International Geologiical Congress - Oslo 2008

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AAB-01 Arctic and Antarctic records of deglaciation since the Last Glacial Maximum: Processes, timing and causes

 

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

 

Robert Larter, British Antarctic Survey (United Kingdom)
Claus-Dieter Hillenbrand, British Antarctic Survey (United Kingdom)
James Smith, British Antarctic Survey (United Kingdom)
Alastair Graham, British Antarctic Survey (United Kingdom)
Tara Deen, British Antarctic Survey (United Kingdom)
Julian Dowdeswell, Scott Polar Research Institute (United Kingdom)
Jeff Evans, University of Loughborough (United Kingdom)
Karsten Gohl, Alfred Wegener Institute for Polar and Marine Research (Germany)
Gerhard Kuhn, Alfred Wegener Institute for Polar and Marine Research (Germany)
Colm O' Cofaigh, Durham University (United Kingdom)
Carol Pudsey, University of Dundee (United Kingdom)
 

 

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 ice extent is shown by swath bathymetry data from outer parts of cross-shelf troughs that reveal relict elongated subglacial bedforms. These show that the troughs were paths of fast-flowing ('streaming') ice. Pervasive post-glacial ploughing by icebergs has erased geomorphological evidence of ice flow over intervening banks. However, seismic profiles across the banks show widespread shelf edge progradation and numerous glacial unconformities, indicating ice extended across them many times during the Pleistocene, and before.

Subglacial tills in the outer parts of troughs are overlain by up to 2 m of postglacial sediments, which are no older than LGM in any core yet dated. A layer of 'soft', intermediate shear strength (12-25 kPa) diamicton, interpreted as deformation till, underlies these sediments. This is consistent with streaming ice having 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 shelf, AMS 14C dates have been obtained on samples from near the base of postglacial sediments in several troughs. Samples were 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 near the grounding line. Modern 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 ages are significantly affected by reworked fossil carbon, as shown by apparent ages from AIOM in modern sediments that range up to ∼6 ka. Thus 14C dates 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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