The biogeochemistry and biomarkers at contrasting sites of terrestrial serpentinization

Cook, Melissa (2022) The biogeochemistry and biomarkers at contrasting sites of terrestrial serpentinization. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The range of aqueous and gaseous geochemistry and how it impacts potential metabolisms and microbial community composition at sites of serpentinization remains poorly understood. Therefore, the goal of this thesis was to characterize the aqueous and gaseous geochemistry and microbial communities at three distinct sites of serpentinization: The Cedars (CA, USA), Aqua de Ney (CA, USA), and the Tablelands (NL, CAN). The results demonstrate that there are many commonalities between these sites including ultra-basic, reducing fluids with high concentrations of methane. Isotopic data (i.e., δ¹³C and δ²H), however, suggests that the methane from The Cedars is at least partially microbial, the Tablelands is non-microbial, and Aqua de Ney is abiogenic. The geochemical results were used to create thermodynamic predictions (Aᵣ) of reactions that could support microbial metabolisms. The Aᵣ results demonstrate that although the values for each reaction are similar among sites, there is variation in available energy between reactions. Potential microbial metabolisms were tested in microcosm experiments and while there was little evidence of the expected metabolisms (i.e., carbon monoxide oxidation, hydrogen oxidation), microbial methanogenesis was observed in experiments from The Cedars. Fluid and carbonate samples from The Cedars were analyzed for lipid biomarkers to determine the microbial community composition and identify lipids indicative of microbial methanogenesis. Although no archaeal lipids, including those indicative of methanogens, were detected in samples, chlorophyl a and pheophytin a were detected in carbonate samples but not fluids. These pigments could indicate the presence of cyanobacteria in carbonates at The Cedars. Finally, Winterhouse Canyon Microbial Observatory (WHCMO) was established to understand the microbial community composition free from surface contributions. Results from the phospholipid fatty acid analysis suggested that the microbial community at WHCMO was largely eukaryotic and likely not representative of ultra-basic subsurface fluids. Winterhouse Canyon 2, a site where subsurface and surface fluids mix, was also sampled and the microbial community composition was determined to be largely non-eukaryotic and likely dominated by ultra-basic subsurface fluids. Overall, this thesis advanced the knowledge of serpentinizing systems by quantifying the range of geochemistry, the potential energy for metabolisms, and the microbial community composition and metabolisms.

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