Extreme enrichment in atmospheric (15)N(15)N

Molecular nitrogen (N(2)) comprises three-quarters of Earth’s atmosphere and significant portions of other planetary atmospheres. We report a 19 per mil (‰) excess of (15)N(15)N in air relative to a random distribution of nitrogen isotopes, an enrichment that is 10 times larger than what isotopic equilibration in the atmosphere allows. Biological experiments show that the main sources and sinks of N(2) yield much smaller proportions of (15)N(15)N in N(2). Electrical discharge experiments, however, establish (15)N(15)N excesses of up to +23‰. We argue that (15)N(15)N accumulates in the atmosphere because of gas-phase chemistry in the thermosphere (>100 km altitude) on time scales comparable to those of biological cycling. The atmospheric (15)N(15)N excess therefore reflects a planetary-scale balance of biogeochemical and atmospheric nitrogen chemistry, one that may also exist on other planets.