The extent of glaciation at the northern margin of the Canadian/Greenland high-latitude Arctic region over the past 30,000 years is uncertain. Geological arguments have been made for Greenland and Ellesmere Island ice sheets that coalesced to block the Nares Strait, and for restricted ice sheets on the two islands leaving the strait open, as it is today. Distinguishing between these two possibilities would provide significant constraints on present understanding of the past circulation between the Arctic and Atlantic oceans, on estimates of past ice-volume, and on the response of the Greenland ice sheet to climate change. Radiocarbon analyses provide dates for the deglaciation of the islands' coasts, but do not yield information on whether ice filled the strait. Here we present measurements of cosmogenic 36Cl that has accumulated in situ in erratics and glacially polished bedrock on islands within the Nares Strait. These data allow us to determine the time for which the rocks have been recently exposed to the atmosphere, and thus the age of the final deglaciation of the strait. We show that Greenland and Ellesmere ice sheets retreated from the Nares Strait about 10,000 years ago. The strait was filled with ice during the last glaciation, blocking this connection between the Arctic and Atlantic oceans, and supporting the model of extensive and long-lasting ice on land and sea in this region.
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