Calcium - Geochemical Cycling

Geochemical Cycling

Calcium provides an important link between tectonics, climate and the carbon cycle. In the simplest terms, uplift of mountains exposes Ca-bearing rocks to chemical weathering and releases Ca2+ into surface water. This Ca2+ eventually is transported to the ocean where it reacts with dissolved CO₂ to form limestone. Some of this limestone settles to the sea floor where it is incorporated into new rocks. Dissolved CO₂, along with carbonate and bicarbonate ions, are referred to as dissolved inorganic carbon (DIC).

The actual reaction is more complicated and involves the bicarbonate ion (HCO₃-) that forms when CO₂ reacts with water at seawater pH:

Ca2+ + 2HCO−
3 → CaCO₃ (limestone) + CO₂ + H₂O

Note that at ocean pH most of the CO₂ produced in this reaction is immediately converted back into HCO−
3. The reaction results in a net transport of one molecule of CO₂ from the ocean/atmosphere into the lithosphere.

The result is that each Ca2+ ion released by chemical weathering ultimately removes one CO₂ molecule from the surficial system (atmosphere, ocean, soils and living organisms), storing it in carbonate rocks where it is likely to stay for hundreds of millions of years. The weathering of calcium from rocks thus scrubs CO₂ from the ocean and atmosphere, exerting a strong long-term effect on climate. Analogous cycles involving magnesium, and to a much smaller extent strontium and barium, have the same effect.

As the weathering of limestone (CaCO₃) liberates equimolar amounts of Ca2+ and CO₂, it has no net effect on the CO₂ content of the atmosphere and ocean. The weathering of silicate rocks like granite, on the other hand, is a net CO₂ sink because it produces abundant Ca2+ but very little CO₂.