Rate-dependent Mg isotope fractionation in shells of Glycymeris (Bivalvia)

Magnesium (Mg) isotopes in biogenic marine carbonates offer a promising proxy for reconstructing past seawater Mg isotope compositions, thereby providing insights into the mechanisms driving Cenozoic climate cooling. This study assesses the bivalve Glycymeris as an alternative geochemical archive to foraminifera and corals, which have yielded inconsistent results in previous work. Specimens from three modern species— Glycymeris bimaculata , G. nummaria , and G. pilosa —spanning a range of ontogenetic ages from 1 to 16 were collected from the Adriatic Sea. Shell samples were taken from the ventral margins of individuals, with one representative individual analyzed per species. Magnesium was rigorously purified using a three-step chromatographic protocol that combined a semi-automated prepFAST system with manual ion-exchange chromatography. Results show that Mg isotope fractionation increases with ontogenetic age in both ventral-margin and intra-shell samples. A geometric model based on relative shell areas was developed to quantify precipitation rates that reveals a precipitation-rate dependence, which is consistent with abiogenic calcite experiments. These findings indicate that Mg isotope fractionation in Glycymeris shells is not species-specific and that these bivalves are potential archives for reconstructing past seawater Mg isotope variations. • δ 26 Mg shows consistent trends across three Glycymeris species with ontogenetic age. • A geometric growth model was developed for the shells. • Intra-shell δ 26 Mg variations correlate with shell growth rate. • Glycymeris shells show potential as archives of past seawater δ 26 Mg.