Variability in magnesium, carbon and oxygen isotope compositions of brachiopod shells: Implications for paleoceanographic studies

• MC-ICP-MS magnesium isotope compositions measured in brachiopod shells
• Variability of the magnesium isotope composition through the shell
• δ26Mg seems to be affected by kinetic effects.
• δ26Mg of the innermost calcite layer lie in the isotopic equilibrium field.

This study presents the carbon, oxygen, and magnesium isotope compositions of two modern brachiopods, Terebratalia transversa and Frieleia halli, and one fossil specimen (2.3 Ma), Terebratula scillae. The aim of this study is to investigate the variability of these isotopic compositions and to evaluate the potential of brachiopods as a proxy of past seawater δ26Mg values. The two investigated brachiopod shells present the same range of δ26Mg variation (up to 2‰). This variation cannot be ascribed to changes in environmental parameters (temperature or pH). As has been previously observed, the primary layer of calcite shows the largest degree of oxygen and carbon isotope disequilibrium relative to seawater. In contrast, the δ26Mg value of the primary layer is comparable to that of the secondary calcite layer value. In both T. scillae and T. transversa, negative trends are observable between magnesium isotopic compositions and oxygen and carbon isotopic compositions. These trends can be explained by kinetic effects linked to changes in growth rate during the brachiopod life. The innermost calcite layer of T. transversa is in isotopic equilibrium for both oxygen and magnesium and could therefore be the best target for reconstructing past δ26Mg values of seawater.