The Calcite-Carbonate-Equilibrium in Marine Aquatic Systems

The Calcite-Carbonate-Equilibrium in Marine Aquatic Systems 

The description of Sections 9.3.1 and 9.3.2 will assume in principle a solution at infinite dilution in order to reduce the system to really important reactions. The Sections 9.3.3 and 9.3.4 provide examples calculated for seawater, including all constituents of quantitative importance. In such calculations, two different approaches are possible  

The use of measured concentrations (calcitrm, carbonate, pH) together with so-called apparent equilibrium constants. These apparent constants (e g. Goyet and Poisson 1989 Roy et al. 1993 Millero 1995) take the difference between activities and concentrations into account, as well as 

The other approach employs a geochemical computer model, such as PHREEQC (Parkhurst 1995 also Chap. 15) with an input of a complete seawater analysis. Such a model will then calculate the activity coefficients and the species distribution of the solution according to the complete analysis and the constants of the thermodynamic database used. These constants are well known with an accuracy which is usually better than the accuracy of most of our analyses at least for the major aquatic species. Together with the real constant of the solubility product a reliable saturation index (SI = log Q) is then calculated. The constants of solubility products are not accurately known for some minerals, but for calcite, and also for most other carbonates, these constants and their dependence on temperature and pressure are very well documented. 

Both approaches used in the analysis of seawater lead to identical results, thus leaving it undetermined which method should be preferred for seawater analysis. However, for marine pore water the first approach is not valid, since quite different concentrations of complexes or ion pairs are likely to occur, for instance, sulfate reduction and/or an increase of alkalinity. Therefore we rely on the second approach in our examples and strongly recommend it for the marine geochemistry. 

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