| Summary: | The chronology and processes that led to the formation of the Jules Geosol were previously not well understood. This last glacial paleosol developed in thick loess deposits (Peoria Silt) proximal to the Illinois River Valley around the time of the last glacial maximum. Two sites, New Cottonwood School and Thomas Quarry, which contain the Jules Geosol, were selected along the Illinois Valley to develop new chronologies of Peoria Silt deposition and measure changes in geochemical and geophysical properties, as well as gastropod assemblages across the Jules Geosol and bounding Peoria silt units. Peoria Silt was deposited between 28,500 and 18,800-16,000 cal yrs BP and the Jules Geosol formed between 23,735 ± 295 and 22,010 ± 240 cal yrs BP, almost 5,000 years older than prior measurements. Clay mineralogy, grain size, and magnetic susceptibility show primary loess was finer and from a distal Mississippi Valley source during Jules Geosol development. Mutual climatic range (MCR) assessments and relative abundances of gastropods indicate mean July temperatures were fairly constant through the Jules Geosol at both New Cottonwood School and Thomas Quarry. Measurements of shell δ18O also exhibit no significant changes during the formation of the Jules Geosol at New Cottonwood School. However, δ18O values of shells from Thomas Quarry decrease by 2‰ from the lower Peoria to the middle Peoria coincident with an abrupt shift in magnetic susceptibility related to the diversion of the Mississippi River and regional ice sheet advance. We thus conclude the Jules Geosol formed due to a decrease in the supply of Illinois Valley silt rather than climate variability. However, a coincident abrupt shift in glacial sediment source and δ18O at the time of the Mississippi diversion suggests a tight coupling between regional ice sheet dynamics, loess deposition, and climate in this region during the Last Glacial period.
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