A HIGH-RESOLUTION ENVIRONMENTAL HISTORY FROM A SMALL LAKE BASIN IN CENTRAL WISCONSIN

Emrick Lake in central Wisconsin is a small, varved, kettle lake in the recessional moraine complex of the Green Bay Lobe. The lake has an area of 37 ha and a maximum depth of 23.5 m. The hydrologically-isolated lake basin provides a detailed record of Holocene climate and vegetation change as well a record of fire and anthropogenic influence near the lake. It is unusual for a lake this far south to have annually-laminated sediments, and thus offers the opportunity of a high-resolution environmental history where others are lacking. Varve counts and AMS 14C dates offer robust age control for the sedimentary sequence. Basal sediments on a 9.4 m core from the deepest portion of the lake are dated between 11,600 ± 320 and 9930 ± 70 14C yr B.P. At present, lake sediments are being analyzed for organic matter/carbonate content (LOI), environmental magnetic properties (IRM, ARM, K), pollen and charcoal.

Environmental magnetism relates lake-sediment magneto-stratigraphy to changes in climate and lake productivity and provides additional independent evidence of erosional events in the lake basin. At Emrick, the magnetic properties reveal an increase in concentration-dependent parameters in the mid-Holocene, suggesting greater inputs of terrigenous materials from runoff or eolian sources. More pronounced peaks in the early Holocene portion of the record are likely due to slope wash from sparsely vegetated adjacent hillsides and slumping within the lake. A rise in magnetic properties at the top of the core is temporally consistent with the onset of human activity in the region and probably signifies enrichment from fire, farming and aerosol pollution. Emrick Lake is situated near the modern-day prairie-forest ecotone (tension zone). Pollen analysis in progress will offer an unusual glimpse of the position of this tension zone through the Holocene. This high resolution record will allow evaluation of direct impacts on hydrology at fine temporal scales (decadal or less). Furthermore, the length of this record is sufficiently long to examine the response and recovery of this lake basin to a variety of environmental changes and will help clarify uncertainties about mid-Holocene climatic conditions in southern Wisconsin.