LAUSR

Abstract The Tablelands massif in Western Newfoundland is part of a Phanerozoic ophiolite sequence and is a terrestrial site of serpentinization. Similar to other Phanerozoic ophiolite sequences, the Tablelands possess environmental conditions that are conducive to all three established pathways of methanogenesis – abiogenic, microbial, and thermogenic – or a combination thereof. Sourcing methane from the Tablelands has thus far been limited because of the low dissolved methane concentrations. We tested dissolved gas extraction and concentration methods (vacuum extraction and gas stripping) for their effect on isotopic fractionation, and we applied these methods to sample dissolved methane for carbon and hydrogen isotopes (δ13CCH4 and δDCH4) as well as doubly substituted “clumped” isotopologue (13CH3D) analyses. Clumped isotope thermometry of methane estimated an apparent temperature of 85 ± 7 °C. The carbon isotope value of methane (δ13CCH4) for samples collected in 2017 was −27.9 ± 0.5‰, consistent with previously measured values (−27.3 ± 0.5‰) dating back to 2009. The hydrogen isotope value of methane (δDCH4) in 2017 was −175 ± 5‰. On a carbon deuterium (CD) plot, the Tablelands methane data plotted outside of the microbial field, but within an area where abiogenic and thermogenic fields overlap. A conclusive discrimination between abiogenic and thermogenic methanogenic pathways remains challenging. Nevertheless, a combination of sedimentary organic matter characterization of the underlying sedimentary units and comparison of geochemical characteristics with those described in experimental and theoretical studies suggest that the methane extracted from the Tablelands is likely a result of slow production of methane at ~85 °C from background organic sources.