Characteristics of Spatial–Temporal Variations and Controlling Factors of Chemical Weathering in the Han River Basin

Abstract

Watershed weathering provides a critical pathway for understanding the feedback mechanisms between continental rock chemical weathering and global climate change. As the longest tributary of the Yangtze River, the Han River plays a crucial role, where samples were collected from the mainstem and tributaries in spring, summer, and autumn to analyze major ion compositions and calculate chemical weathering rates using graphical methods and a forward model. Results show carbonate weathering dominated solute sources (75.7%), followed by silicates (14.8%), with minimal atmospheric and anthropogenic inputs. Spatially, carbonate weathering rate (CWR) and CO₂ consumption rate (ΦCO_2car) increase downstream with lithological variations, while silicate weathering rate (SWR) and CO₂ consumption rate (ΦCO_2sil) exhibit the opposite trend. Basin-wide averages were 9.4 ± 1.2 t/km²/yr (CWR) and 1.3 ± 0.3 t/km²/yr (SWR), with CO₂ consumption rates of 262.6 × 10³ and 55.5 × 10³ mol/km²/yr for carbonates and silicates, respectively. Seasonally, CWR and ΦCO_2car peaked in summer, while SWR and ΦCO_2sil were lower in summer than in spring and autumn. This seasonal pattern suggests that cooler temperatures limit weathering in spring and autumn, while increased summer runoff favors carbonate dissolution. The findings highlight the need for seasonal sampling to accurately assess weathering rates and CO₂ drawdown.

1. Introduction

Chemical weathering is a key process linking the lithosphere, hydrosphere, and atmosphere, shaping global biogeochemical cycles [1,2]. It shapes landform evolution, governs riverine solute fluxes [3], and also regulates Earth’s long-term climate by consuming atmospheric CO₂ through silicate weathering. This process is a critical carbon sink that influences deep-time carbon cycling and climate transitions, such as the Cenozoic cooling [4,5,6]. On contemporary timescales, chemical weathering continuously modulates regional water quality, soil formation, and nutrient availability [7]. Moreover, its resultant solute fluxes, particularly alkalinity derived from silicate weathering, are essential for quantifying riverine carbon export to oceans, resolving modern global carbon budgets, and predicting climate feedback [8,9,10].

Accurate assessment of chemical weathering rates and associated carbon fluxes remains challenging due to complex controls. These include lithology (rock composition and mineralogy), climate (temperature and precipitation), tectonics (topography and erosion rates), hydrology (discharge, flow velocity, and water-rock contact time), and increasingly significant anthropogenic perturbations (land-use change, acid deposition, fertilization, and dam operations) [11,12,13,14]. Monsoon-driven precipitation seasonality exerts particularly strong control over weathering dynamics [15,16,17]. Pronounced dry-wet seasonality induces substantial fluctuations in hydrological conditions (discharge, water level), hydrochemistry (pH, ion concentrations), and water-rock interaction intensity [18,19]. Consequently, weathering rates, product ratios (e.g., HCO₃⁻/SO₄²⁻), and solute fluxes exhibit marked seasonal variability [20,21]. Annual flux timing based on limited seasonal data may miss these variations, leading to inaccurate assessments of carbon sinks. [10,22].

The Han River—the largest tributary of the Yangtze River and situated within the East Asian Monsoon core—provides an ideal natural laboratory for investigating monsoon-modulated seasonal weathering dynamics. The basin exhibits characteristic monsoon hydroclimatic patterns: concentrated rainfall during warm months, distinct dry-wet seasons, and highly variable annual discharge and sediment loads [23]. Such hydrological seasonality profoundly impacts chemical weathering processes [24,25]. Furthermore, anthropogenic influences are substantial, with the upper basin serving as the water source for the Middle Route of the South-to-North Water Diversion Project and the middle-lower reaches experiencing intensive agricultural/industrial activities [26,27]. Despite its ecological and socioeconomic significance, systematic studies of chemical weathering in the Han River Basin remain limited. Existing work primarily addresses water quality analysis, land use, and transport patterns of specific elements [28,29,30], often employing sampling strategies restricted to a single season (typically wet or dry period) without complete hydrological-year coverage [31,32]. This approach does not adequately capture how weathering responds to hydrological changes. It also frequently fails to account for the impacts of extreme events (e.g., storms, droughts) and seasonal human activities (e.g., applying fertilizer, regulating reservoirs). These impacts, which can be brief or cumulative, significantly alter weathering signals and solute fluxes [13,27]. Consequently, this shortcoming limits our mechanistic understanding and reduces the accuracy of carbon sink estimates.

