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

Fossils of rhynchonelliform brachiopods (marine invertebrates) constitute similar to 70% of the samples used for delineating a well-known Phanerozoic trend in oxygen isotope composition (delta(18)O) of low-Mg calcite shells. The trend represents secular variations in temperature and/or delta(18)O 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 delta(18)O 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 delta(18)O 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. Citation: Yamamoto, K., R. Asami, and Y. Iryu (2011), Brachiopod taxa and shell portions reliably recording past ocean environments: Toward establishing a robust paleoceanographic proxy, Geophys. Res. Lett., 38, L13601, doi:10.1029/2011GL047134.