Potential sources of dissolved methane at the Tablelands, Gros Morne National Park, NL, CAN: A terrestrial site of serpentinization
Introduction
Methane can have a microbial, thermogenic, or abiogenic origin. Multiple lines of evidence are required to determine the source of methane (Etiope and Sherwood Lollar, 2013). Stable carbon and hydrogen isotope ratios have been used as one of the lines of evidence. Traditionally, the carbon-deuterium (CD) plot (a diagram of the stable hydrogen isotope values of CH4 (δDCH4) plotted against the stable carbon isotopes value of CH4 (δ13CCH4)), and the Bernard plot (the ratio of CH4 to the sum of ethane, propane, and butane plotted against the δ13CCH4) have been used by oil and gas researchers to source microbial and thermogenic methane (Schoell, 1988), and to demonstrate mixing ratios when both methane sources are present (Hunt, 1996), respectively. On the CD plot, empirically derived fields have been drawn encompassing data collected from laboratory and field-based studies for microbial (including autotrophic CO2 reduction, and heterotrophic acetate fermentation pathways), abiogenic, and thermogenic methane (Etiope and Sherwood Lollar, 2013 and references therein). However, these fields are not mutually exclusive, and overlaps exist.
Another method used to identify the source of methane is the CD fractionation plot, which graphs the stable isotope fractionation factors of carbon between dissolved inorganic carbon (DIC), total inorganic carbon (TIC), or carbon dioxide (CO2) and CH4 on one axis and the stable isotope fractionation of hydrogen between water (H2O) and CH4 on the other axis ((Kohl et al., 2016; Sherwood Lollar et al., 2008) and references therein). Plotting the fractionation factors allows a direct comparison among the isotope effects associated with different methanogenic processes, as it removes the effect of the stable isotope values of substrates or reactants on those of the product methane.
In addition to the compound-specific isotope ratios, the relative abundance of doubly-substituted (“clumped”) methane isotopologues (e.g., 13CH3D) can further constrain the origin of methane by providing information about the temperature at which the sample of methane was formed or last equilibrated (Ono et al., 2014; Stolper et al., 2014). Methane clumped isotope analysis can therefore serve as a geothermometer for methane formed in near-equilibrium processes.
The surficial occurrence of ultramafic rocks on Earth takes place at subduction zones, mid-ocean ridges, in the Precambrian Shield, and ophiolites. These occurrences promote serpentinization by exposing ultramafic lithologies to aqueous alteration. Of these geologic settings, ophiolites are the most accessible and are therefore ideal targets for the investigation of methane production at sites of serpentinization. Methane may be produced via abiogenic, thermogenic, and/or microbial synthesis within an ophiolite's geologic setting (Szponar et al., 2013).
Methane of proposed abiogenic origins has been detected in ophiolites in the Philippines, Italy, Greece, Turkey, Oman, New Zealand, and Japan (Etiope and Sherwood Lollar, 2013 and references therein), as well as the Precambrian Shield in Canada (Sherwood Lollar et al., 2002). The serpentinization process creates an environment conducive to methane production via hydrothermal alterations of the originally anhydrous constituent minerals of an ultramafic rock, namely olivine, orthopyroxene, and clinopyroxene (Barnes et al., 1967). The hydration of these minerals results in the production of serpentine, iron and magnesium hydroxides, and H2. The H2 may react with inorganic carbon to produce methane abiogenically (Sleep et al., 2004), as has been reported at numerous sites with serpentinizing conditions (Etiope and Sherwood Lollar (2013) and references therein). However, experimental studies have shown that the kinetics may be sluggish (McCollom, 2016; McCollom and Donaldson, 2016).
Another possible source of methane is microbial methanogenesis; the process of serpentinization may result in both favorable and unfavorable environments for biological methane production. The water-rock reactions result in the production of reducing aqueous environments (Eh of ≤−500 mV) where methanogens can flourish. In such reducing environments, methanogens can thrive via autotrophic CO2 reduction, heterotrophic organic acid fermentation or a combination thereof (Kohl et al., 2016 and references therein). For example, a few terrestrial serpentinization sites (e.g., The Cedars and the Samail Ophiolite of Oman) have been suggested to support microbial methanogenesis. Genomic (Suzuki et al., 2013), geochemical (Morrill et al., 2013), and laboratory studies (Kohl et al., 2016) all demonstrated the potential for microbially sourced methane at The Cedars; and 16S ribosomal RNA (rRNA) analysis detected Methanobacterium sp. (methanogens that use the CO2 reduction pathway) from well water associated with the Samail Ophiolite in Oman (Miller et al., 2016). However, other conditions resulting from the serpentinization process such as high pH can be detrimental to methanogens. Indeed, besides the few exceptions mentioned above, there is generally limited evidence of microbial methanogenesis within sites of terrestrial serpentinization associated with Phanerozoic ophiolites (Etiope and Sherwood Lollar, 2013 and references therein).
