| Carbon cycle,as a significant interconnection part of the earth's spheres,is one of the key sections to the global climate change and geochemical cycles.The Qinghai-Tibetan Plateau,as a hotspot of global change research,playing an important role in global climate change as well as carbon cycles between sea and land,which has attracted many attentions and interests from the scientists.River is one of the most significant components of global carbon cycle and water cycle research.On the one hand,the hydrochemical characteristics of rivers could reflect the water-rock interactions and the patterns of transformation of elements in the river basin to some extent.On the other hand,the degassing(CO2)phenomenon of rivers is common.Carbon dioxide(CO2),escaping from the river water and participating in the carbon cycle actively,making the river be an important channel for connecting the terrestrial,marine and atmosphere ecosystems as three carbon pools and the key link of biogeochemical cycles.Therefore,studying the carbon cycle of rivers on the Qinghai-Tibetan Palteau is not only related to the world problem of the balance between uplift of Tibetan Plateau and atmospheric CO2,but also the hotspot of land-sea interaction under the changes of river's material flux,which is also of great significance to know the role of Qinghai-Tibetan Plateau in global carbon cycle and ecological climate change.Furthermore,it also provides the scientific theoretical basis for sustainable development of the water resource on the Qinghai-Tibetan Plateau.As a typical plateau river on the Qinghai-Tibetan Plateau,the Yarlung Zangbo River is less affected by human activities.Thus,it can reflect the geochemical characteristics of the plateau river more truthfully.In this research,the Yarlung Zangbo River basin on the plateau was taken as the study area,the representative months of different seasons(dry season and the flood season)were selected for field investigation and sampling for laboratory testing.The hydrochemical characteristics,chemical weathering rate and CO2 absorption flux of the Yarlung Zangbo River basin were analyzed.Meanwhile,the carbon fluxes,carbon transport and transformation,the degassing effect of CO2 at water-air interface and carbon cycle model of the river basin were also discussed.The main study conclusions are as follows:(1)Water of the Yarlung Zang'bo River was mainly derived from the meteoric precipitation and glacial snowmelt.The temporal and spatial distributions of hydrogen and oxygen isotope values of the river water reflected the evaporation effect of the river as well as the elevation effect and continental effect of the local meteoric precipitation.The tritium excess values showed the different water vapor sources of different seasons and the characteristics of river water affecting by the water-rock interaction.(2)The Yarlung Zangbo River water was generally alkaline and its hydrochemical type was Ca2+-HCO3-type,which mainly influenced by the rock weathering.According to the mass balance method,the contribution of each end-member to river water composition can be arrayed as:carbonate source(56.23%)>silicate source(23.66%)>evaporative source(16.91%),indicating the dominative effect of chemical weathering of carbonate rock.The average contribution of atmospheric precipitation was least.(3)The average rock weathering rate and CO2 consumption rate of the Yarlung Zangbo River basin was 42.12 t/km2/a and 407.16×103 mol/km2/a,repectively,which were relatively higher comparing with that of other large rivers in the world.The annual average CO2 consumption flux of the Yarlung Zangbo River basin accounted for about 0.36%of the total CO2 consumption flux of global rock weathering,showing strong chemical weathering rate and CO2 consumption rate in the plateau basin,which is related to the continuous exposure of tresh rocks due to its unique geology and climatic conditions in the plateau.(4)The average dissolved inorganic carbon concentration(DIC)and carbon dioxide partial pressure(pCO2)of the Yarlung Zangbo River were 2.05 mmol/L and 460.4 ?atm,respectively.The pCO2 was at a lower level comparing with that of other large rivers in the world.DIC was mainly derived from the rock weathering and the soil CO2.The quantitative analysis of inorganic carbon stable isotope(?13CDIC)showed that about 60.51%of DIC came from the carbonate rock and the remaining DIC(39.49%)was from soil CO2 during the flood season.The average dissolved organic carbon concentration(DOC)and particulate organic carbon concentration(POC)were 0.15 mmol/L and 0.19 mmol/L,respectiveliy,which were mainly derived from the extraneous sources.Due to the poor soil and low temperature in the watershed,the content of organic carbon was relatively low,which was about 17%of the DIC concentrations.Therefore,the main form of carbon in the Yarlung Zangbo River was DIC.The carbonate system,dominated by high DIC concentration,was the main factor influencing the pCO2 in the river.(5)The floating chamber method and thin boundary layer model of water chemistry were both used to calculate the CO2 exchange flux(FCO2)at the water-air interface,which showed a certain degree of consistency.The FCO2 value calculating from the floating chamber method was adopted as the true value and it showed that the average FCO2 values in the dry and wet seasons were 2.95 mmol/m2.d and 8.59 mmol/m2.d,respectively,indicating that the Yarlung Zangbo River acted as the "weak source" of atmospheric CO2.The CO2 exchange flux was at a lower level compared with that of other large rivers worldwide,showing the weak decarbonization effect of Plateau Rivers.(6)Statistical ananlysis indicated that many factors affected the CO2 exchange flux of the Yarlung Zangbo River.Among them,the carbonate equilibrium system,dominating by the rock weathering,was the maj or influence factor.The barren soil of basin and low dissolved organic carbon(DOC)concentration showed that bilolgical effect had relatively less impact on CO2 exchange flux of the Yarlung Zangbo River.(7)Based on the above analysis,the carbon cycle model of the Yarlung Zangbo River basin was constructed for the first time.It showed that Yarlung Zangbo River transported 90.45%of carbon to lower reaches of the plateau.Among them,DIC accounted for the major part(about 79.50%).The overall degassing effect was weak(1.07%)and the CO2 consumption flux consumed by the river basin was about 51 times of CO2 emission flux from the water body,which showed the significant carbon sink effect in the Yarlung Zangbo River basin and revealed the different carbon cycle system and mechanism of the Plateau Rivers.(8)According to the previous research,the soil CO2 and primary productivity are usually considered to be the major factor driving CO2 supersaturation in most rivers.However,the carbonate system with high DIC concentration,which dominated by the strong rock weathering effect in the plateau,was the major factor controlling CO2 concentration and emission in the Yarlung Zangbo River.The relatively weak biological decarbonization effect and carbon sequestration effect were mainly due to the weak biological effect in the plateau.Therefore,the proportions of degassing of CO2 and organic carbon burial were much lower that that of other inland waters in the world.The Yarlung Zangbo River acted most likely a"pipeline"and transported the biogenic matters providing from chemical weathering of the plateau basin to the downstream and even the ocean.Hence,it palyed an important role on the material replenishment and ecological environment of the low reaches.In addition,the significant rock weathering effect in the river basin,causing by the unique climate and geological conditions of the Qinghai-Tibet plateau,playing an important role in regulating and balancing the atmospheric CO2 concentration as well as the global ecological climate change,which might also supported the hypothesis of"climate change driven by tectonic lift"to some extent. |
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