New geological constraints on Holocene retreat-readvance of the West Antarctic Ice Sheet in the Weddell Sea Embayment

Predicting future change to the Antarctic Ice Sheets requires high quality data to constrain numerical ice sheet models. A major uncertainty stems from a lack of knowledge regarding the late Holocene trajectory of the West Antarctic Ice Sheet (WAIS). There are two hypotheses regarding the late Holocene behaviour of the WAIS. A) Steady retreat throughout the Holocene with stabilisation at or near the present-day position (ice relaxation hypothesis) or, B) retreat to a smaller-than-present configuration with subsequent readvance to the present-day position (the retreat-readvance hypothesis). The two hypotheses represent profoundly different ice sheet trajectories. These hypotheses have been discussed with particular reference to the Amundsen, Ross and Weddell Sea sectors of the WAIS.

Initial studies proposing the retreat-readvance model suggested that GIA related uplift caused re-grounding of ice rises in the Weddell Sea, increasing ice shelf buttressing and leading to grounding line re-advance. In the southern Weddell Sea major ice streams are currently at threshold positions on reverse bed slopes where they are vulnerable to Marine Ice Sheet/Cliff Instabilities. As this region drains ~22% of Antarctica the lack of geological constraint on the current ice sheet trajectory contributes significant uncertainty to future predictions. Any groundling line retreat beyond present day limits would be accompanied by up-stream ice sheet thinning thus retreat to a smaller-than-present configuration would be accompanied by thinning of the ice sheet surface below the present-day level. Consequently, determining whether sub-glacial rock samples from the Weddell Sea sector have been exposed in the recent past can robustly test for a smaller-than-present ice sheet configuration.

We present an update on two field seasons where, using a modified Winkie Drill, we recovered sub-glacial rock samples from the Ellsworth Mountains and Pensacola Mountains. These mountain ranges bracket the proposed zone of retreat and can thus provide limiting data points on the extent and duration of any retreat. The subglacial cores are to be analysed using in situ ¹⁴C and luminescence to test for any past exposure to cosmic rays and sunlight respectively. We will present a summary of the field season outcomes and preliminary analytical data along with initial interpretations.