Geochemistry Of Major Elements In Bed Sediments From Inlets Of The Four Rivers To Dongting Lake,China

The geochemistry of elements in river sediments is very important and critical for us to understand the surface processes of"From the Source to Sink",which represents the most important aspects in study of the natural processes in a river basin.In this study,X-ray fluorescence(XRF)technique was used to systematically analyze the geochemical characteristics of major elements in the sediments from the Xiangjiang,Zijiang,Yuanjiang,and Lishui of to Dongting Lake,to understand the element composition characteristics and temporal and spatial variation of the sediments entering the lake,and then to discuss the important epigenetic information such as the provenance of sediments,the weathering of rocks in the source area,and the hydrodynamic conditions of the river where the sediments were formed.To reveal the characteristics of supergene(natural)action in the“Four Rivers”watershed,Achieve the following innovative understanding:(1)The contents of SiO₂,Al₂O₃,Fe₂O₃,K₂O,LOI in the sediments from the“Four Rivers”of Xiangjiang,Zijiang,Yuanjiang,and Lishui are relatively stable,and there is no significant difference between the rivers,while the contents of Mn O,Mg O,Ca O,Na₂O are significantly different between the rivers.Compared with the North American Shale,the sediments from Dongting Lake are relatively enriched in Mn O and LOI,while depleted in Mg O,Ca O,K₂O,Na₂O.The enrichment and depletion characteristics of major elements in the lacustrine sediments of different rivers are different,which may be related to the different natural action characteristics of the basin.(2)From Xiangjiang and Zijiang to Yuanjiang and Lishui River sediments,the contents of major elements TiO₂,Mg O,Ca O and Na₂O increased while Mn O tended to decrease.The major element contents of the sediments varied greatly with the depth are the Xiangjiang River and the Lishui River,while the sediments from Zijiang and Yuanjiang River varied slightly with the depth.(3)The analysis of that result of solving SiO₂-K₂O/Na₂O,SiO₂/Al₂O₃-K₂O/(Ca O+Na₂O)and discriminant function(F1-F2)by the major element discriminant graph show that,The provenance of the sediments from Dongting Lake has the tectonic background of passive continental margin and active continental margin.The rock types are mainly felsic sedimentary rocks,and the source rocks have the characteristics of continental upper crust.The results of provenance tracer analysis show that the sediments of Xiangjiang and Zijiang are a mixture of clastic sedimentary rocks and weathering products of granite,and the element composition of the sediments has the contribution characteristics of the source rocks of the upper crust+granite assemblage.The element composition of the Yuanjiang sediments is relatively uniform,indicating that the large area of Slate in Xuefeng Mountain contributed to the provenance of the sediments.The element composition of sediments in the Lishui River Basin is characterized by the average element composition of the upper crust,which reflects the influence of sedimentary clastic rocks such as black shale,sandstone and siltstone widely distributed in the upper reaches of the Lishui River Basin on the element composition of sediments.That is to say,the element geochemical composition of the river sediments from the“Four Rivers”into the lake is obviously affected by the basin geological background.(4)The CIA(Chemical index of Alteration)values of the sediments from XR and ZR(78 and 80 on average)were significantly higher than those from YR and LR(72 and 70 on average).However,the average WIP(Weathering Index of Parker,25 and 27 on average)was significantly lower than that of the YR and LR sediment(29 and 29 on average).It shows that the chemical weathering degree of the rocks in the source area of Xiangjiang and Zijiang basins is high,while the chemical weathering degree of the rocks in the Yuanjiang and Lishui basin is low.The results show that the difference of chemical weathering degree reflected by sediments may be the reflection of lithology difference of source rocks in the basin.The results of principal component analysis show that the stable components SiO₂,Al₂O₃,K₂O and the active elements Mg O and Ca O are the key principal element components indicating the weathering degree of source rocks in the basin.(5)Diagrams of Zr/Al₂O₃-Zr,Al₂O₃/TiO₂-Zr/TiO₂ of the sediments from“Four Rivers”show that the chemical composition of sediments is influenced by mineral composition and hydrodynamic sorting.The hydrodynamic conditions of sediment formation and the degree of mineral sorting decrease successively from the Lishui River,Xiangjiang River,Zijiang River and Yuanjiang River.(6)The obvious difference of major element composition of the“Four Rivers”into the lake is a comprehensive reflection of the lithology,chemical weathering and hydrodynamic sorting of the source rocks in the basin.The lithology of basin source rock is the main factor that affects the composition characteristics of sedimentary elements.