Radiocarbon dating

Radiocarbon dating, also called carbon dating or carbon-14 dating, is a way to find out how old something is if it was once alive, like a piece of wood or bone. It works by measuring a special kind of carbon called radiocarbon. This method helps us learn when plants and animals lived long ago.

The idea for radiocarbon dating began in the late 1940s at the University of Chicago by Willard Libby. Radiocarbon is made in the Earth's atmosphere when cosmic rays hit nitrogen. It then becomes carbon dioxide, which plants take in through photosynthesis, and animals get it by eating plants. When a plant or animal dies, it stops getting new carbon, and the radiocarbon inside slowly fades away. By measuring how much radiocarbon is left, we can estimate when the organism died. The half-life of radiocarbon is about 5,730 years, so this method works best for objects up to about 50,000 years old.

Over time, scientists have learned more about how much radiocarbon has been in the air. They use this information to get better age estimates. There are also some things that can change the amount of radiocarbon, like burning fossil fuels or nuclear testing. Today, scientists use a technique called accelerator mass spectrometry to count radiocarbon atoms directly.

Radiocarbon dating changed the field of archaeology greatly. It lets experts date old things more accurately and compare events from far apart. It helped date big changes in history, like the end of the last ice age and the start of the Neolithic and Bronze Age in different places. Because of this, some call it the “radiocarbon revolution.”

Background

History

In 1939, scientists Martin Kamen and Samuel Ruben began experiments to see if some elements could help in medical research. They made carbon-14 and found its half-life was longer than people thought. Later, scientist Serge A. Korff said that cosmic rays could create carbon-14 in the atmosphere. During World War II, Willard Libby thought carbon-14 might help figure out how old things are.

In 1945, Libby moved to the University of Chicago and began working on radiocarbon dating. In 1946, he published a paper saying that living things might contain carbon-14. Libby and his team tested this using methane from sewage works. They found that this methane had carbon-14, unlike methane from petroleum. Their results were published in 1947, showing that it might be possible to date materials that contain carbon.

Libby and James Arnold tested their idea by dating samples with known ages. They dated samples from ancient Egyptian tombs and got results close to the known dates. These results were published in 1949. Within 11 years, many labs around the world were using radiocarbon dating. In 1960, Libby won the Nobel Prize in Chemistry for this work.

Physical and chemical details

Main article: Carbon-14

Carbon comes in three types. Carbon-12 and carbon-13 are stable and not radioactive. Carbon-14, also called radiocarbon, is radioactive. The half-life of carbon-14 is about 5,730 years. Carbon-14 is made in the atmosphere by cosmic rays hitting nitrogen atoms. These cosmic rays create neutrons that turn nitrogen-14 into carbon-14. Carbon-14 combines with oxygen to form carbon dioxide, which plants absorb through photosynthesis. Animals then eat these plants, spreading radiocarbon throughout the natural world. The ratio of carbon-14 to carbon-12 in the air is about 1 part carbon-14 to 1 trillion parts carbon-12.

When a living thing dies, it stops taking in new carbon-14. The carbon-14 inside it continues to decay, so the ratio of carbon-14 to carbon-12 gets smaller over time. By measuring this ratio, scientists can figure out how long ago the organism died. The decay follows a known pattern, allowing scientists to calculate the age of the sample.

Principles

While a plant or animal is alive, it keeps a balance of carbon-14 the same as in the air or ocean around it. When it dies, it stops taking in new carbon-14, but the carbon-14 already inside continues to decay. This means the ratio of carbon-14 to carbon-12 gets smaller over time. Because we know how fast carbon-14 decays, we can use this to figure out how long ago the organism died.

Scientists use a special formula to calculate this. They know how much carbon-14 was in the air when the organism was alive and how much is left now. By measuring the current amount of carbon-14, they can calculate how many years have passed since the organism died. The half-life of carbon-14 is about 5,700 years, meaning after that time, only half of the original carbon-14 will be left. This helps scientists estimate ages for things like ancient wood, bones, or cloth.

Carbon exchange reservoir

Carbon moves around the atmosphere, oceans, and living things. The air, where carbon-14 is made, holds about 1.9% of all carbon. The carbon-14 in the air mixes quickly, within seven years. The ocean holds more carbon, but the surface water has less carbon-14 than the air because it mixes with deeper, older water. This means sea creatures have about 400 years more carbon-14 than land creatures. Land plants and animals match the carbon-14 levels in the air more closely.

Dating considerations

Radiocarbon dating helps us learn how old something is by looking at a special kind of carbon called carbon-14. But sometimes, this isn’t easy because a few things can change the results.

