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 peridotites 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 8.5-11.2) gradients within the 300-400 m deep boreholes. The serpentinites and fluids host an active subsurface ecosystem. Microbial cell abundances in serpentinite vary at least six orders of magnitude, from ≤3.5 × 10 1 to 2.9 × 10 7 cells/g. Low levels of biological sulfate reduction (2-1,000 fmol/ cm 3 /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 lowtemperature water/rock interaction, and how biosignatures of in situ microbial activity are generated.Plain Language Summary Ultramafic rocks undergoing water/rock interaction, and storing fluids that are far from chemical equilibrium, may be one of the most common habitats in our solar system. Through the Oman Drilling Project we collected >1 km of intact serpentinite in contact with groundwaters. These cores capture parts of the rock-hosted biosphere and show how cells are distributed within serpentinites that vary in their mineralogical, physical and chemical properties. The cores are also biologically active, enabling us to detect specific metabolisms, such as when microorganisms combine hydrogen as reductant and sulfate as an oxidant to fuel their metabolism. Although the distribution of microbial cells in the rock cores is very heterogeneous, there are many intervals where the abundance of cells constitutes robust biomass. In the deeper cores, slow, albeit detectable, microbial sulfate reduction proceeds. We suggest that this pervasive biological activity releases byproducts such as sulfide that can react with the serpentinite and change the optical and chemical properties of the rocks. The feedbacks between the rock alteration and microbial activity produce markers that enable us to focus our sea...

Carbon Geochemistry of the Active Serpentinization Site at the Wadi Tayin Massif: Insights From the ICDP Oman Drilling Project: Phase II

“…Finally, the MBio subsamples were transferred into an N 2 glovebox, washed two times with N 2 -purged deionized water, wrapped in aluminum foil, sealed into mylar bags flushed with N 2 together with an O 2 -scrubbing sachet (BD GasPak 260001), and stored at 4°C. A description of sample handling procedures during the OmanDP can be found in the OmanDP Methods and explanatory notes in Kelemen et al (2020) and Templeton et al (2021). In our laboratory, the MBio samples (rock chips or core pieces) were cleaned first by ultrasonication in 100 ml dichloromethane (DCM) for 5 min at room temperature and subsequently dried for 24 h at room temperature in a pre-combusted glass container covered with combusted aluminum foil.…”

“…Recent microbiological and metagenomic studies showed that microbial life can be sustained within the lithosphere and that microbial utilization of H 2 , CH 4 , and formate may be common in different serpentinization systems (Brazelton et al, 2012;Curtis et al, 2013;Daae et al, 2013;Etiope & Sherwood Lollar, 2013;Lang et al, 2018;Ohara et al, 2012;Schrenk et al, 2013). Furthermore, recent studies on drillcores from Site BA1B and BA3A showed cell counts ranging from as high as 10 7 cells g −1 in heavily veined areas down to 10 1 cells g −1 in the interior of some peridotites (Templeton et al, 2021). These cell counts are comparable and, in part, higher than previously reported from other areas such as the Atlantis Massif (10 1 -10 3 cells g −1 , Früh-Green et al, 2018;Orcutt et al, 2017), the Atlantis Bank at the Southwest Indian Ridge (10 2 -10 3 , Li et al, 2020), and the deep continental basalts in the Decca traps (10 5 cells g −1 ; Dutta et al, 2018).…”

Tracing Carbonate Formation, Serpentinization, and Biological Materials With Micro‐/Meso‐Scale Infrared Imaging Spectroscopy in a Mars Analog System, Samail Ophiolite, Oman

“…These travertine springs are often hyperalkaline, with pH of 10-12 (e.g., Canovas et al, 2017;Chavagnac et al, 2013;Stanger, 1984, 1985;Paukert et al, 2012). The subsurface and near-surface aquifers within these diverse lithologies are habitats that host an array of chemosynthetic life (Rempfert et al, 2017;Templeton et al, 2021). Here, we investigate a range of sites within the ophiolite: two serpentinite sites with extensive veining (Al Qaryatayn and Wadi Dima); a listvenite site (complete carbonation of the ultramafic protolith); and a travertine/serpentine site where groundwater springs come to the surface (Figure 1), subset from a wider set of acquisitions (e.g., Pinet et al, 2017).…”

“…A handful of sites on Mars with both Mg carbonate and Mg serpentine have been identified from orbital data, including those in the vicinity of the Mars-2020 landing site at Jezero crater with its potential extended mission toward Nili Planum and Northeast Syrtis (Brown et al, 2020;Ehlmann & Mustard, 2012;Ehlmann et al, 2008Ehlmann et al, , 2010Michalski & Niles, 2010). These carbonate-serpentine sites are of particular interest because such carbonate/serpentine systems on Earth host chemosynthetic life and produce abiotic hydrogen and methane gases (e.g., Newman et al, 2020;Rempfert et al, 2017;Schulte et al, 2006;Templeton et al, 2021). Furthermore, serpentinization of ultramafic rocks can sequester carbon dioxide in carbonate minerals, removing it from the atmospheric and hydrologic reservoirs (e.g., Edwards & Ehlmann, 2015;Kelemen & Matter, 2008).…”

Linking Serpentinization and Weathering of Peridotite: A Study on the Mineralogical and Geochemical Evolution of the Sta. Cruz Nickel Laterite Deposit, Zambales, Philippines

“…Because of this, and the abundance of ultramafic rocks in the Archean Earth [20], serpentinization systems have been important study sites for the biogeochemical origin of life on Earth and even other planets [21,22]. Until recently [e.g., 23,24], most studies about serpentinization focus on the relatively higher temperature (>100°C) processes, while low temperature serpentinization, in particular its relationship and continuity to the chemical weathering of ultramafic rocks exposed on land, have not received as much attention. Complete laterite profiles preserve spatiotemporal information on the mineralogy and geochemistry of these rocks as they undergo serpentinization and weathering of ultramafic rocks and are thus key sites for understanding the link between these two processes.…”