheet basins. One of the principle challenges associated with radar altimetry comes from the relatively large ground footprint of conventional pulse-limited radars, which limits their capacity to make reliable measurements in areas of complex topographic terrain. In recent years, progress has been made towards improving ground resolution, through the implementation of Synthetic Aperture Radar (SAR), or Delay-Doppler, techniques. In 2010, the launch of CryoSat heralded the start of a new era of SAR altimetry, although full SAR coverage of the polar ice sheets will only be achieved with the launch of the first Sentinel-3 satellite in January 2016. Because of the heritage of SAR altimetry provided by CryoSat, current SAR altimeter processing techniques have to some extent been optimized and evaluated for water and sea ice surfaces. This leaves several outstanding issues related to the development and evaluation of SAR altimetry for ice sheets, including improvements to SAR processing algorithms and SAR altimetry waveform retracking procedures. Here we will outline SPICE (Sentinel-3 Performance Improvement for Ice Sheets), a 2 year project which began in September 2015 and is funded by ESA's SEOM (Scientific Exploitation of Operational Missions) programme. This project aims to contribute to the development and understanding of ice sheet SAR altimetry through the emulation of Sentinel-3 data from dedicated CryoSat SAR acquisitions made at several sites in Antarctica. More specifically, the project aims to (1) evaluate and improve the current Delay-Doppler processing and SAR waveform retracking algorithms, (2) evaluate higher level SAR altimeter data, and (3) investigate radar wave interaction with the snowpack. We will provide a broad overview of

  2. Cryosphere Science Outreach using the Ice Sheet System Model and a Virtual Ice Sheet Laboratory

    NASA Astrophysics Data System (ADS)

    Cheng, D. L. C.; Halkides, D. J.; Larour, E. Y.

    2015-12-01

    Understanding the role of Cryosphere Science within the larger context of Sea Level Rise is both a technical and educational challenge that needs to be addressed if the public at large is to trulyunderstand the implications and consequences of Climate Change. Within this context, we propose a new approach in which scientific tools are used directly inside a mobile/website platform geared towards Education/Outreach. Here, we apply this approach by using the Ice Sheet System Model, a state of the art Cryosphere model developed at NASA, and integrated within a Virtual Ice Sheet Laboratory, with the goal is to outreach Cryospherescience to K-12 and College level students. The approach mixes laboratory experiments, interactive classes/lessons on a website, and a simplified interface to a full-fledged instance of ISSM to validate the classes/lessons. This novel approach leverages new insights from the Outreach/Educational community and the interest of new generations in web based technologies and simulation tools, all of it delivered in a seamlessly integrated web platform. This work was performed at the California Institute of Technology's Jet Propulsion Laboratory undera contract with the National Aeronautics and Space Administration's Cryosphere Science Program.

  3. Simulating the Antarctic ice sheet in the late-Pliocene warm period: PLISMIP-ANT, an ice-sheet model intercomparison project

    NASA Astrophysics Data System (ADS)

    de Boer, B.; Dolan, A. M.; Bernales, J.; Gasson, E.; Goelzer, H.; Golledge, N. R.; Sutter, J.; Huybrechts, P.; Lohmann, G.; Rogozhina, I.; Abe-Ouchi, A.; Saito, F.; van de Wal, R. S. W.

    2015-05-01

    In the context of future climate change, understanding the nature and behaviour of ice sheets during warm intervals in Earth history is of fundamental importance. The late Pliocene warm period (also known as the PRISM interval: 3.264 to 3.025 million years before present) can serve as a potential analogue for projected future climates. Although Pliocene ice locations and extents are still poorly constrained, a significant contribution to sea-level rise should be expected from both the Greenland ice sheet and the West and East Antarctic ice sheets based on palaeo sea-level reconstructions. Here, we present results from simulations of the Antarctic ice sheet by means of an international Pliocene Ice Sheet Modeling Intercomparison Project (PLISMIP-ANT). For the experiments, ice-sheet models including the shallow ice and shelf approximations have been used to simulate the complete Antarctic domain (including grounded and floating ice). We compare the performance of six existing numerical ice-sheet models in simulating modern control and Pliocene ice sheets by a suite of five sensitivity experiments. We include an overview of the different ice-sheet models used and how specific model configurations influence the resulting Pliocene Antarctic ice sheet. The six ice-sheet models simulate a comparable present-day ice sheet, considering the models are set up with their own parameter settings. For the Pliocene, the results demonstrate the difficulty of all six models used here to simulate a significant retreat or re-advance of the East Antarctic ice grounding line, which is thought to have happened during the Pliocene for the Wilkes and Aurora basins. The specific sea-level contribution of the Antarctic ice sheet at this point cannot be conclusively determined, whereas improved grounding line physics could be essential for a correct representation of the migration of the grounding-line of the Antarctic ice sheet during the Pliocene.

  4. Deciphering the evolution of the last Eurasian ice sheets

    NASA Astrophysics Data System (ADS)

    Hughes, Anna; Gyllencreutz, Richard; Mangerud, Jan; Svendsen, John Inge

    2016-04-01

    Glacial geologists need ice sheet-scale chronological reconstructions of former ice extent to set individual records in a wider context and compare interpretations of ice sheet response to records of past environmental changes. Ice sheet modellers require empirical reconstructions on size and volume of past ice sheets that are fully documented, specified in time and include uncertainty estimates for model validation or constraints. Motivated by these demands, in 2005 we started a project (Database of the Eurasian Deglaciation, DATED) to compile and archive all published dates relevant to constraining the build-up and retreat of the last Eurasian ice sheets, including the British-Irish, Scandinavian and Svalbard-Barents-Kara Seas ice sheets (BIIS, SIS and SBKIS respectively). Over 5000 dates were assessed for reliability and used together with published ice-sheet margin positions to reconstruct time-slice maps of the ice sheets' extent, with uncertainty bounds, every 1000 years between 25-10 kyr ago and at four additional periods back to 40 kyr ago. Ten years after the idea for a database was conceived, the first version of results (DATED-1) has now been released (Hughes et al. 2016). We observe that: i) both the BIIS and SBKIS achieve maximum extent, and commence retreat earlier than the larger SIS; ii) the eastern terrestrial margin of the SIS reached its maximum extent up to 7000 years later than the westernmost marine margin; iii) the combined maximum ice volume (~24 m sea-level equivalent) was reached c. 21 ka; iv) large uncertainties exist; predominantly across marine sectors (e.g. the timing of coalescence and separation of the SIS and BKIS) but also in well-studied areas due to conflicting yet equally robust data. In just three years since the DATED-1 census (1 January 2013), the volume of new information (from both dates and mapped glacial geomorphology) has grown significantly (~1000 new dates). Here, we present the DATED-1 results in the context of the

  5. Sea-level feedback lowers projections of future Antarctic Ice-Sheet mass loss.

    PubMed

    Gomez, Natalya; Pollard, David; Holland, David

    2015-11-10

    The stability of marine sectors of the Antarctic Ice Sheet (AIS) in a warming climate has been identified as the largest source of uncertainty in projections of future sea-level rise. Sea-level fall near the grounding line of a retreating marine ice sheet has a stabilizing influence on the ice sheets, and previous studies have established the importance of this feedback on ice age AIS evolution. Here we use a coupled ice sheet-sea-level model to investigate the impact of the feedback mechanism on future AIS retreat over centennial and millennial timescales for a range of emission scenarios. We show that the combination of bedrock uplift and sea-surface drop associated with ice-sheet retreat significantly reduces AIS mass loss relative to a simulation without these effects included. Sensitivity analyses show that the stabilization tends to be greatest for lower emission scenarios and Earth models characterized by a thin elastic lithosphere and low-viscosity upper mantle, as is the case for West Antarctica.

  6. Sea-level feedback lowers projections of future Antarctic Ice-Sheet mass loss.

    PubMed

    Gomez, Natalya; Pollard, David; Holland, David

    2015-01-01

    The stability of marine sectors of the Antarctic Ice Sheet (AIS) in a warming climate has been identified as the largest source of uncertainty in projections of future sea-level rise. Sea-level fall near the grounding line of a retreating marine ice sheet has a stabilizing influence on the ice sheets, and previous studies have established the importance of this feedback on ice age AIS evolution. Here we use a coupled ice sheet-sea-level model to investigate the impact of the feedback mechanism on future AIS retreat over centennial and millennial timescales for a range of emission scenarios. We show that the combination of bedrock uplift and sea-surface drop associated with ice-sheet retreat significantly reduces AIS mass loss relative to a simulation without these effects included. Sensitivity analyses show that the stabilization tends to be greatest for lower emission scenarios and Earth models characterized by a thin elastic lithosphere and low-viscosity upper mantle, as is the case for West Antarctica. PMID:26554381

  7. Troughs on Martian Ice Sheets: Analysis of Their Closure and Mass Balance

    NASA Technical Reports Server (NTRS)

    Fountain, A.; Kargel, J.; Lewis, K.; MacAyeal, D.; Pfeffer, T.; Zwally, J.

    2000-01-01

    At the Copenhagen workshop on Martian polar processes, Ralf Greve commented that the flow regime surrounding scarps and troughs of the Martian polar ice sheets cannot be modeled using traditional "plan view" ice-sheet models. Such models are inadequate because they typically use reduced equations that embody certain simplifications applicable only to terrestrial ice sheets where the upper ice sheet surface is smooth. In response to this suggestion, we have constructed a 2-dimensional, time dependent "side view" (two spatial dimensions: one horizontal, one vertical) model of scarp closure that is designed to overcome the difficulties described by Greve. The purpose of the model is to evaluate the scales of stress variation and styles of flow closure so as to estimate errors that may be encountered by "plan view" models. We show that there may be avenues whereby the complications associated with scarp closure can be overcome in "plan view" models through appropriate parameterizations of 3-dimensional effects. Following this, we apply the flow model to simulate the evolution of a typical scarp on the North Polar Cap of Mars. Our simulations investigate: (a) the role of "radiation trapping" (see our companion abstract) in creating and maintaining "spiral-like" scarps on the ice sheet, (b) the consequences of different flowlaws and ice compositions on scarp evolution and, in particular, scarp age, and (c) the role of dust and debris in scarp evolution.

  8. Commitments to future retreat of Antarctic and Greenland ice sheets

    NASA Astrophysics Data System (ADS)

    DeConto, Robert; Pollard, David

    2016-04-01

    The agreement reached at the COP21 United Nations Conference on Climate Change is aimed at limiting future increases in global mean temperature below 2°C. Here, we use a continental ice sheet/shelf model with new treatments of meltwater-enhanced calving (hydrofracturing) and marine terminating ice-cliffs, to explore future commitments to sea-level rise given limits of global mean warming between 1 and 3°C. In this case, ice-sheet model physics are calibrated against past ice-sheet response to temperatures warmer than today. The ice-sheet model is coupled to highly resolved atmosphere and ocean-model components, with imposed limits on future warming designed to mimic the idealized limits discussed at COP21. Both the short and long-term potential rise in global mean sea level are discussed in light of the range of allowances agreed in Paris. We also explore the sensitivity of Greenland and Antarctic ice sheets to plausible ranges of atmospheric versus ocean warming consistent with global mean temperatures between 1 and 3°C; and the resulting long-term commitments to sea-level rise over the coming centuries and millennia.

  9. The future of ice sheets and sea ice: Between reversible retreat and unstoppable loss

    PubMed Central

    Notz, Dirk

    2009-01-01

    We discuss the existence of cryospheric “tipping points” in the Earth's climate system. Such critical thresholds have been suggested to exist for the disappearance of Arctic sea ice and the retreat of ice sheets: Once these ice masses have shrunk below an anticipated critical extent, the ice–albedo feedback might lead to the irreversible and unstoppable loss of the remaining ice. We here give an overview of our current understanding of such threshold behavior. By using conceptual arguments, we review the recent findings that such a tipping point probably does not exist for the loss of Arctic summer sea ice. Hence, in a cooler climate, sea ice could recover rapidly from the loss it has experienced in recent years. In addition, we discuss why this recent rapid retreat of Arctic summer sea ice might largely be a consequence of a slow shift in ice-thickness distribution, which will lead to strongly increased year-to-year variability of the Arctic summer sea-ice extent. This variability will render seasonal forecasts of the Arctic summer sea-ice extent increasingly difficult. We also discuss why, in contrast to Arctic summer sea ice, a tipping point is more likely to exist for the loss of the Greenland ice sheet and the West Antarctic ice sheet. PMID:19884496

  10. The Changing Albedo of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Human, J. M.; Box, J. E.

    2009-12-01

    The study evaluates Greenland ice sheet surface albedo sensitivity to surface melt intensity, air pollution, and precipitation using data from the MODIS and MOPITT sensors operating on the NASA Terra satellite 2000-2009. Precipitation rates are simulated by the Polar WRF climate model running in data assimilation mode. Statistical regression facilitates ranking the relative importance of each of the albedo forcings in space and time. Further, quantitative estimates of the albedo sensitivity to its forcing factors are made, for the first time and over the observed inter-annual range. The work investigates regional patterns in detail to quantify melt water production associated with absorbed solar radiation variability. In-situ records are used to evaluate the cloud radiative effect as another important factor of absorbed solar radiation and ice melt. Insight into Greenland ice sheet melt-precipitation-pollution-albedo feedback is gained, key in better understanding the mass balance response of the ice sheet to future climate change.

  11. Determining Greenland Ice Sheet Accumulation Rates from Radar Remote Sensing

    NASA Technical Reports Server (NTRS)

    Jezek, Kenneth C.

    2002-01-01

    An important component of NASA's Program for Arctic Regional Climate Assessment (PARCA) is a mass balance investigation of the Greenland Ice Sheet. The mass balance is calculated by taking the difference between the areally Integrated snow accumulation and the net ice discharge of the ice sheet. Uncertainties in this calculation Include the snow accumulation rate, which has traditionally been determined by interpolating data from ice core samples taken from isolated spots across the ice sheet. The sparse data associated with ice cores juxtaposed against the high spatial and temporal resolution provided by remote sensing , has motivated scientists to investigate relationships between accumulation rate and microwave observations as an option for obtaining spatially contiguous estimates. The objective of this PARCA continuation proposal was to complete an estimate of surface accumulation rate on the Greenland Ice Sheet derived from C-band radar backscatter data compiled in the ERS-1 SAR mosaic of data acquired during, September-November, 1992. An empirical equation, based on elevation and latitude, is used to determine the mean annual temperature. We examine the influence of accumulation rate, and mean annual temperature on C-band radar backscatter using a forward model, which incorporates snow metamorphosis and radar backscatter components. Our model is run over a range of accumulation and temperature conditions. Based on the model results, we generate a look-up table, which uniquely maps the measured radar backscatter, and mean annual temperature to accumulation rate. Our results compare favorably with in situ accumulation rate measurements falling within our study area.

  12. Laurentide Ice Sheet surging as modeled with PISM

    NASA Astrophysics Data System (ADS)

    Ziemen, Florian; Chlond, Andreas; Rodehacke, Christian; Mikolajewicz, Uwe

    2010-05-01

    The climate during the last glacial showed a much higher variability compared to the holocene. The strongest variations were caused by Heinrich events with a reoccurence interval of 7 000 yrs. They are manifested in ice rafted debris layers in North Atlantic sediment cores. The debris stems mainly from the Laurentide Ice Sheet (LIS), which has experienced massive surges. We use the Parallel Ice Sheet Model (PISM) to study these events; this model combines the traditional Shallow Ice Approximation (SIA) for non-sliding ice with the Shallow Shelf Approximation (SSA) for the sliding portions of the ice sheet and thus allows for a more realistic representation of the sliding areas as well as the transitions between deforming and sliding parts of the ice sheet. We show how the surging of the LIS depends on the climate state and how it is influenced by the basal sliding parameterization. One parameterization we employ makes use of the perfectly plastic till assumption, which cannot be applied in SIA-only models.

  13. Unusual radar echoes from the Greenland ice sheet

    NASA Technical Reports Server (NTRS)

    Rignot, E. J.; Vanzyl, J. J.; Ostro, S. J.; Jezek, K. C.

    1993-01-01

    In June 1991, the NASA/Jet Propulsion Laboratory airborne synthetic-aperture radar (AIRSAR) instrument collected the first calibrated data set of multifrequency, polarimetric, radar observations of the Greenland ice sheet. At the time of the AIRSAR overflight, ground teams recorded the snow and firn (old snow) stratigraphy, grain size, density, and temperature at ice camps in three of the four snow zones identified by glaciologists to characterize four different degrees of summer melting of the Greenland ice sheet. The four snow zones are: (1) the dry-snow zone, at high elevation, where melting rarely occurs; (2) the percolation zone, where summer melting generates water that percolates down through the cold, porous, dry snow and then refreezes in place to form massive layers and pipes of solid ice; (3) the soaked-snow zone where melting saturates the snow with liquid water and forms standing lakes; and (4) the ablation zone, at the lowest elevations, where melting is vigorous enough to remove the seasonal snow cover and ablate the glacier ice. There is interest in mapping the spatial extent and temporal variability of these different snow zones repeatedly by using remote sensing techniques. The objectives of the 1991 experiment were to study changes in radar scattering properties across the different melting zones of the Greenland ice sheet, and relate the radar properties of the ice sheet to the snow and firn physical properties via relevant scattering mechanisms. Here, we present an analysis of the unusual radar echoes measured from the percolation zone.

  14. Glaciology and the Ice Age.

    ERIC Educational Resources Information Center

    Carozzi, Albert V.

    1984-01-01

    Discusses: (1) the beginning of glaciology; (2) origin of erratic boulders, meteorites, volcanic explosions, floods, and drift; (3) ice age hypothesis in Europe and the United States; (4) development of glacial theory; (5) and a unified explanation of glacial events. A bibliography of classical research on glaciology is included. (BC)

  15. Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

    NASA Astrophysics Data System (ADS)

    Naish, Tim R.; Woolfe, Ken J.; Barrett, Peter J.; Wilson, Gary S.; Atkins, Cliff; Bohaty, Steven M.; Bücker, Christian J.; Claps, Michele; Davey, Fred J.; Dunbar, Gavin B.; Dunn, Alistair G.; Fielding, Chris R.; Florindo, Fabio; Hannah, Michael J.; Harwood, David M.; Henrys, Stuart A.; Krissek, Lawrence A.; Lavelle, Mark; van der Meer, Jaap; McIntosh, William C.; Niessen, Frank; Passchier, Sandra; Powell, Ross D.; Roberts, Andrew P.; Sagnotti, Leonardo; Scherer, Reed P.; Strong, C. Percy; Talarico, Franco; Verosub, Kenneth L.; Villa, Giuliana; Watkins, David K.; Webb, Peter-N.; Wonik, Thomas

    2001-10-01

    Between 34 and 15million years (Myr) ago, when planetary temperatures were 3-4°C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1-23.7Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40kyr) and eccentricity (125kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).

  16. Ice Sheet Oscillations During the Last Deglaciation in Western Norway

    NASA Astrophysics Data System (ADS)

    Lohne, O. S.; Mangerud, J.; Svendsen, J.; Gyllencreutz, R.

    2009-12-01

    About 25-26ka cal BP, the Scandinavian Ice Sheet merged with the British Ice Sheet on the continental shelf in the northern part of the North Sea (Sejrup et al., 2009). The Norwegian Channel, that hosted a major ice stream, became finally deglaciated at around 18.5ka cal BP. The deglaciation of western Norway started first in the south and not later than 17-16 ka cal BP, the Jæren region was partly ice-free (Knudsen, 2006). The early deglaciation history in the Hardangerfjord-Bergen area was characterized by some rapid ice-margin oscillations. The extent and timing of the ice front variations are not well constrained, but probably they occurred sometime between 16-14.5ka cal BP. A main difficulty with the exact timing is the radiocarbon plateau around 12.3 ka 14C BP. Subsequently, during the Allerød, the ice sheet withdrew more than 50 km from the coastline, until it again started to re-advance. The recorded sea-level response of this re-advance indicates that the re-growth of the ice sheet started in the mid-Allerød, approximately at 13.6ka cal BP (Lohne et al., 2007), and it seems clear that the resulting ice sheet advance continued until the very end of the Younger Dryas when the prominent Herdla-Halsnøy Moraine was formed. In the 500-800 m deep Hardangerfjord the re-advance stopped at a bedrock threshold where the Halsnøy Moraine subsequently formed. As appear from the sediment stratigraphy in a lake basin on Halsnøy the advancing ice-margin reached the island slightly before the Vedde Ash fall (c. 12.1ka cal BP). The ice-front then halted on the bedrock sill, but stratigraphic evidence indicates that the fjord-glacier continued to grow in thickness. Marginal moraines from Halsnøy can be traced up to a level of about 1000 m a.s.l., 40 km further inland, reflecting a massive buildup of the ice sheet. The advancing ice-sheet reached its maximum position, both in the Bergen and Hardangerfjorden area, late in the YD. A more precise dating by radiocarbon is

  17. The Ice Sheet Mass Balance Inter-comparison Exercise

    NASA Astrophysics Data System (ADS)

    Shepherd, A.; Ivins, E. R.

    2015-12-01

    Fluctuations in the mass of ice stored in Antarctica and Greenland are of considerable societal importance. The Ice Sheet Mass Balance Inter-Comparison Exercise (IMBIE) is a joint-initiative of ESA and NASA aimed at producing a single estimate of the global sea level contribution to polar ice sheet losses. Within IMBIE, estimates of ice sheet mass balance are developed from a variety of satellite geodetic techniques using a common spatial and temporal reference frame and a common appreciation of the contributions due to external signals. The project brings together the laboratories and space agencies that have been instrumental in developing independent estimates of ice sheet mass balance to date. In its first phase, IMBIE involved 27 science teams, and delivered a first community assessment of ice sheet mass imbalance to replace 40 individual estimates. The project established that (i) there is good agreement between the three main satellite-based techniques for estimating ice sheet mass balance, (ii) combining satellite data sets leads to significant improvement in certainty, (iii) the polar ice sheets contributed 11 ± 4 mm to global sea levels between 1992 and 2012, and (iv) that combined ice losses from Antarctica and Greenland have increased over time, rising from 10% of the global trend in the early 1990's to 30% in the late 2000's. Demand for an updated assessment has grown, and there are now new satellite missions, new geophysical corrections, new techniques, and new teams producing data. The period of overlap between independent satellite techniques has increased from 5 to 12 years, and the full period of satellite data over which an assessment can be performed has increased from 19 to 40 years. It is also clear that multiple satellite techniques are required to confidently separate mass changes associated with snowfall and ice dynamical imbalance - information that is of critical importance for climate modelling. This presentation outlines the approach

  18. Are longitudinal ice-surface structures on the Antarctic Ice Sheet indicators of long-term ice-flow configuration?

    NASA Astrophysics Data System (ADS)

    Glasser, N. F.; Jennings, S. J. A.; Hambrey, M. J.; Hubbard, B.

    2014-07-01

    Continent-wide mapping of longitudinal ice-surface structures on the Antarctic Ice Sheet reveals that they originate in the interior of the ice sheet and are arranged in arborescent networks fed by multiple tributaries. Longitudinal ice-surface structures can be traced continuously down-ice for distances of up to 1200 km. They are co-located with fast-flowing glaciers and ice streams that are dominated by basal sliding rates above tens of m yr-1 and are strongly guided by subglacial topography. Longitudinal ice-surface structures dominate regions of converging flow, where ice flow is subject to non-coaxial strain and simple shear. Associating these structures with the AIS' surface velocity field reveals (i) ice residence times of ~ 2500 to 18 500 years, and (ii) undeformed flow-line sets for all major flow units analysed except the Kamb Ice Stream and the Institute and Möller Ice Stream areas. Although it is unclear how long it takes for these features to form and decay, we infer that the major ice-flow and ice-velocity configuration of the ice sheet may have remained largely unchanged for several thousand years, and possibly even since the end of the last glacial cycle. This conclusion has implications for our understanding of the long-term landscape evolution of Antarctica, including large-scale patterns of glacial erosion and deposition.

  19. Improving Surface Mass Balance Over Ice Sheets and Snow Depth on Sea Ice

    NASA Technical Reports Server (NTRS)

    Koenig, Lora Suzanne; Box, Jason; Kurtz, Nathan

    2013-01-01

    Surface mass balance (SMB) over ice sheets and snow on sea ice (SOSI) are important components of the cryosphere. Large knowledge gaps remain in scientists' abilities to monitor SMB and SOSI, including insufficient measurements and difficulties with satellite retrievals. On ice sheets, snow accumulation is the sole mass gain to SMB, and meltwater runoff can be the dominant single loss factor in extremely warm years such as 2012. SOSI affects the growth and melt cycle of the Earth's polar sea ice cover. The summer of 2012 saw the largest satellite-recorded melt area over the Greenland ice sheet and the smallest satellite-recorded Arctic sea ice extent, making this meeting both timely and relevant.

  20. Climate Data Records (CDRs) for Ice Motion and Ice Age

    NASA Astrophysics Data System (ADS)

    Tschudi, M. A.; Fowler, C.; Maslanik, J. A.; Stroeve, J. C.

    2011-12-01

    Climate Data Records (CDRs) for remotely-sensed Arctic sea ice motion and sea ice age are under development by our group at the University of Colorado, Boulder. The ice motion product, archived at NSIDC, has a considerable history of use, while sea ice age is a relatively new product. Our technique to estimate sea ice motion utilizes images from SSM/I, as well as SMMR and the series of AVHRR sensors to estimate the daily motion of ice parcels. This method is augmented by incorporating ice motion observations from the network of drifting buoys deployed as part of the International Arctic Buoy Program. Our technique to calculate ice age relies on following the actual age of the ice for each ice parcel, categorizing the parcel as first-year ice, second-year, ice, etc. based on how many summer melt seasons the ice parcel survives. Both of these research-grade products have been interpolated onto 25x25 km grid points spanning the entire Arctic Ocean using the Equal-Area Scalable Earth (EASE) grid. Datasets generated from this program have shown that the Arctic ice cover has experienced a significant (> 70%) decline in multiyear ice over the last 20 years, leaving a younger ice cover in 2011. By comparing ice age derived by the Lagrangian tracking method to ice thickness estimated by Ice, Cloud and land Elevation Satellite (ICESat) Geoscience Laser Altimeter System (GLAS) data, it is observed that ice age is linearly related to ice thickness, up to an age of 10 years. Therefore, the shift in dominance of multiyear ice to first-year ice relates to a significant thinning of the ice. This thinning is estimated to correspond to a 40% reduction in ice volume in the last 20 years. An ancillary dataset (APP-X) produced by the University of Wisconsin, Madison has been combined with the ice motion product to monitor the properties of the sea ice parcels tracked by the ice motion product. This dataset includes ice surface and 2-meter air temperature, albedo, downwelling shortwave

  1. Pleistocene marine ice sheets and ice shelves at the East Siberian continental margin

    NASA Astrophysics Data System (ADS)

    Niessen, Frank; Kuk Hong, Jong; Hegewald, Anne; Matthiessen, Jens; Stein, Rüdiger; Kim, Sookwan; Jensen, Laura; Jokat, Wilfried; Nam, Seung Il

    2014-05-01

    RV "Polarstern" cruise ARK-XIII/3 (2008) and RV "Araon" cruise ARA03B (2012) investigated an area in the Arctic Ocean located between the Chukchi Borderland and the East Siberian Sea (between 165°W and 170°E). Based on swath bathymetry, sediment echosounding, seismic profiling and sediment coring we present evidence that the western Arctic Ocean had a glaciated continental margin during several glacial periods of the Pleistocene (Niessen et al. 2013). At the southern end of the Mendeleev Ridge and on the Chukchi and East Siberian continental slopes ice sheets and ice shelves grounded in up to 1200 m present water depth. We found mega-scale glacial lineations (MSGL) associated with deposition of glaciogenic wedges and debris-flow deposits indicative of sub-glacial erosion and deposition close to the former grounding lines. Glacially lineated areas are associated with large-scale erosion, capped with diamicton and draped by, in places, several metres of pelagic sediments. On the Arlis Plateau, a detailed bathymetric map exhibits several generations of MSGL, which we interpret as relicts of different Pleistocene glaciations. Traces of former grounding line positions suggest that an ice shelf of approximately 900 m in thickness has spread across the Southern Mendeleev Ridge in a north-easterly direction. According to our results, ice sheets of more than one km in thickness continued onto, and likely centered over, the East Siberian Shelf. A preliminary age model suggests that the youngest and shallowest grounding event of an ice sheet should be within Marine Isotope Stage (MIS) 3 and clearly predates the Last Glacial Maximum. The oldest and deepest event predates MIS 6. The youngest grounding event on the Arlis Plateau is tentatively dated to have occurred during MIS 4. These results have important implication for the former distribution of thick ice masses in the Arctic Ocean during the Pleistocene. They are relevant for albedo, ocean-atmosphere heat exchange

  2. SPH non-Newtonian Model for Ice Sheet and Ice Shelf Dynamics

    SciTech Connect

    Tartakovsky, Alexandre M.; Pan, Wenxiao; Monaghan, Joseph J.

    2012-07-07

    We propose a new three-dimensional smoothed particle hydrodynamics (SPH) non-Newtonian model to study coupled ice sheet and ice shelf dynamics. Most existing ice sheet numerical models use a grid-based Eulerian approach, and are usually restricted to shallow ice sheet and ice shelf approximations of the momentum conservation equation. SPH, a fully Lagrangian particle method, solves the full momentum conservation equation. SPH method also allows modeling of free-surface flows, large material deformation, and material fragmentation without employing complex front-tracking schemes, and does not require re-meshing. As a result, SPH codes are highly scalable. Numerical accuracy of the proposed SPH model is first verified by simulating a plane shear flow with a free surface and the propagation of a blob of ice along a horizontal surface. Next, the SPH model is used to investigate the grounding line dynamics of ice sheet/shelf. The steady position of the grounding line, obtained from our SPH simulations, is in good agreement with laboratory observations for a wide range of bedrock slopes, ice-to-fluid density ratios, and flux. We examine the effect of non-Newtonian behavior of ice on the grounding line dynamics. The non-Newtonian constitutive model is based on Glen's law for a creeping flow of a polycrystalline ice. Finally, we investigate the effect of a bedrock geometry on a steady-state position of the grounding line.

  3. A renewed argument for crystal size control of ice sheet strain rates

    NASA Astrophysics Data System (ADS)

    Cuffey, K. M.; Thorsteinsson, T.; Waddington, E. D.

    2000-12-01

    At present, it is generally believed that crystal size has no direct influence on strain rate in the ice sheets and that the fraction of strain rate enhancement there which is not ascribable to c axis fabric is due to impurity content. Here we challenge this view because it is not consistent with recent results from analyses of deformation at Meserve Glacier and instead ascribe residual enhancement in the ice sheets to variations in crystal size. We resurrect the idea that variations of crystal size can be an important part of the total shear enhancement in the ice sheets, though agree with Paterson that this effect is generally dominated by variations of crystal fabric. We propose that the enhanced shear strain rate of ice age ice in southern Greenland, as inferred from tilt of the Dye 3 borehole, can be explained as a result of combined fabric variations and crystal size variations, with these two ice properties accounting for roughly 70% and 30% of the average enhancement, respectively. Permitting a grain size dependence of ice viscosity also resolves the quandary concerning closure and tilt of the Agassiz Ice Cap borehole.

  4. 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

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

    NASA Astrophysics Data System (ADS)

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

    1998-01-01

    accumulation rates do not indicate a definite trend in the region during this century. The Alfred Wegener Institute has in recent years employed both airborne and ground-penetrating ice radar systems to map the bedrock around deep drilling sites in Central and North Greenland, as well as in a planned Antarctic site in Dronning Maud Land. The radar also records shallow and deep internal echoes, caused by rapid variation in density and ice acidity in layers of certain ages, allowing isochrones to be traced over wide reaches of the ice sheet. Disturbances in regular stratigraphic layering, due to ice flow over an irregular bed, were observed in the lowest 200-300 m of the GRIP and GISP2 ice cores. Since the aim of the new NGRIP coring program is to obtain an ice core reaching further back in time than the Central Greenland cores, this site was chosen in a region where the bedrock is relatively flat. Echo-sounding surveys between GRIP and NGREP show that the isochrones lie 100-200 in higher above the bed at NGRIP, indicating that the Eemian layer is unlikely to have been disturbed by ice flow at this location. Due to the flow pattern of ice sheets, layers forming a vertical sequence in the interior regions of an ice sheet can, under favorable conditions, be traced on horizontal profiles at the margins. Some meaningful correlations have already been established between Greenland deep ice core climatic records and corresponding records from ice margins. In these regions, a clear contrast is observed between ice of Holocene origin and significantly darker-looking ice dating from the Wisconsin glacial period, which displays summertime ablation rates 2-4x higher than the Holocene ice. This difference is due to higher concentrations of dust and other impurities in the Wisconsin ice, by 1-2 orders of magnitude, leading to reduced albedo. Furthermore, smaller crystal sizes in the Wisconsin ice lead to a more homogeneous distribution of impurities on the surface, which probably contributes

  6. CLIVAR Exchanges No. 62: Sea Level Rise, Ocean/Ice Shelf Interactions and Ice Sheets

    SciTech Connect

    Pirani, Anna; Danabasoglu, Gokhan; Griffies, Stephen; Marsland, Simon

    2013-08-01

    This special issue of CLIVAR Exchanges is devoted to presenting a selection of the science contributed by both speakers and poster presenters at the CLIVAR Workshop on Sea Level Rise, Ocean/Ice Shelf Interactions and Ice Sheets at CSIRO Marine and Atmospheric Research in Hobart, Australia, on 18-20 February 2013. The workshop brought together leading international scientists and early-career researchers from the ocean, ice-sheet, ice-shelf, and sea-level rise modelling and observational communities to explore the state-of-science and emerging pathways for development of the next generation of coupled climate models.

  7. Ice Sheet Roughness Estimation Based on Impulse Responses Acquired in the Global Ice Sheet Mapping Orbiter Mission

    NASA Astrophysics Data System (ADS)

    Niamsuwan, N.; Johnson, J. T.; Jezek, K. C.; Gogineni, P.

    2008-12-01

    The Global Ice Sheet Mapping Orbiter (GISMO) mission was developed to address scientific needs to understand the polar ice subsurface structure. This NASA Instrument Incubator Program project is a collaboration between Ohio State University, the University of Kansas, Vexcel Corporation and NASA. The GISMO design utilizes an interferometric SAR (InSAR) strategy in which ice sheet reflected signals received by a dual-antenna system are used to produce an interference pattern. The resulting interferogram can be used to filter out surface clutter so as to reveal the signals scattered from the base of the ice sheet. These signals are further processed to produce 3D-images representing basal topography of the ice sheet. In the past three years, the GISMO airborne field campaigns that have been conducted provide a set of useful data for studying geophysical properties of the Greenland ice sheet. While topography information can be obtained using interferometric SAR processing techniques, ice sheet roughness statistics can also be derived by a relatively simple procedure that involves analyzing power levels and the shape of the radar impulse response waveforms. An electromagnetic scattering model describing GISMO impulse responses has previously been proposed and validated. This model suggested that rms-heights and correlation lengths of the upper surface profile can be determined from the peak power and the decay rate of the pulse return waveform, respectively. This presentation will demonstrate a procedure for estimating the roughness of ice surfaces by fitting the GISMO impulse response model to retrieved waveforms from selected GISMO flights. Furthermore, an extension of this procedure to estimate the scattering coefficient of the glacier bed will be addressed as well. Planned future applications involving the classification of glacier bed conditions based on the derived scattering coefficients will also be described.

  8. Glaciological investigations on modern ice sheet response in South Greenland

    NASA Astrophysics Data System (ADS)

    Mayer, C.; Bøggild, C. E.; Podlech, S.; Olesen, O. B.

    2003-04-01

    The reaction of the large ice sheets to global climate change is still in the focus of scientific debate. Recent investigations have shown pronounced thinning in the southern part of the Greenland Ice Sheet. In order to investigate the cause of the observed thinning in this area and to judge the sensitivity of this part of the ice sheet a combined field work, remote sensing and modelling project was designed. A glaciological transect was established in May 2001 on one of the main outlet glaciers in South Greenland, where the first data series have been collected. This transect aims on monitoring the modern climatic conditions in an area of the ice sheet which is strongest affected by the recently observed thinning. A mass balance/elevation relation has been calculated from the field data, which is used as input to a regional ice dynamic model for investigating the dynamic conditions of this area. Modelled surface velocities are compared to GPS field measurements in order to constrain model parameters. Results show that the sensitivity of the ice sheet to climatic changes varies considerably along the southern margin. The history of the glacier variations in the same area during the last 40 years has been reconstructed on the basis of aerial photographs and satellite images. Comparing the development of the retreat in this area over the last decades with the maximum extent of the ice sheet from the 1890s indicates a strong increase in the recession of the margin. The estimated thinning from these observations is in good agreement with the recent laser altimeter measurements. The observed surface lowering is also responsible for the fast disintegration of several glacier tongues of tide water glaciers in the area.

  9. Holocene Fluctuations of North Ice Cap, a Proxy for Climate Conditions along the Northwestern Margin of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Kelly, M. A.; Osterberg, E. C.; Lasher, G. E.; Farnsworth, L. B.; Howley, J. A.; Axford, Y.; Zimmerman, S. R. H.

    2015-12-01

    North Ice Cap (~76.9°N, 68°W, summit elevation 1322 m asl), a small, independent ice cap in northwestern Greenland, is located within ~25 km of the Greenland Ice Sheet margin and Harald Molkte Bræ outlet glacier. We present geochronological, geomorphic and sedimentological data constraining the Holocene extents of North Ice Cap and suggest that its past fluctuations can be used as a proxy for climate conditions along the northwestern margin of the Greenland Ice Sheet. Prior work by Goldthwait (1960) used glacial geomorphology and radiocarbon ages of subfossil plants emerging along shear planes in the ice cap margin to suggest that that North Ice Cap was not present during the early Holocene and nucleated in the middle to late Holocene time, with the onset of colder conditions. Subfossil plants emerging at shear planes in the North Ice Cap margin yield radiocarbon ages of ~4.8-5.9 cal kyr BP (Goldthwait, 1960) and ~AD 1000-1350 (950-600 cal yr BP), indicating times when the ice cap was smaller than at present. In situ subfossil plants exposed by recent ice cap retreat date to ~AD 1500-1840 (450-110 cal yr BP) and indicate small fluctuations of the ice cap margin. 10Be ages of an unweathered, lichen-free drift <100 m from the present North Ice Cap margin range from ~500 to 8000 yrs ago. We suggest that the drift was deposited during the last ~500 yrs and that the older 10Be ages are influenced by 10Be inherited from a prior period of exposure. We also infer ice cap fluctuations using geochemical data from a Holocene-long sediment core from Deltasø, a downstream lake that currently receives meltwater from North Ice Cap. The recent recession of the North Ice Cap margin influenced a catastrophic drainage of a large proglacial lake, Søndre Snesø, that our field team documented in August 2012. To our knowledge, this is the first significant lowering of Søndre Snesø in historical time.

  10. Greenland ice sheet melting during the last interglacial

    NASA Astrophysics Data System (ADS)

    Langebroek, Petra M.; Nisancioglu, Kerim H.

    2016-04-01

    During the last interglacial period (LIG) peak temperatures over Greenland were several degrees warmer than today. The Greenland ice sheet (GIS) retreated causing a global sea-level rise in the order of several meters. Large uncertainties still exist in the exact amount of melt and on the source location of this melt. Here we examine the GIS response to LIG temperature and precipitation patterns using the SICOPOLIS ice sheet model. The LIG climate was simulated by forcing the Norwegian Earth System Model (NorESM) with the appropriate greenhouse gases and orbital settings. The resulting LIG ice volume evolution strongly depends on the chosen value of uncertain model parameters for the ice sheet (e.g. basal sliding parameter, PDD factors, and atmospheric temperature lapse rate). We reduce the uncertainty by evaluating an ensemble of model results against present-day observations of ice sheet size, elevation and stability, together with paleo information from deep ice cores. We find a maximum GIS reduction equivalent to 0.8 to 2.2m of global sea-level rise. In this model set-up most of the melting occurs in southwestern Greenland.

  11. Modelling the Laurentide Ice Sheet using improved ice margin chronologies and glacio-isostatic observations

    NASA Astrophysics Data System (ADS)

    Gowan, Evan; Tregoning, Paul; Purcell, Anthony; Lambeck, Kurt

    2013-04-01

    Creating models of the Laurentide ice sheet is challenging, due to the deficiency of chronological constraints and the uneven spatial resolution of data to determine the evolution of the glacio-isostatic response after deglaciation. Previous models relied on uncalibrated radiocarbon constrained margins that proved to have deficiencies in recent studies. Additionally, many recent Laurentide ice sheet models have been developed by incorporating climatic parameters that are poorly resolved for the late glacial period. We present a new ice sheet model by an iterative process of changing basal shear stress values and ice sheet margin location. A particular focus of this study is to determine the thickness and extent of the western Laurentide ice sheet, where there were few well dated observations of glacio-isostatic motion until recently. The volume of an ice sheet during long periods depends mostly on basal shear stress and margin position, which are the main parameters that we vary to fit our model to glacio-isostatic observations. We build our ice model using the assumption of perfectly plastic, steady-state conditions, with variable basal shear stress. Basal shear stress values depend on the surficial geology underlying the ice, and are at a minimum in offshore regions that have soft, deformable sediments, and at a maximum in areas with exposed crystalline bedrock. This approach may not capture dynamic and short lived features of the ice sheet, such as ice streams and stagnant ice, but gives an approximation of average conditions to produce ice volumes that fit geophysical observations. We adjust the margin location when the shear stress conditions alone cannot account for the observed glacio-isostatic response. The constraints on the response include relative sea level benchmarks, sea level highstand positions and proglacial lakes. We repeat the analysis using different rheological profiles to determine the dependence the Earth model has on the estimation of ice

  12. Millennial scale chronology of the south-central Laurentide Ice Sheet

    NASA Astrophysics Data System (ADS)

    Curry, B.; Lowell, T. V.

