Footprints of obliquely incident internal solitary waves and internal tides near the shelf break in the northern South China Sea

Abstract

A mooring system and two sites of bottom currents were deployed over the slope and near the shelf break on the propagating paths of internal solitary waves (ISWs), west off Dongsha Atoll in the northern South China Sea. Data indicated that energetic ISWs obliquely shoaled onto the shelf west off Dongsha Atoll in an approximately 290° direction, causing strong reversing currents (some exceeding 80 cm/s) near the bottom. Two types of sandwaves and short scour channels are discernible on the seafloor near the shelf break, which have reasonable correlations with the obliquely incident ISWs and internal tides. Type 1 sandwaves, featured by ISWs at the depths of 130–150 m, have flat crests interacting with the isobaths at an angle of nearly 45° which slightly incline and migrate upslope. Type 2 sandwaves are associated with internal tides, which have crests parallel to the isobaths and distinctly incline and migrate downslope. Short channels are parallel to the depth contours and truncate the strata, which could be formed and maintained by along‐slope currents that are probably produced by the obliquely ISWs on a large gradient (γ > 0.8°). The ISWs can move coarse grains or suspend fine grains but do not change the long‐term trend of sediment transport on the seabed with larger gradients (γ/c > 1), which is dominated by internal tides. These features are likely widespread near the shelf break in the northern South China Sea and other seas but are limited on mild slopes where ISWs do not break.

Distinguishing between Deep-Water Sediment Facies: Turbidites, Contourites and Hemipelagites

“…(including baroclinic tides) oscillate along the interface between two water masses of different densities. These are common throughout the oceans but are especially marked and energetic at the depth of the thermocline [59]. Bottom currents are also affected by intermittent processes such as giant eddies, benthic storms, flow cascading and tsunamis ( Figure 4).…”

Different origins of seafloor undulations in a submarine canyon system, northern South China Sea, based on their seismic character and relative location

“…They are the result of the interaction between strong tidal currents and the abrupt local bathymetry (Zhao et al, 2004;Li et al, 2008). Several fields of sediment wave have been reported on the continental shelf and slope off the Dongsha Islands, and they were interpreted as generated by internal waves interacting with the continental margin (Reeder et al, 2011;Ma et al, 2016).Our study area is just located on the western continental slope of the Dongsha Islands (Fig. 1), where internal waves have been documented by Ma et al(2016).…”

“…Four main hypotheses have been proposed for their genesis based on the environment settings in which seafloor undulations occur, their morphological characteristics, and internal architectures. These four hypotheses include: (1) bottom currents (Masson et al, 2002;Baldwin et al, 2017;Belde et al, 2017); (2) turbidity currents (Lewis and Pantin, 2002;Normark et al, 2002;McCave, 2017); (3) internal waves (Karl et al, 1986;Droghei et al, 2016;Ma et al, 2016;Ribó et al, 2016); and (4) gravity-driven downslope submarine creeps (Lee and Chough, 2001;Shillington et al, 2012;Li, et al, 2016a). In the following sections, the formation mechanisms of seafloor undulations are discussed for the study area based on the criteria proposed by Wynn and Stow (2002) and Symons et al (2016).…”

Bumpy Topographic Effects on the Transbasin Evolution of Large‐Amplitude Internal Solitary Wave in the Northern South China Sea

“…Nevertheless, our emphasis here is on the role of the prominent bumps in modulating the response of shoaling ISWs. Because of the strong bottom currents induced by shoaling ISWs, many sand dunes composed of fine to medium sand are reported to be formed on the upper continental slope region in the northern SCS (Ma et al, ; Reeder et al, ). The further response of shoaling ISWs to this kind of topographic bumps, which are at least 1 order of magnitude smaller than the prominent bumps studied here, is unclear as yet due to the limit of model resolution and the complexity of associated sediment resuspension and transport processes (Boegman & Stastna, ; Stastna & Lamb, ).…”