14C Dating in the Iron Age

Our ability to date the archaeology of Iron Age people stems from what happens in outer space. Particles from space continuously enter our atmosphere and interact with the nitrogen (approximately 80% of our atmosphere) to form a tiny amount of radioactive carbon called radiocarbon or 14C. These radiocarbon atoms very quickly bond with oxygen (approximately 20% of our atmosphere) to form radiocarbon dioxide (14CO2), which constitutes a very small proportion of the total atmospheric carbon dioxide. This very quickly mixes with all the other stable (non-radioactive) carbon dioxide in the atmosphere (12CO2 and 13CO2). The fact that radioactive carbon is produced might seem frightening but the quantity is very small: there is only around one radioactive carbon atom for every million million stable carbon atoms in our atmosphere.

Radiocarbon half-life

A small proportion of the carbon dioxide in the atmosphere (including the 14CO2) is taken up by terrestrial plant life in the process called photosynthesis, whereby the carbon dioxide is transformed into plant material and at the same time, oxygen, which sustains life on Earth, is produced. Thus, the plants become labelled with radioactive carbon dioxide. Plants are eaten by animals and so they too become labelled. While the plants and animals remain alive, they continue to take up radiocarbon dioxide through photosynthesis and direct consumption of plant material, respectively. Therefore, they retain a fixed concentration of radiocarbon in their tissues, similar to that of their contemporary atmosphere. However, when the plants or animals die, they can no longer take up new carbon into their tissues.

Because the radiocarbon atoms are radioactive, they undergo what is termed radioactive decay, whereby they convert back to nitrogen. The important point here is that this radioactive decay occurs at a fixed rate, whereby the amount of radiocarbon in the plant or animal halves in a fixed period of time. This is termed the half-life and for radiocarbon, this is 5730 years.

We can use this to date how long ago something (or someone) died. Because a dead plant or animal cannot continue to take up radiocarbon, the amount within it will start to decline as soon as the tissue dies. So, if we know how much radiocarbon should have been in a sample in the first place, and we also know how much radiocarbon is in the sample now, we can use our knowledge of how fast radiocarbon decays (5730 years for the amount to half) to calculate how much time passed since the organism died. We know how much was in the plant or animal to start with because radiocarbon is formed at an almost constant rate, so by measuring the amount of radiocarbon in living tissue and comparing it to the amount in our sample, we can calculate the time that has elapsed since death, which is reported as a conventional radiocarbon age (CRA) before present (BP). BP is the year 1950.