Manned submersible dives in the northwest South China Sea encountered substantial amounts of plastic litter accumulated at the base of scours along the floor of a submarine canyon, which may associate with the depositional behaviors of turbidity currents. In this study, we conduct numerical simulations using field-scale bathymetry to investigate the relationship between the canyon floor morphology, flow processes, and the locations and sizes of the plastic litter piles. The consistent deposition pattern caused by the numerical turbidity currents with different input parameters indicate that morphology of the canyon may exert a dominant influence on turbidite deposition. This is attributed to a significant reduction in shear velocity as simulated turbidity currents flowing through the scours on the canyon floor. Spatial correspondence between deposits of turbidity currents and plastic litter accumulation suggests that suspended sediments and plastic may undergo simultaneous dynamic processes during the transportation of turbidity currents. The issue of marine plastic litter has attracted wide attention, particularly in terms of its transportation mechanisms and locations of accumulation on the ocean floor. Turbidity currents are subaqueous sediment-gravity flows that can transport large amounts of sediment, nutrients and pollutants into the deep sea, yet there is sparse research on the dynamics of plastic litter transport under the control of turbidity currents, and its accumulation in the deep sea. Here, we present a series of numerical simulations of turbidity currents in a submarine canyon with various input parameters, when combined with observational data on topography and plastic litter distribution, confirm that turbidity currents constitute a plausible mechanism for the transport of plastic litter and for its accumulation in response to changes in flow associated with scours. In addition, we find that the concavity of the scours is also necessary for plastic litter accumulation, since it induces significant fluctuations in the shear velocity and the corresponding depositional process. Numerical simulations were applied to investigate turbidity currents as a cause for plastic litter accumulations in a submarine canyon The simulated turbidite deposits and observed plastic litter accumulations exhibit a strong spatial correspondence The morphology of the canyon floor may exert dominant influence on the plastic litter accumulations in submarine canyons

Typhoons in the western Pacific have a significant impact on the transport of heat, salt and particles through the Luzon Strait. However, there are very limited field observations of this impact because of extreme difficulties and even dangers for ship-based measurements during the rough weather. Here, we present the preliminary results from analyzing a dataset collected by a glider deployed west of the Luzon Strait a few days prior to the arrival of typhoon MITAG. The gilder data revealed an abnormally salinity (>34.8) subsurface water apparently sourced from Kuroshio intrusion during the typhoon. When typhoon MITAG traveled on the east of the Luzon Strait, the positive wind stress curl strengthened the cyclonic eddy and weakened the anti-cyclonic eddy. This led to a slowdown of Kuroshio and made its intrusion easier. The main axis of the Kuroshio at the northern part of the strait shifted westward after the typhoon and did not return to its original position until a week later. The Ekman transport from persistent northerly wind of typhoon MITAG was significant, but its importance in enhancing the Kuroshio intrusion is only secondary relative to the eddies variations.

Submarine canyons are prominent features on continental margins, acting as major conduits for sediment transport primarily through turbidity currents. Understanding how these currents interact with complex canyon topographies is crucial for deciphering the canyon system evolution, yet challenging due to limited field observations. Focusing on a group of slope-confined canyons within the Pearl River Mouth Basin, northern South China Sea, this study integrates multibeam bathymetry, core analysis, and process-based Computational Fluid Dynamics (CFD) modeling to investigate the influence of realistic canyon topography on turbidity current dynamics and resulting sediment deposition. Analysis of core samples revealed a potential turbidite layer characterized by silts and sandy silts. Using these sediment information and bathymetric data, we conducted a series of three-dimensional CFD simulations. Our findings highlight the significant variability in turbidity current characteristics, particularly flow velocity and sediment concentration, within the canyon groups. This variability is primarily controlled by canyon head depth, the width-to-relief ratio, and slope gradients. Notably, the simulations revealed unique flow structures not typically observed in experimental settings, including unidirectional flows, small-scale helical flows, stacked and mixed flow cells, and flow separation and convergence across inter-canyon ridges. Our CFD simulations also revealed a distinct near-wall pattern of linear deposition of coarse-grained sediments, which is similar to the observed bathymetric changes between 2009 and 2017. Based on the consistent results, we propose a conceptual model wherein differential erosion, driven by oceanographic processes, plays a key role in shaping the observed linear depositional patterns.

The numerical simulation data of turbidity currents.

