The Chemical Weathering of Rocks and Its Carbon Sink Effect in the Naqu River Basin of the Nujiang River Source Area, Southwest China

Abstract

Carbon plays an important role in global climate change. The mechanisms of carbon sources and carbon sinks have also received wide attention from society, and the physical and chemical characteristics of riverine ions can reflect the chemical weathering of rocks and carbon sink capacity of river basins. Based on the data on river, rainwater, and rock samples from 2019, this study used various methods, such as ion ratio diagrams and ternary diagrams, to analyze the chemical characteristics of water; the chemical weathering and carbon sink effects of rocks were also calculated while assuming three scenarios based on the main sources of ions in the Naqu River. The results showed that for the whole catchment, the main ion sources in the river were: carbonate rock chemical weathering > silicate rock chemical weathering > evaporite dissolution > atmospheric precipitation input. According to the calculations, in the three scenarios, the carbonate weathering rates were 16.84, 11.32, and 14.08 t/km²/yr, and the carbon sink capacities were 66.47, 121.13, and 93.80 mol/km²/yr, respectively; the evaporite weathering rates were 2.20, 9.63, and 5.92 t/km²/yr, respectively. The silicate chemical weathering rate and carbon sink capacity did not change significantly in either scenario, with 6.82 t/km²/yr and 248.6 mol/km²/yr, respectively. This study quantified the ion sources in the Naqu River basin and accurately analyzed their chemical genesis, which helps in understanding the role of the rivers of the Qinghai–Tibet Plateau in the global carbon cycle and global climate change, in addition to providing a reference for the scientific development of the Nujing River.

1. Introduction

Chemical weathering plays an important role in the mechanism of carbon sinks. River water can be used as a carrier of chemical weathering products, such as solutes and suspended particles [1]. Solutes in rivers may originate from various factors, including rock weathering, soil erosion, atmospheric precipitation inputs, and anthropogenic influences [2]. Analyzing the chemical characteristics of river water can reflect the chemical weathering capacity and carbon sink capacity of a basin to a certain extent [3,4].

The chemical weathering of rocks is impacted by hydrological processes, and it produces carbon monoxide and carbon dioxide; the ensuing carbon sinks have the potential to address a portion of the global carbon imbalance. The escalation of carbon dioxide concentrations resulting from rock weathering contributes to global warming, and heightened temperatures further stimulate rock weathering and the absorption of more carbon dioxide [5]. The succession of climatic extremes and the phenomena of the drying of land stemming from global warming unavoidably exert both positive or negative impacts on the weathering processes of rocks [6]. There is international recognition of the existence of missing carbon sinks in the global carbon cycle, some of which are stored in solid silicate minerals resulting from silicate weathering. The carbon sinks formed through the chemical weathering of carbonates are also considered as parts of these missing sinks [7]. Beyond accelerating the rate of chemical weathering in rocks, warming has the additional effect of reshaping the total ion content produced by the chemical weathering of rocks in river water [2]. Carbonates demonstrate a quicker pace of weathering kinetics when contrasted with silicates, and in virtually all watersheds, the predominant sources of dissolved inorganic carbon are carbonate rocks [8]. Through photosynthesis, dissolved inorganic carbon undergoes a process with aquatic organisms to generate organic carbon, enhancing the carbon sink effect by converting inorganic carbon into organic forms [9].

At present, there is abundant research about chemical weathering and carbon sequestration in basins. Due to factors such as its high altitude and harsh climatic conditions, it is difficult to study the Qinghai–Tibet Plateau. Yu et al. [10] meticulously gauged the vigor of chemical weathering, the velocity of CO₂ consumption, and the nuanced influences at play in the upstream and midstream sections of the Yarlung Tsangpo. Their discerning results unveiled a narrative of subdued chemical weathering intensity and restrained CO₂ sequestration, standing in stark contrast to the grandiose demeanor of most expansive rivers coursing down the Tibetan Plateau. Yu et al. [11,12,13] studied the central basin of the Qinghai–Tibet Plateau and found that the chemical compositions of the rivers in the Niyaqu and Qugaqie basins mainly included Ca²⁺ and HCO³⁻. Seasonal changes had little effect on the solute sources in the Niyaqu basin, but they had a significant impact on the Qugaqie basin. In addition, glaciation enhanced the chemical weathering of the central Qinghai–Tibet Plateau. Zhang et al. [14] studied the oxidative weathering process of pyrite in the eastern part of the Qinghai–Tibet Plateau, and the results showed that in 2012–2013, the CO₂ emissions due to carbonate weathering coupled with sulfuric acid exceeded the CO₂ consumption by the silicate weathering coupled with carbonic acid, but it did not exceed that in 2013–2014. Kang et al. found that the weathering process of rocks in an alpine basin was predominantly shaped by the chemical weathering of carbonate rocks, with additional influences from climatic factors such as temperature and precipitation [15]. The study of chemical weathering in large watersheds can allow one to understand the overall situation of a region, but the study of small watersheds can allow one to accurately analyze the chemical causes and influencing factors, so this is also very necessary. At present, some studies have been carried out on the source region of the Nujiang River on the Qinghai–Tibet Plateau. Huang et al. [4] found that the tributary of the Salween River had a pH between 8.1 and 8.7, the EC was between 33.7 and 54.0 mS/m, and the conductivity of Yuqu, a tributary of the Salween River, was relatively low. Wu et al. [16] found that, compared with the chemical weathering rates of other rocks in Nujiang, carbonate rock had the highest CO₂ consumption flux due to chemical weathering, reaching 5.9 × 10⁵ mol/km²/yr, and the total consumption was 6.5 × 10¹⁰ mol/yr in their study. By analyzing chemical weathering and carbon sink effects, Tao et al. [17] found that the hydrochemical type of the Nujiang River was mainly that of Ca-HCO₃, and the dissolution of evaporite made little contribution to the Nujiang River. The chemical weathering rate of carbonate rock was higher (33.54 t/km²/yr), and its CO₂ consumption flux was 2.22 × 10⁵ mol/km²/yr in the Nujiang River. Previous research mainly focused on the middle and lower reaches of the Nujiang River, and studies of the carbon sink of the Naqu River watershed, which is the source area of the Nujiang River, are relatively scarce.

Based on hydrochemical data from 2016 to 2018, the predominance of rock weathering in the Naqu River basin was found in our previous study through cluster analysis, triangular plots, correlation analysis, and other methods [18]. But our previous study did not quantitatively analyze the ionic sources in the Naqu River. Therefore, through an examination of the hydrochemical traits within the Naqu River basin, this study postulates three scenarios grounded in the principal origins of ions. Subsequently, both the rate of chemical weathering of rocks and the carbon sink capacity are computed. The overarching objective is to delve into the hydrochemical attributes of the Naqu River, unraveling insights into the intricate processes of rock weathering and the carbon sequestration potential within a prototypical plateau river basin. Such investigations bear profound significance, as they contribute to a comprehensive understanding of the roles played by rivers on the Qinghai–Tibetan Plateau within the larger context of the global carbon cycle and climate dynamics.

