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= Manganese nodule = From Wikipedia, the free encyclopedia Jump to navigationJump to search Polymetallic nodules, also called manganese nodules and ferromanganese nodules, are rock concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core. As nodules can be found in vast quantities, and contain valuable metals, deposits have been identified as having economic interest. Manganese nodule Nodules on the seabed

Nodules vary in size from tiny particles visible only under a microscope to large pellets more than 20 centimetres (8 in) across. However, most nodules are between 3 and 10 cm (1 and 4 in) in diameter, about the size of hen's eggs or potatoes. Their surface textures vary from smooth to rough. They frequently have botryoidal (mammilated or knobby) texture and vary from spherical in shape to typically oblate (flying saucer), sometimes prolate (American football), or are otherwise irregular. The bottom surface, buried in sediment, is generally rougher than the top due to a different type of growth.

Contents

 * 1Occurrence
 * 2Growth and composition
 * 3Proposed mining
 * 4Legal developments in 'The Area'
 * 5Environmental issues and sensitivities
 * 6See also
 * 7References
 * 8Further reading
 * 9External links

Occurrence[edit]
Nodules lie on the seabed sediment, often partly or completely buried. They vary greatly in abundance, in some cases touching one another and covering more than 70% of the sea floor. The total amount of polymetallic nodules on the sea floor was estimated at 500 billion tons by Alan A. Archer of the London Geological Museum in 1981.[citation needed]

Polymetallic nodules are found in both shallow (e.g. the Baltic Sea) and deeper waters (e.g. the central Pacific), even in lakes,[citation needed] and are thought to have been a feature of the seas and oceans at least since the deep oceans oxidised in the Ediacaran period over 540 million years ago.

Polymetallic nodules were discovered in 1868 in the Kara Sea, in the Arctic Ocean of Siberia. During the scientific expeditions of HMS Challenger (1872–1876), they were found to occur in most oceans of the world.

Their composition varies by location, and sizeable deposits have been found in the following areas:


 * Penrhyn Basin near within the Cook Islands.
 * North central Pacific Ocean in a region called the Clarion Clipperton Zone (CCZ) roughly midway between Hawaii and Clipperton Islands.
 * Peru Basin in the southeast Pacific, and
 * Southern tropical Indian Ocean in a region termed the Indian Ocean Nodule Field (IONF) roughly 500 km SE of Diego Garcia Island.
 * In the Eastern Pacific, including the area around Juan Fernández Islands and the abyssal plain offshore Loa River.

The largest of these deposits in terms of nodule abundance and metal concentration occur in the Clarion Clipperton Zone on vast abyssal plains in the deep ocean between 4,000 and 6,000 m (13,000 and 20,000 ft). The International Seabed Authority estimates that the total amount of nodules in the Clarion Clipperton Zone exceeds 21 billions of tons (Bt), containing about 5.95 Bt of manganese, 0.27 Bt of nickel, 0.23 Bt of copper and 0.05 Bt of cobalt.

All of these deposits are in international waters apart from the Penrhyn Basin, which lies within the exclusive economic zone of the Cook Islands.

Growth and composition[edit]
On the seabed the abundance of nodules varies and is likely controlled by the thickness and stability of a geochemically active layer that forms at the seabed. Pelagic sediment type and seabed bathymetry (or geomorphology) likely influence the characteristics of the geochemically active layer.

Nodule growth is one of the slowest of all known geological phenomena, on the order of a centimeter over several million years. Hypothesized nodule growth mechanisms include:


 * Hydrogenous (also called oxic diagenetic growth)
 * Precipitation of metals from seawater: The mobile forms of iron and manganese, Fe2+ and Mn2+, are oxidized in the water column to Fe3+ and Mn4+ and are precipitated onto a nucleus. Nodules that form from hydrogenous growth have a smooth surface and are found on top of deep sea sediment. This process is on the order of millimeters per million years and occurs in areas of the deep ocean with little sedimentation.
 * Diagenetic
 * Remobilization of manganese in the water column: Manganese oxides and iron oxides present in the sediment are reduced to their mobile forms, Fe2+ and Mn2+, by organic matter decomposition in the sediment.  Fe2+ and Mn2+ diffuse towards oxic sediment and are oxidized to Fe3+ and Mn4+. These nodules occur in areas where the sedimentation rate is higher and grow faster, approximately tens of millimeters per million years, and are lumpy in appearance. This growth mechanism dominates for nodules that are partially or completely buried by sediment.
 * Hydrothermal
 * Derivation of metals from hot springs associated with volcanic activity
 * Halmyrolitic
 * Decomposition of basaltic debris by seawater
 * Biogenic
 * Precipitation of metal hydroxides through the activity of microorganisms

