Doubly substituted isotopologues of methane hydrate (13CH3D and 12CH2D2): Implications for methane clumped isotope effects, source apportionments and global hydrate reservoirs
Introduction
As a major energy source and significant greenhouse gas, methane (CH4) plays an important role in the global carbon cycle. Quantifying the formation pathways of this gas will help us better constrain the global methane flux and its influence on global climate (Nisbet et al., 2014, Schaefer et al., 2016, Schwietzke et al., 2016). Given that methane is a crucial component in the early Earth’s ecosystems and climate (Ueno et al., 2006, Sauterey et al., 2020), a full understanding of the present-day methane cycle may enable us better decipher processes associated with early earth history.
Naturally occurring methane may be simply categorized as a mixture of gas from three principal sources (Etiope and Sherwood Lollar, 2013, Stolper et al., 2018): ‘thermogenic CH4’, yielded from thermal-breakdown of large hydrocarbon molecules and other organic matter; ‘microbial CH4’, produced by microbial methanogens; and ‘abiotic CH4’, formed by chemical reactions in the absence of any organic matter and biotic activity. Methods for identifying CH4 sources, such as the ‘δD vs. δ13C space’ (so called ‘Whiticar plot’; Whiticar, 1999) and ‘δ13C vs. C1/(C2 + C3) space’ (‘Bernard plot’; Bernard et al., 1976), were developed and have been widely employed. However, obvious shortcomings exist in these methods, such as the partial overlap of isotopic signatures from different CH4 sources, difficulties in constraining CH4 generated from two or more sources, and the alteration of isotopic compositions via post-generation processes such as gas migration and microbial methanotrophy (Douglas et al., 2017, Milkov and Etiope, 2018). These limitations have provided an impetus for developing additional constraints which can improve on interpretations regarding methane sources and histories. The focus of this investigation is to demonstrate the potential application of methane clumped isotope techniques to refine our understanding of such processes.
The clumped isotope method, which refers to two or more rare isotopes in one isotopologue, was firstly applied in carbonate minerals as a paleo-thermometer (e.g. Ghosh et al., 2006). The first report of methane clumped isotopes in natural samples employed a prototype ultra high resolution isotope ratio mass spectrometry (Thermo Scientific 253 Ultra; hereafter ‘Ultra’) established in the California Institute of Technology (CalTech; Stolper et al., 2014), which can only detect the combined abundance of 13CH3D and 12CH2D2 isotopologues. Meanwhile, another method for measuring the 13CH3D isotopologue was developed by a research group in the Massachusetts Institute of Technology (MIT) which uses a tunable infrared laser direct absorption spectroscopy (TILDAS) (Ono et al., 2014). By 2016, a method for the independent determination of both 13CH3D and 12CH2D2 isotopologues with satisfactory precision was achieved using a larger-radius high resolution isotope ratio mass spectrometry (Nu Instruments Panorama IRMS) at the University of California, Los Angeles (UCLA) (Young et al., 2016, Young et al., 2017). Recently, measurements of 13CH3D and 12CH2D2 isotopologues in the same CH4 sample have also been successfully carried out using updated versions of the Ultra (Eldridge et al., 2019, Dong et al., 2020, Dong et al., 2021, Thiagarajan et al., 2020) and TILDAS (Gonzalez et al., 2019) systems.
Unlike conventional isotopic determinations (δ13C and δD), methane clumped isotope signatures are defined as abundances of multiply substituted mass-18 isotopologues (13CH3D and 12CH2D2) relative to the stochastic distribution of each isotope in the same sample, and the notations are Δ18 (combination of 13CH3D and 12CH2D2), Δ13CH3D and Δ12CH2D2 (Young et al., 2016, Young et al., 2017, Douglas et al., 2017). Clumped isotope values may primarily record formation or equilibration temperatures if the samples which are analyzed present intra-species coincident temperatures (i.e., Δ13CH3D and Δ12CH2D2 values located along the curve of thermodynamic equilibrium). Alternatively, disequilibrium clumped isotope signatures might be a proxy for identifying kinetic isotope processes during methane generation and other post-generation processes such as mixing, migration, oxidation, bond re-ordering (Wang et al., 2015, Young et al., 2017, Giunta et al., 2019, Giunta et al., 2021, Labidi et al., 2020, Warr et al., 2021). Thus, disequilibrium clumped isotope signatures can potentially provide additional information for interpreting the cycling of methane in natural systems.
