Abiotic N₂ reduction in submarine hydrothermal systems could quickly fertilize prebiotic oceans

NH₃ or NH₄⁺ is an essential component of the abiotic synthesis of organic compounds for the origin of life and an efficient greenhouse gas to address the faint young Sun paradox on the early Earth. Sustainable NH₃ or NH₄⁺ on the N₂-dominated prebiotic Earth's surface requires potent abiotic N₂ reduction (ANR) in hydrothermal systems, which has not been detected in the geological record despite numerous laboratory demonstrations. Here we report high concentrations and extreme ¹⁵N depletions of NH₄⁺ in hydrothermal veins in oceanic crusts drilled from the South China Sea basin. Our data indicate that abundant ¹⁵N-depleted NH₄⁺ was produced by ANR in deep fluid but progressively overprinted by ¹⁵N-enriched biogenic NH₄⁺ toward the surface. Modeling suggests that ANR could supply up to 9.0 - 10.8×10¹⁰mol⋅year^-1 NH₄⁺ to global oceans, which is minor to the large nitrogen inventory in modern oceans, but could quickly fertilize the oceans and supply NH₃ to the atmosphere in the prebiotic Earth.