ENHANCING THE DEGLACIATION CHRONOLOGY OF WISCONSIN USING IN-SITU COSMOGENIC RADIONUCLIDE

During the late Wisconsin Glaciation, the Laurentide Ice Sheet (LIS) advanced across much of the northern United States. Synchronous with a rise in summer insolation at the end of the Last Glacial Maximum, ice retreated from its southernmost extent in Illinois, Indiana, and Ohio shortly after ~23 ka, with significant retreat underway by ~20 ka. These dates are synchronous with the onset of LIS retreat over New England, suggesting that the timing of retreat was consistent for much of the LIS. While the geologic record and geomorphology of Wisconsin glaciation has been widely studied, the deglacial chronology is poorly constrained. Traditional radiocarbon dating methods have been applied to the region, but such minimum limiting dates do not adequately assess the timing of deglaciation onset because of uncertainties in the time-lag of the arrival of vegetation sometime after ice retreat. Surface exposure dating using in-situ cosmogenic radionuclide offers a way around these confounding limitations. However, use of this technique for the Wisconsin deglacial chronology has been limited to only 4 bedrock cosmogenic dates from well within the ice margin. The mean of these 4 dates indicate that the retreat of the Green Bay Lobe was underway sometime prior to ~20 ka, but unfortunately these dates are far from the terminal ice margin and do not adequately date the onset of retreat. We have collected new boulder samples from at or near the terminal ice extent of the Chippewa, Langlade and Green Bay Lobes. These preliminary cosmogenic beryllium-10 dates better constrain the overall onset of deglaciation in Wisconsin to be at ~22 ka, implying much earlier retreat than indicated by minimum-limiting radiocarbon dates. This timing of retreat onset is consistent with other data from the LIS southern margin and argues for an insolation trigger of ice sheet retreat.