Diffuse flow hydrothermal fluids from 9° 50' N East Pacific Rise: Origin, evolution and biogeochemical controls

Diffuse flow and high temperature hydrothermal fluids were sampled from three areas between 9°49-51'N on the East Pacific Rise, in the area known as the BIOGEOTRANSECT, between 1991-2000 in order to define the relationship between these two types of hydrothermal fluids, and to understand their temporal evolution, and relationship to biological communities. As all of the diffuse flow fluids have Cl concentrations not equal to the ambient seawater value, they must contain a proportion of high temperature vent fluid that has undergone phase separation, requiring temperatures >389°C. In comparison to previously reported data for diffuse flow fluids, these fluids have higher temperatures, and typically have higher concentrations of Si, H₂S, Fe, CO₂, CH₄ and H₂. Each of the three areas has its own distinct chemistry, and follows its own unique evolutionary path. In most cases, the diffuse flow fluids show a loss of H₂S, H₂ and gain of CH₄ relative to the high temperature fluids. We interpret these differences as indicating subsurface biological activity at these sites. If this gain of CH₄ and loss of H₂S and H₂ is common in submarine hydrothermal systems worldwide, it would imply that the fluxes of these elements for diffuse flow cannot be calculated simply based on the high temperature data, but that CH₄ fluxes would need to be revised upward, and H₂S and H₂ revised downward.