Radiocarbon - (n.)
Carbon-14 (14C), radioactive with a half-life near 6000 years, is used as a tracer for assessment of rates of change and for dating. It is generated in the atmosphere, by interaction of 14N with neutrons in cosmic radiation, and eventually decays back to 14N. The ocean receives 14C at its surface, through exchange of carbon with the atmosphere. Since its deep waters are isolated from the atmosphere, the loss of 14C from decay is not replenished there. Thus, the distribution of radiocarbon in the deep ocean defines the age of deep water masses, their direction and rate of motion, as well as the overall mixing time of the ocean.