Chlorine-36

Chlorine-36 is an isotope of chlorine. Chlorine has two stable isotopes and one radioactive environmental isotope: the cosmogenic isotope 36Cl. The ratio of 36Cl to stable 37Cl in the environment is ~700×10-15. Its half-life is 301,000 ± 4,000 years. The long half-life of 36Cl makes it useful to date groundwater up to one million years old. There also are observed natural variations in 37Cl. Most such variations in 37Cl values of hydrologic systems are related to diffusion processes.

Chlorine-36 is produced in the upper atmosphere through spallation reactions and in solid materials on the surface of the earth in three ways: spallation reactions, muon reactions and thermal neutron adsorption. Spallation reactions also occur when gamma rays interact with minerals in the top several meters of the Earth's surface. It can also be produced through muon reactions and thermal neutron absorption. 36Cl was also produced during nuclear bomb testing in the middle of the 20th century.

Trace amounts of radioactive 36Cl exist in the environment, in a ratio of about 7x10−13 to 1 with stable isotopes. 36Cl is produced in the atmosphere by spallation of 36Ar by interactions with cosmic ray protons. In the subsurface environment, 36Cl is generated primarily as a result of neutron capture by 35Cl or muon capture by 40Ca. 36Cl decays to 36S and to 36Ar, with a combined half-life of 308,000 years. The half-life of this hydrophilic nonreactive isotope makes it suitable for geologic dating in the range of 60,000 to 1 million years. Additionally, large amounts of 36Cl were produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of 36Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water, 36Cl is also useful for dating waters less than 50 years before the present. 36Cl has seen use in other areas of the geological sciences, including dating ice and sediments.