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

Duck Lake in Lower Michigan is the focus of a multi-proxy study to identify changes in geochemical parameters in lake sediments and their relationship to regional climate variations and local land use variations. A Livingstone piston corer was used to collect an 11 m core section of lacustrine sediment and samples were taken at the sediment-water interface using the freeze-core method. Core sediment samples, taken at 5 cm intervals over an estimated 14,000 year record, were analyzed for total, inorganic and organic carbon, and total nitrogen.  Radiocarbon dates indicate low accumulation rates (0.05 cm/year) from 3800 to 9000 ¹⁴C years BP and higher accumulation rates (0.2 to 0.3 cm/year) from 3800 ¹⁴C years BP to the present. Age depth models and geochemical analyses of the core suggest change in the watershed about 3800 ¹⁴C years BP that caused more rapid accumulation rates. Other studies from this region show low sedimentation rates from 8000 to 4000 years BP due to drought and increased sedimentation rates from 4000 years BP to the present. The Duck Lake core is characterized by repeating packages of sediment (1 to 10 cm thick) that grade from light to dark from the surface to a depth of about 7.5 m; these packages represent changes in the proportion of organic matter and carbonate deposition. Over the past 9000 ¹⁴C years, total carbon varied from 13.5% to 45% and total nitrogen varied from 0.4% to 3.7%. These analyses show roughly centennial scale cycling of increased carbon and nitrogen values over the past 3800 ¹⁴C years and millennial scale cycling of increased carbon and nitrogen values from 3800 to 9000 ¹⁴C years BP.  Gas chromatograph analysis of CO₂ indicates a range of 0.05% to 10.6% inorganic carbon over the past 9000 ¹⁴C years. Organic carbon ranges from 4% to 45% and organic C:N ratios range from 3 to 11 over the same interval. C:N ratios have generally increased in the past 2000 ¹⁴C years indicating a transition towards greater proportions of terrestrial organic matter input into the basin. Sediment samples have been prepped for phosphorus analysis, including occluded, mineral, and organic phosphorus and for bulk geochemistry, including Ca, Mg, Al, Fe, S, Ti, Pb, Cd, Ba, Cu, Cr and Zn. The results of this study have implications for regional-scale insight into Late Holocene climate change as well as the impact of humans in the recent past.