Pleistocene molluscan assemblages to aid understanding of paleoenvironment, paleoclimate and chronology; examples from Illinois

Across the southern Midwest region, Pleistocene mollusks are locally found in relatively unaltered loess or lacustrine sediment (particularly slackwater deposits) synchronous with central U.S.A. glaciations. More rarely, molluscan faunas are preserved in interglacial lake deposits. The assemblage of gastropods and bivalves provides important information about past environments, climate, and ecology that complements other paleoenvironmental data. In addition, the shell material can be used in chronological studies. Paleoenvironmental interpretations aided by molluscan occurrences include differentiation of terrestrial vs. aquatic conditions; seasonal vs. perennial lake; deep vs. shallow water; and flowing vs. quiet water. Faunas tend to vary spatially and stratigraphically between distal and ice-proximal environments. Such observations can complement or refine interpretations from sedimentological, geomorphic, and other fossil records (e.g., pollen, ostracodes), thus providing a clearer picture of the paleoenvironment. In some cases, such as in oxidized sediments, molluscan assemblages are preserved where other records are lacking or poorly preserved. Some species of minute terrestrial gastropod genera (Vertigo, Pupilla, Columella, Gastrocopta) have climatic sensitivity and, by comparisons with modern distribution patterns (Nekola and Coles, 2010), can aid in general paleoclimate estimations. Aquatic species are generally less affected by climate but some species have sensitivity. Carbon and oxygen isotopic analyses of shell carbonate can aid with interpretations of paleovegetation, paleotemperature, or paleohumidity, though other factors should be considered. Chronologically, radiocarbon dating of Wisconsin Episode terrestrial mollusks in loess or lakeshore records are more precise and considered by many to be more accurate than luminescence dating. With the development of multiple amino acid assays in recent years, amino acid geochronology (using terrestrial or aquatic species) is proving valuable in differentiating Wisconsin, Illinois, and pre-Illinois episode deposits in Illinois, as was initially suggested in western Indiana (Miller et al. 1987). Comparisons of glutamic and aspartic acid D/L ratios with Succinea and Hendersonia are most fruitful thus far.