HR: 10:20h
AN: PP42A-01    [Abstracts]
TI: Deglaciation Chronology of the Southern Laurentide Ice Sheet and Implications for Meltwater Routing bracketing the Younger Dryas Event
AU: * Lowell, T V
EM: Thomas.Lowell@uc.edu
AF: Department of Geology, 500 Geology/Physics University of Cincinnati, Cincinnati, OH 45221, United States
AU: Fisher, T G
EM: tfisher4@utnet.utoledo.edu
AF: Department of Environmental Sciences, University of Toledo, Toledo, OH 43606, United States
AU: Waterson, N
EM: watersnj@email.uc.edu
AF: Department of Geology, 500 Geology/Physics University of Cincinnati, Cincinnati, OH 45221, United States
AU: Hajdas, I
EM: hajdas@phys.ethz.ch
AF: HPK/H27 ETH Hönggerberg, CH-8093, Zurich, OH 45226, Switzerland
AB: During the last glacial hemicycle the intricate interplay of ice sheet 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 the large meltwater reservoir in the center of North America known as 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 channels 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 the location of the ice margin at the beginning of the Younger Dryas is still being studied. However, in order for the ice margin to retreat sufficiently to produce a volume of Lake Agassiz suggested 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 some 25 m/yr then increase to ~200 to 300 m/yr as the ice margin calving 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 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 after the start of the Younger Dryas.
DE: 0726 Ice sheets
DE: 4901 Abrupt/rapid climate change (1605)
DE: 4926 Glacial
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Joint Assembly