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Extent and dynamics of ice on the southeast Pacific margin of West Antarctica during the Last Glacial Maximum, and its subsequent retreat history

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Larter, R. D. , Hillenbrand, C. D. , Smith, J. A. , Graham, A. G. C. , Deen, T. J. , Dowdeswell, J. A. , Evans, J. , Gohl, K. , Kuhn, G. , O'Cofaigh, C. and Pudsey, C. J. (2008): Extent and dynamics of ice on the southeast Pacific margin of West Antarctica during the Last Glacial Maximum, and its subsequent retreat history , 33rd International Geological Congress, 6-14 August 2008, Oslo. .
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Marine geoscience data indicate that during the Last Glacial Maximum (LGM) grounded ice extended to the shelf edge along most, if not all, of the 2500 km-long continental margin from the northern Antarctic Peninsula to the Amundsen Sea. Past extent of grounded ice is indicated by swath bathymetry data from the outer parts of cross-shelf troughs, which reveal relict elongated subglacial bedforms. The bedforms show that the troughs were paths of fast-flowing (streaming) ice. Geomorphological evidence regarding the nature of ice flow over intervening outer shelf banks has been erased through pervasive post-glacial ploughing by icebergs. However, seismic profiles across the banks reveal widespread shelf edge progradation and numerous glacial unconformities that indicate grounded ice has extended across them many times during the Pleistocene, and before. Subglacial tills in the outer parts of shelf troughs are overlain by up to 2 m of postglacial sediments, which are no older than the LGM in any core yet dated. A layer of soft, intermediate shear strength (12¬25 kPa) till, interpreted as deformation till, underlies the postglacial sediments in cores in the troughs. These observations are consistent with the interpretation that streaming ice extended along the troughs during the LGM, but the duration of such flow, and whether or not it spanned the entire period when ice extended to the outer shelf remains undetermined.To determine when, and how rapidly, ice retreated from the continental shelf, ages of core samples from near the base of postglacial sediments in several troughs have been determined by AMS radiocarbon dating. Samples to constrain glacial retreat have been taken from either the base of muds deposited in seasonally open-marine conditions similar to today, or underlying sandy muds interpreted as having been deposited close to the grounding line. Modern sea-floor sediments on some parts of the margin contain sufficient calcareous microfossils for dating to constrain the local marine 14C reservoir correction. However, even where they occur, contents of planktonic foraminifera decrease downcore, and most deglaciation ages have been obtained from acid insoluble organic material (AIOM). In some areas these ages are significantly affected by reworked fossil carbon, as shown by apparent ages from AIOM in modern sea-floor sediments that range up to ~6000 years. Thus radiocarbon results from this margin must be treated with caution and there is a clear need for development of alternative dating methods.Notwithstanding these uncertainties, deglaciation ages obtained thus far suggest variable retreat histories along the margin. Results from the Antarctic Peninsula shelf and Amundsen Sea embayment suggest relatively rapid post-LGM ice retreat from the outer and middle shelf, followed by slower Holocene retreat to the present day ice margin. However, initial results from the Bellingshausen Sea (Belgica Trough) suggest a slower, progressive retreat commencing about 25 ka (corrected radiocarbon years). These results show that local factors are important in controlling the rate of ice retreat, and this needs to be taken into account in numerical models that attempt to predict the dynamic behaviour of large ice sheets.

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