Manganese nodule

thumb|Manganese [[Nodule (geology)|nodule]]

thumb|Nodules on the [[seabed]]

thumb|[[Ferromanganese nodules found on the seafloor]]

Manganese nodules, also called polymetallic nodules, are mineral 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 a potential economic interest. Depending on their composition and authorial choice, they may also be called ferromanganese nodules. Ferromanganese nodules are mineral concretions composed of silicates and insoluble iron and manganese oxides that form on the ocean seafloor and terrestrial soils. The formation mechanism involves a series of redox oscillations driven by both abiotic and biotic processes. As a byproduct of pedogenesis, the specific composition of a ferromanganese nodule depends on the composition of the surrounding soil. The formation mechanisms and composition of the nodules allow for couplings with biogeochemical cycles beyond iron and manganese. The high relative abundance of nickel, copper, manganese, and other rare metals in nodules has increased interest in their use as a mining resource.

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

Occurrence

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 surface. 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.

Polymetallic nodules are found in both shallow (e.g. the Baltic Sea) and deeper waters (e.g. the central Pacific), even in lakes, and are thought to have been a feature of the seas and oceans at least since the deep oceans were oxygenated 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 within the Cook Islands.
• North central Pacific Ocean in a region called the Clarion–Clipperton zone (CCZ) roughly midway between Hawaii and Clipperton Islands. 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 . 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. Several processes are hypothesized to be involved in the formation of nodules, including the precipitation of metals from seawater, the remobilization of manganese in the water column (diagenetic), the derivation of metals from hot springs associated with volcanic activity (hydrothermal), the decomposition of basaltic debris by seawater and the precipitation of metal hydroxides through the activity of microorganisms (biogenic). The sorption of divalent cations such as Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, and Cu²⁺ at the surface of Mn- and Fe-oxyhydroxides, known to be strong sorbents, also plays a main role in the accumulation of these transition metals in the manganese nodules. These processes (precipitation, sorption, surface complexation, surface precipitation, incorporation by formation of solid solutions...) may operate concurrently or they may follow one another during the formation of a nodule.

Manganese nodules are essentially composed of hydrated phyllomanganates. These are layered Mn-oxide minerals with interlayers containing water molecules in variable quantities. They strongly interact with trace metals (Co²⁺, Ni²⁺) because of the octahedral vacancies present in their layers. The particular properties of phyllomanganates explain the role they play in many geochemical concentration processes. They incorporate traces of transition metals mainly via cation exchange by formation of inner sphere complexes at the oxide surface as it is also the case with hydrous ferric oxides, HFO. Slight variations in their crystallographic structure and mineralogical composition may result in considerable changes in their chemical reactivity.

thumb|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 nodules from the Clarion–Clipperton zone (CCZ) 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 type of manganese minerals and the size and characteristics of their core. Those of greatest economic interest contain manganese (27–30 wt. %), nickel (1.25–1.5 wt. %), copper (1–1.4 wt. %) and cobalt (0.2–0.25 wt. %). Other constituents include iron (6 wt. %), silicon (5 wt. %) and aluminium (3 wt. %), 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. In a given manganese nodule, there is one part of iron oxide for every two parts of manganese dioxide.

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.

In the 2010s, increased demand for nickel and other metals prompted commercial interest in seabed nodules. The International Seabed Authority has granted new exploration contracts and is progressing development of a mining code for the area, with most interest being in the Clarion–Clipperton zone.

Since 2011, a number of commercial companies have received exploration contracts. These include subsidiaries of larger companies including Lockheed Martin, DEME (Global Sea Mineral Resources, GSR), Keppel Corporation, The Metals Company, and China Minmetals, and smaller companies like Nauru Ocean Resources, Tonga Offshore Mining and Marawa Research and Exploration.

In July 2021, Nauru announced a plan to exploit nodules in this area, which requires the International Seabed Authority, which regulates mining in international waters, to finalize mining regulations by July 2023. Environmentalists have criticized this move on the grounds that too little is known about seabed ecosystems to understand the potential impacts of deep-sea mining, and some of the major tech companies, including Samsung and BMW, have committed to avoid using metals derived from nodules.