Previous studies of the clumped isotope signatures of thermogenic gas have presented reasonable and apparently consistent temperatures in most cases (Douglas et al., 2017, Young et al., 2017, Stolper et al., 2018, Giunta et al., 2019, Giunta et al., 2021, Xie et al., 2021), indicating that they can record temperatures related to either methane formation or post-bond-re-ordering. Nonetheless, disequilibrium values have also been observed in some laboratory samples which were thermogenically produced both from the pyrolysis of shale and coal (Shuai et al., 2018), and hydrocarbons (Dong et al., 2021). The disequilibrium signal may be possibly derived from a statistical combinatorial effect and/or from a kinetic isotope effect (KIE) occurring during thermal-cracking in the laboratory and it is possible that natural hydrocarbon systems may approach equilibrium during more long-term maturation processes. Additional investigations of natural thermogenic samples are necessary to further improve these isotopic models (Xia and Gao, 2019, Dong et al., 2021, Xie et al., 2021).
Biogenic CH4 is more complex and presents variable clumped isotope signatures. Natural biogenic CH4 gases can be produced via various microbial methanogenesis pathways, such as hydrogenotrophic, acetoclastic and methylotrophic methanogenesis (Conrad, 2020, Kurth et al., 2020). To date, microbial CH4 gases produced in laboratory cultivation experiments have consistently yielded clumped isotope compositions lower than the expected equilibrium values for the respective culture temperatures, regardless of their production pathways (Stolper et al., 2015, Wang et al., 2015, Young et al., 2017, Gruen et al., 2018, Giunta et al., 2019, Taenzer et al., 2020, Douglas et al., 2020). Dramatic depletion in 12CH2D2 (Δ12CH2D2: from −60‰ to −10‰) is mainly ascribed to the quantum tunneling and/or combinatorial effect (Young et al., 2017, Young, 2019, Cao et al., 2019, Taenzer et al., 2020), while relatively mild depletion in 13CH3D (Δ13CH3D: from −4‰ to +4‰) is probably produced by quantum tunneling and/or KIE which scales to the rate of methanogenesis (Wang et al., 2015, Young, 2019, Cao et al., 2019, Douglas et al., 2020). Additionally, Lloyd et al. (2021) suggested that even lower values associated with methylotrophic methanogens when compared to hydrogenotrophic methanogenesis may be partly inherited from methyl substrates depleted in both 13CH2D and 12CHD2.
