Manganese near sediment-water interface

Deep-sea manganese nodules can widely be observed in all oceans. The majority of them is located on the sediment surface so that the upper side is in contact with the sea-water and the lower side in the nearly liquid mud of the sediment/water interface. Only a small part of nodules gets buried during their growth. 

The mobilized metals are reprecipitated in the nearly liquid layer on the sediment water interface. They repr ipitate partly on the surface of manganese nodules located there. The other pirt precipitates within the sediment, forming again micronodules or disseminated precipitates. A portion of the metals again diffuses into bottom-near sea-water. 

Manganese-oxidizing bacteria could also be identified . As manganese in the sediments has to be reduced to Mn ions to become soluble and thus to get mobilized, the very important question arises how or where it is reoxidized. Generally, the bottom-near sea-water of Central Pacific is so rich in dissolved oxygen that manganese and iron will be oxidized when coming into the contact with sea-water (at the sediment-water interface and in the mobile, nearly liquid uppermost sediment layer). 

Another surhcial deposit type includes the manganese-rich crusts and nodules that occur on basalt near mid-ocean ridges, and near the sediment-water interface. Whereas the ultimate source of manganese and iron (as well as the associated nickel, copper, and cobalt) is the hydrothermal alteration of MOR basalt, the fields of manganese-rich encrustations and nodules that cover large areas of sediment-starved ocean floor are the result of surficial authigenic upward remobilization of metals and the fixing of those metals at the sediment-water interface. 

In the St. Lawrence Estuary, Gendron et al. (1986) found evidence for different release mechanisms near the sediment-water interface - the profiles for cobalt resemble those for manganese and iron with increased levels downwards, suggesting a mobilization of these elements in the reducing zone and a reprecipitation at the surface of the sediment profile. On the other hand, cadmium appears to be released at the surface, probably as a result of the aerobic remobilization of organically bound cadmium. 

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