Paper No. 12
Presentation Time: 4:35 PM
NEW EVIDENCE FOR LAKE AGASSIZ DRAINAGE THROUGH THE ST. LAWRENCE AT YOUNGER DRYAS ONSET
RAYBURN, John A., Dept. of Geological Sciences, SUNY New Paltz, 1 Hawk Drive, New Paltz, NY 12561, CRONIN, Thomas M., U.S. Geological Survey, 926A National Center, 12201 Sunrise Valley Drive, Reston, VA 20192, THUNELL, Robert C., Department of Earth and Ocean Sciences, University of South Carolina, Columbia, SC 29208 and FRANZI, David A., Center for Earth and Environmental Science, State University of New York, 101 Broad Street, Plattsburgh, NY 12901, rayburnj@newpaltz.edu
Broecker and others (1989) published a landmark paper in
Nature suggesting that a catastrophic discharge event from glacial Lake Agassiz was routed eastward through the glacial Great Lakes and entered the North Atlantic via the Champlain Sea around 13.0 ka calibrated BP. They argued that the influx of fresh water may have interrupted the North Atlantic Deep Water (NADW) thermohaline circulation thereby abruptly initiating the Younger Drays (YD) cold interval. Thorough work in the eastern Agassiz basin by Thorleifson (1983) and Warman (1991) suggested that Lake Agassiz level dropped from the higher Lockhart to the lower Moorhead Phase around this time, however there were no reliable ages associated with the event in Lake Agassiz and all of the large outlet channels observed connecting Agassiz to the Superior basin post-dated the Moorhead Phase. Fifteen years later, Lowell, Fisher, Broecker, Teller and others concluded there was no outlet on the basis of field evidence (EOS, 2005). In fact, dated ice margin positions made the eastern drainage route seem unlikely.
Our work at the eastern end of the Great Lakes in glaciomarine Champlain Sea sediments has determined that there is evidence for a large discharge event originating from the mid-continent that entered the North Atlantic near the onset of the YD. Significant basin-wide freshening of the Champlain Sea well constrained to 13.0 ka BP is evident in both micro faunal species variation (ostracodes and foraminifera), as well as the δ18O signal from their tests. The dramatic scale of the marine freshening is best explained by an influx of fresh water large enough to have potentially affected the NADW and Lake Agassiz remains the best candidate for its source. We note that the lack of an outlet channel should not necessarily be cause for abandoning the Lake Agassiz eastern discharge hypothesis. In the eastern end of the Great Lakes, the largest glacial Lake Iroquois flood into glacial Lake Vermont left no apparent outlet channel either, while a lesser flood had produced a significant channel.