Atmospheric variation

Early scientists thought the amount of carbon-14 in the air stayed the same for thousands of years. But later, they found this wasn’t always true. By studying tree rings, they learned the amount of carbon-14 in the air has changed over time. Burning coal and oil, which have very little carbon-14, made the air have less carbon-14. Nuclear tests also added more carbon-14 to the air for a short time.

Isotopic fractionation

Plants take in carbon from the air, but they take in carbon-12 more easily than carbon-13 or carbon-14. This can make the carbon-14 levels in plants look different from what they really are. Scientists measure the carbon-13 levels to figure out how much this affects the dating.

Reservoir effects

The amount of carbon-14 isn’t the same everywhere. In the ocean, it can take a long time for carbon-14 to mix all the way through, so deep water can seem much older. This also happens in freshwater areas near rocks that don’t have carbon-14, making things there seem older too.

Contamination

If a sample gets extra carbon from somewhere else, it can make the dating wrong. Modern carbon makes things seem younger, while very old carbon makes things seem older. Scientists have to be very careful to keep samples clean.

Samples

To find out how old something is using radiocarbon dating, the sample needs to be changed into a form that can be measured. This might be a gas, a liquid, or a solid.

Different materials need special care. For example, wood can be tested whole or just a part of it can be used. Bone can be tested, but it works best when a certain part of it is used. Shells are made of minerals that can change over time, so they need careful checking. Peat, soil, and many other things like paper, seeds, and even charred food can also be tested.

Sometimes, scientists increase the amount of a certain carbon in the sample to get more accurate results. The sample then needs to be turned into the right form for the testing tool — this could be a gas like CO₂, a liquid like benzene, or a solid like graphite.

Measurement and results

For many years, the only way to measure carbon-14 in a sample was by watching its radioactive decay. This method is called "beta counting." It measures how often the sample decays. In the late 1970s, a new method called accelerator mass spectrometry (AMS) was created. AMS counts carbon-14 and carbon-12 atoms directly. This makes it more accurate and faster than beta counting. AMS can test smaller samples and give results in minutes.

Beta counting

The first device for beta counting was made by Willard Libby. Later, other tools like gas counters and liquid counters were made. These tools detect tiny flashes or changes caused by carbon-14 decay. They need special protection to block other radiation and ensure correct results.

Accelerator mass spectrometry

AMS counts carbon-14 and carbon-12 atoms directly. The sample is prepared, often as graphite. Ions are created and moved through a machine. This process separates the carbon isotopes, letting scientists count them accurately. AMS helps tell carbon isotopes apart from other similar atoms, making measurements precise.

Calculations

The way scientists find a sample's age depends on the measurement method. For beta counting, they compare the sample's radioactivity to a standard. For AMS, they use the ratio of different carbon isotopes. These measurements are adjusted for many factors to be accurate, and a special formula calculates the sample's age in years.

Errors and reliability

The accuracy of radiocarbon dating can be improved by testing samples longer. This reduces errors. Radiocarbon dating works best for samples up to about 50,000 years old, though special methods can extend this. Results are usually given with a range to show the possible age, showing how confident scientists are in the measurement.

Calibration

To get exact calendar dates, radiocarbon dates need to be adjusted using calibration curves. These curves are made by comparing radiocarbon dates with dates from tree rings and other known records. Over time, scientists have made better calibration curves, such as IntCal, which help change radiocarbon years to calendar years more accurately. These curves fix changes in carbon-14 levels over time, giving better date guesses.

Reporting dates

Scientists report radiocarbon dates in certain formats. Uncalibrated dates are the lab's guess of the age, with an error range. Calibrated dates show the likely calendar years, often with a confidence level. These reports include details about the lab, sample, and methods used to make sure the dating is clear and accurate.

Use in archaeology

When scientists use radiocarbon dating in archaeology, they look at objects found together at a site. Sometimes, they can take a sample right from the object they want to date. Other times, they cannot—like with metal grave goods. In these cases, they might date something found with the object, like charcoal or a coffin, assuming it was placed at the same time. This helps give an estimate for when the grave goods were placed there.

Care is needed when dating very old materials, because new carbon can make objects seem younger than they are. Wood samples can also be tricky because only the outer ring exchanges carbon with the environment. This means a piece of wood might look older than when the tree was cut down, especially if it was used for different things over time. Similar problems can happen with other materials like bitumen, which can make objects seem older than they really are.

Use outside archaeology

Radiocarbon dating is not just for archaeology. It is also used in geology, sedimentology, and lake studies. Scientists can date very small samples thanks to a method called AMS. This helps scientists date pollen, tiny bits of plants, or charcoal from sediments.

Radiocarbon dating also helps us understand when carbon is released from ecosystems. This is important for studying how human actions or climate change affect stored carbon in soils. New techniques even let scientists date methane and carbon dioxide, which are important gases that affect our climate.