5.1 Atmospheric and anthropogenic inputs

To evaluate atmospheric inputs to river water, chloride is the most commonly used reference. Generally, water samples that have the lowest Cl⁻ concentrations are employed to correct the proportion of atmospheric inputs in a river system (Négrel et al., 1993; Gaillardet et al., 1997; Viers et al., 2001; Xu and Liu, 2007). In pristine areas, the concentration of Cl⁻ in river water is assumed to be entirely derived from the atmosphere, provided that the contribution of evaporites is negligible (e.g., Stallard and Edmond, 1981; Négrel et al., 1993). In the SECRB, the lowest Cl⁻ concentration was mainly found in the headwater of each river. According to the geologic setting, no salt-bearing rocks were found in these headwater areas (FJBGRM, 1985; ZJBGMR, 1989). In addition, these areas are mainly mountainous and sparsely populated. Therefore, we assumed that the lowest Cl⁻ concentration of samples from the headwater of each major river came entirely from the atmosphere.

Table 2Chemical compositions of precipitation at different sites located within the studied area (in µmol L⁻¹ and molar ratio).

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The proportion of atmosphere-derived ions in river water can then be calculated by the element ∕ Cl ratios of the rain. Chemical compositions of rain in the studied area have been reported at different sites, including Hangzhou, Jinhua, Nanping, Fuzhou and Xiamen (Zhao, 2004; M. Zhang et al., 2007; Huang et al., 2008; Cheng et al., 2011; Xu et al., 2011) (Fig. 1). The volume-weighted mean concentration of ions and Cl-normalized molar ratios are compiled in Table 2. Based on this procedure, 6.6 %–23.4 % (averaging 14.3 %) of total dissolved cations in the major rivers of the SECRB originated from rain. Among the anions, ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ and ${\mathrm{NO}}_{\mathrm{3}}^{-}$ in the rivers are mainly from the atmospheric input, averaging 73.2 % for ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ and 75.8 % for ${\mathrm{NO}}_{\mathrm{3}}^{-}$, respectively.

As one of the most developed and populated areas in China, the chemistry of river water in the SECRB could be significantly impacted by anthropogenic inputs. Cl⁻, ${\mathrm{NO}}_{\mathrm{3}}^{-}$ and ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ are commonly associated with anthropogenic sources and have been used as tracers of anthropogenic inputs in watershed. High concentrations of Cl⁻, ${\mathrm{NO}}_{\mathrm{3}}^{-}$ and ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ can be found in the lower reaches of rivers in the SECRB, and there is an obvious increase after it had flowed through plains and cities. This tendency indicates that the river water chemistry is affected by anthropogenic inputs while passing through the catchments. After correcting for the atmospheric contribution to river water, the following assumption is needed to quantitatively estimate the contributions of anthropogenic inputs, which is that Cl⁻ originates from only atmospheric and anthropogenic inputs, and the excess of atmospheric Cl⁻ is regarded to present anthropogenic inputs and is balanced by Na⁺.

5.2 Chemical weathering inputs

Water samples were plotted with Na-normalized molar ratios (Fig. 3). The values of the world's major rivers (Gaillardet et al., 1999) are also shown in the figure. The best correlations between elemental ratios were observed for ${\mathrm{Ca}}^{\mathrm{2}+}/{\mathrm{Na}}^{+}$ vs. ${\mathrm{Mg}}^{\mathrm{2}+}/{\mathrm{Na}}^{+}$ (R²=0.95, n=120) and ${\mathrm{Ca}}^{\mathrm{2}+}/{\mathrm{Na}}^{+}$ vs. ${\mathrm{HCO}}_{\mathrm{3}}^{-}/{\mathrm{Na}}^{+}$ (R²=0.98, n=120). The samples cluster on a mixing line, mainly between silicate and carbonate end-members, closer to the silicate end-member and show little evaporite contribution. This corresponds with the rock type distributions in the SECRB. In addition, all water samples have equivalent ratios of $({\mathrm{Na}}^{+}+{\mathrm{K}}^{+})/{\mathrm{Cl}}^{-}$ larger than one, indicating silicate weathering as the source of Na⁺ and K⁺ rather than dissolution of chloride evaporites.

Figure 3Mixing diagrams using Na-normalized molar ratios: (a) ${\mathrm{HCO}}_{\mathrm{3}}^{-}/{\mathrm{Na}}^{+}$ vs. ${\mathrm{Ca}}^{\mathrm{2}+}/{\mathrm{Na}}^{+}$ and (b) ${\mathrm{Mg}}^{\mathrm{2}+}/{\mathrm{Na}}^{+}$ vs. ${\mathrm{Ca}}^{\mathrm{2}+}/{\mathrm{Na}}^{+}$ for the SECRB. The samples mainly cluster on a mixing line between silicate and carbonate end-members. Data for the world's major rivers are also plotted (data from Gaillardet et al., 1999).

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The geochemical characteristics of the silicate and carbonate end-members can be deduced from the correlations between elemental ratios and referred to literature data for catchments with well-constrained lithology. After correction for atmospheric inputs, the ${\mathrm{Ca}}^{\mathrm{2}+}/{\mathrm{Na}}^{+}$, ${\mathrm{Mg}}^{\mathrm{2}+}/{\mathrm{Na}}^{+}$ and ${\mathrm{HCO}}_{\mathrm{3}}^{-}/{\mathrm{Na}}^{+}$ of the river samples ranged from 0.31 to 30, 0.16 to 6.7 and 1.1 to 64.2, respectively. According to the geological setting (Fig. 1), there are some small rivers draining purely silicate areas in the SECR drainage basins. Based on the elemental ratios of these rivers, we assigned the silicate end-member for this study to ${\mathrm{Ca}}^{\mathrm{2}+}/{\mathrm{Na}}^{+}$ $=\mathrm{0.41}\pm \mathrm{0.10}$, ${\mathrm{Mg}}^{\mathrm{2}+}/{\mathrm{Na}}^{+}$ $=\mathrm{0.20}\pm \mathrm{0.03}$ and ${\mathrm{HCO}}_{\mathrm{3}}^{-}/{\mathrm{Na}}^{+}$ $=\mathrm{1.7}\pm \mathrm{0.6}$. The ratio of $({\mathrm{Ca}}^{\mathrm{2}+}+{\mathrm{Mg}}^{\mathrm{2}+})/{\mathrm{Na}}^{+}$ for the silicate end-member was 0.61±0.13, which is close to the silicate end-member for the world's rivers ($({\mathrm{Ca}}^{\mathrm{2}+}+{\mathrm{Mg}}^{\mathrm{2}+})/{\mathrm{Na}}^{+}$ $=\mathrm{0.59}\pm \mathrm{0.17}$, Gaillardet et al., 1999). Moreover, previous research has documented the chemical composition of rivers, such as the Amur and the Songhuajiang in northern China, the Xishui in the lower reaches of the Changjiang, and major rivers in South Korea (Moon et al., 2009; Liu et al., 2013; Wu et al., 2013; Ryu et al., 2008; Shin et al., 2011). These river basins have similar lithological settings to the study area, so we could further validate the composition of the silicate end-member with their results. ${\mathrm{Ca}}^{\mathrm{2}+}/{\mathrm{Na}}^{+}$ and ${\mathrm{Mg}}^{\mathrm{2}+}/{\mathrm{Na}}^{+}$ ratios of silicate end-member were reported for the Amur (0.36 and 0.22), the Songhuajiang (0.44 ± 0.23 and 0.16), the Xishui (0.6 ± 0.4 and 0.32 ± 0.18), the Han (0.55 and 0.21) and six major rivers in South Korea (0.48 and 0.20) in the studies above, well bracketing our estimation for the silicate end-member.

Figure 4Calculated contributions (in %) from the different reservoirs to the total cationic load for major rivers and their main tributaries in the SECRB. The cationic loads are the sum of Na⁺, K⁺, Ca²⁺ and Mg²⁺ from different reservoirs.

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However, some samples show high concentrations of Ca²⁺, Mg²⁺ and ${\mathrm{HCO}}_{\mathrm{3}}^{-}$, indicating the contribution of carbonate weathering. The samples from the upper reaches (Sample 12 and 13) of the Qiantang fall close to the carbonate end-member documented for the world's major rivers (Gaillardet et al., 1999). In the present study, the ${\mathrm{Ca}}^{\mathrm{2}+}/{\mathrm{Na}}^{+}$ ratio of 0.41±0.10 and ${\mathrm{Mg}}^{\mathrm{2}+}/{\mathrm{Na}}^{+}$ ratio of 0.20±0.03 for the silicate end-member are used to calculate the contribution of Ca²⁺ and Mg²⁺ from silicate weathering. Finally, residual Ca²⁺ and Mg²⁺ are attributed to carbonate weathering.

5.3 Chemical weathering rate in the SECRB

Based on the above assumption, a forward model is employed to quantify the relative contribution of the different sources to the rivers of the SECRB in this study (e.g., Galy and France-Lanord, 1999; Moon et al., 2007; Xu and Liu, 2007, 2010; Liu et al., 2013). The calculated contributions of different reservoirs to the total cationic load for major rivers and their main tributaries in the SECRB are presented in Fig. 4. On average, the dissolved cationic loads of the rivers in the study area originate dominantly from silicate weathering, which accounts for 39.5 % (17.8 %–74.0 %) of the total cationic loads in molar unit. Carbonate weathering and anthropogenic inputs account for 30.6 % (3.9 %–62.0 %) and 15.7 % (0 %–41.1 %), respectively. Contributions from silicate weathering are high in the Ou (55.6 %), the Huotong (54.5 %), the Ao (48.3 %) and the Min (48.3 %) river catchments, which are dominated by granitic and volcanic bedrock. In contrast, a high contribution from carbonate weathering is observed in the Qiantang (54.0 %), the Jin (52.2 %) and the Jiulong (44.8 %) river catchments. The results manifest the lithology control on river solutes of drainage basin.

The chemical weathering rate of rocks is estimated by the mass budget, basin area and annual discharge (data from Hydrological data of river basins in Zhejiang, Fujian province and Taiwan region, Annual Hydrological Report of 2010, P. R. China, Table 3), expressed in t km⁻² a⁻¹. The silicate weathering rate (SWR) is calculated using major cationic concentrations from silicate weathering and assuming that all dissolved SiO₂ is derived from silicate weathering (Xu and Liu, 2010), as in the equation below:

The assumption about Si could lead to an overestimation of the silicate weathering rate, as part of the silica may come from dissolution of biogenic materials rather than the weathering of silicate minerals (Millot et al., 2003; Shin et al., 2011). Thus, the cationic silicate weathering rates (Cat_sil) were also calculated.

The carbonate weathering rate (CWR) is calculated based on the sum of Ca²⁺, Mg²⁺ and ${\mathrm{HCO}}_{\mathrm{3}}^{-}$ from carbonate weathering, with half of the ${\mathrm{HCO}}_{\mathrm{3}}^{-}$ coming from carbonate weathering and being derived from the atmosphere CO₂, as in the equation below:

The chemical weathering rate and flux are calculated for major rivers and their main tributaries in the SECRB, and the results are shown in Table 3. Silicate and carbonate weathering fluxes of these rivers (SWF and CWF) range from 0.02×10⁶ to 1.80×10⁶ t a⁻¹ and from 0.004×10⁶ to 1.74×10⁶ t a⁻¹, respectively. Out of the rivers, the Min has the highest silicate weathering flux, and the Qiantang has the highest carbonate weathering flux. On the whole SECRB scale, 3.95×10⁶ and 4.09×10⁶ t a⁻¹ of dissolved solids originating from silicate and carbonate weathering, respectively, are transported into the East and South China seas by rivers in this region. Compared with the largest three river basins (the Changjiang, the Huanghe and the Xijiang) in China, the flux of silicate weathering calculated for the SECRB is lower than the Changjiang (9.5×10⁶ t a⁻¹, Gaillardet et al., 1999) but higher than the Huanghe (1.52×10⁶ t a⁻¹, Fan et al., 2014) and the Xijiang (2.62×10⁶ t a⁻¹, Xu and Liu, 2010).

