An early Holocene oxygen isotope record from the Olson buried forest bed, southern Lake Michigan

Analysis of a 3.5 m vibracore from the Olson buried forest bed in the southern Lake Michigan basin provides new paleolimnological data for the early Holocene. The core records a rise in lake level from the Chippewa low water phase toward the Nipissing high water phase. Deepening of the water level at the core site is suggested by a trend toward decreasing organic carbon content up core that is interpreted as a response to increasing distance between terrestrial debris sources and the core site. Published data from deep water cores from the southern Lake Michigan basin suggest there had been an inflow of isotopically light water from glacial Lake Agassiz into the southern basin between 10.5-11 ka (A1 event). The data also indicate a second flood of isotopically light water between 8-9 ka (A2 event). Three new C-14 dates from the Olson site core suggest that most of the sediment was deposited between 8.45 ka and 8.2 ka, an interval roughly coeval with the second pulse of O-18-depleted water (A2) from Lake Agassiz into the southern basin. Oxygen isotope ratio analysis of shell aragonite from the gastropods Probythinella lacustris and Marstonia decepta shows increasingly negative values up core. This trend in sigma O-18 values suggests that O-18-depleted water entered the southern basin about 8.4 ka. The Olson site core thus provides a chronology of events in the southern Lake Michigan basin associated with the draining of glacial Lake Agassiz.