[U-11] Biogeochemistry of CO world

[U11-P12] Laboratory simulation of electricity generation in deep-sea hydrothermal vent environments

*Norio Kitadai¹ (1.Japan Agency for Marine-Earth Science and Technology)

Keywords:Astrobiology, Origin of Life

Recent electrochemical survey of Okinawa trough hydrothermal fields and the subsequent follow up laboratory simulations have revealed that the geoelectrochemical process in deep-sea hydrothermal vent environments has served as a crucial source of autotrophic energy through the origin and evolution of life. In the deep-sea systems, electrons are catalytically provided by the oxidation of reductive hydrothermal fluid chemicals (for example, H2S and H2) at the fluid-mineral interface and are transported to the mineral-seawater interface via electrically conductive sulfide rocks across the redox gap between the fluid and seawater. The electric potentials for H2 and H2S oxidations are thermodynamically predictable based on the temperature, pH, and composition of hydrothermal fluids. However, the naturally available electric potentials and current must also depend on the catalytic performance of vent-constituting minerals, which remains largely unknown. Here we conducted potential measurement and linear sweep voltammetry of iron sulfide, a major sulfide constituent of deep-sea vents and surrounding deposits, under HS– and H2-containing alkaline hydrothermal fluids. It was found that iron sulfide controls the electric potential through the mackinawite-pyrite equilibrium (FeS + HS– => FeS2 + H+ + 2e–). The resultant pyrite can be reduced back to the original oxidation state through the chemical reaction with H2 (FeS2 + H2 => FeS + HS– + H+), thus realizes a sustained electron 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 25oC) under naturally realistic alkaline hydrothermal conditions. The linear sweep voltammetry showed that higher temperatures lead to greater current under oxidative potentials. These facts indicate that in deep-sea vent systems, iron sulfide works as a natural buttery by flexibly changing its oxidation state in response to the local electric potentials at the reductive hydrothermal fluids-oxidative seawater interface.