Episodic deposition of Illinois Valley Peoria silt in association with Lake Michigan Lobe fluctuations during the last glacial maximum

The chronology and cause of millennial depositional oscillations within last glacial loess of the Central Lowlands of the United States are uncertain. Here, we present a new age model that indicates the Peoria Silt along the Illinois River Valley accumulated episodically from ~28,500 to 16,000 cal yr BP, as the Lake Michigan Lobe margin fluctuated within northeastern Illinois. The age model indicates accelerated loess deposition coincident with regional glacial advances during the local last glacial maximum. A weakly developed paleosol, the Jules Geosol, represents a period of significantly slower deposition, from 23,700 to 22,000 cal yr BP. A gastropod assemblage-based reconstruction of mean July temperature shows temperatures 6-10°C cooler than modern during Peoria Silt deposition. Stable oxygen and carbon isotope values (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. This δ¹⁸O decrease suggests a coupling of regional summer hydroclimate and ice lobe position during the late glacial period.