Cosmogenic-nuclide exposure ages from the Pensacola Mountains adjacent to the Foundation Ice Stream, Antarctica

Abstract

We describe glacial-geological observations and cosmogenic-nuclide exposure ages from the Schmidt, Williams, and Thomas Hills in the Pensacola Mountains of Antarctica adjacent to the Foundation Ice Stream (FIS). Our aim is to learn about changes in the thickness and grounding line position of the Antarctic Ice Sheet in the Weddell Sea embayment between the Last Glacial Maximum (LGM) and the present. Glacial-geological observations from all three regions indicate that currently-ice-free areas were covered by ice during one or more past ice sheet expansions, and that this ice was typically frozen to its bed and thus non-erosive, permitting the accumulation of multiple generations of glacial drift. Cosmogenic-nuclide exposure-age data from glacially transported erratics are consistent with this interpretation in that we observe both (i) samples with Holocene exposure ages that display a systematic age-elevation relationship recording LGM-to-present deglaciation, and (ii) samples with older and highly scattered apparent exposure ages that were deposited in previous glacial-interglacial cycles and have experienced multiple periods of surface exposure and ice cover. Holocene exposure ages at the Thomas and Williams Hills, upstream of the present grounding line of the FIS, show that the FIS was at least 500 m thicker prior to 11 ka, and that 500 m of thinning took place between 11 and 4 ka. However, exposure-age data from the Schmidt Hills, downstream of the present grounding line of the FIS, show no evidence for LGM thickening of the FIS and, in fact, provide some evidence that the FIS could have been no more than 200 m thicker than present at the LGM. If all these observations are correct, they imply that the LGM and early Holocene ice surface slope in the vicinity of the present grounding line was steeper than present, which is inconsistent with glaciological model predictions of possible LGM ice sheet configurations. Specifically, scenarios in which the LGM grounding line of the FIS advanced to the outer continental shelf appear inconsistent with exposure-age data from the Schmidt Hills, whereas scenarios in which the FIS grounding line did not advance at the LGM appear inconsistent with exposure-age data from the Williams and Thomas Hills.