A MULTI-PROXY CLIMATE RECONSTRUCTION OF WESTERN ILLINOIS BASED ON LATE WISCONSIN EPISODE LOESS

Thick loess deposits east of the Illinois Valley, containing some of the thickest Peoria Silt in Illinois, can be used to reconstruct the last glacial maximum (LGM) climate of western Illinois. A core taken near the Cottonwood School Section contains 21 m of Wisconsin Episode loess (Peoria and Roxana Silts). The sequence includes 13 m of Peoria Silt and also an amalgamation of buried A-horizons within the Jules Geosol. Here we report results from this core and an outcrop of the upper part of the Peoria Silt (including the Jules Geosol) 200 meters from the core. The Jules Geosol zone, ~60 cm thick, is slightly finer-grained, darker, and has granular structure. A new age model, based on radiocarbon ages of terrestrial gastropod shell aragonite, suggests deposition of Peoria Silt between 30,000 and 17,000 cal yr BP, and formation of the Jules Geosol at 22,000-23,000 cal yrs BP. These ages are comparable to those in Great Plains loess. The main source of the middle and upper Peoria Silt here was the Lake Michigan Lobe, the only lobe supplying sediment to the Illinois River watershed after the Mississippi River diversion (~ 24,500 cal yr BP).

This study seeks to understand the climatic conditions under which the Jules Geosol formed. A multi-proxy approach, including time series of grain size, magnetic susceptibility, clay mineralogy, d¹⁸O and d¹³C of gastropod carbonate, and gastropod paleoecology data indicate vegetation and climate varied on millennial timescales during and just after the maximum extent of the Wisconsin Episode glaciers in Illinois. A fossil assemblage composed of land snails in the Peoria Silt also suggests multiple environmental changes in western Illinois between 25,000 and 20,000 cal yr BP. The presence of Columella alticola and Vertigo modesta just above the Jules suggest a shift towards colder climate. Other gastropods including Succinea sp., Vertigo elatior, Carychium exile, Punctum sp., and Euconolus fulvus are ubiquitous in the New Cottonwood School fossil record and indicate that the local area was forested throughout the time of loess deposition. Future research will apply these methods at multiple sites to gain a better idea of the regional LGM climate along the Illinois Valley.