    2012-12-01

    The Laurentide Ice Sheet (LIS) was the largest ice sheet to form and decay during the Quaternary, and knowledge of its behavior over millennial time scales is an essential contribution to an understanding of progressive change of the larger Earth System. The LIS extended closer to the equator (39.1°N) than any other ice sheet, and defines the timing of the Last Glacial Maximum (LGM). The southern, terrestrial based, ice sheet was comprised of various lobes and the largest of these were the Great Lakes lobes. Here fossils of spruce trees and tundra plants, as well as buried A and AO horizons (paleosols) yield radiocarbon ages that we summarize below to document the millennial-scale ice decay and growth of the LGM and ice-margin retreat patterns thereafter. The ice sheet reached south of the present Great Lakes by 27.4-28.1 ka cal and continued to its maximum locations by 24.8 ka cal. In the Lake Erie basin the ice sheet underwent regional oscillations with advances at 24.8, 23.1, and 20.9 ka cal. After 20.8 ka cal both margins of the Erie and Lake Michigan lobes began retreating to near modern shorelines of the present lakes by 17.5 and 16.5 ka cal respectively. Subsequent ice-margin retreats were relatively rapid partly because of calving into the glacial Great Lakes. At about 14.1-13.5 ka cal, the margin of the Lake Michigan lobe locally stabilized with a later stabilization about 11.3 ka cal. Another significant part of the southern Laurentide, the Des Moines lobe has a similar space-temporal pattern. An expansion phase was reached by 24.0 ka cal with its maximum at 16.6 ka al and retreatal pauses at 14.4 and 14.0 ka cal. The causes for near-synchrony of ice-margin fluctuations have been debated. The similar pattern at millennial time scales argues for a first order climate control. The differences in areal extent imply either changing climate conditions or localized ice dynamics. In either event, a glaciological model of the Laurentide ice sheet must consider

  13. SPICE: Sentinel-3 Performance Improvement for Ice Sheets

    NASA Astrophysics Data System (ADS)

    Benveniste, Jérôme; Escolà, Roger; Roca, Mònica; Ambrózio, Américo; Restano, Marco; McMillan, Malcolm; Escorihuela, Maria Jose; Shepherd, Andrew; Thibaut, Pierre; Remy, Frederique

    2016-07-01

    Since the launch of ERS-1 in 1991, polar-orbiting satellite radar altimeters have provided a near continuous record of ice sheet elevation change, yielding estimates of ice sheet mass imbalance at the scale of individual ice sheet basins. One of the principle challenges associated with radar altimetry comes from the relatively large ground footprint of conventional pulse-limited radars, which limits their capacity to make reliable measurements in areas of complex topographic terrain. In recent years, progress has been made towards improving ground resolution, through the implementation of Synthetic Aperture Radar (SAR), or Delay-Doppler, techniques. In 2010, the launch of CryoSat-2 by the European Space Agency heralded the start of a new era of SAR altimetry, although full SAR coverage of the polar ice sheets will only be achieved with the launch of the first Sentinel-3 satellite in February 2016. Because of the heritage of SAR altimetry provided by CryoSat-2, current SAR altimeter processing techniques have been optimized and evaluated for water and sea ice surfaces. This leaves several outstanding issues related to the development and evaluation of SAR altimetry for ice sheets, including improvements to SAR processing algorithms and SAR altimetry waveform retracking procedures. Here we will present interim results from SPICE (Sentinel-3 Performance Improvement for Ice Sheets), a 2 year project that focuses on the expected performance of Sentinel-3 SAR altimetry over the Polar ice sheets. The project, which began in September 2015 and is funded by ESA's SEOM (Scientific Exploitation of Operational Missions) programme, aims to contribute to the development and understanding of ice sheet SAR altimetry through the emulation of Sentinel-3 data from dedicated CryoSat SAR acquisitions made at several sites in Antarctica and Greenland. More specifically, the project aims to (1) evaluate and improve the current Delay-Doppler processing and SAR waveform retracking

  14. Late pleistocene ice age scenarios based on observational evidence

    SciTech Connect

    DeBlonde, G. ); Peltier, W.R. )

    1993-04-01

    Ice age scenarios for the last glacial interglacial cycle, based on observations of Boyle and Keigwin concerning the North Atlantic thermohaline circulation and of Barnola et al. concerning atmospheric CO[sub 2] variations derived from the Vostok ice cores, are herein analyzed. Northern Hemisphere continental ice sheets are simulated with an energy balance model (EBM) that is asynchronously coupled to vertically integrated ice sheets models based on the Glen flow law. The EBM includes both a realistic land-sea distribution and temperature-albedo feedback and is driven with orbital variations of effective solar insolation. With the addition of atmospheric CO[sub 2] and ocean heat flux variations, but not in their absence, a complete collapse is obtained for the Eurasian ice sheet but not for the North American ice sheet. We therefore suggest that further feedback mechanisms, perhaps involving more accurate modeling of the dynamics of the mostly marine-based Laurentide complex appears necessary to explain termination I. 96 refs., 12 figs., 2 tabs.

  15. Ice sheet (de)stabilization via grounding zone processes (Invited)

    NASA Astrophysics Data System (ADS)

    Christianson, K. A.; Horgan, H.; Parizek, B. R.; Alley, R. B.; Anandakrishnan, S.; Jacobel, R. W.; Keisling, B. A.; Dalla Santa, K. L.; Craig, B.; Walker, R. T.

    2013-12-01

    Much of the threshold behavior of marine ice sheets is thought to result from processes occurring at the grounding zone, where the ice sheet transitions into the ice shelf. At short time-scales (decades to centuries), grounding zone behavior is likely to be influenced by ongoing sediment deposition, which can stabilize the grounding zone position. Tidally driven flexure just inland of an ice shelf can further enhance stabilization by compacting subglacial till and thereby locally increasing basal shear stress. However, this competes with ocean-driven melt across a several-kilometers-wide grounding zone, where warm ocean water infiltration around bedrock obstacles can result in rapid grounding line retreat. Here we present a suite of geophysical observations (ice-penetrating radar, active-source seismic, GPS, and laser altimetry data) and data-assimilated modeling for one relatively stable (Whillans Ice Stream) and one potentially unstable (Thwaites Glacier) grounding zone in West Antarctica. The geophysical data show that estuaries occur beneath ice sheet grounding zones, where interactions between ocean water, subglacial hydrology, sediment, and tidal processes are complex and occur across a several-kilometers-wide grounding zone. Our modeling results indicate that ice stream stabilization on bedrock highs narrower than the length of the tidally-influenced grounding zone may be ephemeral if circulating warm ocean waters reduce basal resistance and enhance melt across the grounding zone. Stabilization is, however, significantly enhanced by effectively plastic beds and zones of high basal shear stress, which can be created via till compaction from tidal flexure. Thus accurate future projections of sea level require correct understanding of till rheology and local grounding zone processes (interaction of sediment, ocean water, subglacial water, and tidal processes), which are not presently included in modern whole-ice-sheet models.

  16. Pliocene paleoclimate and East Antarctic ice-sheet history from surficial ash deposits

    SciTech Connect

    Marchant, D.R.; Lux, D.R.; West, D.P. Jr.; Denton, G.H. ); Swisher, C.C. III )

    1993-04-30

    The preservation, age, and stratigraphic relation of an in situ ashfall layer with an underlying desert pavement in Arena Valley, southern Victoria Land, indicate that a cold-desert climate has persisted in Arena Valley during the past 4.3 million years. These data indicate that the present East Antarctic Ice Sheet has endured for this time and that average temperatures during the Pliocene in Arena Valley were no greater than 3[degrees]C above present values. One implication is that the collapse of the East Antarctic Ice Sheet due to greenhouse warming is unlikely, even if global atmospheric temperatures rise to levels last experienced during mid-pliocene times.

  17. Caterpillar-like ice motion in the ablation zone of the Greenland ice sheet

    NASA Astrophysics Data System (ADS)

    Ryser, C.; Lüthi, M. P.; Andrews, L. C.; Catania, G. A.; Funk, M.; Hawley, R.; Hoffman, M.; Neumann, T. A.

    2014-10-01

    Current understanding of ice dynamics predicts that increasing availability and variability of meltwater will have an impact on basal motion and therefore on the evolution and future behavior of the Greenland ice sheet. We present measurements of ice deformation, subglacial water pressure, and surface velocity that show periodic and episodic variations on several time scales (seasonal, multiday, and diurnal). These variations, observed with GPS and sensors at different depths throughout the ice column, are not synchronous but show delayed responses of ice deformation with increasing depth and basal water pressure in antiphase with surface velocity. With the help of a Full-Stokes ice flow model, these observations are explained as ice motion in a caterpillar-like fashion. Caused by patches of different basal slipperiness, horizontal stress transfer through the stiff central part of the ice body leads to spatially varying surface velocities and ice deformation patterns. Variation of this basal slipperiness induces characteristic patterns of ice deformation variability that explain the observed behavior. Ice flow in the ablation zone of the Greenland ice sheet is therefore controlled by activation of basal patches by varying slipperiness in the course of a melt season, leading to caterpillar-like ice motion superposed on the classical shear deformation.

  18. The Role of Glacial Erosion in Limiting Ice Sheet Extents

    NASA Astrophysics Data System (ADS)

    Jamieson, S.; Hulton, N.

    2007-12-01

    We aim to identify and quantify feedbacks between ice dynamics and glacial erosion. Whilst geological and geomorphological evidence indicates that ice sheets generally oscillate in time with orbital forcing, their extents are not necessarily a direct function of the amplitude of this forcing. Benthic δ18O records document glacial-interglacial fluctuations and indicate that maximum Pleistocene global ice volume occurs around 400 ka. However, geomorphological evidence in a number of regions is contradictory, with the most extensive ice masses often occurring 100's of kyrs prior to peaks in the δ18O record. For example, the glacial landforms of Patagonia preserve a record of just such behaviour with each successive glacial advance since 1.15 Ma covering an area less extensive than the previous expansion. This implies that other processes are modifying the linkages between ice sheets and climate. We ask: Could glacial erosion of bedrock have caused ice sheets to self-regulate their extents? Ground-breaking experiments by Oerlemans (1984) demonstrated that erosion induced margin retreat was indeed possible. He showed that retreat could be achieved but only where eroding ice streams were smaller in width than the wavelength of lithospheric response. In Patagonia however, the scales of retreat are much larger than this lithospheric wavelength - but could erosion still be an important factor? We use the GLIMMER 3-D thermomechanical ice sheet model (Payne, 1999) with an added erosion component to simulate long-term landscape evolution under theoretical ice sheets (Jamieson et al., 2007). We show that models of glacial erosion can generate feedbacks on a significant scale such that ice sheets can self-limit their extents over periods of 105 - 106 years regardless of the flexural response of the land surface. Erosion around the ELA enables increasingly efficient ice drainage, and the mass balance of the ice sheet thus shifts towards a more negative state. At the same time

  19. A Reconciled Estimate of Ice-Sheet Mass Balance

    NASA Technical Reports Server (NTRS)

    Shepherd, Andrew; Ivins, Erik R.; Geruo, A.; Barletta, Valentia R.; Bentley, Mike J.; Bettadpur, Srinivas; Briggs, Kate H.; Bromwich, David H.; Forsberg, Rene; Galin, Natalia; Horwath, Martin; Jacobs, Stan; Joughin, Ian; King, Matt A.; Lenaerts, Jan T. M.; Li, Jilu; Ligtenberg, Stefan R. M.; Luckman, Adrian; Luthcke, Scott B.; McMillan, Malcolm; Meister, Rakia; Milne, Glenn; Mouginot, Jeremie; Muir, Alan; Nicolas,Julien P.; Paden, John; Payne, Antony J.; Pritchard, Hamish; Rignot, Eric; Rott, Helmut; Sorensen, Louise Sandberg; Scambos, Ted A.; Yi, Dohngui; Zwally, H. Jay

    2012-01-01

    We combined an ensemble of satellite altimetry, interferometry, and gravimetry data sets using common geographical regions, time intervals, and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of Earth's polar ice sheets. We find that there is good agreement between different satellite methods-especially in Greenland and West Antarctica-and that combining satellite data sets leads to greater certainty. Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by -142 plus or minus 49, +14 plus or minus 43, -65 plus or minus 26, and -20 plus or minus 14 gigatonnes year(sup -1), respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 plus or minus 0.20 millimeter year(sup -1) to the rate of global sea-level rise.

  20. High export of dissolved silica from the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Meire, L.; Meire, P.; Struyf, E.; Krawczyk, D. W.; Arendt, K. E.; Yde, J. C.; Juul Pedersen, T.; Hopwood, M. J.; Rysgaard, S.; Meysman, F. J. R.

    2016-09-01

    Silica is an essential element for marine life and plays a key role in the biogeochemistry of the ocean. Glacial activity stimulates rock weathering, generating dissolved silica that is exported to coastal areas along with meltwater. The magnitude of the dissolved silica export from large glacial areas such as the Greenland Ice Sheet is presently poorly quantified and not accounted for in global budgets. Here we present data from two fjord systems adjacent to the Greenland Ice Sheet which reveal a large export of dissolved silica by glacial meltwater relative to other macronutrients. Upscaled to the entire Greenland Ice Sheet, the export of dissolved silica equals 22 ± 10 Gmol Si yr-1. When the silicate-rich meltwater mixes with upwelled deep water, either inside or outside Greenland's fjords, primary production takes place at increased silicate to nitrate ratios. This likely stimulates the growth of diatoms relative to other phytoplankton groups.

  1. A Smoothed Particle Hydrodynamics Model for Ice Sheet and Ice Shelf Dynamics

    SciTech Connect

    Pan, Wenxiao; Tartakovsky, Alexandre M.; Monaghan, Joseph J.

    2012-02-08

    Mathematical modeling of ice sheets is complicated by the non-linearity of the governing equations and boundary conditions. Standard grid-based methods require complex front tracking techniques and have limited capability to handle large material deformations and abrupt changes in bottom topography. As a consequence, numerical methods are usually restricted to shallow ice sheet and ice shelf approximations. We propose a new smoothed particle hydrodynamics (SPH) model for coupled ice sheet and ice shelf dynamics. SPH is a fully Lagrangian particle method. It is highly scalable and its Lagrangian nature and meshless discretization are well suited to the simulation of free surface flows, large material deformation, and material fragmentation. In this paper SPH is used to study ice sheet/ice shelf behavior, and the dynamics of the grounding line. The steady state position of the grounding line obtained from the SPH simulations is in good agreement with laboratory observations for a wide range of simulated bedrock slopes, and density ratios similar to those of ice and sea water. The numerical accuracy of the SPH algorithm is further verified by simulating the plane shear flow of two immiscible fluids and the propagation of a highly viscous blob of fluid along a horizontal surface. In the experiment, the ice was represented with a viscous newtonian fluid. For consistency, in the described SPH model the ice is also modeled as a viscous newtonian fluid. Typically, ice sheets are modeled as a non-Newtonian fluid, accounting for the changes in the mechanical properties of ice. Implementation of a non-Newtonian rheology in the SPH model is the subject of our ongoing research.

  2. Greenland ice sheet albedo feedback: mass balance implications

    NASA Astrophysics Data System (ADS)

    Box, J. E.; Tedesco, M.; Fettweis, X.; Hall, D. K.; Steffen, K.; Stroeve, J. C.

    2012-12-01

    Greenland ice sheet mass loss has accelerated responding to combined glacier discharge and surface melt water runoff increases. During summer, absorbed solar energy, modulated at the surface primarily by albedo, is the dominant factor governing surface melt variability in the ablation area. NASA MODIS data spanning 13 summers (2000 - 2012), indicate that mid-summer (July) ice sheet albedo declined by 0.064 from a value of 0.752 in the early 2000s. The ice sheet accordingly absorbed 100 EJ more solar energy for the month of July in 2012 than in the early 2000s. This additional energy flux during summer doubled melt rates in the ice sheet ablation area during the observation period. Abnormally strong anticyclonic circulation, associated with a persistent summer North Atlantic Oscillation extreme 2007-2012, enabled 3 amplifying mechanisms to maximize the albedo feedback: 1) increased warm (south) air advection along the western ice sheet increased surface sensible heating that in turn enhanced snow grain metamorphic rates, further reducing albedo; 2) increased surface downward shortwave flux, leading to more surface heating and further albedo reduction; and 3) reduced snowfall rates sustained low albedo, maximizing surface solar heating, progressively lowering albedo over multiple years. The summer net infrared and solar radiation for the high elevation accumulation area reached positive values during this period, contributing to an abrupt melt area increase in 2012. A number of factors make it reasonable to expect more melt episodes covering 100% of the ice sheet area in coming years: 1) the past 13 y of increasing surface air temperatures have eroded snowpack 'cold content', preconditioning the ice sheet for earlier melt onset. Less heat is required to bring the surface to melting; 2) Greenland temperatures, have lagged the N Hemisphere average in the 2000s, need to increase further for Greenland to be in phase with the N Hemisphere average. 3) Arctic amplification

  3. Response of the Antarctic ice sheet to ocean forcing using the POPSICLES coupled ice sheet-ocean model

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    We present the response of the continental Antarctic ice sheet to sub-shelf-melt forcing derived from POPSICLES simulation results covering the full Antarctic Ice Sheet and the Southern Ocean spanning the period 1990 to 2010. Simulations are performed at 0.1 degree (~5 km) ocean resolution and ice sheet resolution as fine as 500 m using adaptive mesh refinement. A comparison of fully-coupled and comparable standalone ice-sheet model results demonstrates the importance of two-way coupling between the ice sheet and the ocean. The POPSICLES model couples the POP2x ocean model, a modified version of the Parallel Ocean Program (Smith and Gent, 2002), and the BISICLES ice-sheet model (Cornford et al., 2012). BISICLES makes use of adaptive mesh refinement to fully resolve dynamically-important regions like grounding lines and employs a momentum balance similar to the vertically-integrated formulation of Schoof and Hindmarsh (2009). Results of BISICLES simulations have compared favorably to comparable simulations with a Stokes momentum balance in both idealized tests like MISMIP3D (Pattyn et al., 2013) and realistic configurations (Favier et al. 2014). POP2x includes sub-ice-shelf circulation using partial top cells (Losch, 2008) and boundary layer physics following Holland and Jenkins (1999), Jenkins (2001), and Jenkins et al. (2010). Standalone POP2x output compares well with standard ice-ocean test cases (e.g., ISOMIP; Losch, 2008) and other continental-scale simulations and melt-rate observations (Kimura et al., 2013; Rignot et al., 2013). A companion presentation, "Present-day circum-Antarctic simulations using the POPSICLES coupled land ice-ocean model" in session C027 describes the ocean-model perspective of this work, while we focus on the response of the ice sheet and on details of the model. The figure shows the BISICLES-computed vertically-integrated ice velocity field about 1 month into a 20-year coupled Antarctic run. Groundling lines are shown in green.

  4. Early miocene subglacial basalts, the East antarctic ice sheet, and uplift of the transantarctic mountains.

    PubMed

    Stump, E; Sheridan, M F; Borg, S G; Sutter, J F

    1980-02-15

    Subglacially erupted volcanic rocks from Mount Early and Sheridan Bluff, Antarctica, yield whole-rock potassium-argon dates and argon-40/argon-39 release spectra of Early Miocene age. Field associations suggest the existence of the East Antarctic ice sheet and significant uplift of the Transantarctic Mountains by that time.

  5. Ice-sheet modelling accelerated by graphics cards

    NASA Astrophysics Data System (ADS)

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

    2014-11-01

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

  6. Using ice-penetrating radars to date ice-rise formation and Late Holocene ice-sheet retreat in the Ronne Ice Shelf region, West Antarctica

    NASA Astrophysics Data System (ADS)

    Kingslake, Jonathan; Hindmarsh, Richard; King, Edward; Corr, Hugh

    2015-04-01

    The history of the West Antarctic Ice Sheet in the region currently occupied by the Ronne Ice Shelf is poorly known. This reflects a lack of accessible recently deglaciated surfaces, which prohibits conventional paleo glaciological techniques that can provide evidence of past ice-sheet extent and retreat, for example ocean coring or exposure-dating of geological material. We use a glaciological technique, Raymond Effect Dating, to constrain the retreat of the ice sheet through the Ronne Ice Shelf region. During two Antarctic field seasons, we used a pulse-echo ice-penetrating radar to image the base and internal stratigraphy of four ice rises - areas of grounded ice containing ice divides. Towing the radar with skidoos, we conducted over 2000 km of surveys on the Skytrain, Korff, Henry and Fowler Ice Rises and the ice shelf between them. We also used a step-frequency radar called pRES to measure the vertical ice flow in the vicinity of each ice divide. Isochronal ice layers imaged during the surveys deforming in a predictable way with ice flow, meaning that their shape contains information about past ice flow. Directly beneath ice divides the downward motion of the ice is impeded by an ice-dynamical phenomenon called the Raymond Effect. This causes layers beneath the divides to form 'Raymond Arches' that grow over time. We will present the data and simulate the growth of the Raymond Arches using our pRES-measured vertical ice velocities and date the onset of ice-divide flow at each ice rise by comparing the size of simulated arches to the arches imaged during our radar surveys. We consider the main sources of uncertainty associated with these ice-rise formation dates and discuss what they can tell us about the retreat of the West Antarctic Ice Sheet through this region during the last few thousand years.

  7. Modelling water flow under glaciers and ice sheets

    PubMed Central

    Flowers, Gwenn E.

    2015-01-01

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

  8. Diachronous retreat of the Greenland ice sheet during the last deglaciation

    NASA Astrophysics Data System (ADS)

    Sinclair, G.; Carlson, A. E.; Mix, A. C.; Lecavalier, B. S.; Milne, G.; Mathias, A.; Buizert, C.; DeConto, R.

    2016-08-01

    The last deglaciation is the most recent interval of large-scale climate change that drove the Greenland ice sheet from continental shelf to within its present extent. Here, we use a database of 645 published 10Be ages from Greenland to document the spatial and temporal patterns of retreat of the Greenland ice sheet during the last deglaciation. Following initial retreat of its marine margins, most land-based deglaciation occurred in Greenland following the end of the Younger Dryas cold period (12.9-11.7 ka). However, deglaciation in east Greenland peaked significantly earlier (13.0-11.5 ka) than that in south Greenland (11.0-10 ka) or west Greenland (10.5-7.0 ka). The terrestrial deglaciation of east and south Greenland coincide with adjacent ocean warming. 14C ages and a recent ice-sheet model reconstruction do not capture this progression of terrestrial deglacial ages from east to west Greenland, showing deglaciation occurring later than observed in 10Be ages. This model-data misfit likely reflects the absence of realistic ice-ocean interactions. We suggest that oceanic changes may have played an important role in driving the spatial-temporal ice-retreat pattern evident in the 10Be data.

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

    NASA Astrophysics Data System (ADS)

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

    2014-09-01

    Onshore and offshore studies show that an expanded, grounded ice sheet occupied the Ross Sea Embayment during the Last Glacial Maximum (LGM). Results from studies of till provenance and the orientation of geomorphic features on the continental shelf show that more than half of the grounded ice sheet consisted of East Antarctic ice flowing through Transantarctic Mountain (TAM) outlet glaciers; the remainder came from West Antarctica. Terrestrial data indicate little or no thickening in the upper catchment regions in both West and East Antarctica during the LGM. In contrast, evidence from the mouths of the southern and central TAM outlet glaciers indicate surface elevations between 1000 m and 1100 m (above present-day sea level). Farther north along the western margin of the Ross Ice Sheet, surface elevations reached 720 m on Ross Island, and 400 m at Terra Nova Bay. Evidence from Marie Byrd Land at the eastern margin of the ice sheet indicates that the elevation near the present-day grounding line was more than 800 m asl, while at Siple Dome in the central Ross Embayment, the surface elevation was about 950 m asl. Farther north, evidence that the ice sheet was grounded on the middle and the outer continental shelf during the LGM implies that surface elevations had to be at least 100 m above the LGM sea level. The apparent low surface profile and implied low basal shear stress in the central and eastern embayment suggests that although the ice streams may have slowed during the LGM, they remained active. Ice-sheet retreat from the western Ross Embayment during the Holocene is constrained by marine and terrestrial data. Ages from marine sediments suggest that the grounding line had retreated from its LGM outer shelf location only a few tens of kilometer to a location south of Coulman Island by ˜13 ka BP. The ice sheet margin was located in the vicinity of the Drygalski Ice Tongue by ˜11 ka BP, just north of Ross Island by ˜7.8 ka BP, and near Hatherton Glacier by

  10. The extreme melt across the Greenland ice sheet in 2012

    NASA Astrophysics Data System (ADS)

    Nghiem, S. V.; Hall, D. K.; Mote, T. L.; Tedesco, M.; Albert, M. R.; Keegan, K.; Shuman, C. A.; DiGirolamo, N. E.; Neumann, G.

    2012-10-01

    The discovery of the 2012 extreme melt event across almost the entire surface of the Greenland ice sheet is presented. Data from three different satellite sensors - including the Oceansat-2 scatterometer, the Moderate-resolution Imaging Spectroradiometer, and the Special Sensor Microwave Imager/Sounder - are combined to obtain composite melt maps, representing the most complete melt conditions detectable across the ice sheet. Satellite observations reveal that melt occurred at or near the surface of the Greenland ice sheet across 98.6% of its entire extent on 12 July 2012, including the usually cold polar areas at high altitudes like Summit in the dry snow facies of the ice sheet. This melt event coincided with an anomalous ridge of warm air that became stagnant over Greenland. As seen in melt occurrences from multiple ice core records at Summit reported in the published literature, such a melt event is rare with the last significant one occurring in 1889 and the next previous one around seven centuries earlier in the Medieval Warm Period. Given its rarity, the 2012 extreme melt across Greenland provides an exceptional opportunity for new studies in broad interdisciplinary geophysical research.

  11. Antarctic ice-sheet loss driven by basal melting of ice shelves.

    PubMed

    Pritchard, H D; Ligtenberg, S R M; Fricker, H A; Vaughan, D G; van den Broeke, M R; Padman, L

    2012-04-26

    Accurate prediction of global sea-level rise requires that we understand the cause of recent, widespread and intensifying glacier acceleration along Antarctic ice-sheet coastal margins. Atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves, potentially limiting their ability to buttress the flow of grounded tributary glaciers. Indeed, recent ice-shelf collapse led to retreat and acceleration of several glaciers on the Antarctic Peninsula. But the extent and magnitude of ice-shelf thickness change, the underlying causes of such change, and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted. Here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum-Antarctic pattern of ice-shelf thinning through increased basal melt. We deduce that this increased melt is the primary control of Antarctic ice-sheet loss, through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow. The highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf. Wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of Antarctic ice shelves, through ocean upwelling in the Amundsen and Bellingshausen seas, and atmospheric warming on the Antarctic Peninsula. This implies that climate forcing through changing winds influences Antarctic ice-sheet mass balance, and hence global sea level, on annual to decadal timescales. PMID:22538614

  12. Antarctic ice-sheet loss driven by basal melting of ice shelves.

    PubMed

    Pritchard, H D; Ligtenberg, S R M; Fricker, H A; Vaughan, D G; van den Broeke, M R; Padman, L

    2012-04-25

    Accurate prediction of global sea-level rise requires that we understand the cause of recent, widespread and intensifying glacier acceleration along Antarctic ice-sheet coastal margins. Atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves, potentially limiting their ability to buttress the flow of grounded tributary glaciers. Indeed, recent ice-shelf collapse led to retreat and acceleration of several glaciers on the Antarctic Peninsula. But the extent and magnitude of ice-shelf thickness change, the underlying causes of such change, and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted. Here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum-Antarctic pattern of ice-shelf thinning through increased basal melt. We deduce that this increased melt is the primary control of Antarctic ice-sheet loss, through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow. The highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf. Wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of Antarctic ice shelves, through ocean upwelling in the Amundsen and Bellingshausen seas, and atmospheric warming on the Antarctic Peninsula. This implies that climate forcing through changing winds influences Antarctic ice-sheet mass balance, and hence global sea level, on annual to decadal timescales.

  13. Hydraulic geometry of Greenland Ice Sheet supraglacial streams

    NASA Astrophysics Data System (ADS)

    Chu, V. W.; Smith, L. C.; Rennermalm, A. K.; Forster, R. R.; Gleason, C. J.; Pitcher, L. H.; Moustafa, S.; Overstreet, B. T.; Legleiter, C. J.; Behar, A. E.; Tedesco, M.; Yang, K.

    2012-12-01

    Increasing surface melting on the Greenland ice sheet and rising sea level have heightened the need for understanding the complex pathways transporting meltwater from the ice sheet surface to the ice edge and the ocean. Supraglacial streams are abundant throughout the ablation zone, transporting large volumes of meltwater into moulins and the ice edge, yet these streams remain poorly studied. Here we present a study of supraglacial stream hydraulics and geometry in the ablation zone of western Greenland during summer 2012. We measured flow width, depth, velocity, and water surface slope at different elevations and stream network types at five locations in a 70 km transect spanning the ice edge at 500 m to 1420 m elevation. This transect includes two highly sampled catchments in very different environments, one at 500 m and one at 875 m elevation. Our data show that stream width is correlated with width-depth ratios, providing possibilities for extrapolating depths from high-resolution satellite imagery. Furthermore, we explore using average stream velocities and thalweg depths to determine discharge and provide confidence intervals to these calculations by utilizing over 30 full cross-sectional profiles of flow width, depth, velocity, and discharge. These relationships may be useful for scaling up to larger supraglacial rivers and estimating ablation zone meltwater fluxes at other ice sheet locations using high-resolution satellite imagery.

  14. Smoothed particle hydrodynamics non-Newtonian model for ice-sheet and ice-shelf dynamics

    SciTech Connect

    Pan, W.; Tartakovsky, A. M.; Monaghan, J. J.

    2013-06-01

    Mathematical modeling of ice sheets is complicated by the non-linearity of the governing equations and boundary conditions. Standard grid-based methods require complex front tracking techniques and have limited capability to handle large material deformations and abrupt changes in bottom topography. As a consequence, numerical methods are usually restricted to shallow ice sheet and ice shelf approximations. We propose a new smoothed particle hydrodynamics (SPH) non-Newtonian model for coupled ice sheet and ice shelf dynamics. SPH, a fully Lagrangian particle method, is highly scalable and its Lagrangian nature and meshless discretization are well suited to the simulation of free surface flows, large material deformation, and material fragmentation. In this paper, SPH is used to study 3D ice sheet/ice shelf behavior, and the dynamics of the grounding line. The steady state position of the grounding line obtained from SPH simulations is in good agreement with laboratory observations for a wide range of simulated bedrock slopes, and density ratios, similar to those of ice and sea water. The numerical accuracy of the SPH algorithm is verif;ed by simulating Poiseuille flow, plane shear flow with free surface and the propagation of a blob of ice along a horizontal surface. In the laboratory experiment, the ice was represented with a viscous Newtonian fluid. In the present work, however, the ice is modeled as both viscous Newtonian fluid and non-Newtonian fluid, such that the effect of non-Newtonian rheology on the dynamics of grounding line was examined. The non-Newtonian constitutive relation is prescribed to be Glen’s law for the creep of polycrystalline ice. A V-shaped bedrock ramp is further introduced to model the real geometry of bedrock slope.

  15. ESA Ice Sheets CCI: Overview and surface elevation change results for the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Levinsen, J. F.; Forsberg, R.; Meister, R.; Hogg, A.; Shepherd, A.; Khvorostovsky, K.; Dall, J.; Kusk, A.; Nagler, T.; Scharrer, K.; Andersen, S. B.; Andersen, M. L.

    2013-12-01

    With the climate changing and the Greenland Ice Sheet (GIS) losing mass at an accelerating rate, the European Space Agency has established the Climate Change Initiative (ESA CCI) project in order to provide selected Essential Climate Variables (ECVs) for the GIS. The ECV presented here is the Ice Sheets (http://www.esa-icesheets-cci.org/) in which four parameters are to be determined: Surface elevation changes (SEC), ice velocities, grounding line locations, and calving front locations. The resulting data sets are to be openly distributed via the CCI web-site in transparent and easy-to-use formats. The production of the final data sets, dating from 1991 - present, is well underway. This presentation will provide a status overview of all four ECV parameters. As a preparation for the final data set computations, all ECVs carried out an open Round Robin (RR) exercise in order to find the most optimal method for producing the most reliable estimates. In the RR, the scientific community was asked to submit their best estimate of the given parameters along with a feedback sheet describing methodology, pre- and post-processing steps, etc. We outline the results of the SEC RR, where 11 participants from various US and European institutions provided estimates over the Jakobshavn Isbræ drainage basin. The participants used either Envisat radar or ICESat laser altimetry, and cross-over (XO), repeat-track (RT), and overlapping footprint analyses were made. Especially one of the results, based on Envisat RT, provided the first evidence for the possibility of using radar altimetry to accurately derive SEC estimates in both interior and margin parts of the GIS. This illustrates the potential for the final SEC product, which will be based on Envisat, Cryosat-2, and Sentinel-3 data. Through inter-comparisons and validation of the submissions, the most optimal method for deriving SEC values was found to be a combination of RT and XO algorithms, exploiting the high spatial

  16. Coeval fluctuations of the Greenland Ice Sheet and a local ice cap during the Younger Dryas: implications for late-glacial climate

    NASA Astrophysics Data System (ADS)

    Levy, Laura; Kelly, Meredith; Lowell, Tom; Hall, Brenda; Howley, Jennifer; Smith, Colby

    2016-04-01

    Although the Younger Dryas (YD) has been recorded in ice cores atop the Greenland Ice Sheet, past glacier extents on Greenland dating to the YD are rare. In part, this is due to much of the Greenland Ice Sheet being located offshore until early Holocene time. The Scoresby Sund region (~71°N, 26°W) of central East Greenland, however, is one of only a few locations where the margins of the Greenland Ice Sheet and glaciers independent of the ice sheet were located at least partially on land by late-glacial time. In this region, two distinct sets of moraines, known as the inner and outer Milne Land Stade moraines, have been defined and mark a significant readvance or stillstand during deglaciation from the last glacial maximum. Previous work has dated these moraines to late-glacial and early Holocene time. We present a new 10Be chronology on fluctuations of both the Greenland Ice Sheet and the adjacent Milne Land ice cap from the type locality of the Milne Land Stade moraines in Milne Land. 10Be ages of boulders on bedrock distal to the inner Milne Land Stade moraines range from 12.3 to 11.5 ka and indicate that both ice masses retreated during the YD, likely in response to rising summer temperatures. Since Greenland ice-cores register cold mean annual temperatures throughout the YD, these ice-marginal data support climate conditions characterized by strong seasonality. The mean ages (± 1σ uncertainty) of the inner Milne Land Stade moraines date to 11.4 ± 0.8 ka (Greenland Ice Sheet) and 11.4 ± 0.6 ka (ice cap) indicating that they were formed during Preboreal time or at the end of the YD. Based on these coeval moraine ages, we suggest that both ice masses responded to climate conditions acting on the ice margins, specifically ablation. Moreover, our data show that the ice sheet responded sensitively (i.e., on the same time scale as a small ice cap) to late-glacial and early Holocene climate conditions.

  17. Surface Drifters Track the Fate of Greenland Ice Sheet Meltwater

    NASA Astrophysics Data System (ADS)

    Hauri, Claudine; Truffer, Martin; Winsor, Peter; Lennert, Kunuk

    2014-07-01

    Understanding the fate and influence of glacial meltwater in heavily ice-covered fjord systems has proven difficult because previous measurement platforms were con­strained to deeper water to keep instrumentation safe from drifting icebergs. Now, using novel, satellite-tracked devices that can with­ stand multiple collisions with ice blocks (see Figure 1) without incurring much damage, scientists have obtained new and detailed data about the role of Greenland Ice Sheet meltwater and its trajectories through God­thåbsfjord in western Greenland.

  18. Moulin distribution and formation on the southwest Greenland ice sheet

    NASA Astrophysics Data System (ADS)

    Chu, V. W.; Smith, L. C.; Gleason, C. J.; Yang, K.; Poinar, K.; Joughin, I.; Pitcher, L. H.

    2015-12-01

    River moulins represent a significant connection between surface meltwater generated on the Greenland ice sheet and subglacial drainage networks, where increased meltwater can enhance ice sliding dynamics. In this study, a new high-resolution moulin map is created from WorldView-1/2 imagery acquired during the 2012 record melt year for a 12,500 km2 area near Russell Glacier in southwest Greenland. A total of 1,236 moulins are mapped and categorized as being located: in crevasse fields, along a single ice fracture, within drained lake basins, or having no visible formation mechanism. We find the presence of moulins up to 1787 m elevation, with 11% of moulins found above 1600 m elevation: higher than previously mapped moulins and where glaciological theory suggests few moulins should form. Our study observes moulins in both extensional and compressional ice flow regimes (28% of moulins are found in areas of high extensional strain rate >0.005 yr-1), suggesting that strain rates are not a strong indicator of the likelihood for moulin formation. Overall, moulin density tends to increase with higher bed elevation, thinner ice, lower surface slope, higher velocity, and higher strain rate. In sum, moulins are most common in crevassed, thinner ice near the ice sheet edge, but significant quantities also develop at high elevations. This indicates that future inland expansion of melting may create hydrologic connections between the surface and the bed at higher elevations than previously thought.

  19. Glaciers and ice sheets as a biome.

    PubMed

    Anesio, Alexandre M; Laybourn-Parry, Johanna

    2012-04-01

    The tundra is the coldest biome described in typical geography and biology textbooks. Within the cryosphere, there are large expanses of ice in the Antarctic, Arctic and alpine regions that are not regarded as being part of any biome. During the summer, there is significant melt on the surface of glaciers, ice caps and ice shelves, at which point microbial communities become active and play an important role in the cycling of carbon and other elements within the cryosphere. In this review, we suggest that it is time to recognise the cryosphere as one of the biomes of Earth. The cryospheric biome encompasses extreme environments and is typified by truncated food webs dominated by viruses, bacteria, protozoa and algae with distinct biogeographical structures.

  20. Simulating the Antarctic ice sheet in the Late-Pliocene warm period: PLISMIP-ANT, an ice-sheet model intercomparison project

    NASA Astrophysics Data System (ADS)

    de Boer, Bas; Dolan, Aisling; Bernales, Jorge; Gasson, Edward; Goelzer, Heiko; Golledge, Nick; Sutter, Johannes; Huybrechts, Phillipe; Lohmann, Gerrit; Rogozhina, Irina; Abe-Ouchi, Ayako; Saito, Fuyuki; van de Wal, Roderik

    2015-04-01

    In the context of future climate change, understanding the nature and behaviour of ice sheets during warm intervals in Earth history is of fundamental importance. The Late-Pliocene warm period (also known as the PRISM interval: 3.264 to 3.025 million years before present) can serve as a potential analogue for projected future climates. Although Pliocene ice locations and extents are still poorly constrained, a significant contribution to sea-level rise should be expected from both the Greenland ice sheet and the West and East Antarctic ice sheets based on palaeo sea-level reconstructions. Here, we present results from simulations of the Antarctic ice sheet by means of an international Pliocene Ice Sheet Modeling Intercomparison Project (PLISMIP-ANT). For the experiments, ice-sheet models including the shallow ice and shelf approximations have been used to simulate the complete Antarctic domain (including grounded and floating ice). We compare the performance of six existing numerical ice-sheet models in simulating modern control and Pliocene ice sheets by a suite of four sensitivity experiments. Ice-sheet model forcing fields are taken from the HadCM3 atmosphere-ocean climate model runs for the pre-industrial and the Pliocene. We include an overview of the different ice-sheet models used and how specific model configurations influence the resulting Pliocene Antarctic ice sheet. The six ice-sheet models simulate a comparable present-day ice sheet, although the models are setup with their own parameter settings. For the Pliocene simulations using the Bedmap1 bedrock topography, some models show a small retreat of the East Antarctic ice sheet, which is thought to have happened during the Pliocene for the Wilkes and Aurora basins. This can be ascribed to either the surface mass balance, as the HadCM3 Pliocene climate shows a significant increase over the Wilkes and Aurora basin, or the initial bedrock topography. For the latter, our simulations with the recently

  1. West Antarctic Ice Sheet formed earlier than thought

    NASA Astrophysics Data System (ADS)

    Balcerak, Ernie

    2013-10-01

    About 34 million years ago, Earth transitioned from a warm "greenhouse" climate to a cold "icehouse" climate, marking the transition between the Eocene and Oligocene epochs. This transition has been associated with the formation of a large ice sheet on Antarctica.