To elucidate seasonal controls on chemical weathering in monsoonal rivers, disentangle natural (hydroclimate) versus anthropogenic drivers, and refine weathering-related carbon sink estimates for the Han River Basin, this study implemented a systematic seasonal sampling campaign across the Han River mainstream and major tributaries, covering dry, normal, and wet periods. The core purposes of this study are: (a) to evaluate the respective contributions of different sources (the atmosphere, anthropogenic factors, and chemical weathering) to the dissolved load, (b) to estimate the chemical weathering rate and associated CO₂ consumption rate., and (c) to elucidate the controlling factors of weathering in different seasons.

2. Study Area

2.1. Climate and Hydrology

The Han River Basin (HRB; 32°30′–34°20′ N, 106°15′–114°20′ E), the largest tributary of the Yangtze River, extends 1577 km from its headwaters in the Qinling Mountains (Shaanxi Province) to its confluence at Wuhan City (Figure 1a). Draining an area of 159,000 km² across six provinces [33], the basin exhibits pronounced downstream geomorphic transitions. The upper reaches (upstream of Danjiangkou, ∼925 km) drain 95,200 km² of low-to-medium elevation mountains characterized by deeply incised valleys. Progressing downstream, the middle reaches (Danjiangkou to Zhongxiang, 270 km) traverse 46,800 km² of hilly terrain with intermountain basins. The lower reaches (downstream of Zhongxiang, 382 km) encompass 17,000 km², where the channel narrows progressively while confined between artificial levees. This longitudinal zonation creates distinct hydrological environments governing spatial heterogeneity in weathering processes across the HRB.

The HRB occupies a critical ecotone between northern and southern climate regimes, exhibiting distinct seasonal variability characteristic of a subtropical humid monsoon climate. Meteorological records (1951–2020) reveal significant thermal gradients, with mean monthly temperatures ranging from −2 °C in the northwestern headwaters to 27 °C in the southeastern lowlands. The basin receives 873 mm mean annual precipitation, displaying strong intra-annual heterogeneity—approximately 75% of the total rainfall occurs during the flood season (May–October), frequently generating summer-autumn flood events [28]. Runoff generation is predominantly precipitation-driven (contributing >80% of total discharge), with limited groundwater inputs (<20%), resulting in spatial runoff patterns that closely mirror precipitation distribution. Basin-wide runoff depth varies substantially (300–900 mm), reflecting the combined influences of orographic precipitation effects and longitudinal climate gradients.

2.2. Geologic Setting

The geology of the HRB is very complex, with strata exposed in almost every period from the Precambrian to the Quaternary (Figure 1c). The Paleozoic metamorphic rock system is the most widely distributed, followed by the Cenozoic Tertiary red rock system and the Quaternary loose sedimentary layers, and the Mesozoic strata are the least distributed. The Paleozoic and previous strata are mainly distributed in the upper reaches [32], constituting the Qinling Mountains and Daba Mountains. The Qinling Mountains are dominated by granite and metamorphic rocks. The Daba Mountains are dominated by limestone with some metamorphic rocks. The Cenozoic and Mesozoic strata are mostly distributed in mountain basins, grabens, and low-lying areas, which become medium mountains and hills [31].

Geotectonically, the HRB straddles such first-order geotectonic units as the Qinling fold system, the Yangzi quasi-platform, and the Songpan-Ganzi fold system, with tectonic movements dominated by folds, block movements, and uplift. Stratigraphic ruptures in the basin are large in scale and highly active, and earthquakes are often associated with them, with the largest magnitude of 6.5 occurring in history. Since the Quaternary period, the neotectonic movement has remained relatively active, but seismic activity has been rare, and the Earth crust has been relatively stable [31].

3. Sampling and Analysis

In this study, samples were collected continuously from downstream to upstream in the spring, summer, and autumn in 2023. Sampling points were deployed in the mainstream and major tributaries, which were distributed in different geomorphological units as much as possible. The distribution of sampling points is shown in Figure 1b. A total of 61 river water samples were collected, including 29 in the mainstream of the Han River and 32 in the tributaries. Water samples were generally collected below 10 cm of the water surface, and two samples were collected from each sampling point using 125 mL high-density polyethylene sampling bottles. All of the samples were filtered through 0.22 μm membranes. The cationic water samples were acidified to pH < 2 by adding electronic-grade HNO_3, while the remaining bottle was not acidified for testing anions.

