As humanity continues to release more and more carbon dioxide into our atmosphere, a lot of this carbon dioxide is traded into our hydrosphere which causes ocean acidification. Ocean acidification (OA) happens when carbon dioxide dissolves in water creating carbonic acid which then dissociates to bicarbonate and a free hydrogen atom. The addition of free hydrogen atoms in the ocean is what causes our oceans to become more acidic. However, some of these free hydrogen atoms are also reacting with carbonate ions to make bicarbonate. This process is shown below:

CO₂ + H₂O ↔ H₂CO₃ (carbonic acid)
H₂CO₃ ↔ H⁺ + HCO₃⁻ (bicarbonate)
H⁺ + CO₃²⁻ (carbonate) ↔ HCO₃⁻

The reduction of carbonate ions in the water column is a problem for many calcifying organisms such as coral, mollusks, foraminifera. These organisms play an important role in the formation of limestone. Limestone is a sedimentary rock that is made up of precipitation of calcium carbonate often from biotic factors, such as the calcifying organisms listed above, and accumulation of carbonate particles like those from shells and skeletal material. Limestone is found in oceanic environments under high pressures which allow diagenesis to occur.

Figure 1. Oncolitic limestone (Flickr).

So, how does OA affect the formation of limestone? With less carbonate available, organisms must work harder to create calcium carbonate which reduces calcification and precipitation rates. This can lower the saturation state meaning there is slower precipitation and increases the reliance on tiny seed crystals, mineral fragments, or biological structures like proteins to lead crystal growth in order to overcome energy barriers. This can result in limestone with lower density and a more porous microstructure as well as more amorphous phases due to organisms producing more amorphous calcium carbonate. Organisms begin producing more amorphous calcium carbonate because this requires less energy and in an environment where carbonate is becoming increasingly difficult to find, it’s important they conserve as much energy as possible.

In conclusion, as our oceans take up more and more carbon from our atmosphere, less carbonate becomes available to calcifying organisms. Since limestone is largely made up of precipitation of calcium carbonate from biotic factors, the precipitation rates drop. This causes the formation of limestone with lower calcium and carbonate content and more amorphous and unstable phases. It also affects trace element ratios such as magnesium:calcium and strontium:calcium due to the different precipitation conditions. With less efficient calcification the limestone may also contain more organic matter or clays and overall be more porous and therefore less dense.