Identifying subsurface biologically-mediated processes occurring during modern water/rock interaction in the Samail ophiolite

We have initiated multidisciplinary efforts to assess the distribution, composition and function of the biosphere hosted in subsurface peridotite rocks in Oman. We are exploring the coupling between hydrological, biological, and geochemical processes during the low-temperature hydration of peridotite. Drilling, recovery, and careful preservation of intact peridotite cores for biological analysis was successfully completed in early 2018 during the Oman Drilling Project Phase II. Detailed downhole mineralogical analysis, coupled with aqueous geochemistry, isotope geochemistry, DNA sequencing and metabolic rate assay measurements on fluids pumped from both new and pre-existing boreholes reveal several distinct regimes of biogeochemical activity. The combined suite of water, gas and rock samples intersect large subsurface transitions in water Eh/pH associated with strong contrasts in the availability of reductants (i.e. H₂, CH₄, NH₄⁺, CO) and oxidants (i.e. NO₃^-, SO₄^2-, Fe(III)-phases and CO₂). At borehole site BA1A, where a packer system was deployed for the first time in 2018, we can also spatially resolve distinct differences in the geochemical and biological states of moderately alkaline near-surface fluids vs. deep hyperalkaline and highly reduced fluids.

To date, the majority of our biological data has been derived from the water system, and we are only beginning to extract DNA and lipids, and measure potential rates of C and S transformations, using core samples. Here, we present both 16S rDNA and metagenomic data showing how the microbial diversity and abundance of key functional genes varies across distinct fluid regimes (Type I vs. Type II fluids vs. mixing zones). By coupling this data with targeted metabolic rate assays, as well N, C and S isotope systematics, we also show evidence for biological cycling of nitrate/ammonium, DIC/methane, and sulfate/FeS. Now that we have identified where biological CO₂, NO₃^-and sulfate reduction are occurring, we are using this information to target promising peridotite core samples for microbial diversity, activity and biomarker analysis. We are also querying which mineralogical phases might be a direct product of microbial activity, such as iron sulfide phases that are pervasively distributed in other boreholes of the ODP including BA1A, BA1B, and BA4A.