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 >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 <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 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.

The submarine canyon was the main channel for the transport of sediment to the deep sea, but the transport process in submarine canyon induced by internal solitary waves (ISWs) had not yet been conducted. Based on computational fluid dynamics, this study investigated sediment resuspension and transport induced by the interaction between ISWs and submarine canyon. The simulations showed that sediment particles near the canyon were resuspended due to the propagation of ISWs. The sediment of slope section experienced higher resuspension rates compared to the horizontal section. Sediments on the slope were suspended before the trough of ISW and then transported into the canyon. Sediment resuspension induced by ISWs was more significant in the slope section compared to the horizontal sections and contained 45% more sediment particles than the horizontal section within the canyon. The sediment concentration within the canyon gradually increased in both the horizontal and slope sections, with a particularly pronounced increase at the junction between the horizontal and slope section. The concentration of particles in the canyon was significantly higher than in the horizontal section, with an increase in 2.73 times. Furthermore, the sediment concentration within the canyon was 1.73 times higher than outside the canyon. A sediment water column with higher suspended matter concentration, known as the bottom nepheloid layer, was formed above the submarine canyon. However, these sediment particles could not cross the pycnocline and accumulated in the upper region of the lower water. The sediment distribution of the canyon was much greater than outside the canyon within the transverse section, with three times more sediment particles within the canyon compared to outside. The sediment distribution of slope section was significant more than the horizontal section. Sediment suspended above the canyon gradually moved downstream along the submarine canyon and primarily deposit at the junction between the horizontal and slope sections. Sediment movement also occurred along the submarine canyon in the horizontal section. The interaction between ISWs and submarine canyons intensified sediment resuspension and significantly affected the spatial distribution of sediment particles. This interaction could transport resuspended sediments from the continental shelf to the deep-sea basin, thereby influencing the deep-sea environment.

Marine nepheloid layer is widely distributed in the oceans and marginal seas. The concentration of suspended particles in the nepheloid layer is significantly higher than that of the adjacent layers. Marine nepheloid layers include the surface nepheloid layer (SNL), intermediate nepheloid layer (INL), and bottom nepheloid layer (BNL). As a transport pathway for the particulate matter in the oceans, nepheloid layer is important to the carbon cycle and the source–sink system at the continental margin. This paper focused on the characteristics of the marine nepheloid layer and the mechanism of internal solitary waves that form INL and BNL, providing a reference for further research on the marine ecological environment dynamic process and source–sink system. BNL is formed by suspended seabed sediments with strong currents near the bottom, but the fate of BNL remains unknown. Marine nepheloid layer significantly affects the transportation of materials within the ocean. The transport of sediments by ISWs dramatically affects the formation of deep seabed sedimentary landform. However, the formation process and the transport contribution of INL and BNL are still unclear. The systematic and long-term research on the marine nepheloid layer in the world oceans is still limited.