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 the challenges of C-14 radiocarbon dating
Director of the Radiocarbon Dating Laboratory at the University of Waikato Associate Professor Fiona Petchey talks about radiocarbon dating calibration curves and the challenges they present.
A calibration curve is a technique used to match a C-14 concentration value with samples of a known age (such as a tree ring) to determine a more accurate calendar age.
Questions for discussion
What organic materials have been predominantly used to build up the atmospheric and marine calibration curves? Why were these materials used?
Why is there a different calibration curve for marine samples and artefacts?
Fiona says that one factor that enables radiocarbon labs to be able to build a calibration curve is funding. What do you think she means by this?
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.
Radioactive decay
The graph shows a radioactive decay curve for carbon-14. A plot of the activity of the sample versus time shows that the half-life for carbon-14 is 5,730 years.
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.
Tree rings
Growth rings on a tree cross section. The youngest/newest growth is on the outer rings. Tree ring dating is called dendrochronology.
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 reservoi r 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.
Marine reservoir effect
The marine reservoir effect (MRE) affects radiocarbon dating because the carbon consumed by organisms like shellfish in the ocean is older than the organism and further complications occur from the geographical location and make-up where the organism lived.
Adapted from Alves, E. Q., Macario, K., Ascough, P., & Bronk Ramsey, C. (2018). The worldwide marine radiocarbon reservoir effect: Definitions, mechanisms, and prospects. Reviews of Geophysics, 56, 278–305.
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
Sampling shell and charcoal
Dr Fiona Petchey (left) and C-14 pretreatment technician Katy Anderson on location at a Māori midden. Fiona is sampling shell and charcoal for radiocarbon dating.
Why do calibration curves matter ? Why do experts continue to challenge and refine these curves? Accurate dates matt er 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.
Related content
Learn about wiggle matching and the complexities of radiocarbon dating through research into the volcanic eruption that created Lake Taupō: Explosive lies – how volcanoes can lie about their age, and what it means for us.
This interactive explores the C-14 carbon dating process.
Work to refine calibration curves in the South Pacific has led to more precise dates for the settlement of the North and South Islands in Aotearoa New Zealand. Read more in Ancestral Māori adapted quickly in the face of rapid climate change.
In the 1950s, New Zealand scientist Athol Rafter noticed an increase in radiocarbon in the atmosphere as a result of nuclear weapons testing at that time. This radiocarbon increase was different depending on proximity and other factors related to the testing. Therefore, multiple atmospheric nuclear offsets or ‘bomb curves’ were developed for different geographical zones. These are used for dating artefacts from 1950 onwards.
Activity ideas
The activity Investigating middens helps students understand what ancient middens can tell us about past lives and what discarded rubbish can tell us indirectly about the environment. You could adapt the idea so students also identify the oc rubbish that would survive into the future and could be dated by C-14 methods.
Useful links
Visit the Oxford Radiocarbon Accelerator Unit website to learn more about radiocarbon calibration.
