| Elements geochemistry is an important means of tracing sources and sedimentary environment, which is of significance implication to resource exploration and environment assessment. Based on the data of major and trace elements of 880 samples and granularity of 3251 samples collected from the northwest of the South China Sea, the study was conducted on elements geochemistry at different scales as well as sediment dynamics to reveal the natural variation and the impact of human activaties by using of factoring kriging analysis and grain-size trend analysis.The study area is divided into four categories sedimentary’environments:(Ⅰ) the coarse sandy deposits area nearshore; (Ⅱ) the muddy area, including the area from central part of Beibu Gulf to the west sea of Hainan Island and from west coast of Guangdong to the northeast sea of Hainan Island; (Ⅲ) sandy deposition area in Qiongzhou Strait and its east and west mouth; and (Ⅳ) the residue deposition area. The grain size trends analysis on basis of multivariate geostatistics show that, at local scale(about 33km of radius), there exist generally 18 small grain-size convergence centers of A to R which distribute in all kinds of sedimentary environments, indicating the control of various factors such as river input, terrain and the ocean currents. And at regional scale (about 80km of radius), there exist generally 9 larger grain-size convergence centers of A’ to I’, whose spatial distribution are in consistent with the sedimentary environment of type Ⅰ and Ⅱ, and the spatial distribution of convergence centers is obvious in directivity, reflecting the control effect of regional circulation.Factoring kriging analysis was carried on four elements group:major elements group (Al2O3, Fe2O3, MgO, MnO, TiO2, P2O5, K2O, Na2O and Mz), trace elements group (Ni, Zn, Pb, Cu, Cd, Cr and Mz), SiO2-Sand and CaO-Sr. The result shows there are three-scale spatial structures:nugget, a spherical structure with a range of 70 km (local scale), and a spherical structure with a range of 160 km (regional scale) for the former three groups. Yet, CaO-Sr exists spatially in three scales:nugget, a spherical structure with range of 120 km (local scale), and a spherical structure with range of 250 km (regional scale), which is different from other groups because of marine autogenous attributes of CaO.The major elements group both at local and regional scales, is controlled by the terrigenous clay components and the aqueous Fe-Mn oxide components. At local scale, both the two components exhibit similarity in spatial distribution, and the high value areas distribute mainly in the coarse sandy deposit area nearshore of Ⅰ, coinciding with the grain-size convergence centers of A,E,G,J,N,P, which suggest the control effect of river input. And autogenous precipitation of Fe-Mn oxide in alkaline and oxidizing seawater after river into the sea, leads to more close to the land in spatial distribution than the terrigenous clay components. At regional scale, the terrigenous clay components distributes surround the land and reduce towards the sea, and high value areas distribute mainly in the coarse sandy deposit area nearshore of Ⅰ, which can be speculated it was formed through terrigenous clay components of high sea-level deposited upon the coarse components of low sea-level. The high value areas of Fe-Mn oxide components mainly distribute in the area with strong hydrodynamic conditions, including muddy area of Ⅱ and the east mouth of Qiongzhou Strait, corresponding to the grain-size convergence centers of A’, B’, C’, G’, H’ and I’, indicating the control effect of regional circulation and redox environment.SiO2-Sand group and CaO-Sr group represent respectively siliceous coarse terrigenous components and marine biological components, both of which are not obvious in scale effect of spatial distribution. SiO2 is controlled mainly by hydrodynamic conditions, while CaO is mainly in control of water depth.The trace elements group is affected mainly by human activity at local scale, and Pb,Cd are the indicator elements of human activity. There are 8.4% and 6.5% respectively of total sample stations of Pb,Cd have enriched to moderate to significant degree, demonstrating Pb,Cd have suffered from anthropogenic pollution, and the high value areas of Fangcheng Port, Lianzhou Bay, Jianghong Bay, Zhangjiang Port,Leizhou Bay, Haikou Bay, and Basuo Port, are probably the contaminated areas. At regional scale, the trace elements are dominated by the adsorbed components by terrigenous clay components, which distribute an increasing trend from land to sea, and the high value areas locate in the muddy area of Ⅱ corresponding to the grain-size convergence centers of A’,B’,C’, G’,H’ and I’, indicating the effect of regional circulation. Cd, with an unique geochemistry, is an independent principal component, and the high value areas distribute mainly in Qiongzhou Strait, southwest coast of Hainan Island as well as southern sea area of Moyang River, which was speculated the result of clastic deposits near the source and the interaction between sea and land at low sea-level.In summary, the natural terrigenous components in the study area show the spatial variation at local scale (70km) and regional scale (160km); and at local scale, small and medium rivers input is the dominant effect factor, with depth, terrain and current playing an important role to the migration and deposition of sediments; at regional scale, the sea-level changes and regional circulation are the primary’ controlling factors. Marine biological components exhibit mainly the regional scale variation (250km), with the dominant effect factor of water depth. Anthropogenic components perform the spatial variation at local scale (70km), and mainly controlled by local anthropogenic input. |
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