Isotopic variation of molecular hydrogen in 20°–375°C hydrothermal fluids as detected by a new analytical method

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[1] Molecular hydrogen (H2) is one of the most important energy sources for subseafloor chemolithoautotrophic microbial ecosystems in the deep-sea hydrothermal environments. This study investigated stable isotope ratios of H2 in 20°–375°C hydrothermal fluids to evaluate usefulness of the isotope ratio as a tracer to explore the H2-metabolisms. Prior to the observation, we developed an improved analytical method for the determination of concentration and stable isotope ratio of H2. This method achieved a relatively high sensitivity with a detection limit of 1 nmol H2 within an analytical error of 10‰ in the δDH2 value. The δDH2 values in the high-temperature fluids were between −405‰ and −330‰, indicating the achievement of the hydrogen isotopic equilibrium between H2 and H2O at around the hydrothermal end-member temperature. In contrast, several low-temperature fluids showed apparently smaller δDH2 values than those in the high-temperature fluids in spite of a negligible δDH2 change due to fluid-seawater mixing, suggesting the possibility of δDH2 change in the low-temperature fluids and the surrounding environments. Since the δDH2 change in low-temperature environments is not well explained by the very sluggish abiotic thermal isotopic equilibrium between H2 and H2O, it could be associated with (micro)biological H2-consuming and/or H2-generating metabolisms that would strongly promote the isotopic equilibrium at low temperatures. Our first detection of the δDH2 variation in deep-sea hydrothermal systems presents the availability of the δDH2 value as a new tracer for microbes whose enzymes catalyze D/H exchange in H2.


  • Journal of Geophysical Research: Biogeosciences

    Journal of Geophysical Research: Biogeosciences 115(G3), 2010-08-28

    American Geophysical Union


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