Reconstructing the Pleistocene history of Antarctic Ice Sheet mass loss

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.