2. Materials and Methods

2.1. Study Area

The study area was located in Naqu City, North Tibet Plateau, Tibet Autonomous Region, Southwest China [19], as shown in Figure 1. The Naqu River is the source of the Nujiang River, the geographical position of which is from 30°54′ to 32°43′ N and from 91°12′ to 92°54′ E [18]. The Naqu River, which is 460 km long, covers 16,350 km² of total drainage area [20]. The Naqu River is composed of four main tributaries, which are, Chengqu, Mugequ, Gongqu, and Luoqu from north to south [21]. Most of the Naqu River basin is located in a plateau subfrigid monsoon semi-humid climate zone, with intensive solar radiation and thin air. According to the analysis of the precipitation data, it can be seen that the average precipitation in the Naqu River basin is 478 mm, indicating the total rainfall [22]. The rainy season occurs from July to August and gradually decreases from east to west [23]. From 1980 to 2000, it was shown that the mean annual evaporation was approximately 1058 mm [24]. In addition, the average temperature from 2015 to 2019 ranged from 0.28 to 1.15 °C.

The upper reaches of the Naqu River basin are relatively gentle, and the terrain is flat and open; the lower reaches are relatively steep, with valley-shaped terrain [16]. The carboniferous to Permian, Jurassic, Tertiary, and Quaternary rocks all have different degrees of exposure in the basin. The main exposed rocks are slate, shale, sandstone, graywacke, granite and other magmatic rocks, a small number of metamorphic rocks, and a fourth system of clay, sand, pebbles, gravel, and so on. Both silicate and carbonate rocks are distributed in the upper and lower reaches of the Naqu River, and the middle reaches of the Naqu River are dominated by silicate rocks. The tributaries of the Sangqu, Basuoqu, Mumuqu, Zongqingqu Mugequ, and Gongqu all flow through carbonate-dominated areas [23,25].

The Naqu River basin is an important region of Naqu City, which has a total of 13 townships. The population density is low, and the economic development is dominated by animal husbandry [26]. The main plants in the domain are temperate subtropical alpine meadows, and the main vegetation is alpine tarragon, alpine dwarf tarragon, alpine mat vegetation, and alpine sparse vegetation.

2.2. Sample Collection and Analysis Methods

2.2.1. Sample Collection

The sample for this study was collected in August 2019, and the sampling points are shown in Figure 2. The sampling frequency for atmospheric precipitation was once per month, and 200 mL polyester plastic bottles were used as sampling bottles for the water quality samples. When collecting river samples, the sample bottle was rinsed with river water 2 to 3 times before sampling, and then it was filled with river samples, sealed, and marked. Rock samples were collected near the river sampling sites. A Horiba U50 multi-parameter water quality analyzer (HORIBA Ltd., Kyoto, Japan) was used on site to measure the temperature, dissolved oxygen (DO), electrical conductivity (EC), oxidation–reduction potential (ORP), and total dissolved solids (TDS) of the river. These indicators are commonly used for water quality monitoring. Among them, DO is an important limiting factor for aquatic organisms, as it depends on the concentration of water-soluble salt, biological action, the water temperature, and the partial pressure of atmospheric gases on the water surface. EC is used to determine water quality and detect the presence of chemical substances. The higher the concentration of dissolved minerals in water, the higher the EC value. ORP indicates the cleanliness of water by measuring the oxidation of water, and the lower the ORP value is, the higher the likelihood of contamination will be. All river water samples and rock samples were determined in the laboratory of the First Geological Environmental Investigation Institute of Henan Bureau of Geology and Mineral Exploration and Development. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) (ICAP 6300, Thermo Fisher Scientific, Waltham, MA, USA) was used to measure the concentrations of K⁺, Na⁺, Ca²⁺, and Mg²⁺ in the river, and the concentrations of Cl⁻, HCO³⁻, and CO₃²⁻ were measured through titration with a 50 mL acid burette [27]. The concentrations of SO₄²⁻, NO₃⁻, and SiO₂ in the river were measured with a T9CS dual-beam UV-VIS spectrophotometer (Beijing General Instruments Co., Ltd., Beijing, China) [28], and an inductively coupled plasma mass spectrometer (ICP-MS) (Thermo Fisher Scientific, USA) was used to measure the Sr concentration in the river water samples [29].

If the normalized inorganic charge balance (NICB; NICB = (TZ⁺ − TZ⁻)/TZ⁺) was close to 0, this indicated that the positive and negative charges in the river water were closer to the conservation of charge. If the NICB was large, this indicated that there were ions in river that had not been detected. Figure 3 depicts the relationship between the EC and TZ⁺ in the Naqu River basin. The results show that EC was significantly positive correlated with TZ⁺ in 2019. This good correlation proved that the hydrochemical data obtained in the field and laboratory measurements in this study were reliable because the sum of the cations in the solution was proportional to the conductivity.

Based on the sample data, a Piper diagram was used in this study to determine the hydrochemical types in the river.

2.2.2. Qualitative Methods for Analyzing the Sources of River Solutes

The combined chemical weathering of rocks is one of the main sources of river water ions. According to differences in their chemical properties, weathered surface rocks can be divided into three major categories: carbonate, silicate, and evaporite [30]. The chemical weathering of rocks requires the participation of acidic media. The reaction equations related to the chemical weathering of rocks are as follows.

Hydrolysis of carbonate rocks:

$${(\mathrm{C}\mathrm{a}}_{1-\mathrm{x}}{\mathrm{M}\mathrm{g}}_{\mathrm{x}}){\mathrm{C}\mathrm{O}}_3+{\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\to \left(1-\mathrm{x}\right){\mathrm{C}\mathrm{a}}^{2+}+\mathrm{x}{\mathrm{M}\mathrm{g}}^{2+}+2{\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$$

(1)

Oxidation of pyrite:

$${\mathrm{F}\mathrm{e}\mathrm{S}}_2+7{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{F}\mathrm{e}{\mathrm{S}\mathrm{O}}_4+2{\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_4$$

(2)

Reaction between sulfuric acid formed by the oxidation weathering of pyrite and carbonate rocks:

$${2(\mathrm{C}\mathrm{a}}_{1-\mathrm{x}}{\mathrm{M}\mathrm{g}}_{\mathrm{x}}){\mathrm{C}\mathrm{O}}_3+{\mathrm{H}}_2{\mathrm{S}\mathrm{O}}_4\to 2\left(1-\mathrm{x}\right){\mathrm{C}\mathrm{a}}^{2+}+2\mathrm{x}{\mathrm{M}\mathrm{g}}^{2+}+2{\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}+{\mathrm{S}\mathrm{O}}_4^{2-}$$

(3)

Weathering reaction of anhydrite and halite:

$${\mathrm{C}\mathrm{a}\mathrm{S}\mathrm{O}}_4={\mathrm{C}\mathrm{a}}^{2+}+{\mathrm{S}\mathrm{O}}_4^{2-}$$

(4)

$$\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}={\mathrm{N}\mathrm{a}}^{+}+{\mathrm{C}\mathrm{l}}^{-}$$

(5)

The classical weathering reactions of silicate mineral hydrolysis:

$$2{\mathrm{N}\mathrm{a}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_8+2{\mathrm{C}\mathrm{O}}_2+11{\mathrm{H}}_2\mathrm{O}\to {\mathrm{A}\mathrm{l}}_2{\mathrm{S}\mathrm{i}}_2{\mathrm{O}}_5{\left(\mathrm{O}\mathrm{H}\right)}_4+2{\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}+2{\mathrm{N}\mathrm{a}}^{+}+4{\mathrm{H}}_4\mathrm{S}\mathrm{i}{\mathrm{O}}_4$$