Disequilibrium clumped isotope signatures have also been widely observed in natural biogenic CH4 samples, such as those collected from cow rumen and freshwater ecosystems (Stolper et al., 2015, Wang et al., 2015, Douglas et al., 2016, Young et al., 2017, Douglas et al., 2020). It is however interesting that clumped isotopes of methane recovered from marine sediments, which are supposedly of biogenic origin, typically exhibited reasonable formation temperatures which are reasonably consistent with the local environment (Wang et al., 2015, Inagaki et al., 2015, Young et al., 2017, Ijiri et al., 2018). Mechanisms underlying the equilibrium of methane clumped isotopes naturally occurring in marine sediments are still debatable. Early studies argued that equilibrium clumped isotope signals in biogenic CH4 are the product of low methanogenesis rates combined with high enzymatic reversibility, according to theoretical models of KIE at different degrees of reversibility (Wang et al., 2015), and this has been supported in part by the cultivation experiments and natural observations at much greater rates of methanogenesis (Stolper et al., 2015, Wang et al., 2015, Douglas et al., 2016, Douglas et al., 2020). However, these predictions remain to be tested (by field-based measurements for example) since the direct observations of clumped isotope signatures related to sufficiently low rates of methanogenesis in the laboratory seem rather difficult at this moment. Alternatively, microbial methanotrophy including aerobic and anaerobic oxidation of methane, can alter initial clumped isotope signatures of methane as well (Wang et al., 2015, Ash et al., 2019, Young, 2019, Ono et al., 2021). In particular, reversible bond re-ordering during anaerobic oxidation of methane (AOM) is proposed to be the key process leading to near-equilibrium values based on observations in natural samples (Ash et al., 2019). Preliminary laboratory culture experiments (Young, 2019, Ono et al., 2021) qualitatively support the role of AOM in altering methane clumped isotope signatures, but some of these experimental results do not quantitatively agree with observations in natural samples. For instance, Ono et al. (2021) observed Δ13CH3D values in residual CH4 to be as much as 3.1‰ higher than the expected equilibrium values which may be related to KIE derived from fast AOM rates during AOM incubation, while Young (2019) reported different bond re-ordering trends in the Δ13CH3D vs. Δ12CH2D2 space at different sulfate concentrations. In any case, the detailed mechanism of AOM catalyzed bond re-ordering needs deeper investigation.
The application of the methane clumped isotope approach to the accurate source identification and temperature reconstruction of natural samples requires further resolution of the aforementioned uncertainties and inconsistencies. To improve our knowledge of this novel method, especially in the marine sedimentary system, further laboratory and field studies are required. Here we suggest that gas hydrates recovered from marine sediments, which consist primarily of methane-water clathrate, are ideal materials to potentially provide new insights into the fundamental mechanisms involved in clumped isotope effects. The formation and preservation history of gas hydrates can be fairly complex and a number of processes which potentially influence methane clumped isotope compositions can be evaluated, such as: the mixing of various methane sources (Snyder et al., 2020b), the migration from deep sources to the surface reservoir (Hachikubo et al., 2015), the diffusion or migration between shallow sediment reservoirs (Ginsburg and Soloviev, 1997), the repeated cycling between gas hydrate formation and dissociation (Snyder et al., 2020b), and the potential for bond re-ordering to be catalyzed by AOM (e.g. Ash et al., 2019) and/or by clay minerals (e.g. Giunta et al., 2021). Several preliminary Δ13CH3D results for microbial hydrates recovered from Northern Cascadia Margin have been reported by Wang et al. (2015), presenting equilibrium temperatures which appear to be comparable to the sampling environment, yet the mechanisms leading to equilibrium are still under debate. In this study, we further extend the investigations using the paired clumped isotope approach to include gas hydrates recovered from shallow sediments in the Japan Sea. The common occurrence of AOM in the Japan Sea shallow marine sediment (Matsumoto et al., 2009, Hiruta et al., 2009, Tomaru et al., 2012), existence of both thermogenic and microbial methane sources (Hachikubo et al., 2015, Snyder et al., 2020b), and upward/advective flow of gas-saturated fluid in the gas chimneys and surrounding sediments render this region a natural laboratory for carrying out the above-mentioned study as many ambient parameters have been well documented (Monzawa et al., 2006, Snyder et al., 2007, Snyder et al., 2020a, Snyder et al., 2020b, Okui et al., 2008, Watanabe et al., 2008, Hiruta et al., 2009, Machiyama et al., 2009, Nguyen et al., 2016, Aoki et al., 2018, Zhang et al., 2019).
The paper is organized as follows. 2 Geological and geochemical context of sampling sites, 3 Materials and methods describe details in sampling sites and analytical methods, respectively. In Section 4, isotopic compositions of methane hydrates are reported. The underlying mechanisms for the observed isotopic compositions are discussed in 5.1 Bulk stable isotopes and molecular composition of gas hydrates, 5.2 Quasi-equilibrium signatures of methane clumped isotopes in gas hydrate samples. Using the observed results, we estimate the isotopic compositions of thermogenic and biogenic end-members and their contributions to our methane hydrate samples (Section 5.3). We then briefly discuss the implications for methane clumped isotope systematics and their future applications in methane biogeochemistry in Section 6.
