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

Abstract

The potential for molecular hydrogen ( 2 H ) 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. Depthindiscriminate 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

Advances in Defining Ecosystem Functions of the Terrestrial Subsurface Biosphere

“…Recent work utilizes next generation DNA sequencing for taxonomic and functional classification of borehole fluids in combination with other techniques such as microcosm or enrichment based experiments ( Crespo-Medina et al, 2014 ; Meyer-Dombard et al, 2018 ; Fones et al, 2021 ; Glombitza et al, 2021 ), stable isotope analyses ( Nothaft et al, 2021a , b ), radioisotope labeling ( Fones et al, 2021 ; Glombitza et al, 2021 ; Templeton et al, 2021 ), and ecological/hydrological modeling ( Putman et al, 2021 ). Transcriptomic and single cell genomics studies have also shown mechanisms of adaptation of microbial life to serpentinizing environments ( Fones et al, 2019 , 2021 ; Merino et al, 2020 ; Kraus et al, 2021 ).…”

“…Both host rock composition and hydrologic context influence microbial community composition in subsurface serpentinizing systems, and recent work at the MBO in Oman highlights this concept ( Miller et al, 2016 ; Rempfert et al, 2017 ; Fones et al, 2019 ; Kraus et al, 2021 ; Nothaft et al, 2021a ). The Samail Ophiolite is composed of mafic gabbros and ultramafic peridotites, mainly harzburgite, both presumed to be undergoing active serpentinization, which fuel distinct microbial communities ( Neal and Stanger, 1983 ; Miller et al, 2016 ; Rempfert et al, 2017 ; Fones et al, 2019 ; Nothaft et al, 2021a ).…”

“…Both host rock composition and hydrologic context influence microbial community composition in subsurface serpentinizing systems, and recent work at the MBO in Oman highlights this concept ( Miller et al, 2016 ; Rempfert et al, 2017 ; Fones et al, 2019 ; Kraus et al, 2021 ; Nothaft et al, 2021a ). The Samail Ophiolite is composed of mafic gabbros and ultramafic peridotites, mainly harzburgite, both presumed to be undergoing active serpentinization, which fuel distinct microbial communities ( Neal and Stanger, 1983 ; Miller et al, 2016 ; Rempfert et al, 2017 ; Fones et al, 2019 ; Nothaft et al, 2021a ). Rempfert et al (2017) distinguished between four different fluid types at the Samail ophiolite MBO wells, influenced by host rock lithology and hydrologic flow regime (long v. short residence time): hyperalkaline peridotite (pH > 10), alkaline peridotite (pH 8–10), gabbro, and peridotite/gabbro contact wells.…”

“…Metagenomes from hyperalkaline peridotite wells were more enriched in anaerobic respiration genes, including anaerobic sulfite reductase unit A, AsrA , while genes associated with aerobes were more enriched in the alkaline peridotite wells (< 10 pH) ( Fones et al, 2019 ). The influence of hydrologic regime on geochemical composition, and thus microbial communities, was shown at the Samail Ophiolite through the use of packers to isolate discrete borehole sections in the mantle section of the ophiolite ( Nothaft et al, 2021a ). Nothaft et al (2021a) sampled two boreholes at multiple discrete depths, up to 132 m, and subjected fluids to geochemical, isotopic, and 16s RNA sequencing.…”

“…The influence of hydrologic regime on geochemical composition, and thus microbial communities, was shown at the Samail Ophiolite through the use of packers to isolate discrete borehole sections in the mantle section of the ophiolite ( Nothaft et al, 2021a ). Nothaft et al (2021a) sampled two boreholes at multiple discrete depths, up to 132 m, and subjected fluids to geochemical, isotopic, and 16s RNA sequencing. Fluids isolated from shallower, more oxidized parts of the groundwater contained more heterotrophic organisms capable of aerobic respiration, denitrification, and fermentation, while highly reacted Ca 2+ – OH – groundwaters were dominated by sulfate reducing chemolithoheterotrophs ( Nothaft et al, 2021a ).…”