Solubility of high-magnesium calcite in seawater and its implementation in (Py)CO2SYS

Abstract

The calcium carbonate pump is an important part of the ocean's carbon cycle but our knowledge of it is incomplete. For example, alkalinity data suggest that carbonate mineral dissolution happens at shallow depths where bulk seawater is oversaturated with respect to calcite and aragonite. It has been hypothesised that high-Mg calcites could explain this discrepancy due to their high solubility. However, our knowledge of what depth Mg calcites start dissolving and how they might respond to continuing ocean acidification is limited because their solubility in marine environments is poorly known. Here, we develop an approach to calculate Mg calcite solubility by using published solubility data under standard laboratory conditions and adding dependencies for temperature, salinity and pressure for ranges relevant to the marine environment. We then implement this into the CO2SYS software family (Python and GNU Octave/Matlab versions) and calculate saturation states globally for Mg calcites with different Mg%. Our results reveal that, contrary to previous assumptions, the saturation horizon for many high-Mg calcites often lies deeper than that of aragonite, suggesting that high-Mg calcites are unlikely to account for shallow-water carbonate dissolution. Our model aligns with the few existing in situ particle dissolution measurements of Mg calcites, but many unknowns remain regarding the solubility of Mg calcites in marine environments and future experiments focusing on temperature and pressure dependence are needed to better constrain their role in the marine carbon cycle.