Paleoenvironmental analyses based on carbon and oxygen isotopic compositions of brachiopod shells: A review

Stable carbon and oxygen isotopes in skeletal carbonates are powerful tools for reconstructing various paleoenvironmental conditions. Of many marine invertebrates, articulated brachiopods have been preferentially used as a reliable recorder of carbon and oxygen isotopic compositions in the past oceans because: (1) their dense, low-magnesium calcite shells are less susceptible to diagenetic alteration, (2) their calcitic secondary shell layer is assumed to have been precipitated in or near isotopic equilibrium with ambient seawater, and (3) they have extensive geographic distributions throughout the Phanerozoic. Recent investigations, however, have demonstrated that previously proposed criteria to identify diagenetic alteration in fossil brachiopod shells are not unequivocal and that the secondary shell layer of some living brachiopods is not precipitated in isotopic equilibrium with ambient seawater due to kinetic and/or metabolic effects. However, these results argue against a general rejection of availability of isotopic compositions of brachiopod shells as paleoenvironmental proxies. Further geochemical investigations are needed to reveal temporal and spatial variations in isotopic compositions within brachiopod shells for evaluating kinetic and/or metabolic effects and to clarify isotopic modifications due to diagenetic alteration.