Summary

During the d├ęglaciation of North America after the Last Glacial Maximum, the position of the southern margin of the Laurentide Ice Sheet (LIS) was characterized by millennial-scale oscillations (northward retreat followed by southward advance and subsequent retreat). These oscillations, related to ice-lake/ocean-atmosphere interaction, together with differential isostatic rebound and palaeotopography, caused the drainage of North American runoff to be periodically rerouted to different oceans during deglaciation. Oscillations of the ice margin in the Great Lakes basin between 16.5 and 11.5ka (19,700 and13,500cal. yr) led to at least two switches of ice melt and precipitation runoff between the GulfofMexico and North Atlantic Ocean.After 11.7 ka (13,600ka), ice margin oscillations in the Agassiz basin caused switches of drainage among routes to the Gulf of Mexico, North Atlantic and Arctic oceans, ending with a final large discharge to Hudson Bay and Hudson Strait about 7.7ka (8450cal.yr). Normal baseline runoff, due to precipitation and melting of the LIS, was supplemented on numerous occasions by the release of stored water in ice-marginal lakes, including the proglacial Great Lakes and Lake Agassiz. The timing, magnitude, duration and location ofthese catastrophic releases, and the associated baseline freshwater flows appear to have influenced North Atlantic thermohaline circulation and associated climatic excursions, namely as related to the Younger Dryas, Preboreal Oscillation, and 8.2cal. yr cold events recorded over large areas of the circum-Atlantic region.

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