Last glacial climatic and ecological gradients in the central USA (glaciated Midwest to Gulf Coast) based on fossil terrestrial gastropod assemblages

Terrestrial gastropods preserved in last glacial loess (Peoria Silt) have been studied for well over a century in the central USA. However, a systematic comparison of climatic and ecological conditions (trends) during the last glacial maximum along a north-south transect, from ice-proximal locations ( approximately 40 degrees N) to the Gulf Coast region ( approximately 32 degrees N), has not been attempted. Improved modern-analog gastropod species range maps, accessible climatic databases, and high resolution chronologies now allow for more complete and accurate paleoenvironmental interpretations. Such interpretations are, in part, hinged on the assumption that modern climatic ranges and environments of species are also representative of past conditions. Three examples of cold-indicator species include Vertigo oughtoni, Columella alticolaand Vertigo modesta, which today live in locations with mean July temperatures (MJT) of 22 degrees C), are found in Peoria Silt at sites in southern parts of the study area. Temporal trends in gastropod assemblages have also been documented. Overall, based on mutual climatic range methods with multiple (> or =10) species at each of several sites, we interpret that ice proximal areas in central Indiana and Illinois had a MJT approximately 6 to 9 degrees C cooler than today at about 24 to 20 cal ka, with an ecosystem also vastly different from today. In contrast, the gastropod fauna in southern Mississippi loess suggests a MJT approximately 2 to 5 degrees C cooler than present during the same timeframe. Thus, a steeper regional climatic gradient during the last glacial maximum, likely affected by the southern Laurentide Ice Sheet, is further documented. This regional climatic gradient, nearly double that of today's gradient, is consistent with a dramatic full glacial ecological transition from borderline tundra (ice-proximal) to boreal to mixed boreal-deciduous to a mainly deciduous woodland (Gulf Coast region). Gastropod shell isotopic studies are also in progress and should yield additional insights in relation to hydrologic, ecological, and climatic conditions.