HYDROCLIMATE INFERENCES FOR THE LAST GLACIAL AND PENULTIMATE GLACIAL MAXIMA BASED ON FOSSIL TERRESTRIAL GASTROPOD ASSEMBLAGES AND STABLE ISOTOPES, CENTRAL USA

Terrestrial gastropods are preserved in proximal loess and in lacustrine/palustrine deposits of the Wisconsin Episode (last glaciation) and Illinois Episode (penultimate glaciation) in the central USA. Past hydroclimates can be estimated by both the species assemblages and shells’ isotopic signature. Furthermore, an accurate geochronology can be determined by radiocarbon dating of terrestrial shells, using taxa such as Succineidae or Discus. Values of δ¹⁸O, from last glacial maximum Succineidae shells, range from -1.7 to 2.9‰, with the lowest values in eastern Iowa and northwestern Illinois and highest values in central Indiana. The δ¹⁸O values generally increase towards the east and south; however, the spatial trends likely reflect competing influences of temperature and precipitation source (Gulf versus Pacific moisture). Higher δ¹⁸O values are found in Succineidae shells; thus, corrections are needed in considering multiple genera. For instance, Succineidae shell δ¹⁸O values are ~2‰ higher than Vertigo shell values and ~3‰ higher than Carychium shell values where they co-occur. The gastropod assemblage in the lower Mississippi Valley largely differs from the north, but isotopic correlations can be made with co-occurring genera such as Vertigo and Stenotrema. A strong spatial gradient in temperature, with distance from the southern Laurentide Ice Sheet, is documented from terrestrial gastropod assemblages (by use of modern geographic ranges). Last glacial maximum mean July temperatures are estimated to have been ~9°C cooler than today along the ice margin in central Indiana and ~3°C cooler than today along the Gulf Coast. A comparison of isotope values from modern, Wisconsin Episode, and Illinois Episode terrestrial shells (Succineidae, Vertigo, Carychium) was made from sites with these genera in southwestern Illinois. The δ¹⁸O values were found to be statistically similar for Illinois Episode and Wisconsin Episode shells, implying a similar precipitation source and paleoclimate. In contrast, the δ¹⁸O values of modern Succineidae and Vertigo shells (from early 20^th century collections) are 3 to 4‰ lower than for glacial-age shells. This difference likely reflects a combination of warmer air temperature and lower values of precipitation δ¹⁸O due to smaller global ice volumes today.