LOESS ACCUMULATION AND HYDROCLIMATE VARIABILITY IN ASSOCIATION WITH LAKE MICHIGAN LOBE FLUCTUATIONS DURING THE LAST GLACIAL MAXIMUM

A new age model of Peoria Silt accumulation along the Illinois River Valley (ancestral Mississippi Valley) indicates loess accumulated episodically from ~28,500 to 18,000-16,000 cal yr BP, as the Lake Michigan Lobe margin fluctuated within northeastern Illinois. We find that millennial-scale periods of rapid loess deposition were coincident with regional glacial advances during the local Last Glacial Maximum. Overall, a significant portion of the Peoria Silt accumulated when the Lake Michigan Lobe was at or near its southernmost extent. This finding stands in contrast to previous, limited constraints on the timing of Peoria Silt deposition, which suggested high rates of loess accumulation persisted through the deglaciation, even after the Lake Michigan Lobe had retreated into the Lake Michigan Basin. A weakly developed paleosol, the Jules Geosol, represents a period of significantly slower deposition, from 23,700 to 22,000 cal yr BP. Stable oxygen and carbon isotope ratios (δ¹⁸O and δ¹³C) of gastropod carbonate do not vary significantly across the pedostratigraphic boundary of the Jules Geosol, suggesting slower loess accumulation was a result of reduced glacial sediment supply rather than direct climatic factors. However, a decrease in δ¹⁸O values occurred between 26,000 and 24,000 cal yr BP, synchronous with the Lake Michigan Lobe’s southernmost advance and the Mississippi River’s diversion. This δ¹⁸O decrease suggests a coupling of regional summer hydroclimate and ice lobe position during the late glacial period, and highlights the utility of loess paleoclimate records to gain insight into the interconnectivity of glacial, aeolian, and atmospheric processes.