(1)National Oceanography Centre
Introduction
It is widely accepted that the use of fossil carbon-based fuels is environmentally unsustainable, causing significant and increasingly adverse effects on climates and ecosystems worldwide. As a result, there is a growing demand for alternative sources of energy, that are either renewable or carbon neutral. The aim, set by many industrialised nations, is to reach zero-net carbon by the second half of the 21st Century [1]. This ambition will require a huge increase in production of base metals like copper, nickel, zinc, as well as scarcer minerals and elements critical to enabling a transition to low-carbon energy generation and transport.
Many of these elements are relatively rare or restricted to very few areas of the globe and include: cobalt and lithium (used in batteries for electric vehicles); tellurium (used in cadmium- telluride thin-film, photovoltaic electrical energy generation); neodymium and dysprosium (used for permanent magnets in wind-turbines and electric motors); heavy rare earth elements (used in electronics), and platinum group elements (used in fuel cells and hydrogen catalysers).
For example, and by way of illustration of the magnitude of the demand, we can compare the mineral resources needed to electrify the world’s estimated two billion private cars with today’s worldwide land-based mining production rates for some of these critical and base metals [2].
The best-case scenario, using the latest in battery technologies for electric vehicles, requires 126 years of production for cobalt, 62 years’ of global production for neodymium, 45 years’ of global production for lithium, and 31 years’ of global production for copper. Let us now assume these electric vehicles are powered by zero-carbon renewable energy. If the electricity is generated by wind turbines, then they would require 20 years’ worth of neodymium and dysprosium global production in their permanent magnets [3] or, if using CdTe-type photovoltaic solar energy [4], they would require 2000 years’ of the current global production of tellurium.
Whether this projection proves accurate or not, it is increasingly clear that there is a potential supply crisis for a range of critical elements, currently sourced from land-based mines, to reach any meaningful target for net-zero carbon emissions this century. However, the deep-seafloor hosts billions of tonnes of metalliferous mineral deposits, rich in these critical elements, that may hold part of the solution.
Types of deep-sea minerals of interest
There are three main types of deep-sea mineral deposit: polymetallic nodules, cobalt-rich crusts, and polymetallic massive sulphides.
Polymetallic nodules:
Polymetallic nodules are iron-manganese oxide concretions, rich in nickel, copper, titanium, cobalt, and heavy rare earth elements (Figure 2). They are variable in shape and size, typically 1–12 cm in maximum dimension, and are most abundant on the ocean’s abyssal plains at water depths of 4.000–6.500 m, where they lie on or immediately below the sediment-covered seafloor. Here, Fe and Mn oxide colloids slowly precipitate over millions of years around a hard nucleus, such as a shark’s tooth or fish bone, from the overlying seawater (hydrogenetic nodules) and/or pore waters released from the underlying sediment (diagenetic nodules).
The best-known deposits are from the Clarion-Clipperton Zone (CCZ), between the west coast of Mexico and Hawaii, where there are an estimated 20-30 billion tonnes of nodules. The Peru Basin in the south-eastern Pacific has about of ~6-9 billion tonnes, while the Penrhyn Basin, also in the Pacific and located near the Cook Islands, has an estimated 5-7 billion tonnes of nodules with a resource potential of ~21 million tonnes of cobalt (i.e., 210 years’ worth of global land-based production).
In the Indian ocean, nodule fields contain lower average abundances ~5 kg/m2. Worldwide, polymetallic nodules present an enormous source of metals, with 10 times more manganese, 6 times more cobalt, 4 times more tellurium and yttrium, 3.5 times more nickel, and a third more copper compared with land-based reserves [5].
Cobalt-rich crusts:
Cobalt-rich crusts are another type of iron-manganese oxide-oxyhydroxides deposit. They commonly form layers, up to 25 cm thick (Figure 1), on rocky substrates such as seamounts and underwater ridges (Figure 2). They are rich in cobalt (0.5-1.2%) and other critical elements such as tellurium, platinum, rare earths, titanium, thallium, and nickel [6]. These crusts occur through- out the global ocean, at all depths below 700 m, but are thickest on the oldest areas such as the Prime Crust Zone of the northwest Pacific. They contain more than half the nickel, a fifth of the titanium, over 3 times the yttrium (and other rare earth elements), 7 times the cobalt, 23 times the tellurium, and 3000 times the thallium compared with all known land-based reserves [5].
