黄土区典型小流域矿物化学风化及碳汇效应

黄土中蕴含了巨大的碳库,黄土中碳的转移对区域乃至全球碳循环具有重要影响。本文选择山西省临县一典型黄土小流域青凉寺沟流域进行调查分析,研究黄土水的化学特征及离子来源,分析黄土矿物的溶蚀过程及碳汇效应,并利用正演模型和水化学方法估算不同矿物的离子贡献比例及流域碳汇通量。结果发现,研究区黄土化学成分的含量由高到低依次为SiO_2、Al_2O_3、CaO、Fe_2O_3、MgO、K_2O、Na_2O,表现出贫SiO_2、Fe_2O_3,富CaO、MgO的特点;黄土水的pH呈中性偏碱,阴离子主要以HCO_3~-为主,阳离子以Na~+为主,水化学类型为重碳酸-钠型(HCO_3~--Na~+),水化学组成与黄土的化学组成相对应;流域内不同端元离子来源贡献计算结果表明,大气沉降、人类输入、蒸发盐矿物、硅酸盐矿物和碳酸盐矿物化学风化贡献的溶解物质分别占总溶解物质的1. 66%、6. 32%、10. 38%、62. 23%、19. 31%;黄土区长时间的水-矿物作用是硅酸盐矿物溶解贡献占主导的主要原因,阳离子置换反应、土壤-盐分浸出与蒸发以及人类输入对硅酸盐矿物溶解也有一定的贡献;受黄土区相对低温少雨的影响,黄土矿物的平均化学风化速率较低,为9. 31 t/(km~2·a),低于全球岩石的化学风化速率平均值36 t/(km~2·a),但是其消耗大气CO_2的速率较高,约为6. 34 t CO_2/(km2·a),明显高于同纬度三川河岩溶流域的碳汇速率(5. 28 t CO_2/(km~2·a));利用水化学径流法计算的青凉寺沟黄土小流域的矿物化学风化的大气CO_2消耗量为0. 18×10~4t/a,为中国黄土区大气CO_2消耗量的估算提供基础数据。

Mineral Dissolution and Carbon Sink Effect in a Typical Small Watershed of the Loess Area

The loess is a huge carbon pool, the transfer of carbon in the loess has important impacts on regional and even global carbon cycles. In this study, samples were selected in a typical loess basin in the Linxian County, Shanxi Province to investigate the characteristics of water chemistry in loess water, to analyze the dissolution process and carbon sink effect of loess minerals, and to estimate the carbon flux and the proportion of different mineral contributions of the watershed by using forward modeling and water chemistry methods. The results showed that the list of chemical constituents according to their contents in the loess from high to low in the study area is SiO₂, Al₂O₃, CaO, Fe₂O₃, MgO, K₂O and Na₂O, showing the characteristics of SiO₂ and Fe₂O₃ depleting and CaO and MgO enriching. It was found that pH values of soil water in the loess (loess water) of the study area ranged from neutral to alkaline, the dominated cation in the loess water was Na⁺ and the dominated anion was HCO₃^-, and the water chemistry type of loess water was the HCO₃^--Na⁺ type, and the chemical compositions of loess water correlated closely to the chemical compositions of the loess. A chemical budget method was employed to estimate the contribution of different end-member of ion sources in the basin, the results showed that five end-member components, atmospheric precipitation, human input, evaporating salt minerals, silicate minerals and carbonate minerals could be identified and their contributions to the total dissolved mass of the Qingliangsigou watershed water system were 1.66%, 6.32%, 10.38%, 62.23% and 19.31%, respectively, the major contribution from the silicate minerals dissolution might be resulted mainly from the long-term water-mineral reaction in the loess area and partially from cation exchange reaction, soil-salt leaching and evaporation and impacts of human activities. Due to the relatively low temperature and low rainfall in the loess area, the average chemical weathering rate of loess minerals was 9.31 t/(km²·a), lower than the average chemical weathering rate of global rocks, 36 t/(km²·a). However, the consumption rate of atmospheric CO₂ was about 6.34 t/(km²·a), which is significantly higher than the carbon sink rate of the Sanchuan River karst Basin in the same latitude (5.28 t/(km²·a)). The atmospheric CO₂ consumption of mineral-chemical weathering calculated by the water chemical budget method for the small watershed of Qingliangsigou was 0.18×10⁴ t/a, which provides basic data for the estimation of atmospheric CO₂ consumption in the loess area of China.