海洋に産する鉄-マンガン団塊の成長機構

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Megascopically, nodules are composed of nuclei and concentric shell layers. The hard surface of nuclei is necessary for the start of nodule formation. The concentric shell layers are formed by cyclic embedding of nodules in the sediment. During their embedding, 10 Å manganates are precipitated diagenetically from interstitial water, which are rich in manganese. During their exposure in bottom water, vernadite is formed hydrogenetically, of which concentration of manganese is almost equal to that of iron. Microscopically, concentric shell layers have a zonal structure which is classified as columnar, laminated, mottled, massive and compact. The columnar and mottled zones are the most abundant, and the former is observed in the nodules of hydrogenetic origin and the latter is in those of diagenetic origin. The zonal structure is considered to be formed as follows. When manganese oxides are precipitated, the pH in the micro-environment is lowered and further oxidation of Mn^<2+> is inhibited. During the low pH, hydrated iron oxides and silica are precipitated. After restoration of the initial pH, manganese oxides can again precipitate. Whether ferromanganese concretions are of biological origin or of physicochemical origin is the crux of the problem. Further, there has been a debate on the origin of nodules between procaryotic (bacterial) and eucaryotic (foraminiferal) stromatolites. Although sessile benthic organisms such as foraminifera may prevent cracking of nodules and sediment feeders may move nodules, the direct involvement of these organisms in the growth of nodules is questionable. On the other hand, manganese oxidizing bacteria may contribute to the growth of nodules.
日本海洋学会の論文
1996-11-15

海洋に産する鉄-マンガン団塊の成長機構

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