Internal solitary waves (ISWs) can cause strong vertical and horizontal currents and turbulent mixing in the ocean. These processes affect sediment and pollutant transport, acoustic transmissions and man-made structures in the shallow and deep oceans. Previous studies of the role of ISWs in suspending seafloor sediments and forming marine nepheloid layers were mainly conducted in shallow-water environments. In summer 2017, we observed at least four thick (70–140 m) benthic nepheloid layers (BNLs) at water depths between 956 and 1545 m over continental slopes in the northern South China Sea. We found there was a good correlation between the timing of the ISW packet and variations of the deepwater suspended sediment concentration (SSC). At a depth of 956 m, when the ISW arrived, the near-bottom SSC rapidly increased by two orders of magnitude to 0.62 mg/l at 8 m above the bottom. At two much deeper stations, the ISW-induced horizontal velocity reached 59.6–79.3 cm/s, which was one order of magnitude more than the seafloor contour currents velocity. The SSC, 10 m above the sea floor, rapidly increased to 0.10 mg/l (depth of 1545 m) and 1.25 mg/l (depth of 1252 m). In this study, we found that ISWs could suspend much more sediments on deepwater areas than previously thought. Specifically, we estimated that ISWs could induce and suspend 787 Mt/yr of sediment from shelf to deep-sea areas of the northern South China Sea. The total amount of sediment resuspended by shoaling ISWs was 2.7 times that of river-derived sediment reaching the northern South China Sea. This accounted for 6.1% of the global river-discharged sediment (16.4% of that from Asian rivers) transported to the sea.

The widely recognized global phenomena of bottom nepheloid layer (BNL) and intermediate nepheloid layer (INL) are ubiquitous in the ocean. These phenomena are induced by shoaling internal solitary waves (ISWs), as observed in many studies. In this study, we analyzed the BNLs and INLs induced by shoaling ISWs and their detailed processes using flume experiments and field observations. ISWs suspended seabed sediment by the horizontal velocity in the vortex, and the near‐bottom vertical velocity lifted sediment into the water column to create a BNL, which detached from the slope and diffused along the isopycnals, forming more than one INL. Considering the results of previous researchers, we found that the numbers of BNLs and INLs were principally determined by the relationship between the angle of the ISW group velocity vector (α) relative to horizontal and the slope gradients (γ). In transmissive regions (γ/α < 1), one BNL and more than one INL were formed. In critical regions (γ/α~1), only one BNL and less than one INL were observed. In reflective regions (γ/α > 1), less than one BNL was formed and no INL. The BNL in the critical regions should be the thickest where the sediment resuspension was the greatest. The concentrations and thicknesses of BNLs and INLs were related to the energy and amplitude of ISW and the sediment condition. The results in the transmissive regions were proved by field observation. Our research will help to predict the number and magnitude of transport channels formed by shoaling ISWs from the ocean margin to the ocean interior.

Internal solitary waves (ISWs) can cause strong seafloor sediment resuspension and induce nepheloid layers in both shallow and deep-water environments. However, the roles of ISWs in the &gt;1000 m deep sea sediment resuspension and seafloor geomorphic changes are still unclear. To answer the above question, in the Dongsha area of the northern South China Sea, we measured suspended particulate matter along with a section covering the entire continental slope between 300 and 2000 m water depths, together with high-resolution multibeam bathymetric data for examining geomorphic changes. The results indicate that, on the upper slope with water depth &lt;700 m, seafloor sediments were heavily disturbed and resuspended. We find that ISWs could suspend seabed sediments and shape a bedform at water depths ≲1000 m. The maximum water depth of sediment resuspension by ISWs measured is found as deep as 1500 m. The distribution pattern of the seafloor surface sediments on the east of the Dongsha continental slope (fine and silty sand in &lt;700 m water depth, clayey silt between 700 and 1500 m, and silty clay &gt; 1500 m) also indicates that they are mainly controlled and impacted by ISWs. The wave refraction theory could be applied to the upper slope, but sediment resuspension is related to the seabed topography on the lower slope. Our study shows that the suspension and transport of sediments induced by episodic ISWs on the Dongsha slope of the northern South China Sea could shape the bedform and affect the sedimentary seabed geomorphology. This research will help explain the impacts of the ISWs on the deep-water sediment resuspension and seafloor geopmorphic changes along with the continental slope in the margin sea.

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