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

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 knew approach based on delta C-13-Si/HCO3 systematics of the waters. delta C-13 in the samples ranges from -8.5 parts per thousand to -20.7 parts per thousand and shows a strong linear inverse trend with Si/HCO3 (r(2) = 0.80). The Si/HCO3 ratios in rivers vary from similar to 0.55 to similar to 0.10, the higher value matches the expected ratio for chemical weathering of Deccan basalts based on their composition. The delta C-13-Si/HCO3 trend suggests the mixing of two end members, a carbonate derived endmember with low Si/HCO3 and enriched in C-13 and a silicate derived endmember with higher Si/HCO3 and depleted in C-13. Small rivers of the western ghats are depleted in C-13 (delta C-13: -18.7 +/- 2 parts per thousand) and have high Si/HCO3. These properties are interpreted as signatures of basalt (silicate) weathering with CO2 from C-3 vegetation. Three samples from the Krishna mainstream and two of its larger tributaries, the Bhima and the Ghod, are enriched in C-13 (delta C-13: 8.5 parts per thousand to -15 parts per thousand) 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. (c) 2005 Elsevier B.V. All rights reserved.