An isotope of argon that is useful as a tracer in ocean studies. It is a radioactive inert gas with a half life of 269 years and is produced in the atmosphere by cosmic ray interactino with Argon-40. It is well-mixed through the troposphere and its variation in concentration over the last 1000 years has been estimated to be no more than about 7%. This means that its distribution in the atmosphere and ocean is in steady state.

It enters the ocean by gas exchange with the equilibrium time between the surface mixed layer and the atmosphere being about a month. The equilibrium concentration in surface water is calculated from the solubility of argon, a well known function of temperature and salinity, and the also well known concetration of Ar-39 in the atmosphere. The surface concentration in regions of deep water formation, where the surface water may not equilibrate with the atmosphere due to rapid convection processes, can be determined from measurements. Measurement is at present an onerous process requiring 1500 liters of water, and the concentration measured is reported in % modern, i.e. the Ar-39:Ar:40 ratio of the sample divided by the Ar-39:Ar:40 ratio of the troposphere. The minimum detectable limit is about 5% modern (with an error of 3-5% modern) which corresponds to an age of 1100 years with a resolution of about 50 years.

Argon-39 is an ideal tracer for investigating mixing and circulation in the deep ocean and in the mid to lower thermocline. Its distribution is in steady state and the boundary conditions are well known, i.e. there is no flux across the ocean bottom and the surface water concentration is known everywhere. Its distribution in the ocean interior is affected only by circulation, mixing and radioactive decay process, and since the decay rate is know it serves as a clock for circulation and mixing processes.