Quantitative analysis of the risk of hydrogen sulfide release from gas hydrates

1 Introduction

Pyrites are the most important sulfur sinks and are widely distributed in marine sediments (Lin et al., 2017). They have a significant influence on the sulfur cycle and have resulted in a series of studies involving major scientific issues, such as the evolution of oxygen and the origin of life (Butler et al., 2004; Chen et al., 2006; Formolo and Lyons 2013; Akhoudas et al., 2018). Therefore, many scholars have carried out detailed research on their cause and isotope composition (Hu et al., 2012; Pan et al., 2018). Two pyrite formation pathways have been proposed. One pathway is the polysulfide one:

${\mathrm{F}\mathrm{e}\mathrm{S}}_{\left(\mathrm{s}\right)}\to {\mathrm{F}\mathrm{e}\mathrm{S}}_{\left(\mathrm{a}\mathrm{q}\right)}+{\mathrm{S}}_{\mathrm{n}\left(\mathrm{a}\mathrm{q}\right)}^{2-}={\mathrm{F}\mathrm{e}\mathrm{S}}_{2\left(\mathrm{a}\mathrm{q}\right)}+{\mathrm{S}}_{\mathrm{n}-1}^{2-}\left(\mathrm{a}\mathrm{q}\right)(1)$

The other is the H₂S pathway, initially observed as the reaction of pyrrhotite (Fe_1-xS) with H₂S_(aq):

${\mathrm{F}\mathrm{e}\mathrm{S}}_{\left(\mathrm{s}\right)}\to {\mathrm{F}\mathrm{e}\mathrm{S}}_{\left(\mathrm{a}\mathrm{q}\right)}+{\mathrm{H}}_2{\mathrm{S}}_{\left(\mathrm{a}\mathrm{q}\right)}={\mathrm{F}\mathrm{e}\mathrm{S}}_{2\left(\mathrm{S}\right)}+{\mathrm{H}}_2\left(\mathrm{g}\right)(2)$

There have been many studies related to the formation of pyrite, right from the identification of the macroscopic sedimentary environment to the microscopic microbial culture, and are mainly focused on the redox state of the sedimentary environment and the global C-S-Fe cycle (Peckmann and Thiel 2004; Kraal et al., 2009; Lin et al., 2016). In recent years, the influence of the local depositional environment on the formation of pyrite and its isotopes has received increasing attention, and the idea of using pyrite sulfur isotopes to trace the evolution of depositional environment (such as sea-level changes) has been put forward (Wang et al., 2018a), especially in shallow seas (Peckmann et al., 2001; Lim et al., 2011; Sima et al., 2011). During OSR and SO₄^2--AOM processes, higher HS- concentrations are produced in the pore water and HS- reacts with Fe²⁺ in pore water or sediment to form pyrite. During this process, metal oxides are gradually converted into FeS and finally into pyrite under the action of excessive hydrogen sulfide. Therefore, studying the source of hydrogen sulfide has important scientific significance to understand the change of depositional environments. However, the speciation of H₂S in seawater is complex, with the species most often described in terms of free sulfide (H₂S+ HS⁻+ S^2-). Furthermore, questions remain regarding the role that H₂S plays in the global sulfur cycle, particularly with respect to its presence in the remote oceanic atmosphere and possible transfer across the air/sea interface. Previous studies have found that the main source of hydrogen sulfide for the formation of pyrite in marine sediments is from the anaerobic oxidation of methane (AOM) and organic sulfur reduction (OSR) (Xie et al., 2019; Wei et al., 2020). The production of hydrogen sulfide also occurs in hydrothermal systems due to geochemical processes (Yao and Millero, 1996). Huene and Pecher, (1999) summarized the H₂S concentrations found in a large number of hydrothermal fluids at various locations in the Pacific and Atlantic oceans, whereas the concentrations ranged from 1.1 to 110 mmol kg⁻¹ (Von et al., 1995). However, previous studies have focused more on the mechanism of the formation of hydrogen sulfide and the source of hydrogen sulfide from volcanic eruptions, hydrothermal flux, or from the AOM and OSR (Radford- Knwry and Cutter, 1994, Shen et al., 2008; Wu et al., 2018). Only a handful of studies have focused on hydrogen sulfide contained in hydrates. At present, H₂S has been observed in gas hydrates using Raman spectroscopy at the Hydrate Ridge (Hester et al., 2007), the Nigerian Margin (Chazallon et al., 2007), and the South China Sea (Fang et al., 2019). In marine sediments, there are huge reserves of gas hydrates, which are widely distributed. There are also huge reserves of H₂S in gas hydrates, which are universally present across the Earth. Moreover, H₂S released from gas hydrates has an important influence on the environment. Considering the huge reserves of gas hydrates, the amount of H₂S contained in gas hydrates is also huge. Furthermore, when gas hydrates dissociate, much of the H₂S released from the gas hydrates also affect the environment, the sulfur cycle in the ocean, and the local ecosystem (Kastner et al., 1998). Therefore, it is important to study the role that gas hydrates play in pyrite formation.

