Joint 56th Annual North-Central/ 71st Annual Southeastern Section Meeting - 2022

Paper No. 2-6
Presentation Time: 9:20 AM


GRIMLEY, David, Illinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign, 615 E Peabody Dr., Champaign, IL 61820, DENDY-METZ, Sarah N., University of Illinois, Urbana-Champaign, Department of Geology, 1301 W Green St, Urbana, IL 61801, CONROY, Jessica L., Department of Geology, University of Illinois at Urbana-Champaign, 3081 Natural History Building, 1301 W. Green St., Urbana, IL 61801 and NIELD, Catherine, Department of Geology, University of Cincinnati

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 20th 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.