Chemical weathering and CO2 consumption rates of rocks in the Bishuiyan subterranean basin of Guangxi, China

Physicochemical parameters and total concentrations of dissolved ions

The pH of all of the water samples ranged from 6.68 to 8.33 (Table 1), indicating that the waters were circumneutral to alkaline. Conductivity values ranged from 21.1 to 331 μs/cm. The conductivity values of the R1 and R2 samples were relatively low (21.1–65.4 μs/cm), and reflected waters came from the granite host rock area. These values were also consistent with conductivity values measured upstream in the Zengjiang (42.7–66.9 μs/cm) and Pearl (27.2–78.6 μs/cm) Rivers²⁹. The conductivity values of water samples in the carbonate area (G1, G2) and waters flowing through the carbonate zone (R3) were relatively higher (93.9–331 μs/cm). The result illustrated that the weathering rate of carbonate was higher than the weathering rate of silicate lead to the significant different of physicochemical parameters in samples³⁰.

In natural waters, the total number of cations (Ca²⁺, Mg²⁺, Na⁺, and K⁺) produced during mineral weathering is nearly equivalent to that of anions produced in aggressive medium^31,32. The total cation concentrations of waters analyzed here ranged from 347 to 4,072 μEq/L, in which the result was similar to 60 rivers in the world (TZ⁺ = 300–10,000 μEq/L)²¹. The average value of TZ⁺ is 1855 μEq/L, which is higher than the global average value for rivers (1,250 μEq/L)³³ and Qiantangjiang River (1357 μEq/L)³⁴. The total anion concentrations of water samples ranged from 352–3,732 μEq/L, with an average value of 1803 μEq/L, which was significant higher than the Qiantangjiang River (1,363 μEq/L)³⁴. Equilibrium coefficients (NIBC = (TZ⁺ − TZ⁻)/TZ⁺) ranged from − 9.97 to  + 9.80% with an average value of 1.26%. The typical range of NIBC values is − 10 to  + 10%.

The spatial distribution of primary ionic components

Comparison of water chemical compositions from each cross section of the Bishuiyan subterranean basin indicated that upstream waters were significantly different from those downstream. Cation concentrations of R1, R2, and Q1 upstream waters exhibited trends of Ca²⁺ (0.11–0.31 mmol/L) > Na⁺ + K⁺ (0.07–0.15 mmol/L) > Mg²⁺ (0.01–0.08 mmol/L). The cationic composition was similar to that of Qiantangjiang River basin and Songhua River basin which were mainly composed of exposed silicate^9,34. In contrast, the cation concentrations of G1, G2, R3, and Q2 waters followed trends of Ca²⁺ (0.31–1.45 mmol/L) > Mg²⁺ (0.10–0.64 mmol/L) > Na⁺ + K⁺ (0.07–0.19 mmol/L). This was similar to that of Wujiang River basin which was mainly composed of carbonate³⁵. HCO₃⁻ was the primary anion for all of the river waters, and accounted for 66.7%–95.0% of total anions. HCO₃⁻ ranged from 0.30–0.63 mmol/L in R1, R2, and Q1 and 0.30–3.07 mmol/L for G1, G2, R3, and Q2. The other anions (in descending concentration) were NO₃⁻, SO₄²⁻, and Cl⁻. Ionic concentrations in upstream waters were significantly lower than that in the carbonate area, indicating that corrosion of carbonate considerably influenced the chemical properties of river waters.

Qualitative analysis of ion sources

Chemical analysis of river waters

Water chemical properties can reflect different sources or varying chemical conditions, as exhibited by particular elemental ratios³⁶. Nearly all of the water samples fell above the equilibrium line of Na:Cl = 1 (Fig. 2a). These solute concentrations are influenced by marine aerosols, in addition to other factors³⁷. In particular, the ratio of Ca²⁺ + Mg²⁺ and HCO₃^– is typically used to identify carbonate weathering. The concentration of Ca²⁺ + Mg²⁺ was higher than that of HCO₃⁻ in most of the samples (Fig. 2c). These results implicate the influence of acid from other sources in the weathering of carbonate³⁸.

In addition to the erosive effect of H₂CO₃ derived from the atmospheric CO₂, H₂SO₄ and HNO₃ also make contributions to the rock weathering process (Fig. 2d). Previous studies showed that the chemical weathering by sulfuric acid played an important role in the chemical weathering of karst basin^39,40,41. The sulfuric acid mainly come from atmospheric deposition, evaporate formation (gypsum/anhydrite and MgSO₄) and oxidation of sulfides (pyrite)³⁷. SO₄²⁻ was positively correlated with NO₃⁻ and Cl⁻ in Bishuiyan River waters, while SO₄²⁻ was not obviously correlated with HCO₃⁻. Further, SO₄²⁻ and NO₃⁻ were positively correlated with Na⁺ (Fig. 2b), indicating a similar source of SO₄²⁻ and NO₃⁻ as Cl⁻. Since there is no evaporates in the research area, the source of SO₄²⁻ was not evaporates. It is likely that the allogenic acids in the river primarily derive from human activities and the oxidation of sulfides.

