The influence of the carbonate dissolution rate on the growth and composition of Co-rich ferromanganese crusts from Central Pacific seamount areas |
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| Authors: | P Halbach D Puteanus |
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| Institution: | Fachgebiet “Geochemie, Angewandte Mineralogie und Rohstoffkunde”, Institut für Mineralogie und Mineralische Rohstoffe, Technische Universität Clausthal, D-3392 Clausthal-Zellerfeld F.R.G. |
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| Abstract: | The metal composition of oceanic ferromanganese deposits occurring in seamount regions (Line Islands chain and Mid-Pacific Mountains) varies with water depth and age. The results of metal determinations of carbonate plankton samples suggest that carbonate dissolution in the water column might have an important influence on the accretion and composition of hydrogenetic precipitates. Two ferromanganese crust generations of different age have been observed The precipitation of the older crust took probably place during early Oligocene, the younger crust began to form during middle Miocene. Between the two crust generations periods of carbonate sedimentation and of phosphorite deposition occur. The hydrogenetic formation of the crusts is controlled by the metal supply from the water column, according to the laws of colloidal surface chemistry.Dissolution experiments with carbonate plankton samples show that the main Fe source for the hydrogenetic crust formation are colloidal Fe-hydroxide particles being released in the water column from the dissolution of carbonate plankton skeletons. In the case of Mn, maximum dissolved Mn occurs in the oxygen minimum zone as the result of in-situ break-down of organic matter and the in-situ reduction of Mn-bearing solid phases. Closely beneath the oxygen minimum zone a Fe supply, mobilized within the oxygen minimum zone, has also to be taken into account. In the water column below the oxygen minimum zone, a mixture of colloidal particles of Mn Fe-oxyhydroxide and colloidal AlFe-silicate, precipitate together on the surface of substratum rocks. The mixing ratio of these colloidal phases controlling the metal composition of the ferromanganese precipitates, is depth-dependent and shows also temporal variations. In general, Mn/Fe ratio, Ni, and Co contents decrease with depth down to the calcite compensation depth.The most probable mechanism for the ultimate removal of Co and Ni from the water column might be a surface reaction. δ-MnO2 is specifically able to absorb hydrous Co2+ and Ni2+ ions. Because of the surface enrichment of Co and the strong electrical field of Mn(IV), a subsequent oxidation of Co2+ to Co3+ takes place leading to higher enrichment of Co in comparison to Ni. The most important factor governing the high Co enrichment in the ferromanganese crusts is the growth rate: the lower the growth rate, the higher the Co content. Maximum values of up to 2% Co occurring in samples from water depths between 1500 and 1100 m 1] are related to lower carbonate dissolution rates and corresponding lower Fe supply.The metal supply from the water column is strongly related to distinct environmental factors such as bio-productivity, range of lysocline and calcite compensation depth, rate of carbonate dissolution, and activity of the Antarctic bottom water. Thus, our model shows that the growth periods and the metal composition of hydrogenetic seamount crusts are controlled by changes in the paleoceanography and reflect distinct environmental conditions. |
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