Cosmogenic Surface Exposure Ages for Laurentide Ice Sheet Deglaciation in the Western Slave CratonSunday, October 21, 2018 - 16:30 to 19:00 Multiplex Gym (DND)
The timing of northwest Laurentide ice sheet deglaciation is important for understanding how ice-sheet retreat, and associated meltwater discharge, may have been involved in abrupt climate change and rapid sea-level rise at the end of the last glaciation. However, the deglacial chronology across the western Canadian Shield is poorly understood, with only a handful of minimum-limiting radiocarbon dates constraining the timing and pattern of northwest Laurentide ice-sheet retreat. We use cosmogenic beryllium-10 surface exposure dating of glacial erratics, sampled opportunistically during bedrock mapping campaigns in the western Slave Craton, to directly date the timing of northwest Laurentide ice-sheet retreat during the last deglaciation. Five erratics sampled near the Acasta Gneiss “Discovery Site” have exposure ages between 12.8+/-0.6 and 12.2+/-0.6 thousand years ago (ka), with a weighted mean of 12.4+/-0.2 ka. Five erratics were also sampled 115 km to the east at Point Lake; four exposure ages are currently in-process, but one erratic from this site yielded an exposure age of 11.6+/-0.5 ka. When corrected for decreased atmospheric depth due to isostatic uplift since deglaciation, the data indicate that the Laurentide ice sheet retreated through this western part of the Slave Craton ~13.7-12.8 ka, or ~1000 years earlier than inferred from the canonical compilation of minimum-limiting radiocarbon dates for deglaciation. Additional exposure ages on glacial erratics across the Slave Craton will allow comprehensive testing of hypotheses related to northwest Laurentide ice-sheet retreat and its potential forcing of abrupt deglacial sea-level rise and climate change events.