Effect of exogenous water and acid on karst carbon sink in the Wanhuayan underground river system

The petrochemical weathering process in the basin is an important part of the global carbon cycle.In recent years,increasing attention has focused on the effects of exogenous water (silicate weathering) and exogenous acid on the global carbon cycle in the estimation of hydrochemical carbon sink fluxes in the watershed.In this study,the Wanhuayan underground river system is selected as the study area,where the proportions of silicate rock distribution area and carbonate rock distribution area are 64% and 36%,respectively.In 2017,a one-year sampling monitoring was carried out at the entrance of the cave,and the ion composition of 13 water points in the Wanhuayan underground river system was monitored in April and September,respectively.The chemical weathering rate and CO2 consumption flux of the basin rocks were calculated by using the hydrochemical equilibrium method and Galy model.The process of rock weathering and carbon cycle in the system was analyzed.The results show that the rate of CO2 consumption by rock weathering in the Wanhuayan underground river system is 31.02 t?（km2?a）-1,with dominant carbonate weathering which is 20 times that of silicate rock dissolution.Moreover,carbonate weathering accounts for 92.16% of CO2 consumption in the whole basin.The contribution rate of different rock weathering types to carbon flux is 87.06%,of which carbonate dissolved carbonate rocks is the largest.The external water in the upper reaches of the basin has a great role in promoting the karst carbon sink. After the external water imported,the carbon sink rate of carbonate rock can be twice that of the non-external water sink in the basin.Sulphuric acid dissolved carbonate rocks took the second place,accounting for 9.24%;carbonate weathered silicate rock is the smallest,which is 3.7%.The influence of sulphuric acid on rock weathering should be removed when calculating carbon sink in river basins.