The clay mineral footprint of the West Antarctic Ice Sheet in the Amundsen Sea during the last 6 Ma

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Computer-model based predictions of future sea-level rise caused by the melting of polar ice sheets in response to global warming suffer from large uncertainties arising from limited knowledge of ice-sheet stability. Our proposed research directly addresses this problem by investigating the vulnerability of the West Antarctic Ice Sheet (WAIS) and contributes to a more reliable risk estimation of its future rapid melting ("collapse"). The bed of the WAIS is largely grounded below sea level and slopes towards the interior of the Antarctic continent, making it prone to melting under warm climatic conditions. Current rapid ice loss observed in the Amundsen Sea sector of the WAIS, especially of Thwaites Glacier, which is a major conduit of ice flow within the WAIS, may mark the onset of its collapse over the next few centuries, resulting in dramatic sea level rise over the coming decades. A total WAIS collapse would raise global sea level by 3.3-5 metres and cause flooding of low-lying coastal areas worldwide, with devastating environmental, economic and societal consequences.Our project will investigate changes in the size of the WAIS during the last six million years by investigating very fine grains that its glaciers had scraped off the underlying bedrock, before transporting them to the Antarctic coast. Here, the grains were released into the ocean and then transported further offshore before they were finally deposited on the seafloor of the Amundsen Sea. Our main objective is to find evidence, whether, when and under which climatic conditions the WAIS collapsed in the past. Some reconstructions of past global sea levels, computer-model simulations and a few geological data, suggest that the WAIS collapsed repeatedly during the past. However, all these findings are either conflicting, or do not provide the time of the last WAIS collapse, which makes it impossible to identify the climatic factors triggering its collapse. Moreover, new results suggest that the past sea-level highstands alternatively resulted from collapse of the Greenlandic Ice Sheet in combination with partial melting of the East Antarctic Ice Sheet, the much larger 'sibling' of the WAIS.We propose to analyse two unique, several hundred meters long sedimentary sequences, that were cored on the seabed of the Amundsen Sea during the International Ocean Discovery Program (IODP) cruise Exp379 in early 2019. Here, in the deep sea of the Amundsen Sea, ocean currents had accumulated fine-grained sediment particles into a feature several hundred metres high with a shape resembling that of a dune on a beach or in a desert. The main difference between this so-called "sediment drift" and a dune is that ocean bottom currents, rather than wind, transported and deposited the grains, which then built up the drift over millions of years, and the size of these particles is silt and clay rather than sand. We intend to investigate the mineralogical composition of the clay fraction of these drift sediments. Over previous years, we had mapped the clay mineralogical composition of modern seafloor sediments along the WAIS coast, which allowed us to 'fingerprint' the grains delivered to the ocean by the various glaciers, and to recognise that the particles supplied by Thwaites Glacier are characterised by a distinct fingerprint. Initial shipboard-analyses on the two Exp379 cores revealed that this fingerprint is often present in the drift sediments, but sometimes it is absent, indicating that Thwaites Glacier did not erode rocks during the times, when the corresponding sediments were deposited. This can only be explained by the absence of grounded ice in the area of the continent covered by the glacier today. As ice-sheet models suggest that any disappearance of Thwaites Glacier may initiate a total WAIS collapse, our intended study will not only answer the questions, how (in-)stable the WAIS really is, but also, whether Thwaites Glacier is the "Achilles' heel" of the WAIS.