In the beginning, the oceans were acidic, because they were formed by the condensation of volcanic gases, which consisted of water, carbon dioxide, sulfur dioxide and hydrogen sulfide, hydrogen chloride (i.e. hydrochloric acid) and a few other less abundant acids.
In time, the oceans have become less and less acidic, by dissolving from the volcanic silicate rocks the oxides of the alkaline metals and alkali earth metals, i.e. mainly of sodium, potassium, magnesium and calcium. This dissolution has affected both the rocks on the bottom of the oceans and the continental rocks, where rain has washed the soluble oxides, transporting them through rivers to the oceans.
At some point, so much of the alkaline and alkali earth metals from the volcanic rocks have been dissolved that the oceans have become slightly alkaline instead of acidic, like they are today.
At that time, the carbonates of calcium and magnesium have precipitated from sea water, forming sedimentary rocks. Also around that time, many living beings have evolved mechanisms for controlling this precipitation process, in order to build skeletons for themselves. This has resulted in the fact that many sedimentary rocks are not formed by direct precipitation from sea water, but by precipitation from sea water into skeletons, followed by depositing on the bottom the skeletons of dead living beings.
Now, with increasing concentration of CO2, there is the danger that the oceans will become so acidic as to reverse this, dissolving again a part of the carbonate rocks, including the skeletons of many living beings that are made of carbonates.
There is an equilibrium between the concentration of CO2 in water and in air, depending on temperature and pressure. When the CO2 from water precipitated with calcium or magnesium into rocks, that has drawn more CO2 from air into the water, until a new equilibrium was reached, at a reduced concentration of CO2 in the air. If carbonates would be dissolved by acidic sea water, that would liberate CO2, a part of which would go into the air, further increasing the concentration there.
Thus the formation or destruction of carbonate rocks and skeletons adds a positive feedback to the changes of the CO2 concentration in the air, which has the potential to be bad for us.
Even worse is the fact that this is only one of multiple positive feedback mechanisms that can be triggered by changes in the CO2 concentration in the air, which make very difficult or impossible any long term predictions.