Sampling shells for seasonality: oxygen isotope analysis on shell carbonates of the inter-tidal gastropod Monodonta lineata (da Costa) from populations across its modern range and from a Mesolithic site in southern Britain

Abstract

The degree of resolution of seasonal estimates using stable oxygen isotopes from marine shells is determined by various factors, including the potential of the species selected to track annual shifts of temperatures and the size of the annual growth increments. Where the amount of annual shell growth is small, seasonal resolution of samples of shell material might be affected by time averaging of isotopic signals. There is also a need to control the number of samples analysed to affordable levels, which implies planning of appropriate sampling strategies. In this study, we investigate variations in δ 18O in both modern and archaeological specimens of the inter-tidal gastropod Monodonta lineata (da Costa), in long sequences of samples (across more than one full year of growth) and in short sequences (three samples) taken back from the edge of the shell. Studies on a number of living populations across the modern range of the species, and at two selected localities over a full year, showed that the δ 18O composition of the growing shells generally corresponds to seasonal temperature change. Applying these approaches to shells of M. lineata from the Mesolithic site of Culverwell, Isle of Portland, it was shown that season of death could be demonstrated by interpreting both patterns of edge sequences and actual edge values of δ 18O. For this site, season of death was shown to range mainly from autumn to winter, with the possibility of some early spring collection. For Mesolithic communities relying heavily on shellfish and probably other marine and coastal resources, the identification of this season of death has direct bearing on their scheduling of subsistence behaviour and on patterns of settlement.