Spatial and temporal variations of dissolved silicon isotope compositions in a large dammed river system
Introduction
The global silicon (Si) cycle has attracted considerable interest due to its crucial role in regulating climatic stability via terrestrial silicate weathering on geological time scales (Frings et al., 2016; Conley et al., 2017) and diatom-driven marine biological carbon pumping on the millennial scale (Hawkings et al., 2018). Drainage basins are the main fields for terrestrial silicate weathering, as well as the most important pathways for the delivery of dissolved silicic acid (DSi) released by weathering into the ocean (Tréguer and De La Rocha, 2013). Meanwhile, the diminished connectivity mainly caused by dams and reservoirs (Grill et al., 2019) is placing increasing pressure on the global river systems, which could result in decreased Si flux (Humborg et al., 1997; Maavara et al., 2015) or disproportional Si/N and Si/P ratios (Ran et al., 2018) in estuaries and oceans. Therefore, exploring the responses of the land-to-ocean DSi flux and composition to the ever-increasing human influences is urgently needed for better understanding and constraint of both the terrestrial and marine Si cycles.
DSi in river water is involved in various abiotic and biotic processes, with lighter Si isotopes (28Si and 29Si) preferentially incorporated into neoformed solid products, such as clays (Georg et al., 2007) and biogenic silica (BSi, De La Rocha et al., 1997; Ding et al., 2008), leading to the enrichment of heavier isotopes (30Si) in the DSi pool. Thus, the Si isotope composition of DSi (expressed as δ30Si) in river water serves as a powerful tracer of terrestrial Si geochemical/biogeochemical processes. The δ30Si values in rivers worldwide have been reported to range from −0.14‰ (Hughes et al., 2013) to 4.66‰ (Cockerton et al., 2013), with an average of 1.28‰ ± 0.68‰ (Frings et al., 2016). Despite the distinct distribution tendencies in individual rivers, δ30Si primarily reflects the extent of silicate weathering congruency and biological retention of DSi by diatoms and Si-rich higher plants.
Among the 10 largest rivers worldwide, Changjiang (the Yangtze River) is outstanding in the present Anthropocene Era in view of the high population density and extensive development of river infrastructure (Yang et al., 2014) and agriculture (Bao and Fang, 2007). The huge amounts of DSi (81 × 109 mol/year; Liu et al., 2005) and other macronutrients discharged by Changjiang are of great importance in sustaining the primary production in the coastal waters of the East China Sea (Wang et al., 2018). Since the impoundment of the Three Gorges Reservoir (TGR) in 2003, there have been increasingly many studies of the potential Si retention caused by this world's largest artificial reservoir. During the first impoundment stage (survey conducted in 2003 and 2004), Gong et al. (2006) reported a decreasing trend in the DSi concentration in the Changjiang estuarine region, which was ascribed to the Si retention caused by the TGR. However, work performed during the second (survey conducted in 2007; Ran et al., 2013a) and third impoundments (survey conducted in 2009; Ding et al., 2019) suggested that only 2.8%–3.8% of the DSi inflow into the TGR was trapped there, which could result in a DSi reduction in the river mouth by <2%. In brief, no unanimous agreement has been reached regarding the Si trapping effect of the TGR based on concentration proxies, and more evidence is required.
Another unique feature of Changjiang is the high δ30Si signature (~3‰ at Nantong station locating ~145 km upstream of river mouth, Ding et al., 2004) exported from the river to the estuary, compared to those of other high-discharge rivers (Congo River, Hughes et al., 2011a; Ganges River, Frings et al., 2015; Amazon River, Hughes et al., 2013; Yenisey River, Mavromatis et al., 2016; Lena River, Sun et al., 2018). This high δ30Si in the mainstream of Changjiang was initially ascribed to the wide spread of rice and grass in the basin, which remove Si as phytolith (Ding et al., 2004). However, in a follow-up study carried out a decade later, the δ30Si in the river mouth was observed to range from 0.8‰ to 2.1‰ with an average of 1.6‰ (Ding et al., 2014), lower than the first observation (Ding et al., 2004) by 1.4‰. Such a notable decline in the δ30Si with a temporal variation >2‰ in the Changjiang end-member has exceeded the observed seasonal variations of 0.6‰–1.3‰ in all the other river systems where multiple observations were made (Engström et al., 2010; Hughes et al., 2013; Pokrovsky et al., 2013; Sun et al., 2018). Given the global importance of Changjiang as well as the necessity of well estimation of riverine δ30Si in the studies of marine silicon cycle and paleo-oceanography (Hendry et al., 2016), long-term observations of the Si isotopic composition in the Changjiang basin and explorations of the controlling mechanisms are required.
