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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. [ABSTRACT FROM AUTHOR]

The article highlights the dynamics of sediment plumes near the release from a deep seabed polymetallic nodule mining preprototype collector vehicle was conducted in the Clarion Clipperton Zone in the Pacific Ocean 4500-m deep. It also discusses that that turbidity current dynamics sets the fraction of sediment remaining suspended and the scale of the subsequent ambient sediment plume.

The evolution of midwater sediment plumes associated with deep-sea mining activities is investigated in the passivetransport 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Emerging technologies such as deep-sea mining and geoengineering pose fundamentally new questions regarding the dynamics of gravity currents. Such activities can continuously release dense sediment plumes from moving locations, thereafter propagating as gravity currents. Here, we present the results of idealized numerical simulations of this novel configuration, and investigate the propagation of a gravity current that results from a moving source of buoyancy, as a function of the ratio of source speed to buoyancy velocity. We show that above a certain value of this ratio, the flow enters a supercritical regime in which the source moves more rapidly than the generated current, resulting in a statistically steady state in the reference frame of the moving source. Once in the supercritical regime, the current goes through a second transition beyond which fluid in the head of the current moves approximately in the direction normal to the direction of motion of the source, and the time evolution of the front in the lateral direction is well described by an equivalent constant volume lock-release gravity current. We use our findings to gain insight into the propagation of sediment plumes released by deep-sea mining collector vehicles, and present proof-of-concept tow-tank laboratory experiments of a model deep-sea mining collector discharging dense dyed fluid in its wake. The experiments reveal the formation a wedge-shaped gravity current front which narrows as the ratio of collector-to-buoyancy velocity increases. The time-averaged front position shows good agreement with the results of the numerical model in the supercritical regime. [ABSTRACT FROM AUTHOR]

As the only deep hypersaline, halite‐precipitating lake on Earth today, the Dead Sea is the single modern analog for investigating the mechanisms by which large‐scale and thick salt deposits, known as "salt giants", have accreted in the geological record. We directly measure the hydroclimatic forcing and the physical limnologic processes leading to halite sedimentation, the vertical thermohaline structure, and salt fluxes in the Dead Sea. We demonstrate that changes in these forcing lead to strong seasonal and regional variations in the stratification stability ratio, triggering corresponding spatiotemporal variations in thermohaline staircase formation and diapycnal salt flux, and finally control the thickness of the halite layer deposited. The observed staircase formation is consistent with the mean‐field γ instability, causing layering in double‐diffusive convection. We show that double diffusion and thermohaline staircase formation drive the spatial variability of halite deposition in hypersaline water bodies, underlying and shaping "salt giants" basin architecture. Plain Language Summary: Kilometers‐thick halite sequences in the geological record represent some of the extreme environmental events during Earth's history. The present‐day Dead Sea serves as an analog for halite deposition and the unique limnology/oceanography of those past hypersaline environments. We observe differential downward salt flux in the lake and halite deposition at the deep lakefloor, corresponding to theoretical requirements of water column stability and the evolution of a step‐like thermohaline structure of the water column. These variations are spatiotemporally controlled by regional hydroclimatology, as the salt flux and halite deposition enhanced during the dry summer and farther from the freshwater inflows into the lake, along an increasing salinity gradient. Our findings explain thick halite accretion in global basin depocenters. Key Points: We resolve the mechanisms driving thick salt layers accretion in deep saline waterbodies by direct field measurements in the Dead SeaHydroclimatic variations impact stratification stability, thermohaline staircases, double diffusion salt fluxes, and halite depositionRegional and seasonal drying trends dictate stronger salt flux in the drier and deeper zones and explain the architecture of "salt giants" [ABSTRACT FROM AUTHOR]

