| As the largest river in China,the Changjiang provides a large amout of water and sediment supply to the East China Sea.And the suspended sediments from the Changjiang are the main source of the distal mud deposition in the continental shelf of the East China Sea.therefore,the variation in the sediment load of the Changjiang catchment will inevitably exert important influences on the sedimentary environment of Changjiang subaqueous delta(CSD).However,due to the impounding of the Three Gorges Dam(TGD)in 2003,the sediment load from the Changjiang has been undergoing tremendous changes,including sediment origin and compositions.Above changes might result in far-reaching impacts on the sedimentary dynamic environment of CSD.Furthermore,the resuspension and transport of fine-grained sediment in the CSD also may vary,responding to the Changjiang catchment change.In order to study changes of the dynamics environment of CSD,the Changjiang estuary and coastal waters are selected as the study area.In order to trace the resuspension and transport of sediments in the study area,the 210Pb activity in the sediment,which is easy to be adsorbed by fine-grained sediments,is used as the tracer to investigate the change of sedimentary dynamic environment.In this study,87 surface samples and 22 sediment cores were collected in the study area in September-October,2014.In addition,seven water samples were collected from the main river of the Changjiang in July 2015.Subsequently,the 210Pb activity analysis and particle size analysis were implemented.In order to compare the spatial distribution and vertical distribution characteristics of 210Pb activity before and after 2003,the data of the distribution of 210Pb activity in surface sediments and sediment cores before 2003 were also collected from published literatures.The 2'0Pb activity in suspended sediment of the main river of the Changjiang exhibits a decreasing trend in the downstream direction.In addition,the 210Pb distribution of surface sediments before 2003 showed an increasing trend from the Changjiang mouth to the sea,and the mean value of 210Pb activity was about 15 dpm/g.However,after 2003,the distribution of the 210Pb activity converted to homogenization trend,and the averaged 210Pb activity decreased to 1.9 dpm/g(reduced by 87.3%compared by that before 2003).The vertical distribution of 2'0Pb activity in the sediment core before 2003 showed a significant decay model in the vertical direction;whereas,the vertical distribution of 210Pb activity in the sediment cores after 2003 exert uniform distribution,and the total 210Pb activity is closed to the value of supported 210Pb activity.These changes demonstrated that,the depositional environment is stable before 2003;however,after 2003,the depositional environment became unstable,and the dynamic mixing clearance is enhanced.Compared with the vertical distribution of 210Pb activity in the sediment cores from same sites,the210Pb activity is reduced by more than 50%;whereas,the mixing effect in different stations were recorded,and the decay curves also displayed uniformity trends.Above changes imply that,the mixing effect of the CSD was increased,thereby bring about tremendous challenge to the dating technology using 210Pb.thus,the sediments supply reduction of the Changjiang catchment enhance the mixing effect of the sedimentary environment in the CSD.Meanwhile,the increasing mixing effect in the core sediments results in uniformity of the vertical distribution curves of the 210Pb activity in the core sediments,resulting in more difficult of the calculation of the deposition rate. |
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