HR: 0800h
AN: PP31A-0894    [Abstracts]
TI: Cosmogenic Dating Evidence for the Routing of Lake Agassiz Overflow During the Younger Dryas
AU: * Rinterknecht, V R
EM: vincent@ldeo.columbia.edu
AF: Lamon-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Schaefer, J M
EM: schaefer@ldeo.columbia.edu
AF: Lamon-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Lowell, T V
EM: Thomas.Lowell@UC.EDU
AF: Department of Geology, University of Cincinnati, P.O. Box 0013, Cincinnati, OH 45221 United States
AU: Denton, G H
EM: gdenton@maine.edu
AF: Climate Change Institute , University of Maine, Orono, ME 04469 United States
AU: Teller, J T
EM: tellerjt@ms.umanitoba.ca
AF: Department of Geological Sciences, University of Manitoba, Winnipeg, R3T 2N2 Canada
AU: Broecker, W S
EM: broecker@ldeo.columbia.edu
AF: Lamon-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Fisher, T G
EM: timothy.fisher@utoledo.edu
AF: Department of Earth, Ecological and Environmental Sciences, University of Toledo, 2801 West Bancroft St. , Toledo, OH 43606 United States
AU: Kubik, P W
EM: kubik@phys.ethz.ch
AF: Institut f\"{u}r Teilchenphysik, Schafmattstr. 20 ETH H\"{o}nggerberg, HPK H 30, Z\"{u}rich, 8093 Switzerland
AB: Two sources of fresh water releases in the North Atlantic are possible causes for abrupt climate changes that punctuated the last glaciation. One possible cause is the release of iceberg armadas from a surging Laurentide Ice Sheet (LIS) and the other is the release of meltwater from proglacial Lake Agassiz. This additional fresh water in the North Atlantic would have reduced the surface water density, which would have shut down the thermohaline circulation, resulting in large scale climate changes. The release of iceberg armadas has been well studied in deep sea sediments and has been identified by ice-rafted detritus and called Heinrich events. The sudden release of meltwater could have been the trigger for three cold periods recorded in the $\delta$$^{18}$O records of Greenland ice cores: the Younger Dryas (YD), the Preboreal Oscillation (PBO) and the 8.2 cal ka cold event. The best hypothesis so far to explain these three cold snaps is that outbursts from Lake Agassiz happened via different pathways that abruptly opened during the retreat of the LIS. The final two Greenland isotopic events (PBO and the 8.2 cal ka event) have been correlated to Lake Agassiz bursts, and these occurred through northern outlets: the Athabasca-Mackenzie Valley and Hudson Strait respectively. The isotopic signal associated with the YD event may be related to one of two possible routings: (1) the NW outlet via the Athabasca-Mackenzie Valley and (2) the E outlets into the Great Lakes. Although there is evidence for one flood to the NW around 10 ka, there may have been an earlier event, but that record has not yet been identified. We investigated the second possible routing through Thunder Bay, Ontario, to Lake Superior. Thunder Bay is a strategic area because paleotopographic reconstructions suggest that if ice was not present, meltwater would have overflowed directly into the Lake Superior Basin and from there into the St Lawrence Valley to the North Atlantic Ocean. However, geomorphic evidence for a flood through the Thunder Bay area is not convincing, and an alternative routing may be required. One possibility is that the NW outlet carried overflow during the YD. Another possibility is that flow was eastward, subglacially via the Lake Nipigon Basin to the N, which was buried by the LIS at this time; excellent geomorphic and sediment evidences exist in that area for strong meltwater discharge. To verify this hypothesis we used the surface exposure dating technique and specifically the cosmonuclide $^{10}$Be. We sampled granitic boulders on the Dog Lake moraine (associated with the YD cold event) and granitic boulders from the drainage channels west of Nipigon Lake (presumed to be post-YD in age). Here we present the first results of our investigation.
DE: 9350 North America
DE: 1821 Floods
DE: 1035 Geochronology
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting