Characteristics Of Carbon And Nitrogen Deposition In Plateau Lakes And Traceability Of Their Sources

In the southeast of Qinghai-Tibet Plateau,we collected sediments from the center of Cuopu Lake,which located in Batang County,Ganzi Tibetan Autonomous Prefecture,Sichuan Province in May 2017.After treatment,45 sediment samples were chronologized by 210Pb method.The carbon(TC?IC),nitrogen(TN,NH4-N,NO3-N),n-alkanes and sedimentaty pigments,including chlorophyll and its derivatives(CD),Natural chlorophyll(NC),Oscillaxanthin(Osc)and Myxoxanthophyll(Myx)were detected respectively,we analyzed the variation of their contents with depth and the distribution characteristics of their different components.It was show that the sedimentation rate,flux,carbon-nitrogen deposition characteristics of sediments,mid we also use the total organic carbon to total nitrogen ratio and n-alkanes to analyze the source of organic matter in Cuopu Lake.Cluster analysis and multi-source mixing model for estimating contribution rates of different sources to n-alkanes.Finally,the changes of sedimentary pigments content and their effects on lake were analyzed.The indicators of productivity and sedimentary environment provide some theoretical basis for the tourism development in the area where the Cuopu Lake is located.The main conclusions are as follows:(1)Using 210Pb dating method,the sedimentary age of sediment samples collected from Lake Cuopu is 1841?2017.The range of sedimentary rate is 0.056-1.42 cm a-1,and the average is 0.337 cm a-1,and the overall trend is upward with time;The sedimentary flux is 0.088-0.21 lg cm-2 a-1,and the average value is 0.13g cm-2 a-1,which fluctuates greatly.The sedimentary flux decreases in the small ice age,and after the temperature rises,it remained stable.(2)Total organic carbon(TOC)content ranged from 1.37 mg g-1 to 75.81 mg g-1,with an average value of 36.76 mg g-1;TOC deposition rate ranged from 0.7 to 15.6 mg Cm-2 a-1 and the average value is 6.03 mg cm-2 a-1.The concentration of TOC remained low until 1920,then increased abruptly,and overall increased with time generally.The variation of TOC is dominated by temperature,and the low value corresponds to the small ice age(1850s-1910s),which increases rapidly after the temperature increases.The mutation of TOC is closely related to grazing activities around the lake.Total organic nitrogen is the main form of nitrogen existence in sediments of Cuopu Lake.The total nitrogen concentration ranges from 0.3 mg g-1 to 17.9 mg g-1,with an average value of 7.18 mg g-1.The trend of variation is similar to that of TOC.Total nitrogen deposition rate was lower than that of TOC,ranging from 0.16 to 3.75 g cm-2 a-1 with an average of 1.195 g cm-2 a-1,which rising fluctuations with time.The concentration of TN in Cuopu Lake is much higher than that in eutrophic lakes on the plateau.(3)The concentration of ammonium nitrogen(NH4+-N)ranged from 4.3 to 30.1 mg kg-1 with an average of 10.6 mg kg-1,and the nitrate nitrogen(NO3--N)ranged from 5.8 to 117.8 mg kg-1 with an average of 31.7 mg kg-1.The concentration range of tatol organic nitrogen(TON)in sediments from 0 to 17.9 mg g-1,with an average value of 7.1 mg g-1.The change of organic nitrogen is highly synchronous with organic carbon,which indicates that the source of organic matter in sediments is relatively single.(4)The variation range of TOC/TN value in sediments of Cuopu Lake is 1.6?16,and most of them are between 2 and 8,indicating that the organic matter in sediments of Cuopu Lake is mainly endogenous.The earbon distribution of n-alkanes in sediments is C10?C33,and Cmax are high carbon(C24?C31)which most are C25,and the total concentration of n-alkanes is far below than other plateau lakes.According to the analysis of molecular indicators of n-alkanes substitution,the source of organic matter in Cuopu Lake is mainly phytoplankton and submerged plants,while the input of terrestrial higher plants is less.The contribution rate of algae and submerged aquatic plants was calculated by using the characteristic source from cluster analysis and multi-source mixing model.The results were consistent with the analysis results of molecular indicators of substitution of n-alkanes.The contribution rate of submerged aquatic plants was more than 60%,the contribution rate of algae was less than 50%,and the fitting effect of the model was good(R2=0.85).(5)CD content showed a general downward trend with time;average of NC is 27.8%and the surface NC value was more than 40%which means the preservation of pigments was better.Osc and Myx values showed a highly consistent trend,and there was no sudden increase in history.Pigments have better preservation effect and higher content in surface sediment.NC is not correlated with CD but Osc and Myx.NC has no correlation with CD and there was no significant difference between algae contribution rate and total pigments Content by one-way ANOVA,which indicates that pigments can reflect the primary productivity of lake.According to the change of CD values,the productivity of Cuopu Lake was decreasing as a whole,while the decrease is not significant.(6)The changes of sedimentary pigments indicated that the water depth of the lake decreased and algae in lake are mixed algae,cyanobacteria did not dominate the biological community.The changes of Osc/Myx and C17,C23 values indicate that lakes are poor-nutrient lakes but have a trend of eutrophication in history.The high content of TOC and TON and the low value of Osc/Myx in surface sediments indicate that Cuopu Lake has a trend of eutrophication in recent years.