Distribution and bi-directional transport of surface sediments driven by monsoons on the northwest continental shelf of the South China Sea

The Qizhou Island sea area is located northeast of Hainan Island and is characterized by extensive and steep narrow continental shelves to the northeast and southwest, respectively, it is connected to the Qiongzhou Strait to the west. Its unique geographical location is important for investigating sediment distribution and transport patterns. The present study analyzed the grain size characteristics of 80 surface sediment samples from the Qizhou Island sea area, revealing that the northwest section is predominantly composed of clay, whereas the southeast section is primarily sandy. End-member modeling of grain size identified four distinct components: EM1 represents the offshore transport of terrestrial materials from the northwest, EM2 indicates material input from the southwest, EM3 reflects material input from the northeast, and EM4 may signify residual sediments from a previous low sea level period. Grain size trend analysis revealed that the northeastern area mainly receives material input from the northeast, while the southwestern region is dominated by input from the southwest. Numerical simulation experiments demonstrated that the predominant current direction in the study area is from northeast to southwest. The sedimentation model illustrated that sediment thickness decreases from the northeast to the southwest annually, aligning with material input from the northeast. However, sediment thickness in the southern region is greater between September and November, attributed to a shift in current direction to southwest-northeast during the summer monsoon, which transports substantial riverine sediments generated by summer monsoon and typhoon rainfall from Hainan Island into the study area. The bi-directional sediment transport in this region is driven by seasonal currents influenced by the East Asian monsoon and is further affected by river systems in Southern China and smaller mountainous rivers on Hainan Island. Further investigation involving additional indicators in this area is anticipated to enhance our comprehension of the source-sink system under the combined influence of major rivers and small mountainous rivers globally.