Soil carbon stock of the Central-East Asia in the climate warming

Abstract

Water samples from the Wujiang River, a typical karst river system, were analyzed for major ion concentrations and ~34S values of dissolved sulfate in order to identify the sources of sulfate, quantify the sulfate export flux and understand the role of sulfur cycling in chemical weathering rate of carbonate. Spatial variations in sulfate concentration and sulfur isotopic composition of tributaries over the catchment area are obvious, allowing to decipher S sources between rocks and atmosphere. According to the variations in sulfate concentration and isotopic composition, it is inferred that sulfate ions in the upper-reach river waters may have three sources, rain water, sulfate resultant from oxidation of pyrite in coal, and sulfate from sulfide deposits. In the lower reaches, the S isotopic composition of the samples lies mainly on a mixing trend between evaporite sulfate and rainwater sulfate, the contribution of sulfate from oxidation of pyrite being lesser. A pronounced seasonal variation in both content and isotopic composition of sulfate characterizes the Wujiang River. The average sulfate concentration of the waters is 0.65 mmol/L in winter, 0.17 mmol/L higher than that in summer. River water ~348 values range from -15.7%0 to 18.9%0 in winter, while the 634S values of river waters in summer vary to a lesser extent than in winter, from -11.5%o to 8.3%0. The 834S values of the main stream range from -6.7%0 to -3.9%0 in summer, averaging 3%0 lower than in winter. This indicates that in summer, when the discharge increases, the contribution of a source enriched in light isotopes to the atmosphere or the oxidation of pyrite in coal is more important. The sulfate export flux of the Wujiang River is calculated to be about 172 x 101~ g/a, the export flux in the high-flow period accounting for 80% of the total. Sulfide oxidation (sulfuric acid production) and subsequent weathering of carbonate are important processes in the study area. We determined that in summer about 75% of the riverine sulfate in the Wujiang River could be produced by sulfide oxidation. The relative contributions from rainwater and evaporite sulfate are only 20% and 5%, respectively. As a result, the erosion rate of carbonate by sulfuric acid is on the order of 35.1 t/km2/a (17.5 mm/ka) and the CO2 consumption rate is reduced approximately by 3.66• 105 mol/krn2/a.