The CO2 System in Oceanic Waters

The CO2 system in seawater is an important and complicated balance system in oceans it is composed of some sub-balance systems and is influenced by atmospheric, biological, geologic and other processes. The Pco2 in seawater is an important parameter of the sea s CO2 system and is very sensitive to physicochemical and biological processes in oceans. Its distribution and change are closely related to factors such as water mass and biological activity. 

An important example of non-linearity in a biogeochemical cycle is the exchange of carbon dioxide between the ocean surface water and the atmosphere and between the atmosphere and the terrestrial system. To illustrate some effects of these non-linearities, let us consider the simplified model of the carbon cycle shown in Fig. 4-12. Ms represents the sum of all forms of dissolved carbon (CO2, H2CO3, HCOi" and... 

The view has long been held that hydrogen-ion buffeting in the oceans is due to the CO2—HCO3-— CC>3 equilibrium. Within recent years this view has been challenged, and the importance of aluminosilicate equilibria in maintaining the pH of sea water emphasized. The buffer intensity of a system is of thermodynamic nature and is defined as... 

For example, in the carbon cycle consider the balance between terrestrial photosynthesis and respiration-decay. If the respiration and decay flux to the atmosphere were doubled (perhaps by a temperature increase) from about 5200 x 1012 to 10,400 x 1012 moles y-l, and photosynthesis remained constant, the CO2 content of the atmosphere would be doubled in about 12 years. If the reverse occurred, and photosynthesis were doubled, while respiration and decay remained constant, the CO2 content of the atmosphere would be halved in about the same time. An effective and rapid feedback mechanism is necessary to prevent such excursions, although they have occurred in the geologic past. On a short time scale (hundreds of years or less), the feedbacks involve the ocean and terrestrial biota. As was shown in Chapter 4, an increase in atmospheric CO2 leads to an increase in the uptake of CO2 in the ocean. Also, an initial increase in atmospheric CO2 could lead to fertilization of those terrestrial plants which are not nutrient limited, provided there is sufficient water, removal of CO2, and growth of the terrestrial biosphere. Thus, both of the aforementioned processes are feedback mechanisms that can operate in a positive or negative sense. An increased rate of photosynthesis would deplete atmospheric CO2, which would in turn decrease photosynthesis and increase the oceanic evasion rate of CO2, leading to a rise in atmospheric CO2 content. More will be said later about feedback mechanisms in the carbon system. 

Most natural water systems in contact with calcite (oceans, rivers, lakes, carbonate rock aquifiers) are, however, near equilibrium, and PCO2 dependence cannot be ignored. According to our model, the rate of backward reaction is a significant function of surface pH, and surface pH is determined by calcite equilibrium at the surface PCO2. At the relatively high pH, low PCO2 conditions of most natural waters, the surface pH is least well defined and may depend, in part, on the flux of CO2 between the surface and bulk fluid. 

Equilibrium isotopic fractions rebect the combined, unidirectional kinetic isotopic fractionations. In considering the one-way buxes as in Equation (12), estimates of the one-way kinetic fracbonations are needed. Knowledge of kinetic effects is obtained from controlled experiment with pure CO2 and water or with salt solutions, but for the chemically complex system that is ocean-water, empirical values are adopted. Eor example, laboratory experiments show that the hydration of aqueous CO2 to bicarbonate involves fractionation of 13%c and the dehydration reaction fractionate by 22%c (O Leary et al., 1992). The difference between these two kinetic fractionations, 9%c, corresponds to the equilibrium fractionation depicted above (Marlier and O Leary, 1984 O Leary et al., 1992). In practice, it was estimated... 

The controls on carbon dioxide would have been somewhat different. Today, carbon dioxide is stored in carbonate minerals in the ocean floor and on the continental shelf. Subduction, followed by volcanism, cycles the carbon dioxide to the mantle and then restores the CO2 to the air. Metamorphic decarbonation of the lower crust also returns carbon dioxide. The carbon dioxide is then cycled back to the water, some via rain, some dissolved via wave bubbles. Erosion provides calcium and magnesium, eventually to precipitate the carbonate. In the earliest Archean, parts of this cycle may have been inefficient. The continental supply of calcium may have been limited however, subseafloor hydrothermal systems would have been vigorous and abundant, exchanging sodium for calcium in spilitization reactions, and hence providing calcium for in situ precipitation in oceanic crust. 

---