Reliable dating of the postglacial ice retreat in West Antarctica by an integrated palaeomagnetic and radiocarbon approach

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Major environmental change associated with current global warming represents one of the biggest challenges for mankind during the 21st century. For example, ice-sheet melting in West Antarctica contributes ~11% to the present global sea-level rise of ~1.8 mm per year. A complete melting of the West Antarctic Ice Sheet (WAIS), which is likely to take several hundreds or thousands of years, would raise global sea level by ~5 m. This project aims to study the part of the WAIS that is currently showing the most dramatic indications for ice-sheet melting: the Amundsen Sea sector. Global sea level would rise by additional ~1.4 m, if the ice located in the Amundsen Sea sector would melt completely, which is possible to occur within the next two centuries. This melting would cause devastating flooding in many low-lying cities (e.g. New Orleans, London), agricultural areas (e.g. Netherlands, Bangladesh) and atolls (e.g. Maldives), with immense social, economic and ecological consequences. Until now, however, it is unclear if the melting of the WAIS has been occurring for some time and might continue long into the future, or if it is a relatively short-lived phenomenon. It is also unclear if the present WAIS melting is the result of climate warming, which has been ongoing since the end of the last ice age (~12,000 years ago), or of the mainly man-made greenhouse effect, which has influenced global climate for the last 150 years. Therefore, it is essential to study the history of the WAIS since the last ice age, which is the main scientific objective of the proposed project. In the proposed project the history of WAIS will be reconstructed by studying the sediments that make up the sea floor on the continental shelf that surrounds West Antarctica. These sediments are collected by coring several metres down into the seabed. Work that has already been done on the cores selected for this project shows that the WAIS extended far onto the shelf during the last ice age and retreated since then. The project will attempt to determine the timescale and speed of this retreat by the reliable dating of sediments in several cores. The timing of the ice retreat from a particular core site can be determined by radiocarbon dating of the organic remains of planktonic micro-organisms in the sediments. These organisms lived in the ocean waters that flooded onto the shelf as soon as the ice retreated. When the organisms died, their remains were deposited on the seafloor. Thus, their age reveals when the sediment was formed. Scientists have previously tried to date WAIS retreat from the shelf, but many of their dates are uncertain. This is because the marine sediments contain not only the remains from organisms alive shortly before sediment formation, but also much older, fossil organic remains. These remains were eroded from old sedimentary rocks in the Antarctic hinterland by the ice sheet. Glaciers transported the fossil remains and other detritus to the coast and released them into the sea, where they were deposited together with the remains of those planktonic organisms, which just have died. The fossil organic remains increase the apparent radiocarbon age of a sediment sample. The project will use an improved radiocarbon dating technique, which will overcome this problem and provide more accurate radiocarbon ages. Additionally, the project will apply another (independent) technique for dating the sediments. The intensity of the earth's magnetic field changed in a well-known pattern throughout the last 12,000 years. The project will use sophisticated methods for determining the relative variations of the magnetic intensity of the sediments: these reflect the global magnetic intensity variations. The relative magnetic intensity variations of the sediment cores can then be used to determine their age, and thus the time of ice retreat. The results of the proposed project will aid to a much more accurate prediction of future sea-level rise.