|Natural abundance||7 × 10-13|
|Half-life||301,000 ± 2,000 years|
Chlorine-36 is an isotope of chlorine. Chlorine has two stable isotopes and one radioactive isotope: the cosmogenic isotope 36Cl. Its half-life is 301,000 ± 2,000 years. 36Cl decays primarily (98%) by beta-minus decay to 36Ar, and the balance to 36S.
Trace amounts of radioactive 36Cl exist in the environment, in a ratio of about (7-10) × 10−13 to 1 with stable chlorine isotopes.  This corresponds to a concentration of approximately 1 Bq/(kg Cl).
36Cl is produced in the atmosphere by spallation of 36Ar by interactions with cosmic ray protons. In the top meter of the lithosphere, 36Cl is generated primarily by thermal neutron activation of 35Cl and spallation of 39K and 40Ca. In the subsurface environment, muon capture by 40Ca becomes more important. The production rates are about 4200 atoms 36Cl/yr/mole 39K and 3000 atoms 36Cl/yr/mole 40Ca, due to spallation in rocks at sea level.
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 2 years. 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.
There have been recent reports (2010) identifying fluctuations in the detection rates from 36Cl and other isotopes. At the time of writing it has not been determined if these represent genuine anomalies in the decay rate or have some more mundane explanation.
On August 14, 2012, Purdue University professor of physics Ephraim Fischbach hypothesized these decay fluctuations are tied to solar flare activity. If proven, these fluctuations could be used to predict impending solar flares hours or days in advance. E. Fischbach (2012). "New system could predict solar flares, give advance warning".
- "Interactive Chart of Nuclides". Retrieved 15-Dec-2012.
- M. Zreda et al. (1991). "Cosmogenic chlorine-36 production rates in terrestrial rocks". Earth and Planetary Science Letters 105: 94. Bibcode:1991E&PSL.105...94Z. doi:10.1016/0012-821X(91)90123-Y.
- M. Sheppard and M. Herod (2012). "Variation in background concentrations and specific activities of 36Cl, 129I and U/Th-series radionuclides in surface waters". Journal of Environmental Radioactivity 106: 27–34.
- "Chlorine". Isotopes & Hydrology.
- D. Javorsek II et al. (2010). "Power Spectrum Analyses of Nuclear Decay Rates". Astroparticle Physics 34: 173. arXiv:1007.0924. Bibcode:2010APh....34..173J. doi:10.1016/j.astropartphys.2010.06.011.