Deep-sea mining, dewatering waste, accidental plumes and their potential consequences on trace metal fates in the North Pacific Ocean

9 September 2024

While attention to potential future deep sea mining projects intensifies, very few studies estimated the geochemical and environmental consequences of mining dewatering waste plume and accidental spills from both the riser pipe and the mining platform into the abyssal waters. Xiang and his colleagues (2024, see reference below) conducted laboratory incubation experiments that simulate mining discharge into anoxic waters. The choice of using anoxic waters during the incubation is driven by the fact that oxygen depleted waters (ODZ) are overlying potential mining sites in the North Pacific Ocean. During the ∼100 days for crushed manganese (Mn) nodules to sink through the 600 m-thick ODZ, their results show that cobalt (Co) or copper (Cu) -together with Mn – could be the most enriched trace metal within the waste plume compared to the background seawater concentration. The possible enrichment of dissolved Cu in the waste plume may be a concern given its strong toxicity to some phytoplankton. Its impact on the mesopelagic ecosystem needs to be better evaluated.

**Figure:** Schematic of the trace metal inputs in a potential deep-sea mining dewatering waste plume and accidental spills from both the riser pipe and the mining platform. The section plot is dissolved oxygen concentrations (unit: μmol kg^–1) along 12° N. Dissolved oxygen data are retrieved from the World Ocean Atlas 2018 annual climatology (see reference below) and the gray contours are oxygen concentrations of 30 μmol kg^–1. The most enriched trace metals, in comparison to the background seawater concentrations, that are released from the reductive dissolution of Mn nodules are shown as white, curved arrows.

This paper made the cover of the ACS ES&T Water magazine (see figure below). Congratulations to the authors!

References:

Xiang, Y., Steffen, J. M., Lam, P. J., Gartman, A., Mizell, K., & Fitzsimmons, J. N. (2024). Metal Release from Manganese Nodules in Anoxic Seawater and Implications for Deep-Sea Mining Dewatering Operations. ACS ES&T Water, 4, 2957–2967. Access the paper: 10.1021/acsestwater.4c00166

Boyer, T. P.; García, H. E.; Locarnini, R. A.; Zweng, M. M.; Mishonov, A. V.; Reagan, J. R.; Weathers, K. A.; Baranova, O. K.; Paver, C. R.; Seidov, D.; Smolyar, I. V., World Ocean Atlas 2018. Oxygen. In NOAA National Centers for Environmental Information, 2018.

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