Overall, the large land-sea DSi flux and considerable anthropogenic activity make Changjiang an important and unique area in which to study the terrestrial Si cycle under increased environmental pressure. In this report, we present the first δ30Si datasets obtained in Changjiang basin with regular monitoring of δ30Si in the river mouth since the full operation of the TGR began, with the objective of elucidating the spatial–temporal variations in the riverine δ30Si and the controlling mechanisms. This work may improve understanding of terrestrial Si cycles and constraint of the Si isotope budgets in the ocean. This work is structured as follows: Section 2 describes the study area, sample collection, and analysis methods employed in this work. Section 3 presents the spatial and temporal variation of DSi concentration, δ30Si and BSi in the mainstream and tributaries of Changjiang. Section 4 provides a thorough discussion on the mechanisms controlling the δ30Si in the Changjiang basin, including chemical weathering, biological alteration, and hydrological shift driven by monsoon climate, the impact of TGR on the Si cycle is also discussed in this section. Finally, Section 5 concisely restates the purpose and main results of this study.
Section snippets
Study area
Changjiang rises on the southwest side of Keladandong Peak of Tanggula Mountain in the Qinghai-Tibet Plateau, flows east for >6300 km, and then debouches into the East China Sea at Shanghai. The Changjiang drainage basin is situated at 24.5°–35.8°N and 90.5°–122.4°E with a drainage area of 1.8 × 106 km2. Primarily based on climate and hydrology, Changjiang can be divided into three main sections: the upper reaches from the source to Yichang, the middle reaches from Yichang to Jiujiang, and the
DSi concentration in the Changjiang basin
All the sampling information and measured data are provided in supplementary Table S1. The measured DSi concentrations of the mainstream of Changjiang ranged from 91.3 μM to 125 μM, with an average of 110 μM. The latitudinal evolution of the DSi concentration remained rather stable in the upstream waters before the TGR (Fig. 3a), with an average value of 105 μM. After flowing through the TGR, a slight increase in DSi concentration was observed in the TGR output waters (Yichang) compared to the
Silicate weathering
Preferential incorporation of lighter isotopes into the neoformed clay via silicate weathering or into BSi via biological utilization will lead to 30Si enrichment in the dissolved pool. The extent of isotopic fractionation depends on both the degree of net incorporation of DSi into the secondary phase and the associated fractionation factor 30ε (Bouchez et al., 2013).
The indices of elemental ratio of aluminum (Al) to Si (Al/Si) (Georg et al., 2006; Sun et al., 2018) and Si/(K⁎ + Na⁎) (Na⁎ and K⁎
Conclusions
In this study, we performed a systematic investigation of the dissolved Si isotopic composition in the Changjiang basin, which is a large river system with strong anthropogenic pressure. Based on multiple basin-scale surveys conducted under different hydrological conditions, we elucidated the major factors controlling δ30Si in the main stream and sub-basins. The degree of clay formation superimposed by seasonal biological uptake in the mid-lower reaches played major roles in the evolution of δ30
Declaration of competing interest
None.
Acknowledgement
The authors thank G.S. Zhang for help in analyzing the DSi concentration. We are grateful to Prof. Jerome Gaillardet, Prof. Damien Cardinal and another anonymous reviewer for their comments on this manuscript. This work was supported by the NSFC “Creative Research Team” [Grant No. 40721004, 41021064, and 41906200]; and the China Postdoctoral Science Foundation [Grant No. 2018M642386].
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Geochemical perspective on large dams changing the downstream sediment sources
2022, Journal of Geochemical ExplorationCitation Excerpt :However, river flow and sediment connectivity within the basins have been strongly disturbed by numerous dams and reservoirs, and currently only 23 % of rivers longer than 1000 km worldwide can flow freely to the oceans (Grill et al., 2019). These dams and reservoirs have resulted in significant changes in geomorphology, water and sediment transport, and dissolved and particulate chemistry in large river basins, estuaries and surrounding coastal seas (Li et al., 2016, 2020; Nienhuis et al., 2020; Yang et al., 2019; Zhang et al., 2020). The Changjiang (Yangtze River) is the longest river in Asia, historically transporting ~900 km3/yr of water and ~480 Mt./yr of sediments to the East China Sea (Milliman and Farnsworth, 2011; Yang et al., 2006).