HR: 1340h
AN: OS33A-1001    [Abstracts]
TI: Evidence of Nipigon Phase Overflow From Glacial Lake Agassiz in Northwestern Lake Superior.
AU: * Wattrus, N J
EM: nwattrus@d.umn.edu
AF: Large Lakes Observatory & Dept. of Geological Sciences, University of Minnesota-Duluth, 10 University Dr., Duluth, MN 55812, United States
AU: Colman, S M
EM: scolman@d.umn.edu
AF: Large Lakes Observatory & Dept. of Geological Sciences, University of Minnesota-Duluth, 10 University Dr., Duluth, MN 55812, United States
AU: Gary, J
EM: garyx022@d.umn.edu
AF: Large Lakes Observatory & Dept. of Geological Sciences, University of Minnesota-Duluth, 10 University Dr., Duluth, MN 55812, United States
AB: The Younger Dryas cold reversal is one of the most prominent known abrupt changes in the Earth's recent climate history. This event has been ascribed to a rapid decrease in the production of North Atlantic Deep Water, which resulted when freshwater outflow from Glacial Lake Agassiz was suddenly diverted through the Great Lakes into the North Atlantic about 11,000 14C BP. This inference grew in strength through years of research on Lake Agassiz, the Gulf of Mexico and the North Atlantic. Recently, however, the existence of eastward drainage of Lake Agassiz during this time period has been challenged on a number of fronts, and we interpret the existing evidence as equivocal. Lake Superior is a crucial site for addressing the existence of eastward drainage of Lake Agassiz at the beginning of the Younger Dryas. There is well documented terrestrial evidence of later post-Younger Dryas drainage from easterly outlets through Lake Superior and the Great Lakes to the North Atlantic. We believe that these events (corresponding to the Nipigon Phase of Lake Agassiz) left diagnostic stratigraphic and geomorphic signatures beneath Lake Superior. If so, earlier eastward drainage during the Younger Dryas should have left analogous features. The purported Younger Dryas episode of eastward Lake Agassiz drainage (Morehead phase) was separated from the incontrovertible younger one (Nipigon phase) by the rapid and shortlived Marquette glacial advance (Emerson phase in Lake Agassiz). The thin, fine-grained till of this advance serves as a stratigraphic marker that would separate the younger drainage features from possible older ones. By sequentially examining the seismic stratigraphy of specific discharge locations beneath Lake Superior, and using the Marquette till as a stratigraphic marker, we believe we can perform a rigorous test of whether or not Lake Agassiz drained eastward during the Younger Dryas, and if so, where. We present here results from the recently concluded first field season of the project, which focused on mapping the lake floor expression of the later (Nipigon Phase) overflows. These post-Marquette deposits include subsurface fan deposits and coarse-grained strata that are buried below a thin sequence of Holocene sediments. These overflow deposits are found at the entrance to Nipigon Bay, a drainage pathway from Glacial Lake Agassiz known to be active during the Nipigon Phase of Lake Agassiz, and possibly at other discharge locations. Erosion features suggestive of high discharge scour events also occur in several places in the study area. The seismic stratigraphic model generated from these data will be used to analyze data collected next year off Thunder Bay, where the earlier overflows linked to the Younger Dryas have been postulated.
DE: 3025 Marine seismics (0935, 7294)
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 4299 General or miscellaneous
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting