Determining the habitats of mollusk collection at the Sapelo Island shell ring complex, Georgia, USA using oxygen isotope sclerochronology

Sequential oxygen isotope analysis was performed on Mercenaria spp. (n = 41) and Crassostrea virginica (n = 17) valves excavated from the Sapelo Island Shell Ring complex, coastal Georgia, USA. The data were analyzed to determine the habitats in which the mollusks were harvested in order to gain insight into the subsistence practices of the site occupants. The rationale for this study is based on the observation that temperatures are similar in all of the habitats surrounding the island, but the δ18Owater trends across the habitats co-vary with salinity. Accordingly, the shells grown in these habitats contain sinusoidal oscillations in δ18O through ontogeny due primarily to seasonal temperature variation, but the absolute values reflect the salinity ranges of each habitat. Assuming a relatively constant oxygen isotope/salinity gradient since the time of site occupation, it appears that both of these mollusks were exploited over nearly the full range of their salinity tolerances. While a precise estimate of travel distance during collection cannot be made, these data suggest that mollusks were harvested at distances from the site greater than would be predicted using terrestrial foraging models. In addition to providing insight into past subsistence strategies in this region, this research establishes a new method of assessing catchment area and mobility practices.

► δ18O values in clam and oyster valves vary with the salinity of their habitat.
► We measured δ18O in shells excavated from Sapelo Shell Ring site.
► Inferred salinities suggest site occupants exploited shells from a wide area.
► These data improve subsistence modeling.