(6)

$$2{\mathrm{K}\mathrm{A}\mathrm{l}\mathrm{S}\mathrm{i}}_3{\mathrm{O}}_8+2{\mathrm{C}\mathrm{O}}_2+6{\mathrm{H}}_2\mathrm{O}\to {\mathrm{A}\mathrm{l}}_2{\mathrm{S}\mathrm{i}}_4{\mathrm{O}}_{10}{\left(\mathrm{O}\mathrm{H}\right)}_2+2{\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}+2{\mathrm{K}}^{+}+2{\mathrm{H}}_4\mathrm{S}\mathrm{i}{\mathrm{O}}_4$$

(7)

$${\mathrm{C}\mathrm{a}{\mathrm{A}\mathrm{l}}_2\mathrm{S}\mathrm{i}}_2{\mathrm{O}}_8+2{\mathrm{C}\mathrm{O}}_2+3{\mathrm{H}}_2\mathrm{O}\to {\mathrm{A}\mathrm{l}}_2{\mathrm{S}\mathrm{i}}_2{\mathrm{O}}_5{\left(\mathrm{O}\mathrm{H}\right)}_4+2{\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}+{\mathrm{C}\mathrm{a}}^{2+}$$

(8)

$${\mathrm{M}\mathrm{g}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_4+4{\mathrm{C}\mathrm{O}}_2+4{\mathrm{H}}_2\mathrm{O}\to 4{\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}+2{\mathrm{M}\mathrm{g}}^{2+}+{\mathrm{H}}_4\mathrm{S}\mathrm{i}{\mathrm{O}}_4$$

(9)

2.2.3. Quantification Methods for Analyzing the Sources of River Solutes

According to the data on the concentrations of dissolved ions in river water and atmospheric precipitation, the forward model method was used to analyze the sources of ions [31]. The equation is expressed as follows:

$${\left[\mathrm{X}\right]}_{\mathrm{r}\mathrm{i}\mathrm{v}}={\left[\mathrm{X}\right]}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}+{\left[\mathrm{X}\right]}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}+{\left[\mathrm{X}\right]}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}+{\left[\mathrm{X}\right]}_{\mathrm{s}\mathrm{i}\mathrm{l}}+{\left[\mathrm{X}\right]}_{\mathrm{a}\mathrm{n}\mathrm{t}\mathrm{h}}$$

(10)

In the formula, [X]_riv represents the dissolved ions in river water, [X]_cycl represents the input of atmospheric precipitation, [X]_evap represents the contribution of dissolved minerals in evaporite rocks, [X]_carb represents the contribution of the chemical weathering of carbonate rocks, [X]_sil represents the input of chemical weathering of silicate minerals in the basin, and [X]_anth represents the contribution of human activities in the basin to dissolved ions in river water; the unit of ion X is mmol/L. The application of the forward evolution method was based on the following assumptions: (1) The Cl⁻ in the river only came from evaporite and precipitation; (2) Na⁺ and K⁺ were not affected by carbonate rocks. In this study, the rates of contribution of precipitation, carbonate, silicate, and evaporite to the cations dissolved in river water were quantitatively analyzed based on forward modeling.

Atmospheric precipitation is one of the main sources of river ions. The calculation methods for the input of atmospheric precipitation mainly include the water balance method, rainwater ratio method, and mixing method [30]. According to the available data, the rainwater ratio method was adopted in this study to correct the input ions of atmospheric precipitation. Because the minimum Cl⁻ concentration in the collected river samples was 12.96 μmol/L [18], this value was selected as the Cl⁻ concentration for the input of atmospheric precipitation in the Naqu River basin. The equation is as follows [32]:

$${\mathrm{X}}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}={\mathrm{C}\mathrm{l}}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}\times {\left(\mathrm{X}/\mathrm{C}\mathrm{l}\right)}_{\mathrm{a}\mathrm{t}\mathrm{m}}$$

(11)

In the formula, X = SO₄²⁻, Na⁺, Ca²⁺, Mg², and K⁺ (mmol/L), and (X/Cl)_atm is the ratio of an ion X in rainwater to the concentration of Cl⁻.

The rate of contribution of atmospheric precipitation input to the river water can be calculated with the following formula [32]:

$$\frac{{\mathrm{T}\mathrm{Z}}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}^{+}}{{\mathrm{T}\mathrm{Z}}^{+}}=\frac{{\mathrm{N}\mathrm{a}}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}+{\mathrm{K}}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}+2\times {\mathrm{C}\mathrm{a}}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}+2\times {\mathrm{M}\mathrm{g}}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}}{{\mathrm{T}\mathrm{Z}}^{+}}$$

(12)

$${\mathrm{T}\mathrm{Z}}^{+}={\mathrm{N}\mathrm{a}}_{\mathrm{r}\mathrm{i}\mathrm{v}}+{\mathrm{K}}_{\mathrm{r}\mathrm{i}\mathrm{v}}+2{\mathrm{C}\mathrm{a}}_{\mathrm{r}\mathrm{i}\mathrm{v}}+2{\mathrm{M}\mathrm{g}}_{\mathrm{r}\mathrm{i}\mathrm{v}}$$

(13)

In the formula, the unit of ion X is mmol/L.

The input from the chemical weathering of rocks in the basin is generally composed of three parts: evaporite chemical weathering, silicate chemical weathering, and carbonate chemical weathering.

The output ion concentration and ion contribution rate of the chemical weathering of the evaporite in the Naqu River basin can be calculated according to the following formula:

$${\mathrm{C}\mathrm{l}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}={\mathrm{C}\mathrm{l}}_{\mathrm{r}\mathrm{i}\mathrm{v}}-{\mathrm{C}\mathrm{l}}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}$$

(14)

$${\mathrm{N}\mathrm{a}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}={\mathrm{C}\mathrm{l}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}$$

(15)

$${\left[{\mathrm{S}\mathrm{O}}_4\right]}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}={\mathsf{\alpha }}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}\times \left({\left[{\mathrm{S}\mathrm{O}}_4\right]}_{\mathrm{r}\mathrm{i}\mathrm{v}}-{\left[{\mathrm{S}\mathrm{O}}_4\right]}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}\right)$$

(16)

$${\left[{\mathrm{S}\mathrm{O}}_4\right]}_{\mathrm{s}\mathrm{u}\mathrm{l}\mathrm{f}\mathrm{u}\mathrm{r}\mathrm{i}\mathrm{c} \mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}}=\left(1-{\mathsf{\alpha }}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}\right)\times \left({\left[{\mathrm{S}\mathrm{O}}_4\right]}_{\mathrm{r}\mathrm{i}\mathrm{v}}-{\left[{\mathrm{S}\mathrm{O}}_4\right]}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}\right)$$

(17)

$${\mathrm{C}\mathrm{a}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}={\left[{\mathrm{S}\mathrm{O}}_4\right]}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}$$

(18)

$$\frac{{\mathrm{T}\mathrm{Z}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}^{+}}{{\mathrm{T}\mathrm{Z}}^{+}}=\frac{{\mathrm{C}\mathrm{l}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}+2\times {\left[{\mathrm{S}\mathrm{O}}_4\right]}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}}{{\mathrm{T}\mathrm{Z}}^{+}}$$

(19)

In the formula, α_evap is the proportion of the input of SO₄²⁻ from gypsum to the remaining SO₄²⁻ ions after deducting the inputs of human activities and atmospheric precipitation, while [SO₄]_{sulfuric acid} is the input of SO₄²⁻ into the river from the H₂SO₄ erosion of carbonate rocks. Then, according to the relevant research, it was assumed in this study that, after deducting the inputs of human activities and atmospheric precipitation, the calculation of the proportion of SO₄²⁻ generated through the hydrolysis of gypsum and the action of sulfuric acid in the chemical weathering of rocks was conducted in three different scenarios.

