Changes in the atmospheric helium isotope ratio over the past 40 years



[1] We have compared the helium isotope ratio in five samples of Pacific marine air spanning the 40 year period between 1973 and 2013 against a secondary gas standard. In a separate experiment we directly compared the 3He/4He ratio in air samples collected in 1973 and 2013 at the same location in La Jolla, California, eliminating any geographical bias. Both experiments are consistent with zero time rate of change for atmospheric 3He/4He. Our best estimate for the rate of change of the 3He/4He ratio in Pacific marine air is −0.0014 ± 0.0045%/yr (2σ), indicating that air helium is still a valid standard for terrestrial helium isotope measurements.