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Bedform signature of a West Antarctic palaeo-ice stream reveals a multi-temporal record of flow and substrate control

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Graham, A. G. C. , Larter, R. D. , Gohl, K. , Hillenbrand, C. D. , Smith, J. A. and Kuhn, G. (2009): Bedform signature of a West Antarctic palaeo-ice stream reveals a multi-temporal record of flow and substrate control , First Antarctic Climate Evolution Symposium, GranadaSept 2009. .
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Existing landform models of palaeo-ice stream beds in Antarctica often portray a simple snapshot of former ice-basal conditions, but studies have rarely mapped the bedforms in detail. A better understanding of conditions at the ice-bed interface in palaeo-ice sheets is required because: (a) ice streams determine the discharge from large ice sheets, and (b) knowledge of past ice dynamics can be used to constrain predictions of future ice sheetbehaviour. We use an extensive (9950 km2) marine geophysical dataset, comprising multibeam swath bathymetry, sub-bottom and single-channel seismic reflection profiles, to map the geomorphological signature of a large palaeo-ice stream system in the western Amundsen Sea Embayment, West Antarctica. The bedform imprint of past ice streams comprises morethan 4000 elements, which we divide into five landsystem components: (1) a meltwater assemblage, (2) a composite assemblage of bedforms, (3) a sub-ice stream footprint, (4) grounding line retreat morphology, and (5) a pro-marginal deglacial group. Each group demonstrates different levels of overprinting and preservation, indicating a time-transgressive history for the inner shelf morphology, which implies that bedforms can neither be considered part of a single down-flow continuum nor a direct proxy for palaeo-ice velocity, as suggested previously. A main control on the geomorphology is the subglacial geology of the shelf, which is divided between rough bedrock on the inner shelf, and smooth, dipping, layered sediments on the mid-to-outer shelf. Inner shelf bedform variability reveals information about local, complex basal-ice conditions, meltwater flow, and ice dynamics over time, including detail not apparent at the scale of regional morphological studies. This detail leads us to conclude that past icestreams streamed across the entire shelf, and had onset zones that lay within the interior of the Antarctic Ice Sheet today. In contrast, highly-elongated bedforms on sedimentary strata reveal a timeslice snapshot of the last activity of ice streams on the middle to outer shelf, and may be a truerrepresentation of palaeo-ice velocity in these locations. A revised model for ice streams on the shelf captures multi-temporal bedform patterns associated with a West Antarctic palaeoice stream for the first time, and confirms a dominant substrate control on the flow and geomorphic imprint of this particular ice stream pathway.

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