Abstract

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 °C and 350 °C, 500 bars. The komatiite–H2O–NO3– system also generated H2-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 105–1010 mol/y in the early oceans. If the nitrate concentration in the early oceans was greater than 10 μmol/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 H2-based chemosynthetic ecosystems at the vent-ocean interface but also photosynthetic ecosystems on the early Earth.

Highlights

  • IntroductionThe reduction of abundant dinitrogen (N2 ) to ammonia (NH3 ) is important for prebiotic chemical evolution and the early development of life on Earth

  • Ammonia participates in the synthesis of nitrogen-bearing organic compounds that are essential for cellular functions and self-reproduction

  • During the high temperature water-rock reactions, the pH measured at room temperature

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Summary

Introduction

The reduction of abundant dinitrogen (N2 ) to ammonia (NH3 ) is important for prebiotic chemical evolution and the early development of life on Earth. The production of ammonia is assumed to be a prerequisite for origin-of-life scenarios, e.g., [1,2], and the development of microbial ecosystems in the Hadean and Archean oceans, e.g., [3,4,5]. Previous studies have suggested that nitric oxide (NO) was produced by electrical discharges in the early atmosphere [3,12]. The resulting NO was likely converted into nitric acid (HNO3 ) or nitrous acid (HNO2 ) due to photochemical and aqueous-phase reactions, and it was eventually deposited in the ocean through rain [13]

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