Sedimentary Characteristics And Its Environmental Significance Of The Nianyou Lake In The Middle Reaches Of The Yangtze River In The Past 100 Years

Sample cores TE with a length of 85 cm, ZZ-2 with a length of 89 cm and CB-1 with a length of 91 cm were drilled in the deepest part of Tian’ezhou, Zhongzhouzi and Chiba oxbow Lakes by the Gravity Impact Core Sampler in Jingjiang section of the Yangtze River during November 2012. They were divided into sediment samples in which a set interval is applied to pick about every 1 cm. Dating of the 210 Pb, characteristics of the grain size, frequency dependent susceptibility and element contents(Al and Pb) were analyzed.Three cores disclosed the sedimentary characteristic and evolution process of oxbow lakes, the following conclusions are drawn: By the analyses of grain-size parameters and magnetic parameters of cores TE,ZZ-2 and CB-1, two stages of oxbow lake sediments have been distinguished. They were lower(layer A) which reflected fluvial facies, and upper(layer B) which reflected lacustrine facies.After combining local chronicles with environmental events, such as river/lake transformation events and flood events, which indicated by sediment characteristics, a new dating method with three steps was recommended:(i) According to the 210 Pbex specific activity changes, the average deposition rate was calculated by constant initial concentration(CIC), and the date range of the sediments was determined. The results in the cores of TE, ZZ-2 and CB-1 are 0.88cm/a, 1.26cm/a and 1.49cm/a.(ii) According to the average deposition rate and the diagnostic characteristics of the grain size and frequency dependent susceptibility, the possible sediment depth of the river/lake transformation event layer was found. Then deposition rate was checked by segments. The average deposition rate of layer B(lake sediments) and layer A(fluvial sediments) were obtained respectively before and after river/lake transformation events. In TE core the average deposition rate of layer B is 0.63cm/a and layer A is 1.07cm/a. In ZZ-2 core the average deposition rate of layer B is 0.89cm/a and layer A is 1.85cm/a.(iii) Cross-validation with historical flood events of known age. According to the flood events, age model was checked again.Three cores disclosed the lead enrichment records and its environment significancs of oxbow lakes. The results of the lead enrichment records show that, the characteristics of Pb unnatural enrichment in three cores are mostly identical. They all began in the early 1980 s and declined in the 2000. Based on the high-resolution age of deposition rate and environmental events, which was closely related with the lead pollution history such as the beginning of the lead pollution and 2000 which the year of leaded petrol has been banned in China, it is thought that the result is caused by human activity impacts. By comparing and contrasting three lead enrichment records of three sample cores, the similar trend of three curves were found. It is indicated that oxbow lakes’ river/lake sedimentary facies had no effects on the unnatural enrichment of Pb element. Based on the above study, it reveals that the possibility of using the unnatural enrichment events of Pb element as a time marker in the study area.