2 Geological setting

Qiongdongnan Basin is located in the western part of the northern continental slope of the South China Sea (Figure 1). The northern part of the basin is bound by the Hainan Island, while the west is adjacent to Indochina. Additionally, the east is close to the Pearl River Mouth Basin. The Qiongdongnan Basin covers more than 80,000 km², and approximately 60% of the basin has a water depth of more than 300 m (Wang et al., 2015). The seafloor water temperature at the Qiongdongnan Basin is 2°C–3°C, and the mean geothermal gradient is approximately 40°C/km. However, due to the common gas-bearing fluid activity in the Qiongdongnan Basin, the geothermal gradient in the study area is relatively much higher (Yuan et al., 2009). Oil and gas have been discovered in multiple reservoirs in different structural tectonic belts in the Qiongdongnan Basin (QDNB) (Zhang et al., 2014; Wang et al., 2015; W. Zhang et al., 2015; Zhang et al., 2016; Qin et al., 2019). The sediments deposited during the Pliocene and Quaternary possess favorable conditions for hosting biogenic gases, and these were commonly encountered in the strata shallower than 2,300 m during gas logging. In addition, the coal-measure source rocks deposited in the Oligocene are in the thermal evolution stage of mature-to-high-mature, with favorable conditions for thermogenic gas generation (Huang et al., 2015; Huang et al., 2017). Overpressure was common during the formation of QDNB when the rapidly filling sediments deposited in the Cenozoic became deeply buried under compaction (Shi et al., 2019; Wang et al., 2020). The mud diapirs and gas chimneys caused by overpressure are widely distributed in the deep water, providing an important vertical migration pathway for hydrocarbons and for the formation and accumulation of natural gas hydrates (Zhang et al., 2016; Wang et al., 2020). Furthermore, there is a large concentration of bivalve shells in the methane seep area (Figures 1B–E), indicating that the methane flux is higher. Meanwhile, multiple layers of gas hydrate are also found in the study area (Liang et al., 2019) (Figure 2).

FIGURE 1

FIGURE 1. The methane seep area was observed in Qiongdongnan Basin. [(A): Study Area, (B–E): Biological status of methane leakage area].

FIGURE 2

FIGURE 2. The mode distribution of gas hydrate and sulfate methane transition zone.

3 Methods

In the current study, we used mass conversation equations to calculate the amount of H₂S in the sink and how much H₂S is released from the sink.

The total mass of H₂S at any given time can be described as:

$\frac{d}{dt}{H}_{2}S={H_{2}S}_{RG}+{H_{2}S}_{RA}+{H_{2}SS}_{RO}{+H_{2}S}_{PH}-{H_{2}S}_{FP}-{H_{2}S}_{RM}-{H_{2}S}_O(3)$

where ${H_{2}S}_{RG}$ denotes the H₂S released from gas hydrate dissociation, ${H_{2}S}_{RA}$ denotes the H₂S produced from AOM, ${H_{2}S}_{R0}$ denotes the ${H_{2}S}_{RO}$ produced by OSR, $H_{2}S$ denotes the H₂S released from hydrothermal sources, ${H_{2}S}_{FP}$ denotes the H₂S that reacts with the metal ions to form pyrite, ${H_{2}S}_{RM}$ denotes the H₂S that is oxidized by the Fe (III) oxides, and ${H_{2}S}_O$ denotes the H₂S that is oxidized by oxygen. In the study area, there is no volcanic and hydrothermal activity. Therefore, in the current paper, the ${H_{2}S}_{PH}$ is assumed to be zero.

In the present study, the percentage of H₂S in gas hydrates was confirmed by the relative peak intensities of H₂S to CH₄. in Spectroscopy. Moreover, their Raman quantification factor ratios were calibrated using the crystal established absolute cage occupancies of a pure H₂S sample (Figure 3) (Qin and Kuhs 2013), which is similar to the procedures described in (Qin and Kuhs, 2013) for CH₄ hydrate.

