Publication Date:
2018-08-10
Description:
Palaeo-ice stream beds that are exposed today on polar continental shelves provide unique archives of conditions at the base of ice sheets that are difficult to assess beneath their modern counterparts. During the last decade, several of these palaeo-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 in the entire Pacific sector, or it advanced, at least locally, only to a middle or outer shelf position, is a crucial piece of information required for reconstructing and modeling patterns of ice-sheet change from past to present.
Here we present new 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 palaeo-ice stream trough is incised into the shelf. The seafloor within the western-central part of the trough is characterized by a large grounding zone wedge (GZW), ~70 m thick and ~17 km long, which overlies a high of seaward dipping sedimentary strata. Directly seaward of the GZW a ~20 km wide 80±10 m deep relatively flat basin is mapped. The back-slope of the GZW is characterized by highly elongate streamlined bedforms suggesting fast palaeo-ice flow towards NW. In contrast, the outer shelf seafloor offshore from the GZW is predominantly smooth, at numerous locations scoured by icebergs and characterized by a distinct and ~2 m-deep subbottom reflector. As in other Antarctic shelf sectors, this subbottom reflector is likely to mark the top of a subglacial stiff till that is probably of LGM age, because a calcareous microfossil from the thin layer above the subbottom reflector provided a radiocarbon age of ~9.0 corr. ka BP at 98 cmbsf (centimeter below seafloor). A radiocarbon date from the inner shelf shows that the grounded ice here had retreated landward from the GZW remarkably early (before ~13.0 cal 14C yrs). This early deglaciation is in agreement with other minimum deglaciation ages from the Amundsen Sea embayment (e.g. Pine Island Bay and Dotson-Getz Trough). For the GZW we propose two possible formation scenarios: either (1) that it formed during a prolonged stillstand when the WAIS retreated from the outer shelf following the LGM, or (2) it marks the maximum grounding-line extent at the LGM. Although we rather propose, that grounded ice extended all the way or close to the shelf edge during the LGM, we cannot yet exclude the possibility that the GZW marks the LGM limit of grounded ice. Here we specifically point to the size and volume of the GZW, which are more comparable to grounding-line features marking LGM-positions around Antarctica (e.g. in Prydz Bay), and less similar to GZWs deposited during episodic ice-stream retreat (e.g. in Pine Island Trough and Marguerite Trough). In order to test these two formation scenarios for the GZW and constrain the timing and duration of its formation, we will try to obtain additional ages from seasonal-open marine units in cores seaward of the GZW. This will help to decide whether LGM-or older ice was grounded in the basin directly seaward of the GZW.
We will present preliminary interpretations of these data, which will aim to resolve the extent of the WAIS in this sector, provide new information on GZW formation in this trough and, at the same time, contribute an update on the dynamics of West Antarctic palaeo-ice streams, which drained the former ice sheet.
Repository Name:
EPIC Alfred Wegener Institut
Type:
Conference
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notRev
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