Spatial analysis of the atmospheric helium isotopic composition: Geochemical and environmental implications

Spatial variations in the atmospheric helium isotopic composition (RA = 3He/4Heair = 1.39 × 10−6) might be induced by localized and/or regional inputs of 3He and/or 4He into the air. It has been suggested that inputs of 4He from hydrocarbon exploitation may generate latitudinal variations in atmospheric 3He/4He. In order to test the possibility of such global variations, we performed high precision analyses of the helium isotopic composition (3He/4He) of sixteen air samples collected in 500 cc metal bottles around the world (79°N-75°S) between 2013 and 2015. In most cases, the 3He/4He of these air samples are indistinguishable from that of air sampled around Nancy (France) within ≤2‰ (95% confidence interval). Only two air samples, collected at Dome C (Antarctica) and at Tokyo (Japan), exhibit statistically higher 3He/4He ratios interpreted as potential 3He excesses of (2.0 ± 1.4)‰ and (1.7 ± 1.5)‰ (95% confidence interval), respectively. Excesses such as these suggest that potential helium isotopic variations in air are likely lower than 4‰ and might be generated temporarily by regional and/or local phenomenon (e.g. auroral precipitation, stratospheric to tropospheric exchanges). Thus, this study supports the use of the atmospheric helium isotopic ratio as an inter-laboratory He standard.