The Role of Sulfuric Acid in Continental Weathering: Insights From Dissolved Major Ions and Inorganic Carbon Isotopes of the Teesta River, Lower Brahmaputra System

Abstract

Chemical weathering in the Himalayan river basins plays a key role in the global carbon cycle that controls the climate. In this contribution, the spatial distribution of dissolved major ions and (inorganic) carbon isotopic compositions of the Teesta River, a major tributary of the Brahmaputra River, has been investigated to constrain solute sources, weathering patterns, and acids involved in these processes. Mixing diagrams and piper plots of major ions indicate a dominant solutes supply by weathering of silicates and carbonates. The average sulfate concentration of the Teesta samples (92 ± 47 µM) is significantly higher than that reported earlier for atmospheric deposition (∼5 µM) in this basin. The carbon isotopic compositions of the dissolved inorganic fractions (δ13CDIC) of the samples vary between −11.87‰ and −3.82‰ (mean: −7‰ ± 2‰). Comparison of δ13CDIC – SO4/(SO4 + HCO3) – HCO3/(Ca + Mg) trends with their corresponding values expected for H2SO4 and H2CO3 acid‐mediated weathering indicates that both these acids contribute to chemical weathering in the Teesta river catchment. Inversion modeling of major ions and δ13CDIC data have been used to quantify the impact of carbonic (H2CO3) and sulfuric acids (H2SO4) on the Teesta water chemistry. On average, the fractions of cations derived from H2CO3‐mediated weathering of carbonates (0.27 ± 0.14) and silicates (0.20 ± 0.09) are comparable with H2SO4‐mediated carbonate (0.38 ± 0.18) and silicate (0.15 ± 0.11) weathering in this basin. These results show that the amount of CO2 uptake (via H2CO3‐mediated silicate weathering) and release (via H2SO4‐mediated carbonate weathering) are in balance for the Teesta River.