Reconstructing the Pleistocene history of Antarctic Ice Sheet mass loss

今天，地球的两个半球的极地地区都是巨大的大陆冰盖的家园，但大多数科学家认为情况并不总是如此。这些冰盖是什么时候形成的，为什么形成的？一旦确定，它们的稳定性如何，特别是在比今天更温暖的时候，我们可能会认为它们会融化并上升海平面？这些是我们可以通过从海底收集的沉积物岩芯生成过去气候变化的记录来回答的一些问题。在这项研究中，我们的动机特别是探索南极冰盖(AIS)保存下来的过去在南大洋沉积的海洋沉积物的历史可以告诉我们，它可能容易受到持续的人类诱导的全球变暖的影响。AIS由相当于全球海平面约58米的冰组成，如果这些冰中只有一小部分以水的形式返回海洋，将对生活在世界各地低洼沿海社区的数亿人造成重大后果。我们可以用来追踪AIS过去变化的技术之一是计算被冰川从大陆上剥离并通过冰山崩解和漂移到海底的沙子大小的岩石碎片和矿物，即所谓的冰筏碎片(IRD)。在南大洋充满IRD的海洋记录中，层层显示AIS在过去经历了显著的冰块质量损失。我们可以分析IRD的地球化学和它的沉积学，以确定导致它们沉积的冰山来自南极洲的哪些部分。因此，确定南极沉积物中IRD的来源有助于我们了解AIS的哪些部分是过去地质历史上海平面上升的原因。过去感兴趣的间隔包括：(1)由于全球变暖和大陆冰量变化而导致海平面迅速上升的时期，在过去50万年的晚更新世消融期间，海平面每隔约10万年迅速上升一次，超过约100米；(2)比现在温暖的间冰期气候，其特点是海平面高于今天，在更新世约260万年的历史中，这也是地球气候的常规特征。南大洋的大西洋部分被称为冰山小巷，因为这是今天从南极洲掉落的大多数冰山融化的地方。对10,000至22,000年前最近一次冰川消融期间在冰山小巷沉积的IRD的研究提供了强有力的证据，证明了AIS冰川消融质量损失的快速变化及其对此时海平面上升的贡献。然而，对于这种IRD的来源，或者关于AIS在过去50万年较早的冰川消融事件中的行为，我们一无所知。更少的研究试图确定AIS在比现在更温暖的更新世间冰期可能对海平面上升的贡献。过去，由于缺乏连续的、高分辨率的AIS历史记录和出色的年龄控制，我们解决这些未知问题的能力一直受到阻碍。然而，今年国际海洋发现计划在382次远征期间在冰山小巷的海洋沉积物中谨慎地应用了现代钻探方法，帮助解决了这个问题。这项研究将评估探险382岩芯解决上述问题的可能性，方法是首次记录冰山小巷在关键的晚更新世冰川消退和约120万年前的早更新世间冰期沉积的IRD的来源。

Today, the polar regions of both hemisphere of our planet are home to large continental ice-sheets, but most scientists think this has not always been the case. When and why did these ice sheets form? Once established, how stable were they, especially during times warmer than today when we might expect them to melt and raise sea level? These are some of the questions we can answer by generating records of past climate change from sediment cores collected from the seafloor. In this study, we are motivated in particular by exploring what the history of the Antarctic Ice Sheet (AIS) preserved marine sediments deposited in the Southern Ocean during the past can tell us about its likely vulnerability to ongoing human-induced global warming. The AIS is composed of ice equivalent to ~58 m of global sea-level and if only a small fraction of this ice was returned to the oceans as water there would be major consequences for hundreds of millions of persons living in low-lying coastal communities around the world. One of the techniques we can use to track past changes in the AIS is to count sand-sized rock fragments and minerals that have been scrapped off the continents by glaciers and delivered to the seafloor by calving and drifting of icebergs, known as ice-rafted debris (IRD). Layers in the marine record from the Southern Ocean packed full of IRD indicate times in the past when the AIS experienced significant ice-mass loss. We can analyse the geochemistry of that IRD and its sedimentology to work out which parts of Antarctica the icebergs responsible for their deposition came from. Identifying the sources of IRD in Antarctic sediments can therefore help us to understand which sectors of the AIS have been responsible for sea-level rise in the geological past.Past intervals of interest include: (1) times when sea level rose rapidly due to global warming and changes in continental ice-volume, as it has done so every ~100,000 years by over ~100 m during late Pleistocene deglaciations of the past half a million years and, (2) warmer-than-present interglacial climates characterised by sea level higher than today, which were also a regular feature of Earth's climate throughout the ~2.6 million year history of the Pleistocene.The Atlantic sector of the Southern Ocean is known as Iceberg Alley because this is where most icebergs shed from Antarctica today melt. Studies of IRD deposited in Iceberg Alley during the most recent deglaciation 10,000 to 22,000 years ago provide robust evidence for rapid variations in AIS deglacial mass-loss and its contribution to sea-level rise at this time. Nothing is known, however, about the sources of this IRD, or about the behaviour of the AIS during older deglacial events of the past half a million years. Fewer studies still have attempted to determine the contribution the AIS might have made to sea-level rise during warmer-than-present Pleistocene interglacials.Our ability to address these unknowns has been hampered in the past by the lack of continuous, high-resolution marine records of AIS history with excellent age control. However, careful application of modern drilling methods to marine sediments in Iceberg Alley this year by the International Ocean Discovery Program during Expedition 382 has helped to remedy this problem. This research will evaluate the potential for Expedition 382 cores to address the above issues by generating the first records of the provenance of IRD deposited in Iceberg Alley during key late Pleistocene deglaciations and an early Pleistocene interglacial, ~1.2 million years ago.