Spatial and temporal variations of dissolved silicon isotope compositions in a large dammed river system

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.