Study On Chemical Weathering And Carbon Sink In Xijiang Basin

"The carbon sink effect" caused by chemical weathering of rivers is one of the main components of global atmospheric missing sinks,and also an important part of global carbon cycle research.Taking the main tributaries,main streams and two sub-basins of the Xijiang River Basin as the research objects,Based on regular,high-frequency hydrochemistry,isotope composition and functional bacteria sampling,testing and analysis in the study area,this paper focuses on water cycle(main control factor of river weathering carbon sink),residual dissolved organic carbon(stability of karst carbon sink)and other factors restricting carbon sink effect.Finally,the end-member ratio and final flux of chemical weathering carbon sinks in the Xijiang River basin are estimated based on the study of the Xijiang River basin.The main research results are as follows:(1)In different rainfall-hydrological processes,the controlled factors of river chemical weathering process are different in hydrological processes,but the water cycle control is dominant,and the dynamic change process is similar.1)During three different monitoring periods,inorganic carbon sources were mainly from CCW,CSW and SCW,and the carbon pump effect of aquatic plants could be neglected in the case of large rainfall,flow and low water temperature.2)During the heavy rain-flood process,for the dynamic tribute of inorganic carbon sources,DIC originated from CSW was relatively low and maintained at about 2.6%.The contribution of carbonate to total inorganic carbon was negatively correlated with the exogenous acid,the former was three times as much as that of the latter.During the rainstorm and extreme flood process,the contribution rate of CSW to DIC of three sections increased significantly except for peak period,but the variation of other periods was small,indicating that the inorganic carbon produced by weathering carbonate by silicic acid was relatively stable.The trend of CCW contribution to DIC is similar,showing that the contribution rate increases with the flow and reaches the maximum before flood peak,indicating that karst development is strong in the study area.Rainfall enters into karst pipelines and fissures in various forms and reacts quickly to reach the maximum threshold.The change trend of SCW and CCW contribution rate is similar.The dynamic contribution rate of CCW to DIC varies significantly in the light rain-level water process,indicating that carbonation may be affected by water cycle,degassing and biophotosynthesis.3)The dynamic contribution rate and inorganic carbon flux were calculated by mass balance method.In three different rainfall-hydrological processes,due to the influence of water cycle,the contribution rates of DIC fluxes produced by CCW,CSW and SCW are similar on the whole,and show different changes in special time periods.The contribution rate of DIC flux produced by CSW is basically the same in the three rainfall-hydrological processes.Except for a small increase with the increase of discharge in the process of rainstorm-extreme flood,it is basically stable,indicating that the increase of discharge has little effect on the intensity of chemical weathering of silicate rocks.The contribution rate of DIC fluxes produced by CCW and SCW in the two flood processes is roughly consistent with the flow,indicating that water cycle is the main factor affecting the strength of carbonate weathering;some of the samples and the general variation law are inconsistent,indicating that in addition to the dilution and piston effect caused by water cycle,the sources of exogenous acids and the changes of HCO3-concentration are also obvious influencing factors.The inorganic carbon fluxes produced by CCW,CSW and SCW in three sections during heavy rain-flood and rainstorm-extreme flood process are not significantly different,and the inorganic carbon fluxes produced by the three sources account for a large proportion of the total inorganic carbon fluxes(2.68%-6.63%)in the whole year during the monitoring period.During light rain-flat water process proves that the water cycle is the main factor affecting the weathering of carbonate.(2)Establish the method of residual dissolved organic carbon in Karst water,and analyze the characteristics of AAPB and its effect on residual dissolved organic carbon.1)The redox potential(Eh),dissolved oxygen(DO)and chlorophyll of the water in the study area promoted the trend of bacteria and AAPB abundance.In addition,the bacterial abundance in different seasons was affected by different environmental factors in the study area.From the vertical direction,the bacterial abundance of water samples in each season was the highest at-5 m.2)Establishment of residual dissolved organic carbon testing method suitable for Karst area,found that water body in the study area were afected by bacteria over time,the content of organic carbon gradually fluctuated and decreased,gradually stabilized,and tended to 2mg·L-1 in the later period of culture.With the increase of incubation time,the degradation rate gradually decreased.During the first month of incubation,the average degradation rate of organic carbon in water was about 0.05 mg C.day-1 in Jun and Dec,and 0.27 mg C.day-1 in Sep.3)The threshold value of residual dissolved organic carbon in the water body of the study area is about 2 mg·L-1.When the threshold value is greater than this threshold value,the bacteria in the water body metabolize and degrade organic carbon until its content reaches the threshold range.On the contrary,when the organic carbon in the waters of the study area is less than the threshold range,the bacterial action will maintain the organic carbon in the waters within this threshold through its own regulation.Relative abundance of organic carbon degradation and AAPB showed that the higher the relative abundance of AAPB,the slower the degradation of organic carbon by bacteria.This indicated that the presence of AAPB slowed down the degradation rate of organic carbon by bacteria,made more organic carbon stay in water and increased the cycle of organic carbon in Karst water.