Retreat of the Western Cordilleran Ice Sheet Margin During the Last Deglaciation

The timing of Cordilleran Ice Sheet deglaciation along the central coast of British Columbia, Canada, informs climate forcing and early human migration. Thirty-two Be-10 exposure ages from glacial erratics and local bedrock on the western margin of the former ice sheet represent the earliest exposure ages for the Cordilleran Ice Sheet during the last deglaciation. These data show the western ice margin was retreating by 18.1 +/- 0.2 ka, consistent with the global record of ice mass loss. In contrast, parts of the southern margin reached a maximum at ca. 17.6-16.6 ka, and our data demonstrate a diachronous response of the Cordilleran Ice Sheet during deglaciation. We also show that a low altitude site was exposed by at least 17.7 +/- 0.3 ka, implying that numerous ice-free areas existed along the coastal margin by this time, providing a viable route for the first humans entering the Americas. Plain Language Summary Large ice sheets have advanced and retreated multiple times over the last several hundred thousand years in response to cyclical changes in incoming solar radiation. The Cordilleran Ice Sheet, which once covered much of western Canada, was thought to have advanced and retreated later than others during its last major cycle. The retreat of this ice sheet also opened routes that allowed the first people to enter the Americas. We show that the western margin of the ice sheet retreated earlier than previously thought. Other margins of the ice sheet advanced later, creating a complex picture through time. The early retreat of the western ice margin exposed numerous islands that could have been used by early people migrating southward.