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.