Supersaturation ocean, CaCO

Hence Qsp Ksp, and according to this calculation, the oceans are vastly supersaturated with respect to calcium carbonate. We would therefore expect CaCO to precipitate spontaneously. It turns out that there are two factors which have been overlooked in this simple calculation, namely ionic strength and ion complexation. We will consider these processes. 

Magnesium compounds, Mg(OH)2 in particular, could also contribute to the protective character of calcareous deposits. However, calcimn carbonate is thermodynamically stable in surface seawater, where it is supersaturated, whereas magnesimn hydroxide is unsaturated and less stable. In fact, Mg(OH)2 would precipitate only if the pH of seawater were to exceed approximately 9.5. This is the main reason why the behavior of CaCOs in seawater has been so extensively studied, since calcium carbonate sediments are prevalent and widespread in the oceans. ... 

---