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 sources 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 (super +2) with CO 2 or HCO 3 (super -) 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.

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