Thus, the pH of seawater is strongly related to these cation concentrations.

The chloride ions are not taken up significantly by the sediments, and nearly all chloride remains in solution. As a result, seawater is close to 0.5 M NaCl.

If the carbonates are now considered in Sillen's model, as 0.46 mol of CaCO3 and 0.09 mol of MgCO3, further reactions with the aluminosilicates become possible, but other processes involve precipitation of carbonate solid phases: (two crystalline modifications of CaCO3 (calcite and aragonite), MgCO3, and dolomite, CaMg(CO3)2. Carbonate will also enter into the pH-controlling reactions, but the significance of the model is that the amount of carbonate is small compared with the proton capacity available from alumino-silicates. For 1 liter of seawater in equilibrium with atmospheric CO2, 10~3 mol of strong acid will change the pH from 8 to less than 6 (biological action may modify this). This is a small buffering capacity. However, when the minerals are included, the buffering capacity is about 1 mol/liter.

An additional element of great importance to the system is oxygen as O2. Much of the oxygen remains in the atmosphere, but some is in solution according to Henry's law. This controls the pE of the ocean and determines the states of Fe and Mn to be found in the environment, as discussed in Section 10.6. The entire system is subject to the various physicochemical equilibrium equations, and equilibrium conditions can, at least in principle, be calculated.

For equilibrium models to have any validity, rates of the various reactions must be fast enough to approach equilibrium in the time period in question. Natural reaction rates are complicated by the mixing problem, which has been referred to before.

The length of time a species spends in a given phase (say, seawater) is important in terms of how rapidly appropriate cycles can be completed. This leads to steady-state models rather than equilibrium models of the type discussed earlier. A steady state assumes that input (for the ocean, weathering products brought in by rivers, volcanic activity, atmospheric input) is balanced by removal (sedimentation, ion exchange, biological production of inert material, etc.). That is,

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