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

Abstract

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.