Drazen et al. - PNAS July 28, 2020 117 (30) 17455-17460

Figure: Midwater animal biodiversity: Squid, fish, shrimp, copepods, medusa, filter-feeding jellies, and marine worms are among the midwater creatures that could be affected by deep sea mining. Photos by E. Goetze, K. Peijnenburg, D. Perrine, Hawaii Seafood Council (B. Takenaka, J. Kaneko), S. Haddock, J. Drazen, B. Robison, DEEPEND (Danté Fenolio), and MBARI.

Despite rapidly growing interest in deep-sea mineral exploitation, environmental research and management have focused on impacts to seafloor environments, paying little attention to pelagic ecosystems. Nonetheless, research indicates that seafloor mining will generate sediment plumes and noise at the seabed and in the water column that may have extensive ecological effects in deep midwaters (1), which can extend from an approximate depth of 200 meters to 5 kilometers. Deep midwater ecosystems represent more than 90% of the biosphere (2), contain fish biomass 100 times greater than the global annual fish catch (3), connect shallow and deep-sea ecosystems, and play key roles in carbon export (4), nutrient regeneration, and provisioning of harvestable fish stocks (5). These ecosystem services, as well as biodiversity, could be negatively affected by mining. Here we argue that deep-sea mining poses significant risks to midwater ecosystems and suggest how these risks could be evaluated more comprehensively to enable environmental resource managers and society at large to decide whether and how deep-sea mining should proceed.

Interest in deep-sea mining for sulfide deposits near hydrothermal vents, polymetallic nodules on the abyssal seafloor, and cobalt-rich crusts on seamounts (6) has grown substantially in the last decade. Equipment and system development are already occurring. The International Seabed Authority (ISA), the international organization created under the United Nations Convention on the Law of the Sea (UNCLOS) to manage deep-sea mining beyond national jurisdiction, is developing mineral exploitation regulations, the Mining Code. Currently, 30 ISA exploration contracts cover over 1.5 million square kilometers. In accordance with UNCLOS, the ISA is required to ensure the effective protection of the marine environment, including deep midwater ecosystems, from harmful effects arising from mining-related activities.

Mining strategies will vary significantly between mineral resource types and perhaps between contractors, but they will involve some combination of seafloor collector vehicle with a vertical transport system to carry the ore and sediments to a surface vessel, shipboard separation of ore-bearing materials (dewatering), and subsequent discharge of sediments and water either back into the water column or at the seabed (ref. 7; Fig. 1). Seafloor vehicles will generate noise near the seabed, particularly from grinding hard sulfides or crusts and through hydraulic pumping and rattling of ore in lift pipes throughout the water column. The vehicles will also resuspend seafloor sediments, creating environmentally detrimental plumes that may disperse for tens to hundreds of kilometers from the mining site (8⇓⇓–11), depending greatly on the nature of the deposit, local currents, and the mining technology used.

Of particular importance for the water column is the discharge of the tailings from dewatering of the ore, which will introduce sediment and dissolved metals over potentially large areas. A single polymetallic-nodule mining operation is estimated to discharge 50,000 meters-cubed of sediment, broken mineral fines, and seawater per day (∼8 kilograms per meter-cubed solids) and a hydrothermal vent operation could discharge 22,000 to 38,000 meters-cubed per day (10, 12). These discharges could run continuously for up to 30 years, producing 500,000,000 meters-cubed of discharge over the lifetime of one operation.

Very fine clay sediments could stay in suspension for several years, and along with dissolved metals they could be carried by ocean currents for hundreds of kilometers (11), dispersing far beyond the mining zone in concentrations that are still to be determined. There is currently no regulation or guidance on the depth or manner in which tailings can be discharged into the environment. Given the risk of ecological harm, the need to consider the potential adverse effects from seabed mining to midwater ecosystems and services, and our state of knowledge in evaluating these risks, is critical.

See more: https://www.pnas.org/content/117/30/17455