How Carbon 14 Dating Works

Since its development by J. R. Arnold and W. F. Libby in 1949, radiocarbon dating has proved a valuable resource in dating some archeological and paleontological remains. This form of dating requires some form of previously living matter such as wood or other plant material and animal remains. Substances made from such materials such as paper or papyrus and cloths made from natural fibers are suitable for radiocarbon dating.

Naturally occurring carbon atoms occur as three isotopes carbon-12 and carbon-13, which are stable and the unstable carbon-14 with a half-life of about 5,730 years. The amount of carbon-14 in the atmosphere is maintained by cosmic ray bombardment of the upper atmosphere. Interaction between these cosmic rays and atmospheric nitrogen-14 leads to the formation of carbon-14 and hydrogen. Storms distribute carbon-14 formed in the upper atmosphere throughout the rest of the atmosphere.

Plants metabolize atmospheric carbon incorporating it into carbohydrates. Animals eating plants utilize these carbohydrates in their metabolism. In theory, the carbon used by plants and passed onto animals is in the same isotopic proportions as that found in the atmosphere. Once a plant or animal dies, it ceases to absorb any carbon-14 and the ratio of carbon-14 to the stable isotopes within its remains decreases over time.

By measuring the percentage of carbon-14 in a sample requiring dating and comparing it to that of a living organism an archeologist may obtain an estimate of the time of death and hence the age of the artifact.

The equation

t = [ln (Nf/No) / (-0.693)] * t1/2

provides this estimate. In this equation ln (Nf/No) is the natural logarithm of the percent carbon-14 in the sample compared to the percent carbon-14 in living tissue and t1/2 is the half-life of carbon-14.

Radiocarbon dating assumes that the percentage of atmospheric carbon-14 to be constant. Comparison of dates obtained by carbon dating and methods such as dendrochronolgy (dating using tree-rings) has shown that this is not the case. The amount of cosmic rays acting on the Earth’s upper atmosphere and consequently the percentage of atmospheric carbon-14 has varied over time.  Most dates are now corrected to take account for this variation.

Another intrinsic source of error in radiocarbon dating is the ability of plant to take up carbon-14. Some plants selectively exclude carbon-14 and dates from remains of such plants or animals eating them would appear older than they actually are.

When taking samples for radiocarbon dating scientists must take care to avoid contamination with older or newer organic material. Even the choice of packaging material for the sample may affect the accuracy of a date. Taking a number of samples from an artifact to obtain a range of resuls improves the accuracy of a date.

Some samples are unsuitable for radiocarbon dating. Besides the obvious such as metallic objects or ceramics, which contain no organic carbon remains of oceanic-based plants and animals are unsuitable. Oceanic plants derive much of their carbon from carbonates so they have a naturally low carbon-14 percentage, which they transmit up the food chain. Material over 50,000 years old  is also unsuitable as by the time carbon-14 as gone though 8-9 half-lives, less than 1% remains making any measurement in accurate.

Used with knowledge of its limitation radiocarbon dating still provides valuable dating tool for archeologists and paleontologists.

Reference sources:

NDT resource center

The Regents of the University of California

Carleton College website