Grain size distribution of surface sediments in the area off southeast Hainan Island and its implications for environmental interpretation

377 surface sediment samples from the sea area off the southeast of Hainan Island are analyzed for grain size distribution and sediment classification. Using the multivariate statistical method in addition to transportation trend analysis, the relationship between the grain size distribution patterns and their bearing on source and hydrodynamic conditions is discussed. There are 13 types of sediments in the study area. Sediments are mainly composed of silt and sand and dominated by fine silt (6～8Φ), with a Mz value of 5.73Φ and poor sorting. Obvious differences in sediment grain size are observed in different sea areas. Gravel-bearing sediments occur mainly in the north coast; sandy sediments are distributed in the middle of the study area in a NE-SW direction within a water depth of 80～120 m; and silt and clay sediments in the southwest coast, the north central sea area and the southeast sea area. Based on the differences in geographic locations, sediment sources and hydrodynamic conditions, the study area is divided into three sedimentary districts. The District I is dominated by gravel-bearing coarse-grained sediments, which are mainly formed by runoff and strong wind waves along the Hainan Island. Sediments are sourced from coastal erosion and local rivers such as the Wanquan River and deposited in strong dynamic environments; The District II is dominated by sandy sediments formed by the mixed relict and coastal deposits under the joint action of surface currents, wind wave. runoff and coastal erosion. A small amount of fine-grained materials may come from the Pearl River Basin and other offshore complex sources. The District III is dominated by fine-grained sediments, affected mainly by coastal currents, wind waves and South China Sea warm currents. It is assumed that most of the sediments in this region are sourced from river input, coastal erosion and some fine-grained materials from complex offshore sources by weak hydrodynamic.