Geology Reference
In-Depth Information
Within the ocean, the exchange of material from the dissolved to the
suspended particulate state influences the distribution of several ele-
ments. This scavenging process removes dissolved metals from solution
and accelerates their deposition. The effectiveness of this process is
obvious in the depth profiles of metals, especially those of the surface
enrichment type. Furthermore, the removal can be expressed in terms of
a deepwater scavenging residence time as indicated in Table 10.
The scavenging mechanism can be particularly effective in the water-
sediment boundary region. The re-suspension of fine sediments gener-
ates a very large surface area for adsorption and ion exchange processes.
Within the immediate vicinity of hydrothermal springs, reduced species
of Mn and Fe are released and subsequently are oxidised to produce
colloidal oxyhydroxides, which have a large surface area and very great
sorptive characteristics. Finally, ferromanganese nodules form at the
sediment-water interface and become considerably enriched in a num-
ber of trace metals via surface reactions.
Ferromanganese nodules result from the authigenic precipitation of
Fe and Mn oxides at the seafloor. Two morphological types are recog-
nised, depending upon the growth mechanism. Firstly, spherical encrus-
tations produced atop oxic deep-sea sediments grow slowly, accu-
mulating material from seawater. These seawater nodules exhibit a
relatively low Mn:Fe ratio and are especially enriched with respect to
Co, Fe and Pb. Secondly, discoid shaped nodules develop in nearshore
environments deriving material via diffusion from the underlying anoxic
sediments. Such diagenetic nodules grow faster than deep-sea varieties
and metals tend to be in lower oxidation states. They have a high Mn:Fe
ratio and enhanced content of Cu, Mn, Ni and Zn. Ferromanganese
nodules have concentric light and dark bands in cross-section, related to
Table 10 The deepwater scavenging residence times of some trace elements in
the oceans
Scavenging residence time
(year)
Scavenging residence time
(year)
Element
Element
Sn
10
Mn
51-65
Th
22-33
Al
50-150
Fe
40-77
Sc
230
Co
40
Cu
385-650
Po
27-40
Be
3700
Ce
50
Ni
15 850
Pa
31-67
Cd
117 800
Pb
47-54
Particles
0.365
Source: adapted from Chester, 1990.
1
