yesThe Mg/Ca ratios of biogenic calcite is commonly\ud seen as a valuable palaeo-proxy for reconstructing\ud past ocean temperatures. The temperature dependence of\ud Mg/Ca ratios in bivalve calcite has been the subject of contradictory\ud observations. The palaeoceanographic use of a\ud geochemical proxy is dependent on initial, rigorous calibration\ud and validation of relationships between the proxy\ud and the ambient environmental variable to be reconstructed.\ud Shell Mg/Ca ratio data are reported for the calcite of two bivalve\ud species, Mytilus edulis (common mussel) and Pecten\ud maximus (king scallop), which were grown in laboratory\ud culturing experiments at controlled and constant aquarium\ud seawater temperatures over a range from 10 to 20 C.\ud Furthermore, Mg/Ca ratio data of laboratory- and fieldgrown\ud M. edulis specimens were compared. Only a weak,\ud albeit significant, shell Mg/Ca ratio¿temperature relationship\ud was observed in the two bivalve species: M. edulis\ud (r2=0.37, p<0.001 for laboratory-cultured specimens and\ud r2=0.50, p<0.001 for field-cultured specimens) and P. maximus\ud (r2=0.21, p<0.001 for laboratory-cultured specimens\ud only). In the two species, shell Mg/Ca ratios were not found\ud to be controlled by shell growth rate or salinity. The Mg/Ca\ud ratios in the shells exhibited a large degree of variability\ud among and within species and individuals. The results suggest\ud that the use of bivalve calcite Mg/Ca ratios as a temperature\ud proxy is limited, at least in the species studied to\ud date. Such limitations are most likely due to the presence\ud of physiological effects on Mg incorporation in bivalve calcite.\ud The utilization is further limited by the great variability\ud both within and among shells of the same species that were\ud precipitated under the same ambient condition
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