Carbon isotopic composition of clam shells has been used as an indicator in detecting climate change and ocean acidification, because these shells are composed of calcium carbonate (CaCO3) and ocean acidification will affect the ability of clams to build protective shells and skeletons. When the anthropogenic CO2 sinks into ocean, it will produce HCO3- and CO32- and this affects the carbonate saturation state Ω. A number of investigations suggested that a steady decrease of d13C values throughout a calcifying organism’s life span can be used as signatures of ocean acidification, because ocean acidification will lead to a shift in DIC (dissolved inorganic carbon) equilibria and result in higher CO2 and lower carbonate ion (CO32-) concentrations.
The Pacific geoduck, Panopea abrupta, is an important species and mainly cultured in the southern Puget Sound of Washington State, USA. Geoduck shells are composed of calcium carbonate, and exist in polymorphism as aragonite and calcite. There are also clear rings or layers that precipitated on geoduck shells as annual growth records. The chemical composition and a time series are especially suitable for stable carbon and oxygen isotope ratio analyses (13C/12C or d13C, and 18O/16O or d18O). In this study, we review the theory and practice of using stable carbon isotope ratios in detecting ocean acidification, and report new research examples from geoduck shells from Puget Sound. Two geoduck shell samples were selected, with a length of about 116 and 90 mm from the umbo to the ventral margin, respectively. The 1st geoduck sample was cultured from 2002 for 10 years (2002-2011), and the 2nd sample was cultured from 2011 for 7 years (2011-2017). The d13C values of the 1st shell ranged from -0.62 to +1.28‰, whereas the d13C values of the 2nd shell ranged from +0.12 to +1.22‰. The d13C profiles showed some seasonal variation but more strong decreasing trends over the geoduck’s life span, reflecting a growth event of about 15 years in southern Puget Sound. The d18O values of the shell samples, in contrast, showed a clear life history that was consistent with the annual growth lines on the outside of the shell. In summary, the carbon isotopic composition of geoduck shells revealed isotopic signatures of ocean acidification in Puget Sound, and the shell carbonate are a unique proxy for reconstructing the life history and marine environmental changes that a clam was encountered.