GLACIAL LAKE AGASSIZ: A 5000-YEAR HISTORY OF CHANGE

Lake Agassiz was the largest lake in North America during the last period of deglaciation, extending over a total of 1.5 million km² before it drained about 7.7 ¹⁴C ka (8.4 cal ka). New computer reconstructions, controlled by large beaches, isostatic rebound data, the margin of the Laurentide Ice Sheet, outlet elevations, and modern DEM topographic data, show how variable the size and depth of this lake were during its 4000 ¹⁴C yr (5000 cal yr) history. Abrupt draw downs of 8-110 m in the lake surface occurred on at least 18 occasions, when new outlets were opened, reducing the extent of the lake and sending large outbursts of water to the oceans. These outbursts abruptly added 1600-9500 km³ to the baseline overflow from the lake, and were routed to 3 different ocean basins: the Gulf of Mexico, Arctic, and North Atlantic. After each draw down, the lake transgressed back over the newly exposed lake floor, deepening the water everywhere to the south of the isobase that extended through the outlet. The final drainage of Lake Agassiz abruptly released 163,000 km³ into the Hudson Bay basin and the North Atlantic. Three of the largest outbursts correlate closely in time with large d¹⁸O excursions in the isotopic records of the Greenland ice cap, suggesting that those freshwaters impacted on thermohaline circulation and, in turn, on climate.