First evidence and dating of Glacial Lake Yukon using paleomagnetic dating and cosmogenic nuclides

Yukon Territory has been repeatedly affected by the northern Cordilleran Ice Sheet during the last 2.6 million years, which has significantly affected the landscape. Yukon is unique in Canada in that it has three broad mappable chrono-geomorphic regions representing regionally coherent advances of the northern Cordilleran Ice Sheet and, unlike all other parts of Canada, a large unglaciated area. The oldest surface is a composite of several glaciations that are so old individual limits cannot be resolved. The oldest of these glaciations has occurred 2.6 million years ago and is believed to be responsible for shifting the route of the Yukon River.Geological evidence suggests that the Yukon River, which now flows in northern/western direction into the Bering Sea, was initially flowing south into southwest Yukon and west into the Tanana River basin. Reversal of the Yukon River is believed to be a consequence of the onset of Northern Cordillera glaciations at the beginning of the Quaternary period. Glaciation of the St. Elias Mountains blocked the passage of the paleo-Yukon River and formed a glacially dammed lake. This lake covered an extensive area in central Yukon and then catastrophically drained after overtopping a threshold north of Dawson City. This formed the present route of the Yukon River flowing to the Bering Sea. The presence and timing of the formation of Glacial Lake Yukon has been subject of debate for several decades. However, this hypothesis was widely accepted despite the absence of physical evidence for the glacial lake.The first physical evidence of Glacial Lake Yukon was discovered in 2022 when a succession of lake sediments was exposed in a placer mining operation in the Bonanza Creek Valley (tributary of the Klondike River) at the Lovett Hill site. The sampled section comprises more than 8 m of clays, silts and sands. This section is underlain by the Pliocene ‘White Channel gravel’, and the Quaternary Klondike outwash, with the latter representing first evidence of Quaternary glaciation in the Yukon. The lake sediment succession is overtopped by an erosive gravel unit, which likely marks the drainage of the lake.In order to refine the regional glacial stratigraphy, we utilize a multidisciplinary approach to provide chronological control on the formation of Glacial Lake Yukon. This allows us to test the hypothesis that the reversal of the Yukon River and the formation of Glacial Lake Yukon are associated with the first large Cordilleran Ice Sheet. We combine paleomagnetic measurements and cosmogenic 26Al–10Be isochron dating. In addition, we test the utility of cosmogenic krypton on zircon grains in this setting. Initial paleomagnetic data indicate the lake sequence is reversely magnetized; this combined with a previous burial age of ca. 2.6 Ma for the initiation of Klondike outwash deposition, suggests deposition of the lake sediments during the Matuyama Chron (0.78 to 2.6 Ma). Cosmogenic nuclide data will further refine this age.

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