Terrestrial gastropods are one of the most successful animal groups on Earth. They occupy and thrive in incredibly diverse habitats, ranging from marshes and grasslands to upland forests and arctic tundra. Their distribution in the fossil record is equally diverse. Gastropod shells are commonly found in lake, wetland, alluvial, loess, and glacial deposits, as well as in sediments at archeological sites worldwide. Yet despite their widespread occurrence in the fossil record and the fact that they contain ~12% by weight carbon, gastropod shells are usually avoided for 14C dating because many taxa incorporate 14C-deficient (or "dead") carbon from limestone when building their shells. Geochronologists refer to this as the "Limestone Problem" because the amount of dead carbon incorporated in gastropod shells is variable at all taxonomic levels and can cause 14C ages to be up to 3,000 years too old. However, PIs preliminary results from the American Southwest suggest that some small (maximum body dimension <10 mm) gastropods avoid limestone even when it is readily available. Moreover, these same taxa appear to remain closed systems with respect to carbon over geologic timescales in arid environments. Thus, at least in dry conditions, some small terrestrial gastropod shells may in fact provide reliable 14C ages. The primary research objective of this study is to clearly identify gastropod taxa that are capable of consistently yielding reliable 14C ages, regardless of local lithology, environmental conditions, climate, or ecological habitat. To this end, PIs will conduct a systematic analysis of the 14C content of a suite of modern and fossil terrestrial gastropods that are endemic to North America. They have targeted more than 30 extant species that will be used to answer the following research questions: (1) do individuals of a specific taxon always avoid limestone when building their shells, and (2) if so, do their shells remain closed systems over time regardless of local environmental conditions? The wide taxonomic, ecological, and taphonomic scope of this study will allow PIs to determine what types of gastropod shells can be used to reliably date Quaternary sediments. Moreover, this approach will help other investigators identify additional target species, including those outside North America, by providing a priori guidelines for the identification of taxa that use only metabolically-generated carbon in shell construction and that will hence provide accurate 14C ages.
