Seawater carbonate chemistry and the shell 13C and 18O content and growth rates in the clam Scapharca broughtonii

The effects of elevated CO2 levels on growth and stable carbon and oxygen isotope compositions (delta 13C and delta 18O) of aragonitic shells were experimentally evaluated by rearing the bloody clam, Scapharca broughtonii, at two different temperatures and at six different pCO2 levels (17 °C: 269, 382, 550, 757, 939, and 1114 µatm; 25 °C: 332, 463, 653, 872, 1137, and 1337 μatm). Ambient CO2 levels did not much affect shell growth rates. Mass spectrometric analysis of the outer shell layer indicated a negative correlation between seawater pH and shell delta 18O (equivalently, a positive correlation between seawater pCO2 and shell delta 18O) at both temperatures (regression slopes: –0.70 per-mil ± 0.14 per per-mil/ pH unit at 17 °C; –0.36 per-mil ± 0.17 per-mil / pH unit at 25 °C). A positive correlation was observed between seawater pH and shell delta13C at both temperatures, but the regression slopes were less steep than those between seawater pH and the delta13C of seawater DIC. As CO2 levels rose (and pH dropped to about 7.6), shell delta18O and delta13C increased relative to ambient dissolved inorganic carbon (DIC). Both approached apparent isotopic equilibrium in acidified waters. Shell delta 18O was below that of the ambient mix of bicarbonate and carbonate ions, and usually below apparent isotopic equilibrium with water. Kinetic discrimination against heavy isotopes during CO2 hydration and hydroxylation reactions may contribute to this isotopic “enlightenment”, and elevated ambient CO2 may “wash out” this effect. Our findings thus generally support models of calcification physiology and shell isotopic content, and have implications for isotopic paleontology.