Carbonate equilibria in natural waters

Abstract

Natural waters, which include the ocean and freshwater lakes, ponds and streams, act as a major interface between the lithosphere and the atmosphere, and also between these environmental compart- ments and much of the biosphere. In particular the ocean, with its large volume, serves as both a major reservoir for a number of chemical species; the deep ocean currents also provide an efficient mechanism for the long distance transport of substances. Although it is commonly stated that the composition of natural waters is controlled by a combination of geochemical and biological processes, it is also true that these processes are to some extent affected by the composition of the waters. Among the parameters of water composition, few are more important than the pH and the alkalinity. The latter affects the degree to which waters are buffered against changes in the pH, and it also has some influence on the complexing of certain trace cations. in living organisms. By far the most important of these is carbon in the form of CO2, HCO3− The carbonate system which is the major source of buffering in the ocean and is the main subject of this chapter. The carbonate system encompasses virtually all of the environmental compartments– the atmosphere, hydrosphere, biosphere, and, as CaCO3, major parts of the lithosphere. The complementary processes of photosynthesis and respiration drive a global cycle in which carbon passes slowly between the atmosphere and the lithosphere, and more rapidly between the atmosphere and the oceans. Natural waters contain a variety of weak acids and bases which include the major elements present 3 and CO2−3 .