Carbon isotope ratio of dissolved inorganic carbon (DIC) in rivers draining the Deccan Traps, India: sources of DIC and their magnitudes

Abstract

Chemical weathering of silicate and carbonate rocks by carbonic acid is important in the context of atmospheric CO₂ sequestration and its delivery to the oceans via rivers as dissolved inorganic carbon (DIC). In this work, silicate weathering contribution to DIC in the Krishna river system, draining the Deccan basalts in south-western India has been determined using a new approach based on δ¹³C-Si / HCO₃ systematics of the waters. δ¹³C in the samples ranges from - 8.5% to - 20.7% and shows a strong linear inverse trend with Si / HCO₃ (r² = 0.80). The Si / HCO₃ ratios in rivers vary from ~0.55 to ~0.10, the higher value matches the expected ratio for chemical weathering of Deccan basalts based on their composition. The δ¹³C-Si / HCO₃ trend suggests the mixing of two end members, a carbonate derived endmember with low Si / HCO₃ and enriched in ¹³C and a silicate derived endmember with higher Si / HCO₃ and depleted in ¹³C. Small rivers of the western ghats are depleted in ¹³C (δ¹³C: - 18.7 ± 2%) and have high Si / HCO₃. These properties are interpreted as signatures of basalt (silicate) weathering with CO₂ from C₃ vegetation. Three samples from the Krishna mainstream and two of its larger tributaries, the Bhima and the Ghod, are enriched in ¹³C (δ¹³C: - 8.5% to - 15%) with higher HCO₃ and lower Si / HCO₃ ratios, indicating that a significant fraction of DIC in these samples is derived from carbonate weathering. Carbonate dispersed in river sediments can be a likely source of DIC.