A LONG PALEOENVIRONEMENTAL RECORD FROM DEVILS LAKE, WISCONSIN, USA

Most lakes in the upper Midwest of the U.S. were formed by glaciers at the end of the last Ice Age about 10-11 thousand years (ka) before present (B.P.). Devils Lake, Wisconsin is a rare example of a lake whose sediments extend into the Pleistocene and may include the Last Glacial Maximum (26-19 ka B.P.). Sediment magnetic, geochemical and charcoal records were collected from a 10 meter core taken in the deepest part of the lake. Together, these proxies combine to reveal a history of long-term climatic, vegetative and geologic change during the late Pleistocene-Holocene.

Productivity in the lake was low during the late Pleistocene and increased during the Holocene, as reflected by the sediment lithology, which shows a sudden shift from glacial vivianite-rich and organic-poor clastic-dominated sediments to Holocene diatomaceous sapropels. The silicon to titanium ratio (Si/Ti), a proxy for diatom abundance and productivity, also demonstrates higher values in the Holocene and lower values in the Glacial.

Sediment magnetic parameters show that the predominant magnetic fraction comprises detrital multi domain (MD) magnetite (>1mm) eroded from catchment soils and bedrock. Magnetic concentration and size was high during the glacial period due to erosion by rates in the catchment and input from nearby glacial lakes.

Throughout the Holocene magnetic concentration decreased. The proportion of small (<100nm) single domain (SD) magnetic particles of bacterial origin increased after the deglaciation due to decreased detrital flux and increased biological productivity. The first half of the Holocene is marked by increased charcoal abundance indicating high fire activity. During the mid-Holocene, the fraction of SD particles decreased due to sustained detrital fluxes possibly of eolian origin. Around 3.5 ka BP, there is a shift in magnetic grain size due to a reduced influx of MD grains; at the same time diatom productivity was at its highest sustained level and δ15N values decreased. This could indicate the onset of more humid conditions during the Late Holocene, previously unrecognized in this area. Magnetic grain size and concentration increased again in the most recent part of the record, due to anthropogenic influence in the region including agriculture-related erosion and nutrient inputs.