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

Abstract Spatial variations in the atmospheric helium isotopic composition (R A = 3 He/ 4 He air  = 1.39 × 10 −6 ) might be induced by localized and/or regional inputs of 3 He and/or 4 He into the air. It has been suggested that inputs of 4 He from hydrocarbon exploitation may generate latitudinal variations in atmospheric 3 He/ 4 He. In order to test the possibility of such global variations, we performed high precision analyses of the helium isotopic composition ( 3 He/ 4 He) 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 3 He/ 4 He 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 3 He/ 4 He ratios interpreted as potential 3 He 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.