Isotopic constraints on lightning as a source of fixed nitrogen in Earth's early biosphere

Bioavailable nitrogen is thought to be a requirement for the origin and sustenance of life. Before the onset of biological nitrogen fixation, abiotic pathways to fix atmospheric N-2 must have been prominent to provide bioavailable nitrogen to Earth's earliest ecosystems. Lightning has been shown to produce fixed nitrogen as nitrite and nitrate in both modern atmospheres dominated by N-2 and O-2 and atmospheres dominated by N-2 and CO2 analogous to the Archaean Earth. However, a better understanding of the isotopic fingerprints of lightning-generated fixed nitrogen is needed to assess the role of this process on early Earth. Here we present results from spark discharge experiments in N-2-CO2 and N-2-O-2 gas mixtures. Our experiments suggest that lightning-driven nitrogen fixation may have been similarly efficient in the Archaean atmosphere, compared with modern times. Measurements of the isotopic ratio (delta N-15) of the discharge-produced nitrite and nitrate in solution show very low values of -6 parts per thousand to -15 parts per thousand after equilibration with the gas phase with a calculated endmember composition of -17 parts per thousand. These results are much lower than most delta N-15 values documented from the sedimentary rock record, which supports the development of biological nitrogen fixation earlier than 3.2 billion years ago. However, some Paleoarchean records (3.7 billion years ago) may be consistent with lightning-derived nitrogen input, highlighting the potential role of this process for the earliest ecosystems. Spark discharge experiments suggest lightning was not the main source of bioavailable nitrogen for the established Archaean biosphere, but could have been significant for Earth's earliest ecosystems.