The potential of combined Mg/Ca and δ18O measurements within the shell of the bivalve Pecten maximus to estimate seawater δ18O composition

The combined use of δ18O and another independent palaeotemperature proxy from the same carbonate biomineral archive forms the basis of a much desired palaeosalinity proxy by circumventing the main source of uncertainty in interpreting δ18O palaeorecords, i.e. variations in the isotopic composition of seawater. Contrary to the δ18O proxy, transferring the Mg/Ca–temperature proxy to marine bivalves has proved problematic, since a strong seawater temperature control on shell calcite Mg/Ca has rarely been observed. In this study, a preliminary evaluation of the combined use of δ18Ocalcite and Mg/Ca from Pecten maximus inner shell calcite to estimate δ18Owater is reported. New Mg/Ca data obtained by milling the inner shell regions from P. maximus specimens grown in laboratory aquaria under constant seawater temperatures between 10 and 20 °C exhibit a strong relationship between Mg/Ca and temperature [Mg/Ca = 2.56 (± 0.42) + 0.17 (± 0.03) ∗ T, r2 = 0.77, p < 0.001, N = 54], while the relationship between Sr/Ca and temperature was not strong (r2 = 0.30, p < 0.001, N = 54). Using specimens grown in a separate aquaria validation experiment, estimated Mg/Ca-derived seawater temperature was within 0.48 °C of measured seawater temperature, while estimated δ18Owater was within 0.05 to 0.15‰ of measured seawater δ18O composition, depending on the δ18O–temperature equation used, and with uncertainty estimates of 0.63‰ and 0.35‰. Such observations indicate the strongest potential to date for the development of valid temperature and δ18Owater/salinity proxies in bivalves from the combined application of Mg/Ca and δ18Ocalcite in the inner shell regions of P. maximus. Further studies for a wide range of pectinid species, ages and different natural environmental settings are needed to validate the observations of this study.