『Abstract
The ferromanganese precipitates existing in deep-sea waters of
the Pacific consists of two types of deposits: (1) nodules mainly
are distributed in pelagic basins beneath the CCD (Calcite Compensation
Depth) where the rate of sedimentation is low; (2) polymetallic
encrustations are formed on exposed seamount rocks where currents
prevent normal sediment accumulation. Nodules, being formed in
areas bordering the equatorial zone of high biological productivity,
grow by two different processes: (A) early diagenetic growth by
supply of metals and metal compounds from pore water and (B) hydrogenetic
growth by supply of colloidal particles from near-bottom seawater.
These processes lead to different kinds of oxide and different
metal contents.
The diagenetic growth process takes place under oxidizing to suboxidizing
conditions and is supplied by an ionic solution of Mn2+
and other divalent metal ions. The mobilization of Mn is caused
by the decomposition of organic matter. The growth features of
the early diagenetic nodules show alternating laminae of crystalline
and amorphous material. These rhythmic sequences of different
microlayers are explained by physico-chemical changes (variation
of pH) in the microenvironment of the accreting nodule surface.
The hydrogenetic crust growth on seamounts leads to ferromanganese
precipitates which are in particular rich in Co. The Co concentration
is inversely related to the water depth. Co is positively correlated
to Mn which can be derived from the oxygen minimum zone. Contrary
to the diagenetic nodule growth, the crust accretion is also a
colloidal precipitation process. In the water column below the
oxygen minimum zone, a mixture of particles of Mn-Fe-oxyhydroxide
and silicate accrete together on the surface of substratum rocks.
Surface chemical mechanisms control the enrichment of Ni, Co,
Pb, and other metals from the seawater; for Pr, a coprecipitation
with MnO2 caused by a redox reaction is proposed.
Distinct oceanographical and geological conditions enable or promote,
respectively, the ferromanganese crust formation on seamounts.』
1. Introduction
2. Early diagenetic accretion
2.1 Precipitation by supersaturation
2.2 Colloid chemical precipitation
2.3 Influence of kinetic effects
2.4 Diagenetic uptake of Co
3. Hydrogenetic accumulation
3.1 Metal supply and precipitation from the water column
3.2 Specific metal adsorptin
3.3 Coprecipitation
4. Conclusions
Acknowledgements
References