Kinetics of H-2-O-2-H2O redox equilibria and formation of metastable H2O2 under low temperature hydrothermal conditions

Hydrothermal experiments were conducted to evaluate the kinetics of H-2(aq) oxidation in the homogeneous H-2-O-2-H2O system at conditions reflecting subsurface/near-seafloor hydrothermal environments (55-250 degrees C and 242-497 bar). The kinetics of the water-forming reaction that controls the fundamental equilibrium between dissolved H-2(aq) and O-2(aq) are expected to impose significant constraints on the redox gradients that develop when mixing occurs between oxygenated seawater and high-temperature anoxic vent fluid at near-seafloor conditions. Experimental data indicate that, indeed, the kinetics of H-2(aq)-O-2(aq) equilibrium become slower with decreasing temperature, allowing excess H-2(aq) to remain in solution. Sluggish reaction rates of H-2(aq) oxidation suggest that active microbial populations in near-seafloor and subsurface environments could potentially utilize both H-2(aq) and O-2(aq), even at temperatures lower than 40 degrees C due to H-2(aq) persistence in the seawater/vent fluid mixtures. For these H-2-O-2 disequilibrium conditions, redox gradients along the seawater/hydrothermal fluid mixing interface are not sharp and microbially-mediated H-2(aq) oxidation coupled with a lack of other electron acceptors (e.g. nitrate) could provide an important energy source available at low-temperature diffuse flow vent sites.