Effects of elevated CO2 on 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 (δ¹³C and δ¹⁸O) 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 δ¹⁸O (equivalently, a positive correlation between seawater pCO₂ and shell δ¹⁸O) at both temperatures (regression slopes: -0.70‰ ± 0.14‰/pH unit at 17 °C; -0.36‰ ± 0.17‰/pH unit at 25 °C). A positive correlation was observed between seawater pH and shell δ¹³C at both temperatures, but the regression slopes were less steep than those between seawater pH and the δ¹³C of seawater DIC. As CO₂ levels rose (and pH dropped to about 7.6), shell δ¹⁸O and δ¹³C increased relative to ambient dissolved inorganic carbon (DIC). Both approached apparent isotopic equilibrium in acidified waters. Shell δ¹⁸O 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.