The silicate and carbonate chemical weathering rates for these river watersheds were 14.2–35.8 and 1.8–52.1 t km⁻² a⁻¹, respectively. The total rock weathering rate (TWR) for the whole SECRB is 48.1 t km⁻² a⁻¹, higher than the world average (24 t km⁻² a⁻¹, Gaillardet et al., 1999). The cationic silicate weathering rates (Cat_sil) range from 4.7 to 12.0 t km⁻² a⁻¹ for the river watersheds in the SECRB, averaging at 7.8 t km⁻² a⁻¹. Furthermore, a good linear correlation (R²=0.77, n=28) is observed between the Cat_sil and runoff (Fig. 5), indicating that silicate weathering rates are controlled by runoff as documented in previous research (e.g., Bluth and Kump, 1994; Gaillardet et al., 1999; Millot et al., 2002; Oliva et al., 2003; Wu et al., 2013; Pepin et al., 2013).

Figure 5Plots of the cationic–silicate weathering rate (Cat_sil) vs. runoff for the SECRB.

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Table 3Contribution of each reservoir, fluxes, chemical weathering and associated CO₂ consumption rates for the major rivers and their main tributaries in the SECRB.

Anth. noted for anthropogenic, Sil. for silicate, and Carb. for carbonate. ^a Cat_sil are calculated based on the sum of cations from silicate weathering. ^b SWR, CWR and TWR represent silicate weathering rates (assuming all dissolved silica is derived from silicate weathering), carbonate weathering rates and total weathering rates, respectively. ^c CO₂ consumption rate with assumption that all the protons involved in the weathering reaction are provided by carbonic acid. ^d Estimated CO₂ consumption rate by silicate weathering when H₂SO₄ from acid precipitation was taken into account. ^e Estimated CO₂ consumption rate by silicate weathering when both H₂SO₄ and HNO₃ from acid precipitation were taken into account.

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Figure 6Plots of total cations derived from (a) carbonate and silicate weathering vs. ${\mathrm{HCO}}_{\mathrm{3}}^{-}$ and (b) ${\mathrm{HCO}}_{\mathrm{3}}^{-}+{\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}+{\mathrm{NO}}_{\mathrm{3}}^{-}$ for river water in the SECRB.

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5.4 CO₂ consumption and the role of sulfuric acid

To calculate atmospheric CO₂ consumption by silicate weathering (CSW) and by carbonate weathering (CCW), a charge-balanced state between rock chemical weathering-derived alkalinity and cations was assumed (Roy et al., 1999).

The calculated CO₂ consumption rates by chemical weathering for the rivers in SECRB are shown in Table 3. CO₂ consumption rates by carbonate and silicate weathering are from 17.9 to 530×10³ mol km⁻² a⁻¹ (averaging at 206×10³ mol km⁻² a⁻¹) and from 167 to 460×10³ mol km⁻² a⁻¹ (averaging at 281×10³ mol km⁻² a⁻¹) for major river catchments in the SECRB. The CO₂ consumption rates by silicate weathering in the SECRB are higher than that of major rivers in the world and China, such as the Amazon (174×10³ mol km⁻² a⁻¹, Mortatti and Probst, 2003), the Mississippi and the Mackenzie (66.8 and 34.1×10³ mol km⁻² a⁻¹, Gaillardet et al., 1999), the Changjiang (112×10³ mol km⁻² a⁻¹, Chetelat et al., 2008), the Huanghe (35×10³ mol km⁻² a⁻¹, Fan et al., 2014), the Xijiang (154×10³ mol km⁻² a⁻¹, Xu and Liu, 2010), the Longchuanjiang (173×10³ mol km⁻² a⁻¹, Li et al., 2011), the Mekong (191×10³ mol km⁻² a⁻¹, Li et al., 2014), three large rivers in eastern Tibet (103–121×10³ mol km⁻² a⁻¹, Noh et al., 2009), the Hanjiang in central China (120×10³ mol km⁻² a⁻¹, Li et al., 2009) and the Sonhuajiang in northeastern China (66.6×10³ mol km⁻² a⁻¹, Liu et al., 2013). The high CO₂ consumption rates by silicate weathering in the SECRB could be attributed to extensive distribution of silicate rocks, high runoff and favorable climatic conditions. The regional fluxes of CO₂ consumption by silicate and carbonate weathering are about 47.9×10⁹ mol a⁻¹ (0.57×10¹² g C a⁻¹) and 41.9×10⁹ mol a⁻¹ (0.50×10¹² g C a⁻¹) in the whole SECRB.