  2. Results of the Marine Ice Sheet Model Intercomparison Project, MISMIP

    NASA Astrophysics Data System (ADS)

    Pattyn, F.; Schoof, C.; Perichon, L.; Hindmarsh, R. C. A.; Bueler, E.; de Fleurian, B.; Durand, G.; Gagliardini, O.; Gladstone, R.; Goldberg, D.; Gudmundsson, G. H.; Lee, V.; Nick, F. M.; Payne, A. J.; Pollard, D.; Rybak, O.; Saito, F.; Vieli, A.

    2012-01-01

    Predictions of marine ice-sheet behaviour require models that are able to robustly simulate grounding line migration. We present results of an intercomparison exercise for marine ice-sheet models. Verification is effected by comparison with approximate analytical solutions for flux across the grounding line using simplified geometrical configurations (no lateral variations, no effects of lateral buttressing). Unique steady-state grounding line positions exist for ice sheets on a downward sloping bed, while hysteresis occurs across an overdeepened bed, and stable steady state grounding line positions only occur on the downward-sloping sections. Models based on the shallow ice approximation, which does not resolve extensional stresses, do not reproduce the approximate analytical results unless appropriate parameterizations for ice flux are imposed at the grounding line. For extensional-stress resolving "shelfy stream" models, differences between model results were mainly due to the choice of spatial discretization. Moving grid methods were found to be the most accurate at capturing grounding line evolution, since they track the grounding line explicitly. Adaptive mesh refinement can further improve accuracy, including in fixed-grid models that generally perform poorly at coarse resolution. Fixed grid models with nested grid representations of the grounding line are able to generate accurate steady-state positions, but can be inaccurate over transients. Only one full Stokes model was included in the intercomparison, and consequently the accuracy of shelfy stream models as approximations of full Stokes models remains to be determined in detail, especially during transients.

  3. Results of the Marine Ice Sheet Model Intercomparison Project, MISMIP

    NASA Astrophysics Data System (ADS)

    Pattyn, F.; Schoof, C.; Perichon, L.; Hindmarsh, R. C. A.; Bueler, E.; de Fleurian, B.; Durand, G.; Gagliardini, O.; Gladstone, R.; Goldberg, D.; Gudmundsson, G. H.; Huybrechts, P.; Lee, V.; Nick, F. M.; Payne, A. J.; Pollard, D.; Rybak, O.; Saito, F.; Vieli, A.

    2012-05-01

    Predictions of marine ice-sheet behaviour require models that are able to robustly simulate grounding line migration. We present results of an intercomparison exercise for marine ice-sheet models. Verification is effected by comparison with approximate analytical solutions for flux across the grounding line using simplified geometrical configurations (no lateral variations, no effects of lateral buttressing). Unique steady state grounding line positions exist for ice sheets on a downward sloping bed, while hysteresis occurs across an overdeepened bed, and stable steady state grounding line positions only occur on the downward-sloping sections. Models based on the shallow ice approximation, which does not resolve extensional stresses, do not reproduce the approximate analytical results unless appropriate parameterizations for ice flux are imposed at the grounding line. For extensional-stress resolving "shelfy stream" models, differences between model results were mainly due to the choice of spatial discretization. Moving grid methods were found to be the most accurate at capturing grounding line evolution, since they track the grounding line explicitly. Adaptive mesh refinement can further improve accuracy, including fixed grid models that generally perform poorly at coarse resolution. Fixed grid models, with nested grid representations of the grounding line, are able to generate accurate steady state positions, but can be inaccurate over transients. Only one full-Stokes model was included in the intercomparison, and consequently the accuracy of shelfy stream models as approximations of full-Stokes models remains to be determined in detail, especially during transients.

  4. Reconstruction of late Wisconsinan Ice Sheet and sea-level implications

    NASA Technical Reports Server (NTRS)

    Anderson, John B.

    1993-01-01

    The Ross Sea exhibits north-south oriented troughs associated with modern ice streams and outlet glaciers. Seismic reflection profiles across the troughs show evidence that they were glacially eroded. Seismic records show morphologic features interpreted as till tongues, morainal banks, and possibly glacial deltas formed near the grounding line of the former marine ice sheet. Piston cores from the continental shelf penetrated diamictons whose origin and age is problematic. Detailed petrographic analyses of the minerals and rocks comprising these diamictons were conducted to determine subglacial versus glacial marine origin, and to reconstruct the glacial setting of the Ross Sea during the most recent glacial maximum. The most detailed work, conducted in the western Ross Sea, shows that diamictons do occur in distinct petrologic provinces. This is consistent with deposition from the basal debris zone of either an ice sheet or an ice shelf. Overcompaction, in conjunction with the widespread nature of these deposits, favors deposition from marine ice sheets; ice shelves are believed to deposit their basal debris close to the grounding lines. Other results from the investigation are briefly discussed.

  5. 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.

  6. 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.

  7. Sedimentary record of ice divide migration and ice streams in the Keewatin core region of the Laurentide Ice Sheet

    NASA Astrophysics Data System (ADS)

    Hodder, Tyler J.; Ross, Martin; Menzies, John

    2016-06-01

    The Aberdeen Lake region of central mainland Nunavut is a former core region of the Laurentide Ice Sheet that is characterized by streamlined glacial landforms classified into multiple crosscutting flow sets and near continuous till blanket. The presence of widespread till near the centre of the Keewatin Ice Dome raises questions about its origin. Detailed drillcore logging revealed a complex stratigraphy consisting of at least 6 till units, variably preserved across the study area. Till provenance analysis indicates deposition by near opposite-trending ice flow phases, interpreted as evidence of reconfiguration of the Keewatin Ice Divide. At the surface, large north-northwesterly aligned landforms are present across the study area. The till stratigraphy within these landforms indicates the same NNW ice flow phase is responsible for considerable till production. This ice flow phase is also correlated to a long regional dispersal train of erratics toward the Gulf of Boothia. The production of an extensive, thick (~ 12 m), till sheet during the NNW-trending ice flow phase occurred far from the ice margin at a time of extensive ice cover of mainland Nunavut, likely from an east-west oriented ice divide. A deglacial westerly trending ice flow phase formed small drumlins atop the larger NNW streamlined till ridges and deposited a surficial till unit that is too thin to mask the NNW flow set across the study area. It is proposed that the Boothia paleo-ice stream catchment area propagated deep into the Laurentide Ice Sheet and contributed to significant till production in this core region of the Keewatin Sector prior to the westerly ice flow shift. The apparent relationship between till thickness and the size of the associated or correlated drumlins, flow sets, and dispersal trains indicates complex erosion/deposition interplay is involved in the formation of streamlined subglacial landforms.

  8. Full-depth englacial vertical ice sheet velocities measured using phase-sensitive radar

    NASA Astrophysics Data System (ADS)

    Kingslake, Jonathan; Hindmarsh, Richard C. A.; Adalgeirsdóttir, Gusfinna; Conway, Howard; Corr, Hugh F. J.; Gillet-Chaulet, Fabien; Martín, Carlos; King, Edward C.; Mulvaney, Robert; Pritchard, Hamish D.

    2014-12-01

    We describe a geophysical technique to measure englacial vertical velocities through to the beds of ice sheets without the need for borehole drilling. Using a ground-based phase-sensitive radio echo sounder (pRES) during seven Antarctic field seasons, we measure the temporal changes in the position of englacial reflectors within ice divides up to 900 m thick on Berkner Island, Roosevelt Island, Fletcher Promontory, and Adelaide Island. Recorded changes in reflector positions yield "full-depth" profiles of vertical ice velocity that we use to examine spatial variations in ice flow near the divides. We interpret these variations by comparing them to the results of a full-Stokes simulation of ice divide flow, qualitatively validating the model and demonstrating that we are directly detecting an ice-dynamical phenomenon called the Raymond Effect. Using pRES, englacial vertical ice velocities can be measured in higher spatial resolution than is possible using instruments installed within the ice. We discuss how these measurements could be used with inverse methods to measure ice rheology and to improve ice core dating by incorporating pRES-measured vertical velocities into age modeling.

  9. Geologic controls on the West Antarctic Ice Sheet

    NASA Technical Reports Server (NTRS)

    Anandakrishnan, Sridhar

    1993-01-01

    The stability of the West Antarctic Ice Sheet is intertwined with its geologic history. The sub-ice geology and the possibility of active rifting and associated elevated heat-flux and volcanism might be determining factors in ice-sheet behavior. Seismic monitoring of natural events at the base of Ice Stream C reveals the presence of a young sedimentary basin beneath the ice stream. The sediments are presumed to be of glacio-marine origin, similar to those beneath Ice Stream B and in the Ross Sea. The young sediments are approximately 1/2 km thick at UpC camp, but thin abruptly southward to 100 m or less. We hypothesize the presence of a fault with a throw of 400 m to account for this (though we have not directly detected the fault), rather than invoking unrealistic basement dips. To extend these studies to critical inland regions, we suggest an expanded explosive-source seismic survey of the Byrd Subglacial Basin to determine the extent and character of the hypothesized rift basin. High-resolution seismic monitoring will detect layering in the sedimentary column, as well as possibly imaging faults directly.

  10. ICESHEET 1.0: a program to produce paleo-ice sheet reconstructions with minimal assumptions

    NASA Astrophysics Data System (ADS)

    Gowan, Evan J.; Tregoning, Paul; Purcell, Anthony; Lea, James; Fransner, Oscar J.; Noormets, Riko; Dowdeswell, J. A.

    2016-05-01

    We describe a program that produces paleo-ice sheet reconstructions using an assumption of steady-state, perfectly plastic ice flow behaviour. It incorporates three input parameters: ice margin, basal shear stress and basal topography. Though it is unlikely that paleo-ice sheets were ever in complete steady-state conditions, this method can produce an ice sheet without relying on complicated and unconstrained parameters such as climate and ice dynamics. This makes it advantageous to use in glacial-isostatic adjustment ice sheet modelling, which are often used as input parameters in global climate modelling simulations. We test this program by applying it to the modern Greenland Ice Sheet and Last Glacial Maximum Barents Sea Ice Sheet and demonstrate the optimal parameters that balance computational time and accuracy.

  11. Simulating the Antarctic ice sheet in the Late-Pliocene warm period: PLISMIP-ANT, an ice-sheet model intercomparison project

    NASA Astrophysics Data System (ADS)

    de Boer, B.; Dolan, A. M.; Bernales, J.; Gasson, E.; Goelzer, H.; Golledge, N. R.; Sutter, J.; Huybrechts, P.; Lohmann, G.; Rogozhina, I.; Abe-Ouchi, A.; Saito, F.; van de Wal, R. S. W.

    2014-11-01

    In the context of future climate change, understanding the nature and behaviour of ice sheets during warm intervals in Earth history is of fundamental importance. The Late-Pliocene warm period (also known as the PRISM interval: 3.264 to 3.025 million years before present) can serve as a potential analogue for projected future climates. Although Pliocene ice locations and extents are still poorly constrained, a significant contribution to sea-level rise should be expected from both the Greenland ice sheet and the West and East Antarctic ice sheets based on palaeo sea-level reconstructions. Here, we present results from simulations of the Antarctic ice sheet by means of an international Pliocene Ice Sheet Modeling Intercomparison Project (PLISMIP-ANT). For the experiments, ice-sheet models including the shallow ice and shelf approximations have been used to simulate the complete Antarctic domain (including grounded and floating ice). We compare the performance of six existing numerical ice-sheet models in simulating modern control and Pliocene ice sheets by a suite of four sensitivity experiments. Ice-sheet model forcing fields are taken from the HadCM3 atmosphere-ocean climate model runs for the pre-industrial and the Pliocene. We include an overview of the different ice-sheet models used and how specific model configurations influence the resulting Pliocene Antarctic ice sheet. The six ice-sheet models simulate a comparable present-day ice sheet, although the models are setup with their own parameter settings. For the Pliocene simulations using the Bedmap1 bedrock topography, some models show a small retreat of the East Antarctic ice sheet, which is thought to have happened during the Pliocene for the Wilkes and Aurora basins. This can be ascribed to either the surface mass balance, as the HadCM3 Pliocene climate shows a significant increase over the Wilkes and Aurora basin, or the initial bedrock topography. For the latter, our simulations with the recently

  12. Radiocarbon chronology of the last deglaciation in the Baffin Bay reveals asynchronous melting of Arctic and Laurentide ice sheets

    NASA Astrophysics Data System (ADS)

    Jackson, Rebecca; Kucera, Michal; Vogt, Christoph; Wacker, Lukas

    2016-04-01

    The transition from the last ice age into the Holocene interglacial was characterised by rapid retreat of North American ice sheets, discharging large quantities of meltwater into the Labrador Sea. Whereas the meltwater chronology of the Laurentide Ice Sheet is well documented, the deglacial history of the American Arctic ice sheets (Inuit Ice sheet and northern Greenland Ice Sheet) draining into the Labrador Sea via the Baffin Bay is less well constrained. Here we present the first high-resolution radiocarbon-dated deglacial records from the Canadian and Greenland margins of the central Baffin Bay. Sedimentological and geochemical data confirm the presence during Termination I of two events of enhanced delivery of detrital carbonate (Baffin Bay Detrital Carbonate Events) dated to 14.2-13.7 ka BP and 12.7-11 ka BP. The events are synchronous across the Baffin Bay and their mineralogical signature indicates a common source of detrital carbonate from the Canadian Arctic, with a synchronous clastic source proximal to Greenland. The events postdate Heinrich layers and their onset is not linked to Greenland temperature change. This indicates that the deglaciation of American Arctic ice sheets and associated meltwater discharge were decoupled from the dominant North Atlantic climate mode.

  13. Evidence from Ice-Rafted Debris and Sediment Provenance for a Dynamic East Antarctic Ice Sheet During the Mid-Miocene Climate Transition

    NASA Astrophysics Data System (ADS)

    Williams, T.; Pierce, E. L.; van de Flierdt, T.; Hemming, S. R.; Cook, C. P.; Passchier, S.; Sangiorgi, F.; Bijl, P.

    2015-12-01

    The Antarctic ice sheets underwent a major expansion during the Mid-Miocene Climate Transition, around 14 Ma, lowering eustatic sea level by perhaps 50m, based on evidence from benthic oxygen isotope records and sea level indicators. However, direct evidence of changes in the ice sheet is limited to sites in or close to the Transantarctic Mountains. Here we present evidence for ice sheet change from two widely separated sites offshore of East Antarctica, IODP Site U1356, Wilkes Land, and ODP Site 1165, Prydz Bay. Between 14.1 and 13.8 Ma at these sites, episodic pulses of ice-rafted debris (IRD), including dropstones, were deposited in concentrations exceeding those in the rest of the Miocene. These repeated pulses of IRD-bearing icebergs indicate large and repeated advances and retreats of the ice sheet during the course of the transition to a larger and relatively more stable ice sheet. We conducted provenance analyses on the mid-Miocene IRD and sediments. At Site U1356, 40Ar/39Ar ages of ice-rafted hornblende grains show that a major ice drainage was situated along the inland part of the Mertz Shear Zone and its southward extension along the west side of the Wilkes Subglacial Basin, while Nd isotope data from the terrigenous fine fraction show that the ice margin periodically expanded from high ground well into the Wilkes Subglacial Basin during periods of ice growth. At Site 1165, 40Ar/39Ar on dropstones indicate provenance from both the Lambert Glacier region and the part of Wilkes Land that contains the Aurora Subglacial Basin. The two sites provide a direct record of repeated collapse and re-growth of ice in at least two of East Antarctica's main drainage basins during the mid-Miocene climate transition. We will set our ice-rafted debris and provenance evidence for cryosphere change in the context of mid-Miocene climate records.

  14. Generation of a new Greenland Ice Sheet Digital Elevation Model

    NASA Astrophysics Data System (ADS)

    Nagarajan, S.; Csatho, B. M.; Schenk, A. F.; Babonis, G. S.; Scambos, T. A.; Haran, T. M.; Kjaer, K. H.; Korsgaard, N. J.

    2011-12-01

    Currently available Digital Elevation Models(DEMs) of the Greenland Ice Sheet (GrIS) were originally derived from radar altimetry data, e.g. Bamber (Bamber et al., 2001) and later improved by photoclinometry to fill the regions between orbits (Scambos and Haran, 2002). The elevation error of these DEMs is a few meters in the higher part (above 2000 m) of the ice sheet, but it can be as much as 50-100 meters in marginal regions. The relatively low resolution and accuracy poses a problem, especially for ice sheet modeling. Although accurate elevation data have been collected by airborne and spaceborne laser altimetry (airborne: Airborne Topographic Mapper (ATM) (1993-present), Laser Vegetation Imaging Sensor(LVIS) (2007,2009 and 2011); spaceborne: Ice, Cloud, and land Elevation Satellite (ICESat) (2003-2009)) and DEMs have been derived from stereo satellite imagery (e.g., SPOT (40 m), ASTER (15 m)), a high resolution, consistent DEM of GrIS is not yet available. This is due to various problems, such as different error sources in the data and different dates of data acquisition. In order to overcome these difficulties, we generated a multi-resolution DEM of GrIS, reflecting June 2008 conditions, by fusing a photoclinometry DEM, SPOT and ASTER DEMs as well as elevations from ICESat, ATM and LVIS laser altimetry. The new multi-resolution DEM has a resolution of 40 m x 40 m in the marginal ice sheet regions and 250 m elsewhere. The ice sheet margin is mapped from SPOT and Landsat imagery and SPOT DEMs are used to cover the complex topography of ice sheet marginal regions. The accuracy of SPOT DEMs is approximately ± 6 m except in the areas covered by clouds regions, where the SPOT elevations were replaced by ASTER DEMs. The ASTER DEMs were checked and improved by the DEM derived from aerial photography from the 1980s. A new photoclinometry DEM, derived from Advanced Very High Resolution Radiometer (AVHRR) and Moderate Resolution Imaging Spectroradiometer (MODIS) imagery

  15. Modeling of Firn Compaction for Estimating Ice-Sheet Mass Change from Observed Ice-Sheet Elevation Change

    NASA Technical Reports Server (NTRS)

    Li, Jun; Zwally, H. Jay

    2011-01-01

    Changes in ice-sheet surface elevation are caused by a combination of ice-dynamic imbalance, ablation, temporal variations in accumulation rate, firn compaction and underlying bedrock motion. Thus, deriving the rate of ice-sheet mass change from measured surface elevation change requires information on the rate of firn compaction and bedrock motion, which do not involve changes in mass, and requires an appropriate firn density to associate with elevation changes induced by recent accumulation rate variability. We use a 25 year record of surface temperature and a parameterization for accumulation change as a function of temperature to drive a firn compaction model. We apply this formulation to ICESat measurements of surface elevation change at three locations on the Greenland ice sheet in order to separate the accumulation-driven changes from the ice-dynamic/ablation-driven changes, and thus to derive the corresponding mass change. Our calculated densities for the accumulation-driven changes range from 410 to 610 kg/cu m, which along with 900 kg/cu m for the dynamic/ablation-driven changes gives average densities ranging from 680 to 790 kg/cu m. We show that using an average (or "effective") density to convert elevation change to mass change is not valid where the accumulation and the dynamic elevation changes are of opposite sign.

  16. Recent Changes in Arctic Glaciers, Ice Caps, and the Greenland Ice Sheet: Cold Facts About Warm Ice

    NASA Astrophysics Data System (ADS)

    Abdalati, W.

    2005-12-01

    One of the major manifestations of Arctic change can be observed in the state of balance of Arctic glaciers and ice caps and the Greenland ice sheet. These ice masses are estimated to contain nearly 3 million cubic kilometers of ice, which is more than six times greater than all the water stored in the Earth's lakes, rivers, and snow combined and is the equivalent of over 7 meters of sea level. Most of these ice masses have been shrinking in recent in years, but their mass balance is highly variable on a wide range of spatial and temporal scales. On the Greenland ice sheet most of the coastal regions have thinned substantially as melt has increased and some of its outlet glaciers have accelerated. Near the equilibrium line in West Greenland, we have seen evidence of summer acceleration that is linked to surface meltwater production, suggesting a relatively rapid response mechanism of the ice sheet change to a warming climate. At the same time, however, the vast interior regions of the Greenland ice sheet have shown little change or slight growth, as accumulation in these areas may have increased. Throughout much of the rest of the Arctic, many glaciers and ice caps have been shrinking in the past few decades, and in Canada and Alaska, the rate of ice loss seems to have accelerated during the late 1990s. These recent observations offer only a snapshot in time of the long-term behavior, but they are providing crucial information about the current state of ice mass balance and the mechanisms that control it in one of the most climatically sensitive regions on Earth. As we continue to learn more through a combination of remote sensing observations, in situ measurements and improved modeling capabilities, it is important that we coordinate and integrate these approaches effectively in order to predict future changes and their impact on sea level, freshwater discharge, and ocean circulation.

  17. Late-Holocene Fluctuations of the Greenland Ice Sheet: Insights from a south Greenland threshold lake

    NASA Astrophysics Data System (ADS)

    Sinclair, G.; Carlson, A. E.; Reilly, B.

    2015-12-01

    Several centennial-scale climate fluctuations during the late-Holocene make it an ideal test case for examining the effects of climate change on sea level at societally-relevant timescales. Across much of the Arctic, glaciers and ice sheets reached their maximum late-Holocene extent during the Little Ice Age (LIA, 1400-1900 C.E.), approximately coincident with the global temperature minima observed during this time. However, ongoing work suggests the south Greenland Ice Sheet (sGrIS) may have behaved differently during the late-Holocene, with several outlet glaciers retreating, rather than advancing, during the LIA, possibly due to regional warming in the region different from the Arctic trend. The Qassimiut lobe, a low-lying piedmont-like extension of the sGrIS, may be especially sensitive to late-Holocene climate changes. Geomorphic evidence outboard of Naujaat Sermia, an outlet glacier draining the Qassimiut lobe, suggests three distinct periods of land exposure. We hypothesize these occurred during the last deglacial period, after an advance from near or behind the present margin during the Neoglacial, and during warming following the Little Ice Age in the last 1-2 centuries. Here, we present data from threshold lake cores immediately outboard of the presumed Neoglacial moraine. A sharp contact divides glacial sands and silts from organic gyttja, indicating glacial retreat from the moraine and subsequent meltwater diversion. The contact is accompanied by several geochemical changes, including increased Fe/Ti ratios, increased Br, and decreased Si and K, indicating a switch from more clastic to organic sedimentation. Radiocarbon ages from eight macrofossils immediately above this contact are calibrated to 1350-1950 C.E., suggesting the ice sheet may have retreated from its late-Holocene maximum during the Little Ice Age, but the wide range in ages suggests reworking of organic material may be significant in this region.

  18. Elastic response of a grounded ice sheet coupled to a floating ice shelf.

    PubMed

    Sayag, Roiy; Worster, M Grae

    2011-09-01

    An ice sheet that spreads into an ocean is forced to bend owing to its buoyancy and detaches from the bedrock to form a floating ice shelf. The location of the transition between the grounded sheet and the floating shelf, defined as the grounding line, behaves as a free boundary. We develop a model of an elastic grounded sheet resting on a deformable elastic bed and coupled to an elastic floating shelf. We find that the grounding-line position is determined by the geometry of the bed and the bending-buoyancy length scale of the system. These two contributions depend on the reaction modulus of the bed in opposite ways. We show that the structure of the floating shelf depends on the bending-buoyancy length scale only, allowing us to calculate the bending stiffness of the elastic sheet independently of the properties of the bed. Relations between the structure of the floating shelf and the grounding-line position are also developed. Our theoretical predictions agree with laboratory experiments made using thick elastic sheets and a dense salt solution. Our findings may provide new insights into the dynamics near grounding lines, as well as methods to infer the bending stiffness of ice sheets and the grounding-line position from satellite altimetery that can be applied to elastic sheets in general.

  19. Explosive ice age diversification of kiwi.

    PubMed

    Weir, Jason T; Haddrath, Oliver; Robertson, Hugh A; Colbourne, Rogan M; Baker, Allan J

    2016-09-20

    Molecular dating largely overturned the paradigm that global cooling during recent Pleistocene glacial cycles resulted in a burst of species diversification although some evidence exists that speciation was commonly promoted in habitats near the expanding and retracting ice sheets. Here, we used a genome-wide dataset of more than half a million base pairs of DNA to test for a glacially induced burst of diversification in kiwi, an avian family distributed within several hundred kilometers of the expanding and retracting glaciers of the Southern Alps of New Zealand. By sampling across the geographic range of the five kiwi species, we discovered many cryptic lineages, bringing the total number of kiwi taxa that currently exist to 11 and the number that existed just before human arrival to 16 or 17. We found that 80% of kiwi diversification events date to the major glacial advances of the Middle and Late Pleistocene. During this period, New Zealand was repeatedly fragmented by glaciers into a series of refugia, with the tiny geographic ranges of many kiwi lineages currently distributed in areas adjacent to these refugia. Estimates of effective population size through time show a dramatic bottleneck during the last glacial cycle in all but one kiwi lineage, as expected if kiwi were isolated in glacially induced refugia. Our results support a fivefold increase in diversification rates during key glacial periods, comparable with levels observed in classic adaptive radiations, and confirm that at least some lineages distributed near glaciated regions underwent rapid ice age diversification. PMID:27573837

  20. Explosive ice age diversification of kiwi.

    PubMed

    Weir, Jason T; Haddrath, Oliver; Robertson, Hugh A; Colbourne, Rogan M; Baker, Allan J

    2016-09-20

    Molecular dating largely overturned the paradigm that global cooling during recent Pleistocene glacial cycles resulted in a burst of species diversification although some evidence exists that speciation was commonly promoted in habitats near the expanding and retracting ice sheets. Here, we used a genome-wide dataset of more than half a million base pairs of DNA to test for a glacially induced burst of diversification in kiwi, an avian family distributed within several hundred kilometers of the expanding and retracting glaciers of the Southern Alps of New Zealand. By sampling across the geographic range of the five kiwi species, we discovered many cryptic lineages, bringing the total number of kiwi taxa that currently exist to 11 and the number that existed just before human arrival to 16 or 17. We found that 80% of kiwi diversification events date to the major glacial advances of the Middle and Late Pleistocene. During this period, New Zealand was repeatedly fragmented by glaciers into a series of refugia, with the tiny geographic ranges of many kiwi lineages currently distributed in areas adjacent to these refugia. Estimates of effective population size through time show a dramatic bottleneck during the last glacial cycle in all but one kiwi lineage, as expected if kiwi were isolated in glacially induced refugia. Our results support a fivefold increase in diversification rates during key glacial periods, comparable with levels observed in classic adaptive radiations, and confirm that at least some lineages distributed near glaciated regions underwent rapid ice age diversification.

  1. Hydromechanical Failure Analysis Associated with Laurentide Ice Sheet Glaciations

    NASA Astrophysics Data System (ADS)

    Zhang, Y.; Person, M. A.; Voller, V. R.

    2015-12-01

    Glacial loading by continental ice sheets has been linked to large (> M6) Late Pleistocene and Holocene seismicity in Scandinavia and North America along with sedimentary basin blowout features such as Lake Howe, Manitoba Canada and glacio-tectonic thrust structures. The specter of future glaciations is considered one of the most important factors to consider in siting of high-level nuclear wastes in Switzerland, Canada, and Sweden. To date, continental-scale analysis of crustal failure has focused on mechanical failure which has neglected the effects of pore pressure and permeability changes. We have developed a two-dimensional, Cartesian, elastic, plane strain control volume finite element (CVFEM) model to investigate the effect of pore pressure on the failure potential along a cross-sectional transect stretching from the Hudson Bay to the Gulf of Mexico. Our analysis considers fluid flow and pore pressure eveolution within the upper 10km of the earths crust and mechanical deformation within at 100 km thick lithosphere. We imposed 4 cycles of glaciation of the Laurentide ice sheet during the Last Late Pleistocene. The geomechanical deformation is coupled to the fluid flow through time dependent changes in the mean normal stress. We have conducted a sensitivity study in which we have varied the permeability of the upper crust between a range of 10-20 to 10-14 m2. We solve a series of one-dimensional heat transfer equations to determine regions where the Laurentide ice sheet is wet based or frozen. A Mohr-Coulomb failure criteria was used to analyze the potential of seismicity and permeability changes along pre-existing critically stressed faults. Mechanically failure (neglecting pore pressure evolution, see Figure A attached) caused by ice sheet induced stress perturbation were found to be primarily concentrate in the forebulge region of the ice sheet with the effective Coulomb stress being 4.4 MPa. However, when pore pressure evolution is considered in the

  2. A Grand Design for Future Ice Sheet Projections

    NASA Astrophysics Data System (ADS)

    Edwards, T.

    2014-12-01

    If we are to make robust projections of the probability of sea level rise from the ice sheets, these must be founded in both glaciological theory, represented by dynamical ice sheet models, and statistical inference, i.e. formal uncertainty quantification (UQ). No such studies yet exist for either the Greenland or Antarctic ice sheet. Therefore projections risk being physically implausible, difficult to interpret, or both. More optimistically, ice sheet models have many advantages over climate models with respect to uncertainty quantification. They are computationally cheaper, simpler to understand, and have fewer input parameters and output variables. It is relatively straightforward to switch between different model structures, such as physics approximations, basal drag laws, and resolution. Moreover the ice sheet modelling community is relatively small and is not constrained methodologically or culturally by the legacy - and pitfalls - of the CMIP multi-model "ensemble of opportunity". These advantages present us with a golden opportunity to create a new vision for policy-relevant sea level projections: we can design a grand ensemble that quantifies multiple modelling uncertainties in a statistically rigorous and efficient way. Such an ensemble systematically samples model parameters and structures, initial and boundary conditions, in the most informative way given the available resources and also allows statistical inference. I will review some of the UQ steps that have been taken in ice sheet modelling, such as Bayesian calibration of parameters and projections, history matching (statistically-formalised ruling out of poor parameter values), and emulation (statistical surrogates of numerical models). I will then describe how to implement these and other techniques in a multi-model ensemble design, including: sequential experimental design, which is more efficient than the usual Latin Hypercube; quantifying uncertainty in Full Stokes and high resolution models

  3. Last Glacial Maximum cirque glaciation in Ireland and implications for reconstructions of the Irish Ice Sheet

    NASA Astrophysics Data System (ADS)

    Barth, Aaron M.; Clark, Peter U.; Clark, Jorie; McCabe, A. Marshall; Caffee, Marc

    2016-06-01

    Reconstructions of the extent and height of the Irish Ice Sheet (IIS) during the Last Glacial Maximum (LGM, ∼19-26 ka) are widely debated, in large part due to limited age constraints on former ice margins and due to uncertainties in the origin of the trimlines. A key area is southwestern Ireland, where various LGM reconstructions range from complete coverage by a contiguous IIS that extends to the continental shelf edge to a separate, more restricted southern-sourced Kerry-Cork Ice Cap (KCIC). We present new 10Be surface exposure ages from two moraines in a cirque basin in the Macgillycuddy's Reeks that provide a unique and unequivocal constraint on ice thickness for this region. Nine 10Be ages from an outer moraine yield a mean age of 24.5 ± 1.4 ka while six ages from an inner moraine yield a mean age of 20.4 ± 1.2 ka. These ages show that the northern flanks of the Macgillycuddy's Reeks were not covered by the IIS or a KCIC since at least 24.5 ± 1.4 ka. If there was more extensive ice coverage over the Macgillycuddy's Reeks during the LGM, it occurred prior to our oldest ages.

  4. 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

  5. Simulating the Antarctic ice sheet in the Late-Pliocene warm period: PLISMIP-ANT, an ice-sheet model intercomparison project

    NASA Astrophysics Data System (ADS)

    de Boer, Bas; Dolan, Aisling M.; Hill, Daniel J.; van de Wal, Roderik S. W.

    2014-05-01

    In the context of future climate change, understanding the nature and behaviour of ice sheets during warm intervals in Earth history is of fundamental importance. The Late-Pliocene Warm Period (also known as the PRISM interval: 3.29 to 2.97 million years before present) can serve as a potential analogue for projected future climates, with a global annual mean surface-air temperature warming of 1.76 °C. Although Pliocene ice locations and surface extents are still poorly constrained, a significant contribution to sea-level rise should be expected from Greenland and West and, possibly, East Antarctica based on palaeo sea-level reconstructions. Here, we present results from simulations of the Antarctic ice sheet by means of an international Pliocene Ice Sheet Modeling Intercomparison Project (PLISMIP-ANT). We include an overview of the different ice-sheet models used and how specific model configurations influence the resulting Pliocene Antarctic ice sheet. For the experiments, ice-sheet models including the shallow ice and shelf approximations have been used to simulate the complete Antarctic domain (including grounded and floating ice). We compare the performance of the ice-sheet models in simulating modern control and Pliocene ice sheets by a suite of sensitivity experiments. Ice-sheet model forcing fields are taken from the PlioMIP results incorporating multiple coupled atmosphere-ocean general circulation models (GCM). We show that ice-sheet models simulate a present-day ice sheet which is comparable to the observations, and find no systematic biases introduced when using different GCM forcing relative to observational climate forcing. This project includes multiple ice-sheet models forced with multiple climate model output, from which a comprehensive assessment can be made as to the uncertainties of ice-sheet extent on Antarctica. These results may eventually serve as a new constraint on the extent of the Antarctic ice sheet during the Late-Pliocene Warm Period

  6. Supraglacial bacterial community structures vary across the Greenland ice sheet.

    PubMed

    Cameron, Karen A; Stibal, Marek; Zarsky, Jakub D; Gözdereliler, Erkin; Schostag, Morten; Jacobsen, Carsten S

    2016-02-01

    The composition and spatial variability of microbial communities that reside within the extensive (>200 000 km(2)) biologically active area encompassing the Greenland ice sheet (GrIS) is hypothesized to be variable. We examined bacterial communities from cryoconite debris and surface ice across the GrIS, using sequence analysis and quantitative PCR of 16S rRNA genes from co-extracted DNA and RNA. Communities were found to differ across the ice sheet, with 82.8% of the total calculated variation attributed to spatial distribution on a scale of tens of kilometers separation. Amplicons related to Sphingobacteriaceae, Pseudanabaenaceae and WPS-2 accounted for the greatest portion of calculated dissimilarities. The bacterial communities of ice and cryoconite were moderately similar (global R = 0.360, P = 0.002) and the sampled surface type (ice versus cryoconite) did not contribute heavily towards community dissimilarities (2.3% of total variability calculated). The majority of dissimilarities found between cryoconite 16S rRNA gene amplicons from DNA and RNA was calculated to be the result of changes in three taxa, Pseudanabaenaceae, Sphingobacteriaceae and WPS-2, which together contributed towards 80.8 ± 12.6% of dissimilarities between samples. Bacterial communities across the GrIS are spatially variable active communities that are likely influenced by localized biological inputs and physicochemical conditions. PMID:26691594

  7. 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.

  8. Supraglacial bacterial community structures vary across the Greenland ice sheet.

    PubMed

    Cameron, Karen A; Stibal, Marek; Zarsky, Jakub D; Gözdereliler, Erkin; Schostag, Morten; Jacobsen, Carsten S

    2016-02-01

    The composition and spatial variability of microbial communities that reside within the extensive (>200 000 km(2)) biologically active area encompassing the Greenland ice sheet (GrIS) is hypothesized to be variable. We examined bacterial communities from cryoconite debris and surface ice across the GrIS, using sequence analysis and quantitative PCR of 16S rRNA genes from co-extracted DNA and RNA. Communities were found to differ across the ice sheet, with 82.8% of the total calculated variation attributed to spatial distribution on a scale of tens of kilometers separation. Amplicons related to Sphingobacteriaceae, Pseudanabaenaceae and WPS-2 accounted for the greatest portion of calculated dissimilarities. The bacterial communities of ice and cryoconite were moderately similar (global R = 0.360, P = 0.002) and the sampled surface type (ice versus cryoconite) did not contribute heavily towards community dissimilarities (2.3% of total variability calculated). The majority of dissimilarities found between cryoconite 16S rRNA gene amplicons from DNA and RNA was calculated to be the result of changes in three taxa, Pseudanabaenaceae, Sphingobacteriaceae and WPS-2, which together contributed towards 80.8 ± 12.6% of dissimilarities between samples. Bacterial communities across the GrIS are spatially variable active communities that are likely influenced by localized biological inputs and physicochemical conditions.

  9. Mass Gains of the Antarctic Ice Sheet Exceed Losses

    NASA Technical Reports Server (NTRS)

    Zwally, H. Jay; Li, Jun; Robbins, John; Saba, Jack L.; Yi, Donghui; Brenner, Anita; Bromwich, David

    2012-01-01

    During 2003 to 2008, the mass gain of the Antarctic ice sheet from snow accumulation exceeded the mass loss from ice discharge by 49 Gt/yr (2.5% of input), as derived from ICESat laser measurements of elevation change. The net gain (86 Gt/yr) over the West Antarctic (WA) and East Antarctic ice sheets (WA and EA) is essentially unchanged from revised results for 1992 to 2001 from ERS radar altimetry. Imbalances in individual drainage systems (DS) are large (-68% to +103% of input), as are temporal changes (-39% to +44%). The recent 90 Gt/yr loss from three DS (Pine Island, Thwaites-Smith, and Marie-Bryd Coast) of WA exceeds the earlier 61 Gt/yr loss, consistent with reports of accelerating ice flow and dynamic thinning. Similarly, the recent 24 Gt/yr loss from three DS in the Antarctic Peninsula (AP) is consistent with glacier accelerations following breakup of the Larsen B and other ice shelves. In contrast, net increases in the five other DS of WA and AP and three of the 16 DS in East Antarctica (EA) exceed the increased losses. Alternate interpretations of the mass changes driven by accumulation variations are given using results from atmospheric-model re-analysis and a parameterization based on 5% change in accumulation per degree of observed surface temperature change. A slow increase in snowfall with climate waRMing, consistent with model predictions, may be offsetting increased dynamic losses.

  10. Evaluating ice sheet model spinup procedures using chronological data constraining ice margin positions over time on Greenland

    NASA Astrophysics Data System (ADS)

    Applegate, P. J.; Kirchner, N.; Levy, L.; Kelly, M. A.; Lowell, T. V.; Greve, R.

    2011-12-01

    We compare a recently-published ice sheet model run to field data constraining ice margin positions over time on Greenland, to assess presently-accepted model spinup procedures. Computer models describing ice flow and mass balance are important tools for learning about the future behavior of ice sheets in a warming world. Because ice softness is temperature-sensitive and the thermal field within the ice sheet is mostly unknown, ice sheet models must be "spun up" using paleoclimate data before future changes can be estimated. The models produce ice margin positions over time during the spinup, allowing comparison with field data such as cosmogenic exposure dates and radiocarbon dating of organic matter. If the agreement between modeled and reconstructed ice margin positions is good, we can have increased confidence in the models' ability to forecast future changes. For the present study, we use a model setup from Greve et al. (2011; Annals of Glaciology 52, 23-30; sicopolis.greveweb.net), and a preliminary collection of chronological data. We aggregate the chronological data to the model grid, then plot the data and modeled ice margin positions as time-distance diagrams along west-east transects. Our results have implications for the use of the Summit ice cores to predict mass balance around the margins of the ice sheet and future projections of sea level rise using ice sheet models.

  11. Ice-sheet acceleration driven by melt supply variability.

    PubMed

    Schoof, Christian

    2010-12-01

    Increased ice velocities in Greenland are contributing significantly to eustatic sea level rise. Faster ice flow has been associated with ice-ocean interactions in water-terminating outlet glaciers and with increased surface meltwater supply to the ice-sheet bed inland. Observed correlations between surface melt and ice acceleration have raised the possibility of a positive feedback in which surface melting and accelerated dynamic thinning reinforce one another, suggesting that overall warming could lead to accelerated mass loss. Here I show that it is not simply mean surface melt but an increase in water input variability that drives faster ice flow. Glacier sliding responds to melt indirectly through changes in basal water pressure, with observations showing that water under glaciers drains through channels at low pressure or through interconnected cavities at high pressure. Using a model that captures the dynamic switching between channel and cavity drainage modes, I show that channelization and glacier deceleration rather than acceleration occur above a critical rate of water flow. Higher rates of steady water supply can therefore suppress rather than enhance dynamic thinning, indicating that the melt/dynamic thinning feedback is not universally operational. Short-term increases in water input are, however, accommodated by the drainage system through temporary spikes in water pressure. It is these spikes that lead to ice acceleration, which is therefore driven by strong diurnal melt cycles and an increase in rain and surface lake drainage events rather than an increase in mean melt supply.

