Radiocarbon calibration curves

Radiocarbon dating, also known as carbon-14 dating, is one of several methods referred to as absolute dating – but here’s an interesting fact. There is nothing ‘absolute’ about a radiocarbon date!

Radiocarbon dating does not produce an exact calendar age nor can it determine the date of a sample to a single year. Rather, it is the measure of the isotope C-14 concentration value of the sample or artefact calculated according to set parameters – this is called the conventional radiocarbon age (CRA). The CRA then needs to be interpreted by experts to infer a calendar date. So how do we know that the end date is reliable?

Calibration curves and wiggles

Because C-14 decays at a set rate over time, scientists can measure the C-14 in a sample and calculate how long since that sample stopped interacting with carbon in the atmosphere. In the case of an animal, this would be when the animal died. However, a CRA is not an exact calendar age.

C-14 concentrations fluctuate over time and globally, so experts take the measured CRA and calculate a date using datasets called calibration curves.

Calibration curves have been built up over time using C-14 measurements collected from materials of known age. There are different curves – for example, a shell CRA from a marine environment will have a marine calibration curve applied to get a calendar date. A bone found on land in New Zealand would use the southern hemisphere atmospheric curve – sometimes also referred to as a terrestrial curve.

Experts also use a statistical technique called Bayesian modelling to obtain more accurate radiocarbon dates.

How are calibration curves developed?

Atmospheric curves were assembled from C-14 measurements of tree rings previously dated by dendrochronology (tree ring dating). Every year a tree grows, it forms a new ring, with the first ring in the trunk of a tree dating back to when it first started growing.

The southern hemisphere curve was built up predominantly from ancient kauri tree rings. Associate Professor Fiona Petchey, Director of the Waikato Radiocarbon Dating Lab, says, “Kauri in New Zealand is a perfect material: long-lived trees are found in swamps and have been preserved going back through time.”

Assembling a marine curve is more complex as there aren’t materials like trees/wood that preserve long records. The marine curve has been built up from different materials from all over the world, including corals and sediment layers. It is also complicated by the marine reservoir effect (MRE) – the offset from the terrestrial calibration curve mainly caused by the slow mixing of waters in the deep ocean, resulting in regional variation.

All of the current calibration curves are complicated by other parameters. A key complication is that the proportion of C-14 to C-12 and C-13 in the atmosphere is not uniform over time or space or in its uptake by different organisms. These C-14 fluctuations mean different regions have different atmospheric and marine values over time, impacting the dating of artefacts, so research to refine calibration curves and offsets is continual in radiocarbon dating.

Refining calibration curves

Why do calibration curves matter? Why do experts continue to challenge and refine these curves? Accurate dates matter for scientists, archaeologists and anthropologists looking at the past to document and understand past peoples, their migration patterns and historical environmental conditions and climates.

Fiona Petchey’s work at the Waikato Radiocarbon Dating Laboratory involves dating samples for third parties and her own research to refine C-14 dating for artefacts found in Aotearoa New Zealand and the Pacific by improving the accuracy of curves and offsets used for these areas.