Scenario 1: The SO₄²⁻ ions in the river water of the Naqu River basin were all derived from sulfuric acid after deducting the input from atmospheric precipitation; α_evap = 0.

Scenario 2: The SO₄²⁻ ions in the river water in the Naqu River Basin were all derived from gypsum after deducting the input from atmospheric precipitation; α_evap = 1.

Scenario 3: Gypsum dissolution and H₂SO₄ erosion of carbonate rocks contributed in a ratio of 1:1 with respect to SO₄²⁻ in the water of the Naqu River.

In all three of these scenarios, the rate of chemical weathering of rocks was calculated, and the carbon sink effect of the watershed was analyzed.

According to the forward method, after deducting the inputs of atmospheric precipitation and evaporite rock, the Na⁺ and K⁺ in the river mainly came from the chemical weathering of silicate rock. Therefore, the input of Na⁺ and K⁺ contents due to the chemical weathering of silicate rocks can be calculated according to the following formula [32]:

$${\mathrm{N}\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}={\mathrm{N}\mathrm{a}}_{\mathrm{r}\mathrm{i}\mathrm{v}}-{\mathrm{N}\mathrm{a}}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}-{\mathrm{N}\mathrm{a}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}$$

(20)

$${\mathrm{K}}_{\mathrm{s}\mathrm{i}\mathrm{l}}={\mathrm{K}}_{\mathrm{r}\mathrm{i}\mathrm{v}}-{\mathrm{K}}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}$$

(21)

According to relevant studies, the concentrations of the input of Ca²⁺ and Mg²⁺ due to the chemical weathering of silicate rocks (mainly anorthite) in rivers can be calculated according to the following formula [33]:

$${\mathrm{C}\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}={\mathrm{N}\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}\times {\left(\mathrm{C}\mathrm{a}/\mathrm{N}\mathrm{a}\right)}_{\mathrm{s}\mathrm{i}\mathrm{l}}$$

(22)

$${\mathrm{M}\mathrm{g}}_{\mathrm{s}\mathrm{i}\mathrm{l}}={\mathrm{N}\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}\times {\left(\mathrm{M}\mathrm{g}/\mathrm{N}\mathrm{a}\right)}_{\mathrm{s}\mathrm{i}\mathrm{l}}$$

(23)

The values of (Ca/Na)_sil and (Mg/Na)_sil were approximately 0.08 and 0.03 in river sand samples from the Naqu river. The molar concentration of the input of Si into the river due to the chemical weathering of silicate rock and the rate of contribution of the ion input into the river can be calculated according to the following formula:

$${\mathrm{S}\mathrm{i}}_{\mathrm{s}\mathrm{i}\mathrm{l}}={\mathrm{S}\mathrm{i}}_{\mathrm{r}\mathrm{i}\mathrm{v}}$$

(24)

$$\frac{{\mathrm{T}\mathrm{Z}}_{\mathrm{s}\mathrm{i}\mathrm{l}}^{+}}{{\mathrm{T}\mathrm{Z}}^{+}}=\frac{{\mathrm{N}\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}+{\mathrm{K}}_{\mathrm{s}\mathrm{i}\mathrm{l}}+2{\times \mathrm{C}\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}+2\times {\mathrm{M}\mathrm{g}}_{\mathrm{s}\mathrm{i}\mathrm{l}}}{{\mathrm{T}\mathrm{Z}}^{+}}$$

(25)

The concentration of the input of Ca²⁺ and Mg²⁺ due to the chemical weathering of carbonate rocks can be calculated according to the following formula:

$$\frac{{\mathrm{T}\mathrm{Z}}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}^{+}}{{\mathrm{T}\mathrm{Z}}^{+}}=\frac{2\times \left({\mathrm{C}\mathrm{a}}_{\mathrm{r}\mathrm{i}\mathrm{v}}-{\mathrm{C}\mathrm{a}}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}-{\mathrm{C}\mathrm{a}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}-{\mathrm{C}\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}\right)+2\times \left({\mathrm{M}\mathrm{g}}_{\mathrm{r}\mathrm{i}\mathrm{v}}-{\mathrm{M}\mathrm{g}}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}-{\mathrm{M}\mathrm{g}}_{\mathrm{s}\mathrm{i}\mathrm{l}}\right)}{{\mathrm{T}\mathrm{Z}}^{+}}$$

(26)

2.2.4. Methods for Analyzing Chemical Weathering and Carbon Sink Effects in Rocks

The chemical weathering rates of rock minerals (t/km²/yr) can be measured by using the concentration of dissolved ions that are fed into a river by the weathering products of rock minerals per unit area [34,35]. The contribution of HCO₃⁻ to the chemical weathering of silicate and carbonate rocks can be calculated according to the following formula [36]:

$${{[\mathrm{H}\mathrm{C}\mathrm{O}}_3]}_{\mathrm{s}\mathrm{i}\mathrm{l}}={\mathrm{K}}_{\mathrm{s}\mathrm{i}\mathrm{l}}+{\mathrm{N}\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}+2{\mathrm{C}\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}+{2\mathrm{M}\mathrm{g}}_{\mathrm{s}\mathrm{i}\mathrm{l}}$$

(27)

$${{[\mathrm{H}\mathrm{C}\mathrm{O}}_3]}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}={{[\mathrm{H}\mathrm{C}\mathrm{O}}_3]}_{\mathrm{r}\mathrm{i}\mathrm{v}}-{{[\mathrm{H}\mathrm{C}\mathrm{O}}_3]}_{\mathrm{s}\mathrm{i}\mathrm{l}}$$

(28)

In this formula, [HCO₃]_sil and [HCO₃]_carb are HCO₃⁻ ions produced by the chemical weathering of silicate rock and carbonate rock, respectively. The units of [HCO₃]_sil and [HCO₃]_carb are both mmol/L, and the other symbols have the same meaning as those mentioned above.

The HCO₃⁻ ions produced through the H₂SO₄ erosion of carbonate rocks can be calculated as follows [36]:

$${{[\mathrm{H}\mathrm{C}\mathrm{O}}_3]}_{\mathrm{s}\mathrm{u}\mathrm{l}\mathrm{f}\mathrm{u}\mathrm{r}\mathrm{i}\mathrm{c} \mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}}=2\times \left({{[\mathrm{S}\mathrm{O}}_4]}_{\mathrm{r}\mathrm{i}\mathrm{v}}-{{[\mathrm{S}\mathrm{O}}_4]}_{\mathrm{c}\mathrm{y}\mathrm{c}\mathrm{l}}-{{[\mathrm{S}\mathrm{O}}_4]}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}\right)$$

(29)

In the formula, [HCO₃]_{sulfuric acid} is the HCO₃⁻ ion produced through the erosion of carbonate rock by H₂SO₄, and the unit of each element in the formula is mmol/L.

