17:15 〜 18:30
[MIS14-P16] Clumped isotope signatures of abiotic methane formed via Fischer-Tropsch catalysis and their implications
キーワード：メタン、同位体、凝集同位体
Abiotic CH4 has been widely observed in nature, such as in hydrothermal vents, springs and so on. However, our knowledge on pathways of their formation remains pool. The bulk isotopic compositions of CH4, such as δ13C and δD, are widely used as tracers for identifying the CH4 formation processes. However, obvious defects are existent, such as partly overlapping of isotopic signatures for different CH4 sources, difficulty in constraining CH4 generated from two or more sources, suffering from post-processes and so on (Douglas et al., 2017). Very recently, CH4 clumped isotope signatures have been regarded as helpful constraints to provide additional information on origins of natural CH4 (e.g. Douglas et al., 2017; Young et al., 2017).
The clumped isotope refers to molecules substituted by two or more rare stable isotopes, e.g. 13CH3D and 12CH2D2, whose abundance relative to a random distribution is temperature dependent (Stolper et al., 2014). Therefore, the formation or bond reordering temperatures of CH4 can be obtained if the clumped isotope signatures are presented at intra-species equilibrium. On the other hand, kinetic effect and secondary processes like mixing and diffusion, will introduce disequilibrium values, which can be used in identifying specific process/mechanism (Stolper et al., 2015; Young et al., 2017). Limited data derived from abiotic CH4 synthesized by ruthenium (Ru) so far presented strongly depleted signals in both Δ13CH3D and Δ12CH2D2, and the mechanism has been explained by quantum tunneling effect, combinatory effect and/or kinetic effect (Young et al., 2017; Cao et al., 2019; Young, 2019). Nonetheless, a recent study on abiotic CH4 samples from hydrothermal systems suggested near-equilibrium clumped signals, being attributed to bond reordering at local fluid temperatures (Labidi et al, 2020).
In this study, we will present the clumped isotopic signatures of laboratory produced abiotic CH4 via series of gaseous FTT reactions, applying various experimental parameters and catalysts, such as Ni, Fe, Ru and Co. Both Δ13CH3D and Δ12CH2D2 values are analyzed by a 253 Ultra High Resolution Isotope Ratio Mass Spectrometer established in Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, following the similar procedures described in Eldridge et al. (2019). Preliminary data presented similar depleted signal in Δ12CH2D2, both for Ni and Ru catalyzing reactions at 300-400 °C. The mechanism causing depleted clumped values in laboratory produced abiotic methane will be discussed based on our new results, as well as its implication in the natural abiotic CH4 samples.
References: Douglas et al., 2017, Organic Chemistry; Eldridge et al., 2019, ACS Earth Space Chem.; Stolper et al., 2014, Geochim. Cosmochim. Acta.; Stolper et al., 2015, Geochim. Cosmochim. Acta.; Wang et al., 2018, Geochim. Cosmochim. Acta.; Young et al., 2017, Geochim. Cosmochim. Acta.; Cao et al., 2019, Geochim. Cosmochim. Acta; Young, 2019, Deep Carbon; Past to Present; Labidi et al., 2020, Geochim. Cosmochim. Acta
The clumped isotope refers to molecules substituted by two or more rare stable isotopes, e.g. 13CH3D and 12CH2D2, whose abundance relative to a random distribution is temperature dependent (Stolper et al., 2014). Therefore, the formation or bond reordering temperatures of CH4 can be obtained if the clumped isotope signatures are presented at intra-species equilibrium. On the other hand, kinetic effect and secondary processes like mixing and diffusion, will introduce disequilibrium values, which can be used in identifying specific process/mechanism (Stolper et al., 2015; Young et al., 2017). Limited data derived from abiotic CH4 synthesized by ruthenium (Ru) so far presented strongly depleted signals in both Δ13CH3D and Δ12CH2D2, and the mechanism has been explained by quantum tunneling effect, combinatory effect and/or kinetic effect (Young et al., 2017; Cao et al., 2019; Young, 2019). Nonetheless, a recent study on abiotic CH4 samples from hydrothermal systems suggested near-equilibrium clumped signals, being attributed to bond reordering at local fluid temperatures (Labidi et al, 2020).
In this study, we will present the clumped isotopic signatures of laboratory produced abiotic CH4 via series of gaseous FTT reactions, applying various experimental parameters and catalysts, such as Ni, Fe, Ru and Co. Both Δ13CH3D and Δ12CH2D2 values are analyzed by a 253 Ultra High Resolution Isotope Ratio Mass Spectrometer established in Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, following the similar procedures described in Eldridge et al. (2019). Preliminary data presented similar depleted signal in Δ12CH2D2, both for Ni and Ru catalyzing reactions at 300-400 °C. The mechanism causing depleted clumped values in laboratory produced abiotic methane will be discussed based on our new results, as well as its implication in the natural abiotic CH4 samples.
References: Douglas et al., 2017, Organic Chemistry; Eldridge et al., 2019, ACS Earth Space Chem.; Stolper et al., 2014, Geochim. Cosmochim. Acta.; Stolper et al., 2015, Geochim. Cosmochim. Acta.; Wang et al., 2018, Geochim. Cosmochim. Acta.; Young et al., 2017, Geochim. Cosmochim. Acta.; Cao et al., 2019, Geochim. Cosmochim. Acta; Young, 2019, Deep Carbon; Past to Present; Labidi et al., 2020, Geochim. Cosmochim. Acta