Seasonal and interannual changes of river chemistry in the source region of Yellow River, Tibetan Plateau
Introduction
Over the past few decades, global climate change has drastically affected natural ecosystems and is considered accountable for land degradation in manifold regions. Tibetan Plateau (TP) is one of the most vital and susceptible regions in the earth's climate structure. Over the past half century, TP has experienced more swift warming than other regions (e.g. Kang et al., 2007; Immerzeel et al., 2010; Yang et al., 2014; Li et al., 2015). As a consequence, glacier shrinkage and permafrost degradation are expediting extremely (e.g. Jin et al., 2009; Pohl et al., 2017; Kraaijenbrink et al., 2017), and thus river discharge downstream is regulated by meltwater and/or precipitation released from the TP (e.g. Liu et al., 2009; Lutz et al., 2014; Pritchard, 2017). Besides, with the increase of water supply, the interaction between water and soil/rock strengthens, and hence the chemical weathering process and solutes released to TP rivers must have enhanced (Jin et al., 2009). Moreover, TP is recognized as the ‘water tower’ of Asia because it holds the headwaters of numerous large rivers. Several realms ranging from rivers transport terrestrial materials to downstream aquatic ecosystems, and river chemistry can provide insights to the weathering process (e.g. Zhang et al., 1995; Li et al., 2018), surface water quality (e.g. Fortner et al., 2011; Li et al., 2016; Qu et al., 2019), and the biogeochemical cycles of elements (e.g. Zhang et al., 1995; Chen et al., 2002; Li et al., 2019a). Particularly, major ions in river water are of prime importance because of their dominant production during chemical weathering process. Moreover, the key component of solutes will exports into downstream aquatic ecosystems (e.g. Gaillardet et al., 1999).
In this direction, some studies have been conducted on ion chemistry for large rivers in/around the TP (Dalai et al., 2002; Wu et al., 2008; Huang et al., 2009; Jiang et al., 2015; Li et al., 2018). In the Brahmaputra River, major ions were Ca2+ and HCO3−, and the concentration of total dissolved salts (Ca2+, Na+, K+, Mg2+, Cl−, NO3−, SO42−, HCO3−, and SiO2) was relatively high as compared to water from other basins worldwide (Huang et al., 2009). However, the concentration of NH4+ was generally low, and anthropogenic impacts on water quality were identified (Huang et al., 2009). In the Yamuna River, the dominant ions were Ca2+, Mg2+ and HCO3−, which are predominantly derived from carbonate weathering (Dalai et al., 2002). Moreover, several tributaries in the lower reaches were supersaturated with calcite. River water was mildly alkaline, and total dissolved solids (TDS) had a wide range of concentrations ranging from 32 to 620 mg L−1. This is probably due to the influence of many factors, such as rocks, air temperature and vegetation. Silicate weathering contributed about 25% of total cations, and the rest came from carbonate, evaporite and phosphate (Dalai et al., 2002). In the Lancang River, the predominant ions were Ca2+, Mg2+ and HCO3−, which are derived from carbonate weathering and only 10% came from silicate weathering (Wu et al., 2008). The chemical erosion rate of carbonate was found to be 2 to 27 times higher than that of silicate, and the chemical weathering process in the TP made a minimal contribution to the reduction of atmospheric CO2 (Wu et al., 2008). In the source region of Yangtze River (SRYA), average value of TDS was 778 mg L−1, ranging from 117 to 5496 mg L−1 (Jiang et al., 2015). The sequences of cations and anions were Na+ > Ca2+ > Mg2+ > K+ and Cl− > HCO3− > SO42− > NO3−, respectively. This is consistent with the results from previous findings, in which the enriched cation and anion were Na+ and Cl− due to the weathering of silicate and evaporite as well as the influence of saline lakes (Huang et al., 2009). River chemistry was controlled by the lithogenic weathering process, and has a pronounced regional heterogeneity. Meticulously, the northern rivers were mainly affected by the evaporation and crystallization processes, while the southern rivers were influenced by the weathering of carbonate and silicate (Jiang et al., 2015). In the source region of Yellow River (SRYE), the key ions were Ca2+ and HCO3−, and the median value of TDS (ranging 215–563 mg L−1) was 2.4 times higher than the World median value (Li et al., 2018). The contributions of different sources to dissolved loads followed by the order of carbonate > silicate > evaporite > atmospheric input. The chemical denudation rate ranged from 23.0 to 26.8 t km−2a−1, which is lower than those from the southern TP. The weathering rates of silicate and carbonate were 2.3–2.5 t km−2a−1 and 17.4–19.6 t km−2a−1, respectively. TDS flux was 6.1 × 109 kg a−1, which accounts for 38–47% of the Yellow River export into the sea Li et al., 2018). However, few studies have been performed on the seasonal and annual processes of major ions as well as related weathering processes and controls in the SRYE (Wu et al., 2008; Huang et al., 2009; Lan et al., 2010; Li et al., 2018).
