Abstract

The isotopic composition of nitrogen in the rock record provides valuable evidence of reactive nitrogen sources and processing on early Earth, but the wide range of δ15N values (− 10.2 to + 50.4‰) leads to ambiguity in defining the early Precambrian nitrogen cycle. The high δ15N values have been explained by large fractionation associated with the onset of nitrification and/or fractionation produced by ammonia-ammonium equilibrium and water–air flux in alkaline paleolakes. Previous flux sensitivity studies in modern water bodies report alkaline pH is not a prerequisite and temperature can be the dominate parameter driving water–air flux. Here, I use the chemical and physical components of 1022 modern hydrothermal features to provide evidence that water–air NH3 flux produced a significant source of fixed nitrogen to early Earth’s atmosphere and biosphere. With regard to the modeled average NH3 flux (2.1 kg N m−2 year−1) and outlier removed average flux (1.2 kg N m−2 year−1), the Archean Earth’s surface would need to be 0.0092, and 0.017% terrestrial hydrothermal features, respectively, for the flux to match the annual amount of N produced by biogenic fixation on modern Earth. Water–air NH3 flux from terrestrial hydrothermal features may have played a significant role in supplying bioavailable nitrogen to early life.

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