Methane sources and sinks in continental sedimentary systems: New insights from paired clumped isotopologues 13CH3D and 12CH2D2
Introduction
Unraveling methane sources and sinks is key to understanding carbon cycling over geological timescales. Although it has a relatively short residence time in the atmosphere, methane is an important greenhouse gas. By contrast, methane in the continental crust (e.g. in sedimentary systems or within crystalline rocks) has longer residence times due to the reducing nature of these environments as well as limited exchange with other reservoirs. In such crustal systems, three major methanogenesis pathways are considered: (i) ‘thermogenic methane’ resulting from the thermocatalytic cracking of longer chain hydrocarbons and organic matter (often associated with oil production in sedimentary basins), (ii) ‘microbial methane’ which can result from different type of metabolisms (e.g. CO2 reduction, acetate fermentation), and in rare cases (iii) ‘abiogenic methane’ resulting from Sabatier-type reactions. The identification of these different pools and the resolution of potential mixing between them is traditionally explored using bulk stable isotopic compositions (δ13C and δD) of the methane molecule, combined with the gas wetness ratio C1/C2+ (e.g. Bernard et al., 1976, Schoell, 1988, Sherwood Lollar et al., 1993, Sherwood Lollar et al., 1994, Martini et al., 1996, Whiticar, 1999). Characterization of subsurface methane is strongly constrained by our ability to define these possible endmembers. Isotope ratios are key arbiters in this endeavor. In nature, the variability of reaction pathways, temperatures and isotopic compositions of substrates or chemical precursors can produce a wide spectrum of bulk methane isotopic compositions that can result in overlap in isotope space between different endmembers, making identification of the provenance of the methane challenging (e.g. Martini et al., 1998, Horita and Berndt, 1999, Etiope and Sherwood Lollar, 2013, Vinson et al., 2017).
Recent advances in mass spectrometry have allowed the measurement of multiply substituted clumped isotopologues (i.e. molecules with two or more heavy isotopes). This novel approach allows the investigation of isotope bond ordering in a molecule, which at thermodynamic equilibrium depends on the formation temperature of the molecule. For most molecules, the likelihood to produce a clumped, or multiply-substituted, isotopologue is enhanced with decreasing equilibrium temperature compared to a purely stochastic distribution (Wang et al., 2004, Eiler, 2007). Pioneer studies on clumped isotopologues were initiated on CO2 molecules by measuring the rare abundance of 13C18O16O in atmospheric CO2 (Eiler and Schauble, 2004, Affek et al., 2007) or in carbonate samples (e.g. Ghosh et al., 2006, Ferry et al., 2011). Recently, Stolper et al. (2014a) – using high resolution mass spectrometry – and Ono et al. (2014) – using laser adsorption spectroscopy (TILDAS) – outlined methods to measure 13CH3D, the more abundant of multiply substituted mass-18 clumped isotopologues of methane. The first application to natural samples by Stolper et al. (2014b) analyzed a series of gas samples from different sedimentary systems. While some of their samples were convincingly of thermogenic origin and others presumably affected by microbial activity, they suggested overall reasonably good agreement between temperatures calculated based on the relative abundance of 13CH3D with those occurring within the basin, demonstrating the role 13CH3D can play in determining formation temperature of methane within natural systems. Such good agreement between clumped-based temperatures and environmental temperatures was later supported by Stolper et al. (2015) in a detailed study of gases from the Antrim Shale (Michigan Basin), by Wang et al. (2015) for gas samples from sedimentary environments (Powder River Basin, North Cascadia margin) and from Precambrian cratonic rocks which are considered abiotic in origin (Sherwood Lollar et al., 2002), as well as by Douglas et al. (2016) in Arctic marine sediments. At the same time, both Stolper et al., 2015, Wang et al., 2015 observed that laboratory methanogenic cultures or natural environments known to host methanogenic organisms, can produce methane with unrealistically higher clumped-based temperatures, and even in some cases, clear disequilibrium signatures (occurring when the abundance of a clumped isotopologue is lower than the stochastic distribution). These discrepancies between clumped-based temperatures and expected environmental temperatures were also correlated to large D/H disequilibrium between the methane and the water in which it was formed (Stolper et al., 2015, Wang et al., 2015, Douglas et al., 2016, Gruen et al., 2018). The potential for such large disequilibrium in some samples revealed a fundamental aspect of isotope bond (re-)ordering: in certain cases, methane is formed under thermodynamic equilibrium and the relative distribution of its isotopologues can be predicted and interpreted as an equilibrium temperature; in other cases, methane formation is controlled by kinetic effects yielding a distribution of isotopologues outside of equilibrium. This distinction between equilibrium and disequilibrium was also highlighted by Douglas et al. (2016) who observed a wide variability of clumped isotopologue signatures in a collection of gas seeps from Alaskan lakes, ranging from apparently thermogenic and equilibrated methane, to dramatically disequilibrated methane likely originating from methanogenic organisms, with many samples possibly resulting from mixing between these two ‘pools’.
