By radiocarbon dating
Libby and graduate student Ernest Anderson (1920–2013) calculated the mixing of carbon across these different reservoirs, particularly in the oceans, which constitute the largest reservoir.Their results predicted the distribution of carbon-14 across features of the carbon cycle and gave Libby encouragement that radiocarbon dating would be successful.Korff predicted that the reaction between these neutrons and nitrogen-14, which predominates in the atmosphere, would produce carbon-14, also called radiocarbon.Libby cleverly realized that carbon-14 in the atmosphere would find its way into living matter, which would thus be tagged with the radioactive isotope.In a system where carbon-14 is readily exchanged throughout the cycle, the ratio of carbon-14 to other carbon isotopes should be the same in a living organism as in the atmosphere.However, the rates of movement of carbon throughout the cycle were not then known.Finally, Libby had a method to put his concept into practice. The circular arrangement of Geiger counters (center) detected radiation in samples while the thick metal shields on all sides were designed to reduce background radiation.The concept of radiocarbon dating relied on the ready assumption that once an organism died, it would be cut off from the carbon cycle, thus creating a time-capsule with a steadily diminishing carbon-14 count.
Further research by Libby and others established its half-life as 5,568 years (later revised to 5,730 ± 40 years), providing another essential factor in Libby’s concept.Top of page You read statements in books that such and such a society or archeological site is 20,000 years old.We learned rather abruptly that these numbers, these ancient ages, are not known accurately; in fact, it is at about the time of the First Dynasty in Egypt that the first historical date of any real certainty has been established.” —Willard Libby, Nobel Lecture, 12 December 1960 The concept of radiocarbon dating focused on measuring the carbon content of discreet organic objects, but in order to prove the idea Libby would have to understand the earth’s carbon system.He reasoned that a state of equilibrium must exist wherein the rate of carbon-14 production was equal to its rate of decay, dating back millennia.(Fortunately for him, this was later proven to be generally true.) For the second factor, it would be necessary to estimate the overall amount carbon-14 and compare this against all other isotopes of carbon.