  Carbonate precipitation in oceans is essential for the carbonate-silicate
cycle (inorganic carbon cycle) to maintain temperate climates. By considering
the thermodynamics of carbonate chemistry, we demonstrate that the ocean pH
decreases by approximately 0.5 for a factor of 10 increase in the atmospheric
carbon dioxide content. The upper and lower limits of ocean pH are within 1-4
of each other, where the upper limit is buffered by carbonate precipitation and
defines the ocean pH when the carbon cycle operates. If the carbonate
compensation depth (CCD) resides above the ocean floor, then carbonate
precipitation and the carbon cycle cease to operate. The CCD is deep (>40 km)
for high ocean temperature and high atmospheric carbon dioxide content. Key
divalent carbonates of magnesium, calcium and iron produce an increasingly
wider parameter space of deep CCDs, suggesting that chemical diversity promotes
the carbon cycle. The search for life from exoplanets will benefit by including
chemically more diverse targets than Earth twins.
