The Reconstruction Of Evolution Processes Of Sedimentary Environment Lately In Lake Hulun Based On High Resolution Sedimentary Indicators

A lake,as a relatively independent system,undergoes a long geological history.The continuous deposition coupled with a faster deposition rate can provide abundant information about regional environmental,climatic and human activities in sediments,which has become an important carrier in global climate change research.This s tudy was conducted on the sediment of Lake Hulun,total organic carbon?TOC?,total organic nitrogen?TN?,carbon and nitrogen ratio?C/N?,carbon isotopic composition(?¹³C)in organic matter,total phosphorus?TP?,grain size,and the hydrogen??²H?,oxygen isotopes(?¹⁸O)and chloride ions in pore water of the sediment were analyzed.In combination with the established sediment core chronology using²¹⁰Pb and ¹³⁷Cs measurements,the evolution of the lake environment in recent60 years is reconstructed.Meanwhile,comparing the analysis of the environmental indicators of sediments with the measured data,the indication about environment,climate,and human activities of the lake basin is determined by using indicators of the sediments in the Lake Hulun,and identify effectively the natural and anthropogenic factors caused environmental changes.The main conclusions of this study are as follows:?1?The ²¹⁰Pb_exx results show that high resolution sediment core?41 cm?contains sedimentary record from 1958 to 2015 in the past 58 years using the CIC model,and the sedimentary rate is 0.72 cm/a.This chronology is consistent with the results obtained by ¹³⁷Cs,which shows two significant markers in 1963 and 1975.?2?The results of proportion of land-derived organic matter and lake-derived organic matter in lake sediment was calculated based on C/N ratios and?¹³C values,which shows the proportion is 80%for land-derived organic matter and 20%for lake-derived organic matter in Lake Hulun sediment.?3?The evolution of Lake Hulun can be separate into two periods based on the different variations between the s edimentary indicators((TOC,TN,C/N,?¹³C,TP,C_al,C_au).The lake was in a low nutrition level and the water was in a good quality status due to the limited low TN and TP concentrations before 2000.However,Lake Hulun enhanced growth of algae due to suf ficient nutrients?TN and TP?supply,thus the lake had been suffered from eutrophication after 2000,and the water quality was deteriorated.?4?Comparing the increased processes in land-derived organic matter content and lake-derived organic matter content,which shows the land-derived organic matter increased in 1980's caused by unreasonable grazing and the lake-derived organic matter increased after 2000 due to the decresed river discharge and reduction of water level.The moment of lake change is consistent with the reduction of water level in Lake Hulun in 2000,hence the reduction sharply of water level because of decreased rivers flow could be primary and direct factor caused lake eutrophication and water deterioration.?5?Comparing the sediment grain size distribution in dated sediment core with the peak and valley values of average rainfall changes,which shows the peak and valleys are quite consistent in the same years,.Thus,the changes of grain size in lake sediment can reflect the changes of rainfall and runoff in the short temporal resolution.?6?The grain size parameters of Hulun Lake sediment cores show significant differences at depth of 32-38 cm and other parts,which could effective record the Hulun Lake dam construction event due to the period sediment chronology at depth of 32-38 cm corresponds to the period of dam construction in Hulun Lake.During the blocked period of lake outlet,the large particles was very difficult to transfer to the center of the lake due to the low hydraulic gradient.Furthermore,the small particles can not flow out to the because of no outflow,which resulting in increased clay content and decreased the content of sand in sediment.?7?The analyses and model results of chloride and water stable isotopes show that the solutes in sediment pore water have changed due to the solu tes exchanged with the solutes in overlying lake water by diffusion processes,therefore the solutes in pore water can not be the indicators same as others unchanged indicators stored in the lake sediment.However,the positive values of stable isotopic compositions and high concentration of chloride in lake sediment pore water reflect the low water level state,thus the solutes in lake sediment may supply some useful information for interpreted the past lake environment.