| In the earth's surface system, the chemistry weathering of rocks is a major controlling factor of the balance of longterm atmospheric CO2. Usually when the chemical weathering processes happen between carbonate or silicate rocks and carbonic or sulfuric acid in the terrestrial ecosystem, the weathering based on carbonic acid acts as a sink for atmospheric CO2 while weathering based on sulfuric acid acts as an origin. Because of the difference about regional geology, this kind of CO2 origins is also different, which means in some cases where geology is controlled by carbonate rocks and influenced by sulfide minerals, this sink-origin effect should get more attention. Also, as an important pool of atmospheric precipitation and surface runoff, rivers largely reflect the environmental characters of their drainage areas by river water hydrochemical characters. Therefore, this study chose tributary rivers of the Hongfeng Lake in Middle Guizhou Province of China as the research objects. The rivers belong to a typical karst small watershed, whose terrane is composed by mostly carbonate rocks as well as some silicate rocks and sulfuric coal minerals. Plus, this watershed is a typical acid precipitation area, which has large concentrations of SO42- in rainwater, which could also impact local chemical weathering. According to our analyse of major ions compositions and C, S stable isotopes, we got some cognition about regional chemical weathering characters with the participation of SO42-, which are:(1) Middle Guizhou watershed has a serious condition of atmospheric acid precipitation. Based on information collecting, we calculated the Ca2+ and SO42- contribution rates of river-water in Middle Guizhou watershed from atmospheric precipitation as 5.63% and 5.4% respectively. The samples of rivers in Hongfeng Lake watershed have SO42- concentrations mostly between 295.5～ 1315.6μmol/L, and some between 2323～2866μmol/L as a result of influence by coal minerals. The oxidation of sulfide minerals contributes 63.81% of the SO42- ions in river water while evaporate and atmospheric input contributes 30.79% and 5.4% respectively.(2) Because of distinctness of the fractionation of 13C isotope, we can trace the origins of carbon element easily by theirδ13C values. We processed the data of major ions concentrations, DIC (dissolved inorganic carbon) concentrations and theirδ13C values as well as the relations among them and we figured out that generally, the major ions in the rivers are controlled by both carbonic and sulfuric acid based weathering; the carbon isotopic compositions can mostly be explained by participations of soil CO2 and weathering of carbonate rocks, while other origins and artificial inputs may cause several deviations. It's also apparent that besides carbonic acid based weathering, sulfuric acid based weathering process is another influencing factor about the characters of major ions concentrations and stable isotopic compositions. This effect act more importantly in Taohuayuan River, which is influenced by coal minerals the most seriously, while the other three tributaries are not so much influenced.(3) Analyses about stable sulfur isotopes show that besides local chemical weathering, the atmospheric input also controls the SO42- concentrations andδ34S compositions in the river water of Hongefng Lake watershed. Taohuayuan River shows an obvious differentiation about sulfur concentration and isotopic composition to the other tributaries, which reflects the impact of the sulfur-concentration coal minerals at the origin of the river. Comparatively, Houliu River, Maxian River and Yangchang River mainly show a sulfur character with chemical weathering and atmospheric input influence.(4) In the related domestic and foreign studies, people showed many different ways to calculate the chemical weathering rates. In this study, we used former experiences and existing models as references, and based on the data information we've got about major ions concentrations and stable isotopic compositions, built a new calculation system of carbonate weathering rates. According to the calculation based on this system, we worked out the weathering rates of both carbonic and sulfuric acid based weathering and therefore their contributions to local carbon assimilation and outputting respectively. By the mass conservation operation, we figured out that DIC concentration from weathering between sulfuric acid and carbonate is 4.9909mmol/L, from weathering between carbonic acid and silicate is 2.7339 mmol/L, and from weathering between carbonic acid and carbonate is 7.1206 mmol/L. In other words, the rates of DIC from SCW, CCW and CSW counts 1:3.58:1.37, the total DIC flux is 2.195×1010 molC/a, that is 13.75×106 molC?km-2?yr-1. We can get a inference from these data that the annual CO2 net consumption of Hongfeng Lake watershed is 7.97×109mol/a, in which CCW and CSW processes consume 6.60×109mol/a and 5.06×109mol/a respectively, and SCW process release 3.69×109mol/a. The rate of sulfuric acid based weathering in the total chemical weathering is not large, but their contribution to carbon flux between atmosphere-water systems should not be neglected.
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