LATE QUATERNARY FLUCTUATIONS OF THE EAST ANTARCTIC ICE SHEET IN THE FRAMNES MOUNTAINS

An understanding of past changes in the East Antarctic Ice Sheet is essential for predicting future changes and sea level. The ice sheet has a response time of thousands of years and has not fully adjusted to late Quaternary and Holocene climatic and sea level changes. Ice sheet models are the best tools for understanding time-dependent, continental-scale ice fluctuations. However, the models must explain past changes before they can be considered reliable. Unfortunately, terrestrial field evidence is geographically sparse, and provides a weak horizontal and poorer vertical constraint of past ice extent. Here we present new data from the Framnes Mountains, that stretch from the coast near Mawson Station, to ~50 km inland. 10Be and 26Al analyses were performed on subglacially rounded erratic boulders from summits up to 500 m above the present day ice sheet surface, thus providing a direct constraint on ice volume changes through time. Three former ice levels are identified – 1) Glacial erratics from the last local glaciation show ice sheet thickening of 300-400 m, leaving some summits ice-free throughout this period. 2) Moraines ~200 m above the contemporary ice mark a break in retreat or perhaps a local readvance, and 3) Boulders deposited recently within 100 metres of the present ice sheet indicate relative stability of the ice edge. The cosmogenic dates, due in late 2003, will reveal the timing of these events. If the landforms date from the last global glaciation, our evidence will indicate that the c. 1000 m of ice thickening suggested by some ice sheet models did not occur in this area. Models must address these issues before they are used to assess the future stability of the ice sheet in a warmer world.