FIGURE 3

FIGURE 3. The concentration of H₂S in gas hydrate at Qiongdongnan basin (Referend from huang et al., 2017).

The percentage values of the partial pressure of atmospheric oxygen (P_O2) are determined by Liu et al. (2021) and the present value of P_O2 is 212.28 mbar. The geological time of P_O2 is determined by multiplying the fraction of $P_{O2}$ with 212.28 mbar.

4 Results and discussion

4.1 Influence of the H₂S released from gas hydrates on the formation of pyrite

In marine sediments, the formation of H₂S occurs in a variety of settings. The production of H₂S in the pore water of sediments and the water column of stagnant basins is due to the anaerobic oxidation of methane (Yang et al., 2007). Submarine hydrothermal emissions are also a possible source of H₂S to the ocean. However, previous studies have shown that H₂S released from submarine hydrothermal emissions are not easily transferred into the atmosphere and shallow sediments (Yao and Milero., 1996). Therefore, the organic sulfur reduction (OSR) and anaerobic oxidation of methane (AOM) are the main sources of H₂S for the formation of pyrite (Commeau et al., 1987; Egger et al., 2015; Lin et al., 2015; Xie et al., 2019; Wu 2020; Wu, 2020; Wu, Xie, et al., 2020). However, the question remains as to how to evaluate the quantity of H₂S involved in the process of the formation of pyrites. As we all know, the AOM is widely present in the seepage area of gas hydrates (Wang et al., 2018b). Pyrites are also concentrated in sulfate and methane transition zone. During the process of AOM, the methane diffused from the bottom reacts with the sulfate from the overlying water at the sulfate-methane transition zone (SMTZ). The specific process is as follows:

${\mathrm{C}\mathrm{H}}_4+{\mathrm{S}\mathrm{O}}_4^{2-}\to {\mathrm{H}\mathrm{S}}^{-}+{\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}+{\mathrm{H}}_2\mathrm{O}(4)$

From Eq. 4, assuming the same reaction rate conditions, when 1 mol of methane is reduced by the sulfate, 1 mol of HS⁻ is produced at the same time. However, during the process of pyrite formation, 2 mol HS⁻ is needed to form 1 mol of pyrite (Peckmann et al., 2001; Canet et al., 2006; Lin et al., 2017). The specific reactions are as follows:

${\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}\mathrm{S}}^{-}\to \mathrm{F}\mathrm{e}\mathrm{S}+{\mathrm{H}}^{+}(5)$

$\mathrm{F}\mathrm{e}\mathrm{S}+{\mathrm{H}}_2\mathrm{S}\to {\mathrm{F}\mathrm{e}\mathrm{S}}_2+{\mathrm{H}}_2(6)$

Moreover, 1 mol of hydrogen sulfide is provided by the AOM process, and another 1 mol is provided by another unknown mechanism. The presence of H₂S in the gas hydrates provides an explanation (Qin et al., 2020). Of course, organic sulfur reduction (OSR) also provides part of the missing H₂S. The specific reaction is as follows:

$2{\mathrm{C}\mathrm{H}}_2\mathrm{O}+{\mathrm{S}\mathrm{O}}_4^{2-}\to {2\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}+{\mathrm{H}}_2\mathrm{S}(7)$

However, OSR often takes place in the open environment and shallower sediments as compared to AOM (Xie et al., 2019). Therefore, the hydrogen sulfide produced by the OSR is always diffused upwards under the influence of pressure gradient. Due to this reason, it is difficult for it to be diffused downwards.

In the study area, the SMTZ is mainly distributed at 6–9 mbsf in the Taixinan Basin and the content of pyrite in the sediments is about 1.16%–1.03% at Taixinan Basin (Wu et al., 2019; Wei, et al., 2020). Previous studies have shown that the content of pyrite at the Zhujiangkou Basin, where the SMTZ is present at 5–7 mbsf, is about 2.5%–2.6% (Liu et al., 2016). In the Qiongdongnan Basin, the content of pyrite lies within the range of 1%–3%, and the SMTZ is distributed at about 2–3.9 mbsf. Furthermore, the SMTZ is also very close to the water-sediment seafloor, indicating that the methane flux is high in the Qiongdongnan area (Miao et al., 2021). During the GMGS5 voyages, the methane seep area was found, which covered an area of about 100 km² in the Qiongdongnan Basin (Figure 1). In this area, there is a rich concentration of gas hydrates and pyrites in the sediments. Based on the above analysis, the amount of pyrite in the SMTZ deposits in the methane seep area in the Qiongdongnan Basin can be calculated using the following equation