  12. A dynamic early East Antarctic Ice Sheet suggested by ice-covered fjord landscapes.

    PubMed

    Young, Duncan A; Wright, Andrew P; Roberts, Jason L; Warner, Roland C; Young, Neal W; Greenbaum, Jamin S; Schroeder, Dustin M; Holt, John W; Sugden, David E; Blankenship, Donald D; van Ommen, Tas D; Siegert, Martin J

    2011-06-01

    The first Cenozoic ice sheets initiated in Antarctica from the Gamburtsev Subglacial Mountains and other highlands as a result of rapid global cooling ∼34 million years ago. In the subsequent 20 million years, at a time of declining atmospheric carbon dioxide concentrations and an evolving Antarctic circumpolar current, sedimentary sequence interpretation and numerical modelling suggest that cyclical periods of ice-sheet expansion to the continental margin, followed by retreat to the subglacial highlands, occurred up to thirty times. These fluctuations were paced by orbital changes and were a major influence on global sea levels. Ice-sheet models show that the nature of such oscillations is critically dependent on the pattern and extent of Antarctic topographic lowlands. Here we show that the basal topography of the Aurora Subglacial Basin of East Antarctica, at present overlain by 2-4.5 km of ice, is characterized by a series of well-defined topographic channels within a mountain block landscape. The identification of this fjord landscape, based on new data from ice-penetrating radar, provides an improved understanding of the topography of the Aurora Subglacial Basin and its surroundings, and reveals a complex surface sculpted by a succession of ice-sheet configurations substantially different from today's. At different stages during its fluctuations, the edge of the East Antarctic Ice Sheet lay pinned along the margins of the Aurora Subglacial Basin, the upland boundaries of which are currently above sea level and the deepest parts of which are more than 1 km below sea level. Although the timing of the channel incision remains uncertain, our results suggest that the fjord landscape was carved by at least two iceflow regimes of different scales and directions, each of which would have over-deepened existing topographic depressions, reversing valley floor slopes. PMID:21637255

  13. Late Holocene Sea-levels, Climate and Ice Sheet Dynamics in West Greenland

    NASA Astrophysics Data System (ADS)

    Long, A. J.; Woodroffe, S. A.; Bryant, C.

    2007-12-01

    At the end of the Holocene altithermal, c. 5 ka cal. yr BP, the Greenland Ice Sheet (GIS) had retreated 10-30 km inland of its present position. During the subsequent neoglacial, from c. 4 ka cal yr BP onwards, the GIS re-grew, advancing to reach a maximum extent during the "Little Ice Age". This re-growth saw an increase in load on the earth's surface that, throughout West Greenland, was accompanied by a switch from early and mid Holocene relative sea-level (RSL) fall to late Holocene RSL rise. Evidence for this rise includes drowned archaeological sites, submerged freshwater peats and flooded lake basins. Recently collected sediment cores from flooded lake basins in Disko Bugt and Kangerlussuaq (and from elsewhere in West Greenland) define millennial scale trends in mid and late Holocene RSL that reflect increased ice load during the neoglacial. They provide powerful constraints on geophysical models of Holocene ice sheet history. However, such records lack the resolution required to explore the short-term dynamic interactions between the ice sheet and RSL during the last millennia. In this paper we present a high resolution RSL record from Kangerlussuaq from this period. Our approach uses thin, radiocarbon-dated salt marsh deposits that overlie bedrock. We reconstruct RSL change between c. AD 1400 and the present using seventeen radiocarbon dates on terrestrial plant macrofossils, together with an extensive database of fossil and modern diatom assemblages. Our data indicate a long-term rate of RSL rise over this period of c. 1.3 mm yr-1, close to that suggested by recent GPS observations (when corrected for a twentieth century "eustatic" sea-level rise of c. 1.5-2 mm yr-1). We also identify higher frequency variability in our record that may reflect changes in ice load associated with the "Little Ice Age" in West Greenland. High resolution RSL records, such as this, provide a new target for geophysical models of ice sheet mass balance change and a long term

  14. An ice-sheet-wide framework for englacial attenuation from ice-penetrating radar data

    NASA Astrophysics Data System (ADS)

    Jordan, T. M.; Bamber, J. L.; Williams, C. N.; Paden, J. D.; Siegert, M. J.; Huybrechts, P.; Gagliardini, O.; Gillet-Chaulet, F.

    2016-07-01

    Radar inference of the bulk properties of glacier beds, most notably identifying basal melting, is, in general, derived from the basal reflection coefficient. On the scale of an ice sheet, unambiguous determination of basal reflection is primarily limited by uncertainty in the englacial attenuation of the radio wave, which is an Arrhenius function of temperature. Existing bed-returned power algorithms for deriving attenuation assume that the attenuation rate is regionally constant, which is not feasible at an ice-sheet-wide scale. Here we introduce a new semi-empirical framework for deriving englacial attenuation, and, to demonstrate its efficacy, we apply it to the Greenland Ice Sheet. A central feature is the use of a prior Arrhenius temperature model to estimate the spatial variation in englacial attenuation as a first guess input for the radar algorithm. We demonstrate regions of solution convergence for two input temperature fields and for independently analysed field campaigns. The coverage achieved is a trade-off with uncertainty and we propose that the algorithm can be "tuned" for discrimination of basal melt (attenuation loss uncertainty ˜ 5 dB). This is supported by our physically realistic ( ˜ 20 dB) range for the basal reflection coefficient. Finally, we show that the attenuation solution can be used to predict the temperature bias of thermomechanical ice sheet models and is in agreement with known model temperature biases at the Dye 3 ice core.

  15. The Last Interglacial History of the Antarctic Ice sheet

    NASA Astrophysics Data System (ADS)

    Bradley, Sarah; Siddall, Mark; Milne, Glenn A.; Masson-Delmotte, Valerie; Wolff, Eric; Hindmarsh, Richard C. A.

    2014-05-01

    In this paper we present a summary of the work which was conducted as part of the 'PAST4FUTURE -WP4.1: Sea Level and Ice sheets' project. The overall aim of this study was to understand the response of the Antarctic Ice sheet (AIS) to climate forcing during the Last interglacial (LIG) and its contribution to the observed higher than present sea level during this period. The study involved the application and development of a novel technique which combined East Antarctic stable isotope ice core data with the output from a Glacial Isostatic Adjustment (GIA) model [Bradley et al., 2012]. We investigated if the stable isotope ice core data are sensitive to detecting isostatically driven changes in the surface elevation driven by changes in the ice-loading history of the AIS and if so, could we address some key questions relating to the LIG history of the AIS. Although it is believed that the West Antarctic Ice sheet (WAIS) reduced in size during the LIG compared to the Holocene, major uncertainties and unknowns remain unresolved: Did the WAIS collapse? What would the contribution of such a collapse be the higher than present LIG eustatic sea level (ESL)? We will show that a simulated collapse of the WAIS does not generate a significant elevation driven signal at the EAIS LIG ice core sites, and as such, these ice core records cannot be used to assess WAIS stability over this period. However, we will present 'treasure maps' [Bradley et al., 2012] to identify regions of the AIS where results from geological studies and/or new paleoclimate data may be sensitive to detecting a WAIS collapse. These maps can act as a useful tool for the wider science community/field scientists as a guide to highlight sites suitable to constrain the evolution of the WAIS during the LIG. Studies have proposed that the surface temperature across the East Antarctic Ice Sheet (EAIS) was significantly warmer, 2-5°C during the LIG compared to present [Lang and Wolff, 2011]. These higher

  16. Windblown Pliocene diatoms and East Antarctic Ice Sheet retreat.

    PubMed

    Scherer, Reed P; DeConto, Robert M; Pollard, David; Alley, Richard B

    2016-09-20

    Marine diatoms in tillites along the Transantarctic Mountains (TAMs) have been used to suggest a diminished East Antarctic Ice Sheet (EAIS) during Pliocene warm periods. Updated ice-sheet modelling shows significant Pliocene EAIS retreat, creating marine embayments into the Wilkes and Aurora basins that were conducive to high diatom productivity and rapid accumulation of diatomaceous sediments. Here we show that subsequent isostatic uplift exposed accumulated unconsolidated marine deposits to wind erosion. We report new atmospheric modelling utilizing Pliocene climate and derived Antarctic landscapes indicating that prevailing mid-altitude winds transported diatoms towards the TAMs, dominantly from extensive emerged coastal deposits of the Aurora Basin. This result unifies leading ideas from competing sides of a contentious debate about the origin of the diatoms in the TAMs and their link to EAIS history, supporting the view that parts of the EAIS are vulnerable to relatively modest warming, with possible implications for future sea-level rise.

  17. Windblown Pliocene diatoms and East Antarctic Ice Sheet retreat

    NASA Astrophysics Data System (ADS)

    Scherer, Reed P.; Deconto, Robert M.; Pollard, David; Alley, Richard B.

    2016-09-01

    Marine diatoms in tillites along the Transantarctic Mountains (TAMs) have been used to suggest a diminished East Antarctic Ice Sheet (EAIS) during Pliocene warm periods. Updated ice-sheet modelling shows significant Pliocene EAIS retreat, creating marine embayments into the Wilkes and Aurora basins that were conducive to high diatom productivity and rapid accumulation of diatomaceous sediments. Here we show that subsequent isostatic uplift exposed accumulated unconsolidated marine deposits to wind erosion. We report new atmospheric modelling utilizing Pliocene climate and derived Antarctic landscapes indicating that prevailing mid-altitude winds transported diatoms towards the TAMs, dominantly from extensive emerged coastal deposits of the Aurora Basin. This result unifies leading ideas from competing sides of a contentious debate about the origin of the diatoms in the TAMs and their link to EAIS history, supporting the view that parts of the EAIS are vulnerable to relatively modest warming, with possible implications for future sea-level rise.

  18. Windblown Pliocene diatoms and East Antarctic Ice Sheet retreat.

    PubMed

    Scherer, Reed P; DeConto, Robert M; Pollard, David; Alley, Richard B

    2016-01-01

    Marine diatoms in tillites along the Transantarctic Mountains (TAMs) have been used to suggest a diminished East Antarctic Ice Sheet (EAIS) during Pliocene warm periods. Updated ice-sheet modelling shows significant Pliocene EAIS retreat, creating marine embayments into the Wilkes and Aurora basins that were conducive to high diatom productivity and rapid accumulation of diatomaceous sediments. Here we show that subsequent isostatic uplift exposed accumulated unconsolidated marine deposits to wind erosion. We report new atmospheric modelling utilizing Pliocene climate and derived Antarctic landscapes indicating that prevailing mid-altitude winds transported diatoms towards the TAMs, dominantly from extensive emerged coastal deposits of the Aurora Basin. This result unifies leading ideas from competing sides of a contentious debate about the origin of the diatoms in the TAMs and their link to EAIS history, supporting the view that parts of the EAIS are vulnerable to relatively modest warming, with possible implications for future sea-level rise. PMID:27649516

  19. Windblown Pliocene diatoms and East Antarctic Ice Sheet retreat

    PubMed Central

    Scherer, Reed P.; DeConto, Robert M.; Pollard, David; Alley, Richard B.

    2016-01-01

    Marine diatoms in tillites along the Transantarctic Mountains (TAMs) have been used to suggest a diminished East Antarctic Ice Sheet (EAIS) during Pliocene warm periods. Updated ice-sheet modelling shows significant Pliocene EAIS retreat, creating marine embayments into the Wilkes and Aurora basins that were conducive to high diatom productivity and rapid accumulation of diatomaceous sediments. Here we show that subsequent isostatic uplift exposed accumulated unconsolidated marine deposits to wind erosion. We report new atmospheric modelling utilizing Pliocene climate and derived Antarctic landscapes indicating that prevailing mid-altitude winds transported diatoms towards the TAMs, dominantly from extensive emerged coastal deposits of the Aurora Basin. This result unifies leading ideas from competing sides of a contentious debate about the origin of the diatoms in the TAMs and their link to EAIS history, supporting the view that parts of the EAIS are vulnerable to relatively modest warming, with possible implications for future sea-level rise. PMID:27649516

  20. Discharge of debris from ice at the margin of the Greenland ice sheet

    USGS Publications Warehouse

    Knight, P.G.; Waller, R.I.; Patterson, C.J.; Jones, A.P.; Robinson, Z.P.

    2002-01-01

    Sediment production at a terrestrial section of the ice-sheet margin in West Greenland is dominated by debris released through the basal ice layer. The debris flux through the basal ice at the margin is estimated to be 12-45 m3 m-1 a-1. This is three orders of magnitude higher than that previously reported for East Antarctica, an order of magnitude higher than sites reported from in Norway, Iceland and Switzerland, but an order of magnitude lower than values previously reported from tidewater glaciers in Alaska and other high-rate environments such as surging glaciers. At our site, only negligible amounts of debris are released through englacial, supraglacial or subglacial sediment transfer. Glacio-fluvial sediment production is highly localized, and long sections of the ice-sheet margin receive no sediment from glaciofluvial sources. These findings differ from those of studies at more temperate glacial settings where glaciofluvial routes are dominant and basal ice contributes only a minor percentage of the debris released at the margin. These data on debris flux through the terrestrial margin of an outlet glacier contribute to our limited knowledge of debris production from the Greenland ice sheet.

  1. Modeled variations of precipitation over the Greenland Ice Sheet

    SciTech Connect

    Bromwich, D.H.; Robasky, F.M.; Bolzan, J.F. ); Keen, R.A.

    1993-07-01

    A parameterization of the synoptic activity at 500 hPa and a simple orographic scheme are used to model the spatial and temporal variations of precipitation over the Greenland Ice Sheet for 1963-88 from information from the National Meteorological Center (NMC). Most major spatial characteristics of the observed accumulation distribution are reproduced. The modeled time-averaged total precipitation amount over Greenland is within the range of values determined by other investigators from surface-based observations. A downward trend in simulated ice sheet precipitation over the 26 years is found, supported by a number of lines of evidence. This negative precipitation trend would mean that the Greenland Ice Sheet, depending on its 1963 mass balance state, has over the 1963-88 period either decreased its negative, or increased its positive, contribution to recently observed global sea level rise. Superimposed on the declining simulated precipitation rate for the entire ice sheet is a pronounced 3-5-yr periodicity. This is prominent in the observed and modeled precipitation time series from Summit, Greenland. This cycle shows some aspects in common with the Southern Oscillation. Some deficiencies in the NMC analyses were highlighted by this work. A large jump in simulated precipitation amounts at Summit around 1962, which is not verified by accumulation data, is inferred to be due to an artificial increase in cyclonic activity at 500 hPa associated with the NMC change from manual to numerical analyses. The activity of the storm track along the west coast of Greenland appears to be anomalously low in the NMC analyses, perhaps due to mesoscale cyclogenesis that is not resolved by the NMC analysis scheme. 71 refs., 21 figs., 1 tab.

  2. 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.

  3. Supraglacial fluvial landscape evolution on the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Karlstrom, L.; Yang, K.

    2015-12-01

    In the ablation zone of the Greenland Ice Sheet, melting during the summer drives drainage development in which flow is routed downslope through a network of supgraglacial streams and lakes until it is sequestered by the englacial system or flows off of the glacier. This supraglacial drainage network sets the efficacy by which melt water is transport into the glacier and thus has important implications for coupling between ice sheet sliding and surface melt. Thermal erosion in supraglacial streams is rapid compared to other fluvial environments, raising the possibility that supraglacial topographic evolution is to some extent set by local fluvial incision rather than by underlying bedrock or iceflow. We study a series of supraglacial drainage basins on top of the West Greenland Ice Sheet between 1000-1500 m elevation using a combination of high-resolution images, and concurrent (2 m resolution) DEMs constructed from World View Imagery. Although large-scale topography correlates well with underlying bedrock topography, spectral filtering of the surface also reveals broad, low relief valleys that suggest fluvial modification at all elevations. We extract several hundred supraglacial stream longitudinal profiles per drainage basin, finding many channel segments that are clearly out of equilibrium but also numerous concave up channel segments that are not well correlated with underlying bedrock. These concave up segments have a similar power law exponent, suggesting similarities to equilibrium bedrock and alluvial rivers (although the exponent is different in this setting). We develop a stream-power model to predict equilibrium longitudinal profiles where erosion is due to melting driving by viscous dissipation of heat within streams. We speculate that fluvial erosion driven by viscous dissipation is in part responsible for shaping the Greenland Ice Sheet ablation zone annually, superimposed on long wavelength bedrock control of surface topography and basins.

  4. Abrupt glacial climate shifts controlled by ice sheet changes.

    PubMed

    Zhang, Xu; Lohmann, Gerrit; Knorr, Gregor; Purcell, Conor

    2014-08-21

    During glacial periods of the Late Pleistocene, an abundance of proxy data demonstrates the existence of large and repeated millennial-scale warming episodes, known as Dansgaard-Oeschger (DO) events. This ubiquitous feature of rapid glacial climate change can be extended back as far as 800,000 years before present (BP) in the ice core record, and has drawn broad attention within the science and policy-making communities alike. Many studies have been dedicated to investigating the underlying causes of these changes, but no coherent mechanism has yet been identified. Here we show, by using a comprehensive fully coupled model, that gradual changes in the height of the Northern Hemisphere ice sheets (NHISs) can alter the coupled atmosphere-ocean system and cause rapid glacial climate shifts closely resembling DO events. The simulated global climate responses--including abrupt warming in the North Atlantic, a northward shift of the tropical rainbelts, and Southern Hemisphere cooling related to the bipolar seesaw--are generally consistent with empirical evidence. As a result of the coexistence of two glacial ocean circulation states at intermediate heights of the ice sheets, minor changes in the height of the NHISs and the amount of atmospheric CO2 can trigger the rapid climate transitions via a local positive atmosphere-ocean-sea-ice feedback in the North Atlantic. Our results, although based on a single model, thus provide a coherent concept for understanding the recorded millennial-scale variability and abrupt climate changes in the coupled atmosphere-ocean system, as well as their linkages to the volume of the intermediate ice sheets during glacials.

  5. Abrupt glacial climate shifts controlled by ice sheet changes

    NASA Astrophysics Data System (ADS)

    Zhang, Xu; Lohmann, Gerrit; Knorr, Gregor; Purcell, Conor

    2014-08-01

    During glacial periods of the Late Pleistocene, an abundance of proxy data demonstrates the existence of large and repeated millennial-scale warming episodes, known as Dansgaard-Oeschger (DO) events. This ubiquitous feature of rapid glacial climate change can be extended back as far as 800,000 years before present (BP) in the ice core record, and has drawn broad attention within the science and policy-making communities alike. Many studies have been dedicated to investigating the underlying causes of these changes, but no coherent mechanism has yet been identified. Here we show, by using a comprehensive fully coupled model, that gradual changes in the height of the Northern Hemisphere ice sheets (NHISs) can alter the coupled atmosphere-ocean system and cause rapid glacial climate shifts closely resembling DO events. The simulated global climate responses--including abrupt warming in the North Atlantic, a northward shift of the tropical rainbelts, and Southern Hemisphere cooling related to the bipolar seesaw--are generally consistent with empirical evidence. As a result of the coexistence of two glacial ocean circulation states at intermediate heights of the ice sheets, minor changes in the height of the NHISs and the amount of atmospheric CO2 can trigger the rapid climate transitions via a local positive atmosphere-ocean-sea-ice feedback in the North Atlantic. Our results, although based on a single model, thus provide a coherent concept for understanding the recorded millennial-scale variability and abrupt climate changes in the coupled atmosphere-ocean system, as well as their linkages to the volume of the intermediate ice sheets during glacials.

  6. Abrupt glacial climate shifts controlled by ice sheet changes.

    PubMed

    Zhang, Xu; Lohmann, Gerrit; Knorr, Gregor; Purcell, Conor

    2014-08-21

    During glacial periods of the Late Pleistocene, an abundance of proxy data demonstrates the existence of large and repeated millennial-scale warming episodes, known as Dansgaard-Oeschger (DO) events. This ubiquitous feature of rapid glacial climate change can be extended back as far as 800,000 years before present (BP) in the ice core record, and has drawn broad attention within the science and policy-making communities alike. Many studies have been dedicated to investigating the underlying causes of these changes, but no coherent mechanism has yet been identified. Here we show, by using a comprehensive fully coupled model, that gradual changes in the height of the Northern Hemisphere ice sheets (NHISs) can alter the coupled atmosphere-ocean system and cause rapid glacial climate shifts closely resembling DO events. The simulated global climate responses--including abrupt warming in the North Atlantic, a northward shift of the tropical rainbelts, and Southern Hemisphere cooling related to the bipolar seesaw--are generally consistent with empirical evidence. As a result of the coexistence of two glacial ocean circulation states at intermediate heights of the ice sheets, minor changes in the height of the NHISs and the amount of atmospheric CO2 can trigger the rapid climate transitions via a local positive atmosphere-ocean-sea-ice feedback in the North Atlantic. Our results, although based on a single model, thus provide a coherent concept for understanding the recorded millennial-scale variability and abrupt climate changes in the coupled atmosphere-ocean system, as well as their linkages to the volume of the intermediate ice sheets during glacials. PMID:25119027

  7. Abrupt drainage cycles of the Fennoscandian Ice Sheet

    PubMed Central

    Soulet, Guillaume; Ménot, Guillemette; Bayon, Germain; Rostek, Frauke; Ponzevera, Emmanuel; Toucanne, Samuel; Lericolais, Gilles; Bard, Edouard

    2013-01-01

    Continental ice sheets are a key component of the Earth’s climate system, but their internal dynamics need to be further studied. Since the last deglaciation, the northern Eurasian Fennoscandian Ice Sheet (FIS) has been connected to the Black Sea (BS) watershed, making this basin a suitable location to investigate former ice-sheet dynamics. Here, from a core retrieved in the BS, we combine the use of neodymium isotopes, high-resolution elemental analysis, and biomarkers to trace changes in sediment provenance and river runoff. We reveal cyclic releases of meltwater originating from Lake Disna, a proglacial lake linked to the FIS during Heinrich Stadial 1. Regional interactions within the climate–lake–FIS system, linked to changes in the availability of subglacial water, led to abrupt drainage cycles of the FIS into the BS watershed. This phenomenon raised the BS water level by ∼100 m until the sill of the Bosphorus Strait was reached, flooding the vast northwestern BS shelf and deeply affecting the hydrology and circulation of the BS and, probably, of the Marmara and Aegean Seas. PMID:23569264

  8. Elevation Change of the Southern Greenland Ice Sheet: Update

    NASA Technical Reports Server (NTRS)

    Davis, C. H.; Kluever, C. A.; Haines, B. J.

    1999-01-01

    The overall focus of our research is to document long-term elevation change of the Greenland ice sheet using satellite altimeter data. In addition, we are investigating seasonal and interannual variations in the ice-sheet elevations to place the long-term measurements in context. Specific objectives of this research include: 1) Developing new techniques to significantly improve the accuracy of elevation-change estimates derived from satellite altimetry. 2) Measuring the elevation change of the Greenland ice sheet over a 10-year time period using Seasat (1978) and Geosat GM (1985-86) and Geosat ERM (1986-88) altimeter data. 3) Quantifying seasonal/interannual variations in the elevation-change estimates using the continuous time series of surface elevations from the Geosat GM and ERM datasets. 4) Extending the long-term elevation change analysis to two decades by incorporating data from the ERS-1/2 missions (1991-99) and, if available, the Geosat-Follow On (GFO) mission (1998-??).

  9. Experimental design for three interrelated Marine Ice-Sheet and Ocean Model Intercomparison Projects

    NASA Astrophysics Data System (ADS)

    Asay-Davis, X. S.; Cornford, S. L.; Durand, G.; Galton-Fenzi, B. K.; Gladstone, R. M.; Gudmundsson, G. H.; Hattermann, T.; Holland, D. M.; Holland, D.; Holland, P. R.; Martin, D. F.; Mathiot, P.; Pattyn, F.; Seroussi, H.

    2015-11-01

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

  10. Ice stream reorganization and ice sheet mass balance following the reactivation of Kamb Ice Stream, West Antarctica

    NASA Astrophysics Data System (ADS)

    Bougamont, Marion; Christoffersen, Poul; Price, Stephen; Carter, Sasha

    2015-04-01

    Ice streams in Antarctica account for most of the ice volume discharged to the ocean, and their flow variability greatly influences the mass balance of the ice sheet. Today, the Siple Coast region of West Antarctica is the only one to experience a positive mass balance (~36Gt/yr), as a consequence of the stagnation of Kamb Ice Stream about 170 years ago and the ongoing slowdown of Whillans Ice Stream. However, this positive trend could be temporary; past studies have shown that both ice streams experienced significant flow variability over the past millennia, with stagnation typically followed by reactivation on centennial timescales, occurring in response to internal processes. The impact this variability may have on the future mass balance of the WAIS remains unknown. Here, we explore the future flow variability of the Siple Coast ice streams by using a three-dimensional higher-order ice sheet model (CISM), coupled to a physically-based basal processes model and a model of regional hydrology. To obtain realistic initial flow conditions, we assimilate available velocity data for this region from 1997. We perform forward simulations over a 200 year period, during which the basal properties evolve according to the distribution of meltwater beneath the ice and its drainage/flow through a subglacial till layer. First, we assume that the bed evolves according to ice-till interactions with only local exchange of water between the ice and till. Next, we include a model of the regional basal water system capable of transporting water over long distances, so that meltwater is routed laterally along the bed before interacting with the till layer. We also explore the effect of geothermal heat flux uncertainties. We find that ice discharge to the grounding line is larger and more sustained in time when the regional water system is included in the simulations. Still, in all experiments, the main future perturbation to the current state of flow follows from the reactivation of

  11. 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

  12. New eyes in the sky measure glaciers and ice sheets

    USGS Publications Warehouse

    Kieffer, Hugh; Kargel, Jeffrey S.; Barry, Roger G.; Bindschadler, Robert; Bishop, Michael P.; MacKinnon, David; Ohmura, Atsumu; Raup, Bruce; Antoninetti, Massimo; Bamber, Jonathan; Braun, Mattias; Brown, Ian; Cohen, Denis; Copland, Luke; DueHagen, Jon; Engeset, Rune V.; Fitzharris, Blair; Fujita, Koji; Haeberli, Wilfried; Hagen, Jon Oue; Hall, Dorothy; Hoelzle, Martin; Johansson, Maria; Kaab, Andi; Koenig, Max; Konovalov, Vladimir; Maisch, Max; Paul, Frank; Rau, Frank; Reeh, Niels; Rignot, Eric; Rivera, Andres; De Ruyter de Wildt, Martiyn; Scambos, Ted; Schaper, Jesko; Scharfen, Greg; Shroder, Jack; Solomina, Olga; Thompson, David; van der Veen, Kees; Wohlleben, Trudy; Young, Neal

    2000-01-01

    The mapping and measurement of glaciers and their changes are useful in predicting sea-level and regional water supply, studying hazards and climate change [Haeberli et al., 1998],and in the hydropower industry Existing inventories cover only about 67,000 of the world's estimated 160,000 glaciers and are based on data collected over 50 years or more [e.g.,Haeberli et al., 1998]. The data available have proven that small ice bodies are disappearing at an accelerating rate and that the Antarctic ice sheet and its fringing ice shelves are undergoing unexpected, rapid change. According to many glaciologists, much larger fluctuations in land ice—with vast implications for society—are possible in the coming decades and centuries due to natural and anthropogenic climate change [Oppenheimer, 1998].

  13. A test bed for investigating the flow of outlet glaciers and ice streams embedded in the Greenland ice sheet

    NASA Astrophysics Data System (ADS)

    Calov, Reinhard; Rückamp, Martin; Schlegel, Rebecca; Ganopolski, Andrey; Humbert, Angelika

    2016-04-01

    Here, we define a test bed for fast flow regions and its vicinity embedded in an ice sheet. This test bed is designed for outlet glaciers and ice streams of the Greenland ice sheet. It consists of a fine resolution part with a manufactured basal trough over which the professional software COMSOL (Multiphysics Modeling Software) operates as a full-Stokes model. Results by COMSOL are compared with coarse resolution simulations with the ice-sheet model SICOPOLIS operating in shallow-ice-approximation mode and using parameterizations of the fast flow effects. For simplification, in this preliminary approach, both models run in isothermal mode. Definition of surface mass balance follows the EISMINT intercomparison project with parameters adapted to the Greenland ice sheet. In particular, we inspect with this test bed upstream and lateral flow effects of ice streams and outlet glaciers. We present first simulations with this approach, although presentation of the test bed itself is the main emphasis of this presentation.

  14. Antarctic last interglacial isotope peak in response to sea ice retreat not ice-sheet collapse.

    PubMed

    Holloway, Max D; Sime, Louise C; Singarayer, Joy S; Tindall, Julia C; Bunch, Pete; Valdes, Paul J

    2016-01-01

    Several studies have suggested that sea-level rise during the last interglacial implies retreat of the West Antarctic Ice Sheet (WAIS). The prevalent hypothesis is that the retreat coincided with the peak Antarctic temperature and stable water isotope values from 128,000 years ago (128 ka); very early in the last interglacial. Here, by analysing climate model simulations of last interglacial WAIS loss featuring water isotopes, we show instead that the isotopic response to WAIS loss is in opposition to the isotopic evidence at 128 ka. Instead, a reduction in winter sea ice area of 65±7% fully explains the 128 ka ice core evidence. Our finding of a marked retreat of the sea ice at 128 ka demonstrates the sensitivity of Antarctic sea ice extent to climate warming. PMID:27526639

  15. Antarctic last interglacial isotope peak in response to sea ice retreat not ice-sheet collapse

    PubMed Central

    Holloway, Max D.; Sime, Louise C.; Singarayer, Joy S.; Tindall, Julia C.; Bunch, Pete; Valdes, Paul J.

    2016-01-01

    Several studies have suggested that sea-level rise during the last interglacial implies retreat of the West Antarctic Ice Sheet (WAIS). The prevalent hypothesis is that the retreat coincided with the peak Antarctic temperature and stable water isotope values from 128,000 years ago (128 ka); very early in the last interglacial. Here, by analysing climate model simulations of last interglacial WAIS loss featuring water isotopes, we show instead that the isotopic response to WAIS loss is in opposition to the isotopic evidence at 128 ka. Instead, a reduction in winter sea ice area of 65±7% fully explains the 128 ka ice core evidence. Our finding of a marked retreat of the sea ice at 128 ka demonstrates the sensitivity of Antarctic sea ice extent to climate warming. PMID:27526639

  16. Antarctic last interglacial isotope peak in response to sea ice retreat not ice-sheet collapse.

    PubMed

    Holloway, Max D; Sime, Louise C; Singarayer, Joy S; Tindall, Julia C; Bunch, Pete; Valdes, Paul J

    2016-01-01

    Several studies have suggested that sea-level rise during the last interglacial implies retreat of the West Antarctic Ice Sheet (WAIS). The prevalent hypothesis is that the retreat coincided with the peak Antarctic temperature and stable water isotope values from 128,000 years ago (128 ka); very early in the last interglacial. Here, by analysing climate model simulations of last interglacial WAIS loss featuring water isotopes, we show instead that the isotopic response to WAIS loss is in opposition to the isotopic evidence at 128 ka. Instead, a reduction in winter sea ice area of 65±7% fully explains the 128 ka ice core evidence. Our finding of a marked retreat of the sea ice at 128 ka demonstrates the sensitivity of Antarctic sea ice extent to climate warming.

  17. Preservation of a preglacial landscape under the center of the Greenland Ice Sheet.

    PubMed

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

    2014-04-25

    Continental ice sheets typically sculpt landscapes via erosion; under certain conditions, ancient landscapes can be preserved beneath ice and can survive extensive and repeated glaciation. We used concentrations of atmospherically produced cosmogenic beryllium-10, carbon, and nitrogen to show that ancient soil has been preserved in basal ice for millions of years at the center of the ice sheet at Summit, Greenland. This finding suggests ice sheet stability through the Pleistocene (i.e., the past 2.7 million years). The preservation of this soil implies that the ice has been nonerosive and frozen to the bed for much of that time, that there was no substantial exposure of central Greenland once the ice sheet became fully established, and that preglacial landscapes can remain preserved for long periods under continental ice sheets. PMID:24763587

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

    NASA Technical Reports Server (NTRS)

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

    2014-01-01

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

  19. Shallow ice approximation, second order shallow ice approximation, and full Stokes models: A discussion of their roles in palaeo-ice sheet modelling and development

    NASA Astrophysics Data System (ADS)

    Kirchner, N.; Ahlkrona, J.; Gowan, E. J.; Lötstedt, P.; Lea, J. M.; Noormets, R.; von Sydow, L.; Dowdeswell, J. A.; Benham, T.

    2016-09-01

    Full Stokes ice sheet models provide the most accurate description of ice sheet flow, and can therefore be used to reduce existing uncertainties in predicting the contribution of ice sheets to future sea level rise on centennial time-scales. The level of accuracy at which millennial time-scale palaeo-ice sheet simulations resolve ice sheet flow lags the standards set by Full Stokes models, especially, when Shallow Ice Approximation (SIA) models are used. Most models used in paleo-ice sheet modeling were developed at a time when computer power was very limited, and rely on several assumptions. At the time there was no means of verifying the assumptions by other than mathematical arguments. However, with the computer power and refined Full Stokes models available today, it is possible to test these assumptions numerically. In this paper, we review (Ahlkrona et al., 2013a) where such tests were performed and inaccuracies in commonly used arguments were found. We also summarize (Ahlkrona et al., 2013b) where the implications of the inaccurate assumptions are analyzed for two paleo-models - the SIA and the SOSIA. We review these works without resorting to mathematical detail, in order to make them accessible to a wider audience with a general interest in palaeo-ice sheet modelling. Specifically, we discuss two implications of relevance for palaeo-ice sheet modelling. First, classical SIA models are less accurate than assumed in their original derivation. Secondly, and contrary to previous recommendations, the SOSIA model is ruled out as a practicable tool for palaeo-ice sheet simulations. We conclude with an outlook concerning the new Ice Sheet Coupled Approximation Level (ISCAL) method presented in Ahlkrona et al. (2016), that has the potential to match the accuracy standards of full Stokes model on palaeo-timescales of tens of thousands of years, and to become an alternative to hybrid models currently used in palaeo-ice sheet modelling. The method is applied to an ice

  20. Shallow ice approximation, second order shallow ice approximation, and full Stokes models: A discussion of their roles in palaeo-ice sheet modelling and development

    NASA Astrophysics Data System (ADS)

    Kirchner, N.; Ahlkrona, J.; Gowan, E. J.; Lötstedt, P.; Lea, J. M.; Noormets, R.; von Sydow, L.; Dowdeswell, J. A.; Benham, T.

    2016-03-01

    Full Stokes ice sheet models provide the most accurate description of ice sheet flow, and can therefore be used to reduce existing uncertainties in predicting the contribution of ice sheets to future sea level rise on centennial time-scales. The level of accuracy at which millennial time-scale palaeo-ice sheet simulations resolve ice sheet flow lags the standards set by Full Stokes models, especially, when Shallow Ice Approximation (SIA) models are used. Most models used in paleo-ice sheet modeling were developed at a time when computer power was very limited, and rely on several assumptions. At the time there was no means of verifying the assumptions by other than mathematical arguments. However, with the computer power and refined Full Stokes models available today, it is possible to test these assumptions numerically. In this paper, we review (Ahlkrona et al., 2013a) where such tests were performed and inaccuracies in commonly used arguments were found. We also summarize (Ahlkrona et al., 2013b) where the implications of the inaccurate assumptions are analyzed for two paleo-models - the SIA and the SOSIA. We review these works without resorting to mathematical detail, in order to make them accessible to a wider audience with a general interest in palaeo-ice sheet modelling. Specifically, we discuss two implications of relevance for palaeo-ice sheet modelling. First, classical SIA models are less accurate than assumed in their original derivation. Secondly, and contrary to previous recommendations, the SOSIA model is ruled out as a practicable tool for palaeo-ice sheet simulations. We conclude with an outlook concerning the new Ice Sheet Coupled Approximation Level (ISCAL) method presented in Ahlkrona et al. (2016), that has the potential to match the accuracy standards of full Stokes model on palaeo-timescales of tens of thousands of years, and to become an alternative to hybrid models currently used in palaeo-ice sheet modelling. The method is applied to an ice

  1. Distribution and characteristics of overdeepenings beneath the Greenland and Antarctic ice sheets and their glaciological implications

    NASA Astrophysics Data System (ADS)

    Swift, Darrel; Patton, Henry; Livingstone, Stephen; Jones, Andrew; Clark, Chris; Cook, Simon

    2016-04-01

    Understanding of overdeepening origin and glaciological significance is limited by an absence of quantitative empirical studies. To address this shortcoming, we have mapped the distribution of closed-topographic depressions (i.e. potential overdeepenings) beneath the Antarctic and Greenland ice sheets using automated GIS techniques, and have analysed the resulting database of overdeepening characteristics. The morphologies of a subset of mapped depressions that pass strict quality criteria indicate that overdeepening growth is generally allometric and that topographic confinement of ice flow enhances overdeepening depth. However, we infer that deepening slows with overdeepening age because (a) overdeepening depth is skewed towards shallow values - typically 200 to 300 m; and (b) overdeepening adverse slope steepness declines with overdeepening planform size. Analysis of overdeepening surface ice gradient to bed gradient ratio (the SB ratio) and surface ice velocity shows that velocities are highest for overdeepenings with SB ratios of ~ -1 to -1.5. Further, this ratio is close to the preferred range of SB ratio values exhibited by the dataset. This indicates that ice flow velocity and erosion potential are modulated by the changing efficiency of subglacial drainage and sediment transport that occurs as an overdeepening grows. This is presumed to encourage sediment deposition on the adverse slope, whilst overdeepening enlargement by headward growth (e.g. quarrying) is able to continue, and this presumption is supported by analysis of overdeepening long-profiles, which indicates that overdeepenings are typically asymmetric, with the deepest point skewed toward the overdeepening head. Our observations lead to the conclusion that overdeepening formation enhances ice sheet flow and that thinning during retreat, which will produce even greater negative SB ratios, should result a slowing or stabilisation of ice sheet flow.

  2. Revision of the NW Laurentide Ice Sheet: implications for paleoclimate, the northeast extremity of Beringia, and Arctic Ocean sedimentation

    NASA Astrophysics Data System (ADS)

    England, John H.; Furze, Mark F. A.; Doupé, Jonathan P.

    2009-08-01

    For the past half-century, reconstructions of North American ice cover during the Last Glacial Maximum have shown ice-free land distal to the Laurentide Ice Sheet, primarily on Melville and Banks islands in the western Canadian Arctic Archipelago. Both islands reputedly preserve at the surface multiple Laurentide till sheets, together with associated marine and lacustrine deposits, recording as many as three pre-Late Wisconsinan glaciations. The northwest corner of Banks Island was purportedly never glaciated and is trimmed by the oldest and most extensive glaciation (Banks Glaciation) considered to be of Matuyama age (>780 ka BP). Inside the limit of Banks Glaciation, younger till sheets are ascribed to the Thomsen Glaciation (pre-Sangamonian) and the Amundsen Glaciation (Early Wisconsinan Stade). The view that the western Canadian Arctic Archipelago remained largely ice-free during the Late Wisconsinan is reinforced by a recent report of two woolly mammoth fragments collected on Banks and Melville islands, both dated to ˜22 ka BP. These dates imply that these islands constitute the northeast extremity of Beringia. A fundamental revision of this model is now warranted based on widespread fieldwork across the adjacent coastlines of Banks and Melville islands, including new dating of glacial and marine landforms and sediments. On Dundas Peninsula, southern Melville Island, AMS 14C dates on ice-transported marine molluscs within the most extensive Laurentide till yield ages of 25-49 ka BP. These dates require that Late Wisconsinan ice advanced northwestward from Visount Melville Sound, excavating fauna spanning Marine Isotope Stage 3. Laurentide ice that crossed Dundas Peninsula (300 m asl) coalesced with Melville Island ice occupying Liddon Gulf. Coalescent Laurentide and Melville ice continued to advance westward through M'Clure Strait depositing granite erratics at ≥235 m asl that require grounded ice in M'Clure Strait, as do streamlined bedforms on the channel

  3. Using immersed boundary methods to couple a dynamical ocean model to a dynamical ice sheet/ice shelf model

    NASA Astrophysics Data System (ADS)

    Asay-Davis, Xylar; Lipscomb, William; Price, Steven

    2010-05-01

    The melting of the West Antarctic Ice Sheet (WAIS), the world's largest marine ice sheet, would mean a ~5 meter sea level rise worldwide. About a third of the WAIS lies in the Amundsen Sea Embayment, where small ice shelves provide buttressing for outlet glaciers. Warming oceans may melt the supporting ice shelves leading to accelerated flow of the outlet glaciers. Perhaps more importantly, warming oceans may mean that warm circumpolar deep water can more easily reach the ice sheet grounding line, where it can melt grounded ice directly and force retreat. Previous theoretical work suggests that ice sheets, such as the WAIS, with seabeds that deepen inland may be unstable to grounding line retreat. We present simulations from a coupled dynamical ocean model (based on POP) and dynamical ice sheet/ice shelf model (Glimmer-CISM). This work is a stepping stone toward a global scale simulation of the southern ocean (using CCSM) together with the full Antarctic ice sheet (using Glimmer-CISM). The ocean model uses an immersed boundary method (IBM) to represent the complex, time-evolving geometry of the ice shelf. The IBM allows for accurate representation of the boundary conditions at the ocean/ice interface without the need for a modeling grid that conforms to the boundary or that changes in time. Using simplified seabed and ice shelf geometries, our simulations explore the effects of varying the seabed slope on the stability of ice sheets. We also investigate the melt rates that result from varying levels of seawater warming beneath the ice shelves.