Nonetheless, the expression of such disequilibrium where microbial methanogenesis occurs is not always clear-cut. For example, in the previously mentioned Antrim Shale, where gases have long been considered to be dominated by microbial methane (Martini et al., 1996, Martini et al., 1998, Martini et al., 2003, Waldron et al., 2007), samples interpreted to represent the most the microbial endmember appeared to be close to equilibrium with respect to environmental temperatures (Stolper et al., 2015). Seemingly equilibrated microbial methane has also been reported since then in other continental and marine sedimentary environments (Wang et al., 2015, Douglas et al., 2016, Inagaki et al., 2015 Ijiri et al., 2018). In order to reconcile these observations with laboratory-cultured methanogens (typically producing methane out of equilibrium), Stolper et al. (2015) proposed a model for microbial methanogenesis in which the degree of reversibility of the enzymatic reactions would allow methanogens to produce (near-)equilibrium methane. Accordingly, in the case of the Antrim Shale, Stolper et al. (2015) suggested extremely low substrate availability and slow formation rates would result in the production of near-equilibrated methane over geological timescales. The concept of a variable degree of reversibility was similarly put forward by Wang et al. (2015), who also proposed that methanogens in nature could produce a wide spectrum of clumped signatures, from disequilibrium to (near-)equilibrium, as a function of available free energy. Despite evidence for reversibility of the key enzyme involved in the methanogenesis pathway (Scheller et al., 2010), it is important to note that such near-equilibrium microbial methane has yet to be replicated under controlled (laboratory) conditions.
In the present study, we use the resolved relative concentrations of two rare mass-18 isotopologues, 12CH2D2 and 13CH3D, as a sensitive indicator of the degree of thermodynamic equilibration of methane gas. The Matsuda-type large radius high-resolution mass spectrometer at UCLA has a mass resolving power (> 40,000) that allows the direct measurement of both 13CH3D and 12CH2D2. The latter is another mass-18 isotopologue of methane with extremely low abundance, typically ∼0.1 ppm (Young et al., 2016). This novel dimension has the potential to provide unambiguous formation temperatures of methane in cases where both clumped isotopologue systems yield consistent temperatures, and alternatively, sensitive indicators of kinetic and mixing processes where disequilibrium is indicated. We investigated 13CH3D and 12CH2D2 in natural gas samples from sedimentary strata of Cambrian, Ordovician and Silurian ages in the Southwest Ontario Basin (Canada), and from Silurian and Devonian age strata in the Michigan Basin (USA). The choice of these two geological settings is relevant for a first application of the two mass-18 isotopologues in continental sedimentary systems, because both systems are thought to host significant volumes of microbially derived methane (though under very different salinity regimes) in addition to thermogenic gases (Sherwood Lollar et al., 1993, Martini et al., 1996, Martini et al., 1998, Martini et al., 2003, Clark et al., 2015, Stolper et al., 2015). Furthermore, although Southwest Ontario Basin has been extensively explored and exploited for its hydrocarbon resources, questions remain with respect to the apparent low thermal maturity of the rocks (Barker and Pollock, 1984, Sherwood Lollar et al., 1994) and the significant production of light hydrocarbon gases (C1, C2, C3…) implying more mature hydrocarbon generation, giving us an opportunity to provide additional constraints on the thermal history of this sedimentary system.
Section snippets
Southwest Ontario and Michigan Basins
The Southwest Ontario Basin is a broad sedimentary platform that was deposited during intermittent marine transgression occurring from late Cambrian to Devonian era (Brigham, 1971, Johnson et al., 1992). The sedimentary succession lies on the Algonquin Arch, a Precambrian topographic high trending northeast-southwest, and separating two areas of major subsidence: the Michigan Basin to the northwest and the Appalachian Basin to the southeast (Fig. 1). The Algonquin Arch was formed during the
Gas sample collection
We collected a series (n = 10) of gas samples from Cambrian, Middle Ordovician and Middle Silurian strata in Southwest Ontario. In the Michigan Basin, we collected gas samples from the Devonian Antrim Shale (n = 4), as well as two samples from other Devonian formations (Dundee and Berea), and three samples from the Niagaran Silurian formation which is stratigraphically equivalent to the Silurian formation (Guelph) sampled in Southwest Ontario. In the Michigan Basin, we sampled gases from the
Methane in Southwest Ontario
Gases from Southwest Ontario strata are predominantly composed of methane (40–90%), with variable amounts of ethane (2–21%) and propane (1–11%), as well as nitrogen (up to 15%). These gases show relatively homogeneous C1/C2+ ratios, usually between 5 and 10 (average is 7 ± 3). In this set of samples, methane exhibits significant variation in isotopic composition, ranging from −54‰ to −37‰ in δ13C, and from −281 to −172‰ in δD (Table 1). A general observation is that deeper Cambrian and
Thermogenic, near-equilibrium methane, in Cambrian and Ordovician strata
Samples from the Cambrian and the Ordovician strata in Southwest Ontario plot on, or slightly above the equilibrium curve (Fig. 3c), reflecting temperatures (between 110 °C and 205 °C – see Table 3) that are consistent with what is considered to be the typical thermogenic ‘gas window’ (Tissot and Welte, 1978). The isotopic composition of these gases, between −38 and −42‰ for δ13C, and between −174 and −207‰ for δD, are also in agreement with a thermogenic origin (e.g. Whiticar, 1999). Although
Conclusions
Depending on the processes involved, the methane cycle involves processes controlled by either at thermodynamic equilibrium effects or with large kinetic effects. Therefore, being able to determine whether methane is at thermodynamic equilibrium or not, is key to understanding and identifying subsurface methane sources and sinks. While the measurement of Δ13CH3D alone may provide relevant information on isotope bond ordering, independent estimates of environmental temperatures are required to
Acknowledgment
This study was supported by funding from the Natural Sciences and Engineering Research Council of Canada, the Sloan Foundation Deep Carbon Observatory and the Nuclear Waste Management Organization. The authors wish to thank Dr. Andrew Steele and Dr. Kosala Sirisena at the Carnegie Institution for helping culturing the Methanosarcina barkeri. The authors also wish to thank various industrial partners for their effort and support during sampling.
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