EXPLORING THE DEGLACIATION OF WISCONSIN USING IN-SITU COSMOGENIC RADIONUCLIDES

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. These dates are also 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 glacial record and geomorphology of Wisconsin has been widely studied, the deglacial chronology is poorly constrained. Traditional radiocarbon dating methods have been applied extensively to the region, but such minimum limiting dates do not adequately asses 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 ~18 ka and suggest that ice retreat in Wisconsin may have lagged behind the other southern LIS lobes. To date, no cosmogenic dates are available from the terminal ice margin to adequately date the onset of retreat.

Here we present preliminary cosmogenic beryllium-10 boulder dates from at or near terminal ice extent and propose future sampling strategies/locations to better constrain the overall onset of deglaciation in Wisconsin. We aim to quantify the timing of LIS retreat in Wisconsin relative to other better-dated deglacial chronologies of the LIS.