Stable isotope profiles (Ca,O, C) through modern brachiopod shells of T. septentrionalis and G. vitreus: Implications for calcium isotope paleo-ocean chemistry

Brachiopod shells are widely used as an archive to reconstruct elemental and isotopic composition of seawater. Studies, focused on oxygen and carbon isotopes over the last decades, are increasingly extending to the emerging calcium isotope system. To date, only little attention has been paid to test the reliability of fossil brachiopods on their modern counterparts.In this context, the present study investigates two modern brachiopods, Terebratulina septentrionalis (eastern Canada, 5–30 m depth, 7.1 °C seasonal temperature variation, two-layer shell) and Gryphus vitreus (northern Mediterranean, 200 m depth, constant all-year round temperature, three-layer shell). Both species were sampled along the ontogenetic growth direction and calcium, oxygen, and carbon isotopes as well as elemental concentration were measured. Calcium isotopes were analyzed on TIMS. The elemental composition was analyzed by LA-ICP-MS and ICP-AES.The results indicate an intra-specimen δ^44/40Ca variation ranging from 0.16 to 0.33‰, pointing to a fairly homogenous distribution of calcium isotopes in brachiopod shells. However, in the light of the suggested 0.7‰ increase in calcium isotopes over the Phanerozoic such intra-specimen variations constrain ocean reconstruction. δ^44/40Ca values of T. septentrionalis do not seem to be affected by growth rate. Calcium isotopic values of G. vitreus are heavy in the central part of the shell and trend towards lighter values in peripheral areas approaching the maximum isotopic composition of T. septentrionalis. The maximum inter-species δ^44/40Ca difference of 0.62‰ between T. septentrionalis and G. vitreus indicates that care should be taken when using different taxa, species with different strontium content or brachiopods with specialized shell structure, such as G. vitreus, for ocean water reconstruction in terms of Ca isotopic composition. T. septentrionalis may record Ca isotopic fractionation related to seasonal seawater temperature variations in its shell but this is difficult to resolve at the current analytical precision. Average δ¹⁸O-derived temperatures of the two investigated species are close to on-site measured temperatures.