tion of carbon burial. The situation for nitrogen and phosphorus is complicated
by the increase in fluxes from human activity that has paralleled the loss of wet-
lands. However, it is estimated that restoration of all the wetlands in the Humber
system could remove 25% of the phosphorus and 60% of the nitrogen currently
flowing through the estuary, and preventing it reaching the North Sea where it
can cause eutrophication.
6.3 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 3 - ) and Ca^2 +are biologically cycled (Section 6.4.4), causing vertical gradi-
ents 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.
The Oceans 189
40
20
0
1900 1950
Year
Dissolved oxygen (% saturation)
Fig. 6.6Average autumn dissolved oxygen concentrations in the tidal River Thames. After
Wood (1982).