A portable multi-parameter water quality analyzer (HQ2200, HACH, Loveland, CO, USA) was used on site to determine pH, temperature, total dissolved solids (TDS), dissolved oxygen (DO), and electrical conductivity (EC). The concentrations of cations Ca²⁺, Na⁺, Mg²⁺, K⁺ and Siwere determined by inductively coupled plasma emission spectrometry (ICP-OES, Varian Vista-MPX). Cl⁻, SO₄²⁻, and NO₃⁻ were tested by ion chromatography (IC, Dionex ICS-90). The accuracy of the determination of the major ions is better than ±5% (2σ). The above analysis was carried out in the State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang. The concentration of HCO₃⁻ was analyzed by acid-base titration.

4. Results

4.1. Physico-Chemical Parameters

The chemical composition of the river waters from the HBR is presented in

Table 1. Physical parameters and chemical compositions of water samples in the Han River Basin.

Sample		WT	pH	EC	TDS	Ca2+	Mg2+	K+	Na+	Cl−	SO42−	HCO3−	NO3−	Si
		°C		μs/cm	mg/L
Spring
Mainstream	min	10.7	8.2	215.0	202.7	33.5	7.8	2.4	4.9	2.9	18.4	116.9	4.5	0.1
	max	20.0	9.1	370.0	325.7	50.7	13.8	3.9	20.1	16.1	39.6	198.3	8.7	0.3
	ave	14.8	8.6	287.5	276.0	44.9	10.3	3.2	10.5	7.9	27.2	163.2	5.9	0.2
Tributary	min	7.7	8.2	96.0	105.5	17.0	2.1	1.4	2.5	0.8	9.2	64.8	0.2	0.2
	max	20.0	9.1	796.3	775.4	97.3	38.7	9.7	72.2	117.4	227.9	305.0	16.1	4.8
	ave	13.4	8.5	284.0	279.6	42.9	10.4	3.8	13.2	13.0	36.3	152.6	4.6	2.7
WS-1	wastewater	16.0	7.9	638.3	547.4	74.6	8.9	18.0	52.3	75.4	64.9	227.5	19.3	6.7
Summer
Mainstream	min	23.0	7.7	120.6	185.8	33.0	5.5	2.0	4.2	2.3	15.1	106.1	1.4	0.7
	max	37.3	9.1	320.7	264.5	45.2	11.4	4.4	17.9	20.9	39.2	152.5	4.9	5.3
	ave	29.6	8.3	227.9	228.3	40.0	8.0	2.6	7.6	6.4	23.5	133.1	3.4	3.6
Tributary	min	17.7	7.7	51.9	78.0	11.5	1.4	1.2	2.2	0.5	6.4	45.1	1.1	0.7
	max	34.3	9.2	488.0	472.9	76.6	22.7	6.3	25.8	34.5	75.3	244.0	6.9	6.4
	ave	26.4	8.3	188.9	192.7	31.9	6.7	2.6	6.6	5.3	20.0	112.2	3.3	4.4
WS-1	wastewater	29.2	7.4	648.0	450.2	58.5	7.9	16.4	53.2	64.5	55.7	148.8	35.7	9.4
Autumn
Mainstream	min	10.9	7.8	227.0	213.7	36.3	5.5	1.5	3.1	1.7	15.6	134.7	1.9	2.0
	max	25.0	8.6	376.0	367.7	56.8	13.5	3.6	17.8	25.3	36.8	203.6	9.7	5.7
	ave	16.3	8.1	281.1	262.4	45.2	8.4	2.1	7.4	6.5	23.7	159.2	5.4	4.5
Tributary	min	7.0	7.8	92.0	103.5	16.3	2.3	1.0	2.2	0.6	7.2	64.5	1.7	1.4
	max	25.0	9.0	629.0	521.3	82.1	19.6	5.2	31.0	56.4	119.0	272.4	17.1	5.7
	ave	13.8	8.2	259.4	246.2	41.4	7.9	2.3	7.4	7.6	23.5	147.2	4.9	4.0
WS-1	wastewater	15.3	7.8	778.7	554.6	64.9	9.1	11.2	66.4	81.3	64.1	186.4	62.5	8.7

