Skeletal mineralogy in a high-CO2 world

Increasing atmospheric pCO(2) 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 +/- 54 ppm pCO(2), yielding aragonite saturation states (Omega(A)) 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 pCO(2), but were invariant within monomineralic calcifiers. Calcite Mg/Ca ratios (Mg/Ca-C) also varied with atmospheric pCO(2) 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 pCO(2) continues rising as a result of fossil fuel combustion and deforestation. Paleoceanographic reconstructions of seawater Mg/Ca, temperature, and salinity from the Mg/Ca-C of well-preserved calcitic marine fossils may also be improved by accounting for the effects of paleo-atmospheric pCO(2) on skeletal Mg-fractionation. (C) 2011 Elsevier B.V. All rights reserved.