The chemical weathering rate of evaporative salt rock (EWR), the chemical weathering rate of silicate rock (SWR), and the chemical weathering rate of carbonate rock (CWR) in the Naqu River basin can be calculated according to the following formula [32]:

$$\mathrm{E}\mathrm{W}\mathrm{R}=\frac{\left({\mathrm{C}\mathrm{a}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}+{\mathrm{N}\mathrm{a}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}+{\left[{\mathrm{S}\mathrm{O}}_4\right]}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}+{\mathrm{C}\mathrm{l}}_{\mathrm{e}\mathrm{v}\mathrm{a}\mathrm{p}}\right)\times \mathrm{F}\mathrm{l}\mathrm{o}\mathrm{w}}{\mathrm{A}}$$

(30)

$$\mathrm{S}\mathrm{W}\mathrm{R}=\frac{\left({\mathrm{K}}_{\mathrm{s}\mathrm{i}\mathrm{l}}+{\mathrm{N}\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}+{\mathrm{C}\mathrm{a}}_{\mathrm{s}\mathrm{i}\mathrm{l}}+{\mathrm{M}\mathrm{g}}_{\mathrm{s}\mathrm{i}\mathrm{l}}+{\mathrm{S}\mathrm{i}\mathrm{O}}_2\right)\times \mathrm{F}\mathrm{l}\mathrm{o}\mathrm{w}}{\mathrm{A}}$$

(31)

$$\mathrm{C}\mathrm{W}\mathrm{R}=\frac{\left({\mathrm{C}\mathrm{a}}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}+{\mathrm{M}\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}+0.5{{[\mathrm{H}\mathrm{C}\mathrm{O}}_3]}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}+0.5{{[\mathrm{H}\mathrm{C}\mathrm{O}}_3]}_{\mathrm{s}\mathrm{u}\mathrm{l}}\right)\times \mathrm{F}\mathrm{l}\mathrm{o}\mathrm{w}}{\mathrm{A}}$$

(32)

In the formula, Flow represents the flow rate, A represents the watershed area, and the unit of X_evap, X_sil, and X_carb is mg/L.

The carbon sink effect of the chemical weathering of rocks in the watershed was mainly reflected by the CO₂ in the atmosphere or soil through the HCO₃⁻ ions generated by the chemical weathering of rocks. The CO₂ consumption flux of the chemical weathering of silicate and carbonate in the Naqu River basin can be calculated according to the following formula [32]:

$${\left[{\mathrm{C}\mathrm{O}}_2\right]}_{\mathrm{s}\mathrm{i}\mathrm{l}}=\frac{{{[\mathrm{H}\mathrm{C}\mathrm{O}}_3]}_{\mathrm{s}\mathrm{i}\mathrm{l}}\times \mathrm{F}\mathrm{l}\mathrm{o}\mathrm{w}}{\mathrm{A}}$$

(33)

$${\left[{\mathrm{C}\mathrm{O}}_2\right]}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}=\frac{0.5\times ({{[\mathrm{H}\mathrm{C}\mathrm{O}}_3]}_{\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{b}}-{{[\mathrm{H}\mathrm{C}\mathrm{O}}_3]}_{\mathrm{s}\mathrm{u}\mathrm{l}\mathrm{f}\mathrm{u}\mathrm{r}\mathrm{i}\mathrm{c} \mathrm{a}\mathrm{c}\mathrm{i}\mathrm{d}})\times \mathrm{F}\mathrm{l}\mathrm{o}\mathrm{w}}{\mathrm{A}}$$

(34)

In the formula, [CO₂]_sil and [CO₂]_carb are the CO₂ consumption flux during the chemical weathering of silicate rocks and carbonate rocks, respectively.

3. Results

3.1. Runoff

The precipitation and annual average temperature were 433.2 mm and 0.88 °C in 2019. About 73% of the precipitation occurred in July, August, and September, with the largest precipitation occurring in July, reaching 169.8 mm. The winter temperature in the Naqu River basin was below 0 °C, the average temperature was −9.2 °C, and the lowest temperature reached −10.6 °C.

As can be seen from

Table 1. Drainage and runoff of the main rivers in the Naqu River basin.

River	Drainage Area /km2	Runoff /108 m3	Runoff Depth /mm
Upstream of the main Naqu stream	6412.6	0.15	2.29
Middle of the main Naqu stream	3962	0.16	4.01
Downstream of the main Naqu stream	4338.8	0.32	7.33
Mugequ	2103	0.019	0.92
Chengqu	1090	0.005	0.47
Gongqu	1232	0.012	0.94
Luoqu	1479	0.024	1.60

, in 2019, the areas of the upper-, middle-, and downstream parts of the Naqu River were 6412.6 km², 3962 km², and 4338.8 km², respectively, and the annual runoff was approximately 0.15 × 10⁸ m³, 0.16 × 10⁸ m³, and 0.32 × 10⁸ m³, respectively. The four main tributaries of the Naqu River, Mugequ, Chengqu, Gongqu, and Luoquhave, had drainage areas of 2103 km², 1090 km², 1232 km², and 1479 km², respectively. Their annual runoff was approximately 0.019 × 10⁸ m³, 0.005 × 10⁸ m³, 0.012 × 10⁸ m³, and 0.024 × 10⁸ m³, respectively. Due to the relatively small annual runoff for Mugequ, Chengqu, and Gongqu, this study mainly focused on a runoff analysis of the upper-, middle-, and downstream parts of the Naqu River and its main tributary, Luoqu. The monthly precipitation, temperature, and runoff depth in the downstream area of the main Naqu stream are shown in Figure 4.

The discharge pattern of the Naqu River basin exhibited a singular mode that was characterized by a peak manifesting in the month of August, which coincided with the zenith of air temperatures and the heightened ablation activity experienced by the glaciers. The total runoff of Naqu River from July to September accounted for 70.90% of the water flux. The minimum, which was barely 0.9% of the runoff in August, happened in February. The peak occurred in August and was slightly lower in July and September in the middle of Luoqu. The total runoff from July to September accounted for 77.78% of the water flux. January had the least runoff, which made up just 0.6% of the August runoff. Overall, the seasonal fluctuation in runoff matched the changes in precipitation that the meteorological station recorded.

3.2. Physicochemical Parameters

The statistics of the major ions in the Naqu River basin are shown in

Table 2. Statistical results of the hydrochemical characteristics of the Naqu River.

Parameter	Min	Max	Mean	SD	CV/(%)
Ca2+/(mg/L)	12.1	81.25	37.77	19.23	50.92
Mg2+/(mg/L)	1.96	37.83	13.59	9.76	71.83
K+/(mg/L)	0.39	5.54	2.60	1.81	69.45
Na+/(mg/L)	2.19	63.1	22.42	18.18	81.09
Cl−/(mg/L)	2.48	28.68	7.14	6.74	94.43
HCO3−/(mg/L)	43.63	321.15	173.24	87.65	50.60
SO42−/(mg/L)	11.72	188.85	47.21	44.36	93.95
NO3−/(mg/L)	0.58	3.74	1.58	0.83	52.71
SiO2/(mg/L)	5.04	11.5	7.42	1.99	26.82
Sr/(mg/L)	0.048	1.69	0.27	0.42	1.57
TDS/(mg/L)	53	586	220.57	137.84	62.49
pH	7.65	10.08	8.93	0.66	7.38
Turbidity/NTU	4.36	172	52.17	53.46	102.48
DO/(mg/L)	8.28	31.27	17.26	6.90	40.00
ORP/(mV)	32	181	121.14	35.41	29.23
EC/(μS/cm)	81	915	341.43	213.85	62.63
Temperature/(°C)	10.55	21.7	15.24	3.52	23.10

Note: SD means standard deviation; CV means the coefficient of variation.

