Effects of the coupling between slope morphology and bottom currents on flow erosion and sedimentation at the Dongsha Continental Margin, South China Sea

Abstract

The Dongsha Continental Margin (DCM) projects seaward and is situated in the path of bottom currents coming through the only deep-water exchange passage, the Luzon Strait between the South China Sea (SCS) and the western Pacific Ocean. This provides an opportunity to observe the different interaction between the two wings of the convex margin and the bottom currents, and help understand the corresponding implications for provenance, debris transportation, and sedimentation in such an environment. The convexity of the DCM causes its eastern flank to shrink against upcoming bottom currents and internal solitary waves (ISWs), producing a funneling effect and forming strong erosion grooves or strips, remnant seamounts, and large seafloor coarse debris dunes. The concavity of the western flank induces the expansion of bottom currents that flow around the plateau, resulting in a depositional zone with weak erosion that mainly interacts with bottom currents and gravity flow. The strong erosion on the DCM caused by the bottom current forms the primary provenance of the deep-water environment, while the nepheloid layer that entraps the fine debris of the gravity flow that derives from Taiwan and that is transported by the bottom current is the secondary provenance. The different coupling patterns between the bottom currents and the two flanks determine the different modes of debris transportation and deposition. Debris eroded by the currents is mainly transported by the gravity flow on the eastern flank while sweeping of the outer shelf and upper slope by eddy currents, progradation of the gravity flow, and reworking by the bottom current mainly occur on the western flank. Two types of morphological breaks, namely, continental slope break and bottom-current slope break, have developed on the DCM. They control the evolution of the flow regime of the multi-layer bottom currents and the gravity flow of the DCM as well as the effects of erosion and deposition. These two types of slope breaks are coupled and form an area in front of Dongsha Island with the highest deposition rate in the SCS.