Variations of the stable isotope ratios of carbon and oxygen of the Patella vulgata (Linnæus, 1758) shells according to the vertical zonation of collect

Isotopic investigations performed on carbonated shells from anthropogenic middens provide information about past environments and cultural practices of the human populations who left these vestiges. Patella vulgata , an intertidal gastropod that inhabits northwest Atlantic rocky shores, is commonly occurring as shells in European shell middens, constituting therefore a good candidate for isotopic studies. However, high tidal ranges characterize most of European Atlantic coastlines, and species largely distributed upper to lower foreshore, as Patella sp., may encounter pronounced variations in immersion time depending on their elevation in the intertidal zone, which could affect their shell stable isotopic ratios. In this study, we compared carbon and oxygen stable isotopes of Patella vulgata shells collected in the same station at three different cotidal levels. Isotopic measurements are analyzed in terms of individual growth and aim to characterize the cotidal origin of the specimens. Our results demonstrate the absence of a significant link between the cotidal level on the shore and the growth and δ 18 O profiles of the shell, as high shore and low shore shells show the highest similitude while medium shore shells had a reduced growth with a higher heterogeneity. Consequently, the SST reconstructions were not significantly different. Thus, the use of the δ 18 O of archaeological shells as an SST proxy appears unaffected by the cotidal origin of the specimens. In contrast, the δ 13 C scales from the high shore (lower δ 13 C values) to the low shore (higher δ 13 C), constituting a promising new proxy for determining the precise elevation in the intertidal zone where the shells were collected.