Abstract The dissolved silicon isotope composition (δ30Si) of river water serves as a key parameter in tracing the terrestrial silicon cycle. However, the lack of long-term observations of rivers often makes it difficult to constrain the isotopic features of riverine dissolved silica (DSi) well. In this study, samples were collected during four basin-scale surveys and monthly monitoring at the mouth of Changjiang over the course of two years, during the full operation period of the Three Gorges Dam (TGD), the largest hydropower project in the world, and the DSi concentration and δ30Si were measured. The DSi concentration and δ30Si ranged from 91.3 μM to 124.7 μM and 1.00‰ to 1.86‰ for the main stream of Changjiang, and from 58.4 μM to 194.0 μM and 0.83‰ to 2.27‰ for the tributaries/lakes, respectively. The mid-lower tributaries/lakes possessed mean DSi concentrations comparable to those of the upstream tributaries but higher δ30Si. δ30Si increased by ~0.7‰ in the main stream, accompanied by a decrease in DSi concentration and elemental ratio of dissolved aluminum to Si (Al/Si) downstream from the TGD when the mid-lower tributaries/lakes were in the dry period, suggesting co-influence of enhanced clay formation, productivity of freshwater diatoms as well as influx of 30Si-enriched tributaries/lakes in controlling the evolution of δ30Si in the mainstream. In contrast, little longitudinal variation was observed when the mid-lower stream was in the flooding period, and δ30Si discharged by Changjiang to the estuarine region was mainly controlled by the mixing between the main stream and the mid-lower tributaries/lakes. Groundwater could be relevant source of DSi in the main stream of Changjiang, as indicated by water mass balance calculation, while its impact on δ30Si in the main stream was insignificant. Based on the long-term monitoring at the river mouth, the DSi-flux weighted δ30Si value of Changjiang was re-estimated to be 1.4‰, which represented the best estimation of the riverine δ30Si discharged into the coastal waters by Changjiang at this stage. The temporal variation in δ30Si observed at the river mouth was affected by the annual flow cycles of the upper and mid-lower streams of Changjiang superimposed by seasonal aquatic diatom productivity. Combination with the historical DSi concentrations in the Changjiang end-member suggested that the Three Gorges Reservoir does not play a significant role in direct retaining DSi at this stage, while the severe sediment trapping within the reservoir may cause potential alteration in the silicate weathering and freshwater diatoms productivity in the mid-lower reaches.

中文翻译：

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大坝河流系统溶解硅同位素组成的时空变化

摘要 河水溶解硅同位素组成（δ30Si）是追踪陆地硅循环的关键参数。然而，由于缺乏对河流的长期观测，很难很好地约束河流溶解二氧化硅（DSi）的同位素特征。在这项研究中，在两年的时间里，在世界上最大的水电项目三峡大坝 (TGD) 的全面运行期间，在长江口的四次流域尺度调查和月度监测中收集了样本，并测量了 DSi 浓度和 δ30Si。长江干流 DSi 浓度和 δ30Si 浓度分别为 91.3 μM 至 124.7 μM 和 1.00‰ 至 1.86‰，支流/湖泊分别为 58.4 μM 至 194.0 μM 和 0.83‰ 至 2.27‰。中下游支流/湖泊的平均 DSi 浓度与上游支流相当，但 δ30Si 更高。中下游支流/湖泊处于枯水期时，干流中δ30Si增加~0.7‰，同时伴随着TGD下游DSi浓度和溶解铝与Si元素比（Al/Si）的降低，表明增强粘土形成、淡水硅藻生产力以及富含 30Si 的支流/湖泊的流入对控制干流中 δ30Si 演化的共同影响。相比之下，中下游处于洪水期时纵向变化不大，长江向河口地区排放的δ30Si主要受干流与中下游支流/湖泊的混合控制。水质量平衡计算表明地下水可能是长江干流中DSi的相关来源，而对干流中δ30Si的影响不显着。根据河口长期监测，长江DSi-flux加权δ30Si值重新估算为1.4‰，是现阶段长江排入近海的河流δ30Si的最佳估算值。河口δ30Si的时间变化受长江上、中下游河流年流量循环和季节性水生硅藻生产力叠加的影响。结合长江端元历史 DSi 浓度表明，三峡水库现阶段对 DSi 的直接滞留作用不大，