DETERMINING THE AGE OF THE MOST RECENT ICE-FREE INTERVAL IN THE HUDSON BAY LOWLANDS, CANADA, USING OSL DATING: A SIGNIFICANT REDUCTION OF THE LAURENTIDE ICE SHEET DURING THE MID-WISCONSIN GLACIATION?

Understanding the position and paleogeography of past ice sheets is important for the validation of Earth System Models, which are used to predict future climates. Although we have a good understanding of the timing and extent of the most recent deglaciation, comparatively little is known about the extent of the Laurentide Ice Sheet (LIS) during the majority of the Wisconsin Episode (ca. 71,000 to ca. 11,000 yr BP). Models of ice volume suggest that the LIS was relatively stable and covered large parts of Canada for the majority of the Wisconsin Episode, however there is little land-based evidence from the previously glaciated region to confirm this. Our study area, the Hudson Bay Lowlands, lies near the center of growth for many Pleistocene ice sheets and contains an extensive stratigraphic record spanning several glacial/non-glacial cycles. Attempts to date the non-glacial units using radiocarbon dating suggest that this region may have been ice-free during Marine Isotope Stage 3 (MIS 3; ca. 29,000 to ca. 57,000 yr BP). However, because these radiocarbon dates lie close to the limit of reliability for that method, and these samples are typically found exposed along present-day river banks, there is a possibility that small amounts of modern-day carbon may have been incorporated into these samples, causing otherwise infinite ages to appear finite. Since an accurate chronology for these deposits has important implications for understanding ice sheet history, we used OSL dating to attempt to further strengthen the ages of these deposits. We developed new methods to filter aliquot data and correct for the ‘dim quartz’ issue, where young sediments derived from the fresh-eroded Canadian Shield typically yield very low light emissions under optical stimulation. Water-lain deposits at three sites yield OSL ages ranging from ca. 42,000 to ca. 52,000 yr BP, thus supporting the interpretation of an ice-free interval during MIS 3. Given the location of our study area near the center of growth for Pleistocene ice sheets, these age determinations are a challenge to the perceived stability of the LIS during the Wisconsin Episode. Work is ongoing at three additional sites.