In addition to abiogenic and microbial methanogenesis, methane is also produced via the thermal degradation of buried organic matter. Ophiolites are commonly underlain by a marine metasedimentary mélange consisting of a formerly subduction-related accretionary wedge that originated as seafloor sedimentary deposits. The extensive fracturing inherent to the subduction, subsequent obduction and emplacement of the ultramafic unit creates conduits that can transport thermogenic gases from the subsurface to the surface via groundwater flow. Because the ophiolites are emplaced upon organic-bearing units, methane found in the ophiolites' ultra-basic springs is potentially thermogenic (or partially thermogenic) in source. Methane produced at the Coast Range Ophiolite Microbial Observatory (CROMO) site of continental serpentinization is tentatively classified as thermogenic based on isotopologue thermometry in conjunction with empirical observations at the site (Wang et al., 2015). Contribution of thermogenic methane is also suggested in Chimaera seep in Turkey (Young et al., 2017).
The source of the methane produced at the Tablelands Ophiolite has not been conclusively characterized. On the Bernard plot, the Tablelands' methane plot outside of the common range of microbial methane with relatively high δ13C values (−27 ± 0.5‰) and low ratios (6 to 10) of methane to the sum ethane, propane, butane (Szponar et al., 2013). This tentatively characterizes the Tablelands' methane as non-microbial. Genomic observations, namely the lack of characteristically methanogenic Archaean 16S rRNA (Brazelton et al., 2013), suggest that the microbial community in the Tablelands does not include known methanogens. Laboratory microcosms created using materials collected from the spring sites in the Tablelands did not produce methane despite the favorable conditions for microbial methanogenesis (Morrill et al., 2014). Thus, the geochemical, genomic, and experimental evidence collectively suggest that the methane at the Tablelands is likely not microbial in origin.
While microbial methanogenesis is unlikely at the Tablelands, the available data were not sufficient to conclusively identify or distinguish between thermogenic and abiogenic origins (Morrill et al., 2014; Szponar et al., 2013). Therefore, additional lines of evidence may aid in sourcing the methane. These include measuring the stable hydrogen isotope ratios and the clumped isotopologue abundance of the methane, as well as investigating the thermal history (i.e. Thermal Alteration Index) of the organic matter in the sedimentary units beneath the Tablelands. Initial efforts to measure the stable hydrogen isotope ratios of the dissolved methane (δDCH4) were unsuccessful because the sample concentrations achieved with the sampling protocol employed in our previous study were below the analytical quantification limits (Szponar et al., 2013). A sampling method that collects sufficient amounts of dissolved methane while preserving original isotopic values is therefore required (Morrissey and Morrill, 2016).
Dissolved methane is typically sampled using two methods: gas stripping (Oremland and Des Marais, 1983; Rudd et al., 1974) and vacuum degassing (Sherwood Lollar et al., 1993). Using greater volumes of water and larger vials with these methods may increase the total amount of methane in the gas/headspace to levels above our analytical quantification limits for δDCH4 analyses. However, these sampling methods need to be tested to ensure that the isotopic integrity of the methane sample is maintained.
Therefore, the first objective of this study was to test these sampling methods for isotopic fractionation, and apply them to δDCH4 as well as methane clumped isotopologue (13CH3D) analyses. The second objective was to examine the thermal history of the organic matter in the sedimentary units below the ophiolite to assess their potential of producing thermogenic methane. The final objective was to source the methane at the Tablelands by using new hydrogen isotope and clumped isotopologue data in the context of other geological and geochemical information.
Section snippets
Geologic site description: the Tablelands Massif of the Bay of Islands ophiolite
The Tablelands Massif, a part of the Bay of Islands ophiolite complex and the Humber Arm Allochthon, originated at a peri-Laurentian marine spreading center that was obducted onto the continental margin as a result of the Taconic orogeny. This Taconic orogeny was a subset of the Appalachian orogeny that occurred during the late Cambrian to early Silurian, and it resulted in the accretion of peri-Laurentian marine arcs onto the Laurentian continental margin (van Staal et al., 2007). The
Testing possible isotope effects associated with CH4 gas extraction
The average δ13C value of the laboratory CH4 extracted using the vacuum extraction method was −40.8 ± 0.1‰ (n = 3). The average δ13C value of the laboratory CH4 extracted using the gas stripping method was −40.2 ± 0.1‰ (n = 3). The previously determined δ13C value of the CH4 used to prepare the solution was −40.1 ± 0.5‰. Therefore, neither the vacuum extraction nor the gas stripping method altered the δ13C value of the CH4 beyond the ±0.5‰ analytical error.
The average δD value of the laboratory
Sampling dissolved CH4 using gas stripping for δ13C and δD analyses
Vacuum extraction and gas stripping methods for dissolved methane resulted in δDCH4 and δ13CCH4 values that were within analytical error of the initial methane. Therefore, these methods did not cause significant isotope fractionation during the sampling procedures. Similarly, when methods were used to extract the dissolved methane from the ultra-basic groundwater of the Tablelands, the resulting δDCH4 and δ13CCH4 values were within analytical error of each other. These data suggest that either
Acknowledgements
The authors would like to thank Mark Wilson for his inspirational support and great help over the years, and Geert Van Biesen, Elliot Burden, Steve Emberley, and Jamie Warren for their expertise. Geologic maps were created with the diligent support of David Mercer in Memorial University of Newfoundland's Queen Elizabeth II Map Room. Additionally, we would like to acknowledge Parks Canada for providing access to the site. The authors would like to thank the two anonymous reviewers for their
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