Impact of sources and form of Mg on oyster shell Mg/Ca

Mg/Ca in bivalve shells has been investigated as a promising temperature proxy, but several studies reported compositional shifts that hamper its accuracy. In particular, several models linking shell Mg/Ca and temperature have been published and an empirical difference in the seasonal amplitudes of shell Mg/Ca has been observed between two types of environmental settings: river output, open marine. Here, we investigate several environmental parameters that could be responsible for this difference, using rearing experiments of oysters in natural and artificial seawater. We first tested the influence of Mg form in seawater on shell Mg/Ca and showed that Mg is incorporated in shell regardless of it form (complexed and free). Second, we investigated the sensitivity of shell Mg/Ca to seawater Mg/Ca by artificially increasing the Mg content in seawater. We observe an increase in shell Mg/Ca with seawater Mg/Ca, as previously suggested. Finally, we show that a change in diet induces a significant difference in shell Mg/Ca. Overall, our results suggest that, besides temperature, seawater and food Mg/Ca control shell Mg/Ca. These parameters could be responsible for the discrepancy of Mg/Ca-temperature models, and should be considered for future aquarium and in situ calibration studies, as well as paleoenvironmental reconstructions.