LATE GLACIAL POLLEN SPECTRA FROM SEDIMENTS OF THE GLACIAL LAKE OSHKOSH BASIN, WISCONSIN

Glacial Lake Oshkosh formed in the ancestral drainage basin of the Fox River, east-central Wisconsin, when the northward-flowing Fox River was impounded by the Green Bay Lobe of the Laurentide Ice Sheet. The bulk of the sediment in the basin was deposited as the ice lobe receded from its glacial maximum during the last part of the Wisconsin Glaciation, approximately 25 ka. to 12 ka. B.P. Sediment is thickest (>200 ft) within a buried bedrock drainage system and includes rhythmically laminated (varved) offshore silt and clay, nearshore sand and organic-rich wetland deposits.

In order to acquire paleoenvironmental data for the late glacial period, we have processed more than 300 samples from glacial Lake Oshkosh sediment to determine their pollen content. Most varves contain insufficient pollen concentrations for statistically reliable counts; this presumably results from meltwater flushing of pollen out of the lake. However, a few varves yielded adequate pollen concentrations and typically contain either a tundra, forest-tundra, or spruce (boreal) forest assemblage. Organic-rich nearshore sand and wetland sediments usually yield meaningful pollen concentrations and contain these same three assemblages; they also contain postglacial (Holocene ) deciduous forest and prairie assemblages. Tundra assemblages tend to be among the oldest samples (>14 cal. ka. B.P.) and spruce forest assemblages among the youngest (excluding postglacial samples). Though these pollen samples are not part of continuous lake-sediment or peat sequences, they are, nevertheless, important because they pre-date the earliest late-glacial pollen samples in Wisconsin and they document the existence of tundra vegetation during the early part of deglaciation.