POLLEN FROM THE GLACIAL LAKE OSHKOSH, WISCONSIN SUPPORTS MACROFOSSIL EVIDENCE FOR LATE WISCONSIN VEGETATIONAL HISTORY

Glacial Lake Oshkosh formed in the Fox River lowland of Wisconsin during the Wisconsin Glaciation when the Green Bay Lobe (GBL) of the Laurentide Ice Sheet blocked drainage northward toward Green Bay. In 2000, the Wisconsin Geological and Natural History Survey initiated a project to map the Quaternary geology of the glacial Lake Oshkosh basin, including investigation of the stratigraphy of Quaternary deposits in the basin. We acquired more than 3,000 feet of well-preserved rotosonic drill core which afforded us the opportunity to conduct pollen analysis of the sediments, particularly the lake sediments, some of which are varved. The primary outcome of the pollen analyses is that pollen concentration in these largely minerogenic sediments is extremely low. Of 462 samples analyzed, only two yielded sufficient pollen to permit counts exceeding 300 grains; 55% of the samples were barren. Loss-on-ignition analyses revealed just how inorganic the samples are; the average organic matter content was 0.1% by weight and the maximum value was 0.5%. The low organic matter concentration is at least in part a function of the high sedimentation rates in the lake, which were driven by its proximity to the GBL ice margin. One indicator of the high sedimentation rate is varve thickness, which can reach 30 cm for one couplet. The limited pollen data that we acquired (a total of 1,400 pollen and spores) indicate that trees were absent from the Lake Oshkosh basin during the early (26,000 to 15,500 cal yr BP) and middle (15,500 to 14,000 cal yr BP) phases of the lake, and that spruce grew in the basin by the late phase (14,000 to 13,000 cal yr BP) of the lake. These results agree with previous interpretations based mainly on macrofossils (e.g., Mickelson, et al., 2007).