Revisiting the Jules Geosol and gastropod paleoecology in last glacial maximum loess, western Illinois

A new investigation of the interstadial Jules Geosol in western Illinois has revealed additional insights about its context within a revised chronology and regional glacial history. The Jules Geosol is recognized as a weakly developed paleosol, with abundant secondary carbonate nodules, within the upper-middle portion of last glacial loess (Peoria Silt). The paleosol contains an amalgamation of weak cumulic A horizons associated with a period of reduced, but sporadic, loess deposition. Increased smectite/illite and fine silt/coarse silt ratios in the paleosol zone imply a reduction in locally sourced (Illinois Valley) loess. Radiocarbon ages from terrestrial gastopods (Succinea, Webbhelix) indicate rapid loess deposition immediately preceding (approximately 24.5 to 23.5 cal ka) and following (approximately 22 to 20.5 cal ka) the development of the Jules Geosol. By inference, the Jules Geosol developed during the intervening period (approximately 23.5 to 22 cal ka), significantly older than prior age estimates. The timing of rapid loess deposition corresponds with glacial advances to the prominent Shelbyville-Bloomington and Marseilles Morainic Systems, and likely increased glacial silt production. Paleosol development thus reflects reduced sedimentation and chronologically corresponds with the initial Lake Michigan Lobe recession from its maximum advance. Meltwater discharges during recession may have led to valley incision or erosion rather than silt deposition; thus reducing the availability of fine-grained sediment for eolian entrainment. The assemblage of terrestrial gastropods in the loess sequence include 12 species, notably Succinea sp., Webbhelix multilineata, Columella alticola (rare), Vertigo eliator, Vertigo modesta (rare), Stenotrema hirsutum, Euconolus fulvus, Carychium exile, and Discus whitneyi. This assemblage of woodland species is mainly typical of the boreal forests of western Ontario, north of Lake Superior. Mutual climatic range methods suggest a mean annual temperature of approximately 1 to 3 degrees C for the last glacial maximum in west-central Illinois (approximately 9 degrees C colder than present). A change in temperature or humidity during the time of paleosol formation is possible; however, limited isotopic data from sporadically preserved shells within the Jules Geosol have not shown significant differences from adjacent loess.