${\mathrm{Q}\mathit{u}\mathit{a}\mathit{n}\mathit{t}\mathit{i}\mathit{t}\mathit{y}}_{\mathit{p}\mathit{y}\mathit{r}\mathit{i}\mathit{t}\mathit{e}}={\mathit{D}\mathit{e}\mathit{p}\mathrm{t}\mathit{h}}_{\mathit{S}\mathit{M}\mathit{T}\mathit{Z}}\times {\mathit{S}}_{\mathit{m}\mathit{e}\mathit{t}\mathit{h}\mathit{a}\mathit{n}\mathit{e}\mathit{s}\mathit{e}\mathit{e}\mathit{p}\mathit{a}\mathit{r}\mathit{e}\mathit{a}}\times {\mathit{Q}}_{\mathit{s}\mathit{e}\mathit{d}\mathit{i}\mathit{m}\mathit{e}\mathit{n}\mathit{t}\mathit{d}\mathit{e}\mathit{n}\mathit{s}\mathit{i}\mathit{t}\mathit{y}}\times {\mathit{C}}_{\mathit{t}\mathit{h}\mathit{e}\mathit{p}\mathit{y}\mathit{r}\mathit{i}\mathit{t}\mathit{e}\mathit{p}\mathit{e}\mathit{r}\mathit{c}\mathit{e}\mathit{n}\mathit{t}\mathit{o}\mathit{f}\mathit{s}\mathit{e}\mathit{d}\mathit{i}\mathit{m}\mathit{e}\mathit{n}\mathit{t}\mathit{s}}(8)$

$Mole{H}_{2}S=\frac{{Quantity}_{pyrite}}{m}\times 2(9)$

In this paper, m represents the relative molecular weight of pyrite. The depth of the SMTZ is 1.9 m, whereas the cover area is 100 km². Moreover, Q ranges from 1.64 to 1.9 g/cm³ (Zhang et al., 2015; Liang et al., 2019). We selected a sediment density of 1.8 g/cm³ and a pyrite percentage in the sediment of 2% in the Qiongdongnan Basin (Figure 4) (Mao et al., 2021; A. Haggerty 1991; Lim et al., 2011). The calculated quantity of pyrite is 6.9 × 10¹² g. Therefore, the H₂S needed to form pyrite is 4.06 × 10¹¹ mol. The same methods can be applied to calculate the quantity of H₂S released from the gas hydrates (see Eq. 10).

${Quantity}_{gashydrate}={Depth}_{gas}\times S\times Q_{sedimentdensity}\times Saturationofgashydrate(10)$

FIGURE 4

FIGURE 4. Content of pyrite at sulfate and methane transition zone in different sediments basin.

In this equation, the Depth _{gas hydrate} has been confirmed to be deposited at the three sediments sections (18.55–40.42 mbsf, 43.42 to 56.12 mbsf, and 58.6 to 98.42 mbsf) and was based on coring and sampling results and pilot hole LWD anomalies, which showed a high resistivity, low density, high gamma-ray values, and elevated acoustic velocity (Liang et al., 2019) Herein, the first Depth _gas is 21.87 m, the second Depth _gas is 13 m, and the saturation of gas hydrate is 31%. The quantity of gas hydrate in the first layer is 1.22×10¹⁵ g. The percentage of H₂S in the gas hydrate ranges from 1% to 2%, and, therefore, the quantity of H₂S in the gas hydrate of the first layer is 1.83 × 10¹³ g. The moles of H₂S released from the dissociation of gas hydrates in the first layer can be calculated using the _{following equation:}

$\mathit{M}\mathit{o}\mathit{l}\mathit{e}{\mathit{H}}_2\mathit{S}=\frac{{Quantity}_{H2S}}{d}(11)$

According to this analysis, the released quantity of H₂S is 5.4 ×10¹¹ mol. The H₂S released from the gas hydrate is about 1.3 times the H₂S needed for the formation of pyrite. The calculation results show that the H₂S released from the dissociation of gas hydrates may be the source of H₂S required to form pyrites. What is more, a greater volume and continual release of H₂S from the gas hydrate could cause the H₂S to seep into the water and even into the atmosphere. If so, it is still not clear what its biological and chemical consequences could be.

