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LGM-extent of the West Antarctic Ice Sheet offshore from the Hobbs Coast, based on palaeo-ice stream bed observations

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Klages, J. P. , Kuhn, G. , Hillenbrand, C. D. , Graham, A. G. C. , Smith, J. A. , Larter, R. D. and Gohl, K. (2012): LGM-extent of the West Antarctic Ice Sheet offshore from the Hobbs Coast, based on palaeo-ice stream bed observations , AGU Fall Meeting, San Francisco, California, 3 December 2012 - 7 December 2012 .
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Abstract:

Paleo-ice stream beds that are exposed today on the West Antarctic continental shelf provide unique archives of conditions at the base of the past ice sheet, that are difficult to assess beneath its modern, extant counterpart. During the last decade, several of these paleo-ice stream beds have been studied in detail to reconstruct the extent of the West Antarctic Ice Sheet (WAIS) at the Last Glacial Maximum (LGM), the patterns of ice drainage, and the timing of grounding-line retreat during the last deglaciation. However, despite significant advances, such information still remains poorly constrained in numerous drainage sectors of the WAIS. In particular, the maximum extent of ice at the LGM remains ambiguous for key drainage basins of the ice sheet. Whether the WAIS extended to the shelf break around the continent, or advanced only partially across its sea bed, is a crucial piece of information required for reconstructing and modeling patterns of ice-sheet change from past to present. Here we present marine geological and geophysical data that we collected on R/V “Polarstern” expedition ANT-XXVI/3 in early 2010 to investigate the extent, flow, and retreat of the WAIS, from an especially poorly studied part of the West Antarctic shelf, offshore from the Hobbs Coast in the western Amundsen Sea. Here, a landward deepening paleo-ice stream trough is incised into the shelf. The seafloor within the western-central part of the trough is characterized by a large trough-wide grounding zone wedge, ~70 m thick and ~17 km long, which overlies a high of seaward dipping sedimentary strata. The back-slope of the GZW is characterized by highly elongate streamlined bedforms suggesting fast paleo-ice flow towards NW. The crest of the wedge has been cross-cutted by iceberg keels. In contrast, the outer shelf seafloor offshore the GZW is predominantly smooth and featureless, although there is some evidence locally for iceberg scouring. A radiocarbon age from calcareous microfossils in a core from the inner shelf shows that ice had retreated landward from the GZW before 12.967 cal 14C yrs. There are two possible interpretations for the GZW: either (1) that it formed during a significant stillstand as the WAIS retreated from the shelf edge following the LGM, or (2) it marks the maximum extent of grounded ice at the LGM. Preliminary data appears to support the latter hypothesis. Specifically we point to i) the size and geometry of the GZW which is comparable to other grounding-line features marking LGM-positions around Antarctica (e.g. in the Ross Sea and in Prydz Bay), and less similar to GZWs deposited during episodic ice-stream retreat (e.g. in Pine Island Trough and Marguerite Trough); and ii) the lack of subglacial bedforms on the outer shelf, which may be explained by a thick hemipelagic sediment cover deposited over tens of thousands of years. In order to test these two hypotheses and constrain the timing and duration of GZW formation, additional ages will be obtained from seasonal open-marine sediments overlying subglacial till in cores seaward of the GZW as well as from the outer shelf. We will present preliminary interpretations of these data, which will aim to resolve the extent of the WAIS in this sector and, at the same time, provide new information on the dynamics of paleo-ice streams, which drained the former ice sheet.

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