Another way of writing this is: CaCO3 + H+ ⇌ Ca2+ + The lower the pH, the higher the concentration of bicarbonate will be. This shows how a lower pH can lead to higher alkalinity if the amount of bicarbonate produced is greater than the amount of H+ remaining after the reaction. This is the case since the amount of acid in the rainwater is low. If this alkaline groundwater later comes into contact with the atmosphere, it can lose CO2, precipitate carbonate, and thereby become less alkaline again. When carbonate minerals, water, and the atmosphere are all in equilibrium, the reversible reaction CaCO3 + 2 H+ ⇌ Ca2+ + CO2 + H2O shows that pH will be related to calcium ion concentration, with lower pH going with higher calcium ion concentration. In this case, the higher the pH, the more bicarbonate and carbonate ion there will be, in contrast to the paradoxical situation described above, where one does not have equilibrium with the atmosphere. Oceanic alkalinity There are many methods of alkalinity generation in the ocean. Perhaps the most well known is the dissolution of calcium carbonate to form Ca2+ and (carbonate). The carbonate ion has the potential to absorb two hydrogen ions. Therefore, it causes a net increase in ocean alkalinity. Calcium carbonate dissolution occurs in regions of the ocean which are undersaturated with respect to calcium carbonate; these regions increase in size as a result of a decrease in pH (ocean acidification due to climate change). This can cause great damage to coral reef ecosystems. Lowering of pH due to absorption of CO2 eventually (over millennial timescales) raises mean ocean alkalinity by causing dissolution of carbonates. Physical processes can also affect alkalinity. Freshwater inputs like melting polar ice caps dilute seawater and can serve to decrease oceanic alkalinity. If the ice were to melt, then the overall volume of the ocean would increase. Because alkalinity is a concentration value (mol/L), increasing the volume would decrease AT. However, the actual effect would be much more complicated than this. Biological processes have a much greater impact on oceanic alkalinity on short (decades to centuries) timescales. Aerobic respiration of organic matter can decrease alkalinity by releasing protons during the oxidation of organic nitrogen. Denitrification and sulfate reduction occur in oxygen-limited environments. Both of these processes consume hydrogen ions (thus increasing alkalinity) and release gases (N2 or H2S), which eventually escape