CO2 system in seawater

Lewis E. and Wallace D. W. R. (1998) Basic program for CO2 system in seawater. ORNL/CDIAC-105, Oak Ridge National Laboratory. 

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. 

For very accurate measurements the non-ideal behaviour of CO2 has to be taken into account, Le., fugadty has to be used instead of partial pressure. This is the case if the results are to be used to calculate other parameters of the CO2 system in seawater. The fugacity can be calculated from a knowledge of the virial expression of the equation of state for CO2. For the binary mixture C02-air the fugacity of CO2 (/(CO2)) is given by... 

The interest in the carbonate system is related to attempts to understand the uptake of fossil fuel produced CO2 by the oceans. The carbonate system can be studied by measuring pH, total alkalinity (TA), total inorganic carbon (TCO2), and the fugacity of CO2 (fco )- At least two of these variables are needed (Park, 1969) to characterize the CO2 system in the oceans. Reliable stoichiometric constants (K ) for the carbonate system are needed to determine the concentration, mol (kg solution) of the components of the CO2 system ([HCOa"], [CO2], [COa ]) and the saturation state of CaCOa as a function of salinity, temperature, and pressure (Culberson and Pytkowicz, 1968 Ingle, 1975 Millero, 1995, 2001). This includes constants for the solubility of CO2 in seawater (Weiss, 1974)... 

CO2 exists in seawater in various forms as part of the carbonate system ... 

As described in Chapter 21.7, a system of biogeochemical feedbacks act to stabilize the major ion composition of seawater. Some operate on short time-scale cycles, such as calcite compensation, and others operate over longer periods, such as the basalt-carbonate buffer. The linkages in the crustal-ocean-atmosphere fectory that act on the major ions also influence atmospheric CO2 levels and seawater s pH and alkalinity. 

The distribution of CO2 and the associated carbonic acid system species in the upper ocean (here loosely defined as waters above the thermocline and generally only a few hundred meters in depth) is primarily controlled by the exchange of CO2 across the air-sea interface, biological activity, and circulation of the ocean, mainly through vertical mixing processes. Other factors, such as the temperature and salinity of the water, can also contribute to variations by influencing the solubility of CO2 in seawater and the equilibrium constants of the carbonic acid system. 

An alternative pathway to bicarbonate is possible due to reaction of CO2 and OH in water but is normally less important (Skirrow, 1975). The proton concentration is also inbuenced by total alkalinity and water dissociation, which, in turn, will inbuence the details of the simplibed chemical process depicted above. The relative concentrations of the DIC species in Equation (15) are mainly a function of pH, temperature, and salinity. In seawater, bicarbonate is the dominant species (Skirrow, 1975), and bicarbonate and carbonate are the main components of alkalinity (Broecker and Peng, 1974). The chemical equilibrium for each of the steps in the DIC system in Equation (15) is determined from the specibc temperature sensitive reaction constants (Aix). Note that in the complex system of seawater, empirical Kj values, rather than thermodynamic theoretical values, are usually adopted. Such reacbon constants are of course sensitive to the effects of molecular mass, with molecules containing heavy isotopes favoring slower reactions rates, and are therefore associated with temperature-sensitive isotopic fractionations. Typical equilibrium fractionation values for the carbonate system in dilute solution at 25 °C are (Deuser and Degens, 1967 Mook, 1986 Mook et al., 1974) ... 

In this sense we have already described in Chapters 4 and 7 the equilibrium of the CaC03(s)-H20-C02 system. Specifically, we have used equilibrium models to characterize the concentrations of the carbonate species as a function of pco2 and of pH. We have already shown (Example 7.8) that CaCOj in surface seawater is oversaturated and we calculated in Example 4.10 how the composition of seawater changes as a result of increasing the CO2 concentration in the atmosphere. 

Fig. 2.3.2. Comparison of the range in carbon isotopic composition for hermatypic corals, ahermatypic corals, and marine carbonate precipitated in isotopic equilibrium with the COj" — HCO5 — CO2 system of ambient seawater. All of the corals (111 hermatypes and 59 ahermatypes) were collected from shallow-water, reef surface environments at Heron Island, Australia. The length of the horizontal bars is equivalent to the mean one standard deviation (Weber, 1974).

Equilibrium properties of the C02/sea-water system have been well researched from an experimental standpoint. In particular, the clathrate hydrate forming conditions T < 285K andP>4MPa) are well established. Several experiments have been performed under conditions mimicking the direct injection process and have attempted to study the dissolution rate of CO2 in seawater. Under direct injection conditions, the injected CO2 is in the form of a liquid droplet and a thin spherical shell of CO2 clathrate hydrate of structure I is observed to form around the CO2 drop, separating it from the sea water. The process of hydrate formation has many similarities with that of crystallization, i.e., it can be divided into a nucleation phase and a growth phase. For CO2 clathrates, the nucleation phase involves the formation of a... 

Seawater is slightly alkaline with a pH between 7.8 and 8.3 and is buffered primarily by the carbonate system. The equilibrium reactions between CO2 gas in the atmosphere and seawater are shown in reactions (3.1)—(3.5) ... 

Thus, the empirical characterization of the OV transient produced from OH in seawater leads to a chemical mystery the UV transient definitely interacts with seawater in a pH and CO2-dependent process that occurs in waters of different origin, including artificial seawater. Hence, the effect would appear to be due to the carbonate system itself, not some trace impurity. The following paper tahes up the challenges of finding such an interaction in simpler media than seawater and of explaining the pseudo first-order decay of seawater in terms of these interactions and carbonate system speciation (16). 

A key idea of SONNE is the capture of CO2 from the atmosphere to close the global carbon cycle analogously to the biosphere (Fig. 2.98). According to the fluxes described in Fig. 2.98, the system can be established within a steady state (CO2 flux in = CO2 flux out) or even run in an air abatement mode (CO2 flux in > CO2 flux out). Air capture includes three approaches CO2 capture from ambient air, from seawater and via biomass cultivation (biofarming). All these approaches are interlinked within the global carbon cycle but with different characteristic times. 

The carbonate system plays an important role in moderation of the chemistry of natural aquatic systems (e.g., acts a pH buffer in seawater), which, in turn, influences biotic and chemical activities. Inorganic C is also a major component of the global C cycle its subsequent atmospheric/oceanic interaction plays a major role in atmospheric CO2 levels and hence global warming processes (see above). For example, an oceanic uptake of 40% will only correspond to an average change in DIG of IpmolH therefore, its accurate and precise determination and distribution is essential. 

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