Improved carbon dating tool unlocks secrets of the past

Researchers at the Universities of Sheffield, Belfast, Bristol, Glasgow, Oxford, St Andrews, and Historic England, plus international colleagues, used measurements from almost 15,000 samples from objects dating back as far as 60,000 years ago, as part of a seven-year project.

Using these measurements, they created new international radiocarbon calibration (IntCal) curves, which are fundamental across the scientific spectrum for accurately dating artefacts and making predictions about the future.

Radiocarbon dating is a vital tool in fields such as archaeology and geoscience as it is used to date everything from the oldest modern human bones to historic climate patterns. 

Archaeologists can use that knowledge to restore historic monuments or study the demise of the Neanderthals, while geoscientists on the Intergovernmental Panel on Climate Change (IPCC) rely upon the curves to find out about what the climate was like in the past to better understand and prepare for future changes.

“Radiocarbon dating has revolutionised the field of archaeology and environmental science,” said Professor Paula Reimer, from Queen’s University Belfast and head of the IntCal project. “As we improve the calibration curve, we learn more about our history. The IntCal calibration curves are key in helping answer big questions about the environment and our place within it.”

The team of researchers have developed three curves dependent upon where the object to be dated is found. The new curves, to be published in Radiocarbon, are IntCal20 for the Northern Hemisphere, SHCal20 for the Southern Hemisphere, and Marine20 for the world’s oceans. 

Previous radiocarbon calibration curves, which were developed over the past 50 years, relied heavily on measurements taken from chunks of wood covering 10 to 20 years big enough to be tested for radiocarbon.

But advances in the testing methods mean the updated curves instead use tiny samples, such as tree-rings covering just single years, that provide previously impossible precision and detail in the new calibration curves. Furthermore, improvements in understanding of the carbon cycle have meant the curves have now been extended all the way to the limit of the radiocarbon technique 55,000 years ago. 

Radiocarbon dating is the most frequently used approach for dating the last 55,000 years and underpins archaeological and environmental science. It depends upon two isotopes of carbon called stable 12C and radioactive 14C, which were first developed in 1949. 

While a plant or animal is alive it takes in new carbon, so has the same ratio of these isotopes as the atmosphere at the time. But once an organism dies it stops taking in new carbon – the stable 12C remains but the 14C decays at a known rate. By measuring the ratio of 14C to 12C left in an object, the date of its death can be estimated. 

If the level of atmospheric 14C were constant, this would be easy. However, it has fluctuated significantly throughout history. In order to date organisms precisely, scientists need a reliable historical record of its variation to accurately transform 14C measurements into calendar ages. The researchers said that the new IntCal curves provide this link. 

The curves are created based on collecting a huge number of archives that store past radiocarbon but can also be dated using another method. Such archives include tree-rings from up to 14,000 years ago, stalagmites found in caves, corals from the sea, and cores drilled from lake and ocean sediments. In total, the new curves were based upon almost 15,000 measurements of radiocarbon taken from objects as old as 60,000 years.

“Accurate and high-precision radiocarbon dating underpins the public’s enjoyment of the historic environment and enables better preservation and protection,” said Alex Bayliss, the head of Scientific Dating at Historic England. “The new curves have internationally important implications for archaeological methodology, and for practices in conservation and understanding of wooden built heritage.”

In May this year, archaeologists discovered new medieval and Roman sites while working from home by analysing data from lidar scans.