39Ar

39Ar is a radioactive isotope of argon with mass number 39. It decays by beta emission to 39K with a half-life of 269 years. Because of its known decay rate, 39Ar provides a radiometric clock for ages on the order of decades to centuries. In nature, it is produced mainly by interactions of cosmic rays with atmospheric argon and, to a lesser extent, by neutron-induced reactions in minerals. As a result, trace amounts of 39Ar are present in atmospheric argon and in argon extracted from natural waters and ice.

The natural abundance of 39Ar is extremely small, making direct detection challenging. Its presence in dissolved argon allows dating of groundwater and ice samples that are too young for radiocarbon methods but older than what others can reliably date. Measurement strategies rely on detecting either the beta decays of 39Ar or counting 39Ar atoms directly. Techniques include low-background beta counting after argon extraction, accelerator mass spectrometry, and laser-based atom counting methods such as Atom Trap Trace Analysis (ATTA). ATTA, in particular, can count individual 39Ar atoms with high selectivity and sensitivity, enabling precise age estimates for water and ice samples.

Applications of 39Ar dating center on hydrology and paleoclimatology. It is used to determine the age of groundwater, groundwater renewal rates, and the turnover times of old water in aquifers, as well as to date firn and ice in the late Holocene. Limitations include the need for relatively large sample volumes, sophisticated instrumentation, and careful calibration against known-age materials.

See also: radiometric dating, argon dating, groundwater dating.