PALEO-ENVIRONMENTAL PROXIES FROM ICE-MARGINAL LAKES IN SOUTH-CENTRAL WISCONSIN

The Driftless Area of south-central Wisconsin has remained unglaciated during the Pleistocene, making it a critical refuge during past ice ages. However, the environmental and ecological history of the Driftless Area remains poorly understood. Here we report initial results from sediment cores collected from lakes formed during the late Pleistocene as a result of either ice damming immediately adjacent to the margin of the Green Bay Lobe or outwash dams formed by sediment aggradation along rivers carrying outwash away from the ice margin. Tributaries along the lower Wisconsin River and Black Earth Creek were dammed by aggrading outwash resulting in lake basins collecting slackwater sediment, plant and invertebrate macrofossils, and pollen. None of the lakes are still extant, either because the basin completely filled with sediment or the outwash dam was breached allowing lake drainage. Using a Geoprobe, we drilled into these former lake basins to collect continuous core which we processed for multiple paleo-environmental proxies. Radiocarbon dates indicate that several of these lakes persisted from as early as 26.4 ka to as late as 2.8 ka. On-going research is evaluating these sedimentary archives to assess their suitability for paleoecological and paleoclimatic proxy analyses through the Pleistocene-Holocene transition.

At Marsh Valley, an outwash-dammed tributary of the lower Wisconsin River immediately west of the former ice margin, several cores were analyzed for fossil pollen and charcoal at a fixed interval of 8 cm through the lake sediment, but low pollen and charcoal concentrations were observed in all levels. To better delineate productive zones, we used loss-on-ignition (LOI) at a 1-cm resolution to better constrain portions of the lake sediment that would yield better pollen counts. These analyses identified interbedded organic-rich zones and mineral-rich silty and sandy layers with low pollen accumulation rates.

Similar ice-marginal lakes persisted both behind and in front of the Last Glacial Maximum ice margin during the late Pleistocene through the late Holocene. Additional lacustrine archives from these contexts provide an opportunity to assess the environmental and ecological history of the unglaciated Driftless Area during the last glacial maximum.