Soil gas CO₂ emissions from active faults: a case study from the Anninghe—Zemuhe fault, Southeastern Tibetan Plateau, China

Abstract

Introduction: Carbon dioxide emissions from non-volcanic areas are undervalued in the carbon cycle.

Methods: First estimates of diffuse CO₂ flux from the Anninghe—Zemuhe fault (AZF), Southeastern Tibetan Plateau, China, which suggests this could equal 15% emissions from all volcanoes in China. Following the accumulation chamber method, CO₂ flux was investigated at 1,483 points, and along 67 profiles crossing the AZF.

Results and discussion: Total CO₂ emissions from the AZF were estimated 1.2 Mt yr^-1. The relationship between soil gas CO₂ fluxes, earthquakes, and fault activity was discussed. The intense fault activity in the southern part of the Zemuhe fault (ZMHF) and the northern part of the Anninghe fault (ANH) was inferred, which could have enhanced the porosity of the soil, and accelerated the water-rock interactions and soil gas emission within the fault zone. The chemical and isotopic data indicated that biogenic CO₂ was the primary source of CO₂ from the AZF. Produced by interactions between groundwaters and carbonates, soil gas CO₂ could migrate to the near surface through cracks. Spatial variations of CO₂ flux in soil gas indicate that seismic activity could be responsible for the jumpy variations of CO₂ flux. The diffuse CO₂ from deep faults may contribute considerably to the greenhouse gas cycles.

1 Introduction

Consistently, Global warming is a central focus of climate change and studies have suggested that the growing number of carbon emissions has been considered as an essential factor for global warming (Lee et al., 2016; Isson et al., 2020; Huo et al., 2022). Nowadays, studying the distribution and magnitude of global carbon sources and sinks is an international mainstream for a better understanding of rates and mechanism of carbon cycling in and out of earth (Kämpf et al., 2013; Kang et al., 2020; Hiett et al., 2022; Xu et al., 2022). In addition to human activities, natural factors (e.g., volcanic eruptions, biological respiration, seismic activity) will also contribute to the increase of greenhouse gas concentration in the atmosphere (Inguaggiato et al., 2012; Muirhead et al., 2020). Therefore, a great interest has been focused on studying of CO₂ Earth degassing. There are many targets for such research, such as the relationships between the CO₂ flux and the tectonic structures (Etiope et al., 1999; Jolie et al., 2015), the researches of volcanic degassing (Inguaggiato et al., 2012; Chen et al., 2019; Fischer and Aiuppa, 2020), and the quantification of deeply derived CO₂ discharging into the atmosphere (Lee et al., 2016; Viveiros et al., 2017; Jacome-Paz et al., 2020; Bekaert et al., 2021; Rahilly and Fischer, 2021).

However, the contribution of CO₂ released from non-volcanic areas, particularly deep and large faults, is also worthy of attention (Fischer et al., 2019; Rahilly and Fischer, 2021; Zhang et al., 2021). Based on a study of ³He and CO₂ in springs and wells, Kennedy et al. (1997) estimated a mantle-derived CO₂ flux of ∼0.02 gm^-2 d⁻¹ for the entire San Andreas Fault system. The relationship between active crustal stress and soil CO₂ flux in southern Italy suggested that crustal stress associated with the seism genic process can effectively modulate the gas release in a seismically active area (Camarda et al., 2016). On the Chaozhou fault in China, significant anomalies in soil gas He and Rn were observed before the earthquake, which may reflect changes in the stress field before the earthquake (Fu et al., 2008); The sudden increase in CO₂ and Rn before the earthquake was observed on a fault in the earthquake cluster area in northwestern Bohemia, Czech Republic, which may indicate an increase in porosity and the opening of transport channels due to stress redistribution (Weinlich et al., 2006). Weinlich and others concluded that underground gas components can objectively and sensitively reflect the stress and tectonic activity changes of the earth’s crust, and usually show various anomalies before or after the occurrence of earthquakes.

