Professor Tom Higham getting samples ready to load into the accelerator mass spectrometer (AMS). (Images: courtesy of University of Oxford)
As highlighted countless times in CA over the years (see CA 334 for Joe Flatman’s summary), science has revolutionised archaeological research. But what exactly happens inside the laboratories, away from the field? To those outside the confines of the lab it is often a mystery, and sometimes taken for granted. For this month’s ‘Science Notes’, we went to the Oxford Radiocarbon Accelerator Unit (ORAU ) to explore the enigmatic process behind radiocarbon (14C) dating, sitting down with Professor Tom Higham, the deputy director of ORAU, and Dr David Chivall, the lab’s chemistry manager, to discuss ORAU’s history, laboratory practices, and current research, as well as future prospects.
Founded at the University of Oxford in 1955, the Research Laboratory for Archaeology and the History of Art (RLAHA) – out of which ORAU is based – was one of the first places in the UK to create a laboratory dedicated to the application of science to archaeology. It was not until the 1980s, however, at the dawn of the new era of accelerator mass spectrometry (AMS), that ORAU began. It was one of the world’s first AMS facilities. Today, it is the only radiocarbon lab in the world that specialises in archaeological work, particularly the dating of bone, and over the past two decades, ORAU has seen a boom in the number of samples coming into the lab – reflecting the increased use of radiocarbon dating in archaeological contexts. In 2001, when Tom first came to ORAU, the lab had only dated approximately 10,000 samples since its inception; now the lab dates about 2,500 samples a year.
To meet these increasing demands, ORAU is currently undergoing significant renovations. The lab is being completely redesigned and a new AMS will be installed later this year. Apart from allowing the lab to date even more samples, the new machine will also be more stable and bring higher precision to results.
Dave walked us through the lab process. When a sample first arrives at the facility it is assessed for its dating suitability, which is dependent on the material type, its estimated age, and its degree of preservation. If suitable, the sample then undergoes a series of chemical pre-treatment steps in order to isolate a selected component of the material that is most likely to only contain carbon of the same age as the sample. There are over 50 different pre-treatment methods, depending on the type of organic material being analysed. But if it was once living, it can be dated.
Once the pre-treatment is done, a small amount of the sample is used to analyse the stable carbon isotopes and nitrogen values. While this process informs on the diet and environment of the sample, it is primarily used to identify any traces of marine (or aquatic) trophic systems, which may indicate that the sample is prone to a ‘reservoir effect’. A reservoir effect is caused by large amounts of ancient carbon remaining in the marine system for longer than in the atmosphere – marine organisms, and terrestrial organisms that have a primarily marine diet, take up more of this ancient carbon, skewing their 14C levels. This means they will seem older in date than they actually are.
The current accelerator mass spectrometer
(AMS) at ORAU. It will be replaced later this year by a new machine that is more efficient and precise.
The sample is then converted into graphite and put into the AMS, which acts rather like a giant sieve, targeting only carbon ions. The ratio of 14C (which degrades at a regular rate based on its half-life) to 13C (which remains constant over time) is measured and compared against standard reference materials to calculate the age.
There were many aspects of radiocarbon dating that Tom and Dave wanted to highlight, one of which was that it does not work across the board – its accuracy and precision is dependent on many factors, not least the production of radiocarbon in the atmosphere. Throughout history, there have been periods when the atmospheric level of 14C has remained relatively constant, and without any fluctuations organisms living during this period are unable to be precisely radiocarbon dated. The most pronounced incidence of this in history is called the Hallstatt plateau, between c.800 BC and 400 BC, roughly spanning the late Bronze Age through to the middle of the Iron Age in Britain.
As Tom emphasised, being able to account for variables such as sample preservation, reservoir effects, and time-period inconsistencies means that archaeological context is key in radiocarbon dating – it is a collaborative process and, in order for it to be most effective, its limitations as well as its usefulness need to be understood. But the method is also becoming increasingly reliable and precise, as new techniques are developed. The advances currently being researched at ORAU will be highlighted in next month’s ‘Science Notes’ – watch this space!
This article was published in CA 335.