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doi: 10.7185/geochemlet.1804 The atmospheric helium isotope composition (RA= He/Heair = 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 4He by fossil fuel exploitation could have been at least partly offset by the production of 3He (via the decay of 3H) from nuclear tests. This study supports the suitability of atmospheric helium as an inter-laboratory isotope standard. Received 7 October 2017 | Accepted 29 January 2018 | Published 19 February 2018 1. Centre de Recherches Pétrographiques et Géochimiques, CNRS and Université de Lorraine, Vandœuvre-lès-Nancy, France 2. Commonwealth Scientific and Industrial Research Organisation, Oceans and Atmosphere, Aspendale, Australia * Corresponding author (email: cboucher@crpg.cnrs-nancy.fr; christibouc@gmail.com) Introduction Atmospheric helium is a trace gas used as an international standard, whose abundance and isotope composition may be impacted by anthropogenic activities (e.g., Sano et al., 2010). It is therefore of the utmost importance to check if the atmospheric 3He/4He ratio (RAIR) has, or has not, been constant over time. According to Oliver et al. (1984), the exploitation of natural gases (NG) could have increased the global atmospheric helium content by 1 to 6 ‰ between 1939 and 1981. Because NG are rich in radiogenic 4He generated in the continental crust, the RAIR could have decreased by ≤3 ‰/yr (Brennwald et al., 2013 and references therein; see also Fig. S-1). In line with this possibility, 3He excesses (~3–4 % relative to the present RAIR value) have been reported for old air trapped in vesicles of blast-furnace metallurgical slags (Pierson-Wickman et al., 2001; Sano et al., 2010) and in ancient porcelains (Matsuda et al., 2010), suggesting that pre-industrial air contained less 4He than the present-day air. However, these excesses might also be related to: (i) the release of cosmogenic/nucleogenic 3He from the sample matrix; (ii) isotope fractionation during helium extraction and (iii) the capture of fractionated RAIR during manufacturing (Pierson-Wickman et al., 2001). The occurrence of temporal variations of the RAIR value has been questioned in several studies. Lupton and Evans (2013) did not detect significant variations in Pacific marine air from La Jolla, California, USA (trend of -0.014 ± 0.045 ‰/ yr obtained by direct comparison of La Jolla air collected in 1973 and 2013). From the analysis of air sampled in stainless steel bottles in Tasmania since 1978 (Cape Grim Air Archive, CGAA), Mabry et al. (2015) concluded that the RAIR value has been stable (trend of -0.0095 ± 0.0330 ‰/yr) over the last three decades. These authors argued that the mean helium content of globally produced NG has been overestimated by ~3 times in early studies, so that 4He released by NG may not have impacted the RAIR value within the precision of measurements. Here, we aim to constrain temporal variation of RAIR that could be related to the beginning of the commercial helium production in 1921 (Mohr and Ward, 2014). To do so, we used large amounts of air trapped in stainless steel materials insuring good preservation of helium over time since 1910. The air volume trapped in our selected samples (≥50 cm3), larger than that trapped in vesicles of slags/porcelains, allows repeated measurements and therefore more precision.

Atmospheric helium isotopic ratio from 1910 to 2016 recorded in stainless steel containers