LAKE AGASSIZ OVERFLOW DURING THE YOUNGER DRYAS

Channels west of Lake Superior have long been interpreted as carrying catastrophic overflow (0.3 Sv, 9500 km³ yr-1) from glacial Lake Agassiz into the Great Lakes-St. Lawrence and North Atlantic Ocean ~10.9-10 ¹⁴C ka. This eastward overflow has been implicated in triggering change in thermohaline circulation of the ocean and, in turn, cooling during the Younger Dryas. The widespread subaerial exposure of the floor of Lake Agassiz—resulting from the draw down of the lake at this time—is recorded by dated organics and fluvial sediment. New computer-generated paleotopographic reconstructions, using a modern DEM adjusted for differential isostatic rebound and superposed on Lake Agassiz lake levels show that overflow from Agassiz to the Great Lakes would have occurred during the Younger Dryas if the LIS had retreated from those eastern outlet channels by then. However, recent fieldwork in that region casts doubt on whether catastrophic overflow occurred at that time, because there is an absence of streamlined features and boulders in the channels, some channels have narrow constrictions, and there are no dates in the region older than ~10 ¹⁴C ka.

Perhaps boulders in the channels are buried beneath finer sediment associated with the Marquette ice advance into the Superior basin ~9.9 ¹⁴C ka. Perhaps overflow from Lake Agassiz was not catastrophic, so channel morphology and bedload reflect a non-catastrophic hydrological event. Perhaps overflow from Lake Agassiz during the Younger Dryas was through the NW outlet into the Clearwater-Athabasca valleys to the Arctic Ocean. The latter requires retreat of the LIS from that area, which all reconstructions to date show did not happen until ~10 ¹⁴C ka. However, a distinct boulder gravel below the 10 ¹⁴C ka gravel in the Athabasca Valley may reflect an earlier outburst from Lake Agassiz, with the overlying gravel related to a brief re-damming of the valley at the end of the Younger Dryas when overflow from the lake briefly overflowed through its southern outlet.