Manganese nodule

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Polymetallic nodules, also called manganese 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.[1]

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 (mammillated or knobby) texture and vary from spherical in shape to typically oblate (flying saucer), sometimes prolate (Rugby ball), or are otherwise irregular. The bottom surface, buried in sediment, is generally rougher than the top due to a different type of growth.[2]

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 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.[citation needed]

Polymetallic nodules are found in both shallow (e.g. the Baltic Sea[3]) and deeper waters (e.g. the central Pacific), even in lakes,[citation needed][4] 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.[5]

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.[6]

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

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.[2]

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]

Manganese nodules from the South Pacific Ocean

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.[11] 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.[12] 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 (halmyrolitic) and the precipitation of metal hydroxides through the activity of microorganisms (biogenic).[13] 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[14] in their interlayer like clay minerals and surface complexation[15] by formation of inner sphere complexes at the oxide surface as it is also the case with hydrous ferric oxides, HFO.[16] Slight variations in their crystallographic structure and mineralogical composition may result in considerable changes in their chemical reactivity.[17]

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.[14] 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.[18]

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.[19]

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.[2]

Proposed mining[edit]

Interest in the potential exploitation of polymetallic nodules generated a great deal of activity among prospective mining consortia in the 1960s and 1970s. Almost half a billion dollars was invested in identifying potential deposits and in research and development of technology for mining and processing nodules. These initial undertakings were carried out primarily by four multinational consortia composed of companies from the United States, Canada, the United Kingdom, West Germany, Belgium, the Netherlands, Italy, Japan, and two groups of private companies and agencies from France and Japan. There were also three publicly sponsored entities from the Soviet Union, India and China.[citation needed]

In the late 1970s, two of the international joint ventures collected several hundred-ton quantities of manganese nodules from the abyssal plains (18,000 feet (5.5 km) + depth) of the eastern equatorial Pacific Ocean.[11] Significant quantities of nickel (the primary target) as well as copper and cobalt were subsequently extracted from this "ore" using both pyrometallurgical and hydrometallurgical methods. In the course of these projects, a number of ancillary developments evolved, including the use of near-bottom towed side-scan sonar array to assay the nodule population density on the abyssal silt while simultaneously performing a sub-bottom profile with a derived, vertically oriented, low-frequency acoustic beam.[citation needed] Since then, deep sea technology has improved significantly: including widespread and low cost use of navigation technology such as Global Positioning System (GPS) and ultra-short baseline (USBL); survey technology such as multibeam echosounder (MBES) and autonomous underwater vehicles (AUV); and intervention technology including remotely operated underwater vehicle (ROV) and high power umbilical cables. There is also improved technology that could be used in mining including pumps, tracked and screw drive rovers, rigid and flexible drilling risers, and ultra-high-molecular-weight polyethylene rope. Mining is considered to be similar to the potato harvest on land, which involves mining a field partitioned into long, narrow strips. The mining support vessel follows the mining route of the seafloor mining tools, picking up the about potato-sized nodules from the seafloor.[20][21][22]

In recent times[when?], nickel and other metal supply has needed to turn to higher cost deposits in order to meet increased demand, and commercial interest in nodules has revived. 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.[23]

Since 2011, a number of commercial companies have received exploration contracts. These include subsidiaries of larger companies like Lockheed Martin, DEME (Global Sea Mineral Resources, GSR), Keppel Corporation and China Minmetals, and smaller companies like Nauru Ocean Resources and Tonga Offshore Mining.[11]

In July 2021, Nauru announced a plan to exploit nodules in this area, which requires the International Seabed Authority, who 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.[24]

Ecology[edit]

Very little is known about deep sea ecosystems or the potential impacts of deep-sea mining. Polymetallic nodule fields are hotspots of abundance and diversity for a highly vulnerable abyssal fauna, much of which lives attached to nodules or in the sediment immediately beneath it.[25][24]

Nodule mining could affect tens of thousands of square kilometers of these deep sea ecosystems, and ecosystems take millions of years to recover.[24] It causes habitat alteration, direct mortality of benthic creatures, or smothering of filter feeders by sediment.[26] 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.[27]

See also[edit]

References[edit]

