The Response Of The Oxoyan Lake Sediment To Human Activities In The Middle Reaches Of The Yangtze River In Recent 100 Years

With two oxbow lake- Zhongzhou and Tian’ezhou as its study object,which is located in the areas along the middle reaches of the Yangtze River,and taking the methods of field investigation,comparison among the sedimentary piles,drilling lacustrine deposits core in the centre of lake,laboratory analysis of sediments,as well as the human activities information of land use change in this basin,soil erosion and the pollution of industry and agriculture,this paper focuses on exploring such issues as the human events whose records were kept in the oxbow lake deposits. The conclusions are as follows:1、Sediments in ZZ-2 is mainly consisted of fine silt and clay and other components were less.Magnetic minerals mainly exist in medium silt(16~32μm),magnetic susceptibility values and the average particle size showed a positive correlation.All segments of grain size and magnetic susceptibility corresponding well:when the sediment grain size is coarser,it’s clay content is lower,silt content is higher,and the magnetic susceptibility value is higher relatively;On the contrary,when the grain size is finer,it’s clay content is relatively higher,medium silt content is less,and the magnetic susceptibility value is lower.2、The magnetic and particle size indices in sediments of ZZ-2 have a good response to soil erosion events which caused by human activities(deforestation)。Since the foundation of China to the late 1970 s,large-scale blind reclaiming land from lakes and deforestation resulted in the highly intensity of soil erosion and more coarse particles were brought into the oxw lake.At this time,sediment particle size of ZZ-2 was coarse, clay group content were little,silt content was high,and the magnetic susceptibility values were also high.In the eighties of the 20 th century,although land use intensity was still high,but forest conservation policies were successfully implemented,such as closing hillsides to facilitate afforestation and returning farmland to forest.The loss of soil and water had been well controned.Compared to the previous period, the particles which brought into the lake were finer,the content of clay group in sediments was higer,silt content was less,and the magnetic susceptibility values were also lower.Due to the sampling site of TE is on the apex of oxbow lake which has a high bending rate,the magnetic and particle size indices in sediments of TE have a good response to chemical weathering degree of parent material in basin.When Na/K ratio of sediments is high, It indicates that the weathering degree of catchment surface is low,a large number of incompletely weathering coarse particles were brought into the lake,caused coarser sediment particle size,less clay fractions and higher magnetic susceptibility values.3、There is a same process between metal pollution history of Pb、Zn、Mn in two oxbow sediments and development of industry.After correction by element Al,It reveals that the turning point is at the middle of 1970 s.TE and ZZ-2 in sediments of Pb, Zn, Mn pollution history and basin development of industrial agreement. For Pb, Zn, Mn ratio curves and Al elements of the Al elements after correction, the turning point in the middle of 1970 s.Before the middle of 1970 s,Shishou was dominated by small workshops and individual handicraft industry, industrial development is slow and elements in sediments were mainly from natural source.After the mid-1970 s,with the rapid development of Shishou industry,The wastewater of industry and automobile exhaust emissions have reached the peak at 1980 s to the end of 1990 s.4、There is an obvious increase of element ratio in sediments compared with the previous period.At the beginning of the 21 st century, with the measurements such as energy-saving emission reduction, the establishment of sewage treatment station and the banning of the implementation of the heavily polluted enterprises and monitoring vehicle exhaust emissions well implemented,the industry of Shishou city had a steady development.Then content of elements Pb、Zn and Mn in lake sediments began to decrease.The element Pb in tow oxbow is mainly from the automotive and lead ship exhaust as well as industrial wastewater.Electroplating, non-ferrous metal smelting industry is the main source of pollution of element Mn and Zn in the lake.In addition,the increased amount of coal can also increase the Mn pollution in the lake.4、The nutrient elements(TN,TP and TOC) in sediments have a good response to industrial and agricultural pollution in this basin.TOC and TN had no correlation or lightly correlation with TP,which indicated that N and P were mainly from natural source,and both two oxbow lake were medium or lightly eutrophicated before the 1980 s.After 1980,TOC and TN had a significant positive correlation with TP,and they also had the same trend,which indicated that the two lakes were polluted by anthropogenic activities.With rapid development of industry of Shishou City,A large number of wastewater emissions from industries and agricultures were discharged into lake,and fertilizer loss had become the main form of agricultural non-point source pollution of the lake.Although Shishou began to limit emissions of industrial wastewater at the beginning of the 1990 s,nutrient content in the upper part of the oxbow lake is still high as the endogenous pollution caused by shallow lakes. Therefore, The eutrophication problems in two oxbow lake is still exist.To solve this problem,It also needs to control and manage the internal nutrient in lakes.