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Abstract The West Antarctic Ice Sheet (WAIS) is thinning at an accelerating rate, driven by melting at its margins by warm Circumpolar Deep Water (CDW). However, this understanding is largely based on observations from recent decades, leaving the long-term influence of ocean temperature on WAIS stability uncertain. Here we reconstruct bottom water temperatures and water mass properties over the past 18 kyr using benthic foraminiferal Mg/Ca and δ¹³C records from sediment cores in the Amundsen Sea. Our data indicate that warm CDW occupied the continental shelf between ~ 18.0 and 10.1 kyr BP, coincident with major WAIS retreat from the shelf break to near its present-day grounding-line position along the Marie Byrd Land coast. Bottom waters cooled after ~ 10.1 kyr BP and remained relatively stable thereafter, with no evidence for substantial grounding-line migration. Continued atmospheric warming across West Antarctica until a mid-Holocene thermal maximum (~6–3 kyr BP) without further retreat indicates that ocean heat was the primary driver of WAIS variability since the Last Glacial Maximum. 

Abstract. Benthic foraminiferal assemblages are useful tools for paleoenvironmental studies but rely on the calibration of live populations to modern environmental conditions to allow interpretation of this proxy downcore. In regions such as the region offshore of Thwaites Glacier, where relatively warm Circumpolar Deep Water is driving melt at the glacier margin, it is especially important to have calibrated tracers of different environmental settings. However, Thwaites Glacier is difficult to access, and therefore there is a paucity of data on foraminiferal populations. In sediment samples with in situ bottom-water data collected during the austral summer of 2019, we find two live foraminiferal populations, which we refer to as the Epistominella cf. exigua population and the Miliammina arenacea population, which appear to be controlled by oceanographic and sea ice conditions. Furthermore, we examined the total foraminiferal assemblage (i.e., living plus dead) and found that the presence of Circumpolar Deep Water apparently influences the calcite compensation depth. We also find signals of retreat of the Thwaites Glacier Tongue from the low proportion of live foraminifera in the total assemblages closest to the ice margin. The combined live and dead foraminiferal assemblages, along with their environmental conditions and calcite preservation potential, provide a critical tool for reconstructing paleoenvironmental changes in ice-proximal settings.