Chemical model of seawater

For a first chemical model, we calculate the distribution of species in surface seawater, a problem first undertaken by Garrels and Thompson (1962 see also Thompson, 1992). We base our calculation on the major element composition of seawater (Table 6.2), as determined by chemical analysis. To set pH, we assume equilibrium with CO2 in the atmosphere (Table 6.3). Since the program will determine the HCOJ and water activities, setting the CO2 fugacity (about equal to partial pressure) fixes pH according to the reaction, 

The latter two assumptions are simplistic, considering the number of factors that affect pH and oxidation state in the oceans (e.g., Sillen, 1967 Holland, 1978 McDuff and Morel, 1980). Consumption and production of CO2 and O2 by plant and animal life, reactions among silicate minerals, dissolution and precipitation of carbonate minerals, solute fluxes from rivers, and reaction between convecting seawater and oceanic crust all affect these variables. Nonetheless, it will be interesting to compare the results of this simple calculation to observation. 

To calculate the model with react, we swap C02(g) and 02(g) into the basis in place of H+ and 02(aq), and constrain each basis member. The procedure is 

we define the total dissolved solids (in mg kg-1) for early releases of the REACT program (GWB 6.0 and previous), so the software can correctly convert our input constraints from mg kg-1 to molal units, as carried internally (i.e., variables nii and m.j). The print command causes the program to list in the output all of the aqueous species, not just those in greatest concentration. Typing go triggers the model to begin calculations and write its results to the output dataset. 

The program produces in its output dataset a block of results that shows the concentration, activity coefficient, and activity calculated for each aqueous species (Table 6.4), the saturation state of each mineral that can be formed from the basis, the fugacity of each such gas, and the system s bulk composition. The extent of the system is 1 kg of solvent water and the solutes dissolved in it the solution mass is 1.0364 kg. 

The latter two assumptions are simplistic, considering the number of factors 

TABLE 6.2 Major element composition of seawater (Drever, 1988) 

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Clegg S. L. and Whitfield M. (1995) A chemical model of seawater including dissolved ammonia and the stoichiometric dissociation constant of ammonia in estuarine water and seawater from — 2 to 40 °C. Geochim. Cosmochim. Acta 59, 2403 -2421. 

Dickson, A. G., Friedman, H. L., and Millero, F. J. (1988) Chemical Model of Seawater Systems, a Panel Report, Appl. Geochem. 3, 27-35. 

MacKenzie and Carrels (1966) approached this problem by constructing a model based on a river balance. They first calculated the mass of ions added to the ocean by rivers over 10 years. This time period was chosen because geologic evidence suggests that the chemical composition of seawater has remained constant over that period. They assumed that the river input is balanced only by sediment removal. The results of this balance are shown in Table 10-13. 

The failure to identify the necessary authigenic silicate phases in sufficient quantities in marine sediments has led oceanographers to consider different approaches. The current models for seawater composition emphasize the dominant role played by the balance between the various inputs and outputs from the ocean. Mass balance calculations have become more important than solubility relationships in explaining oceanic chemistry. The difference between the equilibrium and mass balance points of view is not just a matter of mathematical and chemical formalism. In the equilibrium case, one would expect a very constant composition of the ocean and its sediments over geological time. In the other case, historical variations in the rates of input and removal should be reflected by changes in ocean composition and may be preserved in the sedimentary record. Models that emphasize the role of kinetic and material balance considerations are called kinetic models of seawater. This reasoning was pulled together by Broecker (1971) in a paper called "A kinetic model for the chemical composition of sea water."... 

The chemical processes occurring within a black smoker are certain to be complex because the hot, reducing hydrothermal fluid mixes quickly with cool, oxidizing seawater, allowing the mixture little chance to approach equilibrium. Despite this obstacle, or perhaps because of it, we bravely attempt to construct a chemical model of the mixing process. Table 22.3 shows chemical analyses of fluid from the NGS hot spring, a black smoker along the East Pacific Rise near 21 °N, as well as ambient seawater from the area. 

Thompson, M. E., 1992, The history of the development of the chemical model for seawater. Geochimica et Cosmochimica Acta 56, 2985-2987. 

