GLACIAL ADVANCE, RETREAT, AND THE RECORD OF LATE GLACIAL CLIMATE CHANGE IN NORTHEASTERN WISCONSIN

Radiocarbon dates from eastern Wisconsin suggest that late glacial ice-free times separated by periods of ice cover can be recognized. Several recent AMS dates of between 17,125 ± 142 and 18,069±150 BP (cal.) and 15,846± 659 and 16,335± 630 BP (cal.) indicate tundra conditions and permafrost. This is supported by ice wedge casts, floral remains, and pollen. All are under till of the Kewaunee Formation. These ice free times correspond to times of minor warming in the GRIP core and periods of ice margin stability in our ice flow model simulations that are driven by a tuned GRIP temperature record. The model fails to reproduce short-term ice retreats and readvances during deglaciation, perhaps because ice surging played an important role at this time. No radiocarbon dates between 13,893±152 and 15,846±659 BP (cal.) are known at this time from beneath Kewaunee Formation till, likely indicating ice cover between those dates. This was followed by retreat of ice at least to the northern end of Lake Michigan. This retreat of ice corresponds to the time of the Two Creeks Forest Bed (many dates from about 13,200 to 13,800 cal.) when temperature had risen, spruce forest dominated, and Lake Michigan level fell below its present level. These dates lag the rapid warming in the GRIP core that took place about 14,500 BP (cal.), but closely match the retreat of ice predicted by the model. The lag may be due to the time it took for ice to retreat and for spruce forest to establish. (All of the Two Creeks dates are on spruce wood.). The Two Rivers advance blocked the northern end of Lake Michigan and as Lake Michigan rose to the Calumet level the Two Creeks forest at the type locality was drowned about 13,200 cal. years ago.