Chlorine-36 (Cl) is an isotope of chlorine. Chlorine has two stable isotopes and one naturally occurring radioactive isotope, the cosmogenic isotope Cl.  Its half-life is 301,300 ± 1,500 years. Cl decays primarily (98%) by beta-minus decay to Ar, and the balance to S.
Trace amounts of radioactive Cl exist in the environment, in a ratio of about (7–10) × 10 to 1 with stable chlorine isotopes.  This corresponds to a concentration of approximately 1 Bq/(kg Cl).
Cl is produced in the atmosphere by spallation of Ar by interactions with cosmic ray protons. In the top meter of the lithosphere, Cl is generated primarily by thermal neutron activation of Cl and spallation of K and Ca. In the subsurface environment, muon capture by Ca becomes more important. The production rates are about 4200 atoms Cl/yr/mole K and 3000 atoms Cl/yr/mole Ca, due to spallation in rocks at sea level.
The half-life of this isotope makes it suitable for geologic dating in the range of 60,000 to 1 million years. Its properties make it useful as a proxy data source to characterize cosmic particle bombardment and solar activity of the past.
Additionally, large amounts of Cl were produced by irradiation of seawater during atmospheric and underwater test detonations of nuclear weapons between 1952 and 1958. The residence time of Cl in the atmosphere is about 2 years. Thus, as an event marker of 1950s water in soil and ground water, Cl is also useful for dating waters less than 50 years before the present. Cl has seen use in other areas of the geological sciences, including dating ice and sediments.