Section snippets
Geological and geochemical context of sampling sites
The Japan Sea back-arc basin developed between 25 and 15 Ma (Jolivet et al., 1994, Okamura et al., 1995). From early to late Miocene, marine sediment rich in organic matter was rapidly deposited, ultimately becoming the source of oil and gas in this region (Okui et al., 2008, Tomaru et al., 2009, Nguyen et al., 2016). Tectonic inversion was initiated at 4 Ma and ongoing crustal contraction has led to both the generation of hydrocarbon-bearing anticlinal structures and to the development of
Methane hydrate collection, dissociation, and purification
Hydrate samples were collected during the scientific cruises in 2014 (HR14) and 2015 (PS15). Both conventional drill coring and pressure coring techniques were applied (Matsumoto et al., 2017, Snyder et al., 2020b). Hydrate samples were stored in liquid nitrogen immediately on shipboard. Frozen gas hydrates were crushed into small pieces at room temperature using a hammer. These frozen pieces were subsequently transferred into a 50 cm3 syringe connected to an evacuated aluminum-polymer gas
Results
Gas hydrate chemical compositions and methane bulk isotope results are shown in Fig. 4 and Table 1. Methane hydrate samples recovered from UTCW possess the highest δ13C values, ranging from −36.7 to –32.5‰. The δD values of UTCW samples are also relatively high, ranging from −174 to −148‰. On the contrary, samples from the other areas (UTNE, Joetsu Knoll, Oki Trough and Mogami Trough) present lower δ13C (−69.2 to −57.0‰) and δD (−198 to −180‰) values, except for one sample from UTNE which
Bulk stable isotopes and molecular composition of gas hydrates
Most samples collected from Joetsu Knoll, Oki Trough and Mogami Trough fall into the ‘CO2 reduction’ region of the Whiticar plot (Fig. 4A), indicating a major contribution of biogenic CH4 produced via a hydrogenotrophic methanogenesis pathway. This interpretation is further supported by the Bernard plot (Fig. 4B) and ‘δ13C-CH4 vs. δ13C-CO2’ plot (Fig. 4C), in which these samples are mostly located in microbial region as well. Other data lying at the margin of microbial region potentially
Isotope effects related to post-generation processes in deep marine sedimentary systems
One major outstanding problem in recent methane clumped isotope studies is the potential alteration of bulk and clumped isotopes in post-generation processes associated during methane preservation (e.g. Douglas et al., 2017, Young et al., 2017, Labidi et al., 2020, Giunta et al., 2021, Warr et al., 2021). Our new dataset and discussions show that the effects of migration, hydration and dehydration may in fact be insignificant when considering both the bulk and clumped isotope signatures of
Conclusion
In this study, we examined the potential influences of various post-generation processes in methane paired clumped isotopes in gas hydrates recovered from marine sediments and explored how this relatively new approach may be applied to natural hydrocarbon systems. All analyzed hydrate samples recovered from the Japan Sea sediment presented near-intra-species-equilibrium results, corresponding to apparent temperatures ranging from ∼15 to ∼170 °C. Regarding bulk stable isotopes and other
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was financially supported by JSPS KAKENHI funds of JP20K14594, JP17H06105, JP17H06456, and JP20H00195. Mang Lin is supported by the Guangdong Pearl River Talents Program (2019QN01L150). We thank Nina Albrecht (Thermo Fisher Scientific), Hao Xie and Guannan Dong (Caltech) for their helpful support in establishing the clumped isotope analytical method. We acknowledge Shuhei Ono for sharing standard methane samples. The samples taken in the research conducted in 2013 to 2015 under the
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