Geochemical Characteristics And Provenance Of Sediments In The Middle And Lower Reaches Of The Pearl River

Sediments in large basins are greatly affected by climatic conditions and have an important impact on global sea-land material circulation.The Pearl River is located in the southern part of China,with an average annual runoff of 3.41×10¹¹ m³,second only to the Yangtze River,carrying about 7.50×10⁷ t of sediment into the South China Sea every year.Therefore,the main,trace and rare earth elements of sediment samples from the middle and lower reaches of the Pearl River and the main tributaries were measured,and the geochemical characteristics of the sediment samples in the study area were analyzed.The main purpose is to elucidate the elemental composition of sediments in the eastern catchment of the Pearl River basin,reveal the material sources of sediments,and analyze the chemical weathering intensity and its controlling factors in the eastern part of the Pearl River basin.The results show that:1.The content of SiO₂ and Al₂O₃ in the sediment samples of the study area is similar to that of the upper continental crust.The contents of MgO,CaO,Na₂O and K₂O are depleted relative to the continental crust,CaO and Na₂O have the most losses;trace elements such as Zr,Ba,Rb,Pb,Zn,Cu,Cr,Th,Ni,As,Hf,etc.have large differences,the average value of Zr and Ba is between 250 and 450 mg/kg,and the content of Cd is generally around 0.2?g/kg.Among which As,Cs,Hf,Pb,Zn,Cd,Rb,Th and Zr are more concentrated than the upper continental crust and Sr is seriously depleted;the total amount of rare earth elements is higher,the distribution curve is inclined to the right,and the light rare earth is enriched.The?Eu is between0.5 and 0.7,showing a negative anomaly,similar to the characteristics of granite.2.By provenance analysis and discriminant diagrams such as log?SiO₂/Al₂O₃?-log?Na₂O/K₂O??log?SiO₂/Al₂O₃?-log?Fe₂O₃/K₂O??La/Yb-?50?REE?La-Th-Sc?Co/Th-La/Sc?Co/Th-Sc/Th show that the sediments in the study area are mainly the products of granite weathering.The source rock characteristics are closer to the granites in the South China-Youjiang orogenic belt.At the same time,the study area is divided into three sub-areas according to the similarity of sediments:1.middle and lower reaches of the Xijiang River,Pearl River Delta,Dongjiang River Basin;2.Beijiang River Basin;3.Gui,Hejiang River Basin.3.With the CIA value of study area is generally between 75 and 85,the eastern part of the Pearl River Basin mainly suffers from moderate to high degree of chemical weathering.The study area was affected by the typhoon and the rainfall was abundant.The typhoon invasion may cause some of the highly weathered sediments to enter the ocean quickly,thus forming a distribution feature of the middle and high sides of the CIA in the study area.In summary,due to the frequent typhoon in the eastern part of the Pearl River basin,the annual average precipitation is relatively high,mainly showing moderate to high degree of chemical weathering,resulting in serious loss of main and trace elements such as CaO,Na₂O and Sr.The REE partitioning pattern of sediments in the study area is the same as that of granite,indicating that the source rocks are mainly marginal granites,but the degree of similarity is not the same.According to the DF value,it can be divided into three sub-regions.