ABSTRACT: Deglaciation chronology of the glacial Lake Agassiz outlet regions and implications for meltwater routing bracketing the Younger Dryas event.

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Abstract

During the last glacial hemicycle the intricate interplay of ice margin geometry, isostatic rebound, and topography dictated the locations of meltwater storage and routing. Perhaps the least well understood of these is the ice margin chronology. Consequently, we have developed ice margin chronologies at three potential outlets of glacial Lake Agassiz. At the southern outlet of Lake Agassiz several channels cut across the Big Stone Moraine. Radiocarbon dates from depressions and an intermediate level meltwater channel indicate initial lake formation after 13,680 cal yr. Coupled with regional data, this implies a minimum ice margin retreat of ~270 m/yr at the beginning of Bölling/Alleröd warm interval. The chronology of subsequent events and location of the ice margin at the beginning of the Younger Dryas is in progress. However, in order for the ice margin to retreat sufficiently to produce a volume from Lake Agassiz, as in prior reconstructions for the start of the Younger Dryas, requires retreat rates comparable to those of present day catastrophic collapse of the Columbia Glacier (~570 m/yr). The northwest outlet is via the Clearwater spillway of northern Alberta. New geomorphic mapping confirmed fluvial activity within the spillway, but also revealed several unrecognized ice margin positions. According to our reconstructions, retreat rates from 15,000 cal yr to 12,400 cal yr average 25 m/yr, then increase to ~200 to 300 m/yr as the ice margin calved into water. The ice margin position could not allow for northward drainage until about 11,000 cal yr, well after the start of the Younger Dryas. The eastern outlet is of interest as the suggested location of meltwater change (flood and/or simple diversion) from the Gulf of Mexico to the North Atlantic. However, the lack of a clear geomorphic meltwater route is problematic. About 14,000 cal yr the ice sheet retreat rate increased to ~63 m/yr and continued at that pace until 12,100 cal yr at which time the ice margin crossed the lowest topographic pathway out of the Lake Agassiz basin. In this reconstruction the lowest pathway would not be available for meltwater flow until well after the start of the Younger Dryas. Taken together, these results suggest an increasing ice recession rate associated with the Bölling/Alleröd interval with a significant increase after the Younger Dryas. Current time-distance diagrams reveal no offset in overall retreat as might result from a major ice sheet readvance during the Younger Dryas. This work has yet to confirm the hypothesis that the ice sheet geometry was sufficiently retracted to allow any major changes in meltwater storage location or routing just prior to the Younger Dryas. These results imply that Lake Agassiz was not the meltwater trigger for the Younger Dryas cold period.

Bibliography of Canadian Geomorphology