Modern transport and deposition of settling particles in the northern South China Sea: Sediment trap evidence adjacent to Xisha Trough

• TPF of the upper layer are about four times of that of the lower layers.
• Clay mineral assemblage of suspended particles remains relatively stable.
• Taiwan and Red River supply 50–70% and 30–50% particles, respectively.
• Deep water current and coastal currents are key force for sediment transport.
• Currents measured in-situ of both the upper and lower layers flow southward.

Studies on modern sediment transport and deposition, especially studies analyzing settling particles collected with sediment traps, have rarely been carried out in the northern South China Sea. Using sediment trap time series data from Site XS1 (17°24.5′N, 110°55.0′E, water depth 1690 m) adjacent to the Xisha Trough, variations in sediment source through time have been reconstructed. These observations include total particle flux (TPF) and current data, grain size distributions, and clay mineral compositions obtained from two sediment traps deployed in 500 m and 1500 m water depth, respectively. Time series records at Site XS1 changed seasonally for both sampled layers. TPF in the lower layer (426 mg/m2/d) was several times that of the upper layer (113 mg/m2/d) and is affected by lateral transport. However, mean grain size (Mz) of the upper layer is greater that of the lower layer (29 vs10 μm) due to contributions from biogenic materials. There are no clear seasonal changes in clay mineral assemblage in either the upper or lower layers. The annual percentages of four main clay minerals were 82–83% illite, 7–9% kaolinite, 6–8% chlorite and 1–3% smectite. Taiwan was the dominant sediment source (42–74%), while sediment contributions from the Red River and Annamite Chain account for 23–53% and 0–15%, respectively. Sediment supply from Taiwan could be explained by deep water current flow, while coastal currents may aid sediment transport from the Red River and small mountainous rivers of central Vietnam.