4.2 Evaluating the concentration of H₂S released from gas hydrates into the ocean and atmosphere

In general, H₂S can be depleted in seawater via oxidation. Recognized loss processes include photochemical oxidation, reaction with dissolved oxygen (Millero et al., 1987), After hydrogen sulfide (HS=H₂S+HS⁻+S^2-) is produced, it mainly participates in three types of reaction processes: 1) It gradually diffuses to the seabed and enters the bottom seawater body or oxidizes during the diffusion process. The oxidants in this process include oxygen, Iron oxides and manganese-containing oxides, products include elemental sulfur (S₀), complex sulfides (e.g., S₂O₃^2-) and sulfates; 2) Combine with organic matter to form organic sulfur; (three is also the most important reaction process, that is, it combines with iron-bearing minerals to form iron-bearing polysulfides and monosulfides and finally converts to pyrite. This is also the main formation mechanism of pyrite in marine sediments. In addition, a large number of laboratory studies have shown that pyrite can also be formed directly without the transformation of iron polysulfides. However, it is not all HS required for pyrite formation comes from SO₄^2--AOM. (Kelly and Kadish, 1982). However, in marine anoxic sediments, the concentration of oxygen and is low. Yao and Millero, 1996 found that elemental sulfur was the dominant product (95–100%) produced by the oxidation of H₂S by hydrous Fe(III) oxides. In fact, elemental sulfur is seldom found in the marine sediments in the Qiongdongnan Basin. Therefore, in anoxic sediments, the H₂S released from gas hydrates cannot be easily consumed, except in the process of forming pyrite. Nanomolar levels (0.1–2 nM) of H₂S have been found in the surface waters of the oceans (Cutter and Oatts, 1987; Iii and Tsamakis, 1989). However, the mechanism of the production and maintenance of hydrogen sulfide in surface seawater remains unclear. As is discussed in Section 4.1, if the second layer of gas hydrate dissociation results in the release of hydrogen sulfide into seawater, the main controlling factor for the release of hydrogen sulfide into the atmosphere is oxygen. In fact, in anoxic basins, mildly sulfidic deep waters are separated from the atmosphere by an oxygenated surface layer, at the base of which is a sulfide chemocline through which O₂ concentrations fall to zero. Kump et al. (2005) found that a significant buildup of H₂S in the deep sea could have led to toxic emissions of H₂S into the atmosphere, methane accumulation, and global warming Kump et al. (2005). The question is how to evaluate the seepage of H₂S into the water or atmosphere, and that the fundamental characteristic affecting the supply of H₂S into the water or atmosphere is the supply of O₂, whose transport through the surface layer must exceed the upwelling and diffusive flux of the reductant (H₂S) from below. The flux of O₂ must be at least two times the concentration of H₂S given the stoichiometry of the following reaction:

${\mathrm{H}}_2\mathrm{S}+{2\mathrm{O}}_2\to {\mathrm{S}\mathrm{O}}_4^{-2}+2{\mathrm{H}}^{-}(12)$

The fundamental characteristic of a stable chemocline is that the supply of O₂ from the atmosphere across the air-sea interface and its transport through the surface layer must exceed the upwelling and diffusive flux of the reductant (H₂S) from below. Kump et al. (2005) treated the exchange of gases, including O₂, between the atmosphere and ocean by using a piston-velocity formulation, whereby the flux of gases occurs at a rate (in this case, F_O2) that is proportional to the contrast in gas concentrations between the atmosphere and surface ocean with the proportionality constant being the piston velocity (k):

${\mathrm{F}}_{\mathrm{O}2}={\rho }_{\mathrm{o}\mathrm{c}\mathrm{e}}\bullet \mathrm{k}\bullet {\mathrm{K}}_{\mathrm{H}}\left({\mathrm{P}}_{\mathrm{O}2\mathrm{a}\mathrm{t}\mathrm{m}}–{\mathrm{P}}_{\mathrm{O}2\mathrm{o}\mathrm{c}\mathrm{e}}\right)(13)$

The supply of H₂S from below by upwelling (F_H2S) can be written as

${\mathrm{F}}_{\mathrm{H}2\mathrm{S}}={\rho }_{\mathrm{o}\mathrm{c}\mathrm{e}}\bullet \mathrm{u}\bullet \left[{\mathrm{H}}_2\mathrm{S}\right]\mathrm{d}\mathrm{e}\mathrm{e}\mathrm{p}(14)$

where u is the upwelling rate (four in m·yr⁻¹), [H₂S]_deep is the concentration of H₂S in deep waters (in mol·kg⁻¹), ρ_oce is the density of seawater (1,002 kg/m³), and K_H is the Henry’s law constant for O₂ (for warm surface waters, K_H is 10^–3 mol kg⁻¹·bar⁻¹).

