10BE EXPOSURE AGES OF THE LAURENTIDE ICE SHEET RETREAT WEST OF LAKE SUPERIOR

Determining timing of Laurentide Ice Sheet (LIS) deglaciation in the region west of Lake Superior offers valuable insight to understanding climate conditions after the last glacial maximum. Multiple moraine sets from northern Minnesota to the southern shore of Lake Nipigon, southern Ontario, mark LIS marginal positions subsequent to its last glacial maximum extent. Minimum-limiting radiocarbon ages from a south-north transect of this region indicate deglaciation between ~17-11.5 ka (Lowell et al., 2009), from the time of Heinrich Event 1 to end of the Younger Dryas. We present ¹⁰Be ages along the same transect to offer a check on the radiocarbon chronology and to constrain further the timing of LIS deglaciation during this climatically complex time.

We collected ¹⁰Be samples from the Vermillion moraine in northwestern Minnesota and the Steep Rock, Brule, Hartman and Dog Lake moraines in southern Ontario. To test for the possible presence of ¹⁰Be from prior times of exposure, we also sampled from boulder bottoms at some sites. Thirty-two ¹⁰Be ages indicate progressive northward ice retreat and are consistent with the radiocarbon chronology. However, ¹⁰Be ages of individual landforms have a range of ~1000-2000 years, suggesting that post-depositional processes influence sample ages. Boulder bottom sample ¹⁰Be concentrations do not indicate the presence of ¹⁰Be inherited from prior exposure. We suggest that the oldest ¹⁰Be ages for a landform afford an age of deglaciation.

Five ¹⁰Be ages from the Vermillion moraine range from 16.7 ± 0.9 ka to 14.5 ± 0.8 ka and indicate the moraine was likely deposited by ~16.7 ka, which coincides with the timing of Heinrich Event 1 at ~16.8 ka (Hemming, 2004). This age agrees well with a nearby radiocarbon age of 14.7 ¹⁴C kyr [16.8-18.7 ka] (Lowell et al., 2009 and references therein). Six ¹⁰Be ages from the Brule and Steep Rock moraines indicate recession by ~15.8 ka, which is older than nearby radiocarbon ages of ~13 ka. Ten ¹⁰Be ages from the Hartman and Dog Lake moraines indicate ice retreat by ~12.6 ka. Eleven ¹⁰Be samples are in process and will contribute addition information for the LIS deglaciation chronology in this region. Together these ages will offer insights on the relationship between LIS mass balance and climatic events such as Heinrich Events and the Younger Dryas.