Geological Production And Microbial Consumption of Hydrogen During Low-Temperature Water-Rock Reactions

摘要

Geologically derived hydrogen gas supports hydrothermal vent ecosystems on Earth today and may have been an abundant energy source available during the origin and evolution of early life on Earth. The existence of deep subsurface microbial ecosystems is dependent on the ability of the water-rock environment to provide a suitable habitat for life, which has not been well quantified. This dissertation provides insights into the potential for low temperature water-rock reaction systems to support H2-utilizing microbial life and, in turn, for microbial activity to directly affect the water-rock reaction pathways and processes.The partitioning of Fe into secondary minerals is an important control on the generation of H2. However, characterization of these phases is hampered by the small amount of reaction products generated in laboratory scale experiments. A synchrotron x-ray based method integrating micro-spectroscopy and micro-XRF data collection and processing was developed to characterize the speciation of Fe in the rare, microscale solid phase reaction products. In H2- generating experiments conducted at 100°C, Fe(III)-oxides were detected on the surface of spinel particles, while Fe(II)-brucites and talcs were associated with dissolving olivine and pyroxene surfaces. The spinels may be required to mediate electron transfer between Fe(II) and water. Thus, H2-generation is likely a surface controlled process catalyzed by spinels and accommodated by the formation of Fe(III)-oxides. In such a system, microbial colonization of reactive surfaces may be strongly controlled by the heterogeneous distribution of H2 production.To determine if an Archaeal methanogen present in-situ would affect the solid and aqueous geochemistry of water-rock reactions in distinct ways, a water-basalt system amended with Fe0 was inoculated with a methanogen. The reaction products in the abiotic experiment were dominated by Fe-phyllosilicates in contrast to the culture experiment in which an Febearing pyroxene was detected and Fe-phyllosilicates were absent. The unique secondary mineral assemblage in the presence of an active methanogen suggests that H2-utilizing microorganisms do influence the reaction pathways. Therefore, the work presented in this dissertation has helped to advance our understanding of low-temperature water-rock reactions and the potential for microbial activity to survive in and affect these systems.