Abstract
The radioactive isotope 39Ar is an ideal tracer for ocean ventilation, groundwater flow, and for dating mountain glaciers. With a half-life of 269 years, it covers the age range from a few tens to about 1800 years. We evaluate the input history of the atmospheric 39Ar in the past 2500 years. By measuring the 39Ar/Ar ratios of a modern argon sample and two old argon samples collected in 1959 and 1961, respectively, the anthropogenic contribution to the atmospheric 39Ar in the past 60 years is determined to be less than 15%. The temporal variation of the atmospheric 39Ar in the past 2500 years is calculated based on a cosmic-ray record derived from ice cores and tree rings. It is found that the atmospheric 39Ar/Ar ratio has changed by as much as 17% in that period of time. This input variation has to be taken into account and corrected for in future 39Ar dating applications.
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