Geneva Lake of SE Wisconsin, USA (42.561°N, 88.537°W) is useful for reconstructing environmental change due to its old age (formed before 20 cal yr BP by the retreating Laurentide Ice Sheet), 30-meter laminated sediment record we have documented with acoustic imaging, and abundance of ostracodes with calcareous valves that record lakewater chemistry. A ~4.5-meter sediment profile records ~14,500 years of sediment deposition based on 6 14C ages. Bulk sediment geochemistry and mineralogy were examined along with acoustic sub-bottom profiles to infer changes in dominant sediment sources. This included evidence of humans increasing the flux of sediment and several elements including heavy metals; deposition of a unique, lake-wide minerogenic unit at about ~8,200 cal yr BP; and a major transition in sediment composition ~13,400 cal yr BP attributed to changing productivity and influx of loess. Using isotopic (δ18O and δ13C) and trace metal ratio (Sr/Ca and Mg/Ca) analyses of valves of a single abundant ostracode species (Candona ohioensis), we also infer environmental and hydrological change throughout the past ~11,800 years at millennial scale resolution. Changes in these values between 11,800 and 9,700 cal yr BP indicate the lake was more hydrologically closed over this period and experienced a long-term increase in evaporation, temperature, and/or Gulf of Mexico derived precipitation. Increased temperatures and/or precipitation over this time period likely promoted increased lake productivity, recorded in changing sediment chemistry and δ13C profile. From 9,700 cal yr BP to the present, changes in ostracode chemistry were relatively small and possibly driven by small variations in temperature and seasonality of precipitation affecting a more hydrologically open Geneva Lake system. Studies like this are critical for understanding past and future changes in climate and hydrology in the U.S. Midwest.
|Original language||English (US)|
|Title of host publication||AGU Fall Meeting 2019, 9-13 December 2019, San Francisco, California|
|State||Published - 2019|