Abstract
The anthropogenic impact of polymetallic nodule harvesting in the Clarion-Clipperton Fracture Zone is expected to strongly affect the benthic ecosystem. To predict the long-term, industrial-scale impact of nodule mining on the deep-sea environment and to improve the reliability of the sediment plume model, information about the specific characteristics of deep-sea particles is needed. Discharge simulations of mining-related fine-grained (median diameter ≈ 20 μm) sediment plumes at concentrations of 35–500 mg L–1 (dry weight) showed a propensity for rapid flocculation within 10 to 135 min, resulting in the formation of large aggregates up to 1100 μm in diameter. The results indicated that the discharge of elevated plume concentrations (500 mg L–1) under an increased shear rate (G ≥ 2.4 s–1) would result in improved efficiency of sediment flocculation. Furthermore, particle transport model results suggested that even under typical deep-sea flow conditions (G ≈ 0.1 s–1), rapid deposition of particles could be expected, which would restrict heavy sediment blanketing (several centimeters) to a smaller fall-out area near the source, unless subsequent flow events resuspended the sediments. Planning for in situ tests of these model projections is underway.
Highlights
Over the past decade, increasing global consumption and rising market prices of metals, together with technological improvements, have driven several countries and industries to prospect the deep-sea environment for mineral resources, including polymetallic nodules (Hein and Koschinsky, 2013)
Polymetallic nodules that are rich in metals and rare earth elements lie on the surface of the abyssal sediment
To provide more insight into the likely behavior of the sediment plume produced by an industrial-scale mining operation, our research aims were to (1) produce a complete set of sedimentological parameters that are crucial for model reliability and accuracy and (2) discuss the optimal discharge release conditions that could result in a lower impact on the environment
Summary
Over the past decade, increasing global consumption and rising market prices of metals, together with technological improvements, have driven several countries and industries to prospect the deep-sea environment for mineral resources, including polymetallic nodules (Hein and Koschinsky, 2013). The potential future use of mining equipment on the ocean floor and the subsequent return of mining products (e.g., sediment, water, and abraded nodule debris) to the benthic boundary layer (BBL) would create an operational and discharge plume of fine particulate material (Oebius et al, 2001). Potential long-range, lateral and vertical dispersion of fine-grained plume particles would have a lower impact but is expected to spread over several hundreds of kilometers from the disturbance location (“far field”). Both processes would affect the deep-sea ecosystem structure and functioning to a certain, presently unknown, extent (Ramirez-Llodra et al, 2011)
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