『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 on 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.
Keywords: Deccan basalt; carbon isotopes; silicate weathering』
1. Introduction
2. Hydrogeology of the river basins
3. Sampling and analyses
4. Results
5. Discussion
5.1. Dissolution of atmospheric CO2
5.2. Weathering of silicate
5.3. Carbonate weathering
5.4. δ13C in DIC of Deccan rivers: silicate/carbonate
weathering
5.5. Sources of carbonates to DIC
6. Conclusions
Acknowledgements
References