  4. A ``triple sea-ice state'' mechanism for the abrupt warming and synchronous ice sheet collapses during Heinrich events

    NASA Astrophysics Data System (ADS)

    Kaspi, Yohai; Sayag, Roiy; Tziperman, Eli

    2004-09-01

    Abrupt, switch-like, changes in sea ice cover are proposed as a mechanism for the large-amplitude abrupt warming that seemed to have occurred after each Heinrich event. Sea ice changes are also used to explain the colder-than-ambient glacial conditions around the time of the glacier discharge. The abrupt warming events occur in this mechanism, owing to rapid sea ice melting which warmed the atmosphere via the strong sea ice albedo and insulating feedbacks. Such abrupt sea ice changes can also account for the warming observed during Dansgaard-Oeschger events. The sea ice changes are caused by a weak (order of 5 Sv) response of the thermohaline circulation (THC) to glacier discharges. The main point of this work is therefore that sea ice may be thought of as a very effective amplifier of a weak THC variability, explaining the abrupt temperature changes over Greenland. Synchronous ice sheet collapses from different ice sheets around the North Atlantic, indicated by some proxy records, are shown to be possible via the weak coupling between the different ice sheets by the atmospheric temperature changes caused by the sea ice changes. This weak coupling can lead to a "nonlinear phase locking" of the different ice sheets which therefore discharge synchronously. It is shown that the phase locking may also lead to "precursor" glacier discharge events from smaller ice sheets before the Laurentide Ice Sheet discharges. The precursor events in this mechanism are the result rather than the cause of the major glacier discharges from the Laurentide Ice Sheet.

  5. Dating New York - Tracking the Retreat of the Laurentide Ice Sheet

    NASA Astrophysics Data System (ADS)

    Edwards, A. R.; Schaefer, J.; Rinterknecht, V.; Ivy-Ochs, S.

    2004-12-01

    The Laurentide Ice Sheet (LIS) covered the New York City area during the Last Glacial Maximum (LGM) and when it retreated carved out the present topography, exposing fresh bedrock. Although modernization has led to the modification of much of the present landscape, the New York area still features many attributes of the glacial sculpting. These attributes: glacially polished bedrock, erratic boulders and glacial moraines, remain as evidence of past climate change. Long Island represents a terminal moraine of the LIS, while areas in Central Park, Inwood (northern Manhattan) and the Hudson valley display beautiful retreat features. We present surface exposure dates of samples removed from these areas from the measurement of 10Be in them. This allows us to better understand the dynamics of the Laurentide Ice Sheet at the end of the last ice age. We will match the data with existing moraine records from the LIS as well as with mountain glacier records to determine whether or not (i) the LIS and the Scandinavian Ice Sheet were in sync; (ii) the huge LIS reacted more inertial than smaller systems to the climate reorganizations; (iii) the retreat pattern of the LIS.

  6. Geophysical constraints on the dynamics and retreat of the Barents Sea ice sheet as a paleobenchmark for models of marine ice sheet deglaciation

    NASA Astrophysics Data System (ADS)

    Patton, Henry; Andreassen, Karin; Bjarnadóttir, Lilja R.; Dowdeswell, Julian A.; Winsborrow, Monica C. M.; Noormets, Riko; Polyak, Leonid; Auriac, Amandine; Hubbard, Alun

    2015-12-01

    Our understanding of processes relating to the retreat of marine-based ice sheets, such as the West Antarctic Ice Sheet and tidewater-terminating glaciers in Greenland today, is still limited. In particular, the role of ice stream instabilities and oceanographic dynamics in driving their collapse are poorly constrained beyond observational timescales. Over numerous glaciations during the Quaternary, a marine-based ice sheet has waxed and waned over the Barents Sea continental shelf, characterized by a number of ice streams that extended to the shelf edge and subsequently collapsed during periods of climate and ocean warming. Increasing availability of offshore and onshore geophysical data over the last decade has significantly enhanced our knowledge of the pattern and timing of retreat of this Barents Sea ice sheet (BSIS), particularly so from its Late Weichselian maximum extent. We present a review of existing geophysical constraints that detail the dynamic evolution of the BSIS through the last glacial cycle, providing numerical modelers and geophysical workers with a benchmark data set with which to tune ice sheet reconstructions and explore ice sheet sensitivities and drivers of dynamic behavior. Although constraining data are generally spatially sporadic across the Barents and Kara Seas, behaviors such as ice sheet thinning, major ice divide migration, asynchronous and rapid flow switching, and ice stream collapses are all evident. Further investigation into the drivers and mechanisms of such dynamics within this unique paleo-analogue is seen as a key priority for advancing our understanding of marine-based ice sheet deglaciations, both in the deep past and in the short-term future.

  7. Climate Model Dependency in Understanding the Antarctic Ice Sheet during the Warm Late Pliocene

    NASA Astrophysics Data System (ADS)

    Dolan, A. M.; de Boer, B.; Bernales, J.; Hunter, S. J.; Haywood, A.

    2015-12-01

    In the context of future climate change, understanding the nature and behaviour of ice sheets during warm intervals of Earth history is fundamentally important. A warm period in the Late Pliocene (3.264 to 3.025 million years before present) can serve as a potential analogue for projected future climates. Although Pliocene ice locations and extents are still poorly constrained, a significant contribution to sea-level rise should be expected from both the Greenland ice sheet and the West and East Antarctic ice sheets based on palaeo sea-level reconstructions and geological evidence Following a five year international project PLISMIP (Pliocene Ice Sheet Modeling Intercomparison Project) we present the final set of results which quantify uncertainty in climate model-based predictions of the Antarctic ice sheet. In this study we use an ensemble of climate model forcings within a multi-ice sheet model framework to assess the climate (model) dependency of large scale features of the Antarctic ice sheet. Seven coupled atmosphere-ocean climate models are used to derive surface temperature, precipitation and oceanic forcing that drive three ice sheet models (over the grounded and floating domain). Similar to results presented over Greenland, we show that the reconstruction of the Antarctic ice sheet is sensitive to which climate model is used to provide the forcing field. Key areas of uncertainty include West Antarctica, the large subglacial basins of East Antarctica and the overall thickness of the continental interior of East Antarctica. We relate the results back to geological proxy data, such as those relating to exposure rates which provide information on potential ice sheet thickness. Finally we discuss as to whether the choice of modelling framework (i.e. climate model and ice sheet model used) or the choice of boundary conditions causes the greatest uncertainty in ice sheet reconstructions of the warm Pliocene.

  8. Climate Model Dependency and Understanding the Antarctic Ice Sheet during the Warm Late Pliocene

    NASA Astrophysics Data System (ADS)

    Dolan, Aisling; de Boer, Bas; Bernales, Jorge; Hunter, Stephen; Haywood, Alan

    2016-04-01

    In the context of future climate change, understanding the nature and behaviour of ice sheets during warm intervals of Earth history is fundamentally important. A warm period in the Late Pliocene (3.264 to 3.025 million years before present) can serve as a potential analogue for projected future climates. Although Pliocene ice locations and extents are still poorly constrained, a significant contribution to sea-level rise should be expected from both the Greenland ice sheet and the West and East Antarctic ice sheets based on palaeo sea-level reconstructions and geological evidence. Following a five year international project PLISMIP (Pliocene Ice Sheet Modeling Intercomparison Project) we present the final set of results which quantify uncertainty in climate model-based predictions of the Antarctic ice sheet. In this study we use an ensemble of climate model forcings within a multi-ice sheet model framework to assess the climate (model) dependency of large scale features of the Antarctic ice sheet. Seven coupled atmosphere-ocean climate models are used to derive surface temperature, precipitation and oceanic forcing that drive three ice sheet models (over the grounded and floating domain). Similar to results presented over Greenland, we show that the reconstruction of the Antarctic ice sheet is sensitive to which climate model is used to provide the forcing field. Key areas of uncertainty include West Antarctica, the large subglacial basins of East Antarctica and the overall thickness of the continental interior of East Antarctica. We relate the results back to geological proxy data, such as those relating to exposure rates which provide information on potential ice sheet thickness. Finally we discuss as to whether the choice of modelling framework (i.e. climate model and ice sheet model used) or the choice of boundary conditions causes the greatest uncertainty in ice sheet reconstructions of the warm Pliocene.

  9. Surface exposure dating of Little Ice Age ice cap advances on Disko Island, West Greenland

    NASA Astrophysics Data System (ADS)

    Lane, Timothy; Jomelli, Vincent; Rinterknecht, Vincent; Brunstein, Daniel; Schimmelpfennig, Irene; Swingedouw, Didier; Favier, Vincent; Masson-Delmotte, Valerie

    2015-04-01

    Little Ice Age (LIA: 1200-1920 AD) glacier advances in Greenland often form the most extensive positions of Greenland Ice Sheet (GrIS) ice cap and margins since the Early Holocene. Across Greenland these advances are commonly represented by un-vegetated moraines, usually within 1-5 km of the present ice margin. However, chronological constraints on glacier advances during this period are sparse, meaning that GrIS and ice cap behavior and advance/retreat chronology remains poorly understood during this period. At present the majority of ages are based on historical accounts, ice core data, and radiocarbon ages from proglacial threshold lakes. However, developments in the accuracy and precision of surface exposure methods allow dating of LIA moraine boulders, permitting an opportunity to better understand of ice dynamics during this period. Geomorphological mapping and surface exposure dating (36Cl) were used to interpret moraine deposits from the Lyngmarksbræen on Disko Island, West Greenland. A Positive Degree Day (PDD) model was used to estimate Equilibrium Line Altitude (ELA) and mass balance changes for two distinct paleo-glacial extents. Three moraines (M1, M2, and M3) were mapped in the field, and sampled for 36Cl surface exposure dating. The outermost moraine (M1) was of clearly different morphology to the inner moraines, and present only in small fragments. M2 and M3 were distinct arcuate termino-lateral moraines within 50 m of one another, 1.5 km from the present ice margin. The weighted average of four 36Cl ages from M1 returned an early Holocene age of 8.4 ± 0.6 ka. M2 (four samples) returned an age of 0.57 ± 0.04 ka (1441 AD) and M3 (four samples) returned an age of 0.28 ± 0.02 ka (1732 AD). These surface exposure ages represent the first robustly dated Greenlandic ice cap moraine sequence from the LIA. The two periods of ice cap advance and marginal stabilisation are similar to recorded periods of LIA GrIS advance in west Greenland, constrained

  10. Refreezing on the Greenland ice sheet: a model comparison

    NASA Astrophysics Data System (ADS)

    Steger, Christian; Reijmer, Carleen; van den Broeke, Michiel; Ligtenberg, Stefan; Kuipers Munneke, Peter; Noël, Brice

    2016-04-01

    Mass loss of the Greenland ice sheet (GrIS) is an important contributor to global sea level rise. Besides calving, surface melt is the dominant source of mass loss. However, only part of the surface melt leaves the ice sheet as runoff whereas the other part percolates into the snow cover and refreezes. Due to this process, part of the meltwater is (intermediately) stored. Refreezing thus impacts the surface mass balance of the ice sheet but it also affects the vertical structure of the snow cover due to transport of mass and energy. Due to the sparse availability of in situ data and the demand of future projections, it is inevitable to use numerical models to simulate refreezing and related processes. Currently, the magnitude of refrozen mass is neither well constrained nor well validated. In this study, we model the snow and firn layer, and compare refreezing on the GrIS as modelled with two different numerical models. Both models are forced with meteorological data from the regional climate model RACMO 2 that has been shown to simulate realistic conditions for Greenland. One model is the UU/IMAU firn densification model (FDM) that can be used both in an on- and offline mode with RACMO 2. The other model is SNOWPACK; a model originally designed to simulate seasonal snow cover in alpine conditions. In contrast to FDM, SNOWPACK accounts for snow metamorphism and microstructure and contains a more physically based snow densification scheme. A first comparison of the models indicates that both seem to be able to capture the general spatial and temporal pattern of refreezing. Spatially, refreezing occurs mostly in the ablation zone and decreases in the accumulation zone towards the interior of the ice sheet. Below the equilibrium line altitude (ELA) where refreezing occurs in seasonal snow cover on bare ice, the storage effect is only intermediate. Temporal patterns on a seasonal range indicate two peaks in refreezing; one at the beginning of the melt season where

  11. Expanded Late Wisconsinan ice cap and ice sheet margins in the western Queen Elizabeth Islands, Arctic Canada

    NASA Astrophysics Data System (ADS)

    Nixon, F. Chantel; England, John H.

    2014-05-01

    Recent mapping of surficial geology and geomorphology in the western Canadian High Arctic (Melville and Eglinton islands), together with new radiocarbon dates acquired from ice-contact raised marine sediments, document expanded late Wisconsinan ice limits for the northwest Laurentide Ice Sheet and the western Innuitian Ice Sheet. An extension of the northwestern margin of the Laurentide Ice Sheet onto Eglinton Island is proposed based on evidence from till containing erratics derived from the Canadian Shield and a pattern of meltwater channels indicating ice retreat offshore into M'Clure Strait. Expansion of the western Melville Island Ice Cap (part of the western, lowland sector of the Innuitian Ice Sheet) to its offshore late Wisconsinan limit was facilitated by coalescence with the Laurentide Ice Sheet, whose buttressing allowed thickening to occur. Estimates of ice extent and thickness (>500 m) of the western Melville Island Ice Cap are in agreement with high marine limits (≤70 m asl). Lateral and proglacial meltwater channels, moraines and glaciomarine, glaciolacustrine and glaciofluvial deposits indicate radial retreat of the western Melville Island Ice Cap onto central highlands after ˜13.0 cal ka BP. Older marine limit shorelines on southern Eglinton Island (˜13.6 cal ka BP) are broadly synchronous with the early and rapid deglaciation of other areas formerly glaciated by the northwestern Laurentide Ice Sheet to the southeast and southwest (˜14.2-13.6 cal ka BP). The collapse of the northwest Laurentide Ice Sheet in M'Clure Strait beginning at ˜14.2 cal ka BP, in addition to prior inferred thinning, opens the possibility that it made a significant contribution to meltwater pulse 1A.

  12. Decadal slowdown of a land-terminating sector of the Greenland Ice Sheet during sustained climate warming - implications for wider ice sheet hydrology-dynamics coupling

    NASA Astrophysics Data System (ADS)

    Nienow, P. W.; Tedstone, A. J.; Gourmelen, N.; Dehecq, A.; Goldberg, D. N.; Hanna, E.

    2015-12-01

    The relationship between surface melting and ice motion will affect how the Greenland Ice Sheet (GrIS) responds to climate and the structure of the subglacial drainage system may be crucial in controlling how changing melt-rates impact ice motion. Ice sheet motion varies over seasonal time-scales in response to varying surface meltwater inputs to the ice-sheet bed which lubricate the ice-bed interface, resulting in periods of faster ice motion. However, the impact of hydro-dynamic coupling on ice motion over decadal timescales remains poorly constrained. Here we utilise remotely-sensed optical Landsat imagery to generate a record of annual motion spanning three decades extending back to 1985. Our observations cover an ˜8000 km2 area along ˜170 km of predominantly land-terminating margin of the west GrIS, and extend ˜50 km inland to 1100 m asl. We find that that annual ice motion was 12% slower in 2007-2014 compared to 1985-1994, despite a corresponding 50% increase in surface meltwater production. Less than 1/3 of the observed slowdown can be explained by a reduction in internal deformation caused by marginal ice sheet thinning, and we therefore hypothesise that increases in subglacial drainage efficiency, associated with sustained larger melt volumes, have reduced basal lubrication resulting in slower ice flow. Our findings suggest that hydro-dynamic coupling in this section of the ablation zone resulted in net ice motion slowdown over decadal timescales — not speedup as previously postulated. Increases in meltwater production from projected climate warming may therefore further reduce the motion of land-terminating margins of the ice-sheet indicating such margins are more resilient to the dynamic impacts of enhanced meltwater production than previously thought. The implications of these observations for wider ice sheet hydrology-dynamics coupling are considered.

  13. IceBridge Radar as a Tool for Understanding Accumulation Variability throughout the Western Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Lewis, G.; Hawley, R. L.; Osterberg, E. C.; Marshall, H. P.

    2015-12-01

    Understanding the mass balance of the Greenland Ice Sheet (GIS) in a warming climate is of critical interest to scientists and the general public in the context of future sea-level rise. The mass balance of the GIS has been increasingly negative over recent decades, with a conservative estimate of ice sheet mass loss of 2000 Gt yr-1 over the 1988-2014 period. Snow accumulation, the critical input to surface mass balance, varies through time and regionally around Greenland and from the coast to interior, but is constrained by relatively few observations. An improved understanding of temporal and spatial snow accumulation variability will thus reduce uncertainties in GIS mass balance models and sea-level rise projections. Here we quantify spatial and temporal variations in snow accumulation in central and southwest Greenland using data from NASA's Operation IceBridge Accumulation Radar. We estimate depth-density and depth-age relationships using a Herron-Langway density model and Nye flow model, which are iteratively calculated at 50 km intervals along each radar line. We then trace isochrones to calculate spatial and temporal variability in average snow accumulation along several flight paths throughout central and southwestern Greenland. Depth-density, depth-age, and snow accumulation calculations are calibrated at Summit using data from the GISP2, Katie, and SM07C ice cores. Accumulation results will be verified by a series of snow pits, shallow firn cores, and ice-penetrating radar profiles collected on the Greenland Traverse for Accumulation and Climate Studies (GreenTrACS) through southwest Greenland during spring 2016 and 2017.

  14. How does ice sheet loading affect ocean flow around Antarctica?

    NASA Astrophysics Data System (ADS)

    Dijkstra, H. A.; Rugenstein, M. A.; Stocchi, P.; von der Heydt, A. S.

    2012-12-01

    Interactions and dynamical feedbacks between ocean circulation, heat and atmospheric moisture transport, ice sheet evolution, and Glacial Isostatic Adjustment (GIA) are overlooked issues in paleoclimatology. Here we will present first results on how ocean flows were possibly affected by the glaciation of Antarctica across the Eocene-Oligocene Transition (~ 34 Ma) through GIA and bathymetry variations. GIA-induced gravitationally self-consistent bathymetry variations are determined by solving the Sea Level Equation (SLE), which describes the time dependent shape of (i) the solid Earth and (ii) the equipotential surface of gravity. Since the ocean circulation equations are defined relative to the equipotential surface of gravity, only bathymetry variations can influence ocean flows, although the sea surface slope will also change through time due to gravitational attraction. We use the Hallberg Isopycnal Model under late Eocene conditions to calculate equilibrium ocean flows in a domain in which the bathymetry evolves under ice loading according to the SLE. The bathymetric effects of the glaciation of Antarctica lead to substantial spatial changes in ocean flows, and close to the coast, the flow even reverses direction. Volume transports through the Drake Passage and Tasman Seaway adjust to the new bathymetry. The results indicate that GIA-induced ocean flow variations alone may have had an impact on sedimentation and erosion patterns, the repositioning of fronts, ocean heat transport and grounding line and ice sheet stability.

  15. Observational data from the Programme for Monitoring of the Greenland Ice Sheet (PROMICE)

    NASA Astrophysics Data System (ADS)

    Andersen, S. B.; Ahlstrom, A. P.; Andersen, M. L.; Box, J. E.; Citterio, M.; Fausto, R. S.; van As, D.; Forsberg, R.; Skourup, H.; Sandberg, L.; Kristensen, S. S.; Petersen, D.

    2013-12-01

    Climate change in the Arctic has resulted in accelerated mass loss from the Greenland Ice Sheet. The shortage of observations on the Greenland ice sheet infers large uncertainties in estimates of the ice mass loss and in predicting the contribution to sea level rise. For this reason the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) was established in 2007. The aim of the programme is to quantify the mass loss of the Greenland ice sheet and track changes in the extent of the glaciers, ice caps and ice sheet margin. Within PROMICE data sets from several activities are collected. These include: A network of currently 19 automatic weather stations on the margin of the Greenland ice sheet measuring ice ablation and snow fall as well as meteorological parameters. Airborne surveys, yielding surface elevation and ice depth along the entire margin of the Greenland ice sheet. Mapping of all Greenland ice masses, based on the highest detail aero-photogrammetric maps produced from mid-80's aerial photographs. Real-time data from the PROMICE automatic weather station network is shown in at the PROMICE web site www.promice.org and the data is freely available for download. Data from the airborne surveys and mapping activities will also become freely available. Data from PROMICE also contribute to the website www.polarportal.org which is a new Danish web site for providing updated information on the arctic cryosphere to the public.

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

    PubMed Central

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

    2015-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Winkelmann, R.; Levermann, A.; Ridgwell, A.; Caldeira, K.

    2015-12-01

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

  18. The CReSIS Radar Suite for Measurements of the Ice Sheets and Sea Ice during Operation Ice Bridge

    NASA Astrophysics Data System (ADS)

    Leuschen, C.; Gogineni, P. S.; Allen, C.; Paden, J. D.; Hale, R.; Rodriguez-Morales, F.; Harish, A.; Seguin, S.; Arnold, E.; Blake, W.; Byers, K.; Crowe, R.; Lewis, C.; Panzer, B.; Patel, A.; Shi, L.

    2010-12-01

    The University of Kansas, Center for Remote Sensing of Ice Sheets (CReSIS) has developed a suite of radar instrumentation operating at frequencies ranging from 180 MHz to 18 GHz to monitor the major Greenland and Antarctic ice sheets at varying resolution from the surface to the bed as well as the surface topography and snow cover characteristics of sea ice. The radar suite includes a VHF multi-channel coherent radar depth sounder/imager (MCoRDS/I) to measure ice sheet thickness, internal layering, and image ice-bed interface with fine resolution; a UHF accumulation radar to measure shallow (up to 500 m) ice sheet thickness, internal layering, and accumulation rates at decimeter resolutions; an ultra-wideband “snow” radar to measure thickness of snow over sea ice and accumulation rates at centimeter scale resolution; and a wideband Ku-band altimeter to measure surface topography and accumulation layer for comparison with CryoSat-2. To support recent NASA Operation Ice Bridge activities, CReSIS radars were adapted for installation and operation on the NASA DC-8 for deployments primarily to Antarctica for measurements over glacial ice in West Antarctica and sea ice, and on the NASA P-3 for deployments to Greenland and Arctic sea ice. The major integration activities included the development of antenna subsystems specifically tailored for the DC-8 and P-3 aircraft. This included a low-profile radar depth sounder/imager antenna assembly for the DC-8 with a minimal impact on the aircraft range, one of the largest VHF antenna arrays for the P-3, and a set of internal antennas structures and radomes for the P-3 bomb bay. The design and development of the antenna assemblies were focused on meeting a combination of instrument and mission requirements. We will present a description of each radar system including the antenna assemblies, instrument block diagrams, modes of operation, installation specifics for each OIB deployment, example results, and availability of data

  19. Glaciomarine facies from the western sector of the last British ice sheet, Malin Beg, County Donegal, Ireland

    NASA Astrophysics Data System (ADS)

    McCabe, A. Marshall; Bowen, D. Q.; Penney, David N.

    The coastal margins and mountainous core of County Donegal, Republic of Ireland have been areally scoured and deeply-eroded along structural lineaments by major Late Devensian ice streams which moved generally westwards onto the continental shelf. A raised glaciomarine sequence is selectively preserved along a gully at Malin Beg and comprises mud/muddy diamict, sand and diamict. (1) The mud, which was deposited from suspended sediment plumes, passes upwards into muddy diamict, which reflects an increase in ice-rafting. The muds contain a redeposited foraminiferal assemblage of ice-proximal origin pumped into the water column by jets reworking previously deposited outwash. The presence of Middle Devensian shells have been identified from the mud by amino-acid analysis. This suggests a Late Devensian age for the succession. (2) The overlying sand indicates an increase in traction current activity at the site. (3) There is a sharp planar contact between the sands and the overlying tabular diamict unit. (4) A surficial sheet of coarse-grained breccia is attributed to intense periglacial weathering along the slopes of Leahan mountain. This sequence and those at other sites in western Ireland (e.g. Belderg, Co. Mayo) provide a record of deep isostatic depression and sedimentation in a peripheral trough at the north-western margin of the last British ice sheet. Directional indicators including drumlin fields and major troughs indicate that the north-western sector of the ice sheet was drained by major ice-streams. These ended in calving bays. Ice marginal response to ice wastage depended largely on catchment factors such as the magnitude of the ice reservoirs and the length of flow lines. Ultimately calving led to accelerated ice flow and areal down draw and collapse of the north-western sector of the ice sheet.

  20. Learning from the past: Antarctic Eemian ice sheet dynamics as an analogy for future warming.

    NASA Astrophysics Data System (ADS)

    Sutter, Johannes; Thoma, Malte; Grosfeld, Klaus; Gierz, Paul; Lohmann, Gerrit

    2015-04-01

    Facing considerable warming during this century the stability of the West Antarctic Ice Sheet is under increasing scrutiny. Recent observations suggest that the marine ice sheet instability of the WAIS has already started . We investigate the dynamic evolution of the Antarctic Ice Sheet during the last interglacial, forcing a state of the art 3D ice sheet model with Eemian boundary conditions. We elucidate the role of ocean warming and surface mass balance on the coupled ice sheet/shelf and grounding line dynamics. Special focus lies on an ice sheet modeling assessment of Antarctica's potential contribution to global sea level rise during the Eemian. The transient model runs are forced by time slice experiments of a fully coupled atmosphere-ocean global circulation model, as well as different sets of sea level and bedrock reconstructions. The model result show strong evidences for a severe ice-sheet retreat in West Antartica, leading to substantical contribution to global sea level from the Southern Hemisphere. Additionally we compare future warming scenarios of West Antarctic Ice Sheet dynamics to our paleo ice sheet modeling studies.

  1. Paleoclimatic evidence for future ice-sheet instability and rapid sea-level rise

    USGS Publications Warehouse

    Overpeck, J.T.; Otto-Bliesner, B. L.; Miller, G.H.; Muhs, D.R.; Alley, R.B.; Kiehl, J.T.

    2006-01-01

    Sea-level rise from melting of polar ice sheets is one of the largest potential threats of future climate change. Polar warming by the year 2100 may reach levels similar to those of 130,000 to 127,000 years ago that were associated with sea levels several meters above modern levels; both the Greenland Ice Sheet and portions of the Antarctic Ice Sheet may be vulnerable. The record of past ice-sheet melting indicates that the rate of future melting and related sea-level rise could be faster than widely thought.

  2. Controls on Greenland Ice Sheet Runoff from a Land Terminating Glacier

    NASA Astrophysics Data System (ADS)

    Rennermalm, A. K.; Smith, L. C.; Chu, V. W.; Forster, R. R.; Hagedorn, B.; Box, J. E.; van den Broeke, M. R.

    2011-12-01

    Modeling studies show that ice sheet mass loss in the form of meltwater runoff constitutes a large fraction of the total mass loss of the Greenland ice sheet. However, the controls on Greenland ice sheet runoff to the ocean are not well understood in part because few direct observations of ice sheet meteorological conditions and meltwater runoff in rivers draining the ice sheet are available to constrain model development. Here, analysis and modeling using such observational data were carried out. West-central Greenland's ice sheet margin near Kangerlussuaq was monitored between 2008 - 2010, yielding data from two automatic weather stations on the ice sheet, and three pro-glacial riverine discharge stations. These data were used to construct regression models to separate influences from fast flowing supra glacial meltwater runoff, and delayed runoff from meltwater releases from en- and pro-glacial storages. These models show that supra glacial runoff explains the majority of meltwater runoff, with en- and pro-glacial storages also significant and less predictable. Finally, a simple extrapolation model confirms that most meltwater runoff originates from the ice sheet margin, with only a small amount released from the ice sheet interior.

  3. Dynamic Inland Propagation of Thinning Due to Ice Loss at the Margins of the Greenland Ice Sheet

    NASA Technical Reports Server (NTRS)

    Wang, Wei Li; Li, Jun J.; Zwally, H. Jay

    2012-01-01

    Mass-balance analysis of the Greenland ice sheet based on surface elevation changes observed by the European Remote-sensing Satellite (ERS) (1992-2002) and Ice, Cloud and land Elevation Satellite (ICESat) (2003-07) indicates that the strongly increased mass loss at lower elevations (<2000 m) of the ice sheet, as observed during 2003-07, appears to induce interior ice thinning at higher elevations. In this paper, we perform a perturbation experiment with a three-dimensional anisotropic ice-flow model (AIF model) to investigate this upstream propagation. Observed thinning rates in the regions below 2000m elevation are used as perturbation inputs. The model runs with perturbation for 10 years show that the extensive mass loss at the ice-sheet margins does in fact cause interior thinning on short timescales (i.e. decadal). The modeled pattern of thinning over the ice sheet agrees with the observations, which implies that the strong mass loss since the early 2000s at low elevations has had a dynamic impact on the entire ice sheet. The modeling results also suggest that even if the large mass loss at the margins stopped, the interior ice sheet would continue thinning for 300 years and would take thousands of years for full dynamic recovery.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  5. Enthalpy balance methods versus temperature models in ice sheets

    NASA Astrophysics Data System (ADS)

    Calvo, Natividad; Durany, José; Vázquez, Carlos

    2015-05-01

    In this paper we propose and numerically solve an original enthalpy formulation for the problem governing the thermal behaviour of polythermal ice sheets. Although the modelling follows some ideas introduced in Aschwanden and Blatter (2009), nonlinear basal boundary conditions in both cold and temperate regions are also considered, thus including the sliding effects in the frame of a fully coupled shallow ice approximation (SIA) model. One of the main novelties of this work comes from the introduction of the Heaviside multivalued operator to take into account the discontinuity of the thermal diffusion function at the cold-temperate transition surface (CTS) free boundary. Moreover, we propose a duality method for maximal monotone operators to solve simultaneously the nonlinear diffusive term and the free boundary. Some numerical simulation examples with real data from Antarctica are presented and illustrate the small differences between the computed results from the enthalpy formulation here proposed and the alternative formulation in terms of the temperature (Calvo et al., 2001).

  6. Stability of the West Antarctic ice sheet in a warming world

    NASA Astrophysics Data System (ADS)

    Joughin, Ian; Alley, Richard B.

    2011-08-01

    Ice sheets are expected to shrink in size as the world warms, which in turn will raise sea level. The West Antarctic ice sheet is of particular concern, because it was probably much smaller at times during the past million years when temperatures were comparable to levels that might be reached or exceeded within the next few centuries. Much of the grounded ice in West Antarctica lies on a bed that deepens inland and extends well below sea level. Oceanic and atmospheric warming threaten to reduce or eliminate the floating ice shelves that buttress the ice sheet at present. Loss of the ice shelves would accelerate the flow of non-floating ice near the coast. Because of the slope of the sea bed, the consequent thinning could ultimately float much of the ice sheet's interior. In this scenario, global sea level would rise by more than three metres, at an unknown rate. Simplified analyses suggest that much of the ice sheet will survive beyond this century. We do not know how likely or inevitable eventual collapse of the West Antarctic ice sheet is at this stage, but the possibility cannot be discarded. For confident projections of the fate of the ice sheet and the rate of any collapse, further work including the development of well-validated physical models will be required.

  7. Coupled ice sheet-climate interactions during the Last Interglacial simulated with LOVECLIM

    NASA Astrophysics Data System (ADS)

    Goelzer, H.; Huybrechts, P.; Loutre, M.; Fichefet, T.

    2013-12-01

    The Last Interglacial warm period (LIG, ~130 to 115 kyr BP) represents a real-world test case for the stability of the Greenland and Antarctic ice sheets, both thought to have lost considerable amounts of ice compared to their present-day configuration. We use the Earth system model of intermediate complexity LOVECLIM version 1.3 to perform transient simulations over the LIG forced with changes in orbital parameters and greenhouse gases. The model includes thermomechanically coupled models of the Greenland and Antarctic ice sheets, which are interactively coupled with the atmosphere and ocean components. We present modeling results with focus on the evolution of the polar ice sheets and their sea-level contribution, ice-climate interactions and inter-hemispheric coupling. LOVECLIM simulates annual mean temperature and summer temperature anomalies over central Greenland relative to the present day that peak at 5.5 °C and 10.7 °C, respectively. The Greenland ice sheet sea-level contribution peaks at between 1.0 and 2.6 m mainly caused by increased summer temperatures. While changes in surface melting have a negligible effect for the evolution of the Antarctic ice sheet during the LIG, a peak sea-level contribution of 2.1 m is governed by West Antarctic grounding-line retreat. For both ice sheets interactive ice-climate coupling is crucial to produce a temperature evolution over the ice sheets better in line with ice core records.

  8. Observations from the Programme for Monitoring of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Andersen, S. B.; Ahlstrom, A. P.; Andersen, M. L.; Box, J. E.; Citterio, M.; Colgan, W. T.; Fausto, R. S.; van As, D.; Forsberg, R.; Skourup, H.; Sandberg Sørensen, L.; Kristensen, S. S.; Dall, J.; Kusk, A.; Petersen, D.

    2014-12-01

    The Programme for Monitoring of the Greenland Ice Sheet (PROMICE) is as an on-going effort initiated in 2007 to monitor changes in the mass budget of the Greenland Ice Sheet. The aim of the programme is to quantify the mass loss of the Greenland ice sheet and track changes in the extent of the glaciers, ice caps and ice sheet margin. Specifically, PROMICE aims to estimate the mass loss derived from three fundamentally different sources: Surface melt water runoff from the ice sheet margin Iceberg production Mass loss of individual glaciers and ice caps surrounding the ice sheet The first is observed by a network of automatic weather stations (AWS) on the ice sheet margin measuring ice ablation as well as meteorological parameters. The second is determined by establishing a so-called 'flux gate' along the entire ice sheet margin and keeping track of the ice passing through this gate. The flux gate is obtained from airborne surveys of ice sheet surface elevation and thickness. The volume of the ice passing through the gate is derived from maps of the surface velocity of the ice sheet, produced from satellite radar. The third is investigated through regular mapping of area and elevation of the approximately 20.000 individual glaciers and ice caps in Greenland. Mapping is carried out using recent satellite imagery as well as aerial ortho-photos. Within PROMICE data sets from these activities are collected. They include observations from the network of currently about 20 AWS on the margin of the Greenland ice sheet. Airborne surveys, yielding surface elevation and ice depth along the entire margin of the Greenland ice sheet carried out in 2007 and 2011. A map of all Greenland ice masses, based on the highest detail aero-photogrammetric maps produced from mid-80's aerial photographs. Real-time data from the PROMICE AWS network is shown at the web site www.promice.org and the data is freely available for download. Data from the airborne surveys and mapping activities are

  9. Modelling heterogeneous meltwater percolation on the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Ligtenberg, S.

    2015-12-01

    The Greenland Ice Sheet (GrIS) has experienced an increase of surface meltwater production over the last decades, with the latest record set in the summer of 2012. For current and future ice sheet mass balance assessments, it is important to quantify what part of this meltwater reaches the ocean and contributes to sea level change. Meltwater produced at the surface has several options: it can infiltrate the local firn pack, where it is either stored temporarily or refrozen, or it can run off along the surface or via en-glacial drainage systems. In this study, we focus on the first; more specifically, in which manner meltwater percolates the firn column. Over the past years, GrIS research has shown that meltwater does not infiltrate the firn pack homogeneously (i.e. matrix flow), but that inhomogeneities in horizontal firn layers causes preferential flow paths for meltwater (i.e. piping). Although this process has been observed and studied on a few isolated sites, it has never been examined on the entire GrIS. To do so, we use the firn model IMAU-FDM with new parameterizations for preferential flow, impermeable ice lenses and sub-surface runoff. At the surface, IMAU-FDM is forced with realistic climate data from the regional climate model RACMO2.3. The model results are evaluated with temperatures and density measurements from firn cores across the GrIS. By allowing for heterogeneous meltwater percolation, the model is able to store heat and mass much deeper in the firn column. This is, however, in part counteracted by the inclusion of impermeability of ice lenses, which causes part of the meltwater to run off horizontally.

  10. Individual and combined effects of ice sheets and precession on MIS-13 climate

    NASA Astrophysics Data System (ADS)

    Yin, Q. Z.; Berger, A.; Crucifix, M.

    2009-02-01

    Simulations with an Earth System Model of Intermediate Complexity are made to investigate the role of insolation and the size of ice sheets on the regional and global climate for marine isotope stage (MIS) 13. The astronomical forcing is selected at two dates with opposite precession, one when Northern Hemisphere summer (NHS) occurs at perihelion (at 506 ka BP) and the other when it occurs at aphelion (at 495 ka BP). Experiments with five different volumes of the Eurasian and North American ice sheets (ranging from 0 to the Last Glacial Maximum one) are done under these two astronomical conditions. When NHS is at perihelion, the Earth is warmer, the seasonal contrast in Northern (Southern) Hemisphere is larger (smaller) and summer precipitation in Northern Hemisphere monsoon regions is more abundant than when it is at aphelion. The global cooling due to the ice sheets is mainly related to the ice sheet area, little to their height. The regional cooling and warming anomalies caused by the ice sheets get intensified with increasing ice sheet size. The cooling is different whether the NHS occurs at aphelion or at perihelion. Precipitation over different monsoon regions responds differently to the size of the ice sheets. Over North Africa, the ice sheets always reduce precipitation, larger the size less the precipitation. Over East China, when NHS is at perihelion, the ice sheets reinforce the summer precipitation whatever their sizes. But when NHS is at aphelion, there is a threshold in the ice volume beyond which the ice sheets start to reduce the precipitation over East China. This underlies the importance of insolation in shaping the ice sheet impact on the precipitation over the East Asian Summer Monsoon (EASM) region.

  11. Ice sheet runoff and Dansgaard-Oeschger Cycles

    NASA Astrophysics Data System (ADS)

    Hewitt, Ian; Wolff, Eric; Fowler, Andrew; Clark, Chris; Evatt, Geoff; Johnson, Helen; Munday, David; Rickaby, Ros; Stokes, Chris

    2016-04-01

    Many northern hemisphere climate records, particularly those from around the North Atlantic, show a series of rapid climate changes that recurred on centennial to millennial timescales throughout most of the last glacial period. These Dansgaard-Oeschger (D-O) sequences are observed most prominently in Greenland ice cores, although they have a global signature, including an out of phase Antarctic signal. They consist of warming jumps of order 10°C, occurring in typically 40 years, followed generally by a slow cooling (Greenland Interstadial, GI) lasting between a few centuries and a few millennia, and then a final rapid temperature drop into a cold Greenland Stadial (GS) that lasts for a similar period. Most explanations for D-O events call on changes in Atlantic meridional overturning circulation strength, and the majority of such explanations use changes in freshwater delivery from ice sheets as a trigger. Many have relied on large inputs of freshwater from singular events (such as lake outbursts or iceberg armadas) to push the AMOC into its cold state. However the evidence for such events at the right time in each cycle is sparse. Here we investigate mechanisms that would arise from a change in the rate of ice sheet runoff, which would be a natural feedback from each rapid warming or cooling event. Recent work has suggested that AMOC is most easily disrupted by freshwater delivered through the Arctic. We investigate whether the proposed AMOC changes could have occurred as part of a natural oscillation, in which runoff from the Laurentide ice sheet into the Arctic is controlled by temperature around the North Atlantic. The Arctic buffers the salinity changes, but under warm conditions, high runoff eventually leads to water entering the North Atlantic with low enough salinity to switch AMOC into its weaker state. Under the colder conditions now prevailing, the Arctic is starved of runoff, and the salinity rises until a further switch occurs. Contrary to many

  12. Radiative energy transport on the surface of an ice sheet

    NASA Astrophysics Data System (ADS)

    Cathles, Lawrence Maclagan, IV

    Both surface roughness and surface conditions of an ice sheet affect the exchange of energy between the atmosphere and an ice sheet. Potential feedbacks exist where the absorbed energy on the surface changes the surface in such a way that more energy is absorbed. One such feedback occurs when a supraglacial lake absorbs energy and sub-lake melting occurs, which deepens the lake, and reduces the albedo. While another feedback exists from topography which creates non-uniform absorption of sunlight, causing surface geometries to change as melting occurs, affecting how energy is absorbed on the surface. The influence of these feedbacks on the surface energy balance of ice sheet are not well understood, and motivate the work presented here. To assess the influences of surface conditions and surface topography on the radiative budget of an ice sheet, the two feedback cases were studied separately. The effect of the surface condition was determined using a two-stream radiative transfer model for a two layer system, where the top layer could be either ice, snow, or water, and the second layer was ice. An analytical solution for the albedo of a supraglacial lake as a function of water depth was derived, and showed that a supraglacial lake can reduce the albedo by fifty percent. Multispectral analysis is required for surfaces where liquid water is present, but little additional accuracy is gained beyond 18 spectral bands. Complex surface topography has a significant effect on the absorption of energy due to multiple reflections between the surface and its self. A numerical model was developed to determine the absorption of insolation on an arbitrary two-dimensional surface. The absorbed radiation forces an ice ablation model evolving the surface topography through an ablation season. Results from numerical simulations show that there is a strong latitudinal dependence on how surface features evolve through an ablation season. At high latitudes, the aspect ratio of surface

  13. Firn compaction modelling of the Antarctic ice sheet

    NASA Astrophysics Data System (ADS)

    Kallenberg, Bianca; Tregoning, Paul

    2013-04-01

    Satellite altimetry missions detect elevation changes in ice sheets that are not only related to variations in ice mass balance, but also to snow densification. The compaction of snow induces a change in thickness but not in mass and therefore has to be removed from the altimetry measurements when estimating mass loss from height changes. The densification of snow is time dependent and varies with temperature, accumulation rate and depth. Different types of densification processes occur in Antarctica due to the climatic differences from warm and moist coastal areas to a cold and dry desert in the Antarctic interior. The intermediate product between snow and ice is called firn and the transition from snow to ice is a slow process that can take up to millennia in some areas. During the compression snow grains undergo different stages with a density change from around 300 kg/m3 for fresh snow to around 900 kg/m3 for glacier ice. The change in density with temperature and depth is not well known and can only be compared with some snow pits that have been taken at a few locations in Antarctica, thus the density profile is of great importance. The lack of data complicates the generation of an accurate firn compaction model and so far only a few models have been established about expected firn densification processes in Antarctica. We present a time-dependent firn compaction model for Antarctica based on the standard heat-transfer equation after Paterson (1994)* for the temperature profile, and the concept of firn compaction after Zwally & Li (2002)*. By incorporating a time-dependent accumulation rate, our numerical multilayer model considers not only existing snow layers but also freshly deposited accumulation at the surface as a new introduced layer. The initial density profile as been obtained by spinning up the model until the entire firn layer is refreshed. We compare our results with previous firn compaction models and available in-situ measurements of snow pits

  14. Geologic evidence of a leaky Antarctic Ice Sheet: Tracking meltwater influence on ice-sheet retreat in the Ross Sea, Antarctica since the Last Glacial Maximum

    NASA Astrophysics Data System (ADS)

    Simkins, L.; Greenwood, S.; Anderson, J. B.; Prothro, L. O.; Halberstadt, A. R.; Stearns, L. A.; Demet, B. P.