. The WT shows a large range of seasonal variation and varies from 8 to 20 °C in spring, 18–37 °C in summer, and 7–25 °C in autumn. The spatial distribution of WT shows an increasing trend from upstream to downstream, consistent with the elevation of the study area (Figure 2). The pH was mildly alkaline in all three seasons, with higher values ranging from 7.9 to 9.2 in spring, 7.4 to 9.2 in summer, and lower values ranging from 7.8 to 9.0 in autumn. The pH values generally increase from upstream to downstream, except in autumn. The EC varied in the range of 96 to 796 μs/cm in spring, with an average value of 296 μs/cm, 52 to 648 μs/cm in summer, with an average value of 218 μs/cm, and 92 to 779 μs/cm in autumn, with an average value of 279 μs/cm. The considerable variability of TDS (HCO₃⁻, Cl⁻, NO₃⁻, SO₄²⁻, Na⁺, Mg²⁺, K⁺, Ca²⁺, SiO₂) are observed both seasonally and spatially (Figure 2) with concentrations range higher in spring (106–775 mg/L) and lower in summer (78–473 mg/L), highlighting the distinct difference between high-flow and low-flow conditions. The TDS shows an increasing trend along with the flow direction of the mainstream, especially in the Hanzhong plain in spring and autumn, which is similar to EC. This spatial and temporal variability generally demonstrates that riverine solutes in the basin are influenced by different climatic, topographic, and lithological factors [24].

4.2. Major Ions Composition

The cations concentration follows an order of Ca²⁺ > Na⁺ > Mg²⁺ ≫ K⁺ in spring and summer (Table 1), as well in autumn, the order turned to be Ca²⁺ (average 1084 μmol/L, from 408 to 2052 μmol/L) > Mg²⁺ (348 μmol/L, from 96 to 829 μmol/L) ≈ Na⁺ (330 μmol/L, from 80 to 1347 μmol/L) ≫ K⁺ (55 μmol/L, from 11 to 134 μmol/L).

The molar concentration of anions decreases in the order of HCO₃⁻ > SO₄²⁻ > Cl⁻ ≫ NO₃⁻ in the three seasons (Table 1). HCO₃⁻ is the most abundant anion with an average value of 2271 μmol/L (1026–4021 μmol/L), 2016 μmol/L (740–4000 μmol/L), and 2534 μmol/L (1058–4465 μmol/L) in the three seasons, respectively. The second most important anion is SO₄²⁻ with an average concentration of 339 μmol/L (95–2374 μmol/L), 231 μmol/L (67–784 μmol/L), and 247 μmol/L (75–1239 μmol/L) in the three seasons, respectively.

For seasonal variations, Ca²⁺, Mg²⁺, Na⁺, SO₄²⁻, Cl⁻, and NO₃⁻ showed a consistent trend of spring > autumn > summer, whereas HCO₃⁻ was highest in autumn, and K⁺ was lowest in autumn. From the upstream to the downstream, the ion concentration in the source is low, and the overall change in K⁺ concentration is not large; Na⁺, Cl⁻, SO₄²⁻, NO₃⁻ ion concentration and TDS value show the trend of rising (into the Hanzhong Basin) and then decreasing (into the Qinling Mountains) and then rising (into the Jianghan Plain), which is highly consistent with the distribution of the towns and cities and shows that it is affected by anthropogenic activities; Ca²⁺, Mg²⁺, HCO₃⁻ ions are significantly higher in the source area, and then slightly decrease (Ziyang County) and remain relatively stable. A comparison of the stratigraphic lithology of the watershed reveals that the Hanzhong Basin is mainly surrounded by granite and carbonate rocks, and the weathering of the rocks provides a larger amount of Ca²⁺, Mg²⁺, and HCO₃⁻, while the lithology of the rocks from Ziyang to Hankou is dominated by metamorphic rocks (kyanite and schist).

Ca²⁺ and Mg²⁺ together account for 79.4% (in spring), 81.4% (in summer), and 84.4% (in autumn) of the total cations. HCO₃⁻ is the dominant anion, contributing 80% in spring, 82% in summer, and 84% in autumn. Therefore, the vast majority of the river water in the HRB is HCO₃-Ca-Mg type (Figure 3).

5. Discussion

5.1. Source of Major Ions

In this study, a forward model [34] was used to quantify the contribution of each major source to the river solute through the mass balance model. The content of any component X in the river water can be expressed as:

$${\mathrm{X}}_{\mathrm{riv}} = {\mathrm{X}}_{\mathrm{atm}} + {\mathrm{X}}_{\mathrm{anth}} + {\mathrm{X}}_{\mathrm{car}} + {\mathrm{X}}_{\mathrm{sil}}$$

(1)

where riv—river, atm—atmospheric input, anth—anthropogenic input, car—carbonate weathering, and sil—silicate weathering.

