Plume dispersion of low-density clayey suspension turbidity currents created by deep-sea mining

Abstract

Due to the increased demand for materials like cobalt and nickel, there is an interest to mine polymetallic nodules from the deep-sea. These nodules are abundantly distributed along the abyssal plains, e.g., the Clarion Clipperton Zone (CCZ) in the North East Pacific. These nodules lay spread on top of a seabed consisting of very fine clayey sediment and will be collected by a seafloor mining tool (SMT). During the operation, the seabed will be disturbed, resulting in a suspended sediment plume discharged by the SMT. This plume can have a significant environmental impact through a blanketing effect on the abyssal fauna and create a disturbance in the water column affecting the entire food web structure. Hence, identifying the critical processes and quantification of sediment plume dispersion is essential to predict the potential environmental impact better and determine what technologies would enable a lower ecological impact. Spearman et al. (2020) investigated turbidity plumes generated by deep-sea mining experiments and discovered faster settling velocity than the theory described. They hypothesize that this is due to flocculation. The settling speed depends on the density, concentration, shape, and cohesive properties of the sediment. Flocculation in the deep-sea can occur in two ways, by salinity or by organic matter. Gillard et al. (2019) showed that the flocculation response of CCZ sediment strongly depends on the concentration and applied shear rate. To analyze to what extent aggregation could influence the plume dispersion, experiments are conducted in which the effect of aggregation can be adjusted selectively. Lock exchange experiments are used to analyze the impact of aggregation by comparing results with illite based on freshwater, saltwater, pre-existing bed, and added flocculant. Additional experiments with artificial CCZ sediment are done to ensure that the illite experiments are not too idealized. To prove that the settling velocity is increasing in saltwater, settling velocity tests have been performed. I performed experiments with illite suspensions in a settling column. An increase in settling velocity is observed due to the rise in salinity up to 75 g/L of illite. Lock exchange experiments are performed to mimic the particle-driven currents. The lock experiments involve a lock release of a fixed volume suspension of sediment from the mixing section into the outflow section. Similar observations were made by doing lock exchange experiments. A decrease in average head velocity is shown for all experiments up to 75 g/L of illite in saltwater. To further induce flocculation and mimic organic matter in the water column, experiments with flocculant were done. Mixing the flocculant, Zetag 4120 or Zetag 8125, in the mixing section will further decrease average head velocity compared to saltwater. Zetag 4120 even ensures very fast settling as the end of the tank will not be reached For low concentrations in saltwater, flocculation is shown as the tail decreases more rapidly than freshwater. Flocculation with higher mass concentrations of illite is shown when flocculant is added to the mixing section. Adding Zetag 4120 to saltwater makes the current even settle quicker. As the SMT will move over a pre-existing bed, experiments were done to see if a pre-existing clay bed has consequences for the behavior of the flow. A clay bed has been made by running a lock exchange experiment with a concentration of 100g/L. This test then had, depending on the next experiment, one day or two days settling time before it was used for the new experiment. One-day bed experiments overall encountered a decrease in average head velocity. Two-day bed experiments overall experienced an increase in average head velocity. To mimic the existing sediment in the CCZ and determine if illite experiments are not too idealized, experiments were done with artificial CCZ sediment. The currents behaved differently compared to illite, as they produced more coherent flows for low concentration. Also, it showed better effects on lowering the ecological impact when looked at mass concentrations of 100g/L of artificial CCZ sediment as gelling occurred. To minimize the environmental impact created by deep-sea mining, the SMT should produce a discharge between 50 and 100 g/L of artificial CCZ sediment. Gelling will occur, which has a positive effect on minimizing the plume dispersion.