『Abstract
Concentration of manganese in solution and its deposition takes
place by redox-controlled processes in a variety of modern and
ancient geologic and geochemical environments. Modern Mn deposition
occurs predominantly in deep-sea areas rather than shallow-water
domains. Although deep-sea sedimentary deposits dominate, hydrothermal
contribution of Mn to the ocean system may be substantial. Mn
deposition from hydrothermal solutions at or near sea-floor-spreading
centers and less commonly in island-arc areas is known. In addition,
near- and far-field dispersion of Mn from vent sites is also substantial.
Such distributions are controlled by the flow rate and egress
temperature of the solution and the residence time of Mn in seawater.
Thus, even in sedimentary deposit domains, at least partial derivation
of Mn from a hydrothermal source is possible. Sedimentary Fe-Mn
crusts on older volcanic substrates on seamounts form by hydrogenous
deposition of metal concentrated from terrigenous sources in the
mid-water column, oxygen-minimum zones. Thus, the presence or
absence of volcanic rocks is not a clear indication of whether
sedimentary Mn deposits, particularly in the ancient geologic
record, are the result of a totally terrigenous or a totally volcanogenic
source. Abyssal Fe-Mn nodules are considered to form from a basin
water (hydrogenous) and/or pore water (early diagenetic) supply
of metals, but in most cases the extent of supply from either
of the source is unknown. The metal incorporation mechanisms of
free-moving nodules is little understood and it is possible that
in most cases both sources contribute to the nodule composition.
Therefore, no nodule should be considered as totally hydrogenous
or totally early diagenetic based only on its bulk composition.
The determined growth rate giving only an average value cannot
by itself reveal the growth history of the nodules. Biological
participation, directly or indirectly, controls Mn deposition.
The stratified Black Sea demonstrates the concentration of Mn
in solution in an anoxic zone, its advection toward the redox
interface, and its precipitation in an oxygenated condition. Similar
stratified basins are contemplated for ancient Mn deposition in
shallow-water basin-margin areas. Geologic and geochemical signatures
indicate that during sea-level highstands, stratified basins formed
in which Mn was concentrated in solution in the anoxic part. Corresponding
transgression led to the impingement of the redox interface on
the continental shelf, and precipitation of Mn oxides could take
place across the interface during transgression-regression cycles.
Offshore, in anoxic or dysaerobic conditions, mn carbonate could
form by early diagenetic reaction of Mn+2 with CO2 or HCO3- produced
by organic carbon oxidation. Critical Mn deposits occurring in
transgressive, glaciogenic, and black shale-bearing ancient sequences
support this paleoenvironmental model for Mn deposition.』
Introduction
Geochemical Constraints: The Basic Tenet
Processes of Primary Manganese Deposition
Models for Hudrothermal Deposition of Manganese
Modern settings
Ancient settings
Models for sedimentary Deposition of Manganese
Modern environments
Ancient environments
Summary and Conclusions
Acknowledgments
References