5:15 PM - 7:15 PM
[U12-P08] H2-driven pyrite/pyrrhotite redox cycle: a sustained electron generator for the autotrophic origin of life
Keywords:Origin of life, Astrobiology, Deep-sea hydrothermal systems
Electrochemical survey of Okinawa trough hydrothermal fields and the subsequent laboratory simulations have suggested that the electricity generation in deep-sea hydrothermal vent environments has served as a crucial source of autotrophic energy through the origin and evolution of life. However, although thermodynamic calculation has predicted that H2 oxidation in alkaline hydrothermal fluids (H2 => 2H+ + 2e–) is a powerful process for generating strongly negative electric potentials, the factors controlling the potentials and current available in realistic geochemical systems remain largely unknown: there should be a gap between the thermodynamic potentials and the potentials provided to the mineral–seawater interface. Here I show preliminary results of potential measurement and linear sweep voltammetry of iron sulfide under HS– and H2-containing alkaline hydrothermal fluids. It was found that, despite of the presence of H2, iron sulfide controls the electric potential through the mackinawite-pyrite equilibrium (FeS + HS– => FeS2 + H+ + 2e–). H2 chemically reduces the resultant pyrite, realizing a sustained electricity generation. With increasing temperature, the monitored electric potential approached to the thermodynamic potential for the mackinawite/pyrite equilibrium, reaching less than –650 mV versus the standard hydrogen electrode (at 25°C) under naturally realistic alkaline hydrothermal conditions. Additionally, higher temperatures led to greater current generation under oxidative potentials. These results indicate that in deep-sea systems, iron sulfide flexibly changes its oxidation state in response to the local electric potentials, serving as a natural battery between the reductive hydrothermal fluids and oxidative seawater.