  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 N2 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
N2 and O2 and atmospheres dominated by N2 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 the early Earth. Here, we present results from spark discharge experiments
in N2-CO2 and N2-O2 gas mixtures. Our experiments suggest that lightning-driven
nitrogen fixation may have been similarly efficient in the Archaean atmosphere,
compared to modern times. Measurements of the isotopic ratio {\delta}15N of the
discharge-produced nitrite and nitrate in solution show very low values of -6
to -15 permil after equilibration with the gas phase with a calculated
endmember composition of -17 permil. These results are much lower than most
{\delta}15N values documented from the sedimentary rock record, which supports
the development of biological nitrogen fixation earlier than 3.2 Ga. However,
some Paleoarchean records (3.7 Ga) may be consistent with lightning-derived
nitrogen input, highlighting the potential role of this process for the
earliest ecosystems.