    2015-12-01

    Meltwater at the base of an ice sheet has a significant influence on the flow dynamics and stability. Processes that lead to ice-sheet instability are at the forefront of research addressing uncertainty in the fate of modern ice sheets. A better understanding of subglacial meltwater is needed to assess the role of meltwater on ice-sheet instability and place modern observations of meltwater drainage into a longer-term context. Observations of subglacial channels and evidence of meltwater influence on ice-sheet dynamics are sparse in the geologic record around Antarctica. We present subglacial meltwater channels incised into the post-LGM sea scape of the Ross Sea continental shelf with clear relationships to glacial retreat landforms. We combine geomorphic and stratigraphic evidence to test the influence of meltwater on ice-sheet retreat during the last deglaciation. The channel systems are associated with small, closely-spaced ice-marginal landforms suggestive of rapid grounding line retreat. The channels were active during the late stages of retreat based on their positions in the southernmost portions of the troughs and cross-cutting relationships with glacial retreat landforms. Although channelized meltwater appears to be spatially limited, we have identified meltwater deposits within proximal grounding line facies from cores across the continental shelf in the western Ross Sea. Therefore, not only do we observe clear evidence of meltwater under the dwindling ice sheet, but basal meltwater in the Ross Sea is a pervasive feature and associated with geomorphic and sedimentologic expressions of rapid ice-sheet retreat. Ongoing work focuses on quantifying channel flow properties and reasonable meltwater production rates as well as further constraining evidence of meltwater in the Ross Sea and elsewhere around Antarctica.

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

    NASA Astrophysics Data System (ADS)

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

    2013-11-01

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

  16. Geothermal Heat Flux: Linking Deep Earth's Interior and the Dynamics of Large-Scale Ice Sheets

    NASA Astrophysics Data System (ADS)

    Rogozhina, Irina; Vaughan, Alan

    2014-05-01

    Regions covered by continental-scale ice sheets have the highest degree of uncertainty in composition and structure of the crust and lithospheric mantle, compounded by the poorest coverage on Earth of direct heat flow measurements. In addition to challenging conditions that make direct measurements and geological survey difficult Greenland and Antarctica are known to be geologically complex. Antarctica in particular is marked by two lithospherically distinct zones. In contrast to young and thin lithosphere of West Antarctica, East Antarctica is a collage of thick Precambrian fragments of Gondwana and earlier supercontinents. However, recent observations and modeling studies have detected large systems of subglacial lakes extending beneath much of the East Antarctic ice sheet base that have been linked to anomalously elevated heat flow. Outcrop samples from the rift margin with Australia (Prydz Bay) have revealed highly radiogenic Cambrian granite intrusives that are implicated in regional increase of crustal heat flux by a factor of two to three compared to the estimated continental background. Taken together, these indicate high variability of heat flow and properties of rocks across Antarctica. Similar conclusions have been made based on direct measurements and observations of the Greenland ice sheet. Airborne ice-penetrating radar and deep ice core projects show very high rates of basal melt for parts of the ice sheet in northern and central Greenland that have been explained by abnormally high heat flux. Archaean in age, the Greenland lithosphere was significantly reworked during the Early Proterozoic. In this region, the interpretation of independent geophysical data is complicated by Proterozoic and Phanerozoic collision zones, compounded by strong thermochemical effects of rifting along the western and eastern continental margins between 80 and 25 million years ago. In addition, high variability of heat flow and thermal lithosphere structure in central

  17. Dynamic Antarctic ice sheet during the early to mid-Miocene.

    PubMed

    Gasson, Edward; DeConto, Robert M; Pollard, David; Levy, Richard H

    2016-03-29

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

  18. Dynamic Antarctic ice sheet during the early to mid-Miocene

    NASA Astrophysics Data System (ADS)

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

    2016-03-01

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

  19. Dynamic Antarctic ice sheet during the early to mid-Miocene.

    PubMed

    Gasson, Edward; DeConto, Robert M; Pollard, David; Levy, Richard H

    2016-03-29

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

  20. Photophysiology and albedo-changing potential of the ice algal community on the surface of the Greenland ice sheet

    PubMed Central

    Yallop, Marian L; Anesio, Alexandre M; Perkins, Rupert G; Cook, Joseph; Telling, Jon; Fagan, Daniel; MacFarlane, James; Stibal, Marek; Barker, Gary; Bellas, Chris; Hodson, Andy; Tranter, Martyn; Wadham, Jemma; Roberts, Nicholas W

    2012-01-01

    Darkening of parts of the Greenland ice sheet surface during the summer months leads to reduced albedo and increased melting. Here we show that heavily pigmented, actively photosynthesising microalgae and cyanobacteria are present on the bare ice. We demonstrate the widespread abundance of green algae in the Zygnematophyceae on the ice sheet surface in Southwest Greenland. Photophysiological measurements (variable chlorophyll fluorescence) indicate that the ice algae likely use screening mechanisms to downregulate photosynthesis when exposed to high intensities of visible and ultraviolet radiation, rather than non-photochemical quenching or cell movement. Using imaging microspectrophotometry, we demonstrate that intact cells and filaments absorb light with characteristic spectral profiles across ultraviolet and visible wavelengths, whereas inorganic dust particles typical for these areas display little absorption. Our results indicate that the phototrophic community growing directly on the bare ice, through their photophysiology, most likely have an important role in changing albedo, and subsequently may impact melt rates on the ice sheet. PMID:23018772

  1. A comparison of Holocene fluctuations of the eastern and western margins of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Levy, L.; Kelly, M. A.; Lowell, T. V.; Hall, B. L.; Applegate, P. J.; Howley, J.; Axford, Y.

    2013-12-01

    Determining how the Greenland Ice Sheet (GrIS) responded to past temperature fluctuations is important for assessing its future stability in a changing climate. We present a record of the Holocene extents of the western GrIS margin near Kangerlussuaq (67.0°N, 50.7°W) and compare this with the past fluctuations of Bregne ice cap (71°N, 25.6° W), a small ice cap in the Scoresby Sund region 90 km from the eastern GrIS margin, to examine the mechanisms that influenced past ice margin fluctuations. The past extents of the Bregne ice cap are a proxy for the climatic conditions that influenced the nearby GrIS margin. We used glacial geomorphic mapping, 10Be dating of boulders and bedrock, and sediment cores from proglacial and non-glacial lakes. In western Greenland, 10Be ages on the Keglen moraines, 13 km west of the current GrIS margin and the Ørkendalen moraines, ≤2 km west of the current ice margin date to 7.3 × 0.1 ka (n=6) and 6.8 × 0.3 ka (n=9), respectively. Fresh moraines, ≤50 m from the current ice margin date to AD 1830-1950 and are likely associated with advances during the Little Ice Age (LIA). In some areas, the LIA moraines lie stratigraphically above the Ørkendalen moraines, indicating the GrIS was inboard of the Ørkendalen limit from 6.8 ka to the 20th century. In eastern Greenland, 10Be ages show that Bregne ice cap retreated within its late Holocene limit by 10.7 ka. A lack of clastic sediment in a proglacial lake suggests the ice cap was smaller or completely absent from ~10-2.6 ka. A snowline analysis indicates that temperatures ~0.5°C warmer than present would render the entire ice cap into an ablation zone. Glacial silts in the proglacial lake at ~2.6 and ~1.9 cal kyr BP to present indicate advances of Bregne ice cap. Fresh moraines ≤200 m of Bregne ice cap were deposited ≤2.6 cal kyr BP and mark the largest advance of the Holocene. Both the western GrIS margin and Bregne ice cap were influenced by Northern Hemisphere summer

  2. Inter-annual and geographical variations in the extent of bare ice and dark ice on the Greenland ice sheet derived from MODIS satellite images

    NASA Astrophysics Data System (ADS)

    Shimada, Rigen; Takeuchi, Nozomu; Aoki, Teruo

    2016-04-01

    Areas of dark ice have appeared on the Greenland ice sheet every summer in recent years. These are likely to have a great impact on the mass balance of the ice sheet because of their low albedo. We report annual and geographical variations in the bare ice and dark ice areas that appeared on the Greenland Ice Sheet from 2000 to 2014 by using MODIS satellite images. The July monthly mean of the extent of bare ice showed a positive trend over these 15 years, and large annual variability ranging from 89,975 km2 to 279,075 km2, 5% and 16% of the entire ice sheet, respectively. The extent of dark ice also showed a positive trend and varied annually, ranging from 3,575 km2 to 26,975 km2, 4% and 10% of the bare ice extent. These areas are geographically varied, and their expansion is the greatest on the western side, particularly the southwestern side of the ice sheet. The bare ice extent correlates strongly with the monthly mean air temperature in July, suggesting that the extent was determined by snow melt. The dark ice extent also correlates with the air temperature; however, the correlation is weaker. The dark ice extent further correlates negatively with solar radiation. This suggests that the extent of dark ice is not only controlled by snow melt on the ice, but also by changes in the surface structures of the bare ice surface, such as cryoconite holes, which are associated with impurities appearing on the ice surface.

  3. Caterpillar-like flow of the Greenland Ice Sheet: observations of basal control on ice motion

    NASA Astrophysics Data System (ADS)

    Ryser, C.; Luethi, M. P.; Funk, M.; Catania, G. A.; Andrews, L. C.; Hawley, R. L.; Neumann, T.; Hoffman, M. J.

    2012-12-01

    Varying basal motion due to episodic basal water supply is a long-established component of ice flow. However, the physical processes that govern the role of water in basal motion still remain only weakly understood. We instrumented four boreholes at two sites with sensor systems to better understand the processes controlling seasonal flow velocity variations in the marginal zone of the Greenland Ice Sheet. We present measurements of borehole deformation, subglacial water pressure and surface motion during one year (July 2011 to September 2012). Subglacial water pressure and ice deformation show periodic variations on several time scales which are delayed by up to half a period, depending on sensor depth. These observations are interpreted as ice motion in a caterpillar-like fashion, as opposed to the conventionally assumed shear flow. Using a time-dependent, Full-Stokes ice flow model we find that spatially and temporally varying basal motion can explain the observed variations in deformation, and the delayed reaction at different depths. These new data show that the reaction to basal motion is not uniform throughout the ice column, but varies with depth.

  4. Recent Changes in High-Latitude Glaciers, Ice Caps, and Ice Sheets

    NASA Technical Reports Server (NTRS)

    Abdalati, Waleed

    2006-01-01

    The glaciers and ice sheets of the world contain enough ice to raise sea level by approximately 70 meters if they were to disappear entirely, and most of this ice is located in the climatically sensitive polar regions. Fortunately changes of this magnitude would probably take many thousands of years to occur, but recent discoveries indicate that these ice masses are responding to changes in today s climate more rapidly than previously thought. These responses are likely to be of great societal significance, primarily in terms of their implications for sea level, but also in terms of how their discharge of freshwater, through melting or calving, may impact ocean circulation. For millions of years, oceans have risen and fallen as the Earth has warmed and cooled, and ice on land has shrunk and grown. Today is no different in that respect, as sea levels have been rising at a rate of nearly 2 m per year during the last century (Miller and Douglas 2004), and 3 mm/yr in the last 12 years (Leuliette et al. 2004). What is different today, however, is that tens - perhaps hundreds - of millions of people live in coastal areas that are vulnerable to changes in sea level. Rising seas erode beaches, increase flood potential, and reduce the ability of barrier islands and coastal wetlands to mitigate the effects of major storms and hurricanes. The costs associated with a one-meter rise in sea level are estimated to be in the hundreds of billions of dollars in the United States alone. The worldwide costs in human terms would be far greater as some vulnerable low-lying coastal regions would become inundated, especially in poorer nations that do not have the resources to deal with such changes. Such considerations are particularly important in light of the fact that a one meter sea level rise is not significantly outside the 0.09 to 0.88 range of predictions for this century (IPCC 2001), and rises of this magnitude have occurred in the past in as little as 20 years (Fairbanks 1989

  5. Setting a chronology for the basal ice at Dye-3 and GRIP: Implications for the long-term stability of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Yau, Audrey M.; Bender, Michael L.; Blunier, Thomas; Jouzel, Jean

    2016-10-01

    The long-term stability of the Greenland Ice Sheet (GIS) is an important issue in our understanding of the climate system. Limited data suggest that the northern and southern sections extend well back into the Pleistocene, but most age constraints do not definitively date the ice. Here, we re-examine the GRIP and Dye-3 ice cores to provide direct ice core observations as to whether the GIS survived previous interglacials known to be warmer (˜130 ka) or longer (˜430 ka) than the present interglacial. We present geochemical analyses of the basal ice from Dye-3 (1991-2035 m) and GRIP (3020-3026 m) that characterize and date the ice. We analyzed the elemental and isotopic composition of O2, N2, and Ar, of trapped air in these two cores to assess the origin of trapped gases in silty ice. Dating of the trapped air was then achieved by measuring the paleoatmospheric δ40Ar/38Ar and the 17O anomaly (17Δ) of O2. The resulting age is a lower limit because the trapped air may be contaminated with crustal radiogenic 40Ar. The oldest average age of replicates measured at various depths is 970 ± 140 ka for the GRIP ice core and 400 ka ± 170 ka for Dye-3. 17Δ data from Dye-3 also argue strongly that basal ice in this core predates the Eemian. This confirms that the Greenland Ice Sheet did not completely melt at Southern Greenland during the last interglacial, nor did it completely melt at Summit Greenland during the unusually long interglacial ˜430 kyr before present.

  6. Glacial isostatic stress shadowing by the Antarctic ice sheet

    NASA Technical Reports Server (NTRS)

    Ivins, E. R.; James, T. S.; Klemann, V.

    2005-01-01

    Numerous examples of fault slip that offset late Quaternary glacial deposits and bedrock polish support the idea that the glacial loading cycle causes earthquakes in the upper crust. A semianalytical scheme is presented for quantifying glacial and postglacial lithospheric fault reactivation using contemporary rock fracture prediction methods. It extends previous studies by considering differential Mogi-von Mises stresses, in addition to those resulting from a Coulomb analysis. The approach utilizes gravitational viscoelastodynamic theory and explores the relationships between ice mass history and regional seismicity and faulting in a segment of East Antarctica containing the great Antarctic Plate (Balleny Island) earthquake of 25 March 1998 (Mw 8.1). Predictions of the failure stress fields within the seismogenic crust are generated for differing assumptions about background stress orientation, mantle viscosity, lithospheric thickness, and possible late Holocene deglaciation for the D91 Antarctic ice sheet history. Similar stress fracture fields are predicted by Mogi-von Mises and Coulomb theory, thus validating previous rebound Coulomb analysis. A thick lithosphere, of the order of 150-240 km, augments stress shadowing by a late melting (middle-late Holocene) coastal East Antarctic ice complex and could cause present-day earthquakes many hundreds of kilometers seaward of the former Last Glacial Maximum grounding line.

  7. Antarctic ice sheet: preliminary results of first core hole to bedrock.

    PubMed

    Gow, A J; Ueda, H T; Garfield, D E

    1968-09-01

    The Antarctic ice sheet at Byrd Station has been core-drilled to bedrock; the vertical thickness of the ice is 2164 meters. Liquid water-indicative of pressure melting-was encountered at the bed. Heat flow through the base of the ice sheet is estimated at 1.8 microcalories per square centimeter per second. The minimum temperature was -28.8 degrees C at 800 meters; maximum ice density, 0.9206 at 1000 meters. Core studies reveal the existence of a chemically pure, structurally stratified sheet comprising bubbly ice to 900 meters that transforms to bubble-free deformed ice, with substantially vertically orientated c-axis structure, below 1200 meters. Below 1800 meters the deformed ice structure gives way to large annealed crystals. Several thin layers of dirt between 1300 and 1700 meters are tentatively identified as volcanic ash, and horizontally banded debris, including fragments of granite, is present in the basal ice.

  8. The Sea-Level Fingerprints of Ice-Sheet Collapse During Interglacial Periods (Invited)

    NASA Astrophysics Data System (ADS)

    Hay, C.; Mitrovica, J. X.; Gomez, N. A.; Creveling, J. R.; Austermann, J.; Kopp, R. E.

    2013-12-01

    Peak sea-level highstands during previous interglacials provide important insight into the stability of polar ice sheets in a warming world. In most previous analyses of such highstands, site-specific sea-level records are first corrected for the ongoing effects of glacial isostatic adjustment (GIA). The residual sea level is then interpreted as representing the net eustatic signal associated with any excess melting of the polar ice sheets relative to their present-day volumes. However, it is now well understood that the collapse of polar ice sheets produces a distinct geometry, or fingerprint, of sea-level change. These sea-level fingerprints must be accounted for in order to accurately estimate global eustatic sea level from site-specific (GIA-corrected) records. To investigate this issue, we compute fingerprints associated with the collapse of the Greenland Ice Sheet, West Antarctic Ice Sheet, and marine sectors of the East Antarctic Ice Sheet. We demonstrate that these fingerprints are, in contrast to those computed for modern melt scenarios, relatively insensitive to the detailed geometry of ice-sheet collapse. Moreover, using these fingerprints, we isolate regions that would have experienced greater-than-eustatic sea-level change regardless of the melt scenario. Finally, we use the fingerprints to demonstrate that some previous analyses of sea-level records from past interglacials may have overestimated peak eustatic sea level (and thus the net excess melting of polar ice sheets) by 1-2 m.

  9. Dynamic Antarctic ice sheet during the early to mid-Miocene

    PubMed Central

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

    2016-01-01

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

  10. Evidence for smaller extents of the northwestern Greenland Ice Sheet and North Ice Cap during the Holocene

    NASA Astrophysics Data System (ADS)

    Kelly, M. A.; Osterberg, E. C.; Axford, Y.; Bigl, M.; Birkel, S. D.; Corbett, L. B.; Roy, E. P.; Thompson, J. T.; Whitecloud, S.

    2013-12-01

    The Greenland Ice Sheet (GrIS) and local glaciers on Greenland are responding dynamically to warming temperatures with widespread retreat. GRACE satellite data (e.g., Kahn et al., 2010) and the Petermann Glacier calving events document the recent expansion of ice loss into northwestern Greenland. To improve the ability to estimate future ice loss in a warming climate, we are developing records of the response of the northwestern Greenlandic cryosphere to Holocene climatic conditions, with a focus on past warm periods. Our ongoing research includes analyses of glacial geology, sub-fossil vegetation, lake sediment cores, chironomid assemblages and ice cores combined with glaciological modeling. To constrain past ice extents that were as small as, or smaller than, at present, we recovered sub-fossil vegetation exposed at the receding margins of the GrIS and North Ice Cap (NIC) in the Nunatarssuaq region (~76.7°N, 67.4°W) and of the GrIS near Thule (~76.5°N, 68.7°W). We present vegetation types and radiocarbon ages of 30 plant samples collected in August 2012. In the Nunatarssuaq region, five ages of in situ (rooted) vegetation including Polytrichum moss, Saxifraga nathorstii and grasses located <5 m outboard of the GrIS margin are ~120-200 cal yr BP (range of medians of the 2-sigma calibrated age ranges). Nine ages of in situ Polytrichum, Saxifraga oppositafolia and grasses from ~1-5 m inboard of the NIC margin (excavated from beneath ice) range from ~50 to 310 cal yr BP. The growth of these plants occurred when the GrIS and NIC were at least as small as at present and their ages suggest that ice advances occurred in the last 50-120 yrs. In addition to the in situ samples, we collected plants from well-preserved ground material exposed along shear planes in the GrIS margins. In Nunatarssuaq, two Polytrichum mosses rooted in ground material and exposed along a shear plane in the GrIS margin date to 4680 and 4730 cal yr BP. Near Thule, three ages of Salix arctica

  11. Ocean - ice sheet interaction along the NW European margin during the last glacial phases

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

    The NW European continental margin was repeatedly covered by shelf edge glaciations during the last glacial cycles. Here, we present a compilation of new and previously published data from a SW to NE transect of 8 sediment cores raised along the upper continental slope. This study aims to investigate the interaction between sea surface conditions and the variability seen in the British Irish Ice Sheet (BIIS) and the Fennoscandian Ice Sheet (FIS) during the last 13-40 ka BP. Ice Rafted Debris (IRD) counts, IRD flux data, grain size data, the content of the polar planktonic foraminifera Neogloboquadrina pachyderma (sin) and ∂18O measurements were compiled and combined with new Bayesian age models. From 40-24.5 ka BP the build up and consecutive confluence of the BIIS and the FIS are reflected in sediment composition and flux data. Pulses of large quantities of fine material to the southern part of the transect suggest riverine BIIS related influx. The sediment composition in cores close to the Norwegian channel indicates that the Norwegian Channel Ice Stream (NCIS) was only active between 24.5-18.5 ka BP during the last glacial stage. The planktonic foraminifera data during this period strongly suggests a dependence of NCIS extent variability and pulses in warm Atlantic water entering the Nordic Seas. In the northernmost cores rapidly deposited, laminated sediments and ∂18O spikes in planktonic foraminifera dated to 18.5 ka BP were interpreted as meltwater plume deposits. This may reflect NCIS retreat allowing BIIS and FIS to unzip and route ice dammed lake- and meltwater to the margin. In conclusion, the investigation suggests a close co-variation in extent of marine based parts of the BIIS, the FIS and ocean circulation while demonstrating the strong influence of the local glacial history on standard open marine proxies. This suggests that tuning chronologies of single marine records to ice cores in some regions might be more challenging than previously

  12. A model of the western Laurentide Ice Sheet, using observations of glacial isostatic adjustment

    NASA Astrophysics Data System (ADS)

    Gowan, Evan J.; Tregoning, Paul; Purcell, Anthony; Montillet, Jean-Philippe; McClusky, Simon

    2016-05-01

    We present the results of a new numerical model of the late glacial western Laurentide Ice Sheet, constrained by observations of glacial isostatic adjustment (GIA), including relative sea level indicators, uplift rates from permanent GPS stations, contemporary differential lake level change, and postglacial tilt of glacial lake level indicators. The later two datasets have been underutilized in previous GIA based ice sheet reconstructions. The ice sheet model, called NAICE, is constructed using simple ice physics on the basis of changing margin location and basal shear stress conditions in order to produce ice volumes required to match GIA. The model matches the majority of the observations, while maintaining a relatively realistic ice sheet geometry. Our model has a peak volume at 18,000 yr BP, with a dome located just east of Great Slave Lake with peak thickness of 4000 m, and surface elevation of 3500 m. The modelled ice volume loss between 16,000 and 14,000 yr BP amounts to about 7.5 m of sea level equivalent, which is consistent with the hypothesis that a large portion of Meltwater Pulse 1A was sourced from this part of the ice sheet. The southern part of the ice sheet was thin and had a low elevation profile. This model provides an accurate representation of ice thickness and paleo-topography, and can be used to assess present day uplift and infer past climate.

  13. A Maturing Tephra Record in the West Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Dunbar, N. W.; Kurbatov, A.; McIntosh, W. C.

    2011-12-01

    Tephra layers found in many Antarctic ice cores range from sub-centimeter thick, visible layers to cryptotephra consisting of sparse, fine-grained (<10 micron) glass particles. Location of tephra particles has improved with advances in methods of detecting cryptotephra in ice cores. Identification of tephra layers traditionally relied on visible detection or association with sulfate aerosols, but is now supplemented by downhole optical logging (Bay et al., 2001). Improved analytical techniques for glass characterization, such as high quality quantitative electron microprobe analysis, and more complete information on source eruptions has facilitated identification of tephra horizons in ice cores. Two deep ice cores drilled in West Antarctica (Siple Dome and WAIS Divide) contain rich tephra records, with the former containing 37 tephra layers and the latter containing several hundred distinct, visible layers, many of which are likely to be tephra. Most of the tephra layers with strong correlations to sources are derived from Antarctic volcanoes, many from two large West Antarctic stratovolcanoes Mt. Berlin and Mt. Takahe, tephra from which have also been recognized in the marine record (Hillenbrand et al., 1988). A well-defined ash layer is found at a depth of between 190.37-190.39 m depth in the WAIS Divide core, containing 20 um ash shards that are chemically correlated to the the Pleaides volcanoes, in northern Victoria Land. This tephra layer correlates to one found in a Siple Dome (B) ice core (97.2 to 97.7 m depth) and in the Taylor Dome ice core (79.2 m depth). Deeper parts of the WAIS Divide ice core correspond to a time interval of abundant regional volcanism, represented by the large number of visible dust bands and cloudy layers in the core (A. Orsi, pers. comm., 2010). A distinct "visible brown layer" at a depth of 1586.363 m. (8.279 Ky BP preliminary age) is very likely to be from a major eruption of the West Antarctic volcano Mt. Takahe (8.2±5

  14. Global warming and the stability of the West Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Oppenheimer, Michael

    1998-05-01

    Of today's great ice sheets, the West Antarctic Ice Sheet poses the most immediate threat of a large sea-level rise, owing to its potential instability. Complete release of its ice to the ocean would raise global mean sea level by four to six metres, causing major coastal flooding worldwide. Human-induced climate change may play a significant role in controlling the long-term stability of the West Antarctic Ice Sheet and in determining its contribution to sea-level change in the near future.

  15. Modeling North American Ice Sheet Response to Changes in Precession and Obliquity

    NASA Astrophysics Data System (ADS)

    Tabor, C.; Poulsen, C. J.; Pollard, D.

    2012-12-01

    Milankovitch theory proposes that changes in insolation due to orbital perturbations dictate the waxing and waning of the ice sheets (Hays et al., 1976). However, variations in solar forcing alone are insufficient to produce the glacial oscillations observed in the climate record. Non-linear feedbacks in the Earth system likely work in concert with the orbital cycles to produce a modified signal (e.g. Berger and Loutre, 1996), but the nature of these feedbacks remain poorly understood. To gain a better understand of the ice dynamics and climate feedbacks associated with changes in orbital configuration, we use a complex Earth system model consisting of the GENESIS GCM and land surface model (Pollard and Thompson, 1997), the Pennsylvania State University ice sheet model (Pollard and DeConto, 2009), and the BIOME vegetation model (Kaplan et al., 2001). We began this study by investigating ice sheet sensitivity to a range of commonly used ice sheet model parameters, including mass balance and albedo, to optimize simulations for Pleistocene orbital cycles. Our tests indicate that choice of mass balance and albedo parameterizations can lead to significant differences in ice sheet behavior and volume. For instance, use of an insolation-temperature mass balance scheme (van den Berg, 2008) allows for a larger ice sheet response to orbital changes than the commonly employed positive degree-day method. Inclusion of a large temperature dependent ice albedo, representing phenomena such as melt ponds and dirty ice, also enhances ice sheet sensitivity. Careful tuning of mass balance and albedo parameterizations can help alleviate the problem of insufficient ice sheet retreat during periods of high summer insolation (Horton and Poulsen, 2007) while still accurately replicating the modern climate. Using our optimized configuration, we conducted a series of experiments with idealized transient orbits in an asynchronous coupling scheme to investigate the influence of obliquity and

  16. Multiphase flow of the late Wisconsinan Cordilleran ice sheet in Western Canada

    USGS Publications Warehouse

    Stumpf, A.J.; Broster, B.E.; Levson, V.M.

    2000-01-01

    In central British Columbia, ice flow during the late Wisconsinan Fraser glaciation (ca. 25-10 ka) occurred in three phases. The ice expansion phase occurred during an extended period when glaciers flowed westward to the Pacific Ocean and east-southeastward onto the Nechako Plateau from ice centers in the Skeena, Hazelton, Coast, and Omineca Mountains. Initially, glacier flow was confined by topography along major valleys, but eventually piedmont and montane glaciers coalesced to form an integrated glacier system, the Cordilleran ice sheet. In the maximum phase, a Cordilleran ice divide developed over the Nechako Plateau to 300 km inland from the Pacific coast. At this time, the surface of the ice sheet extended well above 2500 m above sea level, and flowed westward over the Skeena, Hazelton, and Coast Mountains onto the continental shelf, and eastward across the Rocky Mountains into Alberta. In the late glacial phase, a rapid rise of the equilibrium line caused ice lobes to stagnate in valleys, and restricted accumulation centers to high mountains. Discordant directions in ice flow are attributed to fluctuations of the ice divide representing changes in the location of accumulation centers and ice thickness. Ice centers probably shifted in response to climate, irregular growth in the ice sheet, rapid calving, ice streaming, and drainage of proglacial and subglacial water bodies. Crosscutting ice-flow indicators and preservation of early (valley parallel) flow features in areas exposed to later (cross-valley) glacier erosion indicate that the ice expansion phase was the most erosive and protracted event.

  17. Future Antarctic Bed Topography and Its Implications for Ice Sheet Dynamics

    NASA Technical Reports Server (NTRS)

    Adhikari, Surendra; Ivins, Erik R.; Larour, Eric Y.; Seroussi, Helene L.; Morlighem, Mathieu; Nowicki, S.

    2014-01-01

    The Antarctic bedrock is evolving as the solid Earth responds to the past and ongoing evolution of the ice sheet. A recently improved ice loading history suggests that the Antarctic Ice Sheet (AIS) has generally been losing its mass since the Last Glacial Maximum. In a sustained warming climate, the AIS is predicted to retreat at a greater pace, primarily via melting beneath the ice shelves.We employ the glacial isostatic adjustment (GIA) capability of the Ice Sheet System Model (ISSM) to combine these past and future ice loadings and provide the new solid Earth computations for the AIS.We find that past loading is relatively less important than future loading for the evolution of the future bed topography. Our computations predict that the West Antarctic Ice Sheet (WAIS) may uplift by a few meters and a few tens of meters at years AD 2100 and 2500, respectively, and that the East Antarctic Ice Sheet is likely to remain unchanged or subside minimally except around the Amery Ice Shelf. The Amundsen Sea Sector in particular is predicted to rise at the greatest rate; one hundred years of ice evolution in this region, for example, predicts that the coastline of Pine Island Bay will approach roughly 45mmyr-1 in viscoelastic vertical motion. Of particular importance, we systematically demonstrate that the effect of a pervasive and large GIA uplift in the WAIS is generally associated with the flattening of reverse bed slope, reduction of local sea depth, and thus the extension of grounding line (GL) towards the continental shelf. Using the 3-D higher-order ice flow capability of ISSM, such a migration of GL is shown to inhibit the ice flow. This negative feedback between the ice sheet and the solid Earth may promote stability in marine portions of the ice sheet in the future.

  18. Northern Cordilleran Ice Sheet Dynamics in Coastal Alaska from MIS 3 to the Present: Initial Results

    NASA Astrophysics Data System (ADS)

    Penkrot, M. L.; Jaeger, J. M.; LeVay, L.; St-Onge, G.; Mix, A. C.; Bahlburg, H.; Davies-Walczak, M.; Gulick, S. P. S.

    2014-12-01

    Establishing the timing of northwestern Cordilleran ice sheet (NCIS) advance-retreat cycles in southern Alaska allows for investigation of global synchronicity in glacial-age climate forcing. Integrated Ocean Drilling Program Expedition 341 targeted the glacial dynamics of the NCIS in the coastal St. Elias range. Sediment cores from Site U1419 encompass times of global ice advance and retreat from MIS 3 to the present, based on a preliminary age model with 5-kya resolution developed using oxygen isotopes from benthic and planktonic foraminifera and stratigraphic correlation with a previously C-14 dated site survey core (Davies et al., 2011; doi:10.1029/2010PA002051). CT images of cores were used to identify sedimentary facies and relative ice sheet proximity. Six sedimentary facies were identified in the images; massive mud with and without lonestones, laminations with and without lonestones, massive and stratified diamict (>1 clast/cm). Elemental scanning XRF data were used to delineate possible downcore changes in sediment provenance using provenance-sensitive transition metals. Diamict and gravelly mud are the most common facies, indicative of persistent glacial input interpreted as marine-terminating glacial systems. Stratified diamicts are interpreted as periods of maximum ice extent (~18-20 ka), whereas massive mud (~14 ka-present) suggests terminus retreat. Intervals of laminated mud with and without lonestones are interpreted as periods of reduced ice cover, with the most recent (~14.5 kya) coinciding with the Bølling Interstade of northern Europe/Greenland (Davies et al., 2011). Downcore changes in Al-normalized metal XRF counts vary along with sedimentary lithoficies, suggesting changes in sediment provenance that may be related to the quantity of glacigenic sediment delivery to this location.

  19. Sonification of cryoconite landscapes over the Greenland ice sheet

    NASA Astrophysics Data System (ADS)

    Tedesco, M.

    2015-12-01

    Sonification is the use of non-speech audio to convey information. In sonification, several elements can be altered, modified or manipulated to change the perception of the sound, and in turn, the perception of the information being transmitted. For example, an increase or decrease in pitch, tempo and amplitude can be used to convey the information but this can also happen by varying other less commonly used components. One of the advantages of using sonification lies in the temporal, spatial, amplitude, and frequency resolution that offer complementary and supplementary possibilities with respect to visualization techniques. Two years ago, the outcomes of the PolarSEEDS project (www.polaseeds.org), consisting of sonification of time series of albedo, melting and surface temperature over the Greenland ice sheet, were presented in this very same session. The work that I will discuss in this presentation builds on the PolarSEEDS experience, focusing on the fascinating microcosm of cryoconite. Cryoconite is a unique and extremely fascinating form of glacial cover consisting of aggregated rock dust, inorganic and detrital organic matter, and active microbial colonies. It can be seen as 'living stones', with this ecosystem containing the only form of life that is sustained on the majestic surface of the Greenland ice sheet. Microbes are, indeed, the catalyst for cryoconite formation and growth. The cryoconite constituents radiate metabolic heat promoting glacier hole development, melt water formation, and decreasing glacier surface albedo. Lower albedos cause a positive feedback that further contributes to glacier ablation. Despite their importance, cryoconite systems are poorly studied and little is known about their evolution. In the talk, I will first present and discuss previous sonification projects whose main focus was on the polar regions; then, I will present new sonifications based on data quantifying the distribution and evolution of cryoconite over the west

  20. Annual accumulation over the Greenland ice sheet interpolated from historical and newly compiled observation data

    USGS Publications Warehouse

    Shen, Dayong; Liu, Yuling; Huang, Shengli

    2012-01-01

    The estimation of ice/snow accumulation is of great significance in quantifying the mass balance of ice sheets and variation in water resources. Improving the accuracy and reducing uncertainty has been a challenge for the estimation of annual accumulation over the Greenland ice sheet. In this study, we kriged and analyzed the spatial pattern of accumulation based on an observation data series including 315 points used in a recent research, plus 101 ice cores and snow pits and newly compiled 23 coastal weather station data. The estimated annual accumulation over the Greenland ice sheet is 31.2 g cm−2 yr−1, with a standard error of 0.9 g cm−2 yr−1. The main differences between the improved map developed in this study and the recently published accumulation maps are in the coastal areas, especially southeast and southwest regions. The analysis of accumulations versus elevation reveals the distribution patterns of accumulation over the Greenland ice sheet.

  1. 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

  2. Subglacial volcanic features beneath the West Antarctic Ice Sheet interpreted from aeromagnetic and radar ice sounding

    USGS Publications Warehouse

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

    2002-01-01

    The West Antarctic Ice Sheet (WAIS) flows through the volcanically active, late Cenozoic West Antarctic rift system. Active subglacial volcanism and a vast (>106 km3) extent of subglacial volcanic structures have been interpreted from aerogeophysical surveys over central West Antarctica in the past decade, combined with results from 1960s and 1970s aeromagnetic profiles over the WAIS. Modelling of magnetic anomalies constrained by radar ice sounding shows volcanic sources at the base of the ice throughout large areas, whose subglacially erupted hyaloclastite edifices have been eroded by moving ice, as in Iceland. The 1800 m-high divide of the WAIS is underlain by the 400 km-long volcanic Sinuous Ridge, which rises above sea level; most hyaloclastite edifices there have also been glacially removed, indicating migration of the ice divide through time. Northeast of the divide of the WAIS there is a 400-nT positive magnetic anomaly over the shallowest, most rugged bedrock topography (elevation +380 m above sea level), probably comprising subaerially erupted flows erupted when the Sinuous Ridge area was deglaciated. Uplift of the Sinuous Ridge may have forced the advance of the WAIS. Other aspects of the subglacial volcanism in Antarctica can be observed in Iceland and have a direct bearing on our understanding of the subglacial conditions of the WAIS and its dynamics.

  3. Late Wisconsinan ice sheet flow across northern and central Vermont, USA

    NASA Astrophysics Data System (ADS)

    Wright, Stephen F.

    2015-12-01

    A compilation of over 2000 glacial striation azimuths across northern and central Vermont, northeastern USA, provides the basis for interpreting a sequence of ice flow directions across this area. The oldest striations indicate widespread ice flow to the southeast, obliquely across the mountains. Similarly oriented striations between northern Vermont and the ice sheet's terminus in the Gulf of Maine suggest that a broad area of southeast ice flow existed at the Last Glacial Maximum. Younger striations with more southerly azimuths on both the mountain ridgelines and within adjacent valleys indicate that ice sheet flow trajectories in most areas rotated from southeast to south, parallel to the North-South alignment of the mountains, as the ice sheet thinned. This transition in ice flow direction was time transgressive from south to north with the Green Mountains eventually separating a thick south-flowing lobe of ice in the Champlain Valley from a much thinner lobe of south-flowing ice east of the mountains. While this transition was taking place yet ice was still thick enough to flow across the mountains, ice flow along a narrow ˜65 km long section of the Green Mountains shifted to the southwest such that ice was flowing into the Champlain Valley. The most likely process driving this change was a limited period of fast ice flow in the Champlain Valley, a short-lived ice streaming event, that drew down the ice surface in the valley. The advancing ice front during this period of fast ice flow may be responsible for the Luzerne Readvance south of Glens Falls, New York. Valley-parallel striations across the area indicate strong topographic control on ice flow as the ice sheet thinned.

  4. Numerical Ice-Sheet Modeling of the Long-Term Development of Prydz Bay, Antarctica: Tectonic Controls on Ice-Sheet Dynamics?

    NASA Astrophysics Data System (ADS)

    Taylor, J.; Hambrey, M. J.; Siegert, M. J.; Payne, A. J.

    2002-12-01

    A large quantity of geological data are now available from both offshore and onshore Prydz Bay and the Lambert Graben, East Antarctica, covering the growth and change of the East Antarctic Ice Sheet (EAIS) since early Oligocene time. We have collated much of this information, in order to constrain the rates of deposition of ice-sheet erosional products in this important sector of the EAIS, together with changes in the limits and styles of glaciation. Sedimentological data and indications of past climate from geological archives therefore formed the basis for constructing time-slice snapshots of possible morphological and climatic settings throughout the past 30-35 Ma. All of these data have then been used to constrain, or been tested by, a three-dimensional numerical ice-sheet model, which incorporates grounding-line physics. The primary concern was to assess likely ice-sheet configurations which can be forced to match the geological data, in particular, examining the possible causes of the onset of ice-stream formation in Prydz Bay after the late Miocene epoch. We suggest that tectonically induced changes in the bathymetry of the Lambert Graben and Prydz Bay are one of the major likely causes of changes in ice-sheet dynamics, and thus ice-sheet extent, in this sector of the EAIS. The results of the numerical ice-sheet modeling show clearly that tectonically induced bathymetric changes are sufficient to alter the glacial environment in this region significantly, in particular by controling the grounding and stability of ice within the Lambert Graben and by focusing ice flow from the surrounding area. The history of positive topographic features such as the bounding Prince Charles Mountains are probably not that significant in controlling ice flow, however. Glacial erosion may also have played a role in excavating the Lambert Graben by promoting fast-flowing ice in a positive feedback. We have also assessed possible changes in mass balance regime (climate) and find

  5. Modelling Greenland ice sheet inception and sustainability during the Late Pliocene

    NASA Astrophysics Data System (ADS)

    Contoux, C.; Dumas, C.; Ramstein, G.; Jost, A.; Dolan, A. M.