5.1.1. Atmospheric Inputs

In the surface water cycle, due to the relatively conservative stability of Cl⁻ and its simple origin, it is often chosen as a reference element in the study of the effect of atmospheric precipitation on river water chemistry. The atmospheric input of dissolved cations is evaluated through multiplication of each cation’s molar concentration with the rainwater ion/Cl⁻ ratio [35]. The ion concentration of the atmospheric input was calculated as follows:

$$\mathrm{X}\mathrm{atm} = {\mathrm{X}}_{\mathrm{r}} \text{×} {\mathrm{Cl}}_{\min }^{-}/{\mathrm{Cl}}_{\mathrm{r}}^{-}$$

(2)

where X_atm represents the ion concentrations of precipitation. X_r and Cl⁻_r represented the concentration of X and Cl⁻ in the rainwater, respectively. The minimal Cl⁻ concentration (Cl⁻_min) in the three seasons was 22.9, 14.1, and 17.0 μmol/L, respectively. By the method of sea salt correction, the ratio of other ions to Cl⁻ in the rainwater collected during the three seasons was used for correction (Table S1).

Analysis of atmospheric contributions to mainstream cations revealed seasonal variations (5.4%, 6.5%, and 4.3% across three seasons; Figure 4a–c), consistent with the 65–80% of annual precipitation in summer. Tributaries showed a similar trend. Spatially, the contributions increased downstream, mirroring precipitation patterns. The Cl⁻ concentration for most samples significantly exceeded rainwater inputs, suggesting additional sources like anthropogenic activities or evaporite dissolution.

5.1.2. Anthropogenic Activities

According to a previous study, anthropogenic pollutants reach river systems through three primary routes: precipitation, agricultural runoff, and industrial effluents [36]. The Han River has experienced significant water quality degradation in recent decades due to rapid industrial and agricultural expansion. Sources of river water sulfate mainly include anthropogenic inputs, atmospheric inputs, gypsum dissolution, and sulfide oxidation [37,38]. There is no geologic evidence of evaporates in the watershed [39,40], as well as little sulfide oxidation [41], so it is assumed that atmospherically corrected SO₄²⁻ is from anthropogenic inputs. Numerous studies have confirmed that nitrate and chloride mainly originate from agricultural inputs, animal waste, and domestic sewage; therefore, these ions have been used as tracers in identifying the degree of anthropogenic contribution [31,42]. The NO₃⁻ concentrations of the mainstream (mean 95.2 umol/L, 54.8 umol/L, and 87.1 umol/L, respectively, in three seasons) are higher than that of the Yangtze River (55.5 μmol/L) [43] and local rainwater (19 μmol/L). Therefore, after deducting atmospheric sources, all NO₃⁻ in river water comes from human activities. Cl⁻ concentrations exhibited an increase from 101 μmol/L [40] to 134 μmol/L [31], ultimately reaching 235 μmol/L in this study. The highest concentrations were consistently recorded in urbanized regions, including Hanzhong, Ankang, and Wuhan, implicating municipal and domestic wastewater as the primary source. This interpretation is further supported by the absence of evaporite in the river basin [39,40]. Consequently, the Cl⁻ exceeding atmospheric contributions originate from anthropogenic sources. In this study, Na/Cl = 1.07, 1.27, and 1.26 were used for correlation calculation, which was from wastewater collected in Ziyang in three seasons (Table 1). In the HRB, the influence of anthropogenic inputs on river solutes was 5%, 3.7%, and 3.7% in the three seasons, respectively.

5.1.3. Rock Weathering

Silicate Weathering Input

Silicate weathering can supply K⁺, Na⁺, Ca²⁺, and Mg²⁺ to river water. For the HRB, Na⁺_sil can be calculated by deducting Na⁺_atm and Na⁺_anth from Na⁺_riv. The primary source of K⁺ in river water is silicate weathering, supplemented by minor atmospheric contributions. For Ca²⁺_sil and Mg²⁺_sil, their values were derived using the (Ca²⁺/Na⁺)_sil and (Mg²⁺/K⁺)_sil ratios, which are 0.35 and 0.2, respectively [41]. Therefore, K⁺_sil, Na⁺_sil, Ca²⁺_sil, and Mg²⁺_sil can be determined by Equations (3)–(6).

$${{\mathrm{Na}}^{+}}_{\mathrm{sil}} ={{\mathrm{Na}}^{+}}_{\mathrm{riv}} - {{\mathrm{Na}}^{+}}_{\mathrm{atm}} - {{\mathrm{Na}}^{+}}_{\mathrm{anth}}$$

(3)

$${{\mathrm{K}}^{+}}_{\mathrm{sil}} ={{\mathrm{K}}^{+}}_{\mathrm{riv}} - {{\mathrm{K}}^{+}}_{\mathrm{atm}}$$

