We test the hypothesis of a North American impact event at ~12.9 cal. yrs. BP (Firestone et al. 2007) with lake sediment records. The impact, hypothesized to have occurred on or over the Laurentide Ice Sheet (LIS) near the Great Lakes, may have contributed to the megafaunal extinction, collapse of the Clovis culture, and destabilization of the LIS resulting in Younger Dryas (YD) cooling. Firestone reported data from ten North American sites, but only one was Midwestern and none were lakes, historically robust sources of data used for paleoenvironmental reconstructions. We collected sediment cores from Appleman and Spicer Lakes, IN, Silver Lake, OH, and Kirchner Marsh, MN, with radiocarbon-dated records spanning the YD. Our sites are ideally situated to record regional environmental responses to a nearby impact. Sediments were analyzed for changes in stratigraphy, charcoal, pollen (including Sporormiella, a dung fungus proxy for megafaunal abundance), loss-on-ignition (LOI), magnetic susceptibility, carbon microspherules, and elemental composition determined from x-ray fluorescence spectrometry (XRF). An impact event should be associated with elevated charcoal, microspherules, magnetics, and concentrations of ET-associated elements, and a decline in Sporormiella.
Results from LOI, magnetics, microspherules, charcoal and XRF do not show the signatures predicted for a YD event. Vegetation reconstructions are still in progress as of the time of this abstract, but at Appleman, the Sporormiella decline precedes the YD. We are continuing analyses to determine whether this apparent discrepancy between the predictions of the YD impact hypothesis and the lack of evidence in lake sediments persist across sites.