Increasing atmospheric pCO₂ reduces the saturation state of seawater with respect to the aragonite, high-Mg calcite (Mg/Ca>0.04), and low-Mg calcite (Mg/Ca<0.04) minerals from which marine calcifiers build their shells and skeletons. Notably, these polymorphs of CaCO₃ have different solubilities in seawater: aragonite is more soluble than pure calcite, and the solubility of calcite increases with its Mg-content. Although much recent progress has been made investigating the effects of CO₂-induced ocean acidification on rates of biological calcification, considerable uncertainties remain regarding impacts on shell/skeletal polymorph mineralogy. To investigate this subject, eighteen species of marine calcifiers were reared for 60-days in seawater bubbled with air-CO₂ mixtures of 409±6, 606±7, 903±12, and 2856±54ppm pCO₂, yielding aragonite saturation states (ΩA) of 2.5±0.4, 2.0±0.4, 1.5±0.3, and 0.7±0.2. Calcite/aragonite ratios within bimineralic calcifiers increased with increasing pCO₂, but were invariant within monomineralic calcifiers. Calcite Mg/Ca ratios (Mg/CaC) also varied with atmospheric pCO₂ for two of the five high-Mg-calcite-producing organisms, but not for the low-Mg-calcite-producing organisms. These results suggest that shell/skeletal mineralogy within some—but not all—marine calcifiers will change as atmospheric pCO₂ continues rising as a result of fossil fuel combustion and deforestation. Paleoceanographic reconstructions of seawater Mg/Ca, temperature, and salinity from the Mg/CaC of well-preserved calcitic marine fossils may also be improved by accounting for the effects of paleo-atmospheric pCO₂ on skeletal Mg-fractionation.
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