AZF is a seismic zone with vigorous seismic activity, which has high slip rates and developed fractures providing a channel for fluid migration deep and CO₂ degassing. Therefore, the estimation of CO₂ degassing from the AZF is of great significance to understand the impact of the Earth’s natural degassing on the environment. Yang et al. (2028) studied three characteristic points in AZF for soil gas CO₂ concentration measurements. The results showed that the mean and maximum values of soil gas CO₂ concentration in the northern section of the ZMHF were much higher than those in the southern section of the ANHF. The soil gas CO₂ geochemistry produced different anomalous features, which, combined with the degree of occlusion in the AZF, reflect the gas-bearing characteristics of the AZF as well as the seismic hazard. However, there is no research on the characteristics of AZF soil gas CO₂ flux and its causes.

Taken together, the relationship between soil gas CO₂ fluxes and active fault tectonic features is considered a hot topic for discussion in the field of geochemistry. In this paper, measurements of flux of soil gas CO₂ were performed at 1,483 sampling points along 67 profile lines oriented perpendicularly to the AZF. The geochemical characteristics of soil gas CO₂ emissions in study areas and the association between the earthquakes and degassing process were investigated. This investigation aims to evaluate the total output of the CO₂ from the AZF and elucidate the tectonic association between the flux of soil gas CO₂ and the activity of earthquakes.

2 Geological setting

The AZF is the main active rift on the eastern boundary of the Sichuan-Yunnan block (Deng et al., 2003; Zhang et al., 2003). Under the combined effect of deep dynamic processes, large shear displacement and deformation zones are formed near the Xianshui fault, the ANHF and the Xiaojiang fault, which control the tectonic pattern of the Sichuan-Yunnan region (Figure 1).

FIGURE 1

The ANHF starts from Shimian in the north, extending through Xichang to Huili (Figure 2), with an overall northsouth trend. The overall NS trend is dominated by left-slip motion, with a maximum left-slip. The maximum left-slip rate since the Cenozoic is about 6.2 years⁻¹ (He and Oguchi, 2008), and the maximum extrusion rate is about 1.4 years⁻¹ (Ran et al. 2008; Ren and Lin, 2010). The Shimian-Xichang area in the southern section of the ANHF was the main rupture site of the earthquake with M_S 7^1/2 in 1,536, but no more significant earthquakes have occurred in the ANHF since an M_s 6 earthquake occurred south of Shimian in 1952 (Wen, 2000).

FIGURE 2

The Zemu River rift zone is connected with the ANHF in the north and the Xiaojiang fault in the south, extending from Xichang to Qiaojia (Figure 2), Yunnan via Puge and Ningnan, with a general strike of 330°. The Zemu River rift zone is dominated by left-slip motion, with a Holocene left-slip rate of about (6.4 ± 0.6) year⁻¹ (Wen, 2000). Historically, there were earthquakes of an M_s 7 in Xichang in 1814 and M_s 7 in Dajiaoliangzi in 1850, and the most recent moderately strong earthquake was the Xichang M_s 5.1 earthquake on 31 October 2018.

3 Materials and methods

3.1 Site description

Soil gas flux CO₂ surveys were performed at ANHF and ZMHF (26.8°N-29.2°N, 102°E – 103°E). During the layout of the CO₂ flux measuring line, a large number of field investigations were carried out on the fault zone in the field. First, we found the location of the active fault fracture zone, and then the suitable measuring line was selected. A total of 67 measuring lines were arranged in ANHF and ZMHF, recording the longitude and latitude of the starting point and end point of the measuring line. Finally, the measuring lines designed for the investigation were drawn on the map indoors, according to the line density and feasibility and the final line scheme was obtained. Overall, sampling for soil gas CO₂ flux was conducted at 1,483 sampling points along the 67 profiles that crossed the target faults (Figure 3). The measuring line is perpendicular to the fracture zone of the active fault zone and each line is generally more than 20 measuring points according to the landform. The spacing of measuring points near the active fault fracture zone is generally 10 m. After leaving the fracture zone, the spacing of measuring points was expanded by tens of meters to hundreds of meters according to the terrain (Figure 4).

