Mg Isotope fractionation in Glycymeris (Bivalvia) shells

Over the past 50 million years, Cenozoic climate cooling has coincided with rising Mg/Ca ratios in seawater. However, the connection between this shift in seawater chemistry and climate change remains debated. Magnesium isotope ratios (δ²⁶Mg) in seawater could help differentiate between possible mechanisms, such as dolomitization, formation of authigenic clays, and variations in silicate and carbonate weathering rates. Past reconstructions of paleo-seawater δ²⁶Mg using foraminifera, corals, and carbonate muds have yielded inconsistent results. In this study, we evaluate bivalves from the genus Glycymeris as alternative archives for paleo-seawater δ²⁶Mg. These bivalves possess several advantages, including evolutionary stability, thick shells, and a fossil record extending back to the Lower Cretaceous—traits that suggest consistent isotopic fractionation over time. We analyzed Mg isotope compositions from the ventral shell margins of three modern Glycymeris species (G. bimaculata, G. nummaria, G. pilosa) from the Adriatic Sea. Our data show increasing isotopic fractionation with ontogenetic age and distinct intra-shell differences depending on sampling location (center vs. margin) at the same developmental stage. Our results show that variations in precipitation rates best explain this behavior of Mg isotopes. Unaltered isotope signatures in fossil shells can thus be clearly identified. This work suggests that the rate-dependence of δ²⁶Mg in Glycymeris can be exploited to derive paleo-seawater δ²⁶Mg.