    2015-08-01

    Understanding the evolution and dynamics of ice sheet growth during past warm periods is a very important topic considering the potential total removal of the Greenland ice sheet. In this regard, one key event is the full glaciation of Greenland that occurred at the end of the Pliocene warm period, which remains partially unexplained. Previous modelling studies succeeded in reproducing this full glaciation either by imposing an unrealistically low CO2 value or by imposing a partial ice sheet over the surface of Greenland. Although they highlight some fundamental mechanisms, none of these studies are fully satisfactory because they do not reflect realistic conditions occurring during the Late Pliocene. Through a series of simulations with the IPSL-CM5A coupled climate model used to force the GRISLI ice sheet model, we show that a drop in CO2 levels does not lead to an abrupt inception of the Greenland ice sheet. High ablation rates in central and northern Greenland combined with low accumulation prevent such an abrupt inception. Ice sheet inception occurs when low summer insolation and CO2 levels below modern values are combined, the Greenland ice sheet being restricted to the southeast region, where high topography favours this build-up. This ice sheet experiences only partial melting during summer insolation maxima combined with high CO2 levels. Further growth of the ice sheet with recoupling experiments is important at 360 and 280 ppm during insolation minima. Thus, the full glaciation at 2.6 Ma could be the result of a cumulative build-up of the Greenland ice sheet over several orbital cycles, leading to progressively more intense glaciations during low summer insolation periods. Although this result could be a shortcoming of the modelling framework itself, the gradual glacial inception interpreted from the oxygen isotope record could support our scenario.

  6. Hydrologic controls on coastal suspended sediment plumes around the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Chu, V. W.; Smith, L. C.; Rennermalm, A. K.; Forster, R. R.; Box, J. E.

    2011-12-01

    Increasing surface melting on the Greenland ice sheet and rising sea level have furthered the need for direct observations of meltwater release from the ice sheet to ocean. Buoyant sediment plumes develop in fjords downstream of outlet glaciers and are controlled by a variety of complex factors, including ice sheet meltwater runoff and fluvial processes. This study classifies average plume suspended sediment concentrations (SSC) around the Greenland ice sheet derived from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery and field data over the period 2000-2009. Spatial and temporal variations in SSC are compared with ice sheet positive-degree-days (PDD), a proxy for ice sheet surface melting, as modeled from the Polar MM5 regional climate model, and outlet glacier environment, as represented by land- or marine-terminating glaciers. Buoyant plume SSCs are successfully retrieved around most of Greenland. Higher ice sheet melting around Greenland produces higher SSCs in surrounding coastal waters. The southwest region, dominated by land-terminating glaciers, experiences highest PDDs and produces plumes with highest SSCs, which typically persist late into the meltwater runoff season. Interannual variations in ice sheet PDD and plume SSC are not coupled as previously demonstrated in Kangerlussuaq Fjord (Chu et al. 2009), suggesting plume dimensions better capture interannual runoff dynamics than SSC. This first exploration of coastal sediment plumes around Greenland demonstrates that while complex factors influence their development and detection, ice sheet hydrology is a dominant control on plume distribution. Satellite remote sensing thus offers a unique methodology for detecting meltwater release from the ice sheet to global ocean.

  7. Ross Sea Till Properties: Implications for Ice Sheet Bed Interaction

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    PubMed Central

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

    2012-01-01

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

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

    PubMed

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

    2012-10-01

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

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

    PubMed

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

    2012-10-01

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

  11. Thermal insights of the Greenland ice sheet perennial firn aquifer

    NASA Astrophysics Data System (ADS)

    Forster, R. R.; Miège, C.; Koenig, L.; Brucker, L.

    2013-12-01

    The Greenland ice sheet hydrology is characterized by a complex system and is triggered essentially by surface melt starting late spring to early summer each year. Understanding the hydrologic system for the ice sheet remains important to address ice dynamics and surface mass balance questions. In April 2011, in Southeast Greenland, field work was conducted and firn-core drilling identified the presence of liquid water persisting through the winter without freezing. This observed feature is named perennial firn aquifer (PFA) and can be mapped by the Accumulation Radar on board of the NASA Operation IceBridge mission. Even if the extent of this feature can be constrained by remote sensing techniques its formation and persistence mechanism remain unclear. Thermal behavior of the PFA is a key parameter to monitor in order to understand melting and refreezing processes at the PFA location. The PFA-13 site (66.18°N, 39.04°W and 1563 m), located near the 2011 site where the PFA was first identified, was revisited in early April 2013 for further investigations of the aquifer. To characterize the PFA thermal regime and seasonal evolution, we installed two thermistor strings . They are used to record the vertical temperature evolution for a year, from the surface to the bottom of the PFA and below. The data are being uploaded daily via satellite link. Progressive heating of the firn pack is observed from June 15th to the end of July 2013, by then, the entire firn column from the surface to 12 m depth (top of PFA) is at 0 °C. This observation brings evidence that meltwater can reach the depth of 12 m in the firn, during one and a half months. By the end of the summer, refreezing is expected from near the surface and the cold surface temperatures will slowly penetrate into the firn. In addition, freshly fallen snow, usually > 2 m over the course of a winter, will help insulate the remaining liquid water within the firn from the surface. We will present the time series of

  12. Improving Climate Literacy Using The Ice Sheet System Model (ISSM): A Prototype Virtual Ice Sheet Laboratory For Use In K-12 Classrooms

    NASA Astrophysics Data System (ADS)

    Halkides, D. J.; Larour, E. Y.; Perez, G.; Petrie, K.; Nguyen, L.

    2013-12-01

    Statistics indicate that most Americans learn what they will know about science within the confines of our public K-12 education system and the media. Next Generation Science Standards (NGSS) aim to remedy science illiteracy and provide guidelines to exceed the Common Core State Standards that most U.S. state governments have adopted, by integrating disciplinary cores with crosscutting ideas and real life practices. In this vein, we present a prototype ';Virtual Ice Sheet Laboratory' (I-Lab), geared to K-12 students, educators and interested members of the general public. I-Lab will allow users to perform experiments using a state-of-the-art dynamical ice sheet model and provide detailed downloadable lesson plans, which incorporate this model and are consistent with NGSS Physical Science criteria for different grade bands (K-2, 3-5, 6-8, and 9-12). The ultimate goal of this website is to improve public climate science literacy, especially in regards to the crucial role of the polar ice sheets in Earth's climate and sea level. The model used will be the Ice Sheet System Model (ISSM), an ice flow model developed at NASA's Jet Propulsion Laboratory and UC Irvine, that simulates the near-term evolution of polar ice sheets (Greenland and Antarctica) and includes high spatial resolution capabilities and data assimilation to produce realistic simulations of ice sheet dynamics at the continental scale. Open sourced since 2011, ISSM is used in cutting edge cryosphere research around the globe. Thru I-Lab, students will be able to access ISSM using a simple, online graphical interface that can be launched from a web browser on a computer, tablet or smart phone. The interface will allow users to select different climate conditions and watch how the polar ice sheets evolve in time under those conditions. Lesson contents will include links to background material and activities that teach observation recording, concept articulation, hypothesis formulation and testing, and

  13. Past ice-sheet behaviour: retreat scenarios and changing controls in the Ross Sea, Antarctica

    NASA Astrophysics Data System (ADS)

    Halberstadt, Anna Ruth W.; Simkins, Lauren M.; Greenwood, Sarah L.; Anderson, John B.

    2016-05-01

    Studying the history of ice-sheet behaviour in the Ross Sea, Antarctica's largest drainage basin can improve our understanding of patterns and controls on marine-based ice-sheet dynamics and provide constraints for numerical ice-sheet models. Newly collected high-resolution multibeam bathymetry data, combined with two decades of legacy multibeam and seismic data, are used to map glacial landforms and reconstruct palaeo ice-sheet drainage. During the Last Glacial Maximum, grounded ice reached the continental shelf edge in the eastern but not western Ross Sea. Recessional geomorphic features in the western Ross Sea indicate virtually continuous back-stepping of the ice-sheet grounding line. In the eastern Ross Sea, well-preserved linear features and a lack of small-scale recessional landforms signify rapid lift-off of grounded ice from the bed. Physiography exerted a first-order control on regional ice behaviour, while sea floor geology played an important subsidiary role. Previously published deglacial scenarios for Ross Sea are based on low-spatial-resolution marine data or terrestrial observations; however, this study uses high-resolution basin-wide geomorphology to constrain grounding-line retreat on the continental shelf. Our analysis of retreat patterns suggests that (1) retreat from the western Ross Sea was complex due to strong physiographic controls on ice-sheet drainage; (2) retreat was asynchronous across the Ross Sea and between troughs; (3) the eastern Ross Sea largely deglaciated prior to the western Ross Sea following the formation of a large grounding-line embayment over Whales Deep; and (4) our glacial geomorphic reconstruction converges with recent numerical models that call for significant and complex East Antarctic ice sheet and West Antarctic ice sheet contributions to the ice flow in the Ross Sea.

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

    PubMed Central

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

    2016-01-01

    The Greenland ice sheet presently accounts for ~70% of global ice sheet mass loss. Because this mass loss is associated with sea-level rise at a rate of 0.7 mm/year, the development of improved monitoring techniques to observe ongoing changes in ice sheet mass balance is of paramount concern. Spaceborne mass balance techniques are commonly used; however, they are inadequate for many purposes because of their low spatial and/or temporal resolution. We demonstrate that small variations in seismic wave speed in Earth’s crust, as measured with the correlation of seismic noise, may be used to infer seasonal ice sheet mass balance. Seasonal loading and unloading of glacial mass induces strain in the crust, and these strains then result in seismic velocity changes due to poroelastic processes. Our method provides a new and independent way of monitoring (in near real time) ice sheet mass balance, yielding new constraints on ice sheet evolution and its contribution to global sea-level changes. An increased number of seismic stations in the vicinity of ice sheets will enhance our ability to create detailed space-time records of ice mass variations. PMID:27386524

  15. Airborne Laser Altimetry Mapping of the Greenland Ice Sheet: Application to Mass Balance Assessment

    NASA Technical Reports Server (NTRS)

    Abdalati, W.; Krabill, W.; Frederick, E.; Manizade, S.; Martin, C.; Sonntag, J.; Swift, R.; Thomas, R.; Wright, W.; Yungel, J.

    2000-01-01

    In 1998 and '99, the Arctic Ice Mapping (AIM) program completed resurveys of lines occupied 5 years earlier revealing elevation changes of the Greenland ice sheet and identifying areas of significant thinning, thickening and balance. In planning these surveys, consideration had to be given to the spatial constraints associated with aircraft operation, the spatial nature of ice sheet behavior, and limited resources, as well as temporal issues, such as seasonal and interannual variability in the context of measurement accuracy. This paper examines the extent to which the sampling and survey strategy is valid for drawing conclusions on the current state of balance of the Greenland ice sheet. The surveys covered the entire ice sheet with an average distance of 21.4 km between each location on the ice sheet and the nearest flight line. For most of the ice sheet, the elevation changes show relatively little spatial variability, and their magnitudes are significantly smaller than the observed elevation change signal. As a result, we conclude that the density of the sampling and the accuracy of the measurements are sufficient to draw meaningful conclusions on the state of balance of the entire ice sheet over the five-year survey period. Outlet glaciers, however, show far more spatial and temporal variability, and each of the major ones is likely to require individual surveys in order to determine its balance.

  16. The Gamburtsev mountains and the origin and early evolution of the Antarctic Ice Sheet.

    PubMed

    Bo, Sun; Siegert, Martin J; Mudd, Simon M; Sugden, David; Fujita, Shuji; Xiangbin, Cui; Yunyun, Jiang; Xueyuan, Tang; Yuansheng, Li

    2009-06-01

    Ice-sheet development in Antarctica was a result of significant and rapid global climate change about 34 million years ago. Ice-sheet and climate modelling suggest reductions in atmospheric carbon dioxide (less than three times the pre-industrial level of 280 parts per million by volume) that, in conjunction with the development of the Antarctic Circumpolar Current, led to cooling and glaciation paced by changes in Earth's orbit. Based on the present subglacial topography, numerical models point to ice-sheet genesis on mountain massifs of Antarctica, including the Gamburtsev mountains at Dome A, the centre of the present ice sheet. Our lack of knowledge of the present-day topography of the Gamburtsev mountains means, however, that the nature of early glaciation and subsequent development of a continental-sized ice sheet are uncertain. Here we present radar information about the base of the ice at Dome A, revealing classic Alpine topography with pre-existing river valleys overdeepened by valley glaciers formed when the mean summer surface temperature was around 3 degrees C. This landscape is likely to have developed during the initial phases of Antarctic glaciation. According to Antarctic climate history (estimated from offshore sediment records) the Gamburtsev mountains are probably older than 34 million years and were the main centre for ice-sheet growth. Moreover, the landscape has most probably been preserved beneath the present ice sheet for around 14 million years.

  17. Greenland Ice Sheet Melt from MODIS and Associated Atmospheric Variability

    NASA Technical Reports Server (NTRS)

    Hakkinen, Sirpa; Hall, Dorothy K.; Shuman, Christopher A.; Worthen, Denise L.; DiGirolamo, Nicolo E.

    2014-01-01

    Daily June-July melt fraction variations over the Greenland Ice Sheet (GIS) derived from the MODerate-resolution Imaging Spectroradiometer (MODIS) (2000-2013) are associated with atmospheric blocking forming an omega-shape ridge over the GIS at 500hPa height (from NCEPNCAR). Blocking activity with a range of time scales, from synoptic waves breaking poleward ( 5 days) to full-fledged blocks (5 days), brings warm subtropical air masses over the GIS controlling daily surface temperatures and melt. The temperature anomaly of these subtropical air mass intrusions is also important for melting. Based on the largest MODIS melt years (2002 and 2012), the area-average temperature anomaly of 2 standard deviations above the 14-year June-July mean, results in a melt fraction of 40 or more. Summer 2007 had the most blocking days, however atmospheric temperature anomalies were too small to instigate extreme melting.

  18. Long-term future contribution of the Greenland ice sheet to sea level rise

    NASA Astrophysics Data System (ADS)

    Calov, Reinhard; Robinson, Alex; Ganopolski, Andrey

    2015-04-01

    We investigate the impact of future cumulative anthropogenic emissions on the fate of the Greenland ice sheet. For this study, we use the polythermal ice sheet model SICOPOLIS, which is bi-directionally coupled with the regional climate model of intermediate complexity REMBO. We constrain our model parameters with simulations over two glacial cycles employing anomalies from the global CLIMBER-2 model. CLIMBER-2 treats the major components or the Earth system, including atmosphere, ocean, terrestrial vegetation and carbon cycle. As constraints we include the cumulative error in ice thickness, the surface mass balance partition (ratio between precipitation and ice discharge) and the ice elevation drop between Eemian and present-day at the NEEM ice core location. Our model includes a new ice discharge parameterization, which describes the ice loss via small-scale outlet glaciers in a heuristic statistical approach. Using the large-ensemble of model versions consistent with our constraints, we estimate the range of the long-term future contribution of the Greenland ice sheet to sea-level rise under global warming. On the 100,000-year time scale, there is a visible modulation over the CO2 signal in the simulated Greenland ice volume caused by the 20,000 years precessional cycle of insolation. In nearly all of our scenarios (500 to 5000 Gt carbon cumulative emissions), the Greenland is sheet fully decays in the future after at least 40,000 years. For the extreme scenario (5000 Gt), the Greenland ice sheet decays much faster - after about 5000 years, while there is still 80% of the ice sheet left after 40,000 years only for the model versions with a low temperature sensitivity and the low cumulative carbon emission scenario (500 Gt). Our results underline that without future negative CO2 emissions, irreversible loss of Greenland ice sheet is essentially unavoidable.

  19. Interaction of ice sheets and climate during the past 800 000 years

    NASA Astrophysics Data System (ADS)

    Stap, L. B.; van de Wal, R. S. W.; de Boer, B.; Bintanja, R.; Lourens, L. J.

    2014-12-01

    During the Cenozoic, land ice and climate interacted on many different timescales. On long timescales, the effect of land ice on global climate and sea level is mainly set by large ice sheets in North America, Eurasia, Greenland and Antarctica. The climatic forcing of these ice sheets is largely determined by the meridional temperature profile resulting from radiation and greenhouse gas (GHG) forcing. As a response, the ice sheets cause an increase in albedo and surface elevation, which operates as a feedback in the climate system. To quantify the importance of these climate-land ice processes, a zonally averaged energy balance climate model is coupled to five one-dimensional ice sheet models, representing the major ice sheets. In this study, we focus on the transient simulation of the past 800 000 years, where a high-confidence CO2 record from ice core samples is used as input in combination with Milankovitch radiation changes. We obtain simulations of atmospheric temperature, ice volume and sea level that are in good agreement with recent proxy-data reconstructions. We examine long-term climate-ice-sheet interactions by a comparison of simulations with uncoupled and coupled ice sheets. We show that these interactions amplify global temperature anomalies by up to a factor of 2.6, and that they increase polar amplification by 94%. We demonstrate that, on these long timescales, the ice-albedo feedback has a larger and more global influence on the meridional atmospheric temperature profile than the surface-height-temperature feedback. Furthermore, we assess the influence of CO2 and insolation by performing runs with one or both of these variables held constant. We find that atmospheric temperature is controlled by a complex interaction of CO2 and insolation, and both variables serve as thresholds for northern hemispheric glaciation.

  20. Interaction of ice sheets and climate during the past 800 000 years

    NASA Astrophysics Data System (ADS)

    Stap, L. B.; van de Wal, R. S. W.; de Boer, B.; Bintanja, R.; Lourens, L. J.

    2014-06-01

    During the Cenozoic, land ice and climate have interacted on many different time scales. On long time scales, the effect of land ice on global climate and sea level is mainly set by large ice sheets on North America, Eurasia, Greenland and Antarctica. The climatic forcing of these ice sheets is largely determined by the meridional temperature profile resulting from radiation and greenhouse gas (GHG) forcing. As response, the ice sheets cause an increase in albedo and surface elevation, which operates as a feedback in the climate system. To quantify the importance of these climate-land ice processes, a zonally-averaged energy balance climate model is coupled to five one-dimensional ice-sheet models, representing the major ice sheets. In this study, we focus on the transient simulation of the past 800 000 years, where a high-confidence CO2-record from ice cores samples is used as input in combination with Milankovitch radiation changes. We obtain simulations of atmospheric temperature, ice volume and sea level, that are in good agreement with recent proxy-data reconstructions. We examine long-term climate-ice sheet interactions by a comparison of simulations with uncoupled and coupled ice sheets. We show that these interactions amplify global temperature anomalies by up to a factor 2.6, and that they increase polar amplification by 94%. We demonstrate that, on these long time scales, the ice-albedo feedback has a larger and more global influence on the meridional atmospheric temperature profile than the surface-height temperature feedback. Furthermore, we assess the influence of CO2 and insolation, by performing runs with one or both of these variables held constant. We find that atmospheric temperature is controlled by a complex interaction of CO2 and insolation, and both variables serve as thresholds for Northern Hemispheric glaciation.

  1. Large-scale interaction between ice sheets and climate during the past 800,000 years

    NASA Astrophysics Data System (ADS)

    Stap, Lennert; Van De Wal, Roderik; De Boer, Bas; Bintanja, Richard; Lourens, Lucas

    2014-05-01

    During the Cenozoic, land ice and climate have interacted on many different time scales. On long time scales, the effect of land ice on global climate and sea level is dictated by large ice sheets on North America, Eurasia, Greenland and Antarctica. The climatic forcing of ice sheets is largely determined by the meridional temperature profile. In their turn, the ice sheets cause an increase in albedo and surface elevation which affects the climate system. To quantify the importance of these climate-land ice processes, a zonally-averaged energy balance climate model is coupled to a one-dimensional ice-sheet model of the four major ice sheets. The benefit of using relatively simple models is that the tested model parameters are easily interpretable. Moreover, the shorter computation time allows for more tests and long transient simulations at geological time scales to be performed. This study focusses on the past 800,000 years, where a high-confidence CO2-record from ice-core samples is used as input. Simulations of atmospheric temperature, ice volume and sea level are obtained, that are in good agreement with recent proxy-data reconstructions (RMSE=20 m over the last glacial cycle). The climate-ice sheet interaction is studied by a comparison of simulations with uncoupled and coupled ice sheets. By performing runs with CO2 or insolation held constant, the influence of these variables is assessed. It is found that atmospheric temperature is controlled by a complex interaction of CO2 and insolation. Finally, we show that the amplification of the climate sensitivity from the long-term ice feedback is a factor 3.

  2. 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

  3. Collapse of the West Antarctic Ice Sheet after local destabilization of the Amundsen Basin.

    PubMed

    Feldmann, Johannes; Levermann, Anders

    2015-11-17

    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

  4. Comparison of ice-sheet satellite altimeter retracking algorithm

    SciTech Connect

    Davis, C.H.

    1996-01-01

    The NASA and ESA retracking algorithms are compared with an algorithm based upon a combined surface and volume (S/V) scattering model. First, the S/V, NASA, and ESA algorithms were used to retrack over 1.3 million altimeter return waveforms from the Greenland and Antarctic ice sheets. The surface elevations from the S/V algorithm were compared with the elevations produced by the NASA and ESA algorithms to determine the relative accuracy of these algorithms when subsurface volume scattering occurs. The results show that the ESA{sub 25%} algorithm produced slightly higher surface elevations than the S/V algorithm. The NASA retracking algorithm produced lower surface elevations than the S/V retracking algorithm, with average differences ranging from {minus}0.3 to {minus}0.9 m. The lower NASA elevations can only account for a portion of previously reported differences between altimeter and geoceiver surface elevations, suggesting that the remainder is probably due to orbital differences. Next, by analyzing several thousand satellite crossover points from the Greenland and Antarctic ice sheets, the author estimated the repeatability of the surface elevations derived from the different retracking algorithms. The elevations derived from the ESA{sub 25%} and S/V algorithm had the smallest standard deviations for the crossover differences for a time period where no significant change in surface elevation should occur. The NASA standard deviations were approximately 0.2 m larger than those from the ESA{sub 25%} and S/V algorithm, which represents an average increase in error of approximately 0.5 m in the datasets.

  5. Using an Earth System Model to Better Understand Ice Sheet Variability Through the Pleistocene

    NASA Astrophysics Data System (ADS)

    Tabor, C. R.; Poulsen, C. J.; Pollard, D.

    2015-12-01

    We use an Earth System model with a dynamic land-ice component to explore several inconsistencies between traditional Milankovitch theory and δ18O sediment records of the Pleistocene. Our model results show that a combination of albedo feedbacks, seasonal offset of precession forcing, and orbital cycle duration differences can explain much of the 41-kyr glacial cycles that characterize the early Pleistocene. The obliquity-controlled changes in annual average high-latitude insolation produce large variations in arctic vegetation-type and sea-ice cover, which amplify the land-ice response. In contrast, the seasonal nature of the precession insolation signal dampens net ice-melt. For instance, when precession enhances ice melt in the spring, it reduces ice melt in the fall, and vice versa. The lower frequency of obliquity cycles in combination with amplified climate sensitivity due to albedo feedbacks help produce a larger ice-volume response to cycles of obliquity compared to precession, despite precession contributing more to variations in high-latitude summer insolation. In addition, we can simulate the appearance of a 100-kyr ice-volume signal by reducing basal sliding in the ice sheet model. Model experiments with enhanced basal drag have greater ice sheet elevation because the ice sheets are not able to flow as quickly, leading to increased ice thickness at the expense of ice extent. These thicker ice sheets have colder surface temperatures, receive more snowfall, and do not readily advance past the ice equilibrium line. Greater high-latitude summer insolation from the combination of high obliquity and precession/eccentricity is then necessary to cause complete ice sheet retreat. This research lends support to the regolith hypothesis, which proposes gradual erosion of high-latitude northern hemisphere regolith by multiple cycles of glaciation helped cause the mid-Pleistocene transition.

  6. Pre-LGM ice dynamics of the Greenland Ice Sheet in Uummannaq Fjord West Greenland, revealed by blockfields, tors and till mantled surfaces

    NASA Astrophysics Data System (ADS)

    Rea, Brice; Roberts, David; Lane, Tim; Rhodés, Angel; Schnabel, Christoph

    2013-04-01

    The future response of the Greenland Ice Sheet has been the focus of much recent modelling work but in order to fully understand the dynamics of this ice mass it is also imperative that the past behaviour of the ice sheet is understood. Indeed it is only through successful hindcasts of past ice geometries and dynamics that confidence in predictions can be achieved. In most glaciated environments determining ice dynamics prior to the Last Glacial Maximum (LGM) and subsequent deglaciation are non-trivial. They rely on fortuitous preservation, more restricted ice cover at LGM than previously or protective cold based ice cover. Here results are presented from hypsometric surfaces in the Uummannaq Fjord region of West Greenland which can provide constrains on the dynamics of ice cover prior to and including the LGM. Uummanaq Fjord is a classic landscape of selective linear erosion containing deeply incised troughs juxtaposed with high elevation plateau where relief approaches 3 km in places. Excavations were made in a number of summit blockfields with samples collected. Results from a morphometric landscape analyses a presented, using both landscape hypsometry and an elevation-range slope-map approach to identify hypsometric surfaces. The hypsometric surfaces are divided into those which are now denuded and classified as regions of areal scour, those with a blockfield cover and areas which are still ice covered. A number of sites have been visited and excavations were made into blockfields. Data are presented indicating minimum depths, granulometry and mineralogy of blockfields which allowed a further subdivision into allochthonous and autochotonous blockfields. Samples were also collected for cosmogenic nuclide exposure analyses and indicate that blockfield boulders and tors exhibit ages extending significantly beyond the LGM. Based on equilibrium profile reconstructions of the LGM ice sheet (constrained by onshore and offshore geomorphology and cosmogenic and 14C ages

  7. Kilometer-thick ice-sheets in the northern mid-latitudes of Mars in the Amazonian: Analogs from the East Antarctic Ice Sheet and the Dry Valleys

    NASA Astrophysics Data System (ADS)

    Head, James W.; Marchant, David R.

    2008-09-01

    Introduction. The strong geomorphic similarities between lobate deposits on the northwest flank of the Tharsis Montes [1,2, 12-13] and along the dichotomy boundary between 30° and 50° [3] with terrestrial cold-based glaciers and glacial deposits has led to new hypotheses for geologically recent (Amazonian-age) low and mid-latitude glaciation on Mars [1,4]. A common theme among many of these studies has been to identify individual landscape elements on Mars and match them with terrestrial counterparts from cold polar deserts on Earth [5,6]. Here, we use the documented long-term history of outlet and alpine glaciers along the East Antarctic Ice Sheet in the Dry Valleys of Antarctica as a suitable analog for glaciation along the martian dichotomy boundary. As a guiding principle we note that, just as for terrestrial glacial landsystems, the most recent ice-related deposit/feature along the dichotomy boundary on Mars need not reflect the maximum in ice volume and/or ice configuration. The Antarctic Dry Valleys (ADV). A terrestrial analog for cold-based glaciation across steppedbedrock topography. To a first order, the large-scale bedrock geomorphology of the Antarctic Dry Valleys (Transantarctic mountain rift-margin upwarp) approximates the martian dichotomy boundary: the valleys occur within, and dissect, a series of broad, coast-facing escarpments (total relief of up to 3000 m) separated by isolated inselbergs. In the middle Miocene, sometime between 14.8 and 12.5 Ma [7, 8], all but the highest mountains in the Dry Valleys were overrun by a major expansion of East Antarctic ice. During this time, ice spilled across bedrock escarpments and flowed out across low-lying valleys toward the continental shelf. A modern-day counterpart for the maximum-overriding stage is seen inland of the ADV, where glacier ice still overrides stepped bedrock topography (Fig. 1). Ice expansion was triggered when the Antarctic cryosphere transitioned from relatively warm and wet (fostering

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

    NASA Astrophysics Data System (ADS)

    Dutton, A.

    2015-12-01

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

  9. Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets.

    PubMed

    Pritchard, Hamish D; Arthern, Robert J; Vaughan, David G; Edwards, Laura A

    2009-10-15

    Many glaciers along the margins of the Greenland and Antarctic ice sheets are accelerating and, for this reason, contribute increasingly to global sea-level rise. Globally, ice losses contribute approximately 1.8 mm yr(-1) (ref. 8), but this could increase if the retreat of ice shelves and tidewater glaciers further enhances the loss of grounded ice or initiates the large-scale collapse of vulnerable parts of the ice sheets. Ice loss as a result of accelerated flow, known as dynamic thinning, is so poorly understood that its potential contribution to sea level over the twenty-first century remains unpredictable. Thinning on the ice-sheet scale has been monitored by using repeat satellite altimetry observations to track small changes in surface elevation, but previous sensors could not resolve most fast-flowing coastal glaciers. Here we report the use of high-resolution ICESat (Ice, Cloud and land Elevation Satellite) laser altimetry to map change along the entire grounded margins of the Greenland and Antarctic ice sheets. To isolate the dynamic signal, we compare rates of elevation change from both fast-flowing and slow-flowing ice with those expected from surface mass-balance fluctuations. We find that dynamic thinning of glaciers now reaches all latitudes in Greenland, has intensified on key Antarctic grounding lines, has endured for decades after ice-shelf collapse, penetrates far into the interior of each ice sheet and is spreading as ice shelves thin by ocean-driven melt. In Greenland, glaciers flowing faster than 100 m yr(-1) thinned at an average rate of 0.84 m yr(-1), and in the Amundsen Sea embayment of Antarctica, thinning exceeded 9.0 m yr(-1) for some glaciers. Our results show that the most profound changes in the ice sheets currently result from glacier dynamics at ocean margins.

  10. Airborne radar evidence for tributary flow switching in Institute Ice Stream, West Antarctica: Implications for ice sheet configuration and dynamics

    NASA Astrophysics Data System (ADS)

    Winter, Kate; Woodward, John; Ross, Neil; Dunning, Stuart A.; Bingham, Robert G.; Corr, Hugh F. J.; Siegert, Martin J.

    2015-09-01

    Despite the importance of ice streaming to the evaluation of West Antarctic Ice Sheet (WAIS) stability we know little about mid- to long-term dynamic changes within the Institute Ice Stream (IIS) catchment. Here we use airborne radio echo sounding to investigate the subglacial topography, internal stratigraphy, and Holocene flow regime of the upper IIS catchment near the Ellsworth Mountains. Internal layer buckling within three discrete, topographically confined tributaries, through Ellsworth, Independence, and Horseshoe Valley Troughs, provides evidence for former enhanced ice sheet flow. We suggest that enhanced ice flow through Independence and Ellsworth Troughs, during the mid-Holocene to late Holocene, was the source of ice streaming over the region now occupied by the slow-flowing Bungenstock Ice Rise. Although buckled layers also exist within the slow-flowing ice of Horseshoe Valley Trough, a thicker sequence of surface-conformable layers in the upper ice column suggests slowdown more than ~4000 years ago, so we do not attribute enhanced flow switch-off here, to the late Holocene ice-flow reorganization. Intensely buckled englacial layers within Horseshoe Valley and Independence Troughs cannot be accounted for under present-day flow speeds. The dynamic nature of ice flow in IIS and its tributaries suggests that recent ice stream switching and mass changes in the Siple Coast and Amundsen Sea sectors are not unique to these sectors, that they may have been regular during the Holocene and may characterize the decline of the WAIS.

  11. The Little Ice Age and Solar Activity

    NASA Astrophysics Data System (ADS)

    Velasco Herrera, Victor Manuel; Leal Silva, C. M. Carmen; Velasco Herrera, Graciela

    We analyze the ice winter severity index on the Baltic region since 1501-1995. We found that the variability of this index is modulated among other factors by the secular solar activity. The little ice ages that have appeared in the North Hemisphere occurred during periods of low solar activity. Seemingly our star is experiencing a new quiet stage compared with Maunder or Dalton minimum, this is important because it is estimated that even small changes in weather can represent a great impact in ice index. These results are relevant since ice is a very important element in the climate system of the Baltic region and it can affect directly or indirectly many of the oceanographic, climatic, eco-logical, economical and cultural patterns.

  12. Laurentide Ice Sheet dynamics in the Bay of Fundy, Canada, revealed through multibeam sonar mapping of glacial landsystems

    NASA Astrophysics Data System (ADS)

    Todd, Brian J.; Shaw, John

    2012-12-01

    Recent multibeam sonar data collected in the Bay of Fundy, Canada, interpreted in conjunction with geophysical profiling and sediment sampling, reveal in unprecedented detail a suite of glacial landforms associated with the southwest margin of the Laurentide Ice Sheet. These landforms constitute four glacial landsystems. 1) Subglacial landsystem I: In southwestern Bay of Fundy, the elongated Grand Manan Basin contains ice-contact sediments of possible mid-Wisconsinan age overlain by late-Wisconsinan ice-contact sediments strongly imprinted by iceberg furrows and pits. In places, possible mid-Wisconsinan glaciomarine sediments have been eroded by late-Wisconsinan ice, creating streamlined landforms. Eroded bedrock and megafluted ice-contact sediment on the flanks of Grand Manan Basin indicate the southwest direction of topographically-steered ice. 2) Subglacial landsystem II: Along the southern margin of the Bay of Fundy, an array of drumlins, with superimposed esker complexes, was formed by glacial ice that emanated northwest from the interior of Nova Scotia and was deflected to the southwest by the ice flowing out of the Bay of Fundy to the Gulf of Maine. The esker complexes formed later when the Nova Scotia ice sheet stagnated and meltwater escaped northwest via topographic gaps. 3) Ice-marginal landsystem I: In northern Bay of Fundy, both small De Geer moraines and larger, basin-bounding moraines were created when retreating late-Wisconsinan ice became grounded in relatively shallow water. New radiocarbon ages show that the Owen Basin Moraine in this landsystem was abandoned prior to c. 14,600 14C yr BP (cal BP 17,015-17,270 [0.7], 17,286-17,405 [0.3]). 4) Ice-marginal landsystem II: This distinctive landsystem consists of numerous arcuate moraines, commonly superimposed on one another. This landsystem was formed by thin (170 m), lightly grounded ice that retreated northeast into the Bay of Fundy. The splayed pattern of the ice margin was a response to a large

  13. Boundary layer stability acts to ballast the mass of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Berkelhammer, M. B.; Noone, D. C.; Steen-Larsen, H. C.; O'Neill, M.; Raudzens Bailey, A.; Cox, C.; Schneider, D. P.

    2014-12-01

    The mass of the Greenland Ice Sheet has been reduced over recent decades as a consequence of warming, the impact of which is already detectable on global sea level. However, temperature projections suggest that at interior high-altitude sites on the ice it could be decades or more before warming forces these regions to transition from a dry to wet snow facies. Shifts in boundary layer dynamics, including atmosphere-ice sheet hydrological exchange and cloud radiative forcing could expedite or delay this transition. These processes are important with respect to future ice sheet stability, yet they remain difficult to constrain. Using continuous in situ measurements of vertical profiles of the isotopic composition of water vapor at Summit Camp, the highest observatory on the ice sheet, we document the presence of a hydrologic balance between surface sublimation and condensation fluxes. This exists because of a nearly persistent temperature inversion, which hinders the efficiency with which surface water vapor mixes into the free atmosphere. In the presence of a strong temperature inversion, fog and ice particles form near the ice-atmosphere interface from surface moisture fluxes. When this condensate precipitates on or settles to the surface, it ballasts the ice sheet's mass. A decade-long trend towards lower annual accumulation at Summit may therefore reflect continuous replacement of the near surface atmosphere due to reduced atmospheric stability. If this tendency toward destabilization continues, it could accelerate mass loss at interior sites on the ice sheet. The role of boundary layer stability in ice sheet hydrological budgets discussed here is applicable beyond the accumulation zone of the Greenland Ice Sheet.

  14. When glaciers and ice sheets melt: consequences for planktonic organisms

    PubMed Central

    SOMMARUGA, RUBEN

    2016-01-01

    The current melting of glaciers and ice sheets is a consequence of climatic change and their turbid meltwaters are filling and enlarging many new proglacial and ice-contact lakes around the world, as well as affecting coastal areas. Paradoxically, very little is known on the ecology of turbid glacier-fed aquatic ecosystems even though they are at the origin of the most common type of lakes on Earth. Here, I discuss the consequences of those meltwaters for planktonic organisms. A remarkable characteristic of aquatic ecosystems receiving the discharge of meltwaters is their high content of mineral suspensoids, so-called glacial flour that poses a real challenge for filter-feeding planktonic taxa such as Daphnia and phagotrophic groups such as heterotrophic nanoflagellates. The planktonic food-web structure in highly turbid meltwater lakes seems to be truncated and microbially dominated. Low underwater light levels leads to unfavorable conditions for primary producers, but at the same time, cause less stress by UV radiation. Meltwaters are also a source of inorganic and organic nutrients that could stimulate secondary prokaryotic production and in some cases (e.g. in distal proglacial lakes) also phytoplankton primary production. How changes in turbidity and in other related environmental factors influence diversity, community composition and adaptation have only recently begun to be studied. Knowledge of the consequences of glacier retreat for glacier-fed lakes and coasts will be crucial to predict ecosystem trajectories regarding changes in biodiversity, biogeochemical cycles and function. PMID:26869738

  15. The dynamic response of the Greenland and Antarctic ice sheets to multiple-century climatic warming

    SciTech Connect

    Huybrechts, P.; Wolde, J. de

    1999-08-01

    New calculations were performed to investigate the combined response of the Greenland and Antarctic ice sheets to a range of climatic warming scenarios over the next millennium. Use was made of fully dynamic 3D thermomechanic ice sheet models, which were coupled to a two-dimensional climate model. The experiments were initialized with simulations over the last two glacial cycles to estimate the present evolution and were subsequently forced with temperature scenarios resulting from greenhouse emission scenarios which assume equivalent CO{sub 2} increases of two, four, and eight times the present (1990 A.D.) value by the year 2130 A.D. and a stabilization after that. The calculations brought to light that during the next century (short-term effect), the background evolution trend would dominate the response of the Antarctic ice sheet but would be negligible for the Greenland ice sheet. On that timescale, the Greenland and Antarctic ice sheets would roughly balance one another for the middle scenario (similar to the IPCC96 IS92a scenario), with respective contributions to the worldwide sea level stand on the order of about {+-}10 cm. On the longer term, however, both ice sheets would contribute positively to the worldwide sea level stand and the most important effect would come from melting on the Greenland ice sheet. Sensitivity experiments highlighted the role of ice dynamics and the height-mass-balance feedback on the results. It was found that ice dynamics cannot be neglected for the Greenland ice sheet, not even on a century timescale, but becomes only important for Antarctica on the longer term. The latter is related to an increased outflow of ice into the ice shelves and to the grounding-line retreat of the west Antarctic ice sheet, which are both found to be sensitive to basal melting below ice shelves and the effective viscosity of the ice shelves. Stretching parameters to their limits yielded a combined maximum rate of sea level rise of 85 cm century

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  17. Heat sources within the Greenland Ice Sheet: dissipation, temperate paleo-firn and cryo-hydrologic warming

    DOE PAGES

    Lüthi, M. P.; Ryser, C.; Andrews, L. C.; Catania, G. A.; Funk, M.; Hawley, R. L.; Hoffman, M. J.; Neumann, T. A.

    2015-01-01

    Ice temperature profiles from the Greenland Ice Sheet contain information on the deformation history, past climates and recent warming. We present full-depth temperature profiles from two drill sites on a flow line passing through Swiss Camp, West Greenland. Numerical modeling reveals that ice temperatures are considerably higher than would be expected from heat diffusion and dissipation alone. The possible causes for this extra heat are evaluated using a Lagrangian heat flow model. The model results reveal that the observations can be explained with a combination of different processes: enhanced dissipation (strain heating) in ice-age ice, temperate paleo-firn, and cryo-hydrologic warmingmore » in deep crevasses.« less

  18. Heat sources within the Greenland Ice Sheet: dissipation, temperate paleo-firn and cryo-hydrologic warming

    SciTech Connect

    Lüthi, M. P.; Ryser, C.; Andrews, L. C.; Catania, G. A.; Funk, M.; Hawley, R. L.; Hoffman, M. J.; Neumann, T. A.

    2015-01-01

    Ice temperature profiles from the Greenland Ice Sheet contain information on the deformation history, past climates and recent warming. We present full-depth temperature profiles from two drill sites on a flow line passing through Swiss Camp, West Greenland. Numerical modeling reveals that ice temperatures are considerably higher than would be expected from heat diffusion and dissipation alone. The possible causes for this extra heat are evaluated using a Lagrangian heat flow model. The model results reveal that the observations can be explained with a combination of different processes: enhanced dissipation (strain heating) in ice-age ice, temperate paleo-firn, and cryo-hydrologic warming in deep crevasses.

