Dating Techniques

Radiocarbon is used to date charcoal, wood, and other biological materials. The range of conventional radiocarbon dating is 30,000–40,000 years, but with sensitive instrumentation this range can be extended to 70,000 years. Radiocarbon (¹⁴C) is a radioactive form of the element carbon. It decays spontaneously into nitrogen-14 (¹⁴N). Plants get most of their carbon from the air in the form of carbon dioxide, and animals get most of their carbon from plants (or from animals that eat plants). Atoms of ¹⁴C and of a non-radioactive form of carbon, ¹²C, are equally likely to be incorporated into living organisms—there is no discrimination. While a plant or animal is alive, the ratio of ¹⁴C/¹²C in its body will be nearly the same as the ¹⁴C/¹²C ratio in the atmosphere. When the organism dies, however, its body stops incorporating new carbon. The ratio will then begin to change as the ¹⁴C in the dead organism decays into ¹⁴N. The rate at which this process occurs is called the half-life. This is the time required for half of the ¹⁴C to decay into ¹⁴N. The half-life of ¹⁴C is 5,730 years. Scientists can tell how many years have elapsed since an organism died by comparing the ¹⁴C/¹²C ratio in the remains with the ratio in the atmosphere. This allows us to determine how much ¹⁴C has formed since the death of the organism.

A problem with radiocarbon dating is that diagenic (after death) contamination of a specimen from soil, water, etc. can add carbon to the sample and affect the measured ratios. This can lead to inaccurate dates. Another problem lies with the assumptions associated with radiocarbon dating. One assumption is that the ¹⁴C/¹²C ratio in the atmosphere is constant though time. This is not completely true. Although ¹⁴C levels can be measured in tree rings and used to correct for the ¹⁴C/¹²C ratio in the atmosphere at the time the organism died, and can even be used to calibrate some dates directly, radiocarbon remains a more useful relative dating technique than an absolute one.