When the density of a gravitationally stable fluid depends on a fast diffusing scalar and a slowly diffusing scalar of opposite contribution to the stability, 'double diffusive' instabilities may develop and drive convection. When the slow diffuser settles under gravity, as is for instance the case for small sediment particles in water, settling-driven double-diffusive instabilities can additionally occur. Such instabilities are relevant in a variety of naturally occurring settings, such as particle-laden river discharges, or underground inflows in lakes. Inspired by the dynamics of the more traditional thermohaline double-diffusive instabilities, we ask whether large-scale 'mean-field' instabilities can develop as a result of sedimentary double-diffusive convection. We first apply the mean-field instability theory of Traxler et al. (J. Fluid Mech., vol. 677, 2011, pp. 530–553) to high-Prandtl-number fluids, and find that these are unstable to Radko's layering instability, yet collectively stable. We then extend the theory of Traxler et al. (2011) to include settling and study its impact on the development of the collective instability. We find that two distinct regimes exist. At low settling velocities, the double-diffusive turbulence in the fingering regime is relatively unaffected by settling and remains stable to the classical collective instability. It is, however, unstable to a new instability in which large-scale gravity waves are excited by the phase shift between the salinity and particle concentration fields. At higher settling velocities, the double-diffusive turbulence is substantially affected by settling, and becomes unstable to the classic collective instability. Our findings, validated by direct numerical simulations, reveal new opportunities to observe settling-driven layering in laboratory and field experiments. [ABSTRACT FROM AUTHOR]

We employ direct numerical simulations in order to analyze the role of double‐diffusive salt fingering in halite precipitation from hypersaline lakes. Guided by field observations from the Dead Sea, which represents the only modern deep stratified lake that precipitates halite under hydrological crisis, we consider a saturated layer of warm, salty brine (epilimnion) overlying a layer of colder, less salty brine (hypolimnion) that is also saturated. The double‐diffusive instability originating in the metalimnion gives rise to an asymmetrical pattern of less salty ascending fingers, accompanied by descending salt fingers that lose heat as they propagate through the metalimnion. The net result is a strong, downward salinity flux that leads to the undersaturation of the epilimnion, while the hypolimnion becomes oversaturated and precipitates halite. These interfacial processes within deep, hypersaline water columns in warm and dry regions suggest a potential route toward the formation of thick halite layers found in the geological record. Key Points: Guided by Dead Sea observations, we simulate double‐diffusive salt fingering in saturated, hypersaline water columnsSalt fingering results in the undersaturation of the epilimnion, as well as halite precipitation in the hypolimnionThe observed interfacial processes suggest a route toward the formation of thick halite layers found in the geological record [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]

We provide a new perspective on the transition of hyperpycnal flows into saline turbidity currents, which permits longer runout lengths than might be otherwise expected. This mechanism relies on the differential turbulent diffusion of salt and sediment, and in contrast to ambient saltwater entrainment it enables the salinification of the freshwater current without diluting the sediment concentration field by a corresponding amount. The freshness‐to‐sediment ratio is introduced in order to quantify the transition process. The results of high‐resolution simulations provide estimates for the transition distance of hyperpycnal model flows into saline turbidity currents. Plain Language Summary: In some near‐coastal areas of the world's oceans, sediment‐rich rivers trigger so‐called hyperpycnal currents, which are currents composed of river‐derived freshwater and sediment that propagate along the seafloor. In general, one may expect such currents to have a short runout length, since sediment deposition will reduce their density until they loft from the seafloor. There is evidence, however, that in some settings hyperpycnal currents can have much larger runout lengths than expected. Here we propose a novel perspective on such large runout lengths, by showing that differential diffusivities can enable the hyperpycnal flow to transition into a saline turbidity current. This mechanism relies on the fact that salt and sediment diffuse at different rates, so that the freshwater current can become salinified without diluting its sediment concentration field. Key Points: Differential diffusion provides a new perspective on the transition of hyperpycnal flows into saline turbidity currentsDifferential diffusion of salt and sediment can render the freshwater current salty, without strongly diluting the sedimentTransition distance estimates are provided for hyperpycnal model flows [ABSTRACT FROM AUTHOR]