Free energy distribution and hydrothermal mineral precipitation in Hadean submarine alkaline vent systems: Importance of iron redox reactions under anoxic conditions

Thermodynamic calculations of mixing between hypothetical seawater and hydrothermal fluid in the Hadean deep ocean were carried out to predict saturation states of mineral precipitates and redox reactions that could occur in Hadean submarine alkaline hydrothermal systems associated with the serpentinization of ultramafic rocks. In the calculations, the seawater was assumed to be weakly acidic (pH = 5.5) and to include carbon dioxide, ferrous iron and silica, with or without nitrate, while the Hadean hydrothermal fluid was assumed to be highly alkaline (pH = 11) and to contain abundant molecular hydrogen, methane and bisulfide, based on the Archean geologic record, the modern low-temperature alkaline hydrothermal vent fluid (Lost City field), and experimental and theoretical considerations. The modeling indicates that potential mineral precipitates in the mixing zone (hydrothermal chimney structures) could consist mainly of iron sulfides but also of ferrous serpentine and brucite, siderite, and ferric iron-bearing minerals such as goethite, hematite and/or magnetite as minor phases. The precipitation of ferric iron-bearing minerals suggests that chemical iron oxidation would be made possible by pH shift even under anoxic condition. In the mixing zone, comprising an inorganic barrier precipitated at the interface of the two contrasting solutions, various redox reactions release free energy with the potential to drive endergonic reactions, assuming the involvement of coupling inorganic protoenzymes. Hydrogenotrophic methanogenesis and acetogenesis - long considered the most ancient forms of biological energy metabolisms - are able to achieve higher maximum energy yield (>0.5 kJ/kg hydrothermal fluid) than those in the modern serpentinization-associated seafloor hydrothermal systems (e.g., Kairei field). Furthermore, the recently proposed methanotrophic acetogenesis pathway was also thermodynamically investigated. It is known that methanotrophic acetogenesis would require additional exergonic reactions to compensate its most endergonic methane-to-methanol conversion reaction at the oxidative entry to the metabolic pathway. Our calculations support the view that this thermodynamic barrier could be overcome by the reduction of nitrate in seawater at low temperature, as previously suggested. However, the thermodynamic calculations also revealed that the reduction of ferric iron-bearing minerals would occur at the outer margin and within the hydrothermal chimney wall. The maximum available energy of iron-reducing methanotrophic acetogenesis was calculated to be 0.25-0.35 kJ/kg hydrothermal fluid. Although this value is lower than theoretically available through nitrate reduction, which approaches similar to 0.45-1.25 kJ/kg hydrothermal fluid on the outer cool margins of a putative Hadean alkaline chimney, it is higher than that of sulfate-reducing anaerobic oxidation of methane in the Lost City field. These results suggest that iron reduction had the potential to drive methanotrophy and that the Hadean hydrothermal mixing zone was energetically more favorable to methanotrophy than previously thought. We conclude that iron oxidation and reduction in oxyhydroxides probably played important roles in the early evolution of energy metabolisms in the Hadean alkaline hydrothermal system. (C) 2015 Elsevier Ltd. All rights reserved.