Polymetallic massive sulphides:
Polymetallic massive sulphides and their metalliferous sediments are the product of intense seafloor volcanic activity and form rapidly from high-temperature hydrothermal fluids. Hydrothermal venting of metal-rich fluids (known as black-smokers) is associated with volcanic activity, typically at the boundaries of tectonic plates such as mid-ocean ridges, and occurs in all the oceans at depths down to 5,000 m (Beaulieu et al. 2013). This phenomenon is one of the most spectacular examples of geology in action where we can witness the release of metal-saturated fluids from the ocean floor at temperatures of up to 450°C where they instantly mix with cold ambient seawater, at 2-4°C , resulting in the formation of spectacular chimneys and mounds of sulphide typically 30-50m high and 100-200 m in diameter (Figure 5).
Previous estimates suggest about 600 million tonnes of accessible seafloor massive sulphide [8], however, hydrothermally inactive deposits may be 10 to 20 times more abundant than active ones with between 3 and 5 times more sulphide under the seafloor than above it [9], hosting 20 to 30 billion tonnes of ore worldwide. One of the largest deposits known is found on the Mid-Atlantic Ridge, at the Semenov Field at 13:30’N, where it forms mounds of sulphide up to 200 m high and 500 m in diameter (Figure 4).
Geophysical studies and drilling show these hydrothermal systems form extensive deposits both on and below the seafloor, down to depths of a few hundred metres. They are primarily rich in iron (up to 32%), zinc (up to 17%), copper (up to 13%), gold (up to 13 ppm), silver (up to 2000 ppm) and have elevated concentration of cadmium, gallium, germanium, antimony, tellurium, thallium, and indium [7].
Challenges
Without deep-sea minerals there is unlikely to be sufficient supply of critical raw material to enable a net zero-carbon economy this century. However, there are three major challenges facing the nascent deep-sea mining industry. First is the uncertainty around the regulations that the United Nations International Seabed Authority is drafting. Second, there remains considerable technological challenge to deep-sea mineral exploration and mining. Thirdly, protecting the environment is key to gaining social license, for which much needs to be done to identify potential harm and how to mitigate it.
Much work is required to enable the technologies for environmentally sustainable extraction as well as environmental monitoring. In response to the urgent need for environmental monitoring, a new research project has been initiated by Europe horizons. Involving researchers from across Europe, collaborators across the world and led by Portugal’s Institute for Technology, INESC TEC. Project Trident is developing the technological knowhow to enable in situ monitoring and forecasting of potential impacts from seafloor activities including deep-sea mining. Such monitoring is essential if there is to be transparency in the work of the operators to allow the public and regulators alike to check on what is being done to the seafloor.
References
[1] The Paris Agreement. https://unfccc.int/process-and-meetings/the-paris-agreement.
[2] McKinsey & Co Metals and Mining. June 2018 https://www.mckinsey.com/~/media/mckinsey/industries/met alt%20a%20tale%20of%20two%20commodities/lithium-and- cobalt-a-tale-of-two-commodities.ashx
[3] Ayman Elshkaki and T.E. Graedel. Dysprosium, the balance problem, and wind power technology. Applied Energy, 136, 548-559, 2014.
[4] Sarah M. Hayes, Erin A. McCullough, Critical minerals: A review of elemental trends in comprehensive criticality studies. Resources Policy, 59, 192-199, 2018.
[5] WOR 3 Marine Resources – Opportunities and Risks. World Ocean Review, 2014.
[6] Paul A. J. Lusty, James R. Hein, and Pierre Josso. Formation and Occurrence of Ferromanganese Crusts: Earth’s Storehouse for Critical Metals, Elements, 14, 313-318, 2018.
[7] Sven Petersen, Berit Lehrmann, and Bramley J. Murton, Modern Seafloor Hydrothermal Systems: New Perspectives on Ancient Ore-Forming Processes. Elements, 14, 307-312, 2018
[8] Hannington, M.D., Jamieson, J., Monecke, T., Petersen, S., Beaulieu, S., 2011. The abundance of seafloor massive sulfide deposits. Geology 39, 1155–1158.
[9] Murton, Bramley J.; Lehrmann, Berit; Dutrieux, Adeline M.; Martins, Sofia; de la Iglesia, Alba Gil; Stobbs, Iain J.; Barriga, Fernando J.A.S.; Bialas, Jörg; Dannowski, Anke; Vardy, Mark E.; North, Laurence J.; Yeo, Isobel A.L.M.; Lusty, Paul A.J., Petersen, Sven. Geological fate of seafloor massive sulphides at the TAG hydrothermal field (Mid-Atlantic Ridge). Ore Geology Reviews, 107. 903-925. https://doi.org/10.1016/j.oregeorev.2019.03.005, 2019