The critical conditions for the balance of O₂ and H₂S must conform to the following equations:

${\mathrm{F}}_{\mathrm{O}2}=2{\mathrm{F}}_{\mathrm{H}2\mathrm{S}}(15)$

${\rho }_{\mathrm{o}\mathrm{c}\mathrm{e}}\bullet \mathrm{k}\bullet {\mathrm{K}}_{\mathrm{H}}\left({\mathrm{P}}_{\mathrm{O}2\mathrm{a}\mathrm{t}\mathrm{m}}–{\mathrm{P}}_{\mathrm{O}2\mathrm{o}\mathrm{c}\mathrm{e}}\right)=2{\rho }_{\mathrm{o}\mathrm{c}\mathrm{e}}\bullet \mathrm{u}\bullet \left[{\mathrm{H}}_2\mathrm{S}\right]\mathrm{d}\mathrm{e}\mathrm{e}\mathrm{p}(16)$

Given these values, and setting the surface water O₂ partial pressure (P_O2oce) to zero, the P_O2oce critical ratio of H₂S in the deep to atmospheric O₂, above which the steady-state surface-water O₂ concentration is zero, is given by Eq. 17.

$\left(\frac{{\mathrm{H}2\mathrm{S}}_{\mathrm{d}\mathrm{e}\mathrm{e}\mathrm{p}}}{{\mathrm{P}}_{\mathrm{O}2,\mathrm{a}\mathrm{t}\mathrm{m}}}\right)=\frac{\mathrm{k}\bullet \mathrm{K}\mathrm{H}}{2\mathrm{u}}=\frac{1000\mathrm{m}\mathrm{y}\mathrm{r}-1\times 10-3\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{k}\mathrm{g}-1\mathrm{b}\mathrm{a}\mathrm{r}-1}{2\times 4\mathrm{m}\mathrm{y}\mathrm{r}-1}\approx 0.13\frac{\mathrm{m}\mathrm{o}\mathrm{l}}{\mathrm{k}\mathrm{g}\mathrm{b}\mathrm{a}\mathrm{r}}(17)$

In the study area, the dissociation of the first gas hydrate layer would cause the H₂S to reach 0.135 mol kg⁻¹, and, at present, P_{O2 atm} is 212.28 mbar. Hence, $\frac{{\mathrm{H}2\mathrm{S}}_{\mathrm{d}\mathrm{e}\mathrm{e}\mathrm{p}}}{{\mathrm{P}}_{\mathrm{O}2,\mathrm{a}\mathrm{t}\mathrm{m}}}$ _present = $\frac{0.135\mathrm{m}\mathrm{o}\mathrm{l}\bullet \mathrm{k}\mathrm{g}-1}{212.28\times 10-3\mathrm{b}\mathrm{a}\mathrm{r}}\approx 0.64\frac{\mathrm{m}\mathrm{o}\mathrm{l}}{\mathrm{k}\mathrm{g}\mathrm{b}\mathrm{a}\mathrm{r}}$. 0.64 mol kg⁻¹∙bar⁻¹ (Figure 5) is greater than the critical value of 0.13 mol kg⁻¹∙bar⁻¹. Due to this reason, the dissociation of the gas hydrate in the first layer would cause a subsequent release of H₂S that could easily enter the atmosphere in the present condition of P_O2. However, during geological time, P_O2 often changes with time, so at different geological times, the critical values would vary (Table 1). In this paper, we calculated the various critical values from the present to 600 Ma. Our results show that, from 0 to 200 Ma, the H₂S easily seeped into the atmosphere through the dissociation of gas hydrate in the first layer. From 200 to 300 Ma, the calculated H₂S _deep/P_O2 values are lower than the standard values. Therefore, it would have been difficult for the H₂S to seep into the atmosphere if the temperature and pressure changed during this geological time.

FIGURE 5

FIGURE 5. (A) Variation in the value of P_O2 from present to 600 Ma BP. (B) Critical values changes from present to 600 Ma BP under the H₂S released from the complete dissociation of the first layer of gas hydrate. Moreover, from the present to 400 Ma, the H₂S released from the complete dissociation of the first layer of gas hydrate hardly moves into the atmosphere.

