effect of methanogenesis on the geochemistry of low temperature water–Fe⁰–basalt reactions

• Published in: Applied geochemistry Vol. 26S; p. S318
• Main Authors: Mayhew, Lisa E, Lau, Graham E, McCollom, Tom M, Webb, Sam, Templeton, Alexis S
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-06-2011
• Subjects: Archaea; chemical composition; energy; geochemistry; headspace analysis; hydrogen; hydrogen production; hydrolysis; iron; laboratory experimentation; methane; methane production; methanogens; rheology; rocks; silicon; temperature; time series analysis
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2011.03.074

Hydrogen gas produced in the subsurface from the hydration of mafic rocks is known to be a major energy source for chemolithotrophic life in extreme environments such as hydrothermal vents. The possibility that in situ anaerobic microorganisms present in the deep subsurface are sustained by low temperature H₂-generating water–rock reactions taking place around them is being investigated. Whether the growth and activity of H₂-utilizing microbes directly influences aqueous geochemistry, rates of mineral dissolution, and the chemical composition of the alteration products is also being quantitatively evaluated. To explore how microorganisms are affected by water–rock reactions, and how their activity may in turn affect reaction progress, laboratory experiments have been conducted to monitor the growth of a methanogenic Archaea in the presence of H₂₍g₎ produced from low temperature water–Fe⁰–basalt reactions. In these systems, the conversion of Fe(II) to Fe(III) and subsequent hydrolysis of water is responsible for the production of H₂₍g₎. To characterize key components of the geochemical system, time series measurements of H₂ and CH₄ gas concentrations, Fe and Si aqueous concentrations, and spatially resolved synchrotron-based analyses of microscale Fe distribution and speciation were conducted. Culture experiments were compared with an abiotic control to document changes in the geochemistry both in the presence and absence of the methanogen. In the control abiotic batch experiment, H₂ was continuously produced, until the headspace became saturated, while in the biotic experiments, microbial consumption of H₂ for methanogenesis draws H₂ down and produces CH₄. Purging the headspace gas reinitiates H₂ and CH₄ production in abiotic and culture experiments, respectively. Mass balance analysis of the amount of CH₄ produced suggests that the total H₂ production in microbial experiments does not exceed the abiotic experiment. Soluble Si concentrations, while buffered to relatively constant values, were higher in culture experiments than the abiotic control. Iron₍ₐq₎ concentrations appear to respond to perturbations of H₂ and CH₄ gas concentrations in both culture experiments and the abiotic control. A pulse of Fe preceded the rise in either H₂ or CH₄ production, and as the gas concentrations increased the Fe₍ₐq₎ decreased. Iron-bearing mineral assemblages change with increasing reaction time and mineral assemblages vary between culture experiments and the abiotic control. These geochemical trends suggest that there are different reaction paths between the culture experiments and the abiotic control. The hydration of mafic rocks is a common geologic reaction and one that has taken place on Earth for the majority of its history and is postulated to occur on Mars. These reactions are important because of their effect on the rheology and geochemistry of the ocean crust. While most often studied at temperatures of ∼250°C, this work suggests that at lower temperatures microorganisms may have a profound effect on what has long been thought to be solely an abiotic reaction, and may produce diagnostic mineral assemblages that will be preserved in the geological record.