|Sind Manganknollen Tiefwasser-Onkoide?|
Frieling, D.; Mrazek, J. (2007). Sind Manganknollen Tiefwasser-Onkoide? Senckenb. Marit. 37(2): 93-128
In: Senckenbergiana Maritima: wissenschaftliche Mitteilungen der Senckenbergischen naturforschenden Gesellschaft. E. Schweizerbart'sche Verlagsbuchhandlung (Nägele u. Obermiller): Stuttgart. ISSN 0080-889X, more
Deep water; Manganese nodules; ISE, Costa Rica [Marine Regions]; Marine
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During the cruise SO 107 of the research vessel SONNE, in 1996, more than 2000 manganese nodules were collected from the easternmost Pacific Ocean by dredging. Densely spaced, potatoe-shaped nodules cover a wide, shallow depression on the top of the Fisher-Ridge, situated more than 100 km off the Pacific coast of Costa Rica near the centralamerican trench, 2800 m below sea level. Some 40 nodules were investigated in detail for their internal structure as well as their mineralogical and geochemical composition. Here only the structural aspect is investigated as to the question: "Are manganese nodules oncoids?" In order to answer this question various oncoids of differing chemical composition were simultaneously studied, i.e. the Cretaceous phophorite nodules and Jurassic manganese nodules from the Northern Calcacerous Alps, the Cretaceous phophorite nodules from Great Britain, the freshwater oncoids from the subalpine Molasse and recent freshwater stromatolites from the German Alpine foreland. Most of the Pacific manganese nodules are irregular ball-like objects with diameters of 10 to more than 80 mm and show rather smooth surfaces. However, rough, warty or cauliflower-like surfaces exist as well. Like other manganese nodules from different ocean floors the nodules from the Fisher Ridge can be subdivided into a core, often containing fragments of volcanic rocks, surrounded by a concentric irregularly layered shell or crust, mainly composed of Fe- and Mn-oxides and -hydroxides. Unlike similar nodule fields, most of the nodules from the Fisher Ridge are "composite nodules" containing more than one core, most of them surrounded by their own layered metaliferous shell. Such clusters of small, initially obviously separately growing nodules are subsequently enveloped by a mutual crust grown in a complicated pattern. Their microstructures consist of finely laminated, concentrically arranged layers as well as small, dome-shaped or columnar elements. This fabric is almost identical with the microstructure of carbonate or phophorite oncoids and stromatolites, growing in marine and freshwater environments. The microstructure of both, oncoids and stromatolites, is controlled by the interplay of the sedimentary environment and of microbial communities and non-cellular organic films on their surface (EPS or mucilage) of mainly photoautotrophic cyanobacteria. The almost identical fabric of carbonate and phosphorite oncoids and manganese nodules serves as a very strong argument for their formation by the activity of bacterial consortia. As within stromatolites, where the microbial communities contain carbonate-precipitating as well as not calcificing species, the manganese nodules may be products of Mn and Fe oxidising as well as reducing bacteria.