Radiocarbon enters the biosphere through natural processes like eating and breathing. Plants and animals absorb both C-12 and C-14 in the course of their natural lifetimes simply by carrying out these basic functions. When they die, they cease to consume them, and the isotope of C-14 begins to revert back to its Nitrogen state at an exponential rate due to its radioactive decay. Comparing the remaining 14C fraction of a sample to that expected from atmospheric 14C allows the age of the sample to be estimated. In addition, scientists know that the half-life of radiocarbon is 5,730 years.
This means that it takes a sample of radiocarbon 5,730 years for half of it to decay back into nitrogen. It takes another 5,730 for half of the remainder to decay, and then another 5,730 for half of what’s left, and so on. After about 10 half-lives, the amount of radiocarbon left becomes too miniscule to measure and so this technique isn’t useful for dating specimens which died more than 60,000 years ago. Another limitation is that this technique can only be applied to organic material such as bone, flesh, or wood. It can’t be used to date rocks directly.
The technique of Carbon Dating was developed by Willard Libby and his colleagues at the University of Chicago in 1949, for which he was awarded the Nobel Prize in 1960. However, it remains controversial for a number of reasons. First, there is the assumption that the half-life of C-14 has remained constant over time. Second, that the ratio of C-12 to C-14 in the atmosphere has remained constant. The ratio can be affected by natural upheaval, human influences, and the fact that C-14 production rates in the atmosphere, which in turn are affected by the amount of cosmic rays penetrating the earth’s atmosphere, are themselves affected by things like the earth’s magnetic field which deflects cosmic rays. Precise measurements taken over the last 140 years have shown a steady decay in the strength of the earth’s magnetic field. This means there’s been a steady increase in radiocarbon production (which would increase the ratio). Nevertheless, it remains the most accurate means of dating the scientific community has discovered to date.
If you’d like more info on Carbon Dating, check out NASA’s Virtual Dating: Isochron and Radiocarbon – Geology Labs On-line, and here’s a link to NASA’s Radiometric Dating Page.
We’ve also recorded an entire episode of Astronomy Cast all about How Carbon Dating Works. Listen here, Episode 164: Inside the Atom.