Deep-sea Sediment Resuspension by Internal Solitary Waves in the Northern South China Sea

Abstract

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.

Utilization of Soot and 210 Po-210 Pb Disequilibria to Constrain Particulate Organic Carbon Fluxes in the Northeastern South China Sea

“…The main current on the northern SCS shelf flows northeastward and the surface current flows southwestward in the northern basin during summer. Internal solitary waves often introduce the bottom nepheloid layer (BNL) on the northern shelf (Zhang et al, 2014) and the intermediate nepheloid layer (INL) on the slope (Jia et al, 2019). Both the BNLs and INLs result in higher particulate matter concentrations in the mesopelagic layer than the euphotic zone in the SCS (Ma et al, 2017;Zhang et al, 2019) and a large amount of shelf-derived sedimentary matter to the basin (Shih et al, 2019).…”

“…Both the BNLs and INLs result in higher particulate matter concentrations in the mesopelagic layer than the euphotic zone in the SCS (Ma et al, 2017;Zhang et al, 2019) and a large amount of shelf-derived sedimentary matter to the basin (Shih et al, 2019). Particles in the widely extended nepheloid layer over the shelf-slope area of the northern SCS are mainly from sediment resuspension (Zhang et al, 2014;Jia et al, 2019). Owing to the variability in the hydrodynamic conditions, the nepheloid layer also showed significant spatiotemporal variability as observed at different depths during different cruises (Zhang et al, 2019).…”

Breaking Location of Internal Solitary Waves Over a Sloping Seabed

“…In this regard, the most likely interactions between ISWs and the seabed would be related to the additional bed shear stress, produced by ISW breaking, which leads to an enhancement of sediment resuspension, as well as seabed erosion and instability (see Y. Jia et al. (2019), and references therein). In particular, during the shoaling process, intense near‐bottom velocities can erode seafloor sediments and generate nepheloid layers, intruding offshore (Bourgault et al., 2014; Dickson & McCave, 1986).…”

A Machine-Learning Model for Forecasting Internal Wave Propagation in the Andaman Sea

“…Large-amplitude IWs can travel for hundreds of kilometers and carry a significant volume of energy toward the coastline. The IWs threaten the safety of underwater navigation [6], enhance ocean mixing [7], affect sediment resuspension [8], and have a significant impact on offshore facilities [9].…”