Manganese Nodules

Manganese concentrations in the seafloor are particularly well developed in deep, calm waters far from continental margins and active volcanic ridges. Manganese deposits form in ocean basins that receive little sediment that would otherwise bury them. Such areas include abyssal plains and elevated areas of the seabed, such as seamounts and isolated shallow shoals. Potential economic concentrations of manganese occur in the marine environment in two forms:

Manganese nodules
Ferromanganese crusts

Manganese nodule is an irregular, black to brown, friable, laminated concretionary mass consisting primarily of manganese-oxide minerals (Mn content 15-30%), alternating with iron oxides. Manganese nodules vary in diameter from a few mm to 25 cm (usually 3-5 cm) and their average weight is about 100 grams, although larger ones are also found (a nodule weighing 770 kg was found). These nodules form extremely slowly, growing at rates of a few millimeters every million years.

Several tens of thousands of square kilometers of the deep sea floor are covered by metal-rich nodules, which mainly contain manganese, but are also economically important due to their content in iron, nickel, cobalt, copper and titanium. Manganese nodules occur widely in sediment-covered depths between 3,500 and 6,500 meters.

Origin of Manganese Nodules

The formation of manganese nodules on the ocean-floor results from the slow precipitate of metallic minerals that are extracted directly from the sea-ﬂoor or from the pore waters in the sea- bottom sediments. Metals like iron and manganese are present in seawater at concentration of less than 1 ppm by weight. Metals enter the oceans via streams carrying material derived from the weathering and decomposition of rocks on the continents and through ocean floor hydrothermal vents, which get their metallic content from subsurface volcanic zones.

While formation of manganese nodule isn’t completely understood, several theories and processes contribute to their creation:

Accumulation and Precipitation: Manganese nodules are thought to form through the slow precipitation of minerals from seawater. This process occurs around a nucleus, which could be a small rock fragment, a shark tooth, or even a fossil. Over time, metal-rich substances accumulate around these nuclei, gradually forming layers and resulting in the growth of the nodule.
Concretion: Some nodules are believed to form through concretion, where dissolved metals in seawater precipitate and accumulate around a central core or nucleus, gradually increasing in size.
Hydrogenous Deposition: Manganese nodules may form through hydrogenous processes, where dissolved metals in seawater, transported by currents, settle on the ocean floor and gradually accumulate to form nodules.
Biogenic Influence: Organic matter, microbial action, or biological processes might also play a role in the initiation or growth of some manganese nodules, although the extent of this influence remains a subject of study and debate.

Precipitation occurs in the form of insoluble iron and manganese oxides and hydroxides in the presence of oxygen in seawater. Concentric layers of iron and manganese hydroxides, as well as copper, nickel, and cobalt oxides, precipitate to the size of potato nodules, giving the ocean floor a cobblestone appearance.

The following factors are required for manganese nodule formation:

Low quantities of suspended material in sea-water and low sedimentation rates, so that the nodules are not buried too rapidly under the sediment cover.
Constant flow of Antarctic bottom currents. These currents flush fine sediment particles away that would otherwise bury the nodules over time. The coarser particles, such as the shells of small marine organisms and clam or nodule fragments left behind act as nuclei for new nodules.
Good oxygen supply. The Antarctic bottom currents, for example, transports oxygen-rich water from the sea surface to greater depths. Without this oxygen supply, the manganese oxide compounds cannot precipitate.
Aqueous sediment. The sediment has to be capable of holding large amounts of pore water. Diagenetic nodule growth can only take place in very aqueous sediments.

Occurrence of Manganese Nodules

It is believed that global manganese nodule occurrences contain far more manganese than all known terrestrial reserves. Economically significant occurrences are situated mostly in the Pacific and Indian Oceans. Individual nodules float loose on the sea floor, but are occasionally buried by a thin silt layer.

Nodules are buried partially or entirely in the sediment that makes up the seabed. Their abundance varies significantly; in certain instances, they cover more than 70 percent of the sea floor. Alan A. Archer of the London Geological Museum estimated in 1981 that there were 500 billion tons of polymetallic nodules on the sea floor.

Polymetallic nodules are found in both shallow (e.g., the Baltic Sea) and deeper waters (e.g., the central Pacific), and are assumed to have been present in the seas and oceans since the deep oceans were oxygenated around 540 million years ago in the Ediacaran period.

In 1868, polymetallic nodules were found in the Kara Sea, in Siberia’s Arctic Ocean. They were discovered in most of the world’s waters during the scientific explorations of HMS Challenger (1872-1876).

Their composition varies depending on location, and significant deposits have been discovered in the following areas:

Penrhyn Basin within the Cook Islands.
The Clarion Clipperton Zone (CCZ), located in the north central Pacific Ocean, roughly halfway between Hawaii and the Clipperton Islands.
The Indian Ocean Nodule Field (IONF) is located in the southern tropical Indian Ocean, approximately 500 kilometers southeast of Diego Garcia Island.
Peru Basin in the southeast Pacific
In the Eastern Pacific, encompassing the area surrounding the Juan Fernández Islands and the abyssal plain off the coast of Loa River.

Manganese Nodule as potential of rare metal resource

The concentration of nickel, copper, cobalt, zinc, iron, and manganese in ferromanganese nodules has resulted in interest in their potential use as a rare metal resource. The Clarion-Clipperton Zone, located in the northern Pacific Ocean, has been identified as having the largest concentration of resource-grade nodules. To be classified as resource-grade, nickel, copper, and cobalt must have a bulk weight of more than 3%. The most ideal rare metal ratio in hydrogenic nodules is produced by nodule formation in oxic waters at or below the carbonate compensation depth. As the grade of ores from surface mines has declined over the years, ferromanganese nodules could be a solution to address the growing worldwide demand for rare metals.

Ferromanganese Crust

Ferromanganese crusts, also known as manganese nodules, are hard, mineral-rich crusts found on the seafloor. These crusts differ from manganese nodules, which are small, potato-shaped concretions. Ferromanganese crusts develop more gradually, layer by layer, on hard surfaces such as rocks, reefs, or even the skeletons of dead organisms on the ocean floor.

The crusts grow by the precipitation of metals dissolved in seawater, and are commonly called ferromanganese crusts, reflecting the fact that their major constituents are iron (Fe) and manganese (Mn), although a host of other minerals occur in them in smaller amounts, including lithium, molybdenum, chromium, zinc, nickel, copper, and particularly cobalt – which is why they are also often called cobalt-rich crusts or cobalt-rich ferromanganese crusts.

Ferromanganese crusts are of interest due to their high concentrations of valuable metals, similar to manganese nodules. However, their extraction poses significant challenges and environmental concerns, much like deep-sea mining for nodules. Protecting deep-sea ecosystems and understanding the potential environmental impacts remain critical considerations in any plans for exploiting these mineral resources.

Manganese Nodules – Origin and Nature of Manganese Deposits on the Seafloor

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