However, in addition to CO₂, the anthropogenic-sourced proton (e.g., H₂SO₄ and HNO₃) is well documented as significant proton providers in rock weathering processes (Galy and France-Lanord, 1999; Karim and Veizer, 2000; Yoshimura et al., 2001; Han and Liu, 2004; Spence and Telmer, 2005; Lerman and Wu, 2006; Xu and Liu, 2007, 2010; Perrin et al., 2008; Gandois et al., 2011). Sulfuric acid can be generated by natural oxidation of pyrite and anthropogenic emissions of SO₂ from coal combustion and subsequently dissolve carbonate and silicate minerals. The riverine nitrate in a watershed can be derived from atmospheric deposition, synthetic fertilizers, microbial nitrification, sewage and manure, etc. (e.g., Kendall, 1998). Although it is difficult to determine the sources of nitrate in river water, we can at least simply assume that nitrate from acid deposition is one of the proton providers. The consumption of CO₂ by rock weathering would be overestimated if H₂SO₄- and HNO₃-induced rock weathering was ignored (Spence and Telmer, 2005; Xu and Liu, 2010; Shin et al., 2011; Gandois et al., 2011). Thus, the role that anthropogenic-sourced protons play on chemical weathering is crucial for an accurate estimation of CO₂ consumption by rock weathering.

Rapid economic growth and increased energy needs have resulted in severe air pollution problems in many areas of China, indicated by high levels of mineral acids (predominately sulfuric) observed in precipitation (Larssen and Carmichael, 2000; Pan et al., 2013; L. Liu et al., 2016). The national SO₂ emissions in 2010 reached 30.8 Tg a⁻¹ (Lu et al., 2011). Previous studies documented that fossil fuel combustion accounts for the most sulfur deposition (∼77 %) in China (L. Liu et al., 2016). The wet deposition rate of nitrogen is the highest over central and southern China, with mean values of 20.2, 18.2 and 25.8 kg N ha⁻¹ a⁻¹ in Zhejiang, Fujian and Guangdong provinces, respectively (Lu and Tian, 2007). Current sulfur and nitrogen depositions in the southeastern coastal region are still among the highest in China (Fang et al., 2013; Cui et al., 2014; L. Liu et al., 2016).

Figure 7(a) δ¹³C_DIC vs. ${\mathrm{HCO}}_{\mathrm{3}}^{-}/({\mathrm{Na}}^{+}+{\mathrm{K}}^{+}+{\mathrm{Ca}}^{\mathrm{2}+}+{\mathrm{Mg}}^{\mathrm{2}+})$^* and (b) $({\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}+{\mathrm{NO}}_{\mathrm{3}}^{-})/({\mathrm{Na}}^{+}+{\mathrm{K}}^{+}+{\mathrm{Ca}}^{\mathrm{2}+}+{\mathrm{Mg}}^{\mathrm{2}+})$^* equivalent ratio in river water draining the SECRB (^* noted concentrations corrected for atmospheric and anthropogenic inputs). The plots show that most water deviates from the three end-member mixing areas (carbonate weathering by carbonic acid and sulfuric/nitric acid and silicate weathering by carbonic acid).

