5:15 PM - 6:30 PM
[MIS14-P15] Physical dependence of stable carbon and hydrogen isotopic ratio of methane gas in gas hydrate
Keywords:gas hydrate, methane gas, icy satellite, stable isotope, isotopic fractionation, physical dependence
Icy satellites have capability of having extra biota due to eruption of plume, which is consisted by sea ice particles including organic matter. The observation using Cassini clarified that this plume included hydrogen molecule (H2), CO2, methane (CH4) and organic matter (e.g. Porco et al. 2006; Waite et al. 2006, 2009, 2017). Especially in plume gas species, methane (CH4) is regarded as one of the candidate of signature of biological activity due to CH4 formation reaction by methanogen. However, CH4 gas in plume is consisted of not only microbial origin but also primitive and/or hydrothermal origin and this fact does not let CH4 detection is sufficient for conclusion for existence of biological activity by microbe. Therefore, we consider that evaluation of the elementary process of CH4 isotopic fractionation (δ13C and δD of CH4) via room experiment is important for isotopic observation of icy satellite in nearly future.
In this study, we focus on CH4 hydrate which is reservoir to produce the plume (Kieffer et al. 2006; Kamata et al. 2019) and plan to evaluate isotopic fractionation with its formation. Isotopic fractionation with hydrate formation has been found at only δD of CH4(liquid_water+CH4: ~5‰; ice+CH4: ~10‰) (P: 0–6 MPa) (Hachikubo et al. 2007; Lapham et al. 2012). In the case of condition of icy satellites, the pressure range is wider than of the Earth, and therefore we need to estimate isotopic fractionation.
We formed clathrate hydrate samples using hydrate formation equipment under the extreme environments, which can hold until 50 MPa. We lowered cell temperature from room temperature to 276 K, and then stirred CH4 gas and pure water at 85 or 400 RPM stirring rotation speed to form hydrate sample. We inserted CH4 gas at slightly higher pressure than the previous studies (4.4–9.7 MPa). We collected gas samples of primitive gas from CH4 gas cylinder and remained gas samples from the high pressure cell after hydrate formation, and then we measured stable isotopic ratio (δ13C and δD of CH4) using GC-IRMS equipment. We used the working standard with δ13C = –48.17‰ and δD = –178‰ for isotopic measurement. Precision of δ13C and δD value were estimated as 0.3‰ (n = 6, 1σ) and 0.7‰ (n = 4, 1σ), respectively, which were based on repeated analyses of the standards.
Results in isotopic measurements showed that the δ13C value of remained gas phase was slight lower than of primitive gas phases (|ε| ~1‰, which was almost same as a few previous studies) and became lower at higher pressure. This suggested that 1) 13CH4 and 12CH4 easily condensed into gas phase in hydrate and remained gas phase with clathrate hydrate formation, respectively, and 2) this condensation became stronger at higher pressure. On the other hand, results in the δD value of remained gas phase maximized at around 6–7 [MPa] (theoretical value). Hachikubo et al. (2007) reported that δD value of hydrate phase became lower than those of remained and primitive gas, and host–guest interaction did not cause the isotopic fractionation of δD. Furthermore, Casco et al. 2015 reported that CH4 adsorption ratio and hydrate formation speed rapidly increased from 6 to 8 [MPa]. Above results and previous studies might suggest that isotopic fractionation of δD with hydrate formation was not caused by water molecule of host cage and CH4 molecule, which had only H atom, was most easily taken at 6–7 [MPa]. Furthermore, results of remained gas phase showed that the δ13C value and pressure of theoretical δD maxima became lower at faster stirring rotation speed (δ13C value at 85 and 400 [RPM]: p < 0.05, n = 8; pressure of theoretical δD maxima: 7.1 and 5.7 [MPa] at 85 and 400 [RPM], respectively). Since Wijayanti (2018) reported that stability and storage capacity of hydrate became higher at higher stirring rotation speed. Thus, above tendencies and previous study might suggest that faster stirring rotation speed caused stronger hydrate stability due to easiness of 13C adsorption, and most easiness statement of only H atom having CH4molecule adsorption easily occurred at lower pressure.
Therefore, we found that physical condition such as pressure and stirring rotation speed, which was concerned to flow velocity of internal ocean, might become important factor of effect of gas hydrate formation at icy satellites condition on transportation and release processes of CH4 gas.
In this study, we focus on CH4 hydrate which is reservoir to produce the plume (Kieffer et al. 2006; Kamata et al. 2019) and plan to evaluate isotopic fractionation with its formation. Isotopic fractionation with hydrate formation has been found at only δD of CH4(liquid_water+CH4: ~5‰; ice+CH4: ~10‰) (P: 0–6 MPa) (Hachikubo et al. 2007; Lapham et al. 2012). In the case of condition of icy satellites, the pressure range is wider than of the Earth, and therefore we need to estimate isotopic fractionation.
We formed clathrate hydrate samples using hydrate formation equipment under the extreme environments, which can hold until 50 MPa. We lowered cell temperature from room temperature to 276 K, and then stirred CH4 gas and pure water at 85 or 400 RPM stirring rotation speed to form hydrate sample. We inserted CH4 gas at slightly higher pressure than the previous studies (4.4–9.7 MPa). We collected gas samples of primitive gas from CH4 gas cylinder and remained gas samples from the high pressure cell after hydrate formation, and then we measured stable isotopic ratio (δ13C and δD of CH4) using GC-IRMS equipment. We used the working standard with δ13C = –48.17‰ and δD = –178‰ for isotopic measurement. Precision of δ13C and δD value were estimated as 0.3‰ (n = 6, 1σ) and 0.7‰ (n = 4, 1σ), respectively, which were based on repeated analyses of the standards.
Results in isotopic measurements showed that the δ13C value of remained gas phase was slight lower than of primitive gas phases (|ε| ~1‰, which was almost same as a few previous studies) and became lower at higher pressure. This suggested that 1) 13CH4 and 12CH4 easily condensed into gas phase in hydrate and remained gas phase with clathrate hydrate formation, respectively, and 2) this condensation became stronger at higher pressure. On the other hand, results in the δD value of remained gas phase maximized at around 6–7 [MPa] (theoretical value). Hachikubo et al. (2007) reported that δD value of hydrate phase became lower than those of remained and primitive gas, and host–guest interaction did not cause the isotopic fractionation of δD. Furthermore, Casco et al. 2015 reported that CH4 adsorption ratio and hydrate formation speed rapidly increased from 6 to 8 [MPa]. Above results and previous studies might suggest that isotopic fractionation of δD with hydrate formation was not caused by water molecule of host cage and CH4 molecule, which had only H atom, was most easily taken at 6–7 [MPa]. Furthermore, results of remained gas phase showed that the δ13C value and pressure of theoretical δD maxima became lower at faster stirring rotation speed (δ13C value at 85 and 400 [RPM]: p < 0.05, n = 8; pressure of theoretical δD maxima: 7.1 and 5.7 [MPa] at 85 and 400 [RPM], respectively). Since Wijayanti (2018) reported that stability and storage capacity of hydrate became higher at higher stirring rotation speed. Thus, above tendencies and previous study might suggest that faster stirring rotation speed caused stronger hydrate stability due to easiness of 13C adsorption, and most easiness statement of only H atom having CH4molecule adsorption easily occurred at lower pressure.
Therefore, we found that physical condition such as pressure and stirring rotation speed, which was concerned to flow velocity of internal ocean, might become important factor of effect of gas hydrate formation at icy satellites condition on transportation and release processes of CH4 gas.