The kinetics of calcium carbonate, nucleation and growth.

During the Phanerozoic, ocean chemistry has oscillated to favour the dominant non-biogenic polymorph of calcium carbonate (CaCO3) to be either calcite or aragonite. The main driving force controlling these aragonite-calcite seas conditions is the Mg:Ca ratio of seawater. However, other parameters such us SO4, temperature and partial pressure of carbon dioxide (pCO2) are also known to influence CaCO3 polymorph formation but are often overlooked in the context of aragonite-calcite seas. The main aim of this research project was to evaluate and quantify the key effects as well as the relative effects of changes in Mg:Ca ratio, SO4 concentration, temperature and pCO2 have on the formation, transformation and structure of CaCO3 phases, in order to provide a better synergistic approach than those taken by previous studies.
We confirm seawater Mg:Ca ratio and SO4 concentration as dominant drivers since small differences in their values can be invoked as controlling the variation in abiotic CaCO3 polymorphs. We propose lower Mg:Ca and SO4 thresholds on seawater chemistry for calcite predominance seas (Mg:Ca≤ 0.65, SO4≤ 10 mM) than those suggested by the geological records (Mg:Ca≈ 2).
Our work demonstrates that there is not a significant response of abiotic marine CaCO3 mineralogy to changing temperature. Our experimental data also reveal that neither changes in pCO2 in the range 400 to 3000 ppm, or their respective changes in seawater pH (8.2, to 7.6) lead to major variations in the stability fields for aragonite and calcite precipitation. Hence, the view of Phanerozoic aragonite-calcite seas should not be temperature and atmospheric pCO2 or seawater pH corrected.
We validate abiogenic calcite Mg:Ca as a reliable and quantitative proxy for past global seawater temperature change if its dependence on both the temperature and the Mg:Ca ratio of the seawater is considered.
We propose SO4 content in abiogenic calcite as a reliable and quantitative proxy for past seawater pH and atmospheric pCO2, but our results indicated that it is not possible to provide a single SO4–pH calibration and the use of seawater SO4-specific equations is needed.
It can also be concluded that switches between aragonite and calcite seas calculated from the stability fields from our experimental results would only match the proxy data for seawater chemistry in the geological record if the lowest suggested seawater SO4 concentrations are considered ([SO4] ≤ 10 mM).
Furthermore, this study shows that Mg, SO4 and temperature significantly influence calcium carbonate morphology and addresses temperature turning out to be an important parameter for the control of particle size.