is an example (Fig. 6.23). These profiles arise because the inputs of these ele-
ments 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 sur-
faces, known by the general term scavenging. Consequently, oceanic concentra-
tions 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 esti-
mated to be a few hundred years, short in comparison with nutrient and conser-
vative elements (Table 6.9). These rapid removal rates imply that river inputs are224 Chapter Six
0 0 1 2 3 4 5
246810Zinc (nmol lā1)Depth (km)0 0 1 2 3 4 5
0.2 0.4 0.6 0.8 1.0Cadmium (nmol lā1)Fig. 6.22Vertical distribution of dissolved zinc and cadmium in the North Pacific. After Bruland (1980).
Table 6.9Concentrations of nutrients and metals in deep (>3000 m) water in the North
Atlantic and North Pacific, together with estimated oceanic residence times.
Estimated oceanic
residence time
Component North Atlantic North Pacific (years)
Nitrate (mmol l-^1 ) 20 40 57 000
Silicon (mmol l-^1 ) 25 170 20 000
DIP (mmol l-^1 ) 1.3 2.8 69 000
Zinc (nmol l-^1 ) 1.7 8.0 4 500
Cadmium (nmol l-^1 ) 0.3 0.9 32 000
Aluminium (nmol l-^1 ) 20 0.4 50
Manganese (nmol l-^1 ) 0.6 0.2 30
DIP, dissolved inorganic phosphorus.