Several of these processes may operate concurrently or they may follow one another during the formation of a nodule. Polymetallic nodules The mineral composition of manganese-bearing minerals is dependent on how the nodules are formed; sedimentary nodules, which have a lower Mn2+ content than diagenetic, are dominated by Fe-vernadite, Mn-feroxyhyte, and asbolane-buserite while diagenetic nodules are dominated by buserite I, birnessite, todorokite, and asbolane-buserite. The growth types termed diagenetic and hydrogenetic reflect suboxic and oxic growth, which in turn could relate to periods of interglacial and glacial climate. It has been estimated that suboxic-diagenetic type 2 layers make up about 50–60% of the chemical inventory of the CCZ nodules whereas oxic-hydrogenetic type 1 layers comprise about 35–40%.The remaining part (5–10%) of the nodules consists of incorporated sediment particles occurring along cracks and pores.

The chemical composition of nodules varies according to the kind of manganese minerals and the size and characteristics of the core. Those of greatest economic interest contain manganese (27–30%), nickel (1.25–1.5 %), copper (1–1.4 %) and cobalt (0.2–0.25 %). Other constituents include iron (6%), silicon (5%) and aluminium (3%), with lesser amounts of calcium, sodium, magnesium, potassium, titanium and barium, along with hydrogen and oxygen as well as water of crystallization and free water.

A wide range of trace elements and trace minerals are found in nodules with many of these incorporated from the seabed sediment, which itself includes particles carried as dust from all over the planet before settling to the seabed.

Freshwater Manganese Nodules
Manganese nodules occur in freshwater lakes at higher latitudes, such as Lake Michigan and Oneida Lake in North America. These nodules, also referred to as concretions, are different from those found in deep sea environments because their mineralogy, composition, shape, and rate of formation differ. The cooler climates of higher latitudes cause an increase in weathering of the igneous rocks and glacial deposits that releases Fe2+ and Mn2+ into porewater and surface water. The Fe2+ and Mn2+ is oxidized and precipitates as iron and manganese oxides onto a nucleus. Redox conditions within a lake are controlled by thermal stratification, turbulence, and wave action. Similar to their marine counterparts, freshwater manganese nodules also contain trace metals. However, the freshwater nodules generally have lower concentrations of trace metals due to their faster growth rates. In addition, differing organic carbon content and the geochemistry of the lake water affect the composition of the nodules. Manganese nodules found in Lake Michigan, for example, have higher arsenic, radium, and barium concentrations compared to deep sea manganese nodules.

Soil Manganese Nodules
Manganese nodules can form in the A horizon of loamy, sandy, and/or clayey soils, or any combination of these three soil types. Soil nodules range in size from 0.5 to 25 mm. Although soil nodules are not continuously covered with water, they still form when soil pore water redox conditions change, such as during alternating wet and dry conditions, similar to diagenetic growth in freshwater and marine nodules. Because oxygen diffuses more slowly in wet soils, reducing conditions are created, but as soil dries out, oxygen is able to diffuse into the soil matrix and the soil becomes more oxidizing. As with marine and freshwater manganese nodules, microbes also play a role in the precipitation of iron and manganese oxides that can result in the formation of nodules.

Environmental Significance of Manganese Nodules
Manganese nodules can have economic value, and the trace elements adsorbed and incorporated into manganese nodules are also of environmental significance. Iron and manganese oxides adsorb or coprecipitate trace elements and organic matter, which removes the metals or organics from the surrounding water or soil. The incorporation of various trace elements into manganese nodules makes them useful for reconstructing paleoclimate. The nodules also remove contaminants from freshwater and soil that would make them a useful remediation tool, although it is also possible that trace metals can be released from manganese nodules, such as in Lake Michigan where arsenic can come from manganese nodules.