Shell sclerochronology of the limpet Patella ferruginea Gmelin, 1791: Implications for growth patterns and reconstruction of past sea surface temperatures

Abstract

Understanding the environmental conditions faced by past human populations is essential to understand their behaviour, and the subsistence strategies that they adopted for survival. The study of oxygen isotope ratios in limpet shells (δ¹⁸O_shell) can provide important information on sea surface temperature (SST), shell growth patterns and the season of shell collection by human populations. Following this approach, in this paper, we assessed δ¹⁸O_shell values of three modern limpets Patella ferruginea Gmelin, 1791 collected alive in Ceuta (northern Africa) as proxies for past SST and to determine the season of shell collection at archaeological sites. Studied shells showed fast growth rates without long periods of growth stops. However, results suggested that the shells did not grow during all tidal immersions. Results also showed higher growth rates between winter and summer, although each shell exhibited its own distinctive patterns. According to the isotope data, studied limpets deposited calcium carbonate to form their shells with an average offset of +0.34 ‰ from expected equilibrium. This offset was higher in summer (0.56 ‰) and lower in winter (0.18 ‰). Reconstructed sea surface temperature (SST_δ18O) exhibited high correlation with satellite temperature. Considering the variability of the oxygen isotope composition of the seawater (δ¹⁸O_sw), past SST_δ18O can be calculated with an uncertainty of +2 °C and −1.4 °C. Our study demonstrates that δ¹⁸O values of P. ferruginea can be used to reconstruct SST provided that the δ¹⁸O_sw is known. Furthermore, the season of shell collection can be estimated from δ¹⁸O_shell curves, which has deep implications for future archaeological investigations.