Assuming that the allogenic acids (H₂SO₄ and HNO₃) derived from human activities or sulfide oxidation were only used to balance Ca²⁺ and Mg²⁺ concentrations in the water, then [Ca²⁺ + Mg²⁺]*([Ca²⁺ + Mg²⁺]* = [Ca²⁺ + Mg²⁺] − [SO₄²⁻ + NO₃⁻]) originates from the weathering of carbonate and silicate. Therefore, the ratio of [Ca²⁺ + Mg²⁺]* to [HCO₃⁻] represents the relative concentration of Ca²⁺ and Mg²⁺ from the weathering of carbonate and silicate, which should exhibit a ratio of less than 1.0. Similarly, the [Na⁺ + K⁺]*([Na⁺ + K⁺]* = [Na⁺ + K⁺] − [Cl⁻]) in the river results from the weathering of carbonate and silicate. Consequently, variation in the ratios of [Ca²⁺ + Mg²⁺]*/[HCO₃⁻] and [Na⁺ + K⁺]*/[HCO₃⁻] reflect the relative contribution of carbonate weathering and silicate weathering to solutes in the river water. The ratios for R1, R2, and Q1 waters fall on both sides of the 1:1 line, indicating that the water chemistry of the tributary water was influenced primarily by the weathering of silicate (Fig. 3). In contrast, water from the Chuanyan tributary and the exposed underground river in the carbonate area exhibited ratios of [Ca²⁺ + Mg²⁺]*/[HCO₃⁻] = 1 and [Na⁺ + K⁺]*/[HCO₃⁻] = 0, indicating that the water chemistry of the underground river was mainly controlled by the weathering of carbonate⁴². The [Ca²⁺ + Mg²⁺]* and [Na⁺ + K⁺]* values were higher than those for HCO₃^– in the first quadrant of the graph, suggesting that excessive cations were not derived from the weathering of silicate and carbonate, but rather may be contributed by human activities. Consequently, it is likely that the anthropogenic contribution to cation concentrations was very small.

Identification of rock weathering source material

Triangular component compositional figures can aid analysis of water chemical data by aiding identification of water chemical compositions, the estimation of relative contributions of primary ions, and also help distinguish sources of solutes and their potential controls. Importantly, the relative contribution of chemical weathering of various rock minerals to dissolved solute loads of waters can be estimated through such analyses⁴³. Triangular ionic compositional analysis of small rivers in the Bishuiyan subterranean basin (Fig. 4) indicated that cations were near the Ca²⁺ endmember at the exit of the Bishuiyan subterranean basin (G1, G2), while anions were reflective of an endmember water from carbonate weathering by H₂CO₃. The main cation in Taiping region water (R1, R2, and Q1) was Ca²⁺, and was also shifted towards the [Na⁺ + K⁺] endmember, while anions fell between the H₂CO₃-weathered carbonate and H₂CO₃-weathered silicate endmembers. The main cation of the Chuanyan region water (R3, Q2) was Ca²⁺, with a more minor contribution of Mg²⁺. However, the anion composition of these water was more atypical, reflecting the common influence from H₂CO₃-weathered carbonate in addition to H₂CO₃-weathered silicate and the H₂SO₄-weathered carbonate. These observations indicated that the solutes of the river water in the Bishuiyan basin were mainly controlled by carbonate weathering, silicate weathering, and atmospheric precipitation. Allogenic acids due to human activity also likely contributed from atmospheric precipitation. In addition, chemical weathering of the rock was primarily due to H₂CO₃-weathered carbonate, followed by H₂CO₃-weathered silicate. The effect of allogenic acids on rock weathering was mainly evident for carbonate, with little apparent effect on silicate.

Quantitative estimation of water chemical constituents in the Bishuiyan river basin

Atmospheric input

The contributions of atmospheric inputs to different river sections of the Bishuiyan subterranean basin were calculated (Table 2). Cation components in the river water clearly varied among regions. Estimated atmospheric contribution rates to the R1, R2, R3, G1, and G2 water were 19.5–45.3% (average 33.2%), 19.7–35.9% (average 26.9%), 4.26–7.94% (average 5.26%), 4.53–5.58% (average 4.93%), and 4.94–10.1% (average 7.99%), respectively. The upper reaches were most affected by atmospheric inputs, while such influences were minimal in water in the carbonate area. In addition, the underground river was more resistant to contributions from atmospheric input compared to the surface river, as indicated by a smaller influence in ionic composition.

