Early deglaciation of the western sector of the Cordilleran Ice Sheet from cosmogenic nuclide exposure dating, central British Columbia coast

Reconstructing the retreat of the Cordilleran Ice Sheet (CIS) during the last deglaciation has implications for sea level variability and the peopling of the Americas, but requires robust glacial chronologies. The southern margin of the CIS advanced to its maximum extent at ca. 16 ka, but no directly dated terrestrial limits presently exist on the central coast to constrain the western ice margin prior to ca. 14 ka. Moreover, published marine sediment cores show intervals of ice-rafted debris interpreted to reflect an unstable, marine-terminating western margin that may have rapidly retreated during deglaciation. Here, we present data from two moraines on Calvert Island, British Columbia (51.6°N, 128.1°W) that demarcate the central-western edge of the CIS during deglaciation. The orientation of the moraines suggests that ice flowed southward along the Hugh Sound-Dean Channel fjord system, a major western outlet of the former ice sheet. Terrestrial cosmogenic 10Be nuclide dating of erratic boulders and bedrock associated with a prominent end moraine in the centre of the island yielded a mean weighted exposure age of 16.5 ± 0.3 ka (n=5). Erratic boulders from a second moraine approximately 8 km to the north and 600 m lower in elevation than the first yielded a weighted mean exposure age of 14.2 ± 0.5 ka (n=4). This latter age supports recently reported bracketing radiocarbon ages for stratigraphically equivalent till in a section exposed in the northwest part of the island. We are currently processing twenty-three further 10Be exposure ages from targeted erratics and bedrock to further constrain the timing and extent of deglaciation within this region. Our current data indicate that if the western CIS reached the outer continental shelf it did so prior to ca. 16.5 ka, and was in retreat by the time the southern margin reached its maximum extent. In addition, these new ages indicate that ice free areas existed along the western coast from at least 16.5 ka, providing further support for a potential coastal route for the first people entering into the Americas.