(4)

$${{\mathrm{Ca}}^{2+}}_{\mathrm{sil}} ={{\mathrm{Na}}^{+}}_{\mathrm{sil}} \text{×} {({\mathrm{Ca}}^{2+}/{\mathrm{Na}}^{+})}_{\mathrm{sil}}$$

(5)

$${{\mathrm{Mg}}^{2+}}_{\mathrm{sil}} ={{\mathrm{Na}}^{+}}_{\mathrm{sil}} \text{×} {({\mathrm{Mg}}^{2+}/{\mathrm{Na}}^{+})}_{\mathrm{sil}}$$

(6)

While silicate weathering contributions in the HRB mainstream vary moderately by season (17% in spring, 16% in summer, 12% in autumn; Figure 4a–c), spatial patterns reveal consistently higher values in the upper reaches where silicate rocks predominate. In particular, the tributaries T-25, T-24, T-23, T-21, and T-20 had the highest silicate weathering contributions, consistent with the geological context of the granitic sub-watersheds they flow through (Figure 4d–f).

Carbonate Weathering Input

Carbonate weathering represents the dominant origin of Ca²⁺ and Mg²⁺ in fluvial environments [44]. After applying corrections above, residual Ca²⁺ and Mg²⁺ concentrations were ascribed to this weathering process. Quantifications of carbonate weathering-derived cations were 73%, 74%, and 80%, respectively, in the mainstream in three seasons. The characteristics are consistent with Figure 5, i.e., samples in summer are closer to the carbonate weathering end member than those in spring. This may be attributed to the increased precipitation during the summer months, which reduces the residence time of water-rock interactions. The shorter water-rock interaction time held in favors carbonate dissolution, which occurs more rapidly. In contrast, silicate alteration requires longer contact to proceed effectively [45,46].

5.2. Chemical Weathering Rate

The weathering rate of silicates and carbonates (SWR and CWR) can be determined by integrating cation inputs with watershed characteristics, including drainage area and river discharge, as follows [15]:

$$\mathrm{SWR} = \left({{\mathrm{K}}^{+}}_{\mathrm{sil}} + {{\mathrm{Na}}^{+}}_{\mathrm{sil}} + {{\mathrm{Ca}}^{2+}}_{\mathrm{sil}} + {{\mathrm{Mg}}^{2+}}_{\mathrm{sil}}\right) \text{×} \mathrm{discharge}/\mathrm{drainage} \mathrm{area}$$

(7)

$$\mathrm{C}\mathrm{WR}=\left({{\mathrm{Ca}}^{2+}}_{\mathrm{car}}+{{\mathrm{Mg}}^{2+}}_{\mathrm{car}}\right) \text{×} \mathrm{discharge}/\mathrm{drainage} \mathrm{area}$$

(8)

where K⁺_sil, Na⁺_sil, Ca²⁺_sil, and Mg²⁺_sil represent cations supplied by silicate weathering, and Ca²⁺_car and Mg²⁺_car are cations from carbonate weathering. The Xiantao hydrological station, closest to Hankou, was used to represent the entire basin, and the main hydrological stations on the mainstream (Wuhou, Yangxian, Ankang, Baihe, Huangjiagang, and Huangzhuang) and the main tributaries (Du River, Ziwu River, Youshui River, and Xushui River) were selected to calculate the rock weathering rate. The discharge data for each of the three seasons was taken from the average of the corresponding sampling period in this year.

The calculated SWR is 1.4 t/km²/yr in spring, 0.9 t/km²/yr in summer, and 1.6 t/km²/yr in autumn (Table S2). The CWR is 5.1 t/km²/yr in spring, 10.0 t/km²/yr in summer, and 13.1 t/km²/yr in autumn. In the HRB, the weathering rates ranked: CWR > SWR and showed an increasing trend and higher values in summer and autumn. The higher weathering rate in autumn compared to summer is due to the increased flow downstream of the Danjiangkou Reservoir during the autumn sampling period due to flooding. Thus, chemical weathering is greatly promoted by high temperature and heavy runoff.

In terms of spatial distribution, both SWR and CWR exhibited a consistent pattern, demonstrating higher values in the Qinling Mountains compared to those observed in the Hanzhong Basin and the Jianghan Plain. Mountain uplift and steeper slopes in tectonically active areas enhance mechanical erosion, exposing fresh mineral surfaces to water and reactive gases [47]. This process drives high weathering rates in orogenic regions, as observed in the Himalayas [48,49] and the Andes [50]. Given that the HRB drains a tectonically active zone, mountain uplift, potentially linked to dynamic uplift or lithospheric processes [51,52], contributes to weathering rates by increasing relief and erosion.

