Researchers Unlock Secrets of the Past with New International Carbon Dating Standard
Data produced by the University of Arizona Accelerator Mass Spectrometry Laboratory are included in the new calibration curves.
P. Brewer/University of Arizona Laboratory of Tree-Ring Research
Locked inside every slice of tree or piece of fossilized bone or ancient article of clothing is a story.
To pin down where those stories fit in the larger history of the world, scientists rely on radiocarbon dating, a technique that is now set to become more accurate than ever, thanks to research done at the University of Arizona, Lawrence Livermore National Laboratory, the University of California, Woods Hole Oceanographic Institution and Cornell University, in collaboration with international partners.
Bristlecone pine tree rings from the Laboratory of Tree Ring Research. Measurements of radiocarbon in single rings from ancient trees like these now cover the period 1700-1500 B.C. in IntCal20.
P. Brewer, Laboratory of Tree-Ring Research, University of Arizona
In a series of three papers, the team of researchers have recalculated and adjusted the international radiocarbon calibration, or IntCal, curves, which are tools used by researchers across many disciplines to accurately date artifacts and make predictions about the future.
Radiocarbon dating works by assessing the ratio of different kinds, or isotopes, of carbon atoms in an object. The method allows archaeologists and environmental scientists to date everything from the oldest modern human bones to historic climate patterns.
"As we improve the calibration curve, we learn more about our history," said Paula Reimer, head of the IntCal project and a professor at Queen's University Belfast. "The IntCal calibration curves are key to helping answer big questions about the environment and our place within it."
The research team used measurements from over 15,000 samples from objects dating back as far as 60,000 years ago, as part of a seven-year project.
Charlotte Pearson
"It's hard to overstate the importance of these new IntCal curves for improving what we know about our past," said Charlotte Pearson, UArizona assistant professor of dendrochronology, anthropology and geosciences, and a member of the IntCal Working Group.
Archaeologists can use the curves to date ancient monuments or study the demise of the Neanderthals, while geoscientists on the Intergovernmental Panel on Climate Change rely upon the curves to find out about what the climate was like in the past to better understand and prepare for future changes.
The team of researchers has developed three curves, based upon where the object to be dated is found – IntCal20 for the northern hemisphere, SHCal20 for the southern hemisphere and Marine20 for the world's oceans.
The new curves are published in the journal Radiocarbon, which is published by the University of Arizona in partnership with Cambridge University Press. The journal began in 1959 and has been published by UArizona since 1989.
Timothy Jull
"The presence of the journal here reflects the great importance of radiocarbon dating at the University of Arizona, which goes back to the mid-1950s when the first lab was established by professor Emil Haury," said UArizona geosciences professor Timothy Jull. "Great changes in technology have occurred since then. IntCal has become an essential tool for accurate calibration of radiocarbon dates and gradually improved over the last 35 years."
The previous radiocarbon calibration curves, developed over the past 50 years, were heavily reliant upon measurements taken from chunks of wood covering 10 to 20 years of consecutive tree ring growth, so they contained enough material to be tested for radiocarbon.
The updated curves instead use tiny samples, such as tree rings covering just single years, that provide previously impossible precision and detail. Thanks to improvements in understanding of the carbon cycle, the curves have now been extended all the way to the approximate limit of the radiocarbon technique, which is 55,000 years ago. Any radioactive carbon older than about 55,000 years will have already decayed.
"This is a really exciting time for radiocarbon research," Pearson said. "Radiocarbon from individual calendar-dated tree rings is not only giving us a more accurate record for calibration but providing new ways to synchronize past timelines and uncover past solar activity. The newly calculated IntCal curves include high-quality data from a range of sources and extend further back in time than ever before."
Pearson and her team recently used annual radiocarbon data from tree rings to constrain the date of the ancient Thera volcano eruption – one of the largest eruptions humanity has ever witnessed.
Radiocarbon dating is the most frequently used approach for dating the last 55,000 years and underpins archaeological and environmental science. It was first developed in 1949. It depends upon two flavors, or isotopes, of carbon called stable carbon – containing six protons – and radioactive carbon – containing eight protons.
While a plant or animal is alive it takes in new carbon, so it 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 carbon remains, but the radioactive carbon decays at a known rate. By measuring the ratio of radioactive carbon to stable carbon left in an object, the date of its death can be estimated.
If the level of atmospheric radioactive carbon 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 radioactive carbon measurements into calendar ages. 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.
