西江上游河水中硫酸盐来源及其对化学风化的影响

Sulfate sources and its impacts on chemical weathering in water of the upper reaches of Xijiang River

硫酸参与的岩石风化过程是流域岩石风化研究的重要组成部分,对全球碳循环具有重要的影响。高频率的采样与分析能够准确评估河水中物质的来源及其迁移转化过程,而前通过高频率的采样分析探讨流域硫酸盐生物地球化学循环过程的研究还较为缺乏。本研究在一个完整水文年内(2013年10月—2014年9月),对我国西江上游南、北盘江和红水河的河水样品进行月份采集与分析。结果表明,流域河水中硫酸盐来源季节性变化受季风气候的影响。与丰水期相比,流域河水硫酸盐在枯水期富集重的同位素。基于硫酸盐的硫氧同位素证据发现,尽管蒸发岩溶解在枯水期是河水硫酸盐的重要端元,但硫化物氧化却是硫酸盐的主要来源。在丰水期,矿山废水可能是河水中硫酸盐最重要的来源,其原因与浅层地表径流快速流动对季风气候下高频率降雨的响应有关。与同一时期的枯水期相比,丰水期硫酸参与碳酸盐岩风化产生的较高的HCO3-含量,来自于硫化物氧化的硫酸参与红水河碳酸盐岩风化产生的HCO3-量,约占河水总HCO3-量的1/4。因此,在一个完整水文年内通过高频率的采样,分析硫酸盐双同位素的"指纹"特征,对准确理解流域河水中硫酸盐的季节性变化与流域风化过程具有重要的作用。

Mineral weathering by sulfuric acid plays an important role in chemical weathering in the catchment, with consequences on global carbon cycle. Highfrequency sampling and analysis can accurately assess the sources and biogeochemical processes of dissolved solutes in the river. However, few studies have focused on the temporal variation of sulfate in rivers and rock weathering by sulfuric acids. Here, a monthly sampling campaign was conducted in the Nanpan, Beipan and Hongshui rivers along the upper reaches of Xijang River during a whole hydrological year （October 2013 to September 2014）. The seasonal variation of riverine sulfate was affected by the monsoon climate in the upper reaches of the Xijang River basin. Heavy isotopes were generally enriched in sulphate during the dry season compared to that in the wet season. The isotopicevidence showed that the riverine sulfate was mainly derived from sulfide oxidation, whereas evaporite dissolution could be an important source during the dry season. Mine drainage would be an important source of sulfate to those rivers during the wet season, mainly due to the responses of the fast surface flow to frequent heavy rain in monsoonal climate regions. A relatively highproportion of HCO3- was derived from rock weathering by sulfuric acid during the wet season compared to that during the dry season. Approximately 1/4 of the HCO3- in the Hongshui River originated from carbonate weathering by sulfuric acid, which derived from the oxidation of sulfide. In all, information from the specific dual isotopic characteristics of riverine sulfate throughout a hydrological year can provide unique evidence for understanding the temporal dynamics of sulfate concentrations and weathering processes in the rivers.