The abyssal ocean possesses vast reserves of minerals such as nickel and cobalt, many times those of all global land-based reserves, in the form of polymetallic nodules. Trillions of these nodules, which took millions of years to accrete, are openly strewn across the abyssal plains. In recent years there has been a substantial increase in activities on the topic of deep-sea mining, with 18 exploration licenses granted in the Clarion Clipperton Fracture Zone (CCFZ), an extensive area of the Pacific Ocean between Hawaii and Mexico. Driving forces behind the ramp-up in activity include an increase in demand for electric vehicles and increasing urbanization. But many have expressed concerns about the potential impact of deep-sea mining activities on the abyssal ocean environment. This article provides an overview of the topic of deep-sea mining, detailing the current state-of-knowledge and consider the outlook in the coming years.
News
- Scientists measure plume stirred up by deep-sea-mining vehicle
Results of a recent field study, published as the cover article in Science Advances, reveals that a fundamental fluid dynamic process, turbidity current dynamics, is a key component for setting the scale of impact of proposed deep-sea mining operations….
Publications
- In situ optical measurement of particles in sediment plumes generated by a pre-prototype polymetallic nodule collector
El Mousadik, S., Ouillon, R., Muñoz-Royo, C., Slade, W., Pottsmith, C., Leeuw, T., Alford, M.H., Mikkelsen O.A., & Peacock, T
Scientific Reports, 14, 23894 (2024)
This study presents in situ, high-resolution optical measurements of particle size distributions (PSD) within sediment plumes generated by a pre-prototype deep seabed nodule collector vehicle operating in the abyssal Pacific Ocean. These measurements were obtained using a cutting-edge instrument, the LISST-RTSSV sensor. The data collected in situ reveal marked differences compared to previously reported laboratory-based, ex situ measurements. The grain size and other key particle shape characteristics are found to be dependent on multiple factors, including the collector vehicle maneuvers, the time elapsed following sediment discharge, and the complex hydrodynamic processes that generate the sediment in suspension. Significantly, the PSD from a highly complex succession of straight-line maneuvers converges to that of the canonical case of a simple straight-line driving maneuver within a timescale of ten minutes. Our results underscore the importance of parameterizing sediment plume transport models based on well-informed, comprehensive PSDs of detrained suspended sediment measured in situ at adequate timescales and in regions no longer strongly influenced by active and complex hydrodynamic processes.
- The fluid mechanics of deep-sea mining
Peacock, T. & Ouillon, R.
Annual Review of Fluid Mechanics, 55, 403-430 (2023)
Fluid mechanics lies at the heart of many of the physical processes associated with the nascent deep-sea mining industry. The evolution and fate of sediment plumes that would be produced by seabed mining activities, which are central to the assessment of the environmental impact, are entirely determined by transport processes. These processes, which include advection, turbulent mixing, buoyancy, differential particle settling, and flocculation, operate at a multitude of spatiotemporal scales. A combination of historical and recent efforts that combine theory, numerical modeling, laboratory experiments, and field trials has yielded significant progress, including assessing the role of environmental and operational parameters in setting the extent of sediment plumes, but more fundamental and applied fluid mechanics research is needed before models can accurately predict commercial-scale scenarios. Furthermore, fluid mechanics underpins the design and operation of proposed mining technologies, for which there are currently no established best practices.
- Advection-diffusion-settling of deep-sea mining sediment plumes. Part 1: Midwater plumes
Ouillon, R., …, & Peacock, T.
FLOW, 2, E22 (2022)
The evolution of midwater sediment plumes associated with deep-sea mining activities is investigated in the passive-transport phase using a simplified advection–diffusion-settling model. Key metrics that characterize the extent of plumes are defined based on a concentration threshold. Namely, we consider the volume flux of fluid that ever exceeds a concentration threshold, the furthest distance from and maximum depth below the intrusion where the plume exceeds the threshold, and the instantaneous volume of fluid in excess of the threshold. Formulas are derived for the metrics that provide insight into the parameters that most strongly affect the extent of the plume. The model is applied to a reference deep-sea mining scenario around which key parameters are varied. The results provide some sense of scale for deep-sea mining midwater plumes, but more significantly demonstrate the importance of the parameters that influence the evolution of midwater plumes. The model shows that the discharge mass flow rate and the concentration threshold play an equal and opposite role on setting the extent of the plume. Ambient ocean turbulence and the settling velocity distribution of particles play a lesser yet significant role on setting the extent, and can influence different metrics in opposing ways.
- Advection-diffusion-settling of deep-sea mining sediment plumes. Part 2: Collector plumes
Ouillon, R., …, & Peacock, T.
FLOW, 2, E23 (2022)
We develop and investigate an advection–diffusion-settling model of deep-sea mining collector plumes, building on the analysis of midwater plumes in Part 1. In the case of collector plumes, deposition plays a predominant role in controlling the mass of sediment in suspension, and thus on setting the extent of the plume. We first discuss the competition between settling, which leads to deposition, and vertical turbulent diffusion, which stretches the plume vertically and reduces deposition. The time evolution of the concentration at the seabed is found to be a highly nonlinear function of time that depends non-trivially on the ratio of diffusion to settling time scales. This has direct implications for the three extent metrics considered, namely the instantaneous area of the seabed where a deposition rate threshold is exceeded, the furthest distance from the discharge where the plume exceeds a concentration threshold and the volume flux of fluid in the water column that ever exceeds a concentration threshold. Unlike the midwater plume, the particle velocity distribution of the sediment has the greatest influence on the extent metrics. The turbulence levels experienced by the plume also markedly affects its extent. Expected variability of turbulence and particle settling velocity yields orders of magnitude changes in the extent metrics.
- Extent of impact of deep-sea nodule mining midwater plumes is influenced by sediment loading, turbulence and thresholds
Munoz-Royo, C., Peacock, T., et al.
Nature Communications Earth & Environment, 2, 148 (2021)
Deep-sea polymetallic nodule mining research activity has substantially increased in recent years, but the expected level of environmental impact is still being established. One environmental concern is the discharge of a sediment plume into the midwater column. We performed a dedicated field study using sediment from the Clarion Clipperton Fracture Zone. The plume was monitored and tracked using both established and novel instrumentation, including acoustic and turbulence measurements. Our field studies reveal that modeling can reliably predict the properties of a midwater plume in the vicinity of the discharge and that sediment aggregation effects are not significant. The plume model is used to drive a numerical simulation of a commercial-scale operation in the Clarion Clipperton Fracture Zone. Key takeaways are that the scale of impact of the plume is notably influenced by the values of environmentally acceptable threshold levels, the quantity of discharged sediment, and the turbulent diffusivity in the Clarion Clipperton Fracture Zone.