. The data were obtained by clustering and analyzing the data from the samples collected from the Naqu River Basin in 2019; since the water samples were from the same river, they received basically the same classification. As seen in Table 2, the SD values of HCO₃⁻, TDS, and EC were large, which indicated that the values of HCO₃⁻, TDS, and EC in the datasets were far away from their mean values, which may have been related to the sampling time. By analyzing the sample data, it was seen that the HCO₃⁻ concentration of the river water in the upper reaches of the main stream of the Naqu River was the maximum value in the whole basin for four consecutive years, and the TDS and EC values of the river water in the Naqu River Basin were the maximum values in the whole basin in each sampling year.

It also can be seen in Table 2 that the measured temperature was 10.55–21.7 °C, and the highest temperature of the river appeared in the upstream area of the Chengqu river. The Naqu River basin was found to be alkaline with an average pH value of 8.93, which varied from 7.65 to 10.08 in August 2019. The coefficient of variation (CV) of the pH in the Naqu River basin was relatively small (7.38%), indicating that the pH value of the river may have changed less as a function of space. The turbidity of the Naqu River basin was between 4.36 and 172 NTU, with an average of 52.17 NTU. Its CV was the highest, as it exceeded 1, indicating that the turbidity may have had a significant variation as a function of space. The EC ranged from 53 to 586 μS/cm, with an average of 220.57 μS/cm. The variation trends of EC and TDS in the Naqu River basin were basically consistent. The value of DO was between 8.28 and 31.27 mg/L, with an average of 17.26 mg/L.

As calculated by using the values in Table 2, the total cationic charge (TZ⁺, TZ⁺ = 2Ca²⁺ + 2Mg²⁺ + K⁺ + Na⁺) of the river samples from the Naqu River basin was between 0.87 meq/L and 10.10 meq/L, with an average value of 4.06 meq/L. The total anionic charge (TZ⁻, TZ⁻ = HCO₃⁻ + Cl⁻ + 2CO₃²⁻ + 2SO₄²⁻) of the anions was between 1.06 meq/L and 10.02 meq/L, with an average value of 4.08 meq/L. Most of the NICB in the river was between −3% and 3% (

Table 3. Hydrochemical types of the main rivers in the Naqu River basin.

River	Elevation	Sampling Time	Hydrochemical Type	NICB
	(m)	(yyyy-mm-dd)
Upstream of the main Naqu stream	4756	2019-08-15	Ca-HCO3	0.0081
Middle of the main Naqu stream	4539	2019-08-15	Mg-HCO3	−0.0117
Downstream of the main Naqu stream	4455	2019-08-16	Na-HCO3	0.0382
Sangqu	4628	2019-08-15	Ca-HCO3	0.0055
Basuoqu	4710	2019-08-15	Ca-HCO3	−0.0106
Mumuqu	4625	2019-08-15	Ca-HCO3	−0.0018
Upstream of Chengqu stream	4522	2019-08-15	Ca-HCO3	0.0036
Downstream of Chengqu stream	4499	2019-08-15	Ca-HCO3	−0.0503
Zongqingqu	4565	2019-08-15	Ca-HCO3	−0.1216
Mugequ main stream	4590	2019-08-14	Na-HCO3	0.0058
Mugequ basin tributary	4697	2019-08-14	Ca-HCO3	−0.0314
Gongqu main stream	4574	2019-08-16	Ca-HCO3	0.2221
Gongqu basin tributary	4489	2019-08-16	Ca-HCO3	−0.0584
Luoqu main stream	4986	2019-08-16	Ca-HCO3	−0.2119

), indicating that the anions and cations may have changed their relative balance and showing that the test results were credible. According to the mean value of the ionic concentration, the order of cation concentration of the Naqu River basin was Ca²⁺ > Na⁺ > Mg⁺ > K⁺, and the order of anion concentration was HCO₃⁻ > SO₄²⁻ > Cl⁻ > NO₃⁻. HCO₃⁻ and SO₄²⁻ were the main anions in the river. The mean concentration of HCO₃⁻ was lower than that in the middle and upper reaches of the Yarlung Tsangpo river (63.44 mg/L) [37] and about 3.34 times the average concentration in rivers around the world (51.8 mg/L) [38]. However, the CV of SO₄²⁻ was large (93.95%), indicating that the content of SO₄²⁻ in the main stream and tributaries of the Naqu River may have varied greatly as a function of space.

The spatial variation in the TDS content in the Naqu River basin is shown in Figure 5. It can be seen in Table 2 and Figure 5 that, in the whole Naqu River basin, the TDS values were between 53 mg/L and 586 mg/L, with an average value of 220.57 mg/L in August 2019. Combined with the analysis of the sampling data from 2016 to 2018, it can be seen that during these three years, the TDS values in the Naqu catchment were between 85.67 and 548.89 mg/L, with an average value of 241.36 mg/L. The TDS values were between 85.67 and 548.89 mg/L in 2016, while they were between 115 and 455 mg/L in 2017, and they were between 120 and 512 mg/L in 2018. It can be seen that from 2016 to 2019, the TDS values in the Naqu River basin had little change. From 2016 to 2018, the minimum value of the TDS content had an upward trend, while the maximum value decreased, which may have been related to the runoff of the Naqu River basin in each sampling year. The maximum TDS values appeared in the upper reaches of the Naqu River, while the TDS showed a decreasing trend from upstream to downstream, which may have been caused by the increasing runoff.

In this study, the hydrochemical types were used to study the hydrochemical characteristics of the Naqu River basin [33,39]. Through the analysis of river sample data collected in August 2019, the chemical characteristics of the main stream and a tributary stream in the Naqu River basin were determined and are shown in Table 3. The milli-equivalent proportions (meq%) of predominant ions are graphically delineated on a Piper diagram [40], which was subsequently extrapolated onto a central diamond field to discern and appraise the hydrogeochemical facies and typologies inherent in these aqueous specimens, as shown in Figure 6.

Considering these results in combination with those of our previous study [22], from 2016 to 2018, the most common type of water chemistry in the Naqu River basin was that with Ca-HCO₃, which accounted for about 79.24%. The hydrochemical type was that of Ca-HCO₃ in the upper reaches of Naqu, Sangqu, Basuoqu, and Mumuqu over the four years of the study (2016–2019); the hydrochemical type in the middle reaches of the Naqu River was that of Mg-HCO₃, which indicated that the weathering of silicate may have affected the sources of ions in this section of the river. Except for 2019, the main stream of the Gongqu River showed the hydrochemical type of Ca-HCO₃; for the other three years, the hydrochemical type was Ca-SO₄, indicating the contribution of evaporative or sulfide minerals in those three years [41].

