Radiocarbon dating

Radiocarbon dating is a scientific method used to determine the age of organic materials. Organic material refers to anything that was once part of a living organism, such as wood, bone, or plant seeds.

This process relies on the unique properties of radiocarbon, which is a radioactive isotope of carbon. By measuring how much radiocarbon remains in a sample, scientists can calculate when the organism died. This discovery transformed archaeology, leading to what many call the "radiocarbon revolution."

The process begins high in the Earth's atmosphere. Cosmic rays from space strike atmospheric nitrogen atoms, causing a nuclear reaction. This reaction transforms nitrogen-14 into radiocarbon, or carbon-14.

This radiocarbon quickly combines with oxygen to form radioactive carbon dioxide. Plants absorb this carbon dioxide during photosynthesis. Animals then acquire the radiocarbon by eating those plants. As long as an organism is alive, it constantly exchanges carbon with its environment. This keeps the ratio of radiocarbon in its body equal to the ratio in the atmosphere.

Once an organism dies, the exchange of carbon stops. The radiocarbon trapped within the biological material begins to undergo radioactive decay. During this decay, a neutron in the carbon-14 nucleus changes into a proton. This process emits a beta particle and an electron antineutrino, turning the atom into stable carbon-12.

Because this decay happens at a constant, predictable rate, it acts like a ticking clock. The older a sample is, the less radiocarbon will be left to detect. Scientists use the known rate of decay to work backward and estimate the time of death.

Researchers use the concept of a "half-life" to measure this decay. The half-life of radiocarbon is approximately 5,730 years. This is the specific amount of time required for half of a given sample to decay. For example, after 5,730 years, only 50 percent of the original radiocarbon remains. After 11,400 years, only one quarter remains. This predictable pattern allows scientists to date objects up to about 50,000 years old. Beyond this limit, the remaining radiocarbon becomes too small to measure reliably.

The history of this method involves several key scientific figures. In 1939, Martin Kamen and Samuel Ruben began studying isotopes for biomedical research. Later, Serge A. Korff predicted that cosmic rays would create radiocarbon in the upper atmosphere. Willard Libby, working at the University of Chicago, developed the dating method in the late 1940s. He proved his theory by testing methane from sewage and samples from Egyptian tombs.

His successful work earned him the Nobel Prize in Chemistry in 1960.

Modern scientists must account for changes in atmospheric carbon levels to ensure accuracy. These changes are corrected using a calibration curve, known as IntCal. One major complication is the burning of fossil fuels, such as coal and oil. Because fossil fuels are so old, they contain almost no radiocarbon. When they are burned, they release carbon dioxide that dilutes the radiocarbon in the atmosphere. Additionally, nuclear tests in the 1950s and 1960s significantly increased atmospheric radiocarbon levels.

Scientists must use these curves to convert radiocarbon years into actual calendar years.

Today, the preferred method for measurement is accelerator mass spectrometry. Older methods used beta-counting devices to detect radiation emitted by decaying atoms. In contrast, an accelerator mass spectrometer counts every single atom in a sample.

This technology is much faster and requires much smaller samples. It can even analyze something as tiny as an individual plant seed. This precision allows archaeologists to compare dates across vast distances and study major historical shifts.