Silicate weathering

The average contributions of silicate weathering to the cation content of water samples in the research area were estimated for R1, R2, R3, G1, and G2 waters as 38.9%, 37.5%, 5.59%, 6.45%, and 10.1%, respectively (Table 2). The R1 sample was from water that were primarily granite-hosted. Hence, the influence of silicate weathering was significantly larger in R1, and the corresponding contribution change was larger than that of other river section water. This result was consistent with those described above, indicating that the weathering rate of silicate was greatly affected by seasonal changes.

Carbonate weathering

The average contribution of carbonate weathering to the cation content of R1, R2, R3, G1, and G2 samples were 27.3%, 35.0%, 89.3%, 88.5%, and 81.7%, respectively (Table 2). The results clearly indicated that during river runoff, increased contact with carbonate resulted in a gradual increase of carbonate components to the river water. Quantitative analysis also indicated that the water chemistry of the surface water or the underground river in the carbonate area was mainly controlled by the carbonate.

In summary, the analyses indicated differences in relative contributions of different endmembers to the solutes of different sections of the river. Silicate contributed most to the R1 and R2 water, followed by carbonate and then atmospheric input. Although there was only a small amount of carbonate in peripheral areas of R1, while the contributions of carbonate weathering and silicate weathering to the river solute were similar, due to the rapid dissolution⁴⁴ or mixed dissolution^45,46. The water chemistry of R1 and R2 may be typically controlled by silicate and carbonate weathering. In contrast, R3, G1, and G2 were primarily influenced by carbonate, silicate, and then atmospheric input. These results are consistent with the geological setting of the Bishuiyan subterranean basin, wherein water chemistry exhibits obvious regional characteristics. Lastly, the water chemistry of the Chuanyan branch water and the exposed underground river in the carbonate area (R3, G1, and G2), were mainly controlled by the weathering of carbonate.

The chemical weathering rate of rocks in the Bishuiyan basin and the consumption of atmospheric CO₂

The chemical weathering rate of rock minerals (t/(km² year)) is generally reflective of the embodiment of the weathering product of the rock minerals in the solutes of the river per unit area. Chemical weathering of carbonate and silicate was the primary control on the water chemical composition of the Bishuiyan subterranean basin. Relevant water chemistry and river flow data for the basin could then be used to calculate the weathering rate of silicate and carbonate, in addition to the consumption of atmospheric CO₂, following previously described methods^9,47 as indicated below.

Silicate weathering rate (SWR):

Carbonate weathering rate (CWR):

CO₂ consumption rate during silicate and carbonate weathering:

The cations produced by the weathering of carbonate and silicate can be calculated from Eqs. (3)–(10). To calculate the weathering rate of carbonate, the corresponding (left[ {{text{HCO}}_{{3}} } right]_{{{text{carb}}}}^{{}}) value is first obtained. When the weathering rate of H₂CO₃-weathered carbonate and CO₂ consumption are calculated, it is necessary to deduct the (left[ {{text{HCO}}_{{3}}^{ – } } right]_{{{text{carb}}}}^{{{text{H}}_{2} {text{SO}}_{4} + {text{HNO}}_{3} }}) released by allogenic acid due to [HCO₃⁻]_carb. If the ions are balanced in the process of silicate solution and erosion by H₂CO₃, then the following equation can be used:

where the [HCO₃⁻]_sil is the HCO₃⁻ produced by silicate that is dissolved and eroded by H₂CO₃ in the water and CO_2sil refers to the atmospheric CO₂ that is consumed in the process of dissolution and erosion.

To determine the [HCO₃⁻]_carb produced during weathering of carbonate (including carbonic acid and allogenic acid dissolution and erosion), the following equation can be used:

where [HCO₃⁻]_total is the total HCO₃⁻ of the water; [HCO₃⁻]_carb is the HCO₃^– produced by dissolution and erosion of carbonate in the water; (left[ {{text{HCO}}_{{3}}^{ – } } right]_{carb}^{{{text{H}}_{2} {text{CO}}_{3} }}) is the HCO₃⁻ produced by carbonate that are dissolved and eroded by H₂CO₃; and (left[ {{text{HCO}}_{{3}}^{ – } } right]_{carb}^{{{text{H}}_{2} {text{SO}}_{4} + {text{HNO}}_{3} }}) is the HCO₃^– produced by carbonate dissolution and erosion by allogenic acids (H₂SO₄ and HNO₃) in the water.

