loading page

Geochemical, biological and clumped isotopologue evidence for substantial microbial methane production under carbon limitation in serpentinites of the Samail Ophiolite, Oman
  • +10
  • Daniel Nothaft,
  • Alexis Templeton,
  • Jeemin H Rhim,
  • David T Wang,
  • Jabrane Labidi,
  • Hannah M Miller,
  • Eric Boyd,
  • Juerg Matter,
  • Shuhei Ono,
  • Edward D Young,
  • Sebastian H Kopf,
  • Peter B Kelemen,
  • Mark E Conrad
Daniel Nothaft
University of Colorado - Boulder

Corresponding Author:[email protected]

Author Profile
Alexis Templeton
University of Colorado Boulder
Author Profile
Jeemin H Rhim
Massachusetts Institute of Technology
Author Profile
David T Wang
Massachusetts Institute of Technology
Author Profile
Jabrane Labidi
University of Tuebingen
Author Profile
Hannah M Miller
University of Colorado - Boulder
Author Profile
Eric Boyd
Montana State University
Author Profile
Juerg Matter
University of Southampton
Author Profile
Shuhei Ono
Massachusetts Institute of Technology
Author Profile
Edward D Young
University of California Los Angeles
Author Profile
Sebastian H Kopf
University of Colorado - Boulder
Author Profile
Peter B Kelemen
Columbia University
Author Profile
Mark E Conrad
Lawrence Berkeley Laboratory
Author Profile

Abstract

In high-pH ($\text{pH}>10$) fluids that have participated in low-temperature ($<150\,^{\circ}\text{C}$) serpentinization, the dominant form of C is often methane (CH$_{4}$), but the origin of this CH$_{4}$ is uncertain. To assess CH$_{4}$ origin during low-temperature serpentinization, we pumped fluids from 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 ($^{13}$CH$_{3}$D and $^{12}$CH$_{2}$D$_{2}$) relative abundances less than equilibrium, consistent with substantial microbial CH$_{4}$ production. Further, we observed an inverse relationship between dissolved inorganic C concentrations and $\delta^{13}\text{C}_{\text{CH}_{4}}$ across fluids bearing microbiological evidence of methanogenic activity, suggesting that the apparent C isotope effect of microbial methanogenesis is modulated by C availability. A second source of CH$_{4}$ is evidenced by the presence of CH$_{4}$-bearing fluid inclusions in the Samail Ophiolite and our measurement of high $\delta^{13}\text{C}$ values of ethane and propane, which are similar to those reported in studies of CH$_{4}$-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 CH$_{4}$ in the ophiolite may further enrich CH$_{4}$ in $^{13}$C. We conclude that substantial microbial CH$_{4}$ is produced under varying degrees of C limitation and mixes with abiotic CH$_{4}$ released from fluid inclusions. This study lends insight into the functioning of microbial ecosystems supported by water/rock reactions.
Oct 2021Published in Journal of Geophysical Research: Biogeosciences volume 126 issue 10. 10.1029/2020JG006025