Dissolved inorganic carbon isotopes of a typical alpine river on the Tibetan Plateau revealing carbon sources, wetland effect and river recharge

The Nyangqu River, the largest right bank tributary of the Yarlung Zangbo River in the Qinghai-Tibet Plateau, was representative of an alpine riverine carbon cycle experiencing climate change. In this study, dissolved inorganic carbon (DIC) spatial and seasonal variations, as well as their carbon isotopic compositions (delta C-13(DIC)) in river water and groundwater were systematically investigated to provide constraints on DIC sources, recharge and cycling. Significant changes in the delta C-13(DIC) values (from -2.9 parts per thousand to -23.4 parts per thousand) of the water samples were considered to be the result of different contributions of two dominant DIC origins: soil CO2 dissolution and carbonate weathering. Three types of rock weathering (dissolution of carbonate minerals by H2CO3 and H2SO4, and silicate dissolution by H2CO3) were found to control the DIC input into the riverine system. In DIC cycling, groundwater played a significant role in delivering DIC to the surface water, and DIC supply from tributaries to the main stream increased from the dry season to the wet season. Notably, the depleted delta C-13(DIC) 'peak' around the 88.9 degrees longitude, especially in the September groundwater samples, indicated the presence of 'special' DIC, which was attributed to the oxidation of methane from the Jiangsa wetland located nearby. This wetland could provide large amounts of soil organic matter available for bacterial degradation, producing C-13-depleted methane. Our study provided insights regarding the role of wetlands in riverine carbon cycles and highlighted the contribution of groundwater to alpine riverine DIC cycles.

Automated summary

The Nyangqu River in the Qinghai-Tibet Plateau is representative of an alpine riverine carbon cycle experiencing climate change. Significant changes in the delta C-13(DIC) values of water samples were attributed to different contributions of soil CO2 dissolution and carbonate weathering. Groundwater played a significant role in delivering DIC to surface water, and the contribution of tributaries to the main stream increased from dry season to wet season. Notably, a depleted delta C-13(DIC) 'peak' around 88.9 degrees longitude indicated the presence of 'special' DIC attributed to nearby wetlands.