Whitfield M (1979) The Extension of Chemical Models for Seawater to include Trace Components at 24 Degrees C and 1 atm Pressure.-Geochmimica et Cosmochimica Acta, 39 pp 1545-1557... 

Obviously the composition of natural waters is markedly influenced by the growth, distribution, and decay of phytoplankton and other organisms. The dominant role of organisms in regulating the oceanic composition and its variation with depth of some of the important sea salt components (i.e., C, N, P, and Si) will be illustrated here by introducing certain aspects of Broecker s kinetic model for the chemical composition of seawater (Broecker and Peng, 1982). We summarize Broecker s line of arguments. 

The concept of chemical modeling of natural hydrologic systems was introduced by Garrels and Thompson Q) in a paper that described the distribution of chemical species in seawater. Their approach was to construct a rigorous thermodynamically based model that was (1) mathematically but not conceptually decoupled from flow, and (2) could provide quantifiable information about the chemical processes active in an aqueous system, such as seawater or groundwater. Their initial model considered 17 species, was restricted to 25 °C and remarkably enough, clearly quantified the predominant ion and ion pair speciation in seawater. This work set the framework for a number of the computer codes used today. 

Bob published a book Mineral Equilibria in 1960 (later expanded to Solutions, Minerals, and Equilibria in 1965 and coauthored with Charles Christ), which brought about a quantum jump in our understanding and approach to the physical chemistiy of natural waters. At about the same time, he conducted a series of outstanding studies with Maty Thompson and Ray Siever at Harvard Universitj especially noteworthy is the work on a chemical model for seawater. In this study, Garrels and Thompson laid the foundation for the calculation of activities in seawater, based primarily on the concept of ion pairing. (This paper has been so influential that it has been designated a classic by Science Citation Index.)... 

Several workers have intended to estimate the chemical compositions of Kuroko ore fluids based on the chemical equilibrium model (Sato, 1973 Kajiwara, 1973 Ichikuni, 1975 Shikazono, 1976 Ohmoto et al., 1983) and computer simulation of the changes in mineralogy and chemical composition of hydrothermal solution during seawater-rock interaction. Although the calculated results (Tables 1.5 and 1.6) are different, they all show that the Kuroko ore fluids have the chemical features (1 )-(4) mentioned above. 

Zirino, A. and Yamamoto, S., A pH-dependent model for the chemical speciation of copper, zinc, cadmium, and lead in seawater, Limnol Oceanogr, 17 (5), 661-671, 1972. 

When they calculated the species distribution in seawater, Garrels and Thompson (1962) were probably the first to apply chemical modeling in the field of geochemistry. Modern chemical analyses give the composition of seawater in terms of... 

Fig. 22.5. Concentrations of components (sulfate, sulfide, carbonate, methane, and acetate) and species (O2 and H2) that make up redox couples, plotted against temperature, during a model of the mixing of fluid from a hot subsea hydrothermal vent with cold seawater. Model assumes redox couples remain in chemical disequilibrium, except between 02(aq) and H2(aq). As the mixture cools past about 38 °C, the last of the dihydrogen from the vent fluid is consumed by reaction with dioxygen in the seawater. At this point the anoxic mixture becomes oxic as dioxygen begins to accumulate.

To model the chemical effects of evaporation, we construct a reaction path in which H2O is removed from a solution, thereby progressively concentrating the solutes. We also must account in the model for the exchange of gases such as CO2 and O2 between fluid and atmosphere. In this chapter we construct simulations of this sort, modeling the chemical evolution of water from saline alkaline lakes and the reactions that occur as seawater evaporates to desiccation. 

In this chapter, in an attempt to devise methods for helping to foresee such unfavorable consequences, we construct models of the chemical interactions between injected fluids and the sediments and formation waters in petroleum reservoirs. We consider two cases the effects of using seawater as a waterflood, taking oil fields of the North Sea as an example, and the potential consequences of using alkali flooding (i.e., the injection of a strong caustic solution) in order to increase oil production from a clastic reservoir. 

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