Organic Carbon And Nitrogen Isotopic Characteristics Of Tengchong Qinghai Lake Sediments During Last 1700 A And Paleo Environmental Reconstruction

Lake as the important basin gathering catchment water which collects various materials,including organic matters,pollutants and nutrients,and thus it is abundant in biological and physico-chemical information.Through the comprehensive analysis of various environmental parameters recorded in lake sediments,we will be able to effectively understand the hydrological characteristics,ecological environment,and pollution status for lake and its catchment,which is important to know the lake environment evolution history and forecast future environment changes.The crater lake in Tengchong area of southwest Yunnan was selected as the research object in this study.The organic carbon isotopics,nitrogen isotopics,C/N ratios,TOC and TN content in the samples of Qinghai Lake columnar sediments were selected.By analyzing and researching the variation characteristics of various environmental proxy indicators in the sediments,environmental meaning indicated,Source of organic matter and palaeoclimatic evolution of in Qinghai Lake 1700a and using the terminal element analysis model,combined with ²¹⁰Pb and ¹³⁷Cs chronology and ¹⁴C test results,the study analysing.Research results show that:?1?The experimental results of various environmental indexes of Qinghai Lake sediments show that the value of organic carbon isotopic(?¹³C)is between-26.966‰^-28.983‰,and the mean value is-27.967‰.The variation range of the organic nitrogen isotopic(?¹⁵N)ranged from-0.191‰to 4.01‰,with an average of 1.898‰.Total organic carbon?TOC?ranges from 3.235%to 9.794%.Through correlation analysis with different particle sizes,the study found that the lake qinghai sediments between TOC content and composition of clay showed obvious negative correlation,and silt composition exist strong positive correlation,and there is a positive correlation between sand composition.The variation range of total nitrogen?TN?was 0.129%⁰.387%,with an average value of 0.246%.It was found that C/N value rangesfrom is wide between 24.536³0.134,and Volatility is the largest and most volatile.?2?Through analyzing the various environmental parameters recorded indicated by environmental significance,and the study found that:the Qinghai Lake sediment's TOC and TN has high correlation,and on the change trend is highly collaborative,therefore,their common reflects the primary productivity of lake level,namely,TOC and TN of high value represents the primary productivity of lake,on the other hand,represents the primary productivity is low.The ratio of C/N reveals that the organic matter in Qinghai Lake is dominated by endogenous input.Through the analysis,the results show that the value of the lake?¹³C is corresponding to the cold period of the climate,while the negative is the warm period.?¹³C and?¹⁵N has good correlation?r=0.81?,therefore,can be thought of?¹³C and?¹⁵N have the same environmental implications,namely?¹⁵N biased on behalf of the warm,moist climate,on the other hand,it is person of environmental conditions.In addition,because of Qinghai Lake unique geological conditions and ecological environment,through the study of sediment grain size parameters,the results show that the regional rainfall is the main factors influencing the sediment particle size distribution,when relatively humid climate condition,the rise of water level in lake,give priority to with fine particles.On the contrary,during the dry period,the drainage area is scarce,the lake is shrinking,and the sediment grain size is coarse.?3?In order to explore the source of organic matter in Qinghai Lake,qualitative analysis was made on the source of organic matter in sediment samples.The results showed that the organic matter of sediments was mainly composed of lakes,and the contribution rate of phytoplankton was the largest,and the proportion of terrestrial organic matter was very small.For small interference by human activities of Qinghai Lake,?¹⁵N changes over time from high value reduced to minimum,especially since the 1950 a,?¹⁵N value is close to the delta N₂ concentration in the atmosphere,may be related to lake basin of vegetation and soil subsidence of reactive nitrogen component fractionation effect,make the lake water body in the delta of?¹⁵N change;The process of gradual partial negative of the value of?¹³C May be related to the continuous increase of temperature,the decrease of the lake's hardness,the increase of primary productivity of the lake,and the absorption of CO₂ dissolved in water after the large number of aquatic plants have multiplied.?4?Based on the analysis of the variation characteristics of various indicators,the climatic features of 1700a in Qinghai Lake were divided into four stages:302⁵24 a AD,and the climatic features of Qinghai Lake were generally cold and humid.524¹072 a AD,in general,the cold and wet ecological environment rapidly changes to warm and semi-humid climate characteristics.1072¹950 a AD,Qinghai Lake is warm and dry climate characteristics;In the period of 1950²017a AD,compared with the previous period,precipitation showed a downward trend and the temperature continued to rise.