A Study On The Composition And Provenance Of The Surface Sediments In Yangtze River Estuary And Its Adjacent Sea Area

Marine sediment provenance study is one of the most important ways which scientists usually take to understand the deposit dynamical processes and trace the diffusion of riverine pollutants in estuary deltas and continental shelves. Much work has been done on the sediment provenance of Changjiang Estuary and its adjacent sea area by many people, most of whom consider the terrestrial flux of Changjiang as the main source of this area. However, there are still no clear conclusions on whether the resuspended sediments come from old Yellow River Estuary can travel across subaqueous Yangtze River Delta and get to Fujian and Zhejiang coast or not.Totally 35 samples are obtained from surface sediment of the area between 122°-124°E and 30°-32°N including subaqueous Yangtze River Delta and its adjacent sea area. Samples from Yangtze River and Yellow River are also collected to identify their end-member composition. After a systemic study of size-fractionated samples on mineralogical and geochemical characteristics by X-ray diffraction methods, ICP-AES (MS) and laser diffraction particle size analysis, some identification signatures of Changjiang and Huanghe sediments are extracted. And then the study area zoning is carried out depend on different sediment provenances.The results indicate that sediments come from the old Yellow River Delta can pass through the subaqueous Yangtze River Delta and reach to Fujian and Zhejiang coast, in which the sediments just located on the directly east of the mud area. The main conclusions can be summarized as follows:There are some differences between the two types of sediments which come from Changjiang and Huanghe both in mineralogy and geochemistry, and the differences will be more apparent in the given grain size.Yangtze River sediment is typical of low content of smectite and high ratio value of"illite/smectite"which is normally higher than 17, while sediment from the Yellow River is characterized by high content of smectite, and low ratio value of"illite/smectite"which is generally lower than 9. Calcite is the Characteristic mineral of the Yangtze River as well as dolomite in the Yellow River. When focus on the sediments that between 4-16μm the largest difference can be found. In the sediments which are finer than 4μm, Ti,Mn,Cd,Cu,Zn,Y,V are relatively rich in Changjiang and Ca,Mg,Sr are richer in Huanghe. And similarly in the sediments of 4-16μm, Changjiang sediments are rich in Ti,Mn,Cd,Co,Pb,Cu,V compared to Huanghe sediments which are rich in Ca and Sr. Generally, Al(<4μm),Fe(<4μm),Mn(4-16μm),Mg (<4μm),Ca(<4μm),4-16μm K,Ti (<4μm),Co(4-16μm)V(<4μm),Sr(<4μm),Ba(4-16μm),Pb(<4μm),C d(4-16μm),Sc(<4μm),Li (<4μm),Be(<4μm),Zr(<4μm),Y(<4μm),U(<4μm)are the characteristic elements to identify Changjiang sediments and Huanghe ones. Illite, chlorite, kaolinite and smectite are the main clay minerals of the sediments in Yangtze River Estuary and its adjacent sea area, in which their average contents are 67.55%,16.71%,12.51% and 3.23% respectively. In the surface sediments finer than 16μm, quartz, K-feldspar, feldspar, calcite and dolomite are the else main compositions, in which quartz11.26%(<4μm),32.15%(4-16μm), K-feldspar 4.55%(<4μm),4.18%(4-16μm), feldspar 2.98%(<4μm),10.84%(4-16μm, calcite 2.24%(<4μm),2.38%(4-16μm)and dolomite 0.99%(<4μm),1.58%(4-16μm)。In the surface sediment of this area that is finer than 4μm, the contents of the major elements range between 0.15- 9.45 %, existing as Al>Fe>K>Mg>Ca>Ti>Mn, while the contents of minor elements are about 0.3-469μg/g existing as Ba>Zn>V>Sr>Zr>Cr >Li> Ni >Cu>Pb>Y>Co >Sc >Th>U>Be>Cd. When the grain size is between 4-16μm, their averaged contents of major elements are about 0.07-12.19% and the decreasing order is Al>Fe>Ca>K>Mg>Ti>Mn, and minor elements 0.310-422μg/g with an order of Ba>Sr>Zn>Zr>V>Cr>Li>Ni>Pb>Cu> Y>Co>Th>Sc>U>Be>Cd. As the results show, the distributions of minor elements in this area can be divided into 5 (<4μm) or 6 (4-16μm) types.The content ofΣREE in the surface sediments, is about 137.47-282.52μg/g, averaged as 229.09μg/g (<4μm), or 144.1-265.5μg/g, averaged as 188.2μg/g (4-16μm). Chondrite-normalized patterns show that the fractionation patterns are right-declined, and LREE are obviously richer. Ce anomaly is very weak but Eu depleted. Both kind of the different grain sizes sediments have nearly even and slightly right-declined NASC-normalized patterns. The LREE are slightly enriched, and Ce depletion and Eu anomalies are both weak.According to the characteristic minerals, elements and cluster analysis, provenance patterns in this area can be classified into three modes corresponding to A, B, and C three areas. Area A is influenced by sediments come from modern Yangtze River, which is high in dolomite, low in calcite and Ca and Sr. Area B is influenced by sediments that come from old Yellow River which is characterized by high calcite, low dolomite and high Ca, Sr. Area C is the remain deposit area, especially high in calcite but low in dolomite, also high in Ca and Sr, which indicate that the sediments may be influenced by calcareous biological detritus. The above zoning clearly show that sediments from the old Yellow River carried by North Jiangsu coast current can travel through subaqueous Yangtze River Delta and get into Fujian and Jiangsu coast.