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The oceanic bottom mixed layer (BML) is a well mixed, weakly stratified, turbulent boundary layer. Adjacent to the seabed, the BML is of intrinsic importance for studying ocean mixing, energy dissipation, particle cycling and sediment-water interactions. While deep-seabed mining of polymetallic nodules is anticipated to commence in the Clarion-Clipperton Zone (CCZ) of the northeastern tropical Pacific Ocean, knowledge gaps regarding the form of the BML and its potentially key influence on the dispersal of sediment plumes generated by deep-seabed mining activities are yet to be addressed. Here, we report recent field observations from the German mining licence area in the CCZ that characterise the structure and variability of the BML locally. Quasi-uniform profiles of potential temperature extending from the seafloor reveal the presence of a spatially and temporally variable BML with an average local thickness of approximately 250 m. Deep horizontal currents in the region have a mean speed of 3.5 cm s - 1 and a maximum speed of 12 cm s - 1 at 18.63 ms above bottom over an 11 month record. The near-bottom currents initially have a net southeastward flow, followed by westward and southward flows with the development of complex, anticyclonic flow patterns. Theoretical predictions and historical data show broad consistency with mean BML thickness but cannot explain the observed heterogeneity of local BML thickness. We postulate that deep pressure anomalies induced by passing surface mesoscale eddies and abyssal thermal fronts could affect BML thickness, in addition to local topographic effects. A simplified transport model is then used to study the influence of the BML on the interplay between turbulent diffusion and sediment settling in the transport of deep-seabed mining induced sediment plumes. Over a range of realistic parameter values, the effects of BML on plume evolution can vary significantly, highlighting that resolving the BML will be a crucial step for accurate numerical modelling of plume dispersal. Article Highlights: Field observations detail the presence of bottom mixed layer in the abyss of the Clarion-Clipperton Zone in the northeastern tropical Pacific Mean local thickness of the layer is approximately 250 m, with large spatial and temporal heterogeneity that needs further understanding The transport of benthic sediment plumes generated by deep-seabed mining can be greatly influenced by bottom mixed layer variability [ABSTRACT FROM AUTHOR]

Results are presented from highly resolved three-dimensional simulations of turbidity currents down a shallow slope into a stratified saline ambient, for various stratifications and particle settling velocity configurations. The front velocity is computed and the results are compared directly to experimental data by Snow and Sutherland (J Fluid Mech 755:251–273, 2014) and to an existing scaling law for gravity currents in stratified environments but on flat bottoms, with close agreement. The entrainment velocity of the ambient fluid into the current is computed from the DNS data and shows strong space time variability. The intrusion of the current into the ambient fluid is computed and compared to the experimental results of Snow and Sutherland (2014), an existing scaling law proposed by those authors, and a new model with increasing levels of complexity. The numerical results highlight the sensitivity of scaling tools to the choice of entrainment coefficient and their limitations in low entrainment but high settling rate scenarios. The new, simpler scaling relation for the intrusion depth is shown to be more robust and predictive in such cases. The energy budget of the flow is analysed in order to explain the governing processes in terms of energy transfers, with a focus on energy losses and potential to kinetic energy transfers. Particular attention is given to the Stokes losses with varying settling velocities, which validates the prediction of the concentration in the intrusion depth model introduced. [ABSTRACT FROM AUTHOR]

We employ direct numerical simulations of the three-dimensional Navier-Stokes equations, based on a continuum formulation for the sediment concentration, to investigate the physics of turbidity currents in complex situations, such as when they interact with seafloor topography, submarine engineering infrastructure and stratified ambients. In order to obtain a more accurate representation of the dynamics of erosion and resuspension, we have furthermore developed a grain-resolving simulation approach for representing the flow in the high-concentration region near and within the sediment bed. In these simulations, the Navier-Stokes flow around each particle and within the pore spaces of the sediment bed is resolved by means of an immersed boundary method, with the particle-particle interactions being taken into account via a detailed collision model. [ABSTRACT FROM AUTHOR]