Unblocking of the Nares Strait by Greenland and Ellesmere ice-sheet retreat 10,000 years ago

The extent of glaciation at the northern margin of the Canadian/Greenland h igh-latitude Arctic region over the past 30,000 years is uncertain. Geologi cal arguments have been made for Greenland and Ellesmere Island ice sheets that coalesced to block the Nares Strait(1), and for restricted ice sheets on the two islands(2) leaving the strait open, as it is today(3). Distingui shing between these two possibilities would provide significant constraints on present understanding of the past circulation between the Arctic and At lantic oceans(4,5), on estimates of past ice-volume(6), and on the response of the Greenland ice sheet to climate change(7). Radiocarbon analyses prov ide dates for the deglaciation of the islands' coasts, but do not yield inf ormation on whether ice filled the strait. Here we present measurements of cosmogenic Cl-36 that has accumulated in situ in erratics and glacially pol ished bedrock on islands within the Nares Strait. These data allow us to de termine the time for which the rocks have been recently exposed to the atmo sphere, and thus the age of the final deglaciation of the strait. We show t hat Greenland and Ellesmere ice sheets retreated from the Nares Strait abou t 10,000 years ago. The strait was filled with ice during the last glaciati on, blocking this connection between the Arctic and Atlantic oceans, and su pporting the model of extensive and long-lasting ice on land and sea in thi s regions(8-11).