FOSSIL​ ​GASTROPODS​ FOR USE IN ESTIMATING PALEOCLIMATE AND PALEOECOLOGY IN LAST GLACIAL LOESS, SOUTHWESTERN ILLINOIS

During the Pleistocene, thick packages of loess in the central USA were derived from glacial meltwater deposits in large valleys that drained the Laurentide Ice Sheet. The potential to utilize terrestrial gastropod fossils preserved in loess horizons as high-resolution paleoecological and paleoclimatic indicators has not been fully explored in the central USA. Such fossils can assist in our ability to date glacial loess horizons, measure accumulation rates, as well as to reconstruct paleoecological and paleoclimatic conditions. In this study, we collected loess samples, in 25 cm intervals vertically, from a site (Demazenod Section) in southwestern Illinois (Belleville, IL) with thick Peoria Silt. Approximately 1300 terrestrial gastropod shells were extracted from wet sieving of bulk loess sediment.

A diverse fauna, containing fifteen species, is documented at the site. Individual Webbhelix multilineata and Succineidae gastropod shells within the loess were radiocarbon dated at approximately 18,100 to 16,700 radiocarbon years before present (21,800 to 20,100 calibrated years). Known geographic ranges and temperature tolerances of species existing today were extrapolated to estimate paleotemperatures for this interval. The presence of fossil gastropod species such as Stenotrema hirsutum, Hendersonia occulta, W. multilineata, and Allogona profunda imply a forested landscape and mean July temperature > 18°C. However, the occurrence of Vertigo modesta in some zones suggest a mean July temperature < 20°C, compared with 26°C in the same area today. Multi-centennial climatic or ecological oscillations are implied by variations in the gastropod assemblage stratigraphically.