4. Discussion

4.1. Provenance of Solutes in River Water

4.1.1. Qualitative Analysis of Different Sources of Riverine Solutes

Our previous research used ion ratio diagrams and the Pearson correlation coefficient to qualitatively analyze the sources of ions in the Naqu River basin from 2016 to 2018 [22]. Two different analysis methods can be mutually corroborated. Combining the formulas and the Pearson correlation coefficient, it was found that during 2016–2018, Ca²⁺ had the strongest correlation with Mg²⁺ among the three cations of K⁺, Na⁺, and Mg²⁺, indicating that Ca²⁺ and Mg²⁺ may have had a common source. Na⁺ and K⁺ had the strongest correlation, indicating the chemical weathering of silicate rocks containing Na⁺ and K⁺. Through the analysis of the ions sampled in the Naqu River basin in 2019, the same results were found. The difference was that in 2019, the correlation coefficient of SiO₂ in the river was analyzed, and it was found that SiO₂ had the strongest correlation with K⁺ among the four cations of K⁺, Na⁺, Ca²⁺, and Mg²⁺, followed by Na⁺, thus further indicating the chemical weathering of silicate rocks containing Na⁺ and K⁺. SiO₂ and HCO₃⁻ had the strongest correlation among the anions, which further indicated that HCO₃⁻ may have been mainly involved in the chemical weathering of silicate rocks in the Naqu River basin [42].

The analysis based on the Pearson correlation coefficient was also confirmed by ion ratio diagrams. Combining our results with those of previous studies, when analyzing ions in the Naqu River basin using ion ratio diagrams, it was found that during 2016–2018, carbonic-acid-mediated reactions in the Naqu River basin played a leading role in rivers other than the main stream of Gongqu. The ion analysis results of all rivers in the Naqu River basin in 2019 showed that the carbonic-acid-mediated reaction was mainly the result of the chemical weathering of rocks. The analysis of water samples from the Naqu River basin’s middle and upper regions from 2016 to 2018 showed that the chemical weathering of gypsum (CaSO₄·2H₂O) or anhydrite (CaSO₄) had a certain effect on the chemical composition of the Naqu River basin [22]. It was also speculated that the chemical weathering of carbonate rocks had a great influence on the chemical composition of the water of Basuoqu, Mumuqu, Zongqingqu, and Luoqu. The chemical weathering of silicate rock may have had a great impact on the source of ions in the main stream of Mugequ. The water chemistry characteristics of Chengqu, the upstream area of the main Naqu stream, and the downstream area of the main Naqu stream were predominantly governed by the processes of carbonate and silicate weathering.

4.1.2. Quantification of Different Sources of Riverine Solutes

The average elevation of the Naqu River basin is as high as 4500 m, with little human activity. Moreover, the concentration of NO₃⁻ in the rivers of the Naqu River basin was low, with an average value of 1.58 mg/L in 2019. Therefore, the contribution of human activities to dissolved ions in the Naqu River was not considered.

The calculations of the atmospheric precipitation inputs are shown in

Table 4. Ionic concentration due to atmospheric precipitation.

Basin	Ca2+	Mg2+	K+	Na+	SO42−	Average Contribution Rate
	μmol/L					%
Naqu River	4.12	7.95	0.16	1.00	9.39	0.88 (0.25–2.89)

. The rate of contribution of inputs of atmospheric precipitation to the ions in the rivers of the Naqu River basin was between 0.25% and 2.89%, and the average contribution rate was 0.88%.

This study examined the chemical weathering of rocks under the action of gypsum hydrolysis and sulfuric acid in three different scenarios. The corresponding α_evap values were 0, 1, and 0.5, respectively. The calculations are shown in

Table 5. Contributions of solutes in each reservoir of the Naqu River basin.

River	Elevation (m)	Precipitation (%)	Evaporite (%)			Silicate (%)	Carbonate (%)
			Scenario 1	Scenario 2	Scenario 3		Scenario 1	Scenario 2	Scenario 3
Upstream of the main Naqu stream	4756	0.25	7.87	46.64	27.25	25.01	66.87	28.11	47.49
Middle of the main Naqu stream	4539	0.50	5.05	30.65	17.85	33.14	61.32	35.72	48.52
Downstream of the main Naqu stream	4455	0.51	5.60	21.89	13.75	37.03	56.85	40.57	48.71
Sangqu	4628	0.41	3.37	28.73	16.05	26.82	69.40	44.04	56.72
Basuoqu	4710	0.49	2.47	14.31	8.39	13.09	83.95	72.11	78.03
Mumuqu	4625	0.99	2.23	12.98	7.61	14.42	82.36	71.61	76.99
Upstream of Chengqu stream	4522	0.50	2.72	16.54	9.63	37.19	59.58	45.76	52.67
Downstream of Chengqu stream	4499	0.52	3.42	21.68	12.55	32.55	63.51	45.25	54.38
Zongqingqu	4565	1.21	3.68	32.30	17.99	6.02	89.08	60.47	74.78
Mugequ main stream	4590	0.84	7.53	20.55	14.04	39.78	51.85	38.83	45.34
Mugequ basin tributary	4697	1.01	2.67	33.53	18.10	14.43	81.89	51.04	66.46
Gongqu main stream	4574	1.24	4.24	31.06	17.65	7.38	87.14	60.32	73.73
Gongqu basin tributary	4489	1.02	3.10	40.17	21.63	13.84	82.05	44.98	63.51
Luoqu main stream	4986	2.89	8.80	34.60	21.70	3.55	84.75	58.95	71.85

; the average rate of contribution of the chemical weathering of evaporite to the ions in the rivers of the Naqu River basin was the greatest in scenario 2. In scenario 3, the chemical weathering of evaporite played an important role in the upper regions of the Naqu River basin, and its rate of contribution was 27.25%. This also confirmed the qualitative analysis of the chemical weathering in the upstream area of the main Naqu stream. In addition, the chemical weathering of evaporite had a relatively high rate of contribution (21.7%) to the ion composition of the Luoqu River’s primary tributary.

According to the formula that was used for the calculations, the rate of contribution of the chemical weathering of silicate in the Naqu River basin ranged from 3.55% to 39.78%, with an average rate of contribution of 21.73%. The silicate rocks’ chemical weathering had the highest rate of contribution (39.78%) to the ionic composition of the primary tributary of the Mugequ River. This observation aligns seamlessly with the antecedent qualitative scrutiny of the impact arising from chemical weathering [22].

In concordance with the three input scenarios involving evaporite chemical weathering, the percentage of major ions in the Naqu River attributed to the chemical weathering of carbonate rock markedly exceeded that originating from the chemical weathering of evaporite rock and silicate rock. In scenario 3, a preeminence in the Basuoqu River was discerned, with the chemical weathering of carbonate registering the highest rate of contribution of 78.03%. Furthermore, the rates of contribution of the weathering of carbonate rock for Mumuqu, Zongqingqu, the main Gongqu stream, and the main Luoqu stream all exceeded 70%, which was in agreement with the qualitative analysis expounding the impact of chemical weathering on these riverine systems. In general, carbonates weather more easily than silicates do [43]. Consequently, a quantitative analysis of the contributions of different river-water ion sources revealed that carbonates were the most weathered. In the whole Naqu River basin, the sources contributing the main ions were: carbonate rock chemical weathering > silicate rock chemical weathering > evaporite rock chemical weathering > atmospheric precipitation input.

Combining this with the results of our previous qualitative research on the origin of ions in the Naqu River basin, atmospheric precipitation had a small contribution to the Naqu River basin, while the chemical weathering of carbonate rock contributed more to the sources of ions, and the chemical weathering of silicate rock and evaporite rock was also an important ion source [22]. It can be seen that the results of this quantitative analysis are basically consistent with the results of the qualitative analysis, confirming the reliability of this research.

