A thicker-than-present East Antarctic Ice Sheet plateau during the Last Glacial Maximum

Abstract. In this study, we present a surface exposure chronology of past ice thickness change derived from in situ cosmogenic-14C dating at a site in the Grove Mountains, located on the edge of the East Antarctic plateau, 380 km inland from the coastline in the Lambert Glacier–Amery Ice Shelf sector. At the Last Glacial Maximum (LGM), observations and models suggest that increased ice sheet volume was accommodated by thicker ice near the coast and grounding line advance towards the continental-shelf edge. In contrast, the ice sheet interior maintained a relatively stable thickness until present, with ice core evidence even suggesting thinner ice relative to today. However, the magnitude of these thickness changes and the transition point from thicker-than-present to thinner-than-present LGM ice are poorly constrained. Here, we reconstruct changes in the thickness of the East Antarctic Ice Sheet since the LGM at a nunatak in the Grove Mountains using in situ 14C, which circumvents the common issue of long-lived nuclide inheritance that leads to inaccurate records of LGM ice thickness. Samples between 1912 m above sea level (a.s.l.) and the modern ice margin (∼1825 m a.s.l.) yield 14C ages of 0.18-5.26 ka. Samples at and above 1912 m a.s.l. have saturated 14C concentrations, implying exposure of the nunatak summit through the LGM. We therefore place the LGM ice surface in the Grove Mountains ∼70 m higher than at present. The unsaturated samples below 1912 m a.s.l. indicate that gradual thinning began ∼16 ka, with some (25 %–45 %) post-LGM thinning recorded ∼16–11 ka and most (55 %–75 %) recorded during the Holocene. Ice sheet models that do not account for this thickness change would inaccurately characterize the LGM geometry of the EAIS and underestimate its contributions to deglacial sea level rise.