Scavenged behaviour

Elements that are highly particle-reactive, characterized by large z/r ratios (see Section 5.2), often have vertical profiles with surface maxima and decreasing concentrations with depth; aluminium (Al)

Table 6.9 Concentrations of nutrients and metals in deep (>3000m) water in the North Atlantic and North Pacific, together with estimated oceanic residence times.

Component

North Atlantic

North Pacific

Estimated oceanic residence time (years)

Nitrate (|imoll-1)

20

40

57000

Silicon (|imol l-1)

25

170

20000

DIP (|imol l-1)

1.3

2.8

69000

Zinc (nmol l-1)

1.7

8.0

4500

Cadmium (nmol l-1)

0.3

0.9

32000

Aluminium (nmoll-1)

20

0.4

50

Manganese (nmoll-1)

0.6

0.2

30

DIP, dissolved inorganic phosphorus.

DIP, dissolved inorganic phosphorus.

Fig. 6.22 Vertical distribution of dissolved zinc and cadmium in the North Pacific. After Bruland (1980).

is an example (Fig. 6.23). These profiles arise because the inputs of these elements are all in the surface waters, producing concentration maxima there. Poorly understood processes lower these concentrations by removal to particulate phases. The removal processes probably involve adsorption on to particle surfaces, known by the general term scavenging. Consequently, oceanic concentrations of scavenged elements are many orders of magnitude below those predicted from simple mineral solubility considerations.

Scavenged species are all metals and their residence times in seawater are estimated to be a few hundred years, short in comparison with nutrient and conservative elements (Table 6.9). These rapid removal rates imply that river inputs are a a o

Dissolved aluminium (nmol l 1)

Seabed

Fig. 6.23 Vertical 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.

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 CaCO3, producing growth rings similar to those in trees. These rings can be counted and sampled for lead analysis. The lead ion, Pb2+, is almost the same size and charge as Ca2+ and substitutes for it in the CaCO3 coral skeleton, 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 Pb2+ concentrations in seawater. Data collected in recent years show a continued decline in surface water Pb2+ concentrations reflecting declining inputs. Deeper in the water column the Pb2+ profiles are more difficult to interpret because decreasing rates of input are fast compared to the residence time of Pb2+. This means that the profiles do not represent a steady-state distribution, rather they record the

i7oo iS9o i9io i93o i95o i97o i99o

Year

Fig. 6.24 Lead concentrations in dated year bands from a coral collected from the Florida Keys. After Shen and Boyle (1987), with permission from Elsevier Science.

i7oo iS9o i9io i93o i95o i97o i99o

Year

Fig. 6.24 Lead concentrations in dated year bands from a coral collected from the Florida Keys. After Shen and Boyle (1987), with permission from Elsevier Science.

Table 6.10 Removal fluxes of elements by hydrothermal plumes (molyr-1). Based on data in Elderfield and Schultz (1996), with permission from the Annual Review of Earth and Planetary Sciences.

Element

Hydrothermal plume removal flux

River input flux

% removal of river input flux

As Cr

73 33 20 8

system still adjusting to rapid changes in Pb2+ input. Although it is regrettable that humans have caused global-scale lead pollution, chemical oceanographers can at least make use of the information as these transient pollutants act as tracers to allow a better understand of material cycling in the oceans (see also Box 7.1).

In the deep oceans, particle-rich hydrothermal plumes (Plate 6.1, facing p. 138), and the iron- and manganese-rich hydrothermal sediments that fall out from them, cause intense scavenging of some elements from seawater, making mid-ocean ridge environments important sinks for some metals. It is calculated that the entire ocean is stripped of its reactive constituents by hydrothermal plumes in about 2.4 X 105 years (Table 6.5). To do this, the volume of the oceans has to pass through the hydrothermal plumes about 100 times and this means that scavenging in the plume is significant for elements that have oceanic residence times (Box 6.3) in excess of 10000 years. Scavenging in the plume is a 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 HPO42- is incorportated in particles of CaHFe(PO4)2; it is assumed that VO24-, HAsO42- and CrO42- form similar compounds with iron oxyhydroxides.

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