Major ion chemistry of seawater

Having examined the chemistry of estuarine environments, we now turn to global chemical cycling in the open ocean. This chapter began by noting that the major ion chemistry of seawater is different from that of continental surface waters (Table 6.1). Three principal features clearly mark this difference:

1 The high ionic strength of seawater (see Fig. 5.3), containing about 35 g l-1 of salts (Box 6.1).

2 The chemical composition of seawater, with Na+ and Cl- overwhelmingly dominant (Table 6.1).

3 Seawater has remarkably constant relative concentrations of major ions in all the world's oceans. For example the Na+ :Cl- ratio changes by less than 1% from the Arabian Gulf to the Southern Ocean. In the oceans, bicarbonate ions (HCO-) and Ca2+ are biologically cycled (Section 6.4.4), causing vertical gradients in their ratios relative to the other major ions. However, the differences in the ratios to Na+ are small—less than 1% for calcium.

There is evidence that the major ion composition of seawater has varied over many millions of years, linked to very long term geochemical cycling. Evidence from ancient marine evaporite sequences (Box 6.2) sets limits on the possible extent of that variability.

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