Nepheloid Layer Structure and Variability Along the Highly Energetic Continental Margin of the Northern South China Sea

Abstract

Observations of hydrography and suspended particles supply a large amount of information regarding material transport with respect to the changing marginal sea. To investigate the nepheloid layer structure and variability along the highly energetic continental margin of the northern South China Sea, we combined optical data and time series of acoustic backscatter data. The surface nepheloid layers and subsurface nepheloid layers were widely developed, but the bottom nepheloid layers (BNLs) and intermediate nepheloid layers (INLs) showed regional differences. On the western side of Dongsha Island, there was a large variability of strong INLs, with concentrations up to 0.94 mg L−1, and they occasionally formed and exceeded 200 m in a few hours, with concentration changes up to an order of magnitude. On the eastern side, INLs thicker than 100 m were rare. However, the BNL was larger and more stable, and the strongest BNLs with nepheloid inversion had concentrations up to 0.55 mg L−1 and unexpectedly thickened to be greater than 1,500 m, extending upward to the thermocline. Time series data found that the recurring nonlinear internal waves (NLIWs) dominated the enhancement of BNLs in the Dongsha Slope. The NLIW‐induced vertical velocity reached 0.5 m s−1, increasing the concentration of BNLs and resuspending sediment to 10 m above the bottom. The NLIWs strongly erode the sediments and provide a material source for the upward development of BNLs, while centimeter‐scale rippled scours form on the seafloor. These findings provide evidence of strong lateral transport across continental margins.