The quantity of HCO₃⁻ from various sources and the influence of various acids on the weathering of carbonate can be assessed via Eqs. (6) and (7). The weathering rate of H₂CO₃-weathered silicate, the weathering rate of carbonate eroded by carbonic acid and allogenic acids in the Bishuiyan subterranean basin, and the consumption of CO₂ in corresponding process can be calculated using Formulas (2)–(8) (Table 3).

The quantitative calculation of water chemistry resulted in an estimated rock weathering rate for the basin of 73.3 t/(km² year), and an atmospheric CO₂ consumption flux of 668 × 10³ mol/(km² year), which are significant higher than the global rock weathering rate of 36 t/(km² year) and the global atmospheric CO₂ consumption flux of 246 × 10³ mol/(km² year)²¹. The weathering rate and CO₂ consumption flux in this study were slightly higher than the values in Yangtze basin, which were 85 t/(km² year) and 611 × 10³ mol/(km² year), respectively²¹. There are obvious climatic regional difference in the weathering rate and corresponding carbon sink capacity of the basin. For instance, Bishuiyan subterranean basin was subtropical monsoon climate, where the chemical weathering rate and atmospheric CO₂ consumption flux were similar to the Pearl River basin and some tributaries of the Amazon basin (tropical rainforest climate)^18,48. However, the corresponding values were significant lower than those in the Lesser Antilles (hot and humid climate, average annual temp. 24–28 ℃, average annual rainfall 2,400–4,600 mm), where the rock weathering rate and atmospheric CO₂ consumption flux were (100–120 t/(km² year)) and ((1,100–1,400) × 10³ mol/(km² year)), respectively⁴⁹. Meanwhile, the corresponding values in this study were lower than those in northern Okinawa Island (subtropical and humid climate, average annual temp. 22.2℃, average annual rainfall above 2000 mm) with the fluxes of CO₂ consumed by silicate ((334–471) × 10³ mol/(km² year))⁵⁰. The rock weathering rate and atmospheric CO₂ consumption flux of the basin located in the plateau climate and arid and semi-arid climate regions (low rainfall) were lower than those in hot and humid climate (high rainfall). The weathering rate and atmospheric CO₂ consumption flux in Xinjiang rivers (average annual temp. 7–8 ℃, average annual rainfall 100–276 mm) were 0.12–93.6 t/(km² year) and (0.19–284) × 10³ mol/(km² year), respectively²⁹. The weathering rate of rock and atmospheric CO₂ consumption flux in the Songhua River basin (average annual temp. 3–5 ℃, average annual rainfall 500 mm) were (5.79 t/(km² year)) and 190 × 10³ mol/(km² year), respectively⁹. The atmospheric CO₂ consumption flux in upper Yellow River in the Qinghai-Tibet Plateau (average annual temp. 1–8 ℃, average annual rainfall 434 mm) was 268 × 10³ mol/(km² year)⁵¹.

Compared to the other small karst watersheds of in the similar climate, the atmospheric CO₂ consumption flux in the study area was lower than that in Xiangxi Dalongdong underground river (819 × 10³ mol/(km² year), average annual rainfall 1,800 mm) and four underground rivers in upstream of Wushui (878 × 10³ mol/(km² year), average annual rainfall 1,444 mm). Meanwhile, the corresponding value was similar to that of Wanhuayan underground river (705 × 10³ mol/(km² year), average annual rainfall 1565 mm)⁵². However, compared with the north karst of China^53,54, the CO₂ consumption flux in the study area was higher, resulting in the greater contribution to the rock weathering.

The comparison with other climatic zones in the world showed that the CO₂ consumption caused by chemical weathering in the hot and humid climate zone is an important part of regulating atmospheric CO₂ and constituting the global carbon balance. Besides, the small basins in karst (subtropical) area had relatively higher CO₂ consumption. Therefore, the potential of chemical weathering carbon sink in some small subtropical basins with a wide distribution of carbonate is worth additional attention, which would provide some new insights for the scientific assessment of carbon sink effects caused by chemical weathering. Overall, the weathering of carbonate accounts for 71.2% (476 × 10³ mol/(km² year)) of the carbon sink flux of weathered rocks in the Bishuiyan basin, while the weathering of silicate only accounts for 28.3% (192 × 10³ mol/(km² year)). It indicates that more attention should be paid to the accurate assessment of the carbonate carbon sink intensity at the global and regional scales, the role and status of carbonate chemical weathering actively involved in the geological carbon cycle, is worth further study.

Source: Ecology - nature.com