No abstract

International audienceThe helium isotope composition of air might have changed since the industrial revolution due to the release of 4He-rich crustal helium during exploitation of fossil fuels. Thereby, variation of the atmospheric helium isotope ratio (3He/4He) has been proposed as a possible new atmospheric tracer of industrial activity. However, the magnitude of such change is debated, with possible values ranging from 0 to about 2/yr(Sano et al., 1989; Hoffmanand Nier, 1993; Pierson-Wickmann et al., 2001;Brennwald et al., 2013;Lupton and Evans, 2013). A new analytical facility for high precision (2, 2σ) analysis of the 3He/4He ratio of air has been developed at CRPG Nancy (France) capable of investigating permil level variations. Previously, Brennwald et al.(2013)analyzed a selection of air samples archived since 1978 at Cape Grim, Tasmania, by the Commonwealth Scientific and Industrial Research Organisation (CSIRO). They reported a mean temporal decrease of the 3He/4He ratio of 0.23–0.30/yr. Re-analysis of aliquots of the same samples using the new high-precision instrument showed no significant temporal decrease of the 3He/4He ratio (0.0095 ±0.033/yr, 2σ) in the time interval 1978–2011. These new data constrain the mean He content of globally produced natural gas to about 0.034% or less, which is about 3×lower than commonly quoted

No abstract

The atmospheric helium isotope composition (R A = 3 He/ 4 He air = 1.39 × 10 -6 ) could have varied over recent times due to anthropogenic activities. In order to check this possibility, we conducted high-precision helium isotope measurements of air trapped in various stainless steel containers from France (pétanque balls, a float carburettor; 1910-2016) and Cape Grim, Tasmania (archived air tanks; 1978, 1988). We used a double collector mass spectrometer at the Centre de Recherches Pétrographiques et Géochimiques (CRPG, Nancy, France). We found a similar composition between the French and Cape Grim air samples. The temporal variation estimated from all samples including data previously published is not significant, with a trend of +0.002 ± 0.024 ‰/yr over 106 years (2σ). We suspect that the release of radiogenic 4 He by fossil fuel exploitation could have been at least partly offset by the production of 3 He (via the decay of 3 H) from nuclear tests. This study supports the suitability of atmospheric helium as an inter-laboratory isotope standard.