Copper-binding ligands in deep-sea pore waters of the Pacific Ocean and potential impacts of polymetallic nodule mining on the copper cycle

Abstract

The release of potentially toxic metals, such as copper (Cu), into the water column is of concern during polymetallic nodule mining. The bioavailability and thus toxicity of Cu is strongly influenced by its speciation which is dominated by organic ligand (L) complexation in seawater, with L-complexes being considered less bioavailable than free Cu2+. The presence of CuL-complexes in deep-sea sediments has, however, not been systematically studied in the context of deep-sea mining. We thus analyzed the Cu-binding L concentration ([L]) in deep-sea pore waters of two polymetallic nodule provinces in the Pacific Ocean, the Peru Basin and the Clarion-Clipperton-Zone, using competitive ligand equilibration–adsorptive stripping voltammetry. The pore-water dissolved Cu concentration ([dCu]) ranged from 3 to 96 nM, generally exceeding bottom water concentrations (4–44 nM). Based on fitting results from ProMCC and Excel, Cu was predominantly complexed by L (3–313 nM) in bottom waters and undisturbed pore waters. We conclude that processes like deep-sea mining are unlikely to cause a release of toxic Cu2+ concentrations ([Cu2+]) to the seawater as > 99% Cu was organically complexed in pore waters and the [Cu2+] was < 6 pM for 8 of 9 samples. Moreover, the excess of L found especially in shallow pore waters implied that even with a Cu release through mining activities, Cu2+ likely remains beneath toxic thresholds.

Metal Release from Manganese Nodules in Anoxic Seawater and Implications for Deep-Sea Mining Dewatering Operations

“…For context, 18 nM of dCu is higher than typical concentrations of Cu in the open ocean (maximum ∼4−5 nM near the bottom) 79 but falls on the lower end of dCu concentrations in pelagic sediment pore waters in the Pacific. 84,86 Two aspects of added Cu from the waste discharge at mesopelagic depths, beyond complexation by excess organic ligands, that could supplement the findings from our study are (1) The toxicity threshold of Cu for mesopelagic ecosystems (e.g., zooplankton and fishes) is unknown. Such thresholds vary between different plankton communities, 80,87 and better baseline studies of Cu ecotoxicology are needed for mesopelagic communities.…”

“…The possible enrichment of dCu in the waste plume may be a concern given its strong toxicity to some phytoplankton. − Cu is strongly complexed by organic ligands in the ocean, and the toxicity of Cu mostly manifests in its inorganic form. Paul et al detected high concentrations of excess Cu-binding ligands (up to ∼200 nM) in bottom waters and deep-sea pore waters of CCZ sediments, and they argued that excess ligands could chelate some of the Cu through organic complexation and reduce its toxicity to benthic fauna during future mining operations. Concentrations of excess organic Cu-binding ligands within the ODZ are up to 2 nM in the Eastern Tropical South Pacific, which can only bind part of the Cu 2+ we project to be released from the waste plume at mesopelagic depths.…”

Heavy Metals Concentration in the Water of Human-Made Objects

“…The use of algicides to regulate biomass and algae population causes the excess in Cu concentration in the water of the studied ponds (Dudnik et al, 2013). Besides, Cu concentration in the ponds changes as a result of accumulation in sediments as a component of complexes with organic substances and deposition of suspended solids with adsorbed copper forms (Linnik et al, 2012;Paul et al, 2021). Due to chelates, suspended solids and humic acids, Cu concentration may decrease 1.0-1.5 times (Dragun et al, 2009).…”

Ethical opportunities in deep‐sea collection of polymetallic nodules from the Clarion‐Clipperton Zone

“…Yet, currently, there is no evidence that dissolved metals would be released along with the sediments and fines (Muñoz‐Royo et al, 2021 ). Paul et al ( 2021 ) evaluated the risk of toxicity from dissolved copper released from pore water by deep‐sea mining as negligible; they also called for further research on different size fractions of copper, co‐release of several metals, and variations of pH.…”