Shell sclerochronology and stable oxygen isotope ratios from the limpet Patella depressa Pennant, 1777: Implications for palaeoclimate reconstruction and archaeology in northern Spain

Stable oxygen isotope ratios of mollusc shells (δO) offer the possibility to reconstruct coastal resource exploitation patterns and changes in the oceanographic conditions of direct relevance to past human populations. This method relies on the fact that shell carbonate is deposited by molluscs in equilibrium with their surrounding environment and actualistic investigation of modern specimens is needed to ensure that selected species can be used as accurate palaeoclimate indicators. The limpet Patella depressa Pennant, 1777 is one of the most common mollusc species found in Holocene archaeological assemblages along the Atlantic coast of Europe. However, this taxon has not, to date, been tested as a seawater palaeothermometer. Here, we explore the ability of P. depressa to be used as an environmental recorder in this littoral region, specifically in northern Iberia where we obtained live-collected specimens throughout the year. We undertook sclerochronological investigations combining observations of incremental shell growth patterns with δO values. Carbonate samples were taken on (i) the ventral margins of shells collected alive year-round in order to test for isotopic equilibrium and (ii) along the axis of maximum shell growth of four modern specimens to decipher the shell growth pattern of this species. Isotopic data showed that calcium carbonate is precipitated in predictable isotopic disequilibrium with the ambient seawater (R = 0.95; p < 0.0001), reporting a consistent offset of +1.08‰. Some periods of growth cessation were observed in the shells when thermal tolerances were exceeded in winter and occasionally in summer. Nevertheless, estimated seawater temperatures from modern shell δO values agreed closely with instrumentally measured temperatures (R = 0.88–0.93; p < 0.0001) and correctly reflected seasonal temperature patterns. As a result, multi-proxy analysis of P. depressa shells can provide a high-resolution palaeothermometer with significant implications for future palaeoclimate and archaeological studies along the Atlantic coast of Europe