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Miller, Hannah MÌý¹Ìý;ÌýMayhew, LisaÌý²Ìý;ÌýKelemen, PeterÌý³Ìý;ÌýTempleton, AlexisÌý⁴

¹ÌýUniversity of Colorado at Â鶹ӰԺ
²ÌýUniversity of Colorado at Â鶹ӰԺ
³ÌýColumbia University
⁴ÌýUniversity of Colorado at Â鶹ӰԺ

Water stored in peridotite aquifers in the shallow subsurface drives hydration and oxidation (serpentinization) reactions, generating hydrogen gas and releasing Mg2+, Fe2+, Ca2+ and OH- into solution. In the presence of fluids enriched in dissolved CO2, extensive precipitation of carbonate minerals occurs. This reaction has large-scale implications for mitigating climate change by providing a stable, geological carbon repository (Kelemen et al., 2011). The Samail Ophiolite in Oman contains large quantities of ultramafic rocks that are currently undergoing serpentinization at low temperatures (30°C) and forming carbonate minerals (Streit et al., 2012) from hyperalkaline fluids. The production of H2 gas may also provide an electron donor for subsurface chemolithoautotrophic life which can contribute to carbon cycling in the subsurface as microorganisms utilize CO2 as an inorganic carbon source (Nealson et al., 2005). Serpentinization reactions can form H2 gas by the reduction of water, coupled with the oxidation of Fe (II) to Fe (III), but these reactions have not been extensively studied at shallow subsurface temperatures (<120°C) most relevant for microbial life. We believe there is great potential for life in Oman's subsurface fluids subsisting off products of water/rock interactions (H2, CH4, formate), but this hypothesis and constraints on habitability are currently untested. We are studying the extent and mechanisms behind H2 generation in the Oman subsurface by conducting low temperature (40 and 100°C) water-rock reactions with Oman peridotite, measuring H2 production and characterizing the speciation of Fe-bearing minerals before and after water-rock interaction. We are also culturing water from Oman's subsurface to enrich for fermenters, H2-oxidizers/Fe(III)reducers, and methanogens. The experimental water-rock reactions produce significant H2 for two and a half months of reaction, peaking at 400 nmol/gram of reacted peridotite and then steadily decrease with time (Figure 1). This production is followed by a stage of H2 consumption, coupled with a decrease in headspace CO2. This points to the possible formation of low weight molecular organic acids, such as formate or acetate, as H2 reduces of CO2; thus, we are probing for the formation of formate and acetate in our water-rock reactions, which may provide a further source of energy for microbial life. The serpentinization reactions in the Oman peridotite are producing significant levels of H2 at a low temperature within the habitability limit for life, suggesting Oman provides a subsurface niche for H2-utilizing microorganisms. Determining the abundance and diversity of microorganisms is important because they have the potential to control peridotite weathering rates, methane production, and carbonate formation. It also would point to the possibility of deep subsurface life subsisting off water-rock reactions on other planets and suggest possibilities for the origins of life on Earth.

Kelemen, P.B., Matter, J., Streit, E.E., Rudge, J.F., Curry, W.B., and Blusztajn, J. (2011). Rates and Mechanisms of Mineral Carbonation in Peridotite: Natural Processes and Recipes for Enhanced, in situ CO2 Capture and Storage. Annu. Rev. Earth Planet. Sci. 39, 545–576.

Nealson, K.H., Inagaki, F., and Takai, K. (2005). Hydrogen-driven subsurface lithoautotrophic microbial ecosystems (SLiMEs): do they exist and why should we care? Trends Microbiol. 13, 405–410.

Streit, E., Kelemen, P., and Eiler, J. (2012). Coexisting serpentine and quartz from carbonate-bearing serpentinized peridotite in the Samail Ophiolite, Oman. Contrib. Mineral. Petrol. 164, 821–837.