TERRESTRIAL GASTROPOD ASSEMBLAGES FROM LAST GLACIAL LOESS IN SOUTHWESTERN ILLINOIS (KELLER FARM SECTION), IMPLICATIONS TO PALEOCLIMATE AND PALEOECOLOGY

Numerous terrestrial gastropod shells are preserved in a 15 m thick section of last glacial loess (Peoria Silt) at the Keller Farm Section in southwestern Illinois. This important loess section contains ~12 minor paleosols that record slightly warmer and more humid conditions (Wang et al., 2000) representing climatic oscillations of the last glacial maximum. The gastropod taxa were not previously examined for climatic and ecological interpretations. The Peoria Silt likely accumulated between about 29 and 18 cal ka based on shell ages thus far. Loess in this area originated from glacial meltwater floods in the Mississippi River Valley that left behind abundant sediment, in turn was exposed to strong winds. Once the climate warmed and glaciers retreated north of the Mississippi River Basin, loess deposition likely ceased.

Nine samples, between 4.25 and 12.0 m depth, were wet sieved >0.5 mm for fossil shells from two to four liters of sediment. Nine taxa (Discus whitneyi, Discus macclintocki, Punctum minutissimum, Hendersonia occulta, Columella columella, Vertigo hubrichti, Vertigo modesta, Vallonia gracilicosta, Succineidae) were identified. Ecologically, most of these taxa live today in leaf litter or on exposed carbonate rock outcrops, based on current literature. The most common fossil at the Keller Farm Section is V. modesta, a cold-climate species presently living from Hudson Bay to the southern Lake Superior shoreline. Of the 226 total adult individuals identified from all samples, 122 were V. modesta. Four depths from 6.85 to 11.35 m contain mostly fragmented shells and have many secondary carbonate concretions, suggesting greater post-depositional dissolution. These depths include a complex of several minor paleosols that likely indicate warmer and more humid climates. A cool-climate species, V. hubrichti, is only found at 4.25 m. Today, V. hubrichti occurs in the upper Great Lakes region as well as in northeastern Iowa. Using a mutual climatic range method for estimating paleotemperature, the gastropod assemblage suggests a mean July temperature between 17 and 19°C during the last glacial maximum in southwestern Illinois. In comparison, the present-day mean July temperature is ~26°C. Thus, the last glacial maximum is estimated to have been 7 to 9°C cooler than today in this region.