Detecting Ocean Acidification from Stable Isotope Records of Shell Carbonate

The use of stable carbon and oxygen isotope ratio analyses (¹³C/¹²C or d¹³C, and ¹⁸O/¹⁶O or d¹⁸O) in marine bivalve shells, particularly for carbon isotopes in detecting the effects of ocean acidification, has recently received growing attention in fisheries science and management. Bivalve shells are composed of calcium carbonate (CaCO₃), and exist in polymorphism as aragonite and calcite. Although polymorphs have identical chemical composition, they have differences such as in bioavailability, stability, and physical structure. When the anthropogenic CO₂ sinks into ocean, it will produce HCO₃^- and CO₃^2- and this affects the carbonate saturation state (Ω). The process can be detected from carbon sources and the isotopic fractionation of d¹³C between blood DIC (dissolved inorganic carbon) and the shell. In this study, we review the theory and practice of using stable isotopic tools in detecting ocean acidification, and report research examples from Pacific geoduck and sea scallop shells. Overall these results suggest that shell carbonate are good proxies for reconstructing the life history and the environmental changes that the animal experienced (mainly from d¹⁸O); and the isotopic data of clam shells have the potential to examine the effects of ocean acidification (mainly from d¹³C) in the past.