  1. ^ Mero, John (1965). The mineral resources of the sea. Elsevier Oceanography Series.
  2. ^ a b c d International Seabed Authority (2010). A Geological Model of Polymetallic Nodule Deposits in the Clarion-Clipperton Fracture Zone and Prospector's Guide for Polymetallic Nodule Deposits in the Clarion Clipperton Fracture Zone. Technical Study: No. 6. ISBN 978-976-95268-2-2.
  3. ^ Hlawatsch, S.; Neumann, T.; van den Berg, C.M.G.; Kersten, M.; Hari, J.; Suess, E. (2002). "Fast-growing, shallow-water ferro-manganese nodules from the western Baltic Sea: origin and modes of trace element incorporation". Marine Geology. 182 (3–4): 373–387. Bibcode:2002MGeol.182..373H. doi:10.1016/s0025-3227(01)00244-4.
  4. ^ Callender, E.; Bowser, C. (1976). "Freshwater Ferromanganese Deposits". Au, U, Fe, Mn, Hg, Sb, W, and P Deposits. Vol. 7. Elsevier Scientific Publishing Community. pp. 341–394. ISBN 9780444599438.
  5. ^ Fike, D.A.; Grotzinger, J.P.; Pratt, L.M.; Summons, R.E. (2006). "Oxidation of the Ediacaran Ocea". Nature. 444 (7120): 744–747. Bibcode:2006Natur.444..744F. doi:10.1038/nature05345. PMID 17151665. S2CID 4337003.
  6. ^ Murray, J.; Renard, A.F. (1891). Report on Deep-Sea Deposits; Scientific Results Challenger Expedition.
  7. ^ Hein, James; Spinardi, Francesca; Okamoto, Nobuyuki; Mizell, Kira; Thorburn, Darryl; Tawake, Akuila (2015). "Critical metals in manganese nodules from the Cook Islands EEZ, abundances and distributions". Ore Geology Reviews. 68: 97–116. doi:10.1016/j.oregeorev.2014.12.011.
  8. ^ Von Stackelberg, U (1997). "Growth history of manganese nodules and crusts of the Peru Basin". Geological Society, London, Special Publications. 119 (1): 153–176. Bibcode:1997GSLSP.119..153V. doi:10.1144/GSL.SP.1997.119.01.11. S2CID 219189224.
  9. ^ Mukhopadhyay, R.; Ghosh, A.K.; Iyer, S.D. (2007). The Indian Ocean Nodule Field Geology and Resource Potential: Handbook of Exploration and Environmental Geochemistry 10. Elsevier Science.
  10. ^ García, Marcelo; Correa, Jorge; Maksaev, Víctor; Townley, Brian (2020). "Potential mineral resources of the Chilean offshore: an overview". Andean Geology. 47 (1): 1–13. doi:10.5027/andgeoV47n1-3260.
  11. ^ a b c Lipton, Ian; Nimmo, Matthew; Parianos, John (2016). NI 43-101 Technical Report TOML Clarion Clipperton Zone Project, Pacific Ocean. AMC Consultants.
  12. ^ Kobayashi, Takayuki (October 2000). "Concentration profiles of 10Be in large manganese crusts". Nuclear Instruments and Methods in Physics Research Section B. 172 (1–4): 579–582. Bibcode:2000NIMPB.172..579K. doi:10.1016/S0168-583X(00)00206-8.
  13. ^ Blöthe, Marco; Wegorzewski, Anna; Müller, Cornelia; Simon, Frank; Kuhn, Thomas; Schippers, Axel (2015). "Manganese-Cycling Microbial Communities Inside Deep-Sea Manganese Nodules". Environ. Sci. Technol. 49 (13): 7692–7700. Bibcode:2015EnST...49.7692B. doi:10.1021/es504930v. PMID 26020127.
  14. ^ a b Novikov, C.V.; Murdmaa, I.O. (2007). "Ion exchange properties of oceanic ferromanganese nodules and enclosing pelagic sediments". Lithology and Mineral Resources. 42 (2): 137–167. doi:10.1134/S0024490207020034. S2CID 95097062.
  15. ^ Appelo, C.A.J.; Postma, D. (1999). "A consistent model for surface complexation on birnessite (δ−MnO2) and its application to a column experiment". Geochimica et Cosmochimica Acta. 63 (19–20): 3039–3048. Bibcode:1999GeCoA..63.3039A. doi:10.1016/S0016-7037(99)00231-8. ISSN 0016-7037.
  16. ^ Dzombak, David A.; Morel, François M. M. (1990). Surface Complexation Modeling: Hydrous Ferric Oxide. John Wiley & Sons. ISBN 978-0-471-63731-8.
  17. ^ Newton, Aric G.; Kwon, Kideok D. (2018). "Molecular simulations of hydrated phyllomanganates". Geochimica et Cosmochimica Acta. 235: 208–223. Bibcode:2018GeCoA.235..208N. doi:10.1016/j.gca.2018.05.021. ISSN 0016-7037. S2CID 104263989.
  18. ^ Wegorzewski, A.V.; Kuhn, T. (2014). "The influence of suboxic diagenesis on the formation of manganese nodules in the Clarion Clipperton nodule belt of the Pacific Ocean". Marine Geology. 357: 123–138. Bibcode:2014MGeol.357..123W. doi:10.1016/j.margeo.2014.07.004.
  19. ^ Broecker, Wallace (1974). Chemical Oceanography (PDF). Harcourt Brace Jovanovich, Inc. p. 89. Retrieved 22 January 2023.
  20. ^ Volkmann, Sebastian Ernst; Lehnen, Felix (21 April 2017). "Production key figures for planning the mining of manganese nodules". Marine Georesources & Geotechnology. 36 (3): 360–375. doi:10.1080/1064119X.2017.1319448. S2CID 59417262.
  21. ^ Volkmann, Sebastian Ernst; Kuhn, Thomas; Lehnen, Felix (2018-02-21). "A comprehensive approach for a techno-economic assessment of nodule mining in the deep sea". Mineral Economics. 31 (3): 319–336. doi:10.1007/s13563-018-0143-1. ISSN 2191-2203. S2CID 134526684.
  22. ^ Volkmann, Sebastian Ernst (2018). Blue mining - planning the mining of seafloor manganese nodules (Thesis). Vol. RWTH Aachen University. Aachen. doi:10.18154/rwth-2018-230772.
  23. ^ "Deep Seabed Mineral Resources".
  24. ^ a b c "'Deep-sea gold rush' for rare metals could cause irreversible harm". The Guardian. 29 April 2022.
  25. ^ University of Ghent press bulletin, June 7, 2016 Archived June 14, 2016, at the Wayback Machine
  26. ^ Glover, A. G.; Smith, C. R. (2003). "The deep-sea floor ecosystem: current status and prospects of anthropogenic change by the year 2025". Environmental Conservation. 30 (3): 21–241. doi:10.1017/S0376892903000225. S2CID 53666031.
  27. ^ Ozturgut, E.; Trueblood, D. D.; Lawless, J. (1997). An overview of the United States's Benthic Impact Experiment. Proceedings of the International Symposium on Environmental Studies for Deep-Sea Mining. Metal Mining Agency of Japan.

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