Submarine canyons are primary conduits for turbidity currents transporting terrestrial sediments, nutrients, pollutants and organic carbon to the deep sea. The concavity in the longitudinal profile of these canyons (i.e. the downstream flattening rate along the profiles) influences the transport processes and results in variations in turbidite thickness, impacting the transfer and burial of particles. To better understand the controlling mechanisms of canyon concavity on the distribution of turbidite deposits, here we investigate the variation in sediment accumulation as a function of canyon concavity of 20 different modern submarine canyons, distributed on global continental margins. In order to effectively assess the isolated impact of the concavity of 20 different canyons, a series of two-dimensional, depth-resolved numerical simulations are conducted. Simulation results show that the highly concave profile (e.g. Surveyor and Horizon) tends to concentrate the turbidite deposits mainly at the slope break, while nearly straight profiles (e.g. Amazon and Congo) result in deposition focused at the canyon head. Moderately concave profiles with a smoother canyon floor (e.g. Norfolk-Washington and Mukluk) effectively facilitate the downstream transport of suspended sediments in turbidity currents. Furthermore, smooth and steep upper reaches of canyons commonly contribute to sediment bypass (i.e. Mukluk and Chirikof), while low slope angles lead to deposition at upper reaches (i.e. Bounty and Valencia). At lower reaches, the distribution of turbidite deposits is consistent with the occurrence of hydraulic jumps. Under the influence of different canyon concavities, three types of deposition patterns are inferred in this study, and verified by comparison with observed turbidite deposits on the modern or paleo-canyon floor. This study demonstrates a potential difference in sediment transport efficiency of submarine canyons with different concavities, which has potential consequences for sediment and organic carbon transport through submarine canyons.

The evolution of turbidity currents covers multiple physical processes, such as fluid entrainment, self-acceleration, and sediment deposition, which are associated with sediment particle behaviors and yet not well understood. This study uses a fully coupled computational fluid dynamics and discrete element method model to investigate the particle-scale dynamics of turbidity currents and their responses to different bedforms. Results show that the turbidity currents controlled by viscous drag exhibit distinct flow features, including changes in morphology, velocity evolution, and other fluid/particle kinematic indicators, depending on their initial particle volume concentration. An increase in initial particle volume concentration enhances the fluid/particle motions accompanied by generating interfacial Kelvin–Helmholtz waves. The fluid/particle indicators, with the exception of the energy loss for particle–bed collisions, have strong relevance with particle concentrations, which can be described by linear or power-law functions. Furthermore, specific bedforms play unique roles in the propagation process and deposition pattern of turbidity currents. Slope beds enhance the motion, suspension, and collision of sediment particles, and cause wave-shaped sediment deposits along the slope particularly in the high-concentration case. By comparison, weakening of particle migration on obstructed and wavy beds is accentuated by blocking effects, mainly resulting from the convex bed morphology. However, the continuously convex and concave features diminish the blocking effect of wavy beds by intensifying particle motions along the lee sides of wave-shaped bumps. The particle-scale dynamics of turbidity currents is linked to the relative sizes of the underlying bedforms, which should be noted and further studied in our future work.

Basic monitoring of the marine environment is crucial for the early warning and assessment of marine hydrometeorological conditions, climate change, and ecosystem disasters. In recent years, many marine environmental monitoring platforms have been established, such as offshore platforms, ships, or sensors placed on specially designed buoys or submerged marine structures. These platforms typically use a variety of sensors to provide high-quality observations, while they are limited by low spatial resolution and high cost during data acquisition. Satellite remote sensing allows monitoring over a larger ocean area; however, it is susceptible to cloud contamination and atmospheric effects that subject the results to large uncertainties. Unmanned vehicles have become more widely used as platforms in marine science and ocean engineering in recent years due to their ease of deployment, mobility, and the low cost involved in data acquisition. Researchers can acquire data according to their schedules and convenience, offering significant improvements over those obtained by traditional platforms. This study presents the state-of-the-art research on available unmanned vehicle observation platforms, including unmanned aerial vehicles (UAVs), underwater gliders (UGs), unmanned surface vehicles (USVs), and unmanned ships (USs), for marine environmental monitoring, and compares them with satellite remote sensing. The recent applications in marine environments have focused on marine biochemical and ecosystem features, marine physical features, marine pollution, and marine aerosols monitoring, and their integration with other products are also analysed. Additionally, the prospects of future ocean observation systems combining unmanned vehicle platforms (UVPs), global and regional autonomous platform networks, and remote sensing data are discussed.

Sandwaves, a ubiquitous bedform commonly found in most coastal seas and sometimes in deep waters of continental slopes and submarine canyons, are often the subject of marine hazard studies because their frequent migration can pose potential threats to pipelines or other engineering structures in the ocean. Analyses of high-resolution bathymetric data collected along a 136-km multibeam bathymetric survey line demonstrate the co-existence of three different sizes of sandwaves: giant, primary dunes (∼10-15 m in height, up to 2,000 m in length); small, secondary sandwaves (∼1 m in height, ∼50 m in length) riding on the stoss side of the primary dunes; and mid-size dunes (∼5-10 m in height, ∼100-500 m in length) found in the trough of primary dunes. While the tidal current-driven, secondary sandwaves are migrating, the primary and mid-size dunes are immobile. Empirical model results suggest that the primary dunes are relict dunes that were most likely formed in geologic past when sea level was higher or the seabed elevation was lower, but water depth is probably not the only factor in limiting the formation of the primary dunes.