贵州乌江水系河水硫同位素组成特征研究

Sulfur isotope composition characters of Wujiang river water in Guizhou province

对乌江丰水期河水硫酸盐的硫同位素组成特征进行了研究。SO42-平均浓度为0.48 mmol/L,δ34S值为-11.5‰~8.3‰,干流河水δ34S值为-6.7‰^-3.9‰。河水的硫同位素组成主要受岩石风化及大气降水的影响,具有明显的区域性分异特征:上游碳酸盐岩地区河水的SO42-浓度高而δ34S值低,SO42-主要来源于煤中黄铁矿的氧化、矿床硫化物的氧化和大气降水;下游碳酸盐岩夹碎屑岩地区河水中的SO42-浓度低而δ34S值高,SO42-主要来源于大气降水和石膏溶解,煤中黄铁矿氧化生成的硫酸盐所占比重较低。乌江河水向贵州省外输出的SO42-通量为172×1010g/a,丰水期占全年SO42-输出总量的72%。来自煤、硫化物、雨水和蒸发岩的硫对丰水期河水中SO42-的平均贡献分别为:50%、25%、20%和5%。H2SO4对碳酸盐岩的侵蚀速率为35.1 t/km2/a(17.5 mm/ka),由此降低大气CO2消耗速率3.66×105mol/km2/a。

The sulfur isotopic （δ^34 S SO^2- 4-） composition of the water in Wujiang river and its catchment area was measured during high-flow period. The average SO^2- 4 concentration is 0.48 mmol/L. The river water δ^34 S values range from - 11.5‰ to 8.3‰, whereas the 6,34 S values of mainstream show only a narrow range and exhibit negative values from - 6.7‰ to - 3.9‰. The isotopic composition of surface water allows distinguishing the contribution due to the weathering rock and atmospheric precipitation, and the spatial variation in δ^34 S over the catchment area is obvious. The average SO^2- 4 concentration is higher in the upper part of the river than in the lower regions, while the average δ^34 S value is lower. The upper river shows a three end-member mixing between rainwater SO^2- 4 ,SO^2- 4 from oxidation of pyrite in coal and the sulfide deposits. In the lower reaches, the samples lie mainly on the isotopic mixing trend between the evaporitic SO^2- 4 and rainwater SO^2- 4 , the contribution of SO^2- 4 from oxidation of pyrite is less. The SO^2- 4 export flux of Wujiang river is 172 × 10^10 g/a, of which the high-flow period export flux accounts for 80 %. the relative contributions from the oxidation of pyrite in coal, the oxidation of sulfides deposits, the rainwater SO^2- 4 and the evaporitic SO^2-4 are 50 %, 25 %, 20 % and 5 %, respectively. The erosion rate of carbonate by sulfuric acid is 35.1 t/km^2/a （17.5 mm/ka） and the consumption rate reducing CO2 is 3.66 × 10^5 mol/km2/a approximately.