Equilibrium Models of Seawater

Now that we have reviewed some basic aspects of the chemical composition of the ocean, we can turn to a more fundamental question. What processes determine the composition of the ocean Current evidence suggests that rivers are the most important contributors of dissolved substances to the ocean. Since there is geological evidence that the concentration and composition of the ocean has been relatively constant over the last —1.5 Ga, we must conclude that river input must be balanced by removal. 

This is a weathering reaction. Sillen argued that Goldschmidt s reaction could also go in the other direction. The reverse reaction would be called reverse weathering. 

The framework for constructing such multi-component equilibrium models is the Gibbs Phase Rule. This rule is valid for a system that has reached equilibrium and it states that  

In a mathematical sense, / represents the difference between the number of independent variables (including T and P) and the number of constraints (equations). If the number of equations equals the number of unknown variables, we can solve for all the concentrations using equilibrium equations. For more discussion of the phase rule, see Stumm and Morgan (1981). Sillen s approach was to mix components, pick a reasonable set of phases that might be present, and then see how many degrees of freedom there are to be fixed. 

Sillen constructed his models in a stepwise fashion starting with a simplified ocean model of five components [HCl, H2O, KOH, Al(OH)3, and Si02] and five phases (gas, liquid, quartz, kaolinite, and potassium mica) (Sillen, 1967). His complete (almost) seawater model was composed of nine components HCl, H2O, and CO2 are acids that correspond to the volatiles from the Earth KOH, CaO, Si02, NaOH, MgO, and Al(OH)3 correspond to the bases of the rocks. If there was an equilibrium assemblage of nine phases, the system would have only two independent variables. Sillen argued that a plausible set could include a gas phase and a solution phase and the following seven solid phases  

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The thermodynamic data compilations of Sillen and Martell catalyzed rapid advances in equilibrium models of seawater speciation. These works were followed by additional compilations that were critically important to modern sea-water speciation assessments. In view of these developments, and additional extensive experimental analyses appropriate to seawater. Principal Species assessments ten to fifteen years after the pioneering work of Sillen demonstrated a much improved awareness of the importance of hydrolysis in elemental speciation. 

The ratio of sulfide content/sulfate content of ore cannot be explained by equilibrium model of seawater-hydrothermal solution mixing. 

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