Herein, we present a comprehensive data set of the concentrations and/or fluxes of major ions in river water, precipitation and groundwater in the SRYE, with the sampling frequency once every 1–5 days for river water (n = 2334) during three consecutive years (2013–2015). These data, combined with previously published data and results, provide a broad understanding of river chemistry evolution and related chemical weathering in a warm climate. This study primarily focuses on the monthly, seasonal and annual variations of solute concentrations and fluxes, the chemical weathering processes and rates of carbonate and silicate, the solute sources, as well as the controlling factors on solute exports and weathering rates in the SRYE.
Section snippets
Study area
SRYE, above the Tangnaihai gauging section (TNH; with an altitude of 2700 m and a basin area of 121,972 km2), is located in the northeastern TP (Fig. 1; Li et al., 2017). The annual mean temperature was −0.65 °C, and annual precipitation was 440 mm at Maduo, Dari, Guoluo (1991–2013) and Xinghai weather stations from 1967 to 2013 (Fig. 1; Li et al., 2017). The local climate is dominated by westerly jet. The topography includes hills, valleys, lakes, rivers and high plains. Most of lakes (e.g.
Hydrology and climate
Annual mean discharge and TSM were 0.72 × 109 m3a−1 and 0.051 kg m−3a−1 respectively at HHY, 3.25 × 109 m3a−1 and 0.044 kg m−3a−1 at JM, 9.08 × 109 m3a−1 and 0.202 kg m−3a−1 at JG, and 12.8 × 109 m3a−1 and 0.068 kg m−3a−1 at TNH from 2013 to 2015 (Table 1; Fig. 2). Annual precipitation and mean temperature were 325 mm and −2.47 °C respectively at HHY, 587 mm and 0.19 °C at JM, 479 mm and 1.06 °C at JG, and 331 mm and 2.14 °C at TNH during 2013–2015. Over 80% of precipitation occurred during
Controls on river chemistry
Changes in discharge and/or TSM are primarily related to precipitation, groundwater, meltwater and related physical erosion in the SRYE (Li et al., 2017). Significant correlations between daily precipitation and discharge (r > 0.21, p < 0.01; at JM and JG) and TSM (r > 0.14, p < 0.01; at all sections) (Fig. 1g) suggest that precipitation is the key factor in determining the transports of water and sediments. This is well consistent with previous studies (Lan et al., 2010; Zhang et al., 2013a)
Conclusion
River discharge with strong seasonality is controlled by precipitation and/or groundwater. Annual discharge increased from HHY to TNH, but TSM increased from HHY/JM to JG and then decreased to TNH. The area from JM to JG was the dominant sediment-producing area, which is closely related to large annual precipitation and strong erosion. The dominant cations were Na followed by Mg2+, Ca2+ and K+ at HHY and JM, and Ca2+ followed by Mg2+, Na+, and K+ at JG and TNH; HCO3− was the dominant anion,
Declaration of competing interest
The authors below hereby declare that we have no competing financial or non-financial interests when submitting this manuscript. The intellectual property is entirely our own.
Acknowledgements
This work was funded by National Natural Science Foundation of China (41730751, 41671053, 91647102, 41771040, 41671071, 41761017), Open Foundation of State Key Laboratory of Cryospheric Sciences (SKLCS-OP-2019-04), Open Foundation of State Key Laboratory of Frozen Soil Engineering (SKLFSE201901), and Special Fund of State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering (20195025612). R Xu and X Cui are thanked for laboratory analysis and Shreya Mahajan is also thanked for
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