Deep-ocean polymetallic nodules as a resource for critical materials

Abstract

Deep-ocean polymetallic nodules (also known as manganese nodules) are composed of iron and manganese oxides that accrete around a nucleus on the vast abyssal plains of the global ocean 1-6. Polymetallic nodules vary in diameter from less than one to tens of centi metres and acquire economically interesting quantities of critical metals (metals that are essential to the security and economic wellbeing of a nation) from ocean water and/or sediment pore waters. As a result, the enormous quantities of these nodules-which are conservatively estimated to total 21 billion dry tons in the Clarion-Clipperton Zone 7,8 (CCZ; located in the northeast equatorial Pacific Ocean) alone (Fig. 1)-host considerable tonnages of critical metals that are essential for green energy, vehicles and infrastructure, as well as for new technology and military applications. This reservoir of critical metals has, therefore, triggered interest in new deep-ocean mining operations. However, the potential environmental impacts of such activities are of great concern and have hastened extensive research and evaluation in this area 9-11. Polymetallic nodules were first discovered on the seabed at a depth of approximately 4,300-5,500 m during the 1872-1876 voyage of the HMS Challenger to study the deep ocean 12. A number of nodules, which contained numerous shark teeth and cetacean ear bones, were collected from the western end of the CCZ nodule field 12. Early hypotheses suggested that polymetallic nodules form by alteration of volcanic rocks and grains, especially glass, which are transformed into manganese carbonates and, finally, into oxides that are deposited from solution to form nodules on water-rich sediments 12-14. However, later work-aided in part by the 1957-1958 International Geophysical Year-revealed the general source of the metals (seawater and sediment pore water) and the sorption processes that might be involved in the acquisition of minor metals by nodules 15-20. Economic interest in the deep-ocean polymetallic nodules developed in the 1960s 15,16 , which subsequently led to the formation of consortia in Germany,

Characterization of Fines Produced by Degradation of Polymetallic Nodules from the Clarion–Clipperton Zone

“…The average bulk composition of Type 1 and Type 2 nodules is presented in Table 1, 2. As expected, the high Mn/Fe ratio (≈6.4) and high Ni and Cu contents of Type 1 nodules suggest their dominantly diagenetic origin, whereas the low Mn/Fe ratio (≈1.2) and high Co and Ce contents of Type 2 nodules indicate their dominantly hydrogenetic origin [5,29].…”

Application of Soft Data in Nodule Resource Estimation

“…Hydrogenetic formation means direct precipitation of Fe oxyhydroxides and Mn oxides from oxygen-rich near-bottom seawater (Koschinsky and Hein 2003). Diagenetic precipitation occurs within pore-space of deep-sea sediments because of Mn oxide precipitation from almost oxygen-free (suboxic) pore water upon contact with oxic near-bottom water (Hein et al 2020). Both processes lead to the formation of different layers around a nucleus forming the Mn nodule.…”

Seabed mining could come at a high price for a unique fauna

“…Thousands of seamounts (>1,000 m high) with exposed rocks are interspersed across the seabed, and large areas of abyssal plains are covered with mineral deposits (“polymetallic nodules”; Figure 1a). Metals that constitute these nodules, such as manganese, copper, nickel, lithium and rare earth elements, are of great economic interest (Hein, Koschinsky & Kuhn, 2020). They may become an essential source of raw material for batteries and other technological products as demand continues to surge.…”