Response Mechanism Of Chemical Weathering To Climate Change In The Upper And Middle Reaches Of The Yarlung Tsangpo River

In recent years,global warming has become a universally recognized fact,especially in the Tibetan Plateau,known as the"world’s third pole".For the alpine basin with strong sensitivity to climate factors,the composition and change of the ion source of river water under the background of climate change is an important factor leading to the change of chemical weathering of river.In order to study the change characteristics of chemical weathering in the alpine basin and its response mechanism to climate change,the middle and upper reaches of the Yarlung Tsangpo River were selected as the study area in this paper,and the water chemical data of the middle and upper reaches of the Yarlung Tsangpo River in March 2019,September 2019,May2020 and May 2021 were sorted out through field observation,sample collection and laboratory measurement.Based on the hydrochemical model,climate model and stable isotope technology,the seasonal differences of chemical weathering in the peak and dry periods of the basin were clarified,the ion contribution rate and chemical weathering rate of different weathering types were calculated while the source of river ions was identified,and the response mechanism of chemical weathering to climate change in the middle and upper reaches of the Yarlung Tsangpo River was revealed.The research results are of scientific significance for understanding the changes of chemical weathering in the alpine basin under the combined action of multiple factors under the background of climate change.The main research conclusions are as follows:（1）The water sample data in September 2019 were taken as the data of the middle and upper reaches of the Yarlung Tsangpo River during the wet season,and the average of the data sampled three times in March 2019,May 2020 and May 2021were taken as the data of the dry season.The ion sources,influencing factors and seasonal differences of chemical weathering were analyzed in depth.The TDS concentration of the river water in the period of abundance and drought did not change much,but there was a difference in p H.The p H value of the river water in the period of abundance and drought showed a weak acid,while the river water in the period of dry water showed a weak alkaline.The main water body in the basin is HCO₃^--SO₄^2--Ca²⁺.Na⁺and K⁺ions are mainly derived from evaporative karst decomposition,Ca²⁺and Mg²⁺are mainly derived from gypsum dissolution and weathering of carbonate rocks,HCO₃^-is mainly derived from chemical weathering of carbonate rocks,and SO₄^2-is mainly derived from the dissolution of gypsum and glauber’s nitrate.Weathering of carbonate rocks dominated the chemical weathering process in the middle and upper reaches of the Yarlung Tsangpo River,and the chemical weathering rate of carbonate rocks in the middle and upper reaches of the river basin was higher than that of silicate rocks in both dry and wet periods,and the CO₂consumption flux of carbonate rocks was also higher than that of silicate rocks.The CO₂flux consumed by weathering of carbonate rocks in the middle reaches of Yarlung Tsangpo River reached 16.119×10⁴mol/（km²·month）during wet season.（2）Ca²⁺and HCO₃^-have been the dominant ions in the middle and upper reaches of the Yarlung Tsangpo River in recent 50 years.The weathering of silicate rocks and the input of atmospheric precipitation have little effect on ions in the basin,and the weathering of carbonate rocks has been dominant.In 1990,the weathering rates of silicate rocks and carbonate rocks in the upper and middle reaches of the Yarlung Tsangpo River both reached the maximum,which were 3.92t/（km²·a）and2.73t/（km²·a）,respectively.Meanwhile,the amount of CO₂absorbed by chemical weathering also reached the maximum.Generally speaking,the temperature in the basin is on the rise,and the minimum temperature is the fastest increasing rate.The distribution of annual precipitation in the middle and upper reaches of the Yarlung Tsangpo River is extremely uneven.The atmospheric precipitation has a greater influence on the variation trend of the wet season and a smaller influence on the dry season.Both temperature and precipitation affect the chemical weathering process of the basin,and precipitation affects the weathering process and rate of the whole basin mainly through influencing the weathering of carbonate rocks.The future climate change and chemical weathering of the basin were predicted under three different climate scenarios:RCP2.6,RCP4.5 and RCP8.5.When calculating chemical weathering rates and CO₂consumption in the typical years 2030 and 2050,only RCP4.5 is the ideal greenhouse gas emission scenario at present.（3）Due to the influence of water vapor transport and the natural environment and climate conditions of the upper and middle reaches of the Yarlung Tsangpo River valley,theδD andδ¹⁸O values of river water and rainwater in the basin vary widely,and theδD andδ¹⁸O values are generally greater than those of other basins or lakes in China.Due to the unique climatic and geographical characteristics,deuterium surplus(d-excess=δD-8δ¹⁸O)in the upper and middle reaches of the Yarlung Tsangpo River is lower than the global mean precipitation.The equation lineδD=8.24δ¹⁸O+11.63 fitted by the least square method is consistent with the global atmospheric precipitation line,which is an important supply source of the middle and upper reaches of the Yarlung Tsangpo River.Sr²⁺ions and Ca²⁺ions in river basin are the same source,which are mainly provided by weathering of carbonate rocks and silicate rocks.The ⁸⁷Sr/⁸⁶Sr values of the upper and middle reaches of the Yarlung Tsangpo River range from0.70766 to 0.71542,and the ⁸⁷Sr/⁸⁶Sr values of the weathering products of silicate rocks are higher than those of carbonate rocks.There is almost no significant fractionation of Sr isotopic composition in a series of geochemical processes,and only material exchange and water-rock interaction caused by physical processes can significantly change ⁸⁷Sr/⁸⁶Sr,which makes the contribution rate of river water ions calculated by inversion model more accurate.