All in good time: radiocarbon dating

As archaeologists we are used to studying time and placing the archaeological material we excavate into its correct sequence. The identification of material allows us to order the past into a relative sequence, and occasionally certain objects such as coins provide a good level of precision within the historic period. However, if we have no artefacts or are dealing with time before the historic period, we have several clever scientific techniques in our armoury — one of which is radiocarbon dating.

Radiocarbon dating is a technique that was developed from the 1940s onwards and can be used to date anything that was once living (carbon-based life forms), or which was made from organic material. Material frequently found on archaeological sites suitable for radiocarbon dating includes the butchered remains of animals, objects made from bone, or food residues such as dried burnt stew, which is sometimes found lining the sides of cooking pots.

Radiocarbon (14C) is formed in the earth’s atmosphere and is absorbed by plants, where it enters the food chain – as part of the diet of animals and humans. When a living organism dies, they stop taking in carbon, of which there are two types: stable carbon (12C) and radioactive carbon (14C). The latter starts to decay after death, and by measuring the ratio between the two we are able to determine the age of the sample. This involves physics (to understand how carbon atoms decay), chemistry (the composition of the material), some clever maths (statistics) to calculate the date (its range, any possible errors etc.) and an archaeologist (the fun part) to interpret the result and what it means.

We use radiocarbon dating to assign a precise date to an event such as the burial of a person. However, in general radiocarbon provides us not with a single year but with a date range in which an event probably occurred.

Radiocarbon was used to date two otherwise undated human burials to the early Mesolithic

However, over the last 25 years new methods based on mathematical probability – known as Bayes’ Theorem have been used to develop programmes such as OxCal. Put simply, the probable date range of an event can be reduced if we are aware of other useful information. For instance, if we know from fieldwork the order in which a particular group of graves were dug in the past, this allows us to combine the basic radiocarbon dates with known archaeological information, allowing the production of more precise chronologies for a site. For example, we can model a site’s beginning (construction), its end, and its period of use.

We routinely use Bayesian modelling of radiocarbon dates as part of our analysis work when we publish the results of our excavations. Recent examples include work on the A477 St Clears to Red Roses road scheme, on the South Wales Gas Pipeline, where the technique enabled subtle chronological modelling of activity at a henge and within the area surrounding a Bronze Age barrow. Bayesian modelling was also used to provide a more precise chronology for a newly discovered Late Bronze Age ring work at Hill Barton in Devon.  

The henge at Vaynor farm – found on the route of the SW gas pipeline and subjected to r
The henge at Vaynor farm – found on the route of the SW gas pipeline and subjected to radiocarbon analysis

One recent development pioneered at the University of Bristol is the radiocarbon dating of absorbed fats (lipids) that are found in the walls of unglazed pottery vessels. This technique allows us to directly radiocarbon date residues left following the cooking and consumption of food. This technique is still relatively new but the implications for improving our understanding of the development and spread of styles of pottery are huge, not least because pottery is usually the most common type of artefact we find on excavation sites. This new technique will eventually become widespread, especially in areas of the UK with acidic geology, where other organic remains such as animal bone are typically poorly preserved.  

Increasingly, and with the application of Bayesian statistics, radiocarbon dating is not just about time but how we write the history of the past.

Alistair Barclay

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