Carbon flux and C, S isotopic characteristics of river waters from a karstic and a granitic terrain in the Yangtze River system

Abstract

We analyzed dissolved major elements, δ13CDIC and δ34SSO4 in river waters from a karstic and a granitic terrain to examine weathering processes and the influence of basin geology on water quality and associated riverine alkalinity transport. Results show that waters from the karstic terrain have higher total dissolved solids (TDS), major ion concentrations and δ13CDIC values but lower K+, Na+ and Si concentration and δ34SSO4 values than that from the granitic terrain. The compositions of major ions and isotopes indicate that the solutes of the granitic terrain are controlled by processes of silicate and carbonate dissolution by carbonic acid. However, carbonate dissolution by both carbonic and sulfuric acid are dominant processes in karstic terrain. Total erosion rates, dissolved inorganic carbon (DIC) flux and CO2 uptake were 28.0tkm−2yr−1, 2.9×105molkm−2yr−1 and 2.3×105molkm−2yr−1 for the granitic terrain and 70.2tkm−2yr−1, 11.9×105molkm−2yr−1 and 6.4×105molkm−2yr−1 for the karstic terrain, respectively. These results indicated the karstic area plays an important role in uptake of CO2 and DIC flux along the Yangzte River Basin. Our results also revealed that CO2 uptake due to carbonate chemical weathering in both karstic and granitic terrain is very high and provide as much or more DIC flux than that from silicate chemical weathering.