FIGURE 3

FIGURE 4

3.2 Measurement methods of soil CO₂ flux

The details and principles of the closed-chamber method were described by Zhou et al. (2016). Analytical error associated with a single measurement was about ±5% and the reproducibility was about ±10% for the range of 100–10,000 gm⁻² d⁻¹.

Soil temperature at a depth of 10 cm was taken at every site, using a pocket digital thermometer (Henxin AZ8821), which has an accuracy of ±0.1°C. To minimize the inherent variability of gas flux due to soil water and humidity, sampling was performed during dry stable weather. Ambient air temperature and barometric pressure were recorded every 25 samples. Each survey was executed over three consecutive days, and under rain-free conditions, to avoid environmental effects. Both soil temperature and atmospheric pressure, recorded at every measurement, were used for calibrating soil flux values. At each observation site, the wind speed and barometric pressure was measured.

3.3 Measurement methods of carbon isotope

Four samples of CO₂ in soil gas from ANHF and four samples from ZMHF were collected for measurement of values of δ¹³C_CO2(V-PDB). Glass bottles (500 mL) were used to collect soil gas in order to improve the accuracy of carbon isotopes. The gas samples were collected as follows: first, the glass bottle was filled with saturated brine and then inverted into a bucket filled with saturated brine and kept the glass bottle from tipping over. Soil gas is pumped into the glass bottle through a rubber hose. Until the filled saturated brine was replaced by gas in one-half of the bottle, the glass bottles were sealed in water with rubber blocks to avoid air contamination. All gas samples were analyzed at the Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences. Carbon isotope analysis was performed using a Delta Plus XL mass spectrometer. It is manufactured by ThermoFinnigan Inc., consisting of the HP6890 gas chromatograph, combustion/conversion oven, interface and DeltaPlusXP mass spectrometer (Li et al., 2017). The stable carbon isotope composition is generally expressed as δ¹³C and has a precision of ±0.2‰.

4 Results

Overall, 734 points along 32 profiles across the ANHF and 728 points along 35 profiles across the ZMHF were sampled for the CO₂ flux analysis (Figure 5). There are two main patterns in the distribution of soil gas CO₂ fluxes on the AZF, one with anomalously high values near the fractures, which are mainly because the fault fractures are well developed at this location, creating many gas escape routes. The other is where the anomaly occurs on both sides of the fault, because the fault is mature and its fractured core is well developed, while the surrounding fragmentation is so high that a peak occurs above the lateral fault. This bimodal distribution is also observed in the San Andreas Fault profile (Kang et al., 2020).

FIGURE 5

In ANHF, the CO₂ flux in soil gas varied in the range of 0–1996.2 g m²d⁻¹, and the arithmetic mean values were 73.7 g ±2.36 m²d⁻¹ (Figure 6); In ZMHF, the CO₂ flux in soil gas varied in the range of 0–442.3 gm²d⁻¹, and the arithmetic mean values were 87.5 g ±4.21 m²d⁻¹. The δ¹³C_CO2(V-PDB) were in the range of −25.0‰ ∼ −19.7‰ (Table 1). The statistics data on the fluxes of CO₂ in soil gas from the AZF are listed in Table 2. The mean fluxes of CO₂ in each soil gas survey line were in the range of 7.23–272.22 gm⁻² d⁻¹ (Figure 7).

