Paleohydrology of the upper Laurentian Great Lakes from the late glacial to early Holocene

Between 10,500 and 9000 cal yr BP delta O-18 values of benthic ostracodes within glaciolacustrine varves from Like superior range from - 18 to -22%. PDB. In contrast, coeval ostracode and bivalve records from the Lake Huron and Lake Michigan basins are characterized by extreme delta O-18 variations, ranging from values that reflect a source that is primarily glacial (similar to -20% PDR) to Much higher values characteristic of a regional meteoric Source (similar to -5%. PDB). Re-evaluated age models for the Huron and Michigan records yield a more consistent delta O-18 stratigraphy. The striking feature of these records is a sharp drop in delta O-18 values between 9400 and 9000 cal yr BP. In the Huron basin, this low delta O-18 excursion was ascribed to the late Stanley lowstand, and in the Lake Michigan basin to Lake Agassiz flooding. Catastrophic flooding from Lake Agassiz is likely, but a second possibility is that the low delta O-18 excursion records the switching of overflow fro the Lake Superior basin from all undocumented northern Outlet back into the Great Lakes basin. Quantifying freshwater fluxes for this system remains difficult because the benthic ostracodes in the glaciolacustrine varves of Lake superior and Like Agassiz may not record the average delta O-18 value of surface water. (c) 2009 University of Washington. All rights reserved.