The Role of Carbonates in Regulating Atmospheric CO2 on Earth-like Exoplanets

Ocean chemistry plays a key role in the removal of CO2 from the atmosphere-ocean system in the form of carbonates that are eventually subducted to the mantle. Silicate weathering and CO2 dissolution dictate the steady-state ocean chemistry and thereby the carbonate-silicate cycle (inorganic carbon cycle). Data on stellar elemental abundances suggest a strong diversity in the bulk mineralogy of exoplanets. We study the role of weathering-derived divalent cations (Ca++, Mg++) on ocean pH and carbonate compensation depth (CCD) in exoplanet oceans. If CCD is too shallow, carbonates on the seafloor cannot be subducted to the mantle. We find that the presence of carbonates sets the upper bound on ocean pH and CO2 dissolution sets the lower bound on ocean pH. We show that CCD increases with increasing divalent cations supplied by weathering and decreases with CO2 dissolution.