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

• Main Authors: Das, Anirban, Krishnaswami, S., Bhattacharya, S. K.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier Science 2005
• Subjects: Carbonic acid
• Online Access: http://repository.ias.ac.in/16890/; http://linkinghub.elsevier.com/retrieve/pii/S0012821X05003183

Chemical weathering of silicate and carbonate rocks by carbonic acid is important in the context of atmospheric CO 2 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 δ 13 C-Si / HCO 3 systematics of the waters. δ 13 C in the samples ranges from - 8.5% to - 20.7% and shows a strong linear inverse trend with Si / HCO 3 (r 2 = 0.80). The Si / HCO 3 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 δ 13 C-Si / HCO 3 trend suggests the mixing of two end members, a carbonate derived endmember with low Si / HCO 3 and enriched in 13 C and a silicate derived endmember with higher Si / HCO 3 and depleted in 13 C. Small rivers of the western ghats are depleted in 13 C (δ 13 C: - 18.7 ± 2%) and have high Si / HCO 3 . These properties are interpreted as signatures of basalt (silicate) weathering with CO 2 from C 3 vegetation. Three samples from the Krishna mainstream and two of its larger tributaries, the Bhima and the Ghod, are enriched in 13 C (δ 13 C: - 8.5% to - 15%) with higher HCO 3 and lower Si / HCO 3 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.