Thursday, August 9, 2007

PS 64-116: Mid-Holocene vegetation and lake level changes at Fallison Lake, Wisconsin

Randy Calcote1, Sara C. Hotchkiss2, Benjamin H. Von Korff2, Mark D. Shapley1, Jarvis Erickson2, and Jasmine Saros2. (1) University of Minnesota, (2) University of Wisconsin

We analyzed pollen from a series of cores from Fallison Lake in northern Wisconsin to determine Holocene forest history and contrast vegetation changes to lake level history derived from structure, magnetic susceptibility, grain size of sediments, and diatoms. Fallison Lake is a seepage lake on sandy soils high in the Trout Lake watershed and lake levels should be sensitive to water balance fluctuations. Peaty sediments ~8000 yrs ago in shallow water cores indicate low lake levels when local vegetation was dominated by jack and/or red pine savanna or woodland. Mesic hardwoods (elm, ash, ironwood) were also present, presumably in wet to mesic microsites. A peak in magnetic susceptibility in all three cores ~6500 yrs ago represents the beginning of rising lake levels and mobilization of clay and silt from the newly activated shoreline. As the lake level stabilized at a higher equilibrium, clastic transport declined and magnetic susceptibility decreased. White pine and birch increased in response to increased moisture availability. White pine pollen abundance peaked during this wet period, corresponding to the period when eastern hemlock populations spread rapidly across UP Michigan. White pine pollen decreases at the time of the hemlock decline (~5400 yrs ago), and oak, ironwood and jack-red pine pollen increase. This vegetation change suggests drier conditions but there is no indication in grain size or magnetic susceptibility that the lake level changed. Our results agree with other evidence for a long-term increase in moisture availability in the western Great Lakes region over the last 8000 years. Rising lake levels and expansion of white pine populations on sandy soils suggest a stepwise increase in moisture ~6500 yrs when conditions in Minnesota remained warm/dry. The increase in hemlock after ~3500 yrs ago indicates a second step-wise increase in available moisture.

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