Sclerochronology and geochemical variation in limpet shells (Patella vulgata): A new archive to reconstruct coastal sea surface temperature

Climate archives contained in shells of the European limpet, Patella vulgata, accumulated in archaeological deposits can potentially provide much needed information about Holocene environmental change in midlatitude coastal areas. Before reconstructing climate information preserved in these zooarchaeological records, we studied the controls on oxygen and carbon isotope ratios (δ 18O and δ 13C, respectively) in modern specimens. We tested the hypothesis that P. vulgata precipitates its shell in isotopic equilibrium with the ambient water by comparing δ 18OSHELL with predicted values. Predicted δ 18OSHELL was constructed using observed sea surface temperature (SST) records and the equilibrium fractionation equation for calcite and water. We assumed a constant δ 18OWATER value of +0.10‰ (VSMOW) based on published regional measurements. Comparison of δ 18OSHELL with predicted values revealed that δ 18OSHELL values were higher than expected by +1.01 ± 0.21‰. Consequently, estimated SST calculated from δ 18OSHELL was 4.2 ± 2.3°C lower than observed SST. However, because of the relatively uniform offset between observed and expected δ 18O, an adjustment can be made to account for this predictable vital effect. Thus past climate can be reliably reconstructed using this temperature proxy once the offset is taken into account. δ 13C values have a similar cyclicity to the δ 18O variation and therefore vary seasonally. However, δ 13C is slightly out of phase relative to δ 18O. An overall negative shift in δ 13CSHELL over the lifetime of the individual indicates a vital effect associated with ontogeny. Further study of environmental and ecological factors that influence shell δ 13C is required to evaluate fully the potential of carbon isotope ratios as a useful environmental proxy.