THE 8.2 KYR EVENT AS A PALEOHYDROLOGIC SIGNAL IN OSTRACODE RECORDS FROM THE GREAT LAKES REGION, USA

The early Holocene lacustrine records of the Great Lakes region are marked by several short-duration climatic oscillations identified by d18O excursions and changes in mineral and microfossil assemblages. The youngest of these events, the 8.2 kyr event, is linked to Laurentide ice sheet deglaciation activity, which in turn produced a climatic and oceanic circulation response identified worldwide. It can also be found as a paleohydrologic signal in the ostracode records from the Laurentian Great Lakes and from the small lakes to the west of the Great Lakes region.

In northern Lake Huron and northern Lake Michigan, the 8.2 kyr event was identified by a drop in lake level and a strong negative excursion of d18O in Candona subtriangulata valves, marking the arrival of extremely cold, isotopically light meltwater under lowstand conditions (Moore et al., 2000). In small lakes to the west of the Great Lakes in Minnesota and North and South Dakota, other ostracode species record lowstand and also drought conditions during this time (8.4 through 8.0 cal. yr) (Smith et al., 2001; Dean et al., 2002). Drop in lake levels during this time for northern Lake Huron and northern Lake Michigan, and for these smaller basins to the west imply that enhanced westerlies likely played a strong role (Dean et al., 2002).

Using biogeographic information from NANODe (North American Nonmarine Ostracode Database (Forester et al., 2005; http://www.kent.edu/NANODe) the shift in faunal composition during the 8.2 kyr event can be seen as associated with a shift in air mass dominance. Ostracode species associated with the climate features of the Arctic air mass and permanent lake conditions give way to species more tolerant of variable conditions, including wind-driven drought west of the Great Lakes and significant changes in the paleohydrology of the Great Lakes region.