Within‐shell variations in carbon and oxygen isotope compositions of two modern brachiopods from a subtropical shelf environment off Amami‐o‐shima, southwestern Japan

Abstract

This study examines the fidelity of carbon (δ13C) and oxygen (δ18O) isotope compositions of two modern brachiopod species (Kikaithyris hanzawai and Basiliola lucida) to use as proxies of δ13C values of total dissolved inorganic carbon and temperature and/or δ18O values of seawater, respectively. Well‐preserved shells of K. hanzawai and living individuals of B. lucida were collected from a subtropical shelf environment off Amami‐o‐shima, southwestern Japan. Some portions of the shells are in isotopic equilibrium with the ambient seawater, while other portions are not. The degree of disequilibrium differs between the two species and between different portions of each shell. Statistically significant positive correlations are recognized between the δ13C and δ18O values of these samples, which can be ascribed to a kinetic fractionation effect. Far from the posterior shell edge and along the axis of shell growth, the δ18O values of the secondary shell layer of K. hanzawai mostly fall within the expected range of equilibrium calcite. The δ13C values from the inner surface of the secondary shell layer in both species are relatively constant and are equivalent to or very close to equilibrium calcite. Therefore, these portions of the shells are most suitable for collecting reliable environmental proxy data. Although the δ13C and δ18O values of modern brachiopod shells are predominantly controlled by a kinetic fractionation effect, the appropriate selection of species and shell portions that reflect the isotopic composition of ambient seawater can facilitate the reconstruction of secular variations in oceanic δ13C or δ18O values.