CALIBRATING THE OXYGEN STABLE ISOTOPES OF THE ROCKY INTERTIDAL GASTROPODS PATELLA CRENATA AND PHORCUS (OSILINUS) ATRATUS FROM THE CANARY ISLANDS AS PALEOCLIMATIC PROXIES

The calcitic Patella crenata and the aragonitic Phorcus (Osilinus) atratus mollusks from the Canary Islands (27-29°N), have the potential to provide invaluable high-resolution paleoclimatic data in the oxygen isotope composition (δ¹⁸O) of their shells. However, because these species have never been studied in low-latitude environments, where climatic variations are relatively subdued and shell growth may be rapid and continuous throughout all seasons, it is necessary to calibrate and validate this approach by using modern specimens. Live organisms were collected from both species in the rocky-intertidal coast of SE Tenerife along with sea surface temperatures (SST), and water δ¹⁸O values. Results illustrate that P. crenata δ¹⁸O values were 1.3 ± 0.2‰ higher than expected values from isotopic equilibrium, which resulted in estimated SST 5.7 ± 0.6°C lower than observed values. This documented offset, or vital effect, in P. crenata is uniform and predictable, and therefore can be corrected by subtracting 1.3‰ from the shell δ¹⁸O value. In contrast, P. atratus δ¹⁸O values overlapped with predicted values in isotopic equilibrium, corresponding to growing temperatures between 23°C and 18°C. These calculated temperatures from the shell also coincided with observed SST in the study area, and therefore no vital effects are observed for this gastropod species. Both P. crenata and P. atratus are common components of archaeological sites across the Canary Archipelago, and this study suggests that well-preserved shells of both taxa retrieved from aboriginal sites are invaluable high-resolution paleotemperature archives for the tropical/subtropical eastern Atlantic region.