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University of Colorado at Boulder (2021)

Subsurface Microbial Ecosystems and The Origin of Methane In Serpentinites of The Samail Ophiolite, Oman

Nothaft, Daniel Blake

Titre : Subsurface Microbial Ecosystems and The Origin of Methane In Serpentinites of The Samail Ophiolite, Oman

Auteur : Nothaft, Daniel Blake

Université de soutenance : University of Colorado at Boulder

Grade : Doctor of Philosophy (PhD) 2021

Résumé
Serpentinization of ultramafic rocks can produce hydrogen (H2), a strong electron donor that microbes can react with electron acceptors such as carbon dioxide (CO2) to produce methane (CH4) and yield energy. This type of chemosynthetic process is relevant to life on Earth and potentially on other silicate bodies of the Solar System. However, serpentinization results in high-pH (> 11), low-CO2 conditions that can be challenging to life. This dissertation assesses relationships between hydrogeochemical parameters and subsurface microbial communities at a site of low-temperature serpentinization in the Samail Ophiolite, Oman. We pumped groundwaters from deep wells, determined fluid chemical compositions, analyzed taxonomic profiles of microbial communities, and measured isotopic compositions of hydrocarbon gases. For some work, we used a packer system to pump discrete intervals as deep as 108 m to 132 m from two 400 m-deep wells, isolating multiple aquifers ranging in pH from 8 to 11. 16S rRNA gene sequencing of deep groundwaters revealed the presence of an ecosystem dominated by microbial sulfate reduction coupled to oxidation of H2 and small organic acids. In shallower, oxidized groundwaters, heterotrophic aerobes or denitrifiers were more prevalent. However, the majority of this dissertation focused on the origin of CH4 in hyperalkaline fluids. Although it has been argued that CH4 in the ophiolite is abiotic due to its 13C-enriched composition (δ13C commonly −10‰ to +5‰ VPDB), we found that 16S rRNA gene sequences affiliated with methanogens were widespread and in high relative abundance in some samples. Further, we measured clumped isotopologue (13CH3D and 12C H2D2) relative abundances less than equilibrium, consistent with microbial CH4 production. Relationships between CO2 concentrations and δ13CCH4 suggest that the C isotope effect of microbial CH4 production is modulated by C availability, and strongly suppressed at pH > 11. CH4 samples from two wells had 14C contents significantly above analytical blanks, and one well had CH4 of up to 0.304 fraction modern. This is the first proof of active (< 10 ka) conversion of atmospheric CO2 to CH4 in a serpentinizing setting. Although we do infer a second abiotic source of CH4 from high δ13C values of co-occurring ethane and propane, we attribute substantial CH4 production to microbial activity despite high pH and C limitation.

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Page publiée le 23 janvier 2023