Minerals Report - International Seabed Authority

eadily incorporated into iron and manganese metal oxyhydroxides (105).
Zinc contents may be relatively low in crusts compared to nickel because little
zinc may be adsorbed onto crusts after precipitation of the oxyhydroxides,
which follows the order of nickel>cobalt>zinc> copper (106). In contrast,
comparable proportions of manganese: iron: cobalt exist in deep seawater
(0.5-1.0:1:0.02-0.05; (107)) as exist in Fe-Mn crusts (0.6-1.6:1:0.02-0.05; Tables 1,
6; (108)).
The dominant controls on the concentration of elements in
hydrogenetic crusts are the concentration of each element in seawater;
element-particle reactivity; element residence times in seawater; the absolute
and relative amounts of iron and manganese in the crusts, which in turn are
related to their abundance and ratio in colloidal flocs in seawater (109); the
colloid surface charge and types of complexing agents, which will determine
the amount of scavenging within the water column (110); the degree of
oxidation of MnO2 (oxygen/manganese)--the greater the degree of oxidation
the greater the adsorption capacity--which in turn depends on the oxygen
content and pH of seawater (111); the amount of surface area available for
accretion, which at the surface of growing crusts is extremely large (mean 300
m2 /g), but which decreases with maturation of crusts (112); the amount of
dilution by detrital minerals and diagenetic phases; and growth rates.
Elements that form carbonate complexes in seawater behave independently
from those that form hydroxide complexes, which indicates their different
modes of removal from seawater onto crust surfaces (113). Very slow growth
rates promote enrichment of minor elements by allowing time for extensive
scavenging by the major oxyhydroxides. Accretion of oxyhydroxides will be
slower where the OMZ intersects the seafloor than it will be above and below
that zone, because manganese is more soluble in low-oxygen seawater (Fig. 2).
Crusts exposed at the seafloor may not necessarily be actively accreting
oxyhydroxides (114) because of mechanical erosion or, less commonly in the
contemporary oceans, because of seawater oxygen contents that are
insufficient to permit oxidation of the major metals.
INTERNATIONAL SEABED AUTHORITY 225