Distribution Of Surface Sediment In The Bohai Sea And Its Relationship With Sediment Supply And Sedimentary Dynamic Environment

The Bohai Sea is a shallow semi-enclosed epicontinental sea. Its primary sediment supply is the huge amounts of terrestrial materials carried by the surrounding rivers among which the Yellow River plays a dominant role. The sedimentary dynamic environment in the Bohai Sea has significant seasonal variability as modulated by the monsoonal climate. Therefore, the distribution and dispersal mechanism of the terrestrial materials are very complicated and attract a wide spread attention. This paper analyzed the grain-size distribution characteristics of surface sediment and modern sedimentation rate in the Bohai Sea, in order to reveal the impacts of sedimentary dynamic process and sediment supply on the classification and distribution of surface sediment. The work is thus of significance to understand the process of "source to sink" and reveal the sedimentary dynamic mechanism.Based on the grain-size analysis of surface sediments recently sampled in the Bohai Sea, the grain size distribution of surface sediment was characterized and its relationship with sediment supply and hydrodynamic environment were presented. In addition, combined with the published data of sedimentation rate in this region, the correlation between grain-size distribution of surface sediment and modern sedimentation rate was discussed, and then the impact of sediment transport pattern on the distribution of surface sediment was analyzed, which contributes to comprehend the process of "source to sink". The results show that:1) Owing to the influence of sediment supply and hydrodynamic environment, the surface sediment in the Bohai Sea were primarily composed of silty sand and clayey silt. The distribution of silt played a primary role in dominating the distribution of surface sediment the Bohai Sea. The surface sediment in the Bohai Sea were mostly characterized by positive skewness, relatively poor to poor in sorting, wide to relatively wide and very wide kurtosis.2) There was an obvious spatial corresponding relationship in the Bohai Sea among the modern sedimentation rate, the grain-size distribution characteristic of surface sediment and sediment transport pattern. When the grain-size of the surface sediment was relatively coarse, the sedimentation rate of the region became low, the grain-size distribution of surface sediment was scattered and the sediment converged towards fine-grained sediment area; whereas the sedimentation rate was relatively high at the fine-grained sediment area and the grain-size distribution of surface sediment was also scattered. Furthermore, the grain-size trend analysis showed that the fine-grained sediment areas, such as the center of the Bohai Sea and the northern of the Liaodong Bay where sedimentation rates are high, were the convergent center of surface sediment, except for the Bohai Bay and the subaqueous Yellow River Delta where offshore sediment transport was evident.3) The surface sediment in the Bohai Sea mainly came from the Yellow River, thus the grain size and dispersal direction have a significant impact on the deposition of the Bohai Sea. The surface sediment at the subaqueous Yellow River Delta were primarily composed of clayey silt and sandy silt, mostly characterized by positive to strongly positive skewness, relatively poor in sorting, wide to very wide kurtosis. The surface sediments types around the presently active delta lobe were mostly sandy silt, relatively coarse in grain size, whereas the surface sediments types in offshore area away from the present river mouth were clayey silt, relatively fine in grain size. Compared with the investigation in 1980s, the recent surface sediments at the subaqueous Yellow River Delta seemed to be coarser in grain size. The reasons for the changes might be ascribed to the enhanced erosion along the delta coast due to insufficient sediment supply. Meanwhile, the sediment delivered from the river to the sea has been much coarser since the Water-Sediment Regulation in 2002, which resulted in considerable accumulation of coarser sediments around the present river mouth. According to the grain-size trend analysis method and the distribution of sedimentation rate at the subaqueous Yellow River Delta, the river-delivered sediments were mostly accumulated at the present river mouth and along the delta coast, together with northward and northeastward transport corresponding to the accumulation of fine-grained sediment in the central Bohai Sea and sediment export to the Bohai Strait through the Laizhou Bay, respectively. In addition, the high accumulation rate of coarse-grained sediment around the present river mouth implies the trapping effects from the tide-induced shear front. These results indicated that sediment supply and sedimentary dynamic environment played predominant role in the distribution of surface sediment at the subaqueous Yellow River Delta.