Stable isotopic evidence in support of active microbial methane cycling in low-temperature diffuse flow vents at 9°50′N East Pacific Rise

A unique dataset from paired low- and high-temperature vents at 9 degrees 50'N East Pacific Rise provides insight into the microbiological activity in low-temperature diffuse fluids. The stable carbon isotopic composition of CH4 and CO2 in 9 degrees 50'N hydrothermal fluids indicates microbial methane production, perhaps.coupled with microbial methane consumption. Diffuse fluids are depleted in C-13 by similar to 10%. in values of delta C-13 of CH4, and by similar to 0.55%. in values of delta C-13 of CO2, relative to the values of the high-temperature source fluid (delta C-13 of CH4 = -20.1 +/- 1.2%, delta C-13 of CO2 = -4.08 +/- 0.15%.). Mixing of seawater or thermogenic sources cannot account for the depletions in C-13 of both CH4 and CO2 at diffuse vents relative to adjacent high-temperature vents. The substrate utilization and C-13 fractionation associated with the microbiological processes of methanogenesis and methane oxidation can explain observed steady-state CH4 and CO2 concentrations and carbon isotopic compositions. A mass-isotope numerical box model of these paired vent systems is consistent with the hypothesis that microbial methane cycling is active at diffuse vents at 9 degrees 50'N. The detectable C-13 modification of fluid geochemistry by microbial metabolisms may provide a useful tool for detecting active methanogenesis. (c) 2008 Elsevier Ltd. All rights reserved.