Manganese nodules are concretions of manganese and other transition metal oxides which form in marine and freshwater sedimentary environments worldwide. The Clarion-Clipperton zone (CCZ) deposits are on the surficial sediments of the deep seabed in the northeastern tropical Pacific Ocean. In general, manganese nodules are believed to form in aqueous, unconsolidated sedimentary environments which include a flux of disslved or chemically adsorbed manganese and iron through a gradient between relatively reducing conditions where Mn(II) is stable in solution (or as an adsorbed species on particles) and oxidizing conditions where the manganese will precipitate as basically tetravalent oxides.
Two sources have been proposed for the metals in the CCZ deposits: hydrothermal sources from seabed volcanoes, and terrestrial sources from North and Central American rivers and airborne particles. Active vulcanism is significant only in the extreme eastern part of the CCZ, so the metals in almost all of the CCZ deposits must involve some sort of lateral transport from the east for either primary sources. Research completed to date suggests that metals in the CCZ surface waters consist mostly of adsorbed species on fine particles concentrated at the top of the thermocline, moving with the surface currents to the west. Plankton concentrate these adsorbed and fine-grained particulate metals into body parts and fecal pellets which are large enough to settle through the water column to the seabed. Thus accumulations of manganese nodules can be expected to be proportional to the productivity of the overlying surface waters. Growth rates of the nodules appear to vary between about 3 to perhaps 20 mm/My. Deposits are patchy, exhibiting wide variations of sbundance in apparent response to local geographic and oceanographic factors.
Available data for the CCZ cover an interpolated area of 9 million km2, which hold 34 billion metric tons of manganese nodules. Specific estimates of metal resources include the following (units, millions of metric tons): Mn - 7500, Ni - 340, Cu - 265, C0 - 78.』
6.2 Formation processes
6.2.1 Chemical and mineralogical factors
6.2.2 Metal sources and oathways
126.96.36.199 Lateral transport in surface waters
188.8.131.52 Vertical transport
6.3 Nodule growth rates
6.4 Local factors
6.5 Resource assessment
6.5.2 General results
6.5.3 Resource assessment results
6.6 Environmental implications