HOLOCENE HISTORY OF GEORGE VI ICE SHELF, ANTARCTIC PENINSULA: INFERENCES FROM LAKE SEDIMENTS

The recent collapse of several Antarctic Peninsula ice shelves has highlighted the need for a long-term perspective on ice shelf stability. An opportunity to obtain a highly detailed late-Quaternary history of one ice shelf is provided by Moutonnée Lake. Moutonnée Lake (ML) is a large, 50m deep epishelf lake impounded on the eastern side of Alexander Island by George VI Ice Shelf, which flows across George VI Sound from the Antarctic Peninsula. Detailed water chemistry measurements have shown ML to be clearly stratified. A distinct halocline occurs at 30m, where modern freshwater overlies marine water. Our hypothesis is that any changes in the stability of the ice shelf should leave distinct biological, chemical and lithological signatures in the lake sedimentary record. Biologically, ice shelf loss would see the present stratified water column replaced by a purely marine one. Significant changes in sedimentation are also likely to follow any change in the configuration of George VI Ice Shelf. Outcrops of igneous and metamorphic rocks with distinctive chemical and isotopic signatures are found on the western coast of the Antarctic Peninsula. These differ from the rocks of Alexander Island, which are predominantly sedimentary in origin. Restricted ranges of igneous and metamorphic clasts, transported through George VI Ice Shelf to Alexander Island, and deposited with locally derived sedimentary clasts, would be replaced, during periods of ice shelf loss, by a wide lithological assemblage dominated by ice-rafted debris and locally derived sedimentary clasts. To test this hypothesis, high-resolution studies have been performed on two sediment cores extracted from ML. Analyses have included physical (MS, LOI and CaCO3), biological (diatom and foraminifera) and isotopic measurements performed on both bulk sediment samples and individual foraminifera. Results demonstrate the existence of two distinct zones, where marine organisms (foraminifera and marine diatoms) and local sedimentary clasts dominate. Both zones coincide with elevated d13C values, interpreted here as enhanced marine activity. Taken together these data imply that George VI Sound has been free of the ice shelf on at least one occasion during the Holocene. Chronological control will be provided by 11 AMS 14C dates performed on individual foraminifera.