Establishing the dates

Dating skeletons helps us understand when a person lived, worked and died. Radiocarbon dating is the most common and well-known dating technique used by archaeologists. It is based on the decay of carbon-14 (¹⁴C), a radioactive isotope which occurs naturally in the atmosphere.

The carbon cycle

Carbon is present in all living things and in the atmosphere as carbon dioxide. Carbon dioxide is produced by plants and animals, including humans, through respiration, and also released by combustion (for example the burning of fossil fuels). Plants use sunlight to convert water and carbon dioxide into oxygen and carbohydrates. Carbon compounds in plants are eaten by animals and passed through the food chain. When plants and animals die and decompose, carbon compounds in their bodies are re-released into the atmosphere as carbon dioxide.

^{The carbon cycle © Peter Brown, Durham University}

Radio-carbon dating

At any given time, the ratio of ¹⁴C to normal carbon (¹²C) in the air and in all living things is fairly constant. Once a living organism dies the ¹⁴C gradually decays away. It takes around 5,700 years after death for the amount of ¹⁴C in organic remains to halve, but the level of ¹²C remains the same. By measuring the ratio of ¹⁴C to ¹²C in a skeleton it is possible to estimate a year of death. This method can only be used to date organic remains up to around 60,000 years old. Radiocarbon dates are usually written as years BP (before present), which actually means before 1950 (so as to stay consistent with earlier research).

In the case of the individuals discovered at Palace Green, radio-carbon dating was used to establish their date, using the teeth of two individuals who had previously been identified as pipe smokers. The roots of the first and third permanent molar were sampled. These teeth form around 10 years apart.

This gave two dates separated by 10 years for each individual. The possible dates were then narrowed down using a process called ‘wiggle matching’. Because the team had already established roughly how old the individuals were when they died, we could consider the amount of time they had lived after the formation of their teeth.

From this analysis we established that the most likely date for the mass graves was in the mid-1600s.