Igneous rocks as a viable source of fixed nitrogen to the prebiotic world

The origin and early evolution of life on Earth and other habitable worlds requires constant supply of ammonic nitrogen (N). Previously proposed abiotic ammonium sources rely on sporadic and heterogeneously distributed high energy processes, such as lightning, subaerial volcanic degassing, or deep sea hydrothermal vents to generate bioavailable nitrogen from atmospheric N₂ gas. Here we explore weathering of ammonium contained in felsic igneous rocks as an alternative source. We find that this process could have supplied 10⁸–10⁹ mol yr⁻¹ of bioavailable N to surface environments in the early Archean, leading to dissolved concentrations of 0.023 ± 0.017 μM in freshwater and 0.01–0.1 μM in seawater. In terrestrial settings, evaporation paired with elevated N supplies from locally enriched felsic bedrock may have led to concentrations approaching 1 μM. Rock weathering would thus have constituted a smaller flux than the sum of all proposed high energy sources of fixed N, but with the major benefit that it was reliably present, especially in terrestrial settings. Weathering of differentiated igneous rocks should thus be considered in models of the emergence of life on Earth and beyond.