Accessing the subsurface biosphere within rocks undergoing active
low-temperature serpentinization in the Samail ophiolite (Oman Drilling
Project)
Abstract
The Oman Drilling Project established an “Active Alteration”
multi-borehole observatory in dunite and harzburgite undergoing
low-temperature serpentinization in the Samail ophiolite. The highly
serpentinized rocks are in contact with strongly reducing fluids.
Distinct hydrological regimes, governed by differences in rock porosity
and fracture density, give rise to steep redox (Eh +200 to -750 mV) and
pH (pH range 8.5 to 11.2) gradients within the 300 to 400 meter deep
boreholes. The serpentinites and fluids host an active subsurface
ecosystem. Microbial cell abundances vary at least 6 orders of
magnitude, from ≤3.5*101 cells/g to
2.9*107 cells/gram. Low levels of biological sulfate
reduction (2-1000 fmol/cm3/day) can be detected in
rock cores, particularly in rocks in contact with reduced groundwaters
with pH <10.5. Thermodesulfovibrio is the predominant
sulfate reducer identified via metagenomic sequencing of adjacent
groundwater communities. We infer that transport and reaction of
microbially generated sulfide with the serpentine and brucite
assemblages gives rise to optical darkening and sulfide overprinting,
including the formation of tochilinite-vallerite group minerals,
potentially serving as an indicator that this system is inhabited by
microbial life. Olivine mesh-cores replaced with ferroan brucite and
minor awaruite, abundant veins containing hydroandradite garnet and
polyhedral serpentine, and late-stage carbonate veins are suggested as
targets for future spatially-resolved life-detection investigations. The
high-quality whole-round core samples that have been preserved can be
further probed to define how life distributes itself and functions
within a system where chemical disequilibria are sustained by
low-temperature water/rock interaction, and how biosignatures of
in-situ microbial activity are generated.