TABLE 1

TABLE 1. The partial pressure of atmospheric oxygen level at different geological period.

4.3 Evaluation of the influence of gas hydrate dissociation on H₂S release through geological time

Gas hydrates are sensitive to temperature and pressure (Wan et al., 2022). When these factors change, the gas hydrates can dissociate. Previous studies have shown that gas hydrates are sensitive to sea-level changes (Chown et al., 2000; Blendinger 2004; Bangs and Nathan, 2005; Jang-Jun et al., 2011). A decrease in sea level will cause a corresponding change in the pressure. These pressure changes can gradually cause the gas hydrates to dissociate. In fact, in the sediments with normal methane seepage, it is difficult for the hydrogen sulfide produced by AOM to reach the water, but the existence of methane-hydrogen sulfide hydrate makes large-scale hydrate decomposition that may lead to massive hydrogen sulfide release. At this time, it is particularly important to evaluate the conditions under which the hydrogen sulfide gas reaches the water body. In this study, we assume that the gas hydrate in the first layer is more susceptible to changes in the sea-level, resulting in a dissociation of the gas hydrate in the first layer. The gas hydrate in the second layer is assumed to be gradually released, causing a 20%, 40%, 60%, 80%, and 100% release of H₂S. Previous studies have shown that the H₂S released from the dissociation of gas hydrates in the first layer easily seeps into the atmosphere, except during 192–307 Ma BP. When 20% of H₂S is released from the gas hydrate in the second layer, it easily seeps into the atmosphere, except from 199 to 325 Ma. When the H₂S released by the dissociation of gas hydrates in the second layer reaches 40%, it also easily seeps into the atmosphere, except from 199 to 273 Ma (Figure 6). When the H₂S released by the dissociation of gas hydrates in the second layer reaches 60%, it easily seeps into the atmosphere, except from 212 to 293 Ma. When the H₂S released by the dissociation of gas hydrates in the second layer reaches 80%, it easily seeps into the atmosphere, except from 212 to 273 Ma. When the H₂S released by the dissociation of gas hydrates in the second layer reaches 100%, it easily seeps into the atmosphere, except from 232 to 273 Ma. For more precise estimates of the risk of H₂S being released into the atmosphere from the dissociation of gas hydrates, we incorporated the effects of sea-level change from 100 Ma BP to today and identified three risk modes: 1) the high-risk mode indicates that the sea level decreased sharply causing large-scale gas hydrate dissociation. The P_O2 value within this geological period is low. In this case, the H₂S released into the atmosphere is high. 2) The moderate-risk mode indicates that the sea level decreased slower than in the high-risk mode and caused partial gas hydrate dissociation. However, the P_O2 values during geological time are at a higher level. In this case, the amount of H₂S released into the atmosphere was moderate. 3) The low-risk mode indicates that the sea level is high and the P_O2 values during this geological period are also high. In this case, the H₂S released from the gas hydrate would not easily seep into the atmosphere. Based on the three risk modes, we classified 5–12 Ma BP, 25–29 Ma BP, 47–52 Ma, and 57–61 Ma BP into the high-risk mode. In addition, we draw a conclusion through detailed analysis: When geological time of the P_O2 was low and the gas hydrate’s temperature and pressure changed greatly, it caused massive gas hydrate dissociation resulting in a massive amount of H₂S being released from the gas hydrates and diffusing into the pore water from the bottom. During the process, part of the H₂S released from the gas hydrate was oxidized by the Fe (III) oxide metals, and part of it was used to form pyrite. Most of the H₂S entered the ocean and even into the atmosphere. When geological time of the P_O2 is high, the sea-level changed greatly. Meanwhile, massive H₂S was released from the gas hydrate into the pore water. This released H₂S will diffuse from the bottom. During the process, a small part of H₂S was released from the gas hydrate (being the H₂S source) to form pyrite. However, most of the H₂S will enter the ocean and get re-oxidized to become sulfate by the O₂ in the ocean. Furthermore, the H₂S released from the dissociation of the gas hydrates would be oxidized by Fe (III) oxide metals, with much of these metal ions being released into the pore water. In addition, the H₂S that was re-oxidized by the O₂ in the ocean also released much of the SO₄^2-. Therefore, the process also provides metal ions and SO₄^2- into pore water or seawater when the H₂S released from gas hydrate diffuses from the bottom.