FIGURE 6

FIGURE 7

5 Discussion

5.1 Sources of CO₂ degassing

The CO₂ in subsurface fluids can be classified into two categories based on the type of parent material formed, organic and inorganic. The organic mainly formed by organic matter decomposition and bacterial activity, and the inorganic mainly from mantle/magmatic activity, thermal decomposition of carbonate rocks and dissolution of carbonate rocks (Wycherley et al., 1999). Different types of CO₂ exhibit different carbon isotopic signatures. δ¹³C_CO2 (V-PDB) vs. 1/CO₂ is regarded to be an indicator for gas sources region (Sano and Marty, 1995; Yuce et al., 2017; Chen et al., 2020). The values of end members for δ¹³C_CO2 of mantle and crust are −6.5‰ and 0‰, respectively, while for atmospheric and biogenic end members are −7‰ and −26‰, respectively (Dai, 1995; Sano and Marty, 1995; Dogan et al., 2009).

From Figure 8, it can be seen that the values of δ¹³C_CO2 values of the hot spring gas and soil gas samples are located in the composition mixing range between the mixed end of the crustal mantle and the biological end. The δ¹³C_CO2 values of the soil gas samples are located between the hot spring gas and the end of biological, which indicates that the CO₂ in the surface AZF mainly originates from the biological and the mixed end of the crustal mantle. Compared to the shallow depth of soil air circulation, the circulation of hot spring gas can reach depths of tens of kilometers. The CO₂ in soil gas is closer to the end of biological, which indicates that the oxidation of organic matter during aerobic microbial respiration plays the most important role in the production of CO₂ in soil gas.

FIGURE 8

Based on the above results, an attempt is made to explain the conceptual model of CO₂ sources and transport in the active fault zone of AZF. Fluids from the crust and mantle that accumulate in the lower crust and upper mantle rise through deep faults with a small amount of crust- and mantle-derived gas diffusing into near-surface soil gas. Although biogenic CO₂ is the primary source of soil gas, crust-derived and mantle-derived CO₂ may be a secondary source. In addition, air can intrude into faulted soil gas due to fluctuations in air pressure (Tamir et al., 2012; Chen et al., 2020).

5.2 Contribution to the atmosphere from CO₂ degassing in the AZHF

Global warming, which has received widespread social attention, is thought to be closely related to the rapid increase in atmospheric CO₂ concentrations over the past 100 years (Joos et al., 1999; solomon et al.,2009; Italiano et al., 2010). However, many scientists currently question the warming caused by increasing atmospheric CO₂ concentrations, and the main debate focuses on what are the main drivers of warming; how accurate predictions of future climate trends based on existing climate models; and what is the magnitude of the impact of climate change (Italiano et al., 2009; Iqbal et al.,2009), all of which are subject to considerable uncertainty. There is a great deal of uncertainty in these issues. Therefore, it is necessary not only to reduce greenhouse gas emissions caused by human activities, but also to study the contribution of natural factors to atmospheric greenhouse gases, so as to distinguish between natural and anthropogenic factors leading to changes in atmospheric greenhouse gas concentrations, and to correctly understand the impact of Earth degassing on the increase of atmospheric greenhouse gas concentrations. In addition, the degassing areas of seismically active fracture zones are also “natural analogue” sites for studying the leakage of CO₂ geological storage, especially the risk of sudden leakage of stored gases to the biosphere (Jing et al., 2019).

The annual contribution of CO₂ degassing from the fault zone to the atmosphere is equal to the amount of the annual average released flux each section of the fault zone multiplied by the area of each section of the fault zone. Based on the AZF and degassing characteristics, the AZF was divided into 5 segments, and the average value of the length and degassing flux of each segment was calculated based on the measurements (Table 3). The width of the fracture zone of the fault is generally tens of meters to hundreds of meters, and the surface avoidance zone of the active fault is more than 50 m (Xu et al., 2002). After the Wenchuan Ms8.0 earthquake, the width of the surface fracture zone of Longmenshan fault is about 200 m (Zhou et al., 2017), Several earthquakes of magnitude 6 or greater have occurred in the history of AZF, with the strongest one occurring north of Xichang with a magnitude of 7.5. The flux data from all sampling points on the AZF were superimposed on a single graph, which could be seen that the outliers with more than double the variance are concentrated within 200 m (Figure 8). High values of soil gas are usually exhibited near the fracture zone (Yuce et al., 2017; Jacome-paz et al., 2020), So the width of AZF was supposed as 200 m in this study.

