Assessing the role of oceanic forcing in West Antarctic Ice Sheet retreat since the Last Glacial Maximum

如果政府要在全球变暖的情况下为未来做好规划，那么对海平面上升的准确预测至关重要。对于伦敦和英国其他低洼地区来说，知道海平面何时上升以及上升多少将决定何时升级泰晤士河屏障等昂贵的基础设施。政府间气候变化专门委员会已确定冰盖迅速融化是海平面加速上升的主要原因，并指出西南极冰盖的崩溃将导致海平面以远高于目前预测的速度上升，了解流入西南极冰盖阿蒙森海部分的冰川的行为是这种预测准确性的关键。它们占南极西部冰盖总流量的三分之一，目前正在以显著和加速的速度造成海平面上升。人们普遍认为，这一地区冰川融化的增加是由温暖的海水入侵造成的，称为环极深水（CDW）。这些温暖的水流到大陆架上，并在冰川的漂浮部分下方融化冰川冰。测量表明，在过去十年中，阿蒙森海的CDW的温度和体积都有所增加，这与冰川融化和海平面上升的增加相吻合。我们还知道，CDW到达该地区受到海洋天气系统的影响，这意味着CDW对大气条件的变化很敏感。虽然现在已经确定了温水推动冰川退缩的观点，但尚不清楚CDW的体积和温度在较长时间尺度上如何变化（这也是限制我们预测未来变化的一个因素）。目前的预测冰盖模型假设阿蒙森海的冰川融化将在未来保持或增加。然而，由于阿蒙森海只有20年的海洋温度数据，很难确认模型是否准确。考虑到该地区的冰损失率以及对全球海防规划的影响，我们有必要了解CDW入侵的长期历史，以及我们今天看到的海洋温度是独特的还是在过去有很大的变化。这项研究将通过重建过去25,000年来阿蒙森海的海洋温度及其与过去冰盖退缩的关系来直接解决这种知识的缺乏。为了实现这一目标，我们将采用两种独立的方法来重建过去的海洋温度，从阿蒙森海的海洋沉积物岩心。第一种方法使用特定的有机残留物（来自生活在表面沃茨的海洋微生物），而第二种方法使用沉积物中发现的钙质贝壳的化学成分。使用这些不同的技术，我们将能够重建表面，次表面和深水温度，并将其与过去25，000年来冰盖退缩的日期记录进行比较。如果我们的研究结果表明，过去的冰盖退缩与温暖的海洋温度相吻合，那么我们就可以量化CDW入侵和冰盖退缩之间的关系。这可能意味着现代变化是长期“轨迹”的一部分，需要纳入预测模型。另一方面，如果冰盖退缩的时间与温水的存在不一致，或者CDW的入侵在过去有很大的不同，那么这也将对未来的预测产生重大影响。最终，我们的数据将有助于支持下一代冰盖模型，反过来，经过充分验证的冰盖模型将能够更好地预测未来的海平面上升。总的来说，这个项目将大大提高我们对冰盖对温水入侵的敏感性的理解。

Accurate predictions of sea-level rise are critical if governments are to plan for the future in a warming world. For London and other low-lying parts of the UK, knowing when and by how much sea level will rise will determine when costly infrastructure like the Thames Barrier is upgraded. The Intergovernmental Panel on Climate Change has identified rapidly melting ice sheets as the main source of accelerating sea level rise and stated that collapse of the West Antarctic Ice Sheet will cause sea-level to rise at rates much higher than currently predicted.Understanding the behaviour of glaciers flowing into the Amundsen Sea sector of the West Antarctic Ice Sheet is key to the accuracy of such predictions. They represent one-third of the total discharge of the West Antarctic Ice Sheet and are currently contributing to sea level rise at a significant and accelerating rate. It is widely understood that increased glacier melting in this region is driven by incursions of warm ocean water, called Circumpolar Deep Water (CDW). This warm water flows onto the continental shelf and beneath the floating parts of the glaciers where it melts the glacier ice. Measurements have shown that the temperature and volume of CDW in the Amundsen Sea has increased during the past decade, which has coincided with increased glacier melting and sea level rise. We also know that the arrival of CDW to the area is affected by the weather systems over the ocean which means that CDW is sensitive to changes in atmospheric conditions. Although the idea that warm water is driving glacier retreat is now firmly established, it is unclear (and a factor limiting our ability to predict future changes) how the volume and temperature of CDW has varied over longer timescales. The current generation of predictive ice sheet models assume that melting of the glaciers in the Amundsen Sea will be maintained or increase in future. However with only two decades of ocean temperature data from the Amundsen Sea it is difficult to confirm whether the models are accurate. Given the rate of ice loss in this area and the implications for sea defence planning worldwide, there is a fundamental need to understand the long-term history of CDW incursion and whether the ocean temperatures we see today are unique or have varied substantially in the past. This research will directly address this lack of knowledge by reconstructing ocean temperature in the Amundsen Sea over the past 25,000 years and its relationship to past ice sheet retreat. To achieve this we will apply two independent methods to reconstruct past ocean temperatures from well-dated marine sediment cores from the Amundsen Sea. The first method uses specific organic remains (from marine microbes that live in the surface waters) whilst the second method uses the chemical composition of calcareous shells found in the sediments. Using these different techniques we will be able to reconstruct surface, sub-surface and deep water temperatures and compare them to well-dated records of ice sheet retreat over the past 25,000 years. If our results show that past ice sheet retreats coincided with warm ocean temperatures, then we can quantify the relationship between incursions of CDW and ice sheet retreat. One implication of this could be that modern changes are part of a long term 'trajectory' that needs to be incorporated into predictive models. On the other hand, if the timing of ice sheet retreat did not coincide with the presence of warm water, or that incursions of CDW has varied substantially in the past then this would also have significant implications for future predictions. Ultimately our data will help underpin the next generation of ice sheet models and in turn, well-validated ice sheet models will be able to better predict future sea-level rise. Overall this project will deliver significant improvements in our understanding of the sensitivity of ice sheets to incursions of warm water.