The global CO2-carbonic acid-carbonate system of seawater, although certainly a well-researched topic of interest in the past, has risen to the fore in recent years because of the environmental issue of ocean acidification (often simply termed OA). Despite much previous research, there remain pressing questions about how this most important chemical system of seawater operated at the various time scales of the deep time of the Phanerozoic Eon (the past 545 Ma of Earth’s history), interglacial-glacial time, and the Anthropocene (the time of strong human influence on the behaviour of the system) into the future of the planet. One difficulty in any analysis is that the behaviour of the marine carbon system is not only controlled by internal processes in the ocean, but it is intimately linked to the domains of the atmosphere, continental landscape, and marine carbonate sediments.
For the deep-time behaviour of the system, there exists a strong coupling between the states of various material reservoirs resulting in an homeostatic and self-regulating system. As a working hypothesis, the coupling produces two dominant chemostatic modes: (Mode I), a state of elevated atmospheric CO2, warm climate, and depressed seawater Mg∕Ca and SO4∕Ca mol ratios, pH (extended geologic periods of ocean acidification), and carbonate saturation states, and elevated Sr concentrations, with calcite and dolomite as dominant minerals found in marine carbonate sediments (Hothouses, the calcite-dolomite seas), and (Mode II), a state of depressed atmospheric CO2, cool climate, and elevated seawater Mg∕Ca and SO4/Ca ratios, pH, and carbonate saturation states, and low Sr concentrations, with aragonite and high magnesian calcites as dominant minerals found in marine carbonate sediments (Icehouses, the aragonite seas).
Investigation of the impacts of deglaciation and anthropogenic inputs on the CO2–H2O–CaCO3 system in global coastal ocean waters from the Last Glacial Maximum (LGM: the last great continental glaciation of the Pleistocene Epoch, 18,000 year BP) to the year 2100 shows that with rising sea level, atmospheric CO2, and temperature, the carbonate system of coastal ocean water changed and will continue to change significantly. We find that 6,000 Gt of C were emitted as CO2 to the atmosphere from the growing coastal ocean from the Last Glacial Maximum to late preindustrial time because of net heterotrophy (state of gross respiration exceeding gross photosynthesis) and net calcification processes. Shallow-water carbonate accumulation alone from the Last Glacial Maximum to late preindustrial time could account for ~24 ppmv of the ~100 ppmv rise in atmospheric CO2, lending some support to the ‘‘coral reef hypothesis’’. In addition, the global coastal ocean is now, or soon will be, a sink of atmospheric CO2, rather than a source. The pHT (pH values on the total proton scale) of global coastal seawater has decreased from ~8.35 to ~8.18 and the CO32- ion concentration declined by ~19% from the Last Glacial Maximum to late preindustrial time. In comparison, the decrease in coastal water pHT from the year 1900 to 2000 was ~8.18 to ~8.08 and is projected to decrease further from about ~8.08 to ~7.85 between 2000 and 2100. During these 200 years, the CO32- ion concentration will fall by ~ 45%. This decadal rate of decline of the CO32- ion concentration in the Anthropocene is 214 times the average rate of decline for the entire Holocene!
In terms of the modern problem of ocean acidification and its effects, the “other CO2 problem”, we emphasise that most experimental work on a variety of calcifying organisms has shown that under increased atmospheric CO2 levels (which attempt to mimic those of the future), and hence decreased seawater CO32- ion concentration and carbonate saturation state, most calcifying organisms will not calcify as rapidly as they do under present-day CO2 levels. In addition, we conclude that dissolution of the highly reactive carbonate phases, particularly the biogenic and cementing magnesian calcite phases, on reefs will not be sufficient to alter significantly future changes in seawater pH and lead to a buffering of the CO2-carbonic acid system in waters bathing reefs and other carbonate ecosystems on timescales of decades to centuries. Because of decreased calcification rates and increased dissolution rates in a future higher CO2, warmer world with seas of lower pH and carbonate saturation state, the rate of accretion of carbonate structures is likely to slow and dissolution may even exceed calcification. The potential of increasing nutrient and organic carbon inputs from land, occurrences of mass bleaching events, and increasing intensity (and perhaps frequency of hurricanes and cyclones as a result of sea surface warming) will only complicate matters more. This composite of stresses will have severe consequences for the ecosystem services that reefs perform, including acting as a fishery, a barrier to storm surges, a source of carbonate sediment to maintain beaches, and an environment of aesthetic appeal to tourist and local populations. It seems obvious that increasing rates of dissolution and bioerosion owing to ocean acidification will result in a progressively increasing calcium carbonate (CaCO3) deficit in the CaCO3 budget for many coral reef environments. The major questions that require answers are: will this deficit occur and when and to what extent will the destructive processes exceed the constructive processes?