significant removal process for a number of elements, particularly vanadium (V),
phosphorus (P), arsenic (As) and chromium (Cr) (Table 6.10). These elements
appear to coprecipitate during iron oxyhydroxide formation, for example
the oxyanion HPO 42 - is incorportated in particles of CaHFe(PO 4 ) 2 ; it is
assumed that VO 42 - , HAsO 42 - and CrO 42 - form similar compounds with iron
oxyhydroxides.
6.6 The role of iron as a nutrient in the oceans
Although abundant in the Earth’s crust (see Fig. 1.3), iron is present in seawater
at very low concentrations (about 1 nmol l-^1 or less) because the thermodynami-
cally stable Fe(III) species is both insoluble (see Fig. 5.2) and particle reactive,
being a highly charged small ion (Section 6.5.5). Despite this, iron is an essen-
tial component for a number of life-supporting enzyme systems including those
involved in photosynthesis and nitrogen fixation. It may appear surprising that
phytoplankton have evolved with an essential requirement for an element present
at such low oceanic concentrations. However, the primitive algae that eventually
gave rise to modern phytoplankton probably evolved at a time of much lower
global oxygen concentrations in a mildly reducing ocean, when soluble Fe(II) was
the dominant iron species.
It is only relatively recently that oceanographers have been able to measure
accurately dissolved iron concentrations in the oceans; earlier efforts were ham-
pered by a combination of the low dissolved iron concentrations and the ubiqui-
tous sources of iron contamination that compromised sampling and analysis. The
distribution of dissolved iron in the oceanic water column is now known to be
mostly nutrient-like (Fig. 6.25). Surface waters can have concentrations below
0.1 nmol l-^1 due to phytoplankton uptake, increasing to about 1 nmol l-^1 in deep
waters throughout the oceans. These higher deep-water concentrations are main-
tained by complexation of Fe(III) by strong organic ligands (Box 6.4), which
prevent scavenging onto particles. Despite this, the residence time of dissolved
iron is thought to be only a few hundred years, similar to that of other scavenged
elements. Thus the behaviour of iron in seawater is similar to both nutrient and
scavenged elements.
As much of the potential riverwater input of dissolved iron is rapidly stripped
out of water during estuarine mixing, the main external source of iron to the open
oceans is from the (very limited) dissolution of wind-blown soil and dust. This is
material derived primarily from the great Asian, African and Middle Eastern
deserts (Plate 6.2, facing p. 138) of the northern hemisphere. Dust has an atmos-
pheric residence time of only a few days, much shorter than hemispheric atmos-
pheric mixing times. This means that the southern hemisphere oceans receive
much lower atmospheric dust inputs than those in the northern hemisphere. For
example, dust inputs to the North Pacific are 11 times greater than those to the
South Pacific.
In some areas of the oceans, it is now clear that the supply of dissolved iron
from terrestrial-derived atmospheric dust and from upwelling is inadequate to
The Oceans 227
