A new timeline for glacial retreat in Western Canada
Much of western Canada was ice-free as early as 14,000 years ago, a new study reports. The results suggest that the Cordilleran Ice Sheet (CIS) retreated more than a millennium sooner than previous estimates and hold important implications for understanding climate patterns and human migration. Previous estimates suggested that the CIS covered large portions of westernmost Canada as late as 12,500 years ago. Here, Brian Menounos and colleagues collected 76 samples from moraines, parallel ridges of debris deposited along the sides of a glacier, that formed after the CIS retreated. Using beryllium isotopes to date the samples, the authors found that the CIS may have in fact been largely melted by 14,000 years ago, while newer, smaller alpine glaciers appear to have sprung up in pockets after the mountain peaks were initially bared. Next, the authors used simulations to show how warm temperatures likely drove the retreat of the CIS, which alone contributed 2.5 to 3.0 meters of sea level rise between 14,500 and 14,000 years ago. The CIS response to abrupt climate change during the latest Pleistocene provides an analog for the behavior of the Greenland Ice Sheet, which is comparable in size, Menounos et al. say. They also suggest that the melting of the CIS had a substantial cooling effect in the Northern Hemisphere. Lastly, in terms of the peopling of the Americas, the authors suggest that their data does not support the migration of people down the west coast because much of the lower elevations across the area remained covered in ice until about 11,000 years ago. These results are highlighted in a Perspective by Shaun A. Marcott and Jeremy D. Shakun.
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