Microscale magnesium distribution in shell of the Mediterranean mussel Mytilus galloprovincialis: An example of multiple factors controlling Mg/Ca in biogenic calcite

Abstract

Since magnesium concentration (Mg/Ca) in biogenic calcite is considered to reflect water temperature during precipitation, the magnesium-to‑calcium ratio has been examined as a proxy for water temperature in paleoclimate research, although factors other than temperature may also influence Mg/Ca in biogenic calcite, thereby introducing a potential bias in the relationship between Mg/Ca and temperature observed in inorganic systems. To better understand factors controlling Mg incorporation into the calcitic shells of bivalves, the distribution of Mg in the Mediterranean mussel Mytilus galloprovincialis was studied, being compared with ambient sea surface temperature (SST), shell growth rate and the distribution of organic matter. Although a positive relationship between Mg/Ca and SST was observed, Mg/Ca had been influenced by additional factors, including the enhancement of Mg incorporation by organic matter, evidenced by the growth line being more Mg-enriched than the growth increments. Furthermore, Mg/Ca was relatively enriched in the undulating (higher curvature) shell portion, being linked to neither SST, growth rate or organic matter. Zoning of Mg2+/Ca2+ within extrapallial fluid at the time of undulating shell portion formation was hypothesized, and heterogeneous Mg distribution in contemporaneously formed shell portions concluded as limiting the usability of Mytilus shell Mg/Ca as a proxy for water temperature.