Geochemical, Biological, and Clumped Isotopologue Evidence for Substantial Microbial Methane Production Under Carbon Limitation in Serpentinites of the Samail Ophiolite, Oman

Abstract

In hyperalkaline (normalpH>0.25em10) fluids that have participated in low‐temperature (<1500.17em°normalC) serpentinization reactions, the dominant form of C is often methane (CH4), but the origin of this CH4 is uncertain. To assess CH4 origin in serpentinite aquifers within the Samail Ophiolite, Oman, we determined fluid chemical compositions, analyzed taxonomic profiles of fluid‐hosted microbial communities, and measured isotopic compositions of hydrocarbon gases. We found that 16S rRNA gene sequences affiliated with methanogens were widespread in the aquifer. We measured clumped isotopologue (CH313D and CH212D2) relative abundances less than equilibrium, consistent with substantial microbial CH4 production. Furthermore, we observed an inverse relationship between dissolved inorganic C concentrations and δ13CnormalCH4 across fluids bearing microbiological evidence of methanogenic activity, suggesting that the apparent C isotope effect of microbial methanogenesis is modulated by C availability. An additional source of CH4 is evidenced by the presence of CH4‐bearing fluid inclusions in the Samail Ophiolite and our measurement of high δ13normalC values of ethane and propane, which are similar to those reported in studies of CH4‐rich inclusions in rocks from the oceanic lithosphere. In addition, we observed 16S rRNA gene sequences affiliated with aerobic methanotrophs and, in lower abundance, anaerobic methanotrophs, indicating that microbial consumption of CH4 in the ophiolite may further enrich CH4 in 13C. We conclude that substantial microbial CH4 is produced under varying degrees of C limitation and mixes with abiotic CH4 released from fluid inclusions. This study lends insight into the functioning of microbial ecosystems supported by water/rock reactions.

Carbon Geochemistry of the Active Serpentinization Site at the Wadi Tayin Massif: Insights From the ICDP Oman Drilling Project: Phase II

“…Consequently, microbial-mediated dolomite formation would be possible within serpentinized peridotites from Site BA. However, we suggest that a high Mg/ Ca ratio of a Type I groundwater (ranging between 7 and 66, Nothaft, Templeton, Rhim, et al, 2021) is probably the dominant controlling factor, and microbial activity only contributes to a minor amount. More analyses of the influence of microbial activity in forming Mg-rich carbonate in these rocks are necessary to evaluate the potential effect of microbial metabolism on dolomite precipitation in ultramafic rocks.…”

“…) in several wells in the Wadi Tayin Massif (−13.9‰ to −10.9‰, Nothaft, Templeton, Rhim, et al, 2021). Thus, we argue that 13 C-depleted carbon from organic matter respiration, mixed with atmospheric CO 2 (δ 13 C = ∼−7‰, Clark et al, 1992), and possibly bicarbonate from the dissolution of limestones represent the main carbon sources for the carbonate veins.…”

“…Erosion and replacement of the first couple of meters with alluvium may have removed the shallow more 18 O-enriched carbonates at Hole BA3A. Templeton, Rhim, et al (2021). Temperature estimates with δ 18 O Fluid = −3.0‰ result in lower values.…”

“…Two calcite veins at about ∼61 m depth have more 13 C-depleted values (δ 13 C = ∼−24‰, Figures 4c and 6) than most of the other veins. This may indicate a stronger influence of Type II fluids (Ca 2+ -OH − ) with a reported DIC value of −30‰ (NSHQ04, Nothaft, Templeton, Rhim, et al, 2021) or input of 13 C-depleted inorganic carbon from anaerobic oxidation of methane. Nothaft, Templeton, Rhim, et al (2021) reported 16S rRNA data that showed the presence of microbes capable of CH 4 oxidation, which produces highly 13 C-depleted inorganic carbon (Drake et al, 2015;Pedersen et al, 1997).…”

“…This may indicate a stronger influence of Type II fluids (Ca 2+ -OH − ) with a reported DIC value of −30‰ (NSHQ04, Nothaft, Templeton, Rhim, et al, 2021) or input of 13 C-depleted inorganic carbon from anaerobic oxidation of methane. Nothaft, Templeton, Rhim, et al (2021) reported 16S rRNA data that showed the presence of microbes capable of CH 4 oxidation, which produces highly 13 C-depleted inorganic carbon (Drake et al, 2015;Pedersen et al, 1997). Alternatively, the more 13 C-depleted carbonate veins could also be explained by a Rayleigh distillation of the DIC in the fluid caused by progressive carbonate precipitation.…”