THE ROLE OF PERMAFROST ON THE MORPHOLOGY OF AN MIS 3 MORAINE FROM THE SOUTHERN LAURENTIDE ICE SHEET

The Laurentide Ice Sheet (LIS) represents the single largest contributor to global sea level during the late Pleistocene glacial-interglacial cycles. Fluctuations of global sea level prior to the Last Glacial Maximum (LGM; 26-19 ka) are well documented, but the terrestrial extent of the LIS prior to the LGM is uncertain. In central Wisconsin, three closely spaced end moraines of the LIS are preserved. The two innermost moraines are constrained to LGM-age through isotopic dating, yet the outermost moraine is estimated to be up to >125 ka based on relative dating techniques. Here we report surface exposure ages from this outermost moraine (the Arnott Moraine) and demonstrate that the LIS reached its most extensive position in central Wisconsin during Marine Isotope Stage (MIS) 3 at approximately 37 ka. These new age constraints for the Arnott Moraine are significantly younger than prior estimates, which were based on sediment weathering and a highly diffused moraine morphology. To address this dichotomy between the relatively young exposure dates and highly diffuse geomorphology, we apply a landscape diffusion model to the Arnott Moraine and find that during permafrost conditions intense weathering can act to efficiently and quickly smooth the older moraine surface. This study implicates the LIS as a potential source for the 10-30 m drop in sea level during MIS 3 while simultaneously demonstrating the inherent uncertainty associated with relative dating techniques if nonlinear weathering conditions are not accounted for in periglacial terrains.