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

由于对冰盖稳定性的认识有限，基于计算机模型的对全球变暖导致极地冰盖融化导致的未来海平面上升的预测存在很大的不确定性。我们提出的研究通过调查南极西部冰盖（WAIS）的脆弱性直接解决了这一问题，并有助于对其未来快速融化（“崩塌”）进行更可靠的风险估计。西洋带的床大部分位于海平面以下，并向南极大陆内部倾斜，这使得它在温暖的气候条件下容易融化。在西海地区的阿蒙森海地区，特别是作为西海地区冰流主要通道的斯韦茨冰川，目前观测到的冰的迅速融化，可能标志着西海地区在未来几个世纪内开始崩塌，导致未来几十年海平面急剧上升。西海系统的全面崩溃将使全球海平面上升3.3-5米，并导致全球低洼沿海地区的洪水泛滥，造成毁灭性的环境、经济和社会后果。我们的项目将通过调查冰川从基岩上刮下的非常细的颗粒，在将它们运送到南极海岸之前，来调查过去600万年里WAIS大小的变化。在这里，颗粒被释放到海洋中，然后被运送到更远的近海，最后沉积在阿蒙森海的海底。我们的主要目标是找到证据，证明过去WAIS是否、何时以及在何种气候条件下崩溃。对过去全球海平面的一些重建、计算机模型模拟和一些地质数据表明，WAIS在过去曾多次崩塌。然而，所有这些发现要么相互矛盾，要么没有提供最后一次WAIS崩溃的时间，这使得确定引发其崩溃的气候因素成为不可能。此外，新的结果表明，过去的海平面高点可能是由格陵兰冰盖的崩塌和南极东部冰盖的部分融化共同造成的，南极东部冰盖是大得多的“兄弟”。我们建议分析两个独特的数百米长的沉积序列，这些序列是在2019年初国际海洋发现计划（IODP）游轮Exp379期间在阿蒙森海海底取芯的。在这里，在阿蒙森海的深海中，洋流将细粒度的沉积物颗粒堆积成几百米高的特征，形状类似于海滩上或沙漠中的沙丘。这种所谓的“沉积物漂移”和沙丘之间的主要区别在于，海底洋流（而不是风）运输和沉积了颗粒，这些颗粒在数百万年的时间里形成了漂移，这些颗粒的大小是淤泥和粘土，而不是沙子。我们打算研究这些漂流沉积物的粘土组分的矿物学组成。在过去的几年里，我们绘制了沿着斯韦茨海岸的现代海底沉积物的粘土矿物组成图，这使我们能够“指纹”各种冰川输送到海洋的颗粒，并认识到斯韦茨冰川提供的颗粒具有独特的指纹特征。船上对两个Exp379岩芯的初步分析显示，这种指纹经常出现在漂移沉积物中，但有时却不存在，这表明在相应沉积物沉积的时期，斯韦茨冰川没有侵蚀岩石。这只能解释为在冰川覆盖的大陆地区没有接地冰。由于冰盖模型表明，任何Thwaites冰川的消失都可能引发WAIS的全面崩塌，我们的研究不仅会回答这个问题，WAIS到底有多稳定，而且还会回答Thwaites冰川是否是WAIS的“阿喀琉斯之脚跟”。

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.