How radiocarbon dating works

by Jonathan DuHamel

Age dating by measuring the amount of carbon-14 (C14) in a piece of wood and other organic artifacts is widely used in archaeology. The method is valuable for estimating the age of an object, but it is only an estimate.

Carbon-14 years are not the same as calendar years. C14 dates are usually expressed as, for instance, 1200 years BP. The “BP” means “before present” but even that is not entirely true. What it really means is the number of years before 1950, the standard calibration year. Another way to express it is “percent modern” which means the percent C14 in the sample compared to a modern sample value in 1950.

Carbon-14 is produced in the upper troposphere and lower stratosphere by cosmic rays. The cosmic rays generate neutrons which knock a proton out of nitrogen-14 to produce carbon-14. This carbon quickly combines with oxygen to form carbon dioxide. (Carbon-14 decays back to nitrogen-14 by emitting an electron.)

The carbon-14 bearing carbon dioxide, as well as stable C12 carbon dioxide, is taken in by plants and animals, and continuously replaced. The ratio of C14 to C12 remains stable until the plant or animal dies. After death, no more carbon dioxide is taken in, C14 decays, and the ratio of C14 to C12 decreases.

So, how does that give us an age?

A series of overlapping tree rings allow us to count back in time to find the age of that particular ring. We can measure the C14/C12 ratio of each tree ring. Then we measure the C14/C12 ratio of the sample and find a tree ring of known age with the same value (done by computers). The sample is presumed to be the same age as the tree ring.

Carbon-14 dating makes these assumptions:

1) The C-14 content of the atmosphere has remained constant over time. This was the initial assumption, but comparing the C14/C12 ratio of tree rings presumably of the same age yielded different ratio values which showed that the C14 content varied over time. So, calibration curves were constructed to account for these variations. There are several versions of these curves calculated by different researchers.

2) The half-life of decay of C14 is known. When the method was invented by Willard Libby in the late 1940s, the half-life (the time it takes one-half of the C14 to decay) was put at 5568 years, but more recent research puts it at 5730 years. That means all C14 will be gone in about 60,000 years. Some studies still use the old standard.

3) You can accurately measure the C14 in the samples and tree rings. The initial ratio of C14 to C12 is about 1.5 to 1 trillion. This requires great care in the analyses.

4) The fractionation effect: The carbon exchange between atmospheric CO2 and carbonate at the ocean surface is subject to fractionation, with C14 in the atmosphere more likely than C12 to dissolve in the ocean. The result is an overall increase in the C14/C12 ratio in the ocean of 1.5%, relative to the ratio in the atmosphere. Wikipedia has detailed discussion of this and other effects here. The University of Oxford has a radiocarbon laboratory and some simple explanations of the method here.

The bottom line: Radiocarbon dating is a valuable tool, but one subject to many potential sources of error. The resulting age of the sample should be regarded as an estimate only.

 

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