ABRUPT CHANGE IN ATMOSPHERIC CIRCULATION IN THE EARLY HOLOCENE INFERRED FROM A CALCITE OXYGEN-ISOTOPE RECORD FROM A MARL LAKE IN SOUTH-CENTRAL ALASKA

Climate change in high latitude regions has shown sensitive responses to broad-scale forcings at the present and in the past. Here we present multi-proxy data from a carbonate lake in the Matanuska Valley of south-central Alaska to investigate climate change of the last 15 ka (=15,000 cal BP). The tentative chronology of the sediment core was controlled by two AMS dates. Sediment lithology changes from marl at ~15-8 ka to gyttja at 8-0 ka. This marl-to-gyttja transition suggests a climate shift to cool and wet conditions, consistent with other regional records. The d¹⁸O record obtained from Pisidium mollusk shells shows a 4‰ negative shift from -11‰ at ~15-11 ka to -15‰ at 11-8 ka. This surprisingly large and dramatic shift in d¹⁸O values in the early Holocene suggests a major change in atmospheric circulation patterns, which has not been documented elsewhere in the region. Possible causes of this isotopic shift include a change in precipitation seasonality, a shift in regional wind directions, and a change in precipitation source regions modulated by retreating ice sheets/glaciers. A negative excursion in d¹⁸O of ~2‰ occurred at ~13-12 ka, similar in age to the Younger Dryas (YD) event of the North Atlantic region. Our data suggest that the climatic shift in the region during the early Holocene was more abrupt and in greater magnitude than the YD event, implying a differential regional response in south-central Alaska to large-scale climate forcing, possibly caused by the stronger regional feedback processes in high latitudes.