The Effects of Ocean Acidification from Stable Isotope Analyses in Shell Carbonate

The use of stable isotope ratio analyses in shell carbonate, especially carbon isotope ratios (¹³C/¹²C or d¹³C), is a growing area in ocean acidification investigations. As the anthropogenic CO₂ sinks into ocean water, it will produce HCO₃^- and CO₃^2- and then affect 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. Here we report two examples of stable carbon and oxygen isotope ratio (¹⁸O/¹⁶O or d¹⁸O) analyses on Pacific geoduck (Panopea abrupta) and Yesso scallop (Patinopecten yessoensis) shells. The d¹⁸O values of geoduck shells ranged from –1.34 to +0.95‰, while the d¹³C values of the same samples ranged from –2.19 to +0.35‰. There were significant isotopic differences between sampling sites, and the decrease of d¹³C was consistent with the pH and dissolved oxygen variations. The life-time d¹³C values of the scallop shells, in contrast, showed a stable decrease from about +1 to -0.5‰ over the 4-year period. These examples suggest that the shell carbonate is a good proxy for life history studies, and the isotopic data have the potential to detect the effects of ocean acidification in the past.