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Aqueous Geochemical and Microbial Variation Across Discrete Depth Intervals in a Peridotite Aquifer Assessed Using a Packer System in the Samail Ophiolite, Oman
Author(s) -
Nothaft Daniel B.,
Templeton Alexis S.,
Boyd Eric S.,
Matter Juerg M.,
Stute Martin,
Paukert Vankeuren Amelia N.
Publication year - 2021
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
eISSN - 2169-8961
pISSN - 2169-8953
DOI - 10.1029/2021jg006319
Subject(s) - geology , aquifer , biogeochemical cycle , groundwater , geochemistry , peridotite , sulfate , biogeochemistry , environmental chemistry , oceanography , chemistry , basalt , geotechnical engineering , organic chemistry
The potential for molecular hydrogen ( H 2 ) 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 400 m ‐deep, peridotite‐hosted wells in the Samail Ophiolite, Oman. Isolation and pumping of discrete intervals as deep as 108 m to 132 m 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 Ca 2 + − OH − groundwaters bearing up to 4.05 μ mol ⋅ L − 1H 2 , 3.81 μ mol ⋅ L − 1methane ( C H 4 ) and 946 μ mol ⋅ L − 1sulfate ( S O 4 2 − ) revealed an ecosystem dominated by Bacteria affiliated with the class Thermodesulfovibrionia, a group of chemolithoheterotrophs supported by H 2 oxidation coupled toS O 4 2 −reduction. In shallower, oxidized Mg 2 + − HCO 3 − groundwaters, aerobic and denitrifying heterotrophs were relatively more abundant. High δ 13 C and δ D ofC H 4 (up to 23.9 ‰ VPDB and 45 ‰ VSMOW , respectively) indicated microbialC H 4 oxidation, particularly in Ca 2 + − OH − waters with evidence of mixing with Mg 2 + − HCO 3 − 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.