By calculating the average flux of each survey line of ANHF and ZMHF, the annual contribution of ANHF and ZMHF zone to the atmosphere is 1.2 ±0.4 Mt (Table 3). The total flux of greenhouse gases from typical Cenozoic volcanic areas in Chinese Mainland to the atmosphere is about 8.13 Mt · a⁻¹ equivalent to about 6% of the total greenhouse gas emissions caused by global (10² ∼ 10³ Mt·y⁻¹) volcanic activities (Guo et al., 2014). The annual emission of AZF is relatively small compared with that of typical volcanoes in China. However, there are only 8 active volcanoes in China (Liu., 1999), while there are more than 200 active fault zones in the intraplate area of Chinese Mainland (Deng et al., 2003), among which the Tanlu fault zone, Altun fault zone, Haiyuan fault zone and Xianshuihe fault zone are more than 1000 km long. The total emission of CO₂ from all active fault zones would be huge, which may be higher than that from active volcanoes in China. Only a few of researches were aimed to calculate carbon emissions from faults in the world (Table 4), but it is far from enough. From a public health risk assessment perspective, detailed CO₂ degassing maps should also be performed to evaluate the potential for CO₂ release from fault.

5.3 Diffuse degassing in the ANHF and ZMHF

It is well known that understanding of crustal dynamics and degassing processes can be improved by studying soil gas in fracture zones (Ciotoli et al., 2016). Soil gas CO₂ flux is an important indicator of crustal stress accumulation and fault activity (Irwin and Barnes, 1980; Zhou et al., 2010; Yuce et al., 2017). Changes in tectonic stress leads to expansion or contraction of bedrock, altering the degree of water-rock response and thus affecting CO₂ degassing. The strength of degassing along active faults is related to seismic activity, which can reflect the state of stress accumulation to some extent (Rovira and Vallejo, 2008; Manaker et al.,2008). Hence, there could be some degree of correlation between CO₂ emissions and historical earthquakes and nowadays earthquakes.

According to the location and magnitude of historical strong earthquakes on the fault zone, the seismic gap on the fault zone, and the spatial distribution of current seismic activity, the soil gas flux on the AZF has obvious segmented features (Yu et al., 2014; Yu et al., 2018; Wen, 2000; Zhou et al., 2017; Chen et al., 2019; Walia et al., 2010).

In this paper, our results show that high soil gas CO₂ fluxes are found in the northern section of ANHF, southern section of AMHF and the central section of AZF (Figure 9), which well correlates with the sites that have high seismic activities. Notably, the higher soil gas concentration values in the northern section of the ANHF relative to the southern section of the ANHF, combined with the high flux values at point A (Figure 9), may indicate that the higher degree of fracture fragmentation in the area leads to elevated bedrock permeability, which promotes CO₂ migration from deeper to the surface. B point (Figure 9) is located at the intersection of ANHF and ZMHF. On the one hand, the area has a high stress background value under the interaction of multiple sets of ruptures (Wen, 2000); on the other hand, the study shows that the area is highly occluded and belongs to the seismic gap area for both historical and present-day earthquakes (Deng et al., 2003). Therefore, the fault fragmentation in this area is low, the fracture is not fully developed, and the overall soil gas CO₂ flux values are in the middle. C point (Figure 9) is characterized by the same high seismic activity, but the soil gas CO₂ flux is weaker than point A. This may be due to the “Y” shaped intersection of the ZMHF, Daliangshan fault, and Xiaojiang fault in the southern section, which may not have the tectonic conditions for high stress concentration. Although the frequency of seismic activity is high and the magnitude is large, the stress drop value of the earthquake source is generally in the middle. Therefore, the degree of fault fragmentation in this area is weak, resulting in relatively low bedrock permeability and fewer soil gas CO₂ migration channels.

