Constraining the sources and cycling of dissolved inorganic carbon in an alpine river, eastern Qinghai-Tibet Plateau

It is generally acknowledged that riverine dissolved inorganic carbon (DIC) behaviors play a critical role in global carbon cycling and hence have an impact on climate change. However, little is known about the intricate DIC dynamics under various meteorological conditions in the alpine areas. Here, we investigated DIC biogeochemical processes in the Bailong River catchment, eastern Qinghai-Tibet Plateau (QTP), by combining measurements of major ions, stable and radioactive isotopic compositions of DIC (δ¹³C_DIC and Δ¹⁴C_DIC), and physiographic parameters in the Bailong River catchment. Statistics and stoichiometry analyses suggest that multiple biogeochemical processes could affect carbon cycling in the Bailong River catchment. The "old" DIC with low Δ¹⁴C values (-472.4 ± 127.8 ‰, n = 3) and stoichiometry analysis of dissolved ions showed clear evidence that carbonate weathering is primarily responsible for water chemistry in the upstream (elevation >2000 m). However, upstream samples showed that δ¹³C_DIC increased between 5 ‰ and 11 ‰ from the theoretical mixing line, concomitant with increasing pH and decreasing pCO₂, suggesting that isotopic fractionation of DIC due to CO₂ outgassing may be the primary cause of the increased δ¹³C_DIC values. Additionally, the higher Δ¹⁴C values (-285.4 ± 123.5 ‰, n = 12) in the downstream region below 2000 m suggest that allochthonous modern carbon had a great impact on DIC variations. The presence of younger DIC may have important implications for the interpretation of inorganic carbon age in downstream rivers. Our study demonstrates that physiographic conditions can regulate DIC behaviors, which can improve estimations of carbon yield and comprehension of global carbon cycle.