ABSTRACT: Freshwater forcing of abrupt climate change during the last deglaciation.

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Abstract

Using the well-preserved geologic record of the last deglaciation of the Laurentide ice sheet (LIS), combined with ice-sheet and climate model results, we developed new time series of continental runoff from North America to the North Atlantic and Arctic Oceans. Runoff is partitioned among 19 drainage basins and routed to one of five injections sites to the North Atlantic and Arctic Oceans. Our routing scheme, which is constrained by over 150 radiocarbon ages, identifies those times during the last deglaciation when runoff from any one of the basins was rerouted from one injection site to another by ice-margin oscillations, isostatic uplift, or eventual collapse of the ice-sheet interior over Hudson Bay. Our geologically based time series indicates that total runoff from North America to the North Atlantic and Arctic Oceans during the last deglaciation (21-8 ka) remained nearly constant at about 0.3 Sv (1 Sv = 10 6 m 3 s -1 ), decreasing to modern-day rates (0.19 Sv) soon after final collapse of the Laurentide ice sheet. In contrast, large and rapid changes in freshwater fluxes to individual injection sites occurred as ice-margin fluctuations rerouted runoff from one site to another. Most major rerouting events of the last deglaciation occurred along the southern margin of the LIS and involved changes in the freshwater fluxes through the Mississippi River versus to the Hudson or St. Lawrence Rivers.Only late in deglaciation did significant routing-controlled changes in runoff occur through more northerly outlets. Our results suggest that the routing changes of North American runoff, combined with additional abrupt increases in freshwater fluxes to the North Atlantic from Heinrich events 1 and 0 and from late-glacial drainage of proglacial lakes along the southern margin of the Fennoscandian ice sheet, can explain all of the abrupt climate oscillations of the last deglaciation.

Bibliography of Canadian Geomorphology