PRELIMINARY GEOCHEMICAL ANALYSES OF LATE HOLOCENE LACUSTRINE SEDIMENT FROM SOUTH-CENTRAL MICHIGAN

Duck Lake in south-central Michigan is the focus of a multi-proxy study to identify changes in geochemical parameters in lake sediments and examine their relationship to regional climate variations and local land use variations. Duck Lake is a 254-hectare lake with a maximum depth of 15 meters and is located in Calhoun County in the interior of the Lower Peninsula. Extensive development dominates Duck Lake's shorelines and both agricultural and forested areas are found within a mile of the shore. A Livingstone piston corer was used to collect cores with overlapping drives about 1 meter apart to a depth of 11 m. Samples were also taken at the sediment-water interface using the freeze-core method. Repeating packages of sediment, 5 to 10 cm thick, that grade from light to dark are found from the surface to a depth of about 7.5 meters, and are interpreted to represent changes in organic matter and carbonate deposition. From 7.5 to 9 meters, x-radiographs of the Livingston cores show approximately 1.5 meters of laminated sediments, millimeters thick. A twig radiocarbon date of 1920 ± 25 years B.P. (UCIAMS # 10332) from a depth of 292 to 293 cm has been used to estimate a sediment accumulation rate of 1.5 mm/year for the top 3 meters of core. Loss-on-ignition analysis has given a range of 4% to 26% organic carbon over this 3-meter interval with lower values in the top 1.5 m and more variable values below 1.5 m. Water content data range from 65% to 90% water in the top 3 meters of core. The carbon and water content data correlate with gray scale analysis of core x-rays; the lighter layers correspond with low organic carbon values and low water content and the darker layers with high organic carbon values and high water content. Core sediment samples, taken at 5 cm intervals over an estimated 14,000 year core record, have been prepped for bulk geochemistry, mercury analysis, phosphorus extractions and inorganic and organic carbon analysis. The results of a characterization of Duck Lake cores will be important in determining natural changes within the lake basin as well as provide information on the influence of human settlement.