LAST GLACIAL MAXIMUM SUMMER TEMPERATURE GRADIENT IN THE CENTRAL USA FROM TERRESTRIAL GASTROPOD ASSEMBLAGES IN PEORIA SILT (LOESS)

Fossil terrestrial gastropods are locally abundant in thick deposits of Peoria Silt, a last glacial maximum (Wisconsin Episode) loess unit in the Midcontinental USA deposited between ~29,000 and 15,000 years ago (fossiliferous portions mainly > 17,000 years ago). The Peoria Silt is widely distributed from Wisconsin to Louisiana and Nebraska to Ohio. In the central and eastern part of this region, loess is typically thickest and fossiliferous in uplands adjacent to major river valleys that transmitted glacial meltwaters, such as the Missouri, Mississippi, and Ohio Valleys. In the Great Plains (Nebraska to Oklahoma), Peoria loess deposition was primarily associated with semi-arid conditions, but is generally chrono-correlative with the valley-sourced loess. Preservation of fossil shells, dependent on depth of carbonate leaching and accumulation rate, is generally restricted to loess > 3 m thick. In this study, fossil gastropods assemblages were compiled from 55 sites across 14 states in the central USA, mainly from the literature but including 7 newly studied sites. Some sites are based on pooled species data from several sites in a multi-county area. From this compilation, 68 species of terrestrial gastropods are reported from Peoria Silt, with 5 to 30 species present at any given site. Sites with < 5 species of gastropods were not used. Mutual climatic range methods were used to estimate the mean July temperature at each site during the last glacial maximum. The present-day locations of the warmest and coldest July temperatures for each species were determined from published species distribution maps and a 0.5° gridded CRU TS4.07 land temperature dataset. July temperatures were used as snails tend to be more active during the warm, humid summer months. Based on this analysis, last glacial maximum mean July temperature is estimated to have ranged from 15 to 16°C in the north, near the southern Laurentide ice margin, increasing to 25°C near the Gulf Coast. These results imply that, during the last glacial maximum, July temperatures in the central USA were 3 to 9°C cooler than today. Additionally, the last glacial maximum north-south temperature gradient (~1°C /150 km) was much stronger than today’s gradient (~1°C /250km), likely in part due to the presence of glacial ice in the Great Lakes region during the Wisconsin Episode.