Study On Modern Geochemical Processes And CO₂Source And Sink Relationship Of Small Watershed Of Typical Hydrothermal Area In Southern Tibet Collision Orogenic Belt

The collision generated between India and Eurasian plate along Yarlung Zangbo River-Indian River in Southern Tibetan Plateau is one of the most far-reaching tectonic event since the late Mesozoic. Intense Magmatic, metamorphic and extensive geothermal activity formed the famous Himalayan large-scale geothermal fieldsin the region. Hydrogeothermal may lead to large amounts of CO2degassing. At the same time, Raymo and Ruddiman considered that the rapid uplift of the Himalayas and other places, would speed up the rate of chemical weathering, which consumed large amounts of CO2, and finally reduced the global temperature. A very important scientific issue is that whether Rapid uplift of the Qinghai-Tibet plateau is the CO2"sources" or "sink", which has caused many scholars’attention. Using a variety of hydrogeochemical methods, many scholars have conducted extensive research on the issue, and the new achievements constantly emerging, t accumulating. However the issue remained unresolved. Therefore, the strengthening of carbon source and sink in the region is significant for further analysis of this issue. At the same time, as the "third pole", it is the "drive" and "amplifier" of global climate change, researchs realate to CO2in the region are important components of regional response to global change.Two more typical of hydrothermal activity area-Qupu and Targejia are investigated in the region. Based on analysising the geochemical processes in geothermal-small watershed, we try to study the carbon sources and sinks. The results follows:(1) Chemical characteristics of the waters in the study area are controlled by the regional geological background, some similar chemical composition in different types of waters, there are also some differences. The water, such as lakes, rain water, surface water, have low salinity, Ca2+.Na HCO3=-, Cl-are the main ions, is characterized as freshwater and HCO3-Ca type. Based on Gibbs and Ca/Na values, the chemical characteristics of the water in Daggyaima basin is controlled by the weathering of the silicate rock, especially the feldspar of the area..The water temperature is22.8～95.0℃, pH ranges from6.67to8.63, electrical conductivity (EC) varies from 1500to1869μS/cm in Qupu geothermal field. The water temperature is22.8～95℃, pH range between6.67and8.63, EC range between1500and1869μS/cm in Qupu geothermal field. Targejia spring water temperature is from67.0to86.5℃, pH is7.3～8.4, EC is2820～4310μS/cm. HCO3-,C1-k+, Na+are dominated in the water, therefore the hydrochemistry types is Na-HCO3. the Na/Cl value is6.07～7.67in Qupuand and3.83～6.81in Targejia, indicating that the two springs have experienced a strong water-rock interaction. In General, thehydrochemistry types of the two springs are almost the same with with most water in geothermal Southern Tibet.(2) The evolution and geochemical processes in the hydrothermal system are controled by the parent rock and the water-rock interaction between the hot water and surrounding rocks in the study area,. The deuterium and oxygen isotopes indicated that the source of ground and surface water comed from atmospheric precipitation. The hot water supply altitude is4109～5924m, and the depth of hot water circulation is617～917m according to the deuterium and oxygen isotopes. Based on activity diagrams, the spring water of Targejia is mainly affected by the weathering of feldspar and potassium feldspar, however Qupu Springs is mainly controlled by the weathering of MICA minerals. In the study area, the mineral saturation index (SI) of anhydrite, gypsum, SiO2(a), chrysotile asbestos is below0, the SI of calcite and dolomite is basically between-0.5～0.5, and the SI of quartz, chalcedony is above0, indicating that the sediments of hot water may be dominated by Silicon. By simulating the hot water of the main springs at100℃and120℃, we found that calcite, dolomite, chlorite, talc deposited more strongly in the crack pipe and deep system than in surface layer, but jade, quartz and feldspar showed a contrary trend. Based on the LogPCO22vs HCO3diagrams, the Pco2of the water will decreas from deep to the surface, indicating that large amounts of CO2gas released.(3) Using Fick’s first law, we preliminarily estimate the CO2degassing flux value of soil is0.244kg/m2.a in Targejia, the jet holes are1.7499kg/m2.a. The CO2degassing flux values of Qupu hydrothermal soil are0.394kg/m2.a, that of the jet holes is3.417～7.457kg/m2.a. Based on water hydrochemistry, the weathering rates of silicate,evaporated salt and carbonate in small-watershed is3.74t/km2.a,4.09t/km2.a,0.89t/km2.a respectively. Using The overall average CO2consumption of the rocks weathering estimated by the cation method in small watershed is155.26X103mol/km2.a in Daggyaima Co basin.