No evidence for change of the atmospheric helium isotope composition since 1978 from re-analysis of the Cape Grim Air Archive

- High-precision measurements for helium isotopes from the Cape Grim Air Archive are presented.
- No statistically significant temporal decrease of the 3He/4He ratio observed in from 1978-2011.
- The maximum rate of 3He/4He decrease consistent with the data is 0.033-0.042 ‰/yr (2σ).
- This data implies a maximum global mean helium content of natural gas around 0.034%.

The 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; Hoffman and 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.