Estimation of weathering rates and CO_2 drawdown based on solute load: Significance of granulites and gneisses dominated weathering in the Kaveri River basin, Southern India

摘要

The solute load of the Kaveri River (South India) and its tributaries draining diverse Precambrian terrains during pre-monsoon and monsoon periods was determined. Using average annual flow, total drainage area and atmospheric input corrected major ion concentrations of these rivers chemical weathering rates, annual fluxes of different ionic species to the ocean and CO_2 consumption rates were estimated.Bicarbonate is the most dominant ion (27-79% of anion budget) in all the river samples collected during monsoon period followed by Ca~(2+), whereas, in case of pre-monsoon water samples Na~+ is the most dominant ion (in meq/l). Two approaches were adopted to estimate silicate and carbonate weathering rates in the drainage basin. At Musuri silicate weathering rate (SWR) is 9.44±0.29tons/km~2/a and carbonate weathering rate (CWR) is 1.46±0.16tons/km~2/a. More than 90% of the total ionic budget is derived from weathering of silicates in the Kaveri basin.CO_2 consumption rate in the basin for silicate weathering FCO_2sil is 3.83±0.12×10~5mol/km~2/a (upper limit), which is comparable with the Himalayan rivers at upper reaches. For carbonate weathering (FCO2carb) CO_2 consumption rate is 0.15±0.03×10~5mol/km~2/a in the Kaveri basin. The lower limit of CO_2 consumption rate corrected for H_2SO4 during silicate and carbonate weathering is FCO_2sil is 3.24×1005mol/km~2/a and FCO_2carb 0.13×10~5mol/km~2/a respectively. CO_2 sequestered due to silicate weathering in the Kaveri basin is 25.41 (±0.82)×10~9mol/a which represents 0.21 (±0.01)% of global CO_2 drawdown. This may be due to tropical climatic condition, high rainfall during both SW and NE monsoon and predominance of silicate rocks in the Kaveri basin.