The Nujiang–Salween River originates on the Tibetan Plateau. The maximum NO₃⁻ concentration in the Naqu River basin was 3.74 mg/L, which was higher than the maximum NO₃⁻ concentration of 2.82 mg/L in the Nujiang–Salwen River according to previous studies [44,45]. The maximum SiO₂ concentration in the Nujiang–Salwen River was 11.5 mg/L, and the CV was 23.82%. Similarly, the CV of SiO₂ in the Nujiang–Salwen River was 21.62% [46], indicating that the SiO₂ concentration may change relatively smoothly. Moreover, the concentration of SiO₂ in the samples of the Nujiang–Salween River in Hpa’an, Myanmar was mostly higher than that in the primary tributary of the Nujiang–Salween River in China [47], which may have been because the weathering of silicate rocks occurred more intensely in the Nujiang–Salween River of Myanmar [44].

4.2. Analysis of the Chemical Weathering Rates of Rocks and Carbon Sink Effects in Watersheds

Table 6. Chemical weathering rates of rocks in the upstream and middle parts of Naqu River basin in 2019.

EWR			SWR	CWR			Sum
Scenario 1	Scenario 2	Scenario 3		Scenario 1	Scenario 2	Scenario 3	Scenario 1	Scenario 2	Scenario 3
t/km2/yr
2.20	9.63	5.92	6.82	16.84	11.32	14.08	25.87	27.78	26.82

shows the calculation results for the chemical weathering rates of rocks in the Naqu River basin’s mid-upper regions.

In the three scenarios, the rate of chemical alteration of carbonate rocks was the highest. In scenario 1, the maximum rate of chemical weathering of carbonate rocks reached 16.84 t/km²/yr, and the minimum rate of chemical weathering of evaporative rock salt was 2.20 t/km²/yr. In scenario 3, as shown in Table 7, the order of rates of chemical weathering of different lithologies in the Naqu River basin’s mid-upper regions was: carbonate rock > silicate rock > evaporite rock.

The rate of chemical metamorphism of silicate rocks in the middle and upper regions of the main Naqu stream was 6.82 t/km²/yr. As shown in other studies, the rates of chemical weathering of silicate rocks in the Nujiang River basin were 5.90 t/km²/yr according to Wu [35], 4.27 t/km²/yr according to Tao [17], and 5.10 t/km²/yr according Zhang [44]. It can be seen that the rate of chemical weathering of silicate rocks in the middle and upper regions of the main Naqu stream was higher than that in the middle and lower reaches of the Nujiang River.

The carbon sink capacity of chemical weathering In the Naqu River basin’s mid-upper regions is shown in Table 7. Under the three conditions, the CO₂ consumption fluxes of the chemical weathering of silicate and carbonate in the upper and middle reaches of the Naqu River were 315.07 × 103 mol/km²/yr, 369.72 × 103 mol/km²/yr, and 342.40 × 103 mol/km²/yr. According to the analysis, the carbon sink capacity of chemical weathering in the mid-upper regions of the main Naqu stream showed that the CO₂ consumption flux of the chemical weathering of silicate was more intense than that of the chemical weathering of carbonate. Zhang’s study [45] showed that under the alkaline conditions of the Qinghai–Tibetan Plateau, the weathering of siliciclastic evolutionary rocks and alkalization of lakes are mutually reinforcing, resulting in the precipitation of large quantities of carbonate rocks. However, the IPCC’s Fifth Assessment Report emphasized that the chemical weathering of carbonate rocks not only produces carbon sinks, but the resulting sinks are also stable [46]. Thus, inland aquatic systems assume a pivotal role in the intricate dynamics of the global carbon cycle, and their strategic position in combating climate change is gradually coming to the fore, while the strong chemical erosion of the waterways of the Tibetan Plateau and the resulting high rate of carbon dioxide depletion play an important role in slowing down climate warming [47].

The calculation methods for the CO₂ consumption rates of other rivers with corresponding lithologies and climate conditions were analyzed. The CO₂ consumption flux of the chemical weathering of silicate in the middle and upper reaches of the Naqu River was 248.60 × 103 mol/km²/yr, which was 2.26 times that in the Nujiang River basin (110 × 103 mol/km²/yr) [34], 5.43 times that in the Tuotuo River (45.8 × 103 mol/km²/yr) [48], and about 55.41% of that in the Ganges River (448.611 × 103 mol/km²/yr) [34].

Table 7. Comparison of the carbon sink capacities of different catchments.

River	Drainage Area	[CO2]sil		[CO2]carb		Sum		Reference
	106 km2	108 mol/yr	103 mol/km2/yr	108 mol/yr	103 mol/km2/yr	108 mol/yr	103 mol/km2/yr
The upstream and middle areas of Naqu River basin	0.01	25.79	248.60	6.90	66.47	32.69	315.07	Scenario 1
	0.01	25.79	248.60	12.57	121.13	38.36	369.72	Scenario 2
	0.01	25.79	248.60	9.73	93.80	35.52	342.40	Scenario 3
Nujiang River	0.11	120	110	650	590	890	810	Wu et al. [35]
Tuotuo River	0.016	7.3	45.8	21.4	135	-	-	Li et al. [49]
Ganges River	1.05	4710	448.611	2360	224.8	-	692	Gaillardet et al. [50]

Table 7. Comparison of the carbon sink capacities of different catchments.

River	Drainage Area	[CO2]sil		[CO2]carb		Sum		Reference
	106 km2	108 mol/yr	103 mol/km2/yr	108 mol/yr	103 mol/km2/yr	108 mol/yr	103 mol/km2/yr
The upstream and middle areas of Naqu River basin	0.01	25.79	248.60	6.90	66.47	32.69	315.07	Scenario 1
	0.01	25.79	248.60	12.57	121.13	38.36	369.72	Scenario 2
	0.01	25.79	248.60	9.73	93.80	35.52	342.40	Scenario 3
Nujiang River	0.11	120	110	650	590	890	810	Wu et al. [35]
Tuotuo River	0.016	7.3	45.8	21.4	135	-	-	Li et al. [49]
Ganges River	1.05	4710	448.611	2360	224.8	-	692	Gaillardet et al. [50]

5. Conclusions

Based on river samples collected in 2019, in this study, the primary ion fluxes in the main stream and tributaries of the Naqu River were estimated in August of each year from 2016 to 2019. Based on the forward model, the contributions of atmospheric precipitation, the chemical weathering of evaporated saline rocks, the chemical weathering of silicate rocks, and the chemical weathering of carbonate rocks to the dissolved ions in river water were quantitatively estimated, and the rate of chemical weathering of rocks in the mid-upper watersheds of the Naqu River and the carbon sink effect were analyzed; finally, the following conclusions were drawn:

a

The changes in terms of space of the TDS and EC values in the Naqu River watershed were the same; they decreased in concentration from upstream to downstream. The pH changed little as a function of space, but the difference was that the turbidity changed greatly. Ca-HCO₃ was the main type of water chemistry in the Naqu River basin.

b

Based on the forward model, the contributions of different sources of river water ions were quantitatively analyzed, and they are listed in descending order: chemical weathering of carbonate rocks > chemical weathering of silicate rocks > chemical weathering of evaporite saline rocks > atmospheric precipitation input.

c

By analyzing the chemical weathering and carbon sink effects in the watershed, three scenarios were hypothesized based on the percentage of SO₄²⁻ production from rock weathering, with carbonate chemical weathering rates being the largest. In the upstream and middle of the Naqu River basin, the chemical weathering of silicate rocks consumed more CO₂ than that of carbonate rocks did.