Stable Abiotic Production of Ammonia from Nitrate in Komatiite-Hosted Hydrothermal Systems in the Hadean and Archean Oceans

Abiotic fixation of atmospheric dinitrogen to ammonia is important in prebiotic chemistry and biological evolution in the Hadean and Archean oceans. Though it is widely accepted that nitrate (NO3-) was generated in the early atmospheres, the stable pathways of ammonia production from nitrate deposited in the early oceans remain unknown. This paper reports results of the first experiments simulating high-temperature, high-pressure reactions between nitrate and komatiite to find probable chemical pathways to deliver ammonia to the vent-ocean interface of komatiite-hosted hydrothermal systems and the global ocean on geological timescales. The fluid chemistry and mineralogy of the komatiite-H2O-NO3- system show iron-mediated production of ammonia from nitrate with yields of 10% at 250 degrees C and 350 degrees C, 500 bars. The komatiite-H2O-NO3- system also generated H-2-rich and alkaline fluids, well-known prerequisites for prebiotic and primordial metabolisms, at lower temperatures than the komatiite-H2O-CO2 system. We estimate the ammonia flux from the komatiite-hosted systems to be 10(5)-10(10) mol/y in the early oceans. If the nitrate concentration in the early oceans was greater than 10 mu mol/kg, the long-term production of ammonia through thermochemical nitrate reduction for the first billion years might have allowed the subsequent development of an early biosphere in the global surface ocean. Our results imply that komatiite-hosted systems might have impacted not only H-2-based chemosynthetic ecosystems at the vent-ocean interface but also photosynthetic ecosystems on the early Earth.

Automated summary

The study investigates the production of ammonia from nitrate in early oceans and hydrothermal systems through high-temperature, high-pressure reactions between nitrate and komatiite. It suggests that iron-mediated ammonia production from nitrate in komatiite systems might have played a crucial role in the early evolution of biospheres on Earth.