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Ions in Seawater

A large part of the dissolved components of seawater is present as ion pairs, or in complexes, rather than as simple ions. While the major cations are largely uncomplexed, the anions, other than chloride, are to varying degrees present in the form of complexes. About 13% of the magnesium and 9% of the calcium in ocean waters exist as magnesium sulfate and calcium sulfate respectively.

More than 90% of the carbonate, 50% of the sulfate, and 30% of the bicarbonate exist as complexes. Many minor or trace components occur primarily as complexed ions at the pH and the redox potential of seawater. Boron, silicon, vanadium, germanium, and iron form hydroxide complexes. Gold, mercury, and silver, and probably calcium and lead, form chloride complexes. Magnesium produces complexes with fluorides to a limited extent.

Surface seawater characteristically has pH values higher than 8 owing to the combined effects of air-sea exchange and photosynthesis. The carbonate ion concentration is consequently relatively high in surface waters. In fact, surface waters are almost always supersaturated with respect to the calcium carbonate phases, calcite and aragonite.

The introduction of molecular carbon dioxide into subsurface waters during the decomposition of organic matter decreases the saturation state with respect to carbonates. While most surface waters are strongly supersaturated with respect to the carbonate species, the opposite is true of deeper waters that are often under saturated in carbonates.