  19. Morphological properties of tunnel valleys beneath the southern sector of the former Laurentide Ice Sheet

    NASA Astrophysics Data System (ADS)

    Livingstone, Stephen; Clark, Chris

    2016-04-01

    Tunnel valleys have been widely reported on the bed of former ice sheets and are considered an important expression of subglacial meltwater drainage. Although known to have been cut by erosive meltwater flow, the water source and development of channels has been widely debated. Possible mechanisms include: (i) gradual formation by water flow in a subglacially deforming bed into channels under steady-state conditions; (ii) time-transgressive formation close to the ice margin by drainage of supraglacial meltwater to the bed or of meltwater temporarily impounded behind a permafrost wedge; and or (iii) by catastrophic subglacial meltwater floods. We have mapped and analysed the spatial pattern and morphometry of tunnel valleys and associated glacial bedforms along the southern sector of the former Laurentide Ice Sheet from high-resolution digital elevation models. Around 2000 tunnel valleys have been mapped, revealing a well-organised pattern of sub-parallel, semi-regularly spaced valleys that cluster together in distinctive networks. The tunnel valleys are typically <20 km long, and 0.5-3 km wide and preferentially terminate at moraines. They tend to be associated with outwash fans, eskers, glacial curvilineations, giant current ripples, and hill-hole-pairs. A relative age of the tunnel valleys, based on cross-cutting relationships, is used to resolve when individual tunnel valleys and networks were eroded. Our results suggest a time-transgressive origin for most tunnel valleys (i.e. they grow upstream) with some contributions from large meltwater drainage events.

  20. Recent ice dynamic and surface mass balance of Union Glacier in the West Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

    Rivera, A.; Zamora, R.; Uribe, J. A.; Jaña, R.; Oberreuter, J.

    2014-08-01

    Here we present the results of a comprehensive glaciological investigation of Union Glacier (79°46' S/83°24' W) in the West Antarctic Ice Sheet (WAIS), a major outlet glacier within the Ellsworth Mountains. Union Glacier flows into the Ronne Ice Shelf, where recent models have indicated the potential for significant grounding line zone (GLZ) migrations in response to changing climate and ocean conditions. To elaborate a glaciological base line that can help to evaluate the potential impact of this GLZ change scenario, we installed an array of stakes on Union Glacier in 2007. The stake network has been surveyed repeatedly for elevation, velocity, and net surface mass balance. The region of the stake measurements is in near-equilibrium, and ice speeds are 10 to 33 m a-1. Ground-penetrating radars (GPR) have been used to map the subglacial topography, internal structure, and crevasse frequency and depth along surveyed tracks in the stake site area. The bedrock in this area has a minimum elevation of -858 m a.s.l., significantly deeper than shown by BEDMAP2 data. However, between this deeper area and the local GLZ, there is a threshold where the subglacial topography shows a maximum altitude of 190 m. This subglacial condition implies that an upstream migration of the GLZ will not have strong effects on Union Glacier until it passes beyond this shallow ice pinning point.

  1. Decadal slowdown of a land-terminating sector of the Greenland Ice Sheet despite warming.

    PubMed

    Tedstone, Andrew J; Nienow, Peter W; Gourmelen, Noel; Dehecq, Amaury; Goldberg, Daniel; Hanna, Edward

    2015-10-29

    Ice flow along land-terminating margins of the Greenland Ice Sheet (GIS) varies considerably in response to fluctuating inputs of surface meltwater to the bed of the ice sheet. Such inputs lubricate the ice-bed interface, transiently speeding up the flow of ice. Greater melting results in faster ice motion during summer, but slower motion over the subsequent winter, owing to the evolution of an efficient drainage system that enables water to drain from regions of the ice-sheet bed that have a high basal water pressure. However, the impact of hydrodynamic coupling on ice motion over decadal timescales remains poorly constrained. Here we show that annual ice motion across an 8,000-km(2) land-terminating region of the west GIS margin, extending to 1,100 m above sea level, was 12% slower in 2007-14 compared with 1985-94, despite a 50% increase in surface meltwater production. Our findings suggest that, over these three decades, hydrodynamic coupling in this section of the ablation zone resulted in a net slowdown of ice motion (not a speed-up, as previously postulated). Increases in meltwater production from projected climate warming may therefore further reduce the motion of land-terminating margins of the GIS. Our findings suggest that these sectors of the ice sheet are more resilient to the dynamic impacts of enhanced meltwater production than previously thought. PMID:26511580

  2. Decadal slowdown of a land-terminating sector of the Greenland Ice Sheet despite warming

    NASA Astrophysics Data System (ADS)

    Tedstone, Andrew J.; Nienow, Peter W.; Gourmelen, Noel; Dehecq, Amaury; Goldberg, Daniel; Hanna, Edward

    2015-10-01

    Ice flow along land-terminating margins of the Greenland Ice Sheet (GIS) varies considerably in response to fluctuating inputs of surface meltwater to the bed of the ice sheet. Such inputs lubricate the ice-bed interface, transiently speeding up the flow of ice. Greater melting results in faster ice motion during summer, but slower motion over the subsequent winter, owing to the evolution of an efficient drainage system that enables water to drain from regions of the ice-sheet bed that have a high basal water pressure. However, the impact of hydrodynamic coupling on ice motion over decadal timescales remains poorly constrained. Here we show that annual ice motion across an 8,000-km2 land-terminating region of the west GIS margin, extending to 1,100 m above sea level, was 12% slower in 2007-14 compared with 1985-94, despite a 50% increase in surface meltwater production. Our findings suggest that, over these three decades, hydrodynamic coupling in this section of the ablation zone resulted in a net slowdown of ice motion (not a speed-up, as previously postulated). Increases in meltwater production from projected climate warming may therefore further reduce the motion of land-terminating margins of the GIS. Our findings suggest that these sectors of the ice sheet are more resilient to the dynamic impacts of enhanced meltwater production than previously thought.

  3. Revised estimates of Greenland ice sheet thinning histories based on ice-core records

    NASA Astrophysics Data System (ADS)

    Lecavalier, Benoit S.; Milne, Glenn A.; Vinther, Bo M.; Fisher, David A.; Dyke, Arthur S.; Simpson, Matthew J. R.

    2013-03-01

    Ice core records were recently used to infer elevation changes of the Greenland ice sheet throughout the Holocene. The inferred elevation changes show a significantly greater elevation reduction than those output from numerical models, bringing into question the accuracy of the model-based reconstructions and, to some extent, the estimated elevation histories. A key component of the ice core analysis involved removing the influence of vertical surface motion on the δ18O signal measured from the Agassiz and Renland ice caps. We re-visit the original analysis with the intent to determine if the use of more accurate land uplift curves can account for some of the above noted discrepancy. To improve on the original analysis, we apply a geophysical model of glacial isostatic adjustment calibrated to sea-level records from the Queen Elizabeth Islands and Greenland to calculate the influence of land height changes on the δ18O signal from the two ice cores. This procedure is complicated by the fact that δ18O contained in Agassiz ice is influenced by land height changes distant from the ice cap and so selecting a single location at which to compute the land height signal is not possible. Uncertainty in this selection is further complicated by the possible influence of Innuitian ice during the early Holocene (12-8 ka BP). Our results indicate that a more accurate treatment of the uplift correction leads to elevation histories that are, in general, shifted down relative to the original curves at GRIP, NGRIP, DYE-3 and Camp Century. In addition, compared to the original analysis, the 1-σ uncertainty is considerably larger at GRIP and NGRIP. These changes reduce the data-model discrepancy reported by Vinther et al. (2009) at GRIP, NGRIP, DYE-3 and Camp Century. A more accurate treatment of isostasy and surface loading also acts to improve the data-model fits such that the residuals at all four sites for the period 8 ka BP to present are significantly reduced compared to the

  4. Green Mountains and White Plains: the effect of Northern Hemisphere ice sheets on the global energy budget

    NASA Astrophysics Data System (ADS)

    Roberts, William; Valdes, Paul

    2016-04-01

    There are two physical features of a large ice sheet that can fundamentally change the global climate: the topography and albedo. Using a series of climate model experiments we shall show how the climate responds to these features, acting alone and in concert. We shall focus on the global energy budget. We shall use as a tool the HadCM3 climate model. We shall examine three suites of experiments in which we impose the albedo, topography or both of the Laurentide Ice Sheet. In each suite we vary the size of the ice sheet in order that we may examine how the climate's response varies with ice sheet size. Understanding the effect of ice sheets at a size below their maximum is important because, during any glacial period the ice sheets exist at these lesser extents for the majority of the time. We shall show that the albedo of the ice sheet causes a reduction in the incoming shortwave radiation over the ice sheet and that this is balanced by a compensating incoming energy flux into the Southern Hemisphere. The topography of the ice sheet causes an increase in the incoming shortwave radiation over the ice sheet that is balanced by an outgoing energy flux to the south of the ice sheet, with little change in the Southern Hemisphere. The topography and albedo of the ice sheet cause an increase in the outgoing shortwave radiation over the icesheet that is balanced by incoming fluxes to the south of the ice sheet and in the Southern Hemisphere. The magnitude of the cross equatorial atmospheric heat flux shall be related to the position of the ITCZ. We shall show there is a close correlation between the position of the ITCZ and the cross equatorial heat flux, if there is no change in the ice sheet. Changing the ice sheet topography causes this relationship to breakdown.

  5. Peregrinations of the Greenland Ice Sheet divide in the last glacial cycle: implications for central Greenland ice cores

    NASA Astrophysics Data System (ADS)

    Marshall, Shawn J.; Cuffey, Kurt M.

    2000-06-01

    The superb quality of the climate chronology archived in the Summit, Greenland ice cores (GRIP, GISP2) testifies that the Greenland Ice Sheet divide has been generally stable through the last glacial cycle. The ice sheet has experienced a broad range of paleoclimate conditions, ice sheet margin configurations, and internal dynamical adjustments in glacial-interglacial transitions, however. It is unlikely that the Summit region escaped shifts in ice divide position, geometry, elevation, and flow characteristics. Details of this dynamical history are important to several aspects of ice core studies. The magnitudes of pure and simple shearing, reconstruction of vertical ice velocity, the explicit location of the ice divide, and the divide 'residence time' at different locations are all of interest in interpretation of climatic variables and physical properties of ice in the ice cores. We apply a three-dimensional, thermomechanical ice sheet model to examine the evolution of these dynamical variables over the last 160 kyr in central Greenland. While a high-elevation ice dome is present in the Summit region throughout the simulation, ice divide migrations of up to 150 km are predicted. All points in the vicinity of the Summit ice cores, including the modern divide, have been subject to flowline shifts and variable, non-zero shear deformation during the adjustment from glacial to Holocene conditions, from ca. 10 ka to the present. Modelled divide peregrinations and strain rate history are consistent with the observed disturbance of deep ice in the GRIP and GISP2 ice cores, which has muddled paleoclimate reconstructions for the last interglacial (Eemian) period in Greenland. Dynamical excursions are also evident north of the modern summit, where the NGRIP ice core is currently being drilled [Dahl-Jensen et al., J. Glaciol. 43 (1997) 300-306]. However, the prevailing flow direction and deformation regime at the NGRIP site are much more stable than those at GRIP and GISP2

  6. Do climate mode flips control the dynamics of the Laurentide Ice Sheet

    SciTech Connect

    Lowell, T.V. . Dept. of Geology)

    1993-03-01

    Mode flips, major reorganizations of the earth's ocean-atmosphere system, are being debated as the cause of glacial terminations. If these do occur, one important implication is that ice sheets responded to, rather than drove, climate changes. Using the Laurentide Ice Sheet as an example, the author explores this implication and suggest that ice sheets acted in three fundamentally different modes. In the growth mode the LIS expanded under cold conditions, produced little meltwater and left little sediment. Equilibrium profiles were maintained during growth. During it's full extent mode, the ice sheet remained in a quasi-equilibrium state for several thousand years with minor marginal changes but produced little meltwater. In the decay mode, despite its rapid retreat, the ice sheet did most of its geomorphic work largely because of the abundant water produced from a rapid rise in ELA. Surface profiles would be low from the increased surface melting. This model explains the large volumes of meltwater that are necessary for deforming beds, surges, major subglacial erosion, and formation of proglacial lakes. If this hypothesis is correct it implies that the role of ice sheets in GCM models must be modified from driving to driven, and that anthropogenic warming may change the behavior of the Antarctic Ice Sheet by introducing a large ablation zone.

  7. Recent Changes in the Greenland Ice Sheet as Seen from Space

    NASA Technical Reports Server (NTRS)

    Hall, Dorothy K.

    2011-01-01

    Many changes in the Greenland Ice Sheet have been reported in the recent scientific literature and have been attributed to various responses of the ice sheet due to regional (and global) warming. Because melting of the ice sheet would contribute approximately 7 m to sea-level rise, the lives and habitat of hundreds of millions of people worldwide would be directly and indirectly affected if continued ice-sheet melting occurs. As mean-annual global temperatures have increased, there has been an increasing focus on studying the Greenland Ice Sheet using available satellite data, and numerous expeditions have been undertaken. Regional "clear-sky" surface temperature increases since the early 1980s in the Arctic, measured using Advanced Very High Resolution Radiometer (AVHRR) infrared data, range from 0.57+/-0.02 C to 0.72+/-0.10 C per decade. Arctic warming has important implications for ice-sheet mass balance because much of the periphery of the Greenland Ice Sheet is already near O C during the melt season, and is thus vulnerable to more extensive melting if temperatures continue to increase. An increase in melting of the ice sheet would accelerate sea-level rise, an issue of increasing concern to billions of people worldwide. The surface temperature of the ice sheet has been studied in even greater detail using Moderate-Resolution Imaging Spectroradiometer (MODIS) data in the six individual drainage basins as well as for the ice sheet as a whole. Surface temperature trends in the decade of the 2000s have not been strong, according to the MODIS measurements. In addition to surface-temperature increases over the last few decades as measured by AVHRR, other changes have been observed such as accelerated movement of many of Greenland's outlet glaciers and sudden draining of supraglacial lakes. Decreasing mass of the ice sheet since (at least) 2002 has been measured using Gravity Recovery and Climate Experiment (GRACE) data, along with an build-up of ice at the higher

  8. Cenozoic ice sheet history from East Antarctic Wilkes Land continental margin sediments

    USGS Publications Warehouse

    Escutia, C.; De Santis, L.; Donda, F.; Dunbar, R.B.; Cooper, A. K.; Brancolini, Giuliano; Eittreim, S.L.

    2005-01-01

    The long-term history of glaciation along the East Antarctic Wilkes Land margin, from the time of the first arrival of the ice sheet to the margin, through the significant periods of Cenozoic climate change is inferred using an integrated geophysical and geological approach. We postulate that the first arrival of the ice sheet to the Wilkes Land margin resulted in the development of a large unconformity (WL-U3) between 33.42 and 30 Ma during the early Oligocene cooling climate trend. Above WL-U3, substantial margin progradation takes place with early glacial strata (e.g., outwash deposits) deposited as low-angle prograding foresets by temperate glaciers. The change in geometry of the prograding wedge across unconformity WL-U8 is interpreted to represent the transition, at the end of the middle Miocene "climatic optimum" (14-10 Ma), from a subpolar regime with dynamic ice sheets (i.e., ice sheets come and go) to a regime with persistent but oscillatory ice sheets. The steep foresets above WL-U8 likely consist of ice proximal sediments (i.e., water-lain till and debris flows) deposited when grounded ice-sheets extended into the shelf. On the continental rise, shelf progradation above WL-U3 results in an up-section increase in the energy of the depositional environment (i.e., seismic facies indicative of more proximal turbidite and of bottom contour current deposition from the deposition of the lower WL-S5 sequence to WL-S7). Maximum rates of sediment delivery to the rise occur during the development of sequences WL-S6 and WL-S7, which we infer to be of middle Miocene age. During deposition of the two uppermost sequences, WL-S8 and WL-S9, there is a marked decrease in the sediment supply to the lower continental rise and a shift in the depocenters to more proximal areas of the margin. We believe WL-S8 records sedimentation during the final transition from a dynamic to a persistent but oscillatory ice sheet in this margin (14-10 Ma). Sequence WL-S9 forms under a polar

  9. Simulating a dynamic Antarctic ice sheet in the early to middle Miocene

    NASA Astrophysics Data System (ADS)

    Gasson, Edward; DeConto, Robert; Pollard, David; Levy, Richard

    2016-04-01

    A variety of sources of geological data suggest that there were major variations in Antarctic ice sheet volume and extent during the early to middle Miocene. Simulating such large-scale changes is problematic due to a strong hysteresis effect, which results in limited retreat of the terrestrial ice sheet once it has reached continental size. A relatively narrow range of atmospheric CO2 concentrations shown by proxy records exacerbates this problem. Here we use a new asynchronous climate-ice sheet coupling method, using a high-resolution atmospheric component, to account for ice sheet-climate feedbacks. Accounting for these processes results in increased retreat when compared with standard offline simulations. Combined with recently proposed mechanisms for ice retreat into deep subglacial basins, we are able to simulate large-scale variability of the Miocene Antarctic ice sheet. This variability is equivalent to a seawater oxygen isotope signal of 0.52 - 0.66 ‰, or a sea level equivalent change of 30 - 36 m, for a range of atmospheric CO2 between 280 - 500 ppm and a changing astronomical configuration. This result represents a substantial advance in resolving the long-standing model-data conflict of Miocene Antarctic ice sheet and sea level variability, and provides a mechanistic explanation for new ice-proximal records emerging from sedimentological drill cores.

  10. The role of basal hydrology in the surging of the Laurentide Ice Sheet

    NASA Astrophysics Data System (ADS)

    Roberts, William H. G.; Payne, Antony J.; Valdes, Paul J.

    2016-08-01

    We use the Glimmer ice sheet model to simulate periodic surges over the Laurentide Ice Sheet during the Last Glacial Maximum. In contrast to previous studies we use the depth of water at the base of the ice sheet as the switch for these surges. We find that the surges are supported within the model and are quite robust across a very wide range of parameter choices, in contrast to many previous studies where surges only occur for rather specific cases. The robustness of the surges is likely due to the use of water as the switch mechanism for sliding. The statistics of the binge-purge cycles resemble observed Heinrich events. The events have a period of between 10 and 15 thousand years and can produce fluxes of ice from the mouth of Hudson Strait of 0.05 Sv - a maximum flux of 0.06 Sv is possible. The events produce an ice volume of 2.50 × 106 km3, with a range of 4.30 × 106-1.90 × 106 km3 possible. We undertake a suite of sensitivity tests varying the sliding parameter, the water drainage scheme, the sliding versus water depth parameterisation and the resolution, all of which support the ice sheet surges. This suggests that internally triggered ice sheet surges were a robust feature of the Laurentide Ice Sheet and are a possible explanation for the observed Heinrich events.

  11. Modelling the Isotopic Response to West Antarctic Ice Sheet Collapse and Sea Ice Retreat During the Last Interglacial

    NASA Astrophysics Data System (ADS)

    Holloway, M. D.

    2015-12-01

    Ice sheet changes can exert major control over spatial water isotope variations in Antarctic surface snow. Consequently a significant mass loss or gain of the West Antarctic Ice Sheet (WAIS) would be expected to cause changes in the water isotope record across Antarctic ice core sites. Analysis of sea level indicators for the last interglacial (LIG; 130,000 to 115,000 years ago) suggest a global sea level peak 6 to 9 m higher than present. Recent NEEM Greenland ice core results imply that Greenland likely provided a modest 2m contribution towards this global sea level rise. This implies that a WAIS contribution is necessary to explain the LIG sea level maxima. In addition, Antarctic ice core records suggest that Antarctic air temperatures during the LIG were up to 6 °C warmer than present. Climate models have been unable to recreate such warmth when only orbital and greenhouse gas forcing are considered. Thus changes to the Antarctic ice sheet and ocean circulation may be required to reconcile model simulations with ice core data. Here we model the isotopic response to differing WAIS deglaciation scenarios, freshwater hosing, and sea ice configurations using a fully coupled General Circulation Model (GCM) to help interpret Antarctic ice core records over the LIG.

  12. Numeric control on the late-glacial chronology of the southern Laurentide Ice Sheet derived from ice-proximal lacustrine deposits

    NASA Astrophysics Data System (ADS)

    Carson, Eric C.; Hanson, Paul R.; Attig, John W.; Young, Aaron R.

    2012-11-01

    We used a combination of radiocarbon and OSL dating in ice-proximal lacustrine silt and clay and outwash sand to estimate when ice of the Green Bay Lobe of the Laurentide Ice Sheet began retreating from its maximum position in south-central Wisconsin. The radiocarbon ages indicate that lakes had formed in the two tributary valleys by ~ 17.2 and 20.1 ka, respectively. The OSL ages indicate that the Green Bay Lobe was at its maximum position from about 26.4 ± 5.1 ka to 21.4 ± 3.3 ka. These data provide entirely new chronologic control on late Wisconsin (Marine Isotope Stage 2) glacial event in the upper Midwest, as well as the opportunity to directly compare radiocarbon and OSL ages in this setting.

  13. A glimpse of ice sheets in the Early Palaeozoic greenhouse world

    NASA Astrophysics Data System (ADS)

    Armstrong, H. A.; Turner, B. R.

    2012-04-01

    It is a commonly held notion that the Earth had a mild greenhouse climate for much of the Early Palaeozoic, terminated by the Hirnantian Ice Age (c. 434 Ma). Models now predict pCO2 values of 5x present atmospheric level and a global mean air temperature of 15oC, consistent with δ18O values and zooplankton biogeographical studies that indicate a modern-style "cool" world climate for the Late Ordovician. Did icehouse conditions exist in the earlier Ordovician? Studiesof depositional architecture from the tectonically quiescent, subpolarGondwana continental margin,in South Africa and Jordan, provide a well constrained sedimentary record of 4th and, 3rd ordereustatic cyclesduring the Floian and Darriwilian. When fourth order sequences are hypothesized to be paced by the long eccentricity 405-kyr cycle the 3rd order sequences are calculated to be ~1.2-myr and broadly correlate with the global eustatic curve. These intervals are separated by sequences of ~2.4-myr duration. In comparison with Mesozoic and Cenozoic we conclude that the ~1.2-myr cycles correspond with long obliquity cycles predominant in icehouse conditions and the ~2.4-myr cycles with the long eccentricity cycle predominant during greenhouse conditions. We propose Floian and Darriwilian Ice Ages, during which, orbitally induced "cold snaps," caused the expansion and amalgamation of small/medium-scale ice sheets. Based on relative sea level changes of 15 - 30m we hypothesize ice sheets of 8-12 x 106 km3.Placing deposition sequence orders into a high resolution temporal framework (i.e. orbital periodicities) provides a method for identifying icehouse periods throughout the Palaeozoic.

  14. Distinct Seasonal Velocity Patterns Based on Ice-Sheet-Wide Analysis of Greenland Outlet Glaciers

    NASA Astrophysics Data System (ADS)

    Moon, T. A.; Joughin, I. R.; Smith, B. E.; van den Broeke, M. R.; Usher, M.

    2014-12-01

    Mass loss from the Greenland Ice Sheet increased significantly over the last several decades and current mass losses of 260-380 Gt ice/yr contribute 0.7-1.1 mm/yr to global sea-level rise. Greenland mass loss includes ice discharge via marine-terminating outlet glaciers and surface meltwater runoff, the former now making up a third to a half of total ice loss. The magnitude of ice discharge depends in part on ice-flow speed, which has broadly increased since 2000 but varies locally, regionally, and from year to year. Research on a limited set of Greenland glaciers also shows that speeds vary seasonally. However, for much of the west, northwest, and southeast coasts where ice loss is increasing most rapidly, there are few or no records of seasonal velocity variation. Ice velocity is influenced by several key components of the ice-sheet-ocean-climate system: subglacial environment, surface melt and runoff, and ice-ocean interaction at the ice-front (terminus). Thus, knowledge of seasonal velocity patterns is important for predicting annual ice discharge, understanding the effects of increased surface melt on total mass loss, and establishing how ice-flow responds to other climatic changes. We developed 5-year records of seasonal velocity measurements for 55 glaciers around the ice-sheet margin. Among glaciers with significant speed variations, we find three distinct seasonal velocity patterns. One pattern indicates relatively high glacier sensitivity to ice-front position, with seasonal summer speedup sustained through fall. The other two patterns appear to be meltwater controlled and indicate regional differences in which some subglacial systems likely transition seasonally from inefficient, distributed hydrologic networks to efficient, channelized drainage, while others do not. These differences in dominant velocity control mechanisms reveal likely spatiotemporal variations in the dynamic response of the ice sheet to climate change.

  15. Advances in Measuring Antarctic Sea-Ice Thickness and Ice-Sheet Elevations with ICESat Laser Altimetry

    NASA Technical Reports Server (NTRS)

    Zwally, H. Jay

    2004-01-01

    NASA's Ice, Cloud and Land Elevation Satellite (ICESat) has been measuring elevations of the Antarctic ice sheet and sea-ice freeboard elevations with unprecedented accuracy. Since February 20,2003, data has been acquired during three periods of laser operation varying from 36 to 54 days, which is less than the continuous operation of 3 to 5 years planned for the mission. The primary purpose of ICESat is to measure time-series of ice-sheet elevation changes for determination of the present-day mass balance of the ice sheets, study of associations between observed ice changes and polar climate, and estimation of the present and future contributions of the ice sheets to global sea level rise. ICESat data will continue to be acquired for approximately 33 days periods at 3 to 6 month intervals with the second of ICESat's three lasers, and eventually with the third laser. The laser footprints are about 70 m on the surface and are spaced at 172 m along-track. The on-board GPS receiver enables radial orbit determinations to an accuracy better than 5 cm. The orbital altitude is around 600 km at an inclination of 94 degrees with a 8-day repeat pattern for the calibration and validation period, followed by a 91 -day repeat period for the rest of the mission. The expected range precision of single footprint measurements was 10 cm, but the actual range precision of the data has been shown to be much better at 2 to 3 cm. The star-tracking attitude-determination system should enable footprints to be located to 6 m horizontally when attitude calibrations are completed. With the present attitude calibration, the elevation accuracy over the ice sheets ranges from about 30 cm over the low-slope areas to about 80 cm over areas with slopes of 1 to 2 degrees, which is much better than radar altimetry. After the first period of data collection, the spacecraft attitude was controlled to point the laser beam to within 50 m of reference surface tracks over the ice sheets. Detection of ice

  16. Numerical simulations of the Cordilleran ice sheet through the last glacial cycle

    NASA Astrophysics Data System (ADS)

    Seguinot, Julien; Rogozhina, Irina; Stroeven, Arjen P.; Margold, Martin; Kleman, Johan

    2016-03-01

    After more than a century of geological research, the Cordilleran ice sheet of North America remains among the least understood in terms of its former extent, volume, and dynamics. Because of the mountainous topography on which the ice sheet formed, geological studies have often had only local or regional relevance and shown such a complexity that ice-sheet-wide spatial reconstructions of advance and retreat patterns are lacking. Here we use a numerical ice sheet model calibrated against field-based evidence to attempt a quantitative reconstruction of the Cordilleran ice sheet history through the last glacial cycle. A series of simulations is driven by time-dependent temperature offsets from six proxy records located around the globe. Although this approach reveals large variations in model response to evolving climate forcing, all simulations produce two major glaciations during marine oxygen isotope stages 4 (62.2-56.9 ka) and 2 (23.2-16.9 ka). The timing of glaciation is better reproduced using temperature reconstructions from Greenland and Antarctic ice cores than from regional oceanic sediment cores. During most of the last glacial cycle, the modelled ice cover is discontinuous and restricted to high mountain areas. However, widespread precipitation over the Skeena Mountains favours the persistence of a central ice dome throughout the glacial cycle. It acts as a nucleation centre before the Last Glacial Maximum and hosts the last remains of Cordilleran ice until the middle Holocene (6.7 ka).

  17. Intermittent ice sheet discharge events in northeastern North America during the last glacial period

    NASA Astrophysics Data System (ADS)

    Papa, Brian D.; Mysak, Lawrence A.; Wang, Zhaomin

    2006-02-01

    The 3D ice sheet model of Marshall and Clarke, which includes both dynamics and thermodynamics, is used to successfully simulate millennial-scale oscillations within an ice sheet under steady external forcing. Such internal oscillations are theorized to be the main cause of quasi-periodic large-scale ice discharges known as Heinrich Events. An analysis of the mechanisms associated with multi-millennial oscillations of the Laurentide Ice Sheet, including the initiation and termination of sliding events, is performed. This analysis involves an examination of the various heat sources and sinks that affect the basal ice temperature, which in turn determines the nature of the ice sheet movement. The ice sheet thickness and surface slope, which affect the pressure-melting point and strain heating, respectively, are found to be critical for the formation and development of fast moving ice streams, which lead to large iceberg calving. Although the main provenance for Heinrich Events is thought to be from Hudson Bay and Hudson Strait, we show that the more northerly regions around Lancaster Strait and Baffin Island may also be important sources for ice discharges during the last glacial period.

  18. Inland thinning of West Antarctic Ice Sheet steered along subglacial rifts.

    PubMed

    Bingham, Robert G; Ferraccioli, Fausto; King, Edward C; Larter, Robert D; Pritchard, Hamish D; Smith, Andrew M; Vaughan, David G

    2012-07-25

    Current ice loss from the West Antarctic Ice Sheet (WAIS) accounts for about ten per cent of observed global sea-level rise. Losses are dominated by dynamic thinning, in which forcings by oceanic or atmospheric perturbations to the ice margin lead to an accelerated thinning of ice along the coastline. Although central to improving projections of future ice-sheet contributions to global sea-level rise, the incorporation of dynamic thinning into models has been restricted by lack of knowledge of basal topography and subglacial geology so that the rate and ultimate extent of potential WAIS retreat remains difficult to quantify. Here we report the discovery of a subglacial basin under Ferrigno Ice Stream up to 1.5 kilometres deep that connects the ice-sheet interior to the Bellingshausen Sea margin, and whose existence profoundly affects ice loss. We use a suite of ice-penetrating radar, magnetic and gravity measurements to propose a rift origin for the basin in association with the wider development of the West Antarctic rift system. The Ferrigno rift, overdeepened by glacial erosion, is a conduit which fed a major palaeo-ice stream on the adjacent continental shelf during glacial maxima. The palaeo-ice stream, in turn, eroded the 'Belgica' trough, which today routes warm open-ocean water back to the ice front to reinforce dynamic thinning. We show that dynamic thinning from both the Bellingshausen and Amundsen Sea region is being steered back to the ice-sheet interior along rift basins. We conclude that rift basins that cut across the WAIS margin can rapidly transmit coastally perturbed change inland, thereby promoting ice-sheet instability.

  19. Implications of increased surface melt under global warming scenarios: Greenland ice-sheet simulations

    NASA Astrophysics Data System (ADS)

    Parizek, B. R.; Alley, R. B.

    2003-04-01

    The Greenland Ice Sheet represents ~10% (by volume) of the cryosphere and ~7 meters of sea-level equivalence. Citing the inherent stability offered by the long glaciological timescales involved in classical ice-sheet dynamics, the elevation of the bedrock on which the ice sheet is perched, and the extremely cold inland surface temperatures, numerical studies on the future of this ice sheet under various global-warming scenarios have all but dismissed the potential for substantial dynamic changes in the next millennium. Unlike for the setting of the West Antarctic Ice Sheet, there were simply no foreseen mechanisms for rapid switches in Greenland's prevailing ice-flow regime. Recently, field observations near the Swiss Camp in west-central Greenland may have offered the essential link between surface temperatures and ice dynamics at and below the equilibrium line that may require the ice sheet to ``listen'' to climate far more closely than previously envisioned by model parameterizations. Zwally et al. (2002) documented correlation between increased ice velocity and increased surface melt (as parameterized by positive degree days (PDD)). They argued that surface water is piped directly to the bed with little delay, causing increased basal-water pressures and basal-sliding velocities. Using the PSU/UofC thermomechanical flowline model, numerous simulations are being conducted to test a wide variety of parameter spaces that link surface melt with a new sliding law under several global warming scenarios. Initial comparisons to the EISMINT Level 3 global-warming benchmark illustrate an enhanced sensitivity of the ice sheet to surface warming resulting in higher ablation rates, thinning of the margin, and a reduction in ice volume that all lead to a positive contribution to global sea-level rise.

  20. The Ellsworth Subglacial Highlands and the inception and retreat of the West Antarctic Ice Sheet

    NASA Astrophysics Data System (ADS)

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

    2012-04-01

    Laying on a bed in places >2km below sea level, the West Antarctic Ice Sheet (WAIS) is thought to be prone to major rapid decay due to melting from the ocean, which induces grounding line retreat. A feedback may occur, in which migration of the grounding line to deeper regions leads to further ice loss. Highland regions of the subglacial bed will act both as seeding centres for ice sheet growth and points of stability ('pinning points') during ice sheet recession. While several highland regions exist beneath the WAIS, none have been confirmed as ice sheet seeding centres/pinning points. Studies of subglacial East Antarctica have demonstrated the utility of radio-echo sounding (RES) in the identification of glacial geomorphology from which past ice sheet conditions can be appreciated. Here, we characterise the detailed glacial morphology of the Ellsworth Subglacial Highlands (ESH), from ground-based and airborne RES surveys. We document well-preserved classic features associated with restricted, dynamic, marine-proximal alpine glaciation, with hanging tributary valleys feeding significant over-deepened troughs cut by valley (tidewater) glaciers. Fjord-mouth threshold bars down-ice of overdeepenings define the termini of palaeo outlet-glaciers. We show how MODIS satellite imagery of the ice surface reflects the gross subglacial morphology. The imagery reveals numerous glaciated valleys cutting through the ESH, terminating at the edge of the deep Bentley Subglacial Trench. The landscape obviously predates the present ice sheet, and is likely to have been formed by a small dynamic ice cap at times when the marine sections of the WAIS were absent. As well as acting as a key WAIS seeding point, the ESH would be critical for 'pinning' the ice sheet during any large-scale retreat event.

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

    NASA Astrophysics Data System (ADS)

    Marshall, S. J.; Banwell, A.

    2015-12-01

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

  2. Examining the interaction between multi-year landfast sea ice and the Mertz Glacier Tongue, East Antarctica: Another factor in ice sheet stability?

    NASA Astrophysics Data System (ADS)

    Massom, Robert A.; Giles, A. Barry; Fricker, Helen A.; Warner, Roland C.; LegréSy, Benoit; Hyland, Glenn; Young, Neal; Fraser, Alexander D.

    2010-12-01

    The Mertz Glacier tongue (MGT), East Antarctica, has a large area of multi-year fast sea ice (MYFI) attached to its eastern edge. We use various satellite data sets to study the extent, age, and thickness of the MYFI and how it interacts with the MGT. We estimate its age to be at least 25 years and its thickness to be 10-55 m; this is an order of magnitude thicker than the average regional sea-ice thickness and too thick to be formed through sea-ice growth alone. We speculate that the most plausible process for its growth after initial formation is marine (frazil) ice accretion. The satellite data provide two types of evidence for strong mechanical coupling between the two types of ice: The MYFI moves with the MGT, and persistent rifts that originate in the MGT continue to propagate for large distances into the MYFI. The area of MYFI decreased by 50% following the departure of two large tabular icebergs that acted as pinning points and protective barriers. Future MYFI extent will be affected by subsequent icebergs from the Ninnis Glacier and the imminent calving of the MGT. Fast ice is vulnerable to changing atmospheric and oceanic conditions, and its disappearance may have an influence on ice tongue/ice shelf stability. Understanding the influence of thick MYFI on floating ice tongues/ice shelves may be significant to understanding the processes that control their evolution and how these respond to climate change, and thus to predicting the future of the Antarctic Ice Sheet.

  3. Determination of volume and surface scattering from saline ice using ice sheets with precisely controlled roughness parameters

    SciTech Connect

    Bredow, J.W.; Porco, R.L.; Fung, A.K.; Tjuatja, S.; Jezek, K.C.; Gogineni, S.; Gow, A.J.

    1995-09-01

    Experiments were performed at the US Army Cold Regions Research and Engineering Laboratory (CRREL) in Hanover, NH, to precisely determine the relative contributions of surface and volume scattering from saline ice that has well-known surface roughness characteristics. The ice growth phase of the experiment made use of two 6-ft diameter tanks and a 6-ft diameter mold with known roughness statistical parameters of rms height = 0.25 cm and Gaussian correlation (correlation length = 2.0 cm). One tank was used for growing a moderately thick saline ice sheet with very smooth surface, and the other was used for growing a thin layer of freshwater ice over the surface mold. The latter resulted in a layer with one statistically known rough boundary and one smooth boundary. Wide-bandwidth, multiple incidence angle backscattering measurements were performed, first on the bare saline ice sheet and then on the same sheet after the thin freshwater ice sheet was placed on top of it. Results indicate that the surface scattering dominates over saline ice volume scattering at all frequencies for low incidence angles for both the very smooth and Gaussian rough surfaces. The significance of volume scattering depends strongly on angle of incidence, frequency, volume scattering albedo, surface roughness, and surface correlation function.

  4. Earliest Holocene south Greenland ice sheet retreat within its late Holocene extent

    NASA Astrophysics Data System (ADS)

    Carlson, Anders E.; Winsor, Kelsey; Ullman, David J.; Brook, Edward J.; Rood, Dylan H.; Axford, Yarrow; LeGrande, Allegra N.; Anslow, Faron S.; Sinclair, Gaylen

    2014-08-01

    Early Holocene summer warmth drove dramatic Greenland ice sheet (GIS) retreat. Subsequent insolation-driven cooling caused GIS margin readvance to late Holocene maxima, from which ice margins are now retreating. We use 10Be surface exposure ages from four locations between 69.4°N and 61.2°N to date when in the early Holocene south to west GIS margins retreated to within these late Holocene maximum extents. We find that this occurred at 11.1 ± 0.2 ka to 10.6 ± 0.5 ka in south Greenland, significantly earlier than previous estimates, and 6.8 ± 0.1 ka to 7.9 ± 0.1 ka in southwest to west Greenland, consistent with existing 10Be ages. At least in south Greenland, these 10Be ages likely provide a minimum constraint for when on a multicentury timescale summer temperatures after the last deglaciation warmed above late Holocene temperatures in the early Holocene. Current south Greenland ice margin retreat suggests that south Greenland may have now warmed to or above earliest Holocene summer temperatures.

  5. Reprint of: Modeling Antarctic ice sheet and climate variations during Marine Isotope Stage 31

    NASA Astrophysics Data System (ADS)

    DeConto, Robert M.; Pollard, David; Kowalewski, Douglas

    2012-10-01

    Marine Isotope Stage 31 (MIS-31) is one of the major interglacials of the early Pleistocene ~ 1.08 to 1.06 Ma. Data from proximal sediment cores around several sectors of Antarctica indicate strong sea surface warming and ice shelf and sea ice retreat. Benthic deep-sea-core δ18O values at this time are some of the lowest of the Pleistocene, indicating both deep sea warming and reduced global ice volume. A coeval alignment of orbital parameters produces one of the strongest high-latitude summer insolation anomalies of the last several million years. Here we use a 3-D ice sheet-shelf model to simulate the evolution of Antarctic ice sheets through the event, and a global climate model to simulate temperatures and sea ice during peak Antarctic warmth. The ice model predicts nearly complete collapse and subsequent recovery of marine ice in West Antarctica, and the ice and climate model results agree well with proximal sediment core data in the Ross Embayment recovered by the ANDRILL and Cape Roberts drilling projects. The dominant forcing is found to be variations in sub-ice-shelf oceanic melting, with insignificant surface melting of terrestrial ice flanks even during peak warmth. Implications are noted in light of other observations and theories of Pliocene-Pleistocene Antarctic ice sheet variability that do involve surface melt.

  6. Modelling high latitude climates and ice sheets during the mid-Pliocene warm period

    NASA Astrophysics Data System (ADS)

    Hill, D. J.; Haywood, A. M.; Hindmarsh, R. C.; Valdes, P. J.; Lunt, D. J.

    2007-12-01

    Reduction in the polar ice caps and associated climate feedbacks are implicated in the warming of pre- Quaternary palaeoclimates. General Circulation Model (GCM) simulations of the last such warm period, the mid- Pliocene (3.29-2.97 Ma), suggests global surface temperatures were between 1.4°C and 3.6°C warmer than today. However, the changes are amplified in the high latitudes, where the PRISM (Pliocene Research, Interpretation and Synoptic Mapping) palaeoenvironmental reconstruction specifies a 50% reduction in the Greenland Ice Sheet (GrIS) and a 33% reduction in the Antarctic Ice Sheet. These ice sheets configurations are largely based on poorly constrained sea level estimates and are one of the least well-known boundary conditions for the mid-Pliocene. Utilizing a suite of mid-Pliocene GCM experiments, evaluated against available palaeoenvironmental information, and a 3-D thermomechanically coupled ice sheet model, the state of the GrIS and East Antarctic Ice Sheet (EAIS) during this interval has been reconstructed. Ensemble models of Greenland, which compare favourably to evidence of ice-rafted debris and mid-Pliocene vegetation, suggest a reduction in the ice sheet to 30 - 40% of the modern ice volume. In East Antarctica increased surface temperatures during the mid-Pliocene lead to significant melt over the Wilkes and Aurora Subglacial Basins and a reduction in the extent of the ice sheet. These ice losses are partially offset by an increase in snowfall over the Antarctic plateau. Marine diatoms in the Transantarctic Mountains have been used as evidence of major East Antarctic deglaciations during the Pliocene. However, our EAIS predictions show that the modelled mid-Pliocene climate is insufficient to cause the hypothesized magnitude of ice sheet retreat. Finally, the mid-Pliocene has been suggested as a possible palaeoclimate analogue for the climate of the late 21st century. Here we compare predictions of mid-Pliocene ice sheets with observations of