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Origin of Manganese Nodules
Manganese nodules were first discovered on the ocean floor in 1803. After the Second World War, a comprehensive investigation of the Ocean World started, and new data were obtained on a wide distribution of manganese nodules, that made scientists consider modules as one of the major characteristics of the deep oceanic zone. Since the 1960's manganese nodules have been recognized as a potential ore source, investigation of which is stimulated by the progressive depletion of land-based mineral resources.
Here we will discuss three factors about the problems of origin of manganese nodules: the source of ore matter and manganese balance, the genetic classification of manganese nodules and the biogenic aspects in manganese nodule genesis. About the sources of ore material supply to the ocean, there were once three major presumptions:
(1) Main ore material supply to the ocean comes from terrigenous and volcanic, and also hydrothermal material.
(2) Volcanic material is the major source of the supply of ore matter to the ocean.
(3) Terrigenous is the major source of the supply of ore matter to the ocean.
The first is favored by later studies for the role of hydrothermal supply of ore matter is obvious at present, though the share of its components is not unanimously agreed upon resulting in various balance estimates.
Goldschmidt was the first to open the problem of balance for discussion. In 1954, he pointed out that absolute masses of some metals in the ocean exceed their supply from continental rock weathering over geological time. This problem has risen a great interest in geochemistry. It is sustained by the problem of manganese nodule origin and also by the problem of hydrothermal matter supply to the ocean. In 1966, Scientists computed the balance of some elements in the ocean. Their computations revealed an actual excess of manganese over its supply from continental sources. The computation result also revealed that elements such as chlorine, sulfur, bromine, iodine, molybdenum were "excessive." According to the calculating result of Elderfield (1976), dissolved manganese supply to the ocean from various sources is 2.2-4.0 million tons annually; 0.7-7.4 million tons of manganese sink to the ocean bottom annually; 0.5-10 million tons of hydrothermal manganese are supplied to the ocean annually. But according to different estimates, the supply of labile and dissolved manganese to the ocean is 0.2-0.4million tons yearly through river discharge; 0.05-0.3 million tons yearly by atmospheric material; 1-10 million tons by hydrothermal material; and 0.4-4 million tons yearly by diagenetic flow. So, the amplitude of minimum-maximum summary estimates is 1.2-14.7 million tons per year. These are the general profiles of the accumulation of manganese nodules. The accumulation rate of manganese should be studied in individual oceanic regions.
Classification of Manganese Nodules
Oceanic nodules may be formed from the ore material of polygenic origin, although some researchers would emphasize the predominant role of hydrothermal sources. Manganese nodules may grow due to a supply of material both from above (out of oceanic water) and below (out of host sediments). Depending on the major ore elements (manganese, iron and base metals) the nodules contain, three major types were distinguished:
(1) hydrogenous, i.e., formed due to slow deposition of metals out of sea water and characterized by a high concentration of base metals and varying Mn/Fe rations (from 0.5-5)
(2) hydrothermal, i.e., rich in iron and depleted of other metals, and turns out to be having an extremely wide range of Mn/Fe rations
(3) diagenetic, i.e., characterized by high Mn/Fe ratios and relatively low concentration of base metals.
Biogenic Aspects in Manganese Nodule Genesis
In 1928, Butkevich first reported the discovery of some specific iron bacteria in manganese nodules from the Arctic basin bottom. Later, in manganese nodules, other species of microorganisms were found which were capable of accumulating iron and manganese. Although the function of microorganisms in manganese and the mechanism of oxidation are still under debate, it is believed that the biogenic factor determines to a considerable extent the manganese destiny on its ways to the ocean. The fine terrigenous material and the greater part of hydrothermal material that enter into the ocean pass through the biogenic filter before they reach the bottom. Manganese species in water and suspension are associated with the occurrence of organic mater and activity of sea bacteria. At the oceanic bottom, manganese and other metals are also affected by the biogenic factor, beginning with the downward flux of organic matter and terminating with benthic activity. Bacterial activity can affect the growth, structure and composition of nodules.
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