  The evolution of the atmospheric noble gas composition during the past few
decades has hardly been studied because, in contrast to many other atmospheric
gases, systematic time-series measurements have not been available. Based on
theoretical considerations, the atmospheric noble gas isotope composition is
assumed to be stable on time scales of up to about 10^6 years, with the
potential exception of anthropogenic changes predicted for the He concentration
and the 3He/4He ratio. However, experimental assessments of the predicted
changes in the atmospheric He isotope composition are controversial. To
empirically test these assumptions and predictions, we analysed the noble gas
isotope composition in samples of the Cape Grim Air Archive, a well-defined
archive of marine boundary layer air in the southern hemisphere. The resulting
time series of the 20Ne, 40Ar, 86Kr and 136Xe concentrations and 20Ne/22Ne and
40Ar/36Ar ratios during 1978-2011 demonstrate the stability of the atmospheric
Ne, Ar, Kr and Xe composition during this time interval. The He isotope data
provide strong evidence for a decrease in the 3He/4He during the same time
interval at a mean rate of 0.23-0.30 permil per year. This result is consistent
with most model predictions of the rate of decrease in the atmospheric 3He/4He
ratio associated with mining and burning of fossil fuels.
