408 / 2024-09-16 13:55:56

Characteristics and Sources of Surface Sediments on the Northwestern Shelf of the South China Sea: Evidence from Clay Minerals and Geochemistry

Sediment transport；Mountain rivers；Geochemistry；Ecological environment；South China Sea

Sediments serve as a "natural recorder", encapsulating extensive geological and environmental information. The South China Sea, receives 700 Mt/a of sediment from adjacent rivers and islands, establishing it as a focus for international research in recent years. The Qizhou Islands, located northeast of Hainan Island, are bordered by a broad shelf to the northeast and a narrow, steep shelf to the southwest. The northeastern area receives significant sediment input from the Pearl River, while the southwestern region is influenced by smaller rivers originating from the mountainous terrain of Hainan Island. To the west, this region connects to the Qiongzhou Strait. This distinctive geographical configuration is critical for investigating sediment distribution and transport dynamics. 80 surface sediments were collected from the vicinity of the Qizhou Islands, with analyses conducted on grain size, clay minerals, and geochemical elements to identify potential sources, transport trends, and mechanisms. Nearshore sediments predominantly comprise clay and silt, whereas sand is more prevalent in offshore regions. Numerical simulations of tidal currents indicate that the annual mean flow generally follows a northeast-southwest trajectory, aligning with the northeastern input. During summer, under the influence of the southwest monsoon, coastal currents shift, facilitating the transport of sediments from Hainan Island into the study area. Clay minerals are primarily composed of illite and smectite, with illite showing an increased trend offshore, while kaolinite displays an opposite pattern. The clay minerals are sourced from multiple origins, including the Pearl River, Taiwan Island, Hainan Island, and the Leizhou Peninsula. Southwestward coastal currents transport materials from the mountainous rivers of Taiwan and the Pearl River, carrying them nearly a thousand kilometers along the northwestern shelf of the South China Sea. Despite the dominant influence of the Pearl River, the clay minerals in the study area also reflect contributions from distant sources such as Taiwan Island, as well as local inputs from smaller mountainous rivers in Hainan Island and the Leizhou Peninsula. The results of the enrichment factor and pollution load index analyses indicate that Cd and Pb concentrations are influenced by human activities. The enrichment of Cd in the southeastern outer shelf may be attributable to biogenic carbonate contributions or enhanced nearshore material input during the late Pleistocene low sea-level period. These findings enhance the understanding of sediment distribution and transport mechanisms in complex marine environments, contributing to the comprehension of sediment transport patterns influenced by both major rivers and smaller mountainous streams.