5.3. CO₂ Consumption Rate

The involvement of sulfuric acid in the chemical weathering of carbonate minerals results in a net release of CO₂ to the atmosphere, which is a net source of atmospheric CO₂, so the mechanism of sulfuric acid weathering of carbonate minerals and its relationship with the regional carbon cycle has been a scientific issue of concern to scientists [34,53]. Based on the different stoichiometric coefficients, CO₂ consumption rates by silicate and carbonate weathering are calculated using the following equations [15]:

$$\mathsf{\Phi }{\mathrm{CO}}_{2\mathrm{release}} = {{\mathrm{SO}}_4^{2-}}_{\mathrm{anth}} \text{×} \mathrm{discharge}/\mathrm{drainage} \mathrm{area}$$

(9)

$$\mathsf{\Phi }{\mathrm{CO}}_{2\mathrm{sil}}=(2 {{\mathrm{Ca}}^{2+}}_{\mathrm{sil}}+2{{\mathrm{Mg}}^{2+}}_{\mathrm{sil}}+{{\mathrm{K}}^{+}}_{\mathrm{sil}}+{{\mathrm{Na}}^{+}}_{\mathrm{sil}}) \text{×} \mathrm{discharge}/\mathrm{drainage} \mathrm{area}$$

(10)

$$\mathsf{\Phi }{\mathrm{CO}}_{2\mathrm{car}}=({{\mathrm{Ca}}^{2+}}_{\mathrm{car}}+{{\mathrm{Mg}}^{2+}}_{\mathrm{car}} - {{\mathrm{SO}}_4^{2-}}_{\mathrm{anth}}) \text{×} \mathrm{discharge}/\mathrm{drainage} \mathrm{area}$$

(11)

where SO₄²⁻_anth is SO₄²⁻ produced by anthropogenic input. According to the calculation results, the CO₂ release rate in the Han River basin is 9.3 × 10³ mol/km²/yr ~ 3.1 × 10⁵ mol/km/yr (Table S2). The discharge-weighted CO₂ consumption rates by silicate weathering with temporal variability in spring (64.7 × 10³ mol/km²/yr), summer (30.9 × 10³ mol/km²/yr), and autumn (70.7 × 10³ mol/km²/yr). The CO₂ consumption rates by carbonate weathering in the HRB with temporal variability in spring (111.0 × 10³ mol/km²/yr), summer (210.5 × 10³ mol/km²/yr), and autumn (288.4 × 10³ mol/km²/yr). For seasonal variations, as for weathering rate reasons.

In the HRB, carbonate weathering exhibits higher CO₂ consumption rates compared to silicate weathering. Seasonal variations show elevated rates during summer and autumn, driven by increased discharge and higher solute loads, which directly enhance carbon sequestration [1]. This phenomenon can be explained by the basin’s extensive carbonate rock distribution combined with favorable topographic and climatic conditions that promote weathering efficiency.

5.4. Factors Influencing Chemical Weathering in the HRB

Identifying the principal controls on chemical denudation remains challenging, especially in large drainage basins where environmental factors are complex. This difficulty arises from the intricate interrelationships among key controlling factors, including lithology, temperature, runoff/precipitation, physical erosion rate (PER), and vegetation [13,54,55,56].

The Gibbs plot effectively identifies natural hydrochemical influences [57]. Our results show that none of the samples fall within the precipitation-dominated range of the Gibbs diagram (Figure 6), suggesting that precipitation has a limited influence on water chemistry in the basin. Arrhenius-type laws demonstrate that mineral reaction rates follow temperature-dependent exponential functions, a relationship clearly observed in laboratory dissolution studies. However, field expressions of this dependence are frequently masked by interacting environmental variables [58], adding complexity to the long-studied connection between chemical weathering and atmospheric CO₂ through temperature feedbacks [59]. The observed positive relationship between water temperature and both CWR and SWR (Figure 7a) aligns with the Arrhenius law’s prediction of accelerated mineral dissolution at higher temperatures [14]. Thus, the weathering rate in spring and autumn is limited by temperature and is generally lower than in summer.

Multiple studies have established runoff as a key controller of chemical weathering rates across diverse environments [1,60]. This relationship is further supported by Kump et al. (2000), who found that the strongest field correlation exists between weathering rates and runoff intensity [58]. Consistent with these findings, Yde et al. (2014) demonstrated discharge-dependent solute export at catchment scales [61]. In the HRB, the close correspondence between seasonal weathering rate variations and water flux dynamics (Figure 7b) confirms runoff’s dominance in regulating chemical weathering processes. Higher runoff accelerates the rate of exposure to new minerals, which in turn accelerates rock weathering [62].

