Mixing, Reaction and Microbial Activity in the Sub‐Seafloor Revealed by Temporal and Spatial Variation in Diffuse Flow Vents at Axial Volcano

To begin to understand the relationship between microbial communities and the geochemical environment, we have conducted systematic sampling and in situ analysis of a range of seafloor vents on or near the January 1998 lava flow at the summit of Axial Volcano on the Juan de Fuca ridge. The systematics of the chemical composition indicate that low-temperature diffuse fluids (3°C-78°C) at Axial Volcano have a high-temperature (>350°C) reaction-zone component overprinted by lower-temperature reactions. The low-temperature reactions include production of methane, ammonia and particulate elemental sulfur, oxidation of hydrogen sulfide, nitrate reduction, stripping of metals from seawater, and reactions that dissolve iron and produce alkalinity. High concentrations of CO 2 from magmatic degassing maintain acidic pH conditions and may be important in promoting low-temperature hydrolysis reactions. H 2 S oxidation is the dominant chemical energy source for microbial metabolism at Axial Volcano, and the energy available from either methanogenesis or iron oxidation is ∼100 times less. Chemical evidence, genetic signatures of thermophilic, non-seawater organisms, presence of culturable thermophiles, and cell counts elevated above backgound seawater in low-temperature fluids indicate microbial activity below the seafloor. Metabolic activity of organisms identified in venting fluids matches the chemical processes occurring in low-temperature sub-seafloor reservoirs.