The Chemical Weathering And Carbon Biogeochemistry In The Bailongjiang Catchment On The Eastern Qinghai-Tibetan Plateau

Climate change plays a significant role on the global biogeochemical cycle.Water and carbon cycles connect different Earth’s spheres,transfer energy and lay the foundation of matter cycle.As a link between marine and terrestrial ecosystems,rivers are important channels for migration and sites for transformation of materials.Studies on the water and carbon cycles of the river basin are conducive to understanding terrestrial biogeochemical processes.As one of the headwater of the Yangtze River,the Bailong River locates on the eastern margin of the Qinghai-Tibetan Plateau.This area is sensitive to global and/or regional climate change.Thus,the study on the water and carbon cycle of the Bailongjiang catchment will reveal great significance for assessing the regional sustainable development of ecosystems and understanding the mechanism of climate change around the Qinghai-Tibetan Plateau.The Bailongjiang catchment is an ideal area for the study of geochemical processes in the Qinghai-Tibetan Plateau because of the large variations of natural geographical conditions and the limited influence of human activities.In this study,we conducted the field geological and geomorphological investigations of the main stream and five tributaries of the Bailong River,and spatially collected 100 series of river water samples,and analyzed the hydrochemical composition,hydrogen and oxygen isotopic composition,the concentration of different forms of dissolved carbon（DIC,DOC）and their stable isotope(¹²C,¹³C)and radioisotope(¹⁴C)compositions,and UV absorption spectrum analysis.Through analysis mentioned above,we comprehensively studied the hydrological process,hydrochemical characteristics,chemical weathering characteristics and carbon cycle processes in the Bailongjiang catchment.The main results are as follows:（1）The hydrogen and oxygen isotopic composition of the river water in the Bailongjiang catchment varies largely.δ¹⁸O andδD values vary in range of-13.2～-8.6‰and-88.0～-56.8‰,respectively,which shows a good positive correlation(δD=8.4δ¹⁸O+18.9).The hydrogen and oxygen isotopic compositions of water samples are essentially consistent with those of global and local meteoric water,suggesting that the main source of river water in the basin is meteoric water.The correlation between deuterium excess（d）andδ¹⁸O demonstrates effects of both marine and terrestrial air masses.In addition,there is an obvious negative correlation between altitude and the waterδ¹⁸O:H=-559.3δ¹⁸O-3927.8,suggesting that the spatial variation of hydrogen and oxygen isotopic compositions is obviously affected by geographical factors.（2）The pH value of the river water in the Bailongjiang catchment varies from 7.5to 8.7 with an average of 8.4,being weak alkalinity.The mean value of total dissolved solids in the water（320 mg/L,varying from 136 to 2068 mg/L）is higher than that of global rivers（100 mg/L）.The dissolved anions and cations in the river water are mainly HCO₃^-and Ca²⁺,having concentrations of 84.3～328.8 mg/L and 25.6～220.3 mg/L,respectively.The results of forward modeling show that rock weathering is the main source of the solute in river water.The average contribution proportion of different types of rocks to river water are carbonate rock（76.7%）,evaporite rock（8.0%）and silicate rock（7.8%）,whereas the atmospheric precipitation contributes～7.5%.The mass balance calculation reveals that the chemical weathering rates of carbonate rock and silicate rock evaporite are 51.1 t/（km²·yr）and 5.2 t/（km²·yr）,respectively.And the CO₂ consumption rates of carbonate rock and silicate rock are 4.1×10⁵ mol/（km²·yr）and 1.3×10⁵ mol/（km²·yr）,respectively.（3）The average DOC concentration in the river water of the Bailongjiang catchment is 1.4 mg C/L,and the average DIC is 38.5 mg C/L,both of which show a decreasing trend with the decrease of altitude.δ¹³C_DOC andΔ¹⁴C_DOC results demonstrate that the soil organic matter formed by the decomposition of near surface modern C₃plants is the main source of DOC.UV absorption spectrum analysis also confirms that DOC mainly comes from terrestrial organic matter.δ¹³C_DIC andΔ¹⁴C_DIC results show that the sources of DIC are mainly the CO₂ decomposed from soil and CO₂ formed by carbonate weathering.The variation range of CO₂ partial pressure（p CO₂）in the river water of the Bailongjiang catchment is 363.3～1819.5μatm with an average of 620.0μatm,significantly higher than the global and regional atmospheric CO₂ partial pressure,which indicated that the local river release atmospheric CO₂ into the region.The correlation analysis between the content and isotopic composition of DIC and DOC shows the low temperature at high altitude and the weak photosynthesis of water body.In summary,factors such as lithological differences,topographic changes and climatic conditions are important to control water cycle,chemical weathering and carbon cycle in the Bailongjiang catchment.The unique geographical and climatic environment of the Qinghai-Tibet Plateau promotes regional chemical weathering,which plays an important role in regulating atmospheric CO₂ concentration and then affecting global climate change.Chemical weathering products are transported to the downstream and even the ocean through river"pipelines",which participate strongly in the construction of downstream ecosystem and provide the supply of nutrients.This study of water cycle,chemical weathering and carbon cycle in the Bailongjiang catchment is of great significance to understand the regional matter cycle and the impact of the uplift of Qinghai-Tibetan Plateau on the global biogeochemical cycle.