FIGURE 9

The higher degree of active fault fragmentation and the increased permeability of bedrock and soil, provide an upward channel for gas transport (Ciotoli et al., 2016). However, under the action of tectonic stress, the degree of water-rock response varies at different stages of deformation evolution, and the permeability within the fault changes temporally and spatially (Dai et al., 1996; King, 1986; Pei et al., 1998; Neri et al., 2006; Annunziatellis et al., 2008). It is well known that faults can be both an upward pathway for transporting gases and can also block deep gas leaks (Caine et al., 1996; Dai et al., 1996; Sizova et al., 2019). Moreover, fault permeability is subject to self-sealing processes that might limit or inhibit fluid gas motion (Pei et al., 1998). The pore pressure of faulted soils usually depends on the crustal stresses associated with tectonic activity, which promotes the development of fractures and the formation of microfractures, resulting in the migration and redistribution of gases within the pore space (Holub and Brady, 1981; Zhang et al., 2003). CO₂ transport to the surface along well-developed fractures driven by pressure, which explains the higher soil gas CO₂ fluxes are often found near faults.

6 Conceptual model of soil gas in the AZF

The cross-cutting and interaction of fracture zones of different scales on the AZF, with some strongly extruded passages forming blockages, leads to stress concentration, earthquake nucleation and breeding (Shen et al., 2005). The geometric structure and activity habits of different sections of the fault zone is also controlled by the general dynamical background of the east boundary of the Sichuan-Yunnan active block (Chiodini et al., 1998; Toda et al., 2008). This is reflected in the uncoordinated deformation movement of the tectonic units. This incoordination is probably the main reason for the type and size of earthquakes and the local stress distribution pattern. In discussing the seismic activity and stress distribution in this multi-crossing fault zone, it is necessary to consider the characteristics of the cross-faults (Brune et al., 1970; Jiang et al., 2015). The interaction of cross-faults on the local stress field and seismic hazard needs to be considered when discussing the seismic activity and stress distribution characteristics in this multi-cross-fault region.

The northern section of the ANHF meets the Xianshuihe fault and the Daliangshan fault at an acute angle, which has local conditions of high stress concentration, high seismic activity, large magnitude, and high source stress, so the soil gas CO₂ flux is the highest (Figure 10) (Wang et al., 1998; Toda et al., 2008); the ANHF and ZMHF meet at an obtuse angle, which has weak seismic activity and low source stress in general, so the soil gas CO₂ flux is the lowest; the southern section of the AZF, ZMHF meets the Xiaojiang fault and the Daliangshan fault at an obtuse angle, which may not have high source stress (Xu et al., 2022) (Figure 10). The southern section of AZF, ZMHF and Xiaojiang fault and Daliangshan fault meet at an obtuse angle, so they may not have the tectonic condition of high stress concentration, and although the seismic activity is frequent, the source stress is relatively not high (Wu, 2020). Therefore, the overall performance of soil gas CO₂ flux is at medium level.

FIGURE 10

It has been suggested that degassing features similar to the AZF can be found in worldwide geologic extensional contexts (Apollaro et al., 2012), and spatial variations in CO₂ emissions from deep sources may be attributed to tectonic activity (Irwin and Barnes, 1980). In addition, it has also been demonstrated that areas with enhanced crustal permeability may show precursors of CO₂ degassing activity. Therefore, the geochemical exploration method proposed in this paper for AZF would be applied to other CO₂ degassing areas on Earth (Martinelli and Dadomo, 2017). Meanwhile, tectonic activity in possible future earthquakes can be monitored by implementing soil gas CO₂ flux and concentration measurements on active faults.

7 Conclusion

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