Aqueous Geochemical and Microbial Variation Across Discrete Depth Intervals in a Peridotite Aquifer Assessed Using a Packer System in the Samail Ophiolite, Oman

The potential for molecular hydrogen (H2) generated via serpentinization to fuel subsurface microbial ecosystems independent from photosynthesis has prompted biogeochemical investigations of serpentinization-influenced fluids. However, investigations typically sample via surface seeps or open-borehole pumping, which can mix chemically distinct waters from different depths. Depth-indiscriminate sampling methods could thus hinder understanding of the spatial controls on nutrient availability for microbial life. To resolve distinct groundwaters in a low-temperature serpentinizing environment, we deployed packers (tools that seal against borehole walls during pumping) in two 400m-deep, peridotite-hosted wells in the Samail Ophiolite, Oman. Isolation and pumping of discrete intervals as deep as 108m to 132m below ground level revealed multiple aquifers that ranged in pH from 8 to 11. Chemical analyses and 16S rRNA gene sequencing of deep, highly reacted Ca2+-OH- groundwaters bearing up to 4.05 mu mol.L-1H2, 3.81 mu mol.L-1 methane (CH4) and 946 mu mol.L-1 sulfate (SO42-) revealed an ecosystem dominated by Bacteria affiliated with the class Thermodesulfovibrionia, a group of chemolithoheterotrophs supported by H2 oxidation coupled to SO42- reduction. In shallower, oxidized Mg2+-HCO3- groundwaters, aerobic and denitrifying heterotrophs were relatively more abundant. High delta 13C and delta D of CH4 (up to 23.9 parts per thousand VPDB and 45 parts per thousand VSMOW, respectively) indicated microbial CH4 oxidation, particularly in Ca2+-OH- waters with evidence of mixing with Mg2+-HCO3- waters. This study demonstrates the power of spatially resolving groundwaters to probe their distinct geochemical conditions and chemosynthetic communities. Such information will help improve predictions of where microbial activity in fractured rock ecosystems might occur, including beyond Earth.

Automated summary

The study utilized packers to isolate and pump groundwater from peridotite-hosted wells in Oman, revealing multiple aquifers with varying pH levels and chemical compositions. Deep waters with high Ca2+-OH- content showed a dominance of bacteria affiliated with the class Thermodesulfovibrionia, supported by H2 oxidation coupled to SO42- reduction. Shallow waters with Mg2+-HCO3- content were characterized by aerobic and denitrifying heterotrophs. The study highlights the importance of spatially resolving groundwaters to understand their geochemical conditions and the presence of chemosynthetic communities.