Proposed mining areas of manganese nodules

thumb|Research into manganese nodules in the Clarion–Clipperton zone

The Clarion–Clipperton zone serves as the largest and most popular area for mining manganese nodules. Extending from approximately 120W to 160W, the Clarion–Clipperton zone can be located in the Pacific Ocean, lying between Hawaii and Mexico. According to the ISA, it covers an area of about four million square kilometers which almost equals the size of the European Union. The huge potential of the Clarion–Clipperton zone is based on an estimated amount of 21 billion tons of nodules. Around 44 million tons of cobalt are stored in that area alone, which is around three times more than the land reserves could provide. Manganese nodule fields are not equally distributed on the seafloor within the Clarion–Clipperton zone but rather occur in patches. Economically interesting patches with a high distribution of manganese nodules can cover an area of several thousand square kilometers. This rather irregular nodule distribution in the South Pacific can be found as a possible result of the greater topographic and sedimentological diversity of the South Pacific. Some scientists question the prime economical interest in manganese nodules. As far as they are concerned, such biological resources could be an untapped value for biotechnologies and medicines and should therefore be protected at all cost.

Ecology

Ferromanganese nodules are highly redox active, allowing for interaction with biogeochemical cycles primarily as an electron acceptor. Notably, terrestrial nodules uptake and trap nitrogen, phosphorus, and organic carbon. The higher rate of organic carbon uptake allows nodules to enhance a soil's ability to sequester carbon, creating a net sink. Phosphorus concentration in the nodules ranges from 2.5 to 7 times the value of the surrounding soil matrix. Microbes in the soil can utilize the nutrient enrichment on the surface of nodules coupled with their redox potential to fuel their metabolic pathways and release the once immobile phosphorus. Along with nutrients, ferromanganese nodules can sequester toxic heavy metals (lead, copper, zinc, cobalt, nickel, and cadmium) from the soil, improving its quality. Due to the complexity and remoteness of the deep-sea, environmental scientists work in a knowledge poor situation with many gaps and high uncertainty. Nevertheless, there are several sources of cumulative impacts caused within a mining operation that must be considered. These impacts can be directly caused by the mining activities themselves but also occur as indirect impacts such as sedimentation plumes and disposition. Multiple impacts can be caused from the same mining activity but affect the deep-sea environment in different ways.

These could include:

• discharge plumes that have effects of clogging feeding mechanisms of plankton
• Noise and light pollution that cause reduced visibility for predators
• Ecotoxicological or chemical temperature changes in the water quality Scientists found that collection vehicles can have long-lasting physical and biological effects on the seafloor and cause an altering of various deep-sea ecosystems that scientists are still working to understand. This mining method leads to an inevitable loss of life among animals while the plow tracks remained visible decades later. impact study aimed to reveal the potential long-term impacts of deep-sea mining-related disturbances on seafloor integrity by revisiting 26-year-old plough tracks. While nodules appeared outside the tracks dusted with sediments, the plough tracks themselves were clearly devoid of nodules.

In addition to the plumes created by mining activities on the seabed, discharge plumes should also be considered, that will be created by the return of excess water. Excess water occurs during the dewatering process on board of the surface vessel as well as when ore slurries are transported from the mothership to the transport barges. Light pollution is another important factor that causes environmental impacts on sea life. The light that is used to make mining work undersea possible could attract or repel some animal species, bright lights can also blind certain marine animals. Strong lights used at the vessel and ships can influence birds as well as near surface animals. Experimental studies in the 1990s concluded in part that trial mining at a reasonable scale would likely help best constrain real impacts from any commercial mining.

Recovery potential of seabed ecosystems

The slow recovery potential of ecosystems can be seen as one of the major concerns of nodule mining. Seabed areas that contain nodules will be massively disturbed and the recovery of epifauna is exceptionally slow within the mined areas. A significant proportion of the animals are dependent on the nodules, which create a hard substrate for them. These substrates will not return for millions of years until new nodules are formed.