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

Molecular hydrogen (H-2) 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 H-2 in 20 degrees-375 degrees C hydrothermal fluids to evaluate usefulness of the isotope ratio as a tracer to explore the H-2-metabolisms. Prior to the observation, we developed an improved analytical method for the determination of concentration and stable isotope ratio of H-2. This method achieved a relatively high sensitivity with a detection limit of 1 nmol H-2 within an analytical error of 10 parts per thousand in the delta D-H2 value. The dDH(2) values in the high-temperature fluids were between -405 parts per thousand and -33 parts per thousand, indicating the achievement of the hydrogen isotopic equilibrium between H-2 and H2O at around the hydrothermal end-member temperature. In contrast, several low-temperature fluids showed apparently smaller dDH(2) values than those in the high-temperature fluids in spite of a negligible dDH(2) change due to fluid-seawater mixing, suggesting the possibility of dDH(2) change in the low-temperature fluids and the surrounding environments. Since the dDH(2) change in low-temperature environments is not well explained by the very sluggish abiotic thermal isotopic equilibrium between H-2 and H2O, it could be associated with (micro) biological H-2-consuming and/or H-2-generating metabolisms that would strongly promote the isotopic equilibrium at low temperatures. Our first detection of the dDH2 variation in deep-sea hydrothermal systems presents the availability of the dDH2 value as a new tracer for microbes whose enzymes catalyze D/H exchange in H-2.