FIGURE 6

FIGURE 6. Variation in the values from 600 Ma BP under the gradual dissociation of second gas hydrate from 20% to 100%.

5 Conclusion

It is important to evaluate how H₂S is released from gas hydrates. In this study, we established a one-dimensional mathematical model to calculate the amount of H₂S released from multiple layers of gas hydrates in the Qiongdongnan Basin. We investigated the role that H₂S released from the dissociation of gas hydrates plays on the sulfur cycle. Furthermore, we established the relationship between H₂S released from gas hydrate dissociation and the concentration of H₂S in the atmosphere. Our results show that the sulfate and methane transition zone (SMTZ) in the Qiongdongnan Basin contains 2.3 × 10¹² g of pyrite, which requires 4.06 × 10¹¹ mol of H₂S for its formation. The amount of H₂S released from the gas hydrate dissociation is 5.4 ×10¹¹ mol, which is about 1.3 times more than that needed for the formation of pyrite. Therefore, the H₂S released from the dissociation of gas hydrates is an important source of H₂S for the formation of pyrites in the SMTZ in the Qiongdongnan Basin. Based on the flux of H₂S and the partial pressure of O₂ (P_O2) in the atmosphere, we calculated the critical value of the balance between the flux of H₂S and P_O2 to be 0.13 mol kg⁻¹∙bar⁻¹. Furthermore, considering the effects of global sea-level change, we determined three risk modes to evaluate the possible seepage of H₂S from gas hydrate dissociation into the atmosphere. These are as follows: 1) the high-risk mode indicates that the sea level decreased sharply causing large-scale gas hydrate dissociation. The P_O2 values during this geological time are low. In this situation, there was a larger amount of H₂S released into the atmosphere. 2) The moderate-risk mode indicates that the sea level decreased gradually and caused partial gas hydrate dissociation. However, the P_O2 values at this geological time are higher. In this case, the amount of H₂S released into the atmosphere was moderate. 3) The low-risk mode indicates that the sea level is high, and the P_O2 values at this geological time are also higher (Figure 7). Therefore, it was not easy for the H₂S released from the gas hydrate dissociation to seep into the atmosphere. Based on the three risk modes, we classified 5–12 Ma BP, 25–29 Ma BP, 47–52 Ma, and 57–61 Ma BP into the high-risk mode. Furthermore, the H₂S released from the gas hydrate was oxidized by Fe (III) oxide metals, with much of the metal ions being released into the pore water. The H₂S that was re-oxidized by the O₂ in the ocean also released a lot of SO₄^2- (Figure 8). Therefore, the whole process also provides the raw materials for the process itself. This paper provides new insights into the source of H₂S found in the atmosphere and shows that the H₂S contained in gas hydrates possibly plays an important role in the global sulfur cycle.

FIGURE 7

FIGURE 7. Risk ranging from present to 100 Ma BP. The purple curve represents the change in global sea level. The grey area represents the high risk of H₂S going into the atmosphere.

FIGURE 8

FIGURE 8. Mode of the sulfur cycle. The results show that a part of the H₂S released from the dissociation of the gas hydrate is oxidized by Fe (III) oxide metal, with much of the metal ions being released into the pore water. Another part of the H₂S is re-oxidized by the O₂ in the ocean, with much of SO₄^2- released into the seawater. Therefore, the process also provides metal ions and SO₄^2- to pore water or seawater when the H₂S released from the gas hydrate diffuses from the bottom. The difference in A and B is the level of P_O2. In (A), the P_O2 is high and the released hydrogen sulfide from the decomposition of hydrate finds it difficult to enter the atmosphere. In (B), the P_O2 is low and the released hydrogen sulfide from the decomposition of hydrate finds it easy to enter the atmosphere.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

XW: Conceptualization, Methodology, Writing—Original Draft. SL: Data Curation. BZ: Conceptualization, Supervision, Writing—Review and Editing. YY and GW: Supervision. RX: Writing—Review and Editing. YF and ZN: Funding Acquisition, Resources, Supervision. All authors contributed to the manuscript.

Funding

The research was supported by the Project of Sanya Yazhou Bay Science and Technology City (No. SCKJ-JYRC-2022-14), GuangDong Basic and Applied Basic Research Foundation (No. 2020A1515110405), Geology Investigation Project of China Geological Survey (No. DD20221725), National Natural Science Foundation of China (No. 42106078).

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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