In the late 1940s, a chemist at the University of Chicago suspected that he could calculate the age of an object using simple calculations based on its carbon-14 content (or that of the surrounding material). So Willard Libby borrowed a few random objects from museums with verified ages—a sandal found in a cave in Oregon, linen wrappings from the Dead Sea Scrolls and even a charred piece of bread from Pompeii—and tested his theory. The dates matched, and the radiocarbon dating revolution was launched. Libby received a Nobel Prize for the discovery in1960.
Before radiocarbon dating, humans’ ability to date prehistoric objects was extremely limited.
“You could argue that anything before the 6th century B.C. was pretty much a case of educated guesswork,” says Sturt Manning, director of the Cornell Tree Ring Laboratory. “When did the Neolithic show up in Britain? When did people arrive in North America? There was zero basis of knowledge other than it was a really long time ago.”
How Radiocarbon Dating Works
Radiocarbon dating has gone through waves of refinement that make it much more accurate and nimble today, but the basic premise remains the same. Carbon-14 is a radioactive, unstable form of carbon that has two extra neutrons. All living things, from plants to animals, absorb carbon-14. When they die, they stop absorbing it, and the remaining carbon-14 decays at a known rate, or half-life (5,730 years). By measuring the amount of carbon-14 left, scientists can estimate how long ago the organism died.
Carbon-14 dating isn't limited to humans and other animals. Some objects, like the sandal and charred bread that Libby originally tested, were made of plant material, so the method can estimate their age. If an object (like a clay pot) was not made from organic material, scientists can apply radiocarbon dating to organic material found around it.
How Radiocarbon Dating Has Improved
By the 1960s, scientists discovered some complications in the process. They realized that the amount of radiocarbon in the air varies over time, based on solar activity, Manning explains. To control for that, they needed something to calibrate the radiocarbon method against. They settled on tree rings.
The newer method allowed scientists to independently date things—accurately—and created a new understanding of prehistory. For example, they discovered that the Neolithic period is thousands of years older than previously believed.
In one of the first big uses of radiocarbon dating in North America, scientists at a dig in Nevada discovered that the bones of the people who lived there were from a completely different time period than the animals found there. Archeologists had assumed that the animals and people coexisted, but this new information provided an accurate timeline for what is now Tule Springs Fossil Beds National Monument.
New Tech Allows Dating Very Small Samples
By the 1980s, scientists had found another tool that further revolutionized radiocarbon dating: accelerator mass spectrometry (AMS). Although expensive, AMS allowed scientists to more precisely date objects with just a fraction of the previous sample size.
“All sorts of things could now be dated that couldn’t in the past,” Manning says. “That radically changed the ability to ask questions in archeology and get the answer you wanted, such as, ‘When were people living in this particular cave?’”
But they still weren’t able to pinpoint longer stretches of tens of thousands of years.
That started to become possible when scientists incorporated Bayesian statistical analysis into their method. Today, researchers can establish the chronological order and calendar age among past events, which has had far-reaching impacts.
Fields Transformed by Radiocarbon Dating
Archaeology
The impact on the field of archeology was instantaneous, and remains profound today. For example, Manning was working at a major indigenous site near Toronto that had been confidently dated at about 1500-1530. But when his team ran samples from the area using radiocarbon dating, they discovered the time period was much later, around the year 1600.
This meant that interactions between Indigenous peoples and the introduction of European goods occurred much later than previously thought, prompting a reevaluation of that period across northeastern North America.
Oceanography
Scientists have used carbon-14 to study sea life, including plankton. They have even learned how ocean currents circulate around the planet by dating traces of sea life that flowed through them.
Climate science
Radiocarbon dating has been critical in understanding the role of carbon emissions in climate change. Fossil fuels are composed of decomposing plants and animals that are millions of years old, so their carbon-14 levels have nearly completely decayed. When fossil fuels are burned and released into the atmosphere, they dilute carbon-14 levels, providing clear evidence of human impact on the environment.
History
In addition to transforming our understanding of prehistory, radiocarbon dating has been used to verify or refine more recent historical events. For example, researchers have used the technique to establish a more accurate chronology of Egyptian pharaohs dating back to the reign of Djoser, which began between 2691 and 2625 B.C.
Health
Researchers use carbon dating to track different diseases. For example, they can find out when Alzheimer’s plaques started to form in the brain, how chemotherapy shrinks cancer, and the progression of tumors.
Forensics
Radiocarbon dating has even found use in law enforcement, helping to solve cases ranging from homicides to illegal logging. It can help confirm time of death, as well as which year a tree was cut down.
