Evidence for seawater Mg/Ca and dietary control on Mg incorporation in oyster shells

Marie Pesnin, Laurent Emmanuel, Amelie Guittet, Boris Eyheraguibel, Gaëtan Schires, Julien Normand, Vincent Mouchi 

The Mg/Ca ratio in bivalve shells has been investigated as a promising proxy for temperature reconstruction of the seawater. However, important discrepancies exist among the Mg/Ca-temperature calibrations reported for bivalves in the literature. These discrepancies currently limit the use of Mg/Ca for paleoclimate reconstructions based on bivalves and suggest that factors beyond temperature influence Mg incorporation in the shell. In particular, several studies have highlighted empirical differences in the seasonal amplitudes of shell Mg/Ca between two types of environments: river-influenced coastal settings and open marine systems. In this study, we investigate several environmental parameters that may account for these differences, using rearing experiments of oysters in both natural and artificial seawater. First, we tested the influence of Mg speciation in seawater on shell Mg/Ca and showed that Mg is incorporated in the shell regardless of its form (free or complexed with organics). Second, we investigated the sensitivity of shell Mg/Ca to seawater Mg/Ca by artificially increasing the Mg concentration in seawater. We observed an increase in shell Mg/Ca concomitant with the rise in seawater Mg/Ca, confirming the dependance of shell Mg/Ca on seawater Mg content. Finally, we show that a change in diet induces a significant difference in shell Mg/Ca, and that shell Mg/Ca is positively correlated with the Mg/Ca of the diet. Overall, our results suggest that, in addition to temperature, both seawater and dietary Mg/Ca ratios control shell Mg/Ca. These parameters could be responsible for the discrepancy between Mg/Ca-temperature models published in the literature, and should be carefully considered in future aquarium-based and in situ calibration studies, as well as paleoenvironmental reconstructions.