Isotopic compositions of brachiopod shells. Toward a definitive understanding ofδ^<13> C andδ^<18> O evolution of Phanerozoic seawater

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This study examines inter-and intraspecific variations in carbon-and oxygen-isotope compositions(.^<13> C and.^<18> O) of modern brachiopod shells to establish sound basis to use the.^<13> C and.^<18> O values of fossil brachiopods as proxies for.^<13> C values of dissolved inorganic carbon and seawater temperature and/or.^<18> O values(.^<18> OSW). Interspecific variations in.^<13> C and.^<18> O values of six modern-brachiopod species collected from a subtropical to cool-temperate environments showed that the degree of the vital effect is highly variable among species. Intraspecific variation in.^<13> C and.^<18> O values of two modern-brachiopod species(9 and 8 specimens of Basiliola lucida and Terebratulina crossei, respectively) demonstrated that.^<13> C and.^<18> O values of some shell portions show small variations and are identical or close to those of calcite precipitated in isotopic equilibrium with the surrounding seawater. Such portions are useful to reliably reconstruct ancient ocean environments.