Formation of calcium carbonate shells by organisms and increase in the partial pressure of carbon dioxide

　Will expanding coral reefs reduce atmospheric carbon dioxide and mitigate global warming?

　One might think that the formation of calcium carbonate shells in the ocean would reduce the amount of carbon dioxide in the atmosphere, but in fact, the opposite is true.

　Indeed, as corals grow and form calcium carbonate (CaCO₃) particles, total carbonic acid (DIC) in seawater decreases. However, pCO₂ rises.

　To understand how this works, we must consider the changes in the alkalinity of seawater.

Here, the definition of alkalinity is written out again.

【Alkalinity】=（【Na⁺】+【K⁺】+2【Mg²⁺】+2【Ca²⁺】）

－（【Cl^-】+【Br^-】+2【SO₄^2-】+【NO₃^-】）

　Which of the strong electrolyte ionic components that determine alkalinity can be altered by the action of organisms? The first one that seems to have a large effect is Ca²⁺. Coccolithales, corals, shellfish, and foraminifera have shells of calcium carbonate (CaCO₃). As these organisms grow, Ca²⁺ in the seawater is absorbed and alkalinity decreases. When these organisms die and their shells leach into the seawater, the alkalinity increases. To compensate for this change in alkalinity, the charge of the weak electrolyte anions (carbonate ions) increases or decreases.

　The important points are,

①　At the pH of seawater, the majority of weak electrolyte ions in the carbonic acid are HCO₃^-.

②　What we want to know now is the H₂CO₃ concentration (i.e., the pCO₂ at atmospheric equilibrium)