Geochemical, biological and clumped isotopologue evidence for substantial microbial methane production under carbon limitation in serpentinites of the Samail Ophiolite, Oman

Abstract

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Low‐Temperature Hydrogen Formation During Aqueous Alteration of Serpentinized Peridotite in the Samail Ophiolite

“…Recent work has identified low‐diversity microbial communities in well NSHQ‐14, just a few meters from Hole BA3A, as well as in other local wells containing hyperalkaline groundwater. These communities are dominated by Thermodesulfovibrionaceae (likely sulphate reducers), candidate phylum OP1 (potential hydrogenotrophic acetogens), Meiothermus (generally aerobic heterotrophs), and Methanobacterium (methanogens) likely utilizing H 2 as an electron donor (Kraus et al., 2020; Miller et al., 2016; Nothaft et al., 2020, 2021; Rempfert et al., 2017). To date, all the microbiological studies in the Samail ophiolite have focused on characterizing the community structure and activity of organisms present in the fracture fluids.…”

“…Data on the aqueous geochemistry of the Ministry of Water Resources well NSHQ‐14 have previously been reported (Miller et al., 2016; Nothaft et al., 2020; Paukert Vankeuren et al., 2019; Rempfert et al., 2017). Additional samples and measurements were collected in 2020 using similar methods.…”

“…Dissolved silica was analyzed by the silicomolybdate‐molybdenum blue colorimetric assay (ASTM International, 2016). Silica activity was calculated at 25°C, and major ion fluid composition measured from recent sampling of NSHQ14 (Nothaft et al., 2020) and BA3A (SI Table 1), with pH measured at the sampling depth in the well logs (SI Table 2).…”

“…There is active microbial methanogenesis occurring in the peridotite-hosted aquifer in the Samail ophiolite, including in well NSHQ-14 10 m away from Hole BA3A (Fones et al, 2019(Fones et al, , 2020Kraus et al, 2020). Nothaft et al (2020) have used a combination of geochemical, isotopic, and microbial observations to show that microbial methanogenesis, as well as an abiotic source of CH 4 -some of which could be derived from pre-existing fluid inclusions with serpentinized rims in olivine (Klein et al, 2019)contribute important amounts of CH 4 . The extremely low fO 2 in the deeper waters in boreholes BA1B, BA1A, NSHQ-14, BA3A and BA4A (Kelemen, Bach, et al, 2020;Kelemen, Matter, et al, 2020) close to the dissociation limit for H 2 O, the presence of widespread, nanoscale awaruite inclusions in serpentine documented in this study, and the regional observation of streams of H 2 -rich gas emanating from peridotite-hosted alkaline springs in the Samail ophiolite (e.g., Neal & Stanger, 1985;Vacquand et al, 2018) all raise the possibility that abiotic CH 4 is forming via hydrogenation of CO 2 or dissolved carbonate species.…”

Theoretical Predictions Versus Environmental Observations on Serpentinization Fluids: Lessons From the Samail Ophiolite in Oman

“…(2005) interpreted that hyperalkaline fluids in Oman originate in deeper aquifers (>500 m) and manifest at the surface when deep‐seated fluids discharge as springs in fault zones. Recent works on several wells in the Samail ophiolite have documented hyperalkaline fluids occurring at shallower depths (<100 m) despite not being in discharge zones (Miller et al., 2016; Nothaft et al., 2020; Paukert Vankeuren et al., 2019; Rempfert et al., 2017). It is currently unknown whether these hyperalkaline fluids were formed in situ at these depths or were transported from deeper or other sources.…”