The last deglaciation was abruptly interrupted by a millennial-scale reversal to glacial conditions¹, the Younger Dryas cold event. This cold interval has been connected to a decrease in the rate of North Atlantic Deep Water formation and to a resulting weakening of the meridional overturning circulation^{2, 3, 4} owing to surface water freshening. In contrast, an earlier input of fresh water (meltwater pulse 1a), whose origin is disputed^{5, 6}, apparently did not lead to a reduction of the meridional overturning circulation⁴. Here we analyse an ensemble of simulations of the drainage chronology of the North American ice sheet in order to identify the geographical release points of freshwater forcing during deglaciation. According to the simulations with our calibrated glacial systems model, the North American ice sheet contributed about half the fresh water of meltwater pulse 1a. During the onset of the Younger Dryas, we find that the largest combined meltwater/iceberg discharge was directed into the Arctic Ocean. Given that the only drainage outlet from the Arctic Ocean was via the Fram Strait into the Greenland−Iceland−Norwegian seas⁷, where North Atlantic Deep Water is formed today, we hypothesize that it was this Arctic freshwater flux that triggered the Younger Dryas cold reversal.

1. Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7

Correspondence to: Lev Tarasov¹ Correspondence and requests for materials should be addressed to L.T. (Email: lev@atmosp.physics.utoronto.ca).

Received 5 October 2004; Accepted 1 April 2005

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