An Introduction to Environmental Chemistry

these areas has come from a recent experiment in which iron was added to a small

patch of ‘chemically labelled’ surface water in the Southern Ocean. The chemi-

cal label (an artificial compound called SF 6 ) was used to trace the movement of

the iron-enriched patch of water, and within a few days, phytoplankton abundance

had increased markedly (Fig. 6.26).

During the last ice age, approximately 20 000 years ago, dust fluxes to the

oceans were higher because the climate was drier and windier. It has been pro-

posed recently that the resulting enhanced supply of dust-derived iron at this time

increased ocean productivity directly by increasing photosynthesis. The high

dust-derived iron flux may have also promoted enhanced productivity by bacte-

rial nitrogen fixation in the nitrate-deficient central ocean areas, as this enzyme

system has a requirement for iron (see above). If increased productivity did take

place, it would have also consumed CO 2 because of increased photosynthesis (see

eqn. 5.19) lowering global atmospheric CO 2 (see Section 7.2.2) and hence sus-

taining the glacial climate. The proposed linkage between the biogeochemical

cycling of iron and CO 2 is debated hotly between scientists and is the subject of

much ongoing research.

6.7 Ocean circulation and its effects on trace element distribution

element distribution

The preceding discussion of trace elements in seawater has assumed that the

oceans have a uniform, warm, nutrient-depleted surface mixed layer and a static

deep zone. In fact, at high latitudes surface seawater is cold enough to destroy

any density stratification, mixing the oceans to depths of up to 1000 m. This dense

surface water sinks and flows slowly into the centre of the oceans as a layer of

The Oceans 229

Time (days)

Chlorophyll (mg m

–2)

120

(^00481014)
80
26 12
40
Inside patch
Outside patch
Iron-enrichment
commenced
Fig. 6.26Chlorophyll concentrations as indicators of phytoplankton growth, inside and
outside an iron-enriched patch of seawater in the Southern Ocean. After a few days
chlorophyll concentrations had increased markedly in the iron-enriched water indicating
phytoplankton growth. After Boyd et al. (2000), reprinted with permission from Nature.
Copyright (2000) Macmillan Magazines Limited.