Carbon flux in a typical dolomite-dominated drainage basin in humid subtropical climate

doi:10.13745/j.esf.sf.2022.1.43

Abstract

Abstract:

Karstification is directly involved in the global carbon cycle. Understanding the mechanism of karst carbon sink is of great significance for an accurate evaluation of the carbon cycle. Carbonate rocks are widely distributed in China, especially in southwestern China. Although quite amount of research on inorganic carbon sequestration in limestone-dominated watersheds of this area has been carried out, only few involved the dolomite counterparts, which has adverse impacts on the accurate assessment of China’s carbon sink potential in karst systems and the national effort toward carbon neutrality. To fill this research gap, we investigated the Huangzhou River Basin in Shibing, Guizhou Province, a representative domolmite-dominated drainage basin in humid subtropical climate in southwestern China. We conducted a year-long (May 2018 to May 2019) automated hydrological, hydrochemical monitoring study of water-rock-gas interaction and inorganic carbon sequestration in the drainage basin, and obtained high temporal resolution continuous monitoring data on regional rainfall and water flow, water temperature, water conductivity and pH at the outlet area of the Huangzhou River Basin. The saturation indexes of calcite and dolomite and the partial pressure of CO₂ in river water were then calculated based on a chemical thermodynamic model. Our study show that (1) the Huangzhou River Basin is a typical drainage basin in mountainous area that is significantly impacted by rainfall. Although flash flooding happens frequently, the chemical regime of the dolomite-dominated drainage basin is still controlled by its chemostatic behavior. (2) The river is sourced from dolomite aquifer upstream on a residual plateau where good quality water-bearing media made of karstified dolomite are developed on either side of a shallow waterbed; whereas its middle and downstream sections lack well developed karst system, with runoff overland into streams. Due to CO₂ release from river surface, the calcite saturation index of river water is high at the outlet, where travertine deposition could be found at the riverbed. (3) According to the chemical thermodynamic model of open system for dolomite, the $HCO 3 -$ concentration in karst groundwater discharged to the river was calculated to be 5.1 mmol/L using the soil CO₂ data for different land use types. This value is 16% higher than the average concentration of $HCO 3 -$ in river water at the outlet of the Huangzhou River Basin, indicating the dissolved inorganic carbon in the river water is reduced due to turbulence-promoted degassing and aquatic photosynthesis. (4) The carbon flux in the dolomite-dominated watershed, calculated by the load estimation method, was 36.43 t CO₂/(km²·a). This value is not significantly higher than the carbon flux in other carbonate-dominated drainage basins in southern China, suggesting the CO₂ concentration in land cover is still inversely proportional to watershed runoff in the dolomite-dominated drainage basin, so that its carbon flux remains relatively constant even in humid subtropical climate.

Key words: dolomite-dominated watershed, carbon flux in karst system, chemostatic behavior, hydrochemical regime, automated monitoring

CLC Number:

P94
X143

ZENG Cheng, HE Chun, XIAO Shizhen, LIU Zaihua, CHEN Wangguang, HE Jianghu. Carbon flux in a typical dolomite-dominated drainage basin in humid subtropical climate[J]. Earth Science Frontiers, 2022, 29(3): 179-188.

Figures/Tables 5

Fig.1 Simplified hydrogeological map of the Huangzhou River Basin. Modified after [17-18].

Fig.2 Annual variations of soil CO2 concentration for different land use types (bottom plots) and annual variation of average monthly temperature (top plot) in the Huangzhou River Basin

Table 1 Hydrochemical parameters of river water at the outlet of the Huangzhou River Basin from tour gauging

指标	离子浓度/(mg·L^-1)							T/℃	pH值	电导率/ (μS·cm^-1)
指标	Ca²⁺	Mg²⁺	Na⁺	K⁺	$HCO 3 -$	$SO 4 2 -$	Cl^-	T/℃	pH值	电导率/ (μS·cm^-1)
平均值	56.33	23.61	0.72	0.86	264.84	10.42	1.29	15.27	8.60	393.71
	(0.19)	(0.16)	(0.15)	(0.29)	(0.06)	(0.21)	(0.22)	(0.32)	(0.01)	(0.03)

Table 1 Hydrochemical parameters of river water at the outlet of the Huangzhou River Basin from tour gauging

指标	离子浓度/(mg·L^-1)							T/℃	pH值	电导率/ (μS·cm^-1)
指标	Ca²⁺	Mg²⁺	Na⁺	K⁺	$HCO 3 -$	$SO 4 2 -$	Cl^-	T/℃	pH值	电导率/ (μS·cm^-1)
平均值	56.33	23.61	0.72	0.86	264.84	10.42	1.29	15.27	8.60	393.71
	(0.19)	(0.16)	(0.15)	(0.29)	(0.06)	(0.21)	(0.22)	(0.32)	(0.01)	(0.03)

Fig.3 Temporal variations of hydrological, hydrochemical parameters of river water at the outlet of the Huangzhou River Basin during a year-long monitoring study (May 2018-May 2019)

Table 2 Data of hydrological, hydrochemical regime at the outlet of the Huangzhou River Basin

指标	流量/ (m³·s^-1)	水温/℃	pH值	电导率/ (μS·cm^-1)	$HCO 3 -$ 浓度/ (mmol·L^-1)	SI_c	SI_d	$p C O 2$ /Pa	无机碳迁移量 (以C计)/(g·s^-1)
最小值	0.204	5.77	7.96	233.73	2.6	0.20	0.04	34.3	8.1
最大值	31.002	23.93	8.76	421.96	4.7	1.22	2.28	271.0	1 499.2
平均值	0.633	15.12	8.27	385.83	4.3	0.77	1.30	125.7	31.0
	(2.53)	(0.32)	(0.02)	(0.07)	(0.07)	(0.21)	(0.29)	(0.31)	(2.43)

Table 2 Data of hydrological, hydrochemical regime at the outlet of the Huangzhou River Basin

指标	流量/ (m³·s^-1)	水温/℃	pH值	电导率/ (μS·cm^-1)	$HCO 3 -$ 浓度/ (mmol·L^-1)	SI_c	SI_d	$p C O 2$ /Pa	无机碳迁移量 (以C计)/(g·s^-1)
最小值	0.204	5.77	7.96	233.73	2.6	0.20	0.04	34.3	8.1
最大值	31.002	23.93	8.76	421.96	4.7	1.22	2.28	271.0	1 499.2
平均值	0.633	15.12	8.27	385.83	4.3	0.77	1.30	125.7	31.0
	(2.53)	(0.32)	(0.02)	(0.07)	(0.07)	(0.21)	(0.29)	(0.31)	(2.43)

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