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The involvement of protons originating from H₂SO₄ and HNO₃ in the river water can be illustrated by the stoichiometry between cations and anions, shown in Fig. 6. In the rivers of the SECRB, the sum of the cations released by silicate and carbonate weathering could not be balanced by ${\mathrm{HCO}}_{\mathrm{3}}^{-}$ only (Fig. 6a), but were almost balanced by the sum of ${\mathrm{HCO}}_{\mathrm{3}}^{-}$ , ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ and ${\mathrm{NO}}_{\mathrm{3}}^{-}$ (Fig. 6b). This implies that H₂CO₃, H₂SO₄ and HNO₃ are the potential erosion agents in chemical weathering in the SECRB. The δ¹³C values of the water samples showed a wide range, from −11.0 ‰ to −24.3 ‰, with an average of −19.4 ‰. The δ¹³C from soil is dominated by the relative contribution from C₃ and C₄ plants (Das et al., 2005). The studied areas have subtropical temperatures and humidity, and thus C₃ processes are dominant. The δ¹³C of soil CO₂ is derived primarily from δ¹³C of organic material, which typically has a value between −24 ‰ and −34 ‰, with an average of −28 ‰ (Faure, 1986). According to previous studies, the average value for C₃ trees and shrubs are from −24.4 ‰ to −30.5 ‰, and most of them are lower than −28 ‰ in southern China (Chen et al., 2005; Xiang, 2006; Dou et al., 2013). After accounting for the isotopic effect from the diffusion of CO₂ from soil, the resulted δ¹³C (from the terrestrial C₃ plant process) should be $\sim -\mathrm{25}$ ‰ (Cerling et al., 1991). This mean DIC derived from silicate weathering by carbonic acid (100 % from soil CO₂) would yield a δ¹³C value of −25 ‰. Carbonate rocks are generally derived from marine systems and typically have a δ¹³C value close to zero (Das et al., 2005). Thus, the theoretical δ¹³C value of DIC derived from carbonate weathering by carbonic acid (50 % from soil CO₂ and 50 % from carbonate rocks) is −12.5 ‰. DIC derived from carbonate weathering by sulfuric acid are all from carbonate rocks; thus the δ¹³C of the DIC would be 0 ‰. Based on the above discussion, sources of riverine DIC from different end-members in the SECRB were plotted in Fig. 7. Most water samples drift away from the three end-member mixing areas (carbonate and silicate weathering by carbonic acid and carbonate weathering by sulfuric acid) and towards silicate weathering by sulfuric and nitric acid area, clearly illustrating the effect of the anthropogenic-sourced protons (sulfuric and nitric acid) on silicate weathering in the SECRB.

Considering the H₂SO₄ and HNO₃ effects on chemical weathering, CO₂ consumption by silicate weathering can be determined from the equation below (Moon et al., 2007; Ryu et al., 2008; Shin et al., 2011):

where γ is calculated by cation_sil ∕ (cation_sil + cation_carb).

Based on the calculation in Sect. 5.1, ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ and ${\mathrm{NO}}_{\mathrm{3}}^{-}$ in river water were mainly derived from atmospheric inputs. Assuming that ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ and ${\mathrm{NO}}_{\mathrm{3}}^{-}$ in river water derived from atmospheric input (after correction for sea-salt contribution) are all from acid precipitation and considering H₂SO₄ and HNO₃ effects, CO₂ consumption rates by silicate weathering (SNSW) are estimated between 55×10³ and 286×10³ mol km⁻² a⁻¹ for major river watersheds in the SECRB. For the whole SECRB, the actual CO₂ consumption rate by silicate is 191×10³ mol km⁻² a⁻¹ when the effect of H₂SO₄ and HNO₃ is considered. The flux of CO₂ consumption is overestimated by 16.1×10⁹ mol a⁻¹ (0.19×10¹² g C a⁻¹) due to the involvement of H₂SO₄ and HNO₃ from acid precipitation, accounting for approximately 33.6 % of the total CO₂ consumption flux by silicate weathering in the SECRB. It highlights the fact that the drawdown of atmospheric CO₂ by silicate weathering can be significantly overestimated if acid deposition is ignored in long-term perspectives. The result quantitatively shows that anthropogenic activities can significantly affect rock weathering and associated atmospheric CO₂ consumption. The quantification of this effect needs to be well evaluated in Asia and globally, taking into account current and future human activity.

It is noticeable that the chemical weathering and associated CO₂ consumption rates for the study area were calculated by the river water geochemistry of high-flow season. As a subtropical monsoon climate area, the river water of the southeastern coastal rivers is mainly recharged by rain, and the amount of precipitation in the high-flow season accounts for more than 70 % of annual precipitation in the area. The processes in the low-flow season might be different to some extent. It is worth making further efforts to investigate the hydrological and temperature effects on weathering rate and flux, as well as evaluate the anthropogenic impact in different climate regimes and hydrology seasons.