Seawater carbonate chemistry, calcification and dissolution response, and skeletal mineralogy of benthic orhanisms during experiments, 2011

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 ± 54 ppm pCO2, yielding aragonite saturation states 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.大气二氧化碳分压（atmospheric pCO₂）升高会降低海水相对于文石（aragonite）、高镁方解石（high-Mg calcite，Mg/Ca>0.04）以及低镁方解石（low-Mg calcite，Mg/Ca<0.04）矿物的饱和状态——这些矿物是海洋钙化生物构建壳体与骨骼的核心原料。值得注意的是，碳酸钙（CaCO₃）的这些同质多象变体在海水中的溶解度存在显著差异：文石的溶解度高于纯方解石，而方解石的溶解度随其镁含量的升高而增大。尽管近年来针对CO₂诱导的海洋酸化对生物钙化速率的影响研究已取得诸多进展，但海洋酸化对壳体/骨骼矿物相组成的影响仍存在大量不确定性。为探究这一科学问题，研究人员将18种海洋钙化生物置于充有409±6、606±7、903±12及2856±54 ppm CO₂-空气混合气体的海水中培育60天，对应获得的文石饱和状态分别为2.5±0.4、2.0±0.4、1.5±0.3及0.7±0.2。双矿质海洋钙化生物体内的方解石/文石比值随大气pCO₂升高而显著增大，但单矿质钙化生物的该比值未发生明显变化。在5种产高镁方解石的生物中，有2种的方解石Mg/Ca比值（Mg/Ca_C）随大气pCO₂变化而改变，而产低镁方解石的生物的该比值则无显著波动。上述结果表明，随着化石燃料燃烧与森林砍伐导致的大气pCO₂持续升高，部分（而非全部）海洋钙化生物的壳体/骨骼矿物相组成将发生改变。此外，通过量化古大气pCO₂对骨骼镁分馏作用的影响，基于保存完好的钙质海洋化石的方解石Mg/Ca_C开展的海水Mg/Ca、温度及盐度古海洋重建工作，其精度或可得到进一步提升。