A thinner-than-present West Antarctic Ice Sheet in the southern Weddell Sea Embayment during the Holocene

Abstract. Making accurate measurements and predictions of the West Antarctic Ice Sheet’s (WAIS) contribution to present and future sea-level rise fundamentally depends on knowing its trajectory over the last few thousand years. We present new in situ ¹⁴C concentrations from subglacial bedrock cores collected from the southern Weddell Sea sector of the WAIS. Critically, these concentrations are above levels that can be produced under present-day ice thicknesses at the core sites. The cosmogenic nuclide inventories provide clear evidence for the ice sheet being thinner-than present at some point during the Holocene following initial thinning from its Last Glacial Maximum configuration. Forward modelling of nuclide concentrations indicates that the nuclide depth-profiles within our cores are best explained by a 500–3500 year period of (near) total exposure that has occurred since 6–4 ka. We suggest that thinning at our core sites is most likely to reflect a regional, dynamic response to grounding-line retreat rather than a localised change in ice-surface elevation. Our data are the first direct geological evidence for a thinner-than-present WAIS in the Weddell Sea sector and are consistent with Holocene retreat that culminated inboard of present-day limits. Glacio-isostatic adjustment has been inferred as a driving mechanism, causing re-grounding of floating ice and increased buttressing allowing the grounding line to stabilise and readvance. These data allow dynamic retreat-readvance behaviour of this nature to be tested in ice-sheet models, improving predictions of future sea-level rise in this critical sector of West Antarctica.