Carbon isotope composition in modern brachiopod calcite: A case of equilibrium with seawater?

We examined a large number of modern, shallow-water articulated brachiopods representing the orders Terebratulida, Rhynchonellida, Thecideida and one inarticulated brachiopod of the order Craniida from polar to tropical regions for their carbon isotope compositions. Based on our detailed investigation, we recommend avoiding fast growth areas such as the youngest shell increments; in addition, the primary layer and transition zone calcites of brachiopods must be avoided because they are in carbon and oxygen isotope disequilibrium with ambient seawater. After adjusting isotope compositions for the Mg effect, we observed no significant difference (p>0.05) in δ13C values between dorsal and ventral valves of our articulated brachiopods.