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 delta O-18 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 delta O-18 palaeorecords, i.e. variations in the isotopic composition of seawater. Contrary to the delta O-18 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 delta O-18(calcite) and Mg/Ca from Pecten maximus inner shell calcite to estimate delta O-18(water) 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 degrees C exhibit a strong relationship between Mg/Ca and temperature [Mg/Ca = 2.56 (+/- 0.42) + 0.17 (+/- 0.03)*T, r(2) = 0.77, p<0.001, N = 54], while the relationship between Sr/Ca and temperature was not strong (r(2)=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 degrees C of measured seawater temperature, while estimated delta O-18(water) was within 0.05 to 0.15% of measured seawater delta O-18 composition, depending on the delta O-18-temperature equation used, and with uncertainty estimates of 0.63 parts per thousand and 0.35%.. Such observations indicate the strongest potential to date for the development of valid temperature and delta(18)O(water/)salinity proxies in bivalves from the combined application of Mg/Ca and delta O-18(calcite) 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. (C) 2011 Elsevier B.V. All rights reserved.