EXAMPLES OF PALEOECOLOGY AND PALEOCLIMATE INTERPRETATIONS FROM FOSSIL (LATE PLEISTOCENE-HOLOCENE) AQUATIC AND TERRESTRIAL GASTROPODS IN ILLINOIS AND INDIANA

Fossil gastropods can be valuable for use in paleoecological, paleoclimate, stratigraphic, and chronological aspects of Quaternary research. Here, we provide case studies from four different late Quaternary environments. In last glacial loess deposits (Peoria Silt), land snails are used to infer paleoclimate based on the mutual temperature range method. An example is provided from Demazenod Section (SW Illinois near St. Louis) where 17 taxa of gastropods were found in thick loess bordering the Mississippi River valley. Vertical changes in the assemblage and fossil abundance may relate to multi-centennial climate changes during the last glacial maximum, with mean summer temperatures ~ 7°C cooler than today at 20–22 cal ka. The chronology is from radiocarbon dating of Succinea and Webbhelix shells. A second case study is provided from central Indiana (Clayton Section) where 9 taxa of land snails, in palustrine sediment between glacial tills, suggest a boreal to near-tundra environment. Here, minute fossil shells of Columella alticola and Vertigo oughtoni suggest cold conditions at 21.9 cal ka, with summers ~9°C cooler than today. A third example is provided by a new core drilled in a small basin in Bureau County, northwestern Illinois. This core reveals peat (0.5 m thick) over lake sediment (2.9 m thick) over last glacial outwash. The peat and lake sediment contain 9 gastropod taxa, all freshwater aquatic fauna except for Succineidae. A moderate change in the assemblage occurs at 10–13 cal ka, coinciding with the Pleistocene-Holocene boundary. Pomatiopsis cincinnatiensis (an amphibious species) is only found in Holocene sediment. In contrast, Valvata sincera (a cold water lacustrine species) is only found in last glacial sediments, and co-occurs with ostracod species and charophyte oogonium. The geochronology is based on corrected and calibrated radiocarbon ages on Valvata and Succineidae shells. In a Holocene example, core samples from the Sangamon River valley in central Illinois reveal fluvial sequences containing abundant Pleurocera acuta shells at 2–6 m depth. This species tends to live in alkaline, slow flow river environments with a sandy substrate. The age of three fluvial sediment packages ranges from ~0.2–2.5, 3.5–5.0 and 8.2–10.4 cal ka, based on wood and Pleurocera shells (corrected for old carbon).