A new approach to West Antarctic Ice Sheet evolution using blue-ice moraines on nunataks

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Did the West Antarctic Ice Sheet (WAIS) survive the last interglacial? We propose to use nunataks as dipsticks of ice-sheet elevation change to help answer this question. There are currently two conflicting hypotheses. Hypothesis 1: A dynamic WAIS. The hypothesis is that the WAIS disappeared under last-interglacial conditions ~125,000 years ago when climatic and oceanic conditions were slightly warmer than those of the present day. Hypothesis 2: A stable WAIS. The WAIS may have varied in elevation but that it persisted as a coherent ice sheet during the last interglacial. The co-existence of two opposing hypotheses implies that we have much to learn about the principal controls on ice-sheet stability. This uncertainty undermines confidence in our ability to predict the future of the WAIS and its effect on global sea-level change. Important research on the WAIS relies on satellite observations which monitor changes in velocity and elevation over recent decades, while predictions of future changes rely on ice-sheet models. Both approaches would be enhanced if we knew what happened to the WAIS during the last glacial cycle. The longer term perspective tells of the trajectory of change upon which decadal changes are superimposed. Further, a history of elevation changes during a glacial cycle provides data with which to constrain and improve ice-sheet models. Here we propose to test the two hypotheses using moraines that form on nunataks in blue-ice areas. Blue-ice areas result from strong downslope winds which are often funnelled in the vicinity of nunataks and ablate the ice surface. In response the ice flows into such ablation areas, sometimes bringing basal debris to the surface which is then deposited at the ice margin. Relict moraines occur on certain nunatak slopes above the present ice surfaces and are over 400,000 years old, suggesting that there is the potential to obtain a long record of ice elevation change.This project brings together glaciologists, geomorphologists and geophysicists to work in the Heritage Range, a group of nunataks which protrude through the central WAIS dome. We will test predictions of the two competing hypotheses firstly by examining the processes of blue-ice moraine formation today using field survey and radar, and secondly by establishing the form and sediment characteristics of the moraines and their age. The latter will employ exposure-age dating, a technique that measures the time a rock has been at the surface and exposed to cosmic rays. By using more than one isotope we can establish times when a rock surface may have been buried by ice and thus there is the potential to reconstruct a rich history of ice elevation changes. In this way we will assess if the WAIS remained intact, or disappeared during the last interglacial. Our hope is that the approach could be extended to other nunataks in Antarctica and provide widely dispersed evidence of elevation changes in predicting the future response of the WAIS to a changing climate.