Holocene sedimentary evolution of a subaqueous delta off a typical tropical river, Hainan Island, South China

High-resolution seismic surveys were conducted off the eastern coast of Hainan Island to study the distribution and mass budget of Holocene subaqueous clinoform sediments. The Holocene subaqueous clinoform extends 45 km seaward across the shelf and more than 80 km northeastward along the shore. It covers an area of over 3,000 km², which is almost equivalent to the expansion of large river systems. Overlying the transgressive surface, the deltaic sequence can be divided into a transgressive unit and a highstand unit separated by a downlap maximum flooding surface. The volume of sediments within the Holocene subaqueous clinoform is estimated to be 33.12×10⁹ m³, equivalent to an annual accumulation of 3.61–3.31 Mt/yr, which is substantially higher than the global average. The high sediment yield presumably resulted from strong tropical weathering and active tectonics. The sediment flux during the transgression and sea-level highstand were 3.85–3.08 and 3.47 Mt/yr, respectively, suggesting no substantial changes in sedimentation. Compared to river systems originating from high mountain glaciers, the sedimentation variation of small tropical rivers throughout the Holocene is relatively minor. However, the sediment discharge of the Wanquan River has drastically decreased in recent decades because of increased human activity.