An Introduction to Environmental Chemistry

removed mainly in estuaries, where suspended solid concentrations are high
(Section 6.2.1). Consequently, the atmosphere provides the main input of
particle-reactive metals to the surface waters of the central ocean. This
atmospheric flux has a natural component, the fallout of wind-blown dust
particles, which subsequently dissolve in seawater to a small extent (typically a
few per cent). Aluminium (Fig. 6.23) and manganese are examples. The second
source of particles is human activity: lead (Pb) is an example, entrained into the
atmosphere principally from automobile exhaust emissions. Lead use, particularly
as a petrol additive, increased rapidly during the 1950s until concern over the
possible health effects resulted in a dramatic decline in its use from the late 1970s
onward.
We do not have a direct history of dissolved lead concentrations in seawater,
but we do have an indirect record from corals. Coral skeletons are made of
annual layers of CaCO 3 , producing growth rings similar to those in trees. These
rings can be counted and sampled for lead analysis. The lead ion, Pb^2 +, is almost
the same size and charge as Ca^2 +and substitutes for it in the CaCO 3 coral skele-
ton, faithfully documenting the history of lead concentrations in surface
seawater (Fig. 6.24). The coral data have recently been augmented by ocean
water data as we now have the analytical capability to measure Pb^2 +concentra-
tions in seawater. Data collected in recent years show a continued decline in
surface water Pb^2 +concentrations reflecting declining inputs. Deeper in the water
column the Pb^2 +profiles are more difficult to interpret because decreasing rates
of input are fast compared to the residence time of Pb^2 +. This means that the
profiles do not represent a steady-state distribution, rather they record the

The Oceans 225

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Dissolved aluminium (nmol l–1)

Depth (km)

Seabed

Fig. 6.23Vertical distribution of aluminium in the North Pacific showing low values at 1–2
km depth, indicative of scavenging. The deepwater increase in concentration arises from
aluminium inputs from the sea-bed sediment. After Orians and Bruland (1986), with
permission from Elsevier Science.