Formation of H2, CH4 and N-species during low-temperature experimental alteration of ultramafic rocks

Abstract

Hydrogen, methane and ammonium are important chemical species for chemolithotrophic microorganisms that sustain subsurface endolithic communities. It is well known that production of H2 through serpentinization of the primary Fe(II)-containing minerals, olivine and pyroxene, in ultramafic rocks and reduction of CO2 to CH4 are processes that occur at higher temperatures. Knowledge is, however, limited about these reactions under low-temperature conditions, and how they are affected by Fe(II)-containing secondary minerals, such as brucite and serpentine, which are commonly found in these environments. In this experimental study, we explored the formation of H2, CH4 and NH4 during low-temperature (25°C) reactions between deionised water and 1) unaltered, 2) medium altered and 3) highly altered dunites, over a period of 99days.Hydrogen was detected in all three experiments and the concentration increased over time. The O2 concentration decreased over time, but O2 was still present at significant levels when H2 started to form. This suggests that the H2 was formed locally at the surface of mineral grains rather than in the hypoxic solution. The highly altered dunite gave the highest and the unaltered dunite the lowest H2 concentrations, and while the O2 concentration decreased fast with the medium altered dunite, significant amounts of O2 still remained with the unaltered dunite at the end of the experiments. Both the medium and highly altered dunite gave O2 concentrations close to or below the detection limit during the last part of the experiment. Factors that likely affected the H2 production are Fe-content, mineralogy, dissolution rates and surface areas of the minerals as well as the presence of catalytic minerals such as chromite and magnetite.Methane was also detected and increased with time in all experiments. The results indicate that CH4 formation also occurred locally on the surface of minerals. Detection of both NH4 and NO3 in the aqueous solutions indicates that previously absorbed N-species were leached out during the experiment. Increasing NH4 and decreasing NO3 concentrations over time suggest NO3 reduction, likely catalysed by mineral surfaces.The results imply that low-temperature water–rock reactions in moderately to highly altered ultramafic rocks provide reduced chemical species that can be utilized as electron donors in energy-yielding metabolic processes by subsurface microorganisms. Such reactions could play an important role in sustaining subsurface microbial communities, particularly in ophiolites and near seafloor parts of the ultramafic oceanic lithosphere at slow-spreading mid-ocean ridges and possibly also in other rocks and sediments. The results suggest that brucite and possibly also serpentine may be more important sources of Fe(II) and thus H2 formation than olivine in low-temperature ultramafic systems. Formation of H2 in reducing microenvironments on mineral surfaces surrounded by oxic conditions increases the extent of H2 formation and the subsurface habitat for aerobic H2-oxidizing microorganisms.