Analysis Of The Deposition, Mineralization, Burial And Coupling Factors Of Organic Carbon In Lake Sediments

The lake carbon cycle is an important part of the global carbon cycle.Changes in lake biological composition and nutrient levels directly affect the deposition,mineralization and burial of organic carbon in lake sediments.Although the content of organic carbon in sediments can be accurately measured at present,the organic carbon in sediments is continuously mineralized within a certain period of time,and the content of organic carbon in sediments changes dynamically.Therefore,the quantitative evaluation of lake organic carbon deposition,mineralization and their coupling factors is of great significance for inverting the history of lake sediment organic carbon and predicting its future changes.In this study,15 sedimentation columns were collected from three lakes（Dianchi Lake,Fuxian Lake,and Cuopu Lake）in the Yunnan-Guizhou Plateau and Qinghai-Tibet Plateau,and 751 sedimentary samples were obtained;and 24 samples from the indoor culture experiment.A total of 24 indicators in 7 categories,including isotopes,various types of carbon,various nutrients,pigments,short-chain isoprene（ISO）,n-alkanes,and diatoms,were measured,and 37,037 raw data were obtained.This study determined the quantitative relationship between the source of organic carbon and the amount of organic carbon buried by analyzing the current status of organic carbon burial in sediments and performing source analysis.The lake organic carbon history was inverted by constructing a dynamic model of organic carbon deposition and mineralization（BMR-OC）.The history of organic carbon deposition and mineralization in sediments,and the future changes of buried organic carbon are predicted.The key coupling factor TN/IP（Total nitrogen/inorganic phosphorus）and total nitrogen（TN）were selected.The quantitative coupling relationship between TN/IP and TN and organic carbon deposition,mineralization and burial was analyzed.Finally,a system for dynamically evaluating the amount of organic carbon deposition,mineralization and burial in lake sediments was formed,and the quantitative coupling relationship between nutrients and organic carbon in the sediments was analyzed.The main results are as follows:（1）There is an obvious coupling relationship between the burial of organic carbon in lake sediments and the concentration of nutrients.Higher nutrition level in lake water or in sediments is beneficial to the burial of organic carbon in lakes.The average organic carbon concentration in the sediment samples of Cuopu Lake,an oligotrophic lake with high total nitrogen concentration(up to 16 mg g^-1),reached 39.39 mg g^-1,which is much higher than that of the eutrophic lake Dianchi(25.2 mg g^-1)and oligotrophic lake Fuxian Lake(8.84 mg g^-1).（2）The source of organic carbon quantitatively affects the burial of organic carbon in sediments.The factors separated by source analysis methods can quantitatively simulate the amount of organic carbon buried in lake sediments.The burial coefficients of different factors are significantly different.The burial coefficients of the four factors in Cuopu Lake are 0.01,24.43,3.00 and 6.63 respectively.A few factors can explain the burial of most organic carbon in the sediments.（3）The deposition,mineralization and burial of organic carbon in sediments are a dynamic whole linked by time.By constructing a BMR-OC dynamic model,the deposition,mineralization history and future evolution of organic carbon in the sediment can be simulated.Analysis of the degree of fit of the model results showed that the fitting coefficient R between the simulated value and the measured value was about 0.93（n=651）.Sensitivity analysis shows that the model has a certain degree of error tolerance to the dating data in the measured data,and is sensitive to the deposition coefficient and mineralization potential in the model coefficients.It can well reflect the deposition and long-term burial of organic carbon in lake sediments.（4）The amount of organic carbon deposited and buried in different lakes has changed significantly in recent decades.Compared with 1949,by 2000,the organic carbon deposition in Dianchi Lake increased by 40%,and the long-term burial increased by 60%;the Cuopu Lake organic carbon deposition increased by 195%,and the long-term burial increased 208%;the Fuxian Lake organic carbon deposition dropped by 15%,and the long-term burial dropped by 23%.（5）The deposition and mineralization characteristics of organic carbon from different sources in lake sediments are obviously different.The mineralization of organic carbon mainly from bacteria and algae mainly occurs within one year after sedimentation,the mineralization of organic carbon mainly by aquatic plants mainly occurs between 1 and 20 years after sedimentation,and the mineralization of organic carbon mainly from land sources Occurs between 10 and 100 years after deposition.（6）Nutrients in sediments couple the contribution rate and contribution amount of end-source factors through the link of"nutrients→end-source factors→organic carbon",and then couple the amount of organic carbon deposition,mineralization and burial in the sediments.The end-source factors are divided into background factor（BF）and response factor（RF）through feature judgment.The TN/IP has an exponential relationship with the contribution rate of RF（RF/（BF+RF））.The TN has a linear relationship with the contribution amount of RF.The change per unit of TN in Dianchi Lake leads to a change in the contribution of 2.2 units of RF.Changes in BF and RF can explain more than 80%of the changes in organic carbon deposition and mineralization and more than 90%of changes in organic carbon burial in the sediments of Dianchi Lake.