Brachiopod taxa and shell portions reliably recording past ocean environments: Toward establishing a robust paleoceanographic proxy

Fossils of rhynchonelliform brachiopods (marine invertebrates) constitute ∼70% of the samples used for delineating a well-known Phanerozoic trend in oxygen isotope composition (δ¹⁸O) of low-Mg calcite shells. The trend represents secular variations in temperature and/or δ¹⁸O of ancient seawater. However, the use of brachiopods as a paleoceanographic proxy is based on the presupposition that the shell calcite is precipitated in isotopic equilibrium with ambient seawater. Here, we show that high-resolution time series of the shell δ¹⁸O values along the maximum growth axes of two long-lived cool-water brachiopods are identical to, greater than, or less than those of calcite precipitated in isotopic equilibrium with ambient seawater, depending on the difference in shell growth rates. Coupled with δ¹⁸O data from subtropical and warm-temperate brachiopod species examined in our previous studies, we provide a sound framework illustrating which taxa and shell portions reliably recorded past ocean environments.