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The effects of elevated CO 2 levels on growth and stable carbon and oxygen isotope compositions (δ 13 C and δ 18 O) of aragonitic shells were experimentally evaluated by rearing the bloody clam, Scapharca broughtonii , at two different temperatures and at six different p CO 2 levels (17 °C: 269, 382, 550, 757, 939, and 1114 µatm; 25 °C: 332, 463, 653, 872, 1137, and 1337 μatm). Ambient CO 2 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 δ 18 O (equivalently, a positive correlation between seawater p CO 2 and shell δ 18 O) 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 δ 13 C at both temperatures, but the regression slopes were less steep than those between seawater pH and the δ 13 C of seawater DIC. As CO 2 levels rose (and pH dropped to about 7.6), shell δ 18 O and δ 13 C increased relative to ambient dissolved inorganic carbon (DIC). Both approached apparent isotopic equilibrium in acidified waters. Shell δ 18 O 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 CO 2 hydration and hydroxylation reactions may contribute to this isotopic “enlightenment”, and elevated ambient CO 2 may “wash out” this effect. Our findings thus generally support models of calcification physiology and shell isotopic content, and have implications for isotopic paleontology. [ABSTRACT FROM AUTHOR]