(3)Analysis of hydrogeochemical characteristics and calculation of inorganic carbon flux of chemical weathering in Xijiang River Basin1)In order to establish the spatial relationship between Sr and its isotopes and their sources in Xijiang River Basin,we should grasp the differences of Sr and its isotopes caused by the different sources of materials from the spatial characteristics;Further explore the relationship between Sr and its isotopes and the sources of materials,and select semi-variance model and path model simulation to carry out corresponding research.2)The results of semi-variance model and path model show that structural factors play an important role in flood season,while structural and stochastic factors play a dual role in Sr and its isotopes in dry season.Sr and its isotopes differ in material sources between high and low water periods.Structural factors in high water period are dominated by weathering and dissolution of silicate rocks,while weathering and dissolution of dolomite in low water period are dominated by both structural and random factors.3)The structural and stochastic factors of Sr and its isotopes in high and low water periods are analyzed by the path model.The material sources of Sr and its isotopes are different in high and low water periods.The weathering and dissolution of silicate rocks play a dominant role in high water period,while the weathering and dissolution of dolomite play a dominant role in low water period under the dual effects of structural and stochastic factors.4)Selecting an improved inversion model to calculate flux and end members.This method can avoid the influence of traditional human activities and subdivide the end members of each component.It is suitable for calculating CO2 flux and end members in complex stratigraphic and lithologic regions.5)Through discussing the sensitivity of the precipitation end of the improved inversion model,it is found that(1)Although the river water in the study area is affected by oceanic movement to some extent,the main factors affecting the spatial variation of Sr and its isotopes are structural factors.The effect of the local environment is easily neglected when the oceanic factors are considered solely in the precipitation end member;(2)The climate in the study area is characterized by the same period of water and heat.There are some differences in the content of substances between wet and dry seasons.It is easy to neglect the differences by means of calculation.In addition,sensitivity testing involves two different end member components of silicate rocks.It is found that when the inversion model gives the end member components a larger range,the better the results are obtained.If the composition of end member is too narrow,the result may be inconsistent with the actual situation.6)The results of model operation show that the basic element ratios of carbonate in high and low water periods are almost the same as Sr isotope values,while the Sr isotope values of silicate rocks are quite different.There are some differences in the sources of cations in Xijiang River Basin during in high and low water periods:(1)During high water period,rainfall contributes about 15%(0-62%)of the total dissolved cations,and 5%in the low water period.(2)Dolomite accounted for only 5%(0-37%)of the total dissolved cations in the high water period,while 27%(0-54%)in the low water period.(3)The cations input of limestone to Xijiang River water during high water period and low water period is 66%(22%-95%)and 54%(37%-85%)respectively.(4)The cations input of silicate rocks is 14%(4%-21%and 5%-15%).Limestone is the main source of water cations in Xijiang River.The material input of silicate rocks is relatively stable,and the cationic input of precipitation and dolomite end members is quite different in high and low water periods.7)The results show that the ?CO2 in the high and low water period are 306.21(0.00-54058.12)103 mol/km2/yr and 1096.61(12.31-10694.45).103 mol/km2/yr.The ?CO2 in high water period is about 5 times of that in low water period.The assumed average value of ? is about 15%different from that of ?CO2,which indicates that the water system of Xijiang River is mainly affected by carbonation weathering,while sulfuric acid plays a secondary role.The optimum weathering rate of silicate rocks is 7.264-35.551 103 mol/km2/yr,and carbonic acid is the main factor of weathering.8)According to the actual situation of this paper,the calculation methods of silicate and carbonate fluxes and residual dissolved organic carbon fluxes are determined,and the final carbon sink fluxes and the proportion of end member are calculated.During the study period,the CO2 flux consumed by the lithochemical weathering in xijiang basin was 171.21 × 109 mol/yr,of which 120.84 × 109 mol and 50.37 × 109 mol in the high and low water periods,accounted for 70.58%and 29.42%of the total fluxes respectively.The difference in flux is similar to that in Lijiang river basin,and the ratio of flux and flow is similar(mean monthly flow is 9879.47 km3 in high water period,while that in low water period is 2813.62 km3,the former is 3.51 times higher than that in the latter).Therefore,water cycle is the dominant factor in the carbon sink effect in Xijiang River basin.Inert organic carbon flux accounts for 11.99%of the total flux,indicating that biological action plays an important role in carbon sink effect.The flux of CO2 produced by SCW is 23.23 × 109mol CO2/yr,and the effect of sink reduction is obvious.Compared with the predecessors,the final flux,the carbon flux produced by CCW and CSW are all in one order of magnitude.The main innovations of this paper are embodied in:Analysis of microbial indicators,establish a practical method for monitoring residual dissolved organic carbon in Karst areas;the monitoring of organic carbon content in karst surface rivers during flood/rainstorm period was carried out to study the content,source,variation characteristics and carbon flux of organic carbon in flood process.Based on the hydrochemical indices,Sr and its isotopes in different sub-basins of Xijiang River basin,combined with different models,discussed the errors of rock weathering caused by human activities,and refined the process of carbon element in large basin from "inorganic carbon?organic carbon?residual dissolved organic carbon" was clarified,quantified CO2 fluxes corrected by Sr isotopes and the proportion of different end member sources in carbon flux from chemical weathering was identified.