STRATIGRAPHY AND PALEOLIMNOLOGIC RECORD OF LOWER HOLOCENE SEDIMENTS IN NORTHERN LAKE HURON AND GEORGIAN BAY

Seismic reflection profiling and piston coring identified seismic refl ectors in northern Lake Huron and Georgian Bay linked with unconformit ies caused by at least six reductions in lake level. In ascending stra tigraphic order, these lowstands occurred at about 11200 BP, associate d with the Kirkfield outlet from early Lake Algonquin; 10 200 - 9900 B P, associated with the post-Algonquin lake level fall; 9800-9050 BP, t he most extreme lowstand, associated with the main Stanley - Hough dra wdown; and 7800-7450 BP. The concomitant highstands are Lake Algonquin , from about 11 200 - 10 200 BP; early Lake Mattawa, between 9600 and 9350 BP; the main Mattawa phase, 9050-7800 BP; and the Nipissing highs tand, at about 4700 BP. Isotopic and paleoecological data show that al l of the lowstands are characterized by cold, dilute, and isotopically very light(< -20%0) waters from the melting Laurentian ice cap. Highs tands, on the other hand, are characterized by higher dissolved solid concentrations and a much smaller meltwater component. Oxygen isotope values of the waters in these lakes were -15 to -17%0 in Lake Algonqui n, -13 to -14%0 for the early and middle Mattawa stages, -9 to -8%0 fo r the main Mattawa stage, and -7%0 for modern waters. This association of lowstands and not highstands with isotopically light waters is a n ew contribution to early Holocene hydrology of the Great Lakes. The Yo unger Dryas cool episode is coeval with the Lake Algonquin highstand a nd a younger cool episode is generally coeval with the Mattawa highsta nd. This supports the hypothesis of C.F.M. Lewis and T.W. Anderson tha t these large cold lakes were responsible for regional cooling during the early Holocene.