PALEOENVIRONMENTAL AND PALEOCLIMATE INFERENCES FROM GASTROPOD ASSEMBLAGES IN LAST GLACIAL LOESS: ILLINOIS, INDIANA, AND KENTUCKY

High resolution chronologies from terrestrial gastropods in last glacial loess sequences in the midcontinental USA (Peoria Silt; 30–16 cal ka) provide a means to evaluate temporal and spatial variations in sedimentation rates, climate, and environments. From a loess-paleosol section in western Kentucky, stratigraphic changes in gastropod assemblages imply climatic cooling at 29 ka and again at 27 ka, leading into the last glacial maximum. At 30 ka, a temperate summer climate (MJT > 21°C) and forested ecosystem is implied by a diverse assemblage (15 species) that includes Anguispira kochi–Gastrocopta pentodon–Helicodiscus parallelus. At 29–27 ka, the assemblage includes Discus whitneyi, Hendersonia occulta and H. parellelus, but lacks A. kochi and Gastrocopta. After 27 ka, the introduction of Columella alticola, Vertigo modesta, and Vallonia gracilicosta and disappearance of Helicodiscus records a shift to substantially colder (MJT 15–19 °C), boreal conditions. Finer-scale variability in species composition, sedimentation rates, and weak paleosol development may record multi-centennial fluctuations in climate and/or glacial lobe margins.

A strong NE-SW regional climatic gradient at 22–21 cal ka (during last glacial maximum) is suggested by contrasting gastropod assemblages from central Indiana to southwestern Illinois. Resedimented loess between till units in central Indiana (Clayton and Plainfield Sections) contains abundant cold tolerant species Vertigo modesta-V. oughtoni-Columella alticola, reflecting boreal to borderline tundra conditions near the ice margin. At the same latitude (40°N) in loess along the Illinois Valley, the assemblage transitions to a more diverse one, with only rare V. modesta and C. alticola and more southern species. In southwestern Illinois (St. Clair County), the loess fauna has few cold-climate species but contains W. multilineata, Allogona profunda, and Stenotrema hirsutum, suggesting more temperate conditions. We thus envision a steep paleoclimatic gradient from present-day Indianapolis (MJT 14–16°C) to St. Louis, Missouri (MJT 18–20°C). These interpretations are complementary to plant macrofossil and regional pollen records, but enable a more detailed characterization of environmental change across the central USA during the last glacial maximum.