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proximity to continental margins and volcanic arcs are relatively enriched in
Fe and depleted in trace metals, whereas central Pacific crusts are relatively
enriched in Mn and trace metals, with the Johnston Island and Marshall
Islands crusts showing the greatest enrichments; (C) Complete data set used
in the means in B.
Manganese, manganophile elements (e.g. cobalt, nickel, cadmium,
molybdenum), and the iron oxyhydroxide phase generally decrease, whereas detrital
phase-related elements (silicon, aluminium, iron, yttrium) increase with increasing
latitude 94. The intensification of the OMZ in the equatorial zone of high bioproductivity
allows for greater amounts of manganese to remain in solution in seawater and slower
growth rates of crusts. The increase in iron, silicon, aluminium, and yttrium to the north
is partly due to increased supply of detritus by the trade winds. The same trends in
manganese and manganophile elements should occur with longitude (increasing to the
east) in the equatorial region because regional bioproductivity increases in that direction.
However, a paucity of seamounts exists between Hawaii and North America, so that
trend cannot be confirmed Pacific-wide; regardless, the trend is generally poorly
developed regionally (e.g., 95). Contents of manganophile elements in California
continental margin crusts are diluted by detrital and biogenic inputs, which ameliorates
the increased d-MnO2-related elements expected to occur there. The residual biogenic
phase elements generally increase with proximity to the equatorial zone of high
bioproductivity and in the eastern Pacific, where productivity is yet higher 96.
Manganese, manganophile elements, and CFA-related elements decrease,
whereas iron, copper, and detrital-related elements increase with increasing water depth
of occurrence of crusts 97. Cobalt and other manganophile elements are enriched more
than manganese is in shallow water 98. Those relationships have been explained by the
increased dissolved manganese in the OMZ (about 300-1500 m water depths; Fig. 2) and
an increased supply of detrital phases at greater water depths, which contributed iron,
aluminium, and silicon.
3. IRON-MANGANESE CRUST FORMATION
Even though Fe-Mn crusts form by hydrogenetic precipitation, the exact
mechanisms of metal enrichments in the water column and at the crust surface are poorly
understood. The ultimate sources of metals to the oceans are river and eolian input,
hydrothermal input, weathering of ocean-floor basalts, release of metals from sediments,
and extraterrestrial input. Elements in seawater may occur in their elemental form or as
inorganic and organic complexes. Those complexes may in turn form colloids that
interact with each other and with other dissolved metals (e.g., 99). Thermodynamic,
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