钼（Mo）同位素作为非常热门的反演古海洋缺氧历史指标是建立在一个有争议的假设之上，即认为在地质历史时期，陆源输入的δ98/95Mo与地壳岩石平均值(0.0‰)一致，且长期稳定不变。根据现代河流数据显示，河水δ98/95Mo在空间尺度上变化明显（0.15‰ - 2.40‰），且重于地壳岩石平均组成。而目前，关于河水δ98/95Mo在时间尺度上变化特征的工作几乎没有。同时，对河水δ98/95Mo偏重于地壳岩石的成因机制存在争议。
中国科学院广州地球化学研究所韦刚健研究员的研究小组率先建立西江和黄河河水的持续观测，建立起第一个大型水系的持续时间超过一年的δ98/95Mo时间序列，并结合西江颗粒物δ98/95Mo和已发表的长江河水δ98/95Mo数据对河水Mo同位素偏重的机制开展研究。发现西江河水δ98/95Mo季节性变化微弱(1.04‰ - 1.31‰，相对于NIST 3134)，但偏重于世界上绝大多数河流。西江流域土壤选择性截获岩石风化出来较轻δ98/95Mo的Mo是控制西江河水δ98/95Mo偏重特征和季节变化主要因素。同时，发现中国三条主要的河流河水δ98/95Mo呈现出一种梯度（西江>长江>黄河）变化特征，这种特征是由于分布在不同的气候区的三条河流域土壤对岩石风化出来Mo的截获能力的强弱差异而引起的。
相关成果发表于国际主流期刊Chemical Geology。该项研究获得了国家自然科学基金资助。
英文摘要：
Mo isotopic composition in large rivers is very important for understanding global Mo cycle. At present, temporal variation signatures in the δ98/95Mo in large rivers have not been investigated, which hinder a comprehensive understanding on the mechanism for the variations of Mo isotopic compositions in river water. In this study, we report a one-year-long time series (March 2010 to March 2011) of the δ98/95Mo of both the water and suspended particles collected at Guiping, from the middle reaches of the Xijiang River (XJR), and of the water δ98/95Mo from the lower reaches of the Huanghe River (HHR, or Yellow River) collected at Lijin, China. The results indicate that the temporal variations in the concentration and δ98/95Mo of dissolved Mo in the XJR range from 4.32 to 10.5 nmol/L (mean 7.31 nmol/L) and 1.04‰ to 1.31‰ (relative to NIST 3134) (mean 1.20‰), respectively, but that the particulates have a lower δ98/95Mo (-0.18‰ to 0.58‰). Analysis of the suspended particulates and other chemical parameters of the river water suggest that the weathering of silicates and sulfides is the main contributor to the dissolved Mo content in the XJR. Subsequently, the highly efficient preferential trapping of lighter δ98/95Mo by weathering products such as clay minerals, Fe–Mn oxides, and organic materials in soils and saprolites, which are abundant in the tropical catchment, is the key to the heavy Mo isotope signatures in the XJR. Furthermore, it seems that neither Mo-scavenging by suspended particulates during riverine transportation, nor the weathering of sulfates, significantly influence the δ98/95Mo of the XJR water. Cross comparison among the δ98/95Mo in the waters of the three largest rivers in China (the Changjiang (CJR), HHR, and XJR) indicates that the HHR has the highest Mo concentrations, but the lightest δ98/95Mo, the XJR has the lowest Mo concentrations but the heaviest δ98/95Mo, and the values for the CJR are intermediate between those for the other two rivers. This supports the conclusion that the efficiency of the selective trapping of Mo by soils and saprolites is the main factor controlling δ98/95Mo in these large rivers. The tropical/subtropical XJR catchment experiences intense chemical weathering, the semi-arid temperate HHR catchment experiences very little, and the CJR catchment falls somewhere between the two. Such a relationship between water Mo concentrations and δ98/95Mo agrees with previous observations from many large rivers worldwide. It is therefore suggested that chemical weathering on continents is the key to variations in the isotopic composition of Mo in the waters of large rivers.
