11:45 AM - 12:00 PM
[MIS19-05] Experimental investigation of the 13C-13C clumping in thermogenic ethane
Keywords:13C-13C clumped isotope, Thermogenic hydrocarbons, Hydrothermal system
Hydrocarbons in hydrothermal vents may provide insights into biogeochemical processes occurring under subsurface. Generally, hydrocarbons are mainly produced by the thermal decomposition of organic matter (thermogenic hydrocarbons). In contrast, in hydrothermal systems, water–rock reactions may produce hydrocarbons abiologically [Etiope and Sherwood Lollar, 2013]. Carbon isotopic compositions (d13C = (13C/12C)sample/(13C/12C)standard – 1) have been used to distinguish hydrocarbons sources [Des Marais et al., 1981]. However, identifying hydrocarbons sources is often challenging because thermogenic methane has a large d13C variation that overlaps with abiotic methane [Milkov and Etiope, 2018].
The development of multiply-substituted (so-called “clumped”) isotope analysis provides additional information preserved within a single molecule. Recently, we have developed a method for measuring 13C-13C clumped-isotope in ethane [Taguchi et al., 2020; 2021]. Since C-C bond in ethane is hard to recombine in nature, the 13C-13C clumping (D13C13C = (13C13C/12C12C)/(13C12C/12C12C)2 – 1) would provide information directly related to the hydrocarbons formations. A previous study showed that thermogenic ethane has a narrow range of D13C13C values from –0.11±0.03‰ to +0.46±0.05‰ which is distinct from abiotically produced ethane, providing a new way to distinguish hydrocarbons sources [Taguchi et al., 2022]. However, it is still unclear which parameters govern the D13C13C values of thermogenic ethane.
Here, we examined the D13C13C values of ethane from a pyrolysis experiment of organic materials. Lignin and docosane (C22H46) were used as starting materials. Lignin is a polymer of monolignol, while docosane is a straight-chain alkane. For comparison, we have also measured ethane produced by pyrolysis of sediment collected from Okinawa Trough [Kawagucci et al., 2020]. The apparatus used in this study consists of a gold reaction cell equipped with TiO2 fittings and a heating shell. The pyrolysis experiment was conducted at 30 MPa and 320°C with a duration of 3046 hours for lignin and 9000 hours for docosane. The Okinawa Trough sediment was heated at 361°C and 30MPa for 1177 hours using the same apparatus [Kawagucci et al., 2020]. The d13C values of lignin, docosane, and the Okinawa Trough sediment were –29.7‰, –31.5‰, and –22.0‰, respectively [Kawagucci et al., 2020]. The analytical precisions for d13C values were estimated by repeated analyses of in-house standard gases to be less than ±0.3‰.
Ethane from lignin pyrolysis was sampled at 46, 70, 574, and 3046 hours. The D13C13C and d13C values ranged from +0.06±0.07‰ to 0.22±0.09‰ and from –31.2‰ to –28.5‰, respectively. For docosane pyrolysis, ethane was sampled only at 9000 hours. The D13C13C and d13C values are –0.01±0.03‰ and –50.0‰, respectively. From the heated sediment, ethane was sampled at 1177 hours. The D13C13C and d13C values were –0.01‰±0.11‰ and –20.3‰. All the D13C13C values from the pyrolysis experiments fall within the range of the D13C13C values observed from natural gas fields (from –0.11‰ to +0.46‰) and higher than that of experimentally synthesized abiotic ethane (from –0.92±0.04‰ to –0.27±0.08‰) [Taguchi et al., 2020; 2022].
The observed narrow range of D13C13C values indicates that neither the starting material nor thermal maturity does impact significantly the 13C-13C clumping. On the other hand, the d13C variations could originate from the 13C content of the precursor molecules and isotopic effects associated with bond breaking (such as C-C, C-S, and C-O bonds in precursor molecules). Calculated isotope effects for C-C, C-S, and C-O bond rupture indicate a limited range in D13C13C values, which is smaller than observed variations in thermogenic gas in nature [Taguchi et al., 2022 and reference therein]. Therefore, the 13C-13C signature of thermogenic ethane mainly reflects those of organic precursors and could thus be used as a biomarker for organic molecules.
The development of multiply-substituted (so-called “clumped”) isotope analysis provides additional information preserved within a single molecule. Recently, we have developed a method for measuring 13C-13C clumped-isotope in ethane [Taguchi et al., 2020; 2021]. Since C-C bond in ethane is hard to recombine in nature, the 13C-13C clumping (D13C13C = (13C13C/12C12C)/(13C12C/12C12C)2 – 1) would provide information directly related to the hydrocarbons formations. A previous study showed that thermogenic ethane has a narrow range of D13C13C values from –0.11±0.03‰ to +0.46±0.05‰ which is distinct from abiotically produced ethane, providing a new way to distinguish hydrocarbons sources [Taguchi et al., 2022]. However, it is still unclear which parameters govern the D13C13C values of thermogenic ethane.
Here, we examined the D13C13C values of ethane from a pyrolysis experiment of organic materials. Lignin and docosane (C22H46) were used as starting materials. Lignin is a polymer of monolignol, while docosane is a straight-chain alkane. For comparison, we have also measured ethane produced by pyrolysis of sediment collected from Okinawa Trough [Kawagucci et al., 2020]. The apparatus used in this study consists of a gold reaction cell equipped with TiO2 fittings and a heating shell. The pyrolysis experiment was conducted at 30 MPa and 320°C with a duration of 3046 hours for lignin and 9000 hours for docosane. The Okinawa Trough sediment was heated at 361°C and 30MPa for 1177 hours using the same apparatus [Kawagucci et al., 2020]. The d13C values of lignin, docosane, and the Okinawa Trough sediment were –29.7‰, –31.5‰, and –22.0‰, respectively [Kawagucci et al., 2020]. The analytical precisions for d13C values were estimated by repeated analyses of in-house standard gases to be less than ±0.3‰.
Ethane from lignin pyrolysis was sampled at 46, 70, 574, and 3046 hours. The D13C13C and d13C values ranged from +0.06±0.07‰ to 0.22±0.09‰ and from –31.2‰ to –28.5‰, respectively. For docosane pyrolysis, ethane was sampled only at 9000 hours. The D13C13C and d13C values are –0.01±0.03‰ and –50.0‰, respectively. From the heated sediment, ethane was sampled at 1177 hours. The D13C13C and d13C values were –0.01‰±0.11‰ and –20.3‰. All the D13C13C values from the pyrolysis experiments fall within the range of the D13C13C values observed from natural gas fields (from –0.11‰ to +0.46‰) and higher than that of experimentally synthesized abiotic ethane (from –0.92±0.04‰ to –0.27±0.08‰) [Taguchi et al., 2020; 2022].
The observed narrow range of D13C13C values indicates that neither the starting material nor thermal maturity does impact significantly the 13C-13C clumping. On the other hand, the d13C variations could originate from the 13C content of the precursor molecules and isotopic effects associated with bond breaking (such as C-C, C-S, and C-O bonds in precursor molecules). Calculated isotope effects for C-C, C-S, and C-O bond rupture indicate a limited range in D13C13C values, which is smaller than observed variations in thermogenic gas in nature [Taguchi et al., 2022 and reference therein]. Therefore, the 13C-13C signature of thermogenic ethane mainly reflects those of organic precursors and could thus be used as a biomarker for organic molecules.