Dissolved silicon isotope dynamics in large river estuaries
Introduction
The land-to-ocean silicon (Si) flux, mainly occurring via riverine discharge, is of great importance because it strongly stimulates diatom growth in coastal oceans, thereby significantly contributing to global carbon fixation (Laruelle et al., 2009, Treguer and De La Rocha, 2013, Frings et al., 2016). Estuaries, situated between freshwater and marine environments, are key components of the land-ocean aquatic continuum and are highly complex environments with distinct patterns of salinity, pH, nutrients, and turbidity (e.g., Edmond et al., 1985, Nelson and Dortch, 1996, Cai et al., 2004). Estuarine mixing alters the distributions of dissolved and particulate materials delivered by rivers through, for example, adsorption-desorption onto particles (e.g., Nguyen et al., 2019), mineral precipitation and dissolution (e.g., Singurindy et al., 2004), and uptake by organisms (e.g., DeMaster et al., 1996). Dissolved silicate (Si(OH)4) concentrations often deviate from conservative mixing behavior between river water and seawater in estuaries, which can be induced by both biotic and abiotic processes (e.g., Carbonnel et al., 2013, Treguer and De La Rocha, 2013). Diatoms incorporate Si(OH)4 into their frustules during growth and export biogenic silica (bSi) to the sediments, thereby removing Si from the dissolved pool (e.g., DeMaster, 1981, Conley and Malone, 1992). In the sediments, diagenetic reactions via dissolution of bSi and formation of authigenic silicate minerals (i.e., reverse weathering) help to further preserve Si (e.g., Michalopoulos and Aller, 1995, Michalopoulos and Aller, 2004, Rahman et al., 2017). In contrast, the dissolution of fluvial amorphous silica (ASi) adds Si to estuarine environments, which has been demonstrated in both laboratory experiments (e.g., Oelkers et al., 2011, Jones et al., 2012) and field studies (e.g., Pastuszak et al., 2008, Carbonnel et al., 2013, Lehtimäki et al., 2013). Furthermore, anthropogenic activities may decrease (such as damming; Conley, 2002, Hughes et al., 2012) or increase (such as deforestation and climate warming; Conley et al., 2008, Laruelle et al., 2009) Si(OH)4 concentrations and impact the riverine Si flux to the estuaries.
Tropical and subtropical rivers are the major contributors of riverine Si input to the ocean (Zhang et al., 1999, Beusen et al., 2009). Taking into account the strong climatic control on the weathering regime and intensity, processes affecting Si in estuaries associated with changing climate zones are of particular interest. In addition, large river estuaries with high water and sediment discharge commonly exhibit not only high turbidity and complex physical circulation but also significant seasonal variability in nutrient dynamics and phytoplankton bloom development (DeMaster and Pope, 1996).
Despite their potential significance for global Si cycling, our knowledge of the role of large river estuaries is still limited (Weiss et al., 2015, Frings et al., 2016, Sutton et al., 2018). One reason is that the Si concentration distributions only partially reveal Si dynamics. Over the past two decades, the stable isotopic composition of Si (δ30Si) has been developed as a powerful tool for identifying Si sources and tracking Si biogeochemical processes over various temporal and spatial scales (e.g., Varela et al., 2004, Zhang et al., 2019). In comparison to extensive studies conducted in rivers, lakes, and the ocean (Sutton et al., 2018, and references therein), information on factors controlling dissolved Si isotope compositions (δ30SiSi(OH)4) in estuaries is sparse and data have so far only been reported for four estuaries worldwide (Hughes et al., 2012, Delvaux et al., 2013, Weiss et al., 2015, Zhang et al., 2015). In the tropical Tana River Estuary, Si(OH)4 concentrations decrease linearly with increasing salinity, whereas δ30SiSi(OH)4 signatures remain stable due to the absence of processes fractionating Si isotopes (Hughes et al., 2012). A notable increase of δ30SiSi(OH)4 has been observed to occur during diatom growth in the tidal freshwater zone of both the Scheldt River (Delvaux et al., 2013) and the Elbe River estuaries (Weiss et al., 2015). Heavier δ30SiSi(OH)4 signatures induced by biological utilization also occur at high salinities in the Yangtze River Estuary in summer (Zhang et al., 2015). Given the large importance of estuaries, integrated efforts are necessary for the development of a more comprehensive understanding of the global Si cycle (Sutton et al., 2018).
In this study, we investigate for the first time surface water δ30SiSi(OH)4 distributions and their controlling processes in the estuaries of three of the world’s largest rivers, namely the Amazon River (1st) in the tropical zone, the Yangtze River (or Changjiang, 5th) in the temperate zone, and the Pearl River (or Zhujiang, 13th) in the subtropical zone, including both the embayment and adjacent shelf (Fig. 1). The relationship between δ30SiSi(OH)4 and salinity is investigated in each estuarine system to distinguish between chemical and biological fractionation effects and conservative mixing. Given that these rivers represent more than one fifth of the global freshwater discharge to the ocean, their Si isotope dynamics will shed new light on the importance of estuarine processes in the oceanic Si cycle.
Section snippets
Study area
Amazon River Estuary (ARE). The Amazon River, covering over 20° of latitude and extending longitudinally across 3,000 km, is the largest river in the world (Nittrouer and DeMaster, 1996). It discharges 5.8 × 1012 m3 yr−1 of freshwater with peak flow in June and minimum flow in November, which is approximately 20% of the global river water discharge (Meade et al., 1985, DeMaster and Pope, 1996). It also delivers 1.1–1.3 × 109 ton yr−1 of suspended sediments to its lower reaches, contributing up
Distribution of salinity, Si(OH)4, and δ30SiSi(OH)4
In all three estuaries, surface salinities generally increased from the upper reaches or the mouth (<1.0) to the adjacent shelf (Fig. 2a, e, j), reflecting estuarine mixing between the fresh river waters and the saline seawaters. Salinities at the outermost offshore stations were 35.0, 34.0, and 31.1 for the ARE, YRE, and PRE (Table 1), suggesting the dominance of oceanic shelf water in the ARE and YRE and the still significant influence of river plume water in the PRE during the high discharge
Differences in δ30SiSi(OH)4 signatures of river water endmembers between estuaries
The Si(OH)4 concentration and δ30SiSi(OH)4 value at zero salinity of the ARE amounted to 150.2 μmol L−1 and 1.2 ± 0.2‰, respectively, in May 2018 (field measurements at station M147_66-1; Table 1), and reached 125.3 μmol L−1 and 1.3 ± 0.2‰, respectively, in November 2013 (field measurements at station River; Table A1). Therefore, our data show a consistent δ30SiSi(OH)4 composition of the river water endmember despite differing Si(OH)4 concentrations, sampling locations, and sampling periods. A δ
Conclusions
δ30SiSi(OH)4 in the ARE, YRE, and PRE displayed significant differences in distribution patterns, reflecting the role of large river estuaries as either a passage or a reactor in delivering riverine Si to the ocean. While conservative mixing between river water and seawater primarily modulated both surface Si(OH)4 concentrations and δ30SiSi(OH)4 signatures in the YRE and PRE presented in this study, significant Si isotope fractionation during diatom growth was observed over the entire salinity
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was funded by the National Natural Science Foundation of China (NSFC; 91858107 and 41606089) and by the National Key Scientific Research Project (2015CB954003) sponsored by the Ministry of Science and Technology of China. Zhouling Zhang was co-supported by a scholarship under the Graduate School of Xiamen University and the National Key Scientific Research Project (2015CB954001). Henning Kuhnert was supported by the DFG Research Center “The Ocean in the Earth System”. Cristiano M.
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