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


Geomorphic evidence for grounding line retreat in the Amundsen Sea Embayment and significance for west Antarctic deglaciation


Alastair Graham, British Antarctic Survey (United Kingdom)
Robert Larter, British Antarctic Survey (United Kingdom)
Claus-Dieter Hillenbrand, British Antarctic Survey (United Kingdom)
James Smith, British Antarctic Survey (United Kingdom)
Karsten Gohl, Alfred Wegener Institute (Germany)
Gerhard Kuhn, Alfred Wegener Institute (Germany)


Multibeam swath bathymetric and parametric sub-bottom profiler datasets demonstrate evidence for grounding line retreat of a major fast-flowing ice stream, which contributed to the drainage of the West Antarctic Ice Sheet at the Last Glacial Maximum (LGM). Swath bathymetry from the western Amundsen Sea reveals an extensive and pristine ice stream geomorphology in a pronounced 800 m-deep, >120 km-long and up to 75 km-wide topographic trough. Three subordinate ice stream tributaries, originating in the Dotson and Getz ice shelf catchments, fed the main trough, which contains well-preserved subglacial bedforms. An important control on the subglacial morphology is the transition from hard, granitic bedrock on the inner shelf within the tributaries, to a 'soft' sedimentary substrate on the mid-to-outer shelf. One theory to explain the quality of landform preservation in the main trough is that streaming was followed immediately by a rapid decoupling of the ice/bed contact ('lift-off'), preserving sedimentary bedform evidence on the mid-to-outer shelf through initial deglaciation (before ∼16 ka BP). Two types of grounding line deposits are superimposed on the ice stream footprint: (1) On the mid-shelf, grounding line deposition during the deglaciation is characterised by small, laterally discontinuous ramps with subtle relief, over which bedform continuity is interrupted.

Morphologically, these resemble 'sheet' systems, interpreted as minor pauses of the grounding line. These retreat ramps formed during phases of rapid deglaciation and relate to the back-stepped layering of a thin subglacial till during continual ice retreat; (2) By contrast, in one of the landward ice stream tributaries, a series of stacked sedimentary wedges is observed (relief of 30-50 m, extents of 8-12 km). These are interpreted more typically as grounding zone wedges; ice marginal landforms created by laterally-dispersed deposition at the grounding line during stepwise retreat from the mid-shelf (∼16-10 ka BP). We also show landform evidence from elsewhere in the Amundsen Sea as further examples of grounding zone wedges, which add to the overall picture of numerous grounding line positions on this part of the West Antarctic continental shelf. As landforms and depocentres taking some time to form, the presence of potential grounding zone features implies that the grounding line underwent stable or readvancing phases on several occasions during the last deglaciation.

Our preliminary interpretation is that initial rapid retreat of ice streams from the shelf may have been punctuated by periods of relative quiescence. For the Amundsen Sea at least, we suggest that sediment supply, substrate and bed geometries were the most influential factors in halting otherwise rapid periods of 'early' ice margin retreat. These observations hold significance for testing stabilising mechanisms in ice streams, whose style of grounding line deposition may drastically alter ice sheet stability.


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