PER was calculated by multiplying the SPM (suspended particulate material) concentration and water runoff [16]. Using the PER value of river water in the monitoring section to characterize the intensity of physical erosion conditions for the whole basin is reasonable [63]. In this study, enhanced carbonate and silicate weathering rates (CWR and SWR) in summer were positively correlated with PER, demonstrating that increased physical erosion rates promote chemical weathering processes (Figure 7c). Meanwhile, PER correlates better with SWR than CWR because the Han River Basin has sufficient carbonates, which do not need physical erosion rates to provide a source but more water to dissolve, while silicates have fewer sources and need stronger physical erosion rates to bring in the original minerals. Weathering rates exhibit a logarithmic relationship with PER, which is best explained by the dual regimes of “transport-limited” and “kinetic-limited” conditions [45,64]. The chemical weathering regime in this region can be characterized by analyzing mineral saturation indices in river water samples. To assess carbonate mineral dissolution under different topographic and climatic conditions, we calculated saturation indices for calcite (SIc) and dolomite (SId). Figure 8 reveals that most HRB water samples were near equilibrium (~0) or oversaturated, suggesting hydrochemical evolution occurred primarily under a “transport-limited” regime.

5.5. Implications

This study highlights the necessity of seasonal sampling to capture the shifting dominance of temperature and runoff in regulating chemical weathering processes. In the Han River Basin, the observed transition from temperature-limited weathering in spring/autumn to runoff-dominated weathering in summer underscores that single-time sampling would fail to represent the dynamic interplay between these factors. Such seasonally resolved data are critical for accurately quantifying annual weathering fluxes and understanding how climate variability (e.g., warming vs. intensified rainfall) may differentially alter weathering rates. Neglecting seasonal variations could lead to systematic biases in regional carbon budget estimates or predictions of solute transport to oceans.

The findings emphasize that current models assuming uniform controls (e.g., temperature as the primary driver) may oversimplify weathering responses in monsoonal or temperate basins. By demonstrating that runoff governs peak weathering rates during high-discharge summers when silicate/carbonate dissolution and ion transport are most this study argues for incorporating seasonal hydrological thresholds into weathering algorithms. Seasonal sampling thus provides empirical evidence to refine Earth system models, particularly under climate change scenarios where decoupling of temperature and precipitation trends could reshape weathering regimes.

6. Conclusions

Through systematic sampling campaigns conducted in March (spring), August (summer), and November (autumn), this study investigates the spatiotemporal variations in major ion chemistry and quantifies chemical weathering rates along with atmospheric CO₂ consumption rates in the Han River Basin (HRB). The results demonstrate that HRB waters exhibit mild alkalinity with Ca²⁺ and HCO₃⁻ as the dominant ions, reflecting substantial contributions from carbonate weathering. Our model reveals a clear seasonal trend in solute sources. Carbonate weathering accounts for 75.2% of total cations in spring, increasing to 78.1% in summer and 82.3% in autumn, while silicate weathering inputs decline during warmer seasons due to enhanced hydrological flushing effects. Spatially, carbonate weathering predominance intensifies downstream (from 68% to 89%), mirroring the basin’s lithological transition from upstream silicate-rich to mid-lower reach carbonate-dominated terrains. The chemical weathering rates and CO₂ consumption rates have significant seasonal variations, as shown by CWR (5.1 t/km²/yr in spring, 10.0 t/km²/yr in summer, and 13.1 t/km²/yr in autumn) and ΦCO_2car (111.0 × 10³ mol/km²/yr in spring, 210.5 × 10³ mol/km²/yr in summer, and 288.4 × 10³ mol/km²/yr in autumn) gradually increasing, and SWR (1.4 t/km²/yr in spring, 0.9 t/km²/yr in summer, and 1.6 t/km²/yr in autumn) and ΦCO_2sil (64.7 × 10³ mol/km²/yr in spring, 30.9 × 10³ mol/km²/yr in summer, and 70.7 × 10³ mol/km²/yr in autumn) were significantly lower in summer than in spring and autumn. The multivariate analysis identifies water temperature as the primary limiting factor for weathering rates in spring/autumn, whereas summer fluxes are strongly regulated by runoff intensity, with PER exerting significantly greater control on silicate versus carbonate weathering processes. Unlike previous studies, which concluded that chemical weathering processes are controlled by a single factor, this research demonstrates that the controlling factors also vary with the seasons. Because erosion processes vary significantly from season to season, relying on single-season sampling could lead to misleading conclusions. To accurately assess the carbon cycle, studies must include full seasonal coverage.