Radiocarbon dating

Radiocarbon dating, also called carbon dating or carbon-14 dating, is a way to find out how old something is if it once was alive, like a piece of wood or bone. It works by measuring a special kind of carbon called radiocarbon, which is a radioactive type of the element carbon. 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 through radioactive decay. By measuring how much radiocarbon is left, we can estimate when the organism died. The half-life of radiocarbon—how long it takes for half of it to disappear—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 over the past 50,000 years. They use this information to get even 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, which works with very small samples and gives results faster.

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 any common elements in organic matter had isotopes with long enough half-lives to be useful in medical research. They made carbon-14 using a special machine and found its half-life was much longer than people thought before. Later, scientist Serge A. Korff predicted that cosmic rays hitting nitrogen in the upper atmosphere would create carbon-14. During World War II, Willard Libby learned about this and thought it might be possible to use carbon-14 to figure out how old things are.

In 1945, Libby moved to the University of Chicago and started working on radiocarbon dating. In 1946, he published a paper suggesting that living things might contain carbon-14 along with non-radioactive carbon. Libby and his team tested this idea using methane from sewage works in Baltimore. They found that this methane contained carbon-14, unlike methane from petroleum, which was too old to have any. 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. For example, they dated samples from ancient Egyptian tombs and got results close to the known dates. These results were published in 1949. Within 11 years, over 20 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

In nature, 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—the time it takes for half of it to decay—is about 5,730 years. Carbon-14 is constantly made in the atmosphere by cosmic rays hitting nitrogen atoms. These cosmic rays create neutrons that turn nitrogen-14 into carbon-14. Once made, 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 exactly 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. These places are called the carbon exchange reservoir. 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. Dead plants and animals have less carbon-14 because they stop taking in new carbon when they die.

Dating considerations

Radiocarbon dating helps us find out how old something is by looking at a special type of carbon called carbon-14. But sometimes, this isn’t always easy because there are a few things that 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 that this wasn’t always true. By studying tree rings, they learned that 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. Also, nuclear tests 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 a little 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 water from deep down 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, depending on the tools being used. Before this step, the sample must be cleaned to get rid of dirt and other unwanted parts. This includes washing away things like tiny roots that might have gotten into the sample over time.

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 to learn their age.

Sometimes, especially for very old samples, 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. The amount of material needed depends on the testing tool, but very small amounts can be used with the most sensitive tools.

Measurement and results

For many years after the first experiments with radiocarbon dating, the only way to measure the carbon-14 in a sample was by detecting its radioactive decay. This method, known as "beta counting," measures the number of decay events from the sample. In the late 1970s, a new method called accelerator mass spectrometry (AMS) was introduced. AMS directly counts the number of carbon-14 and carbon-12 atoms, which makes it more accurate and faster than beta counting. AMS can also test much smaller samples and provide results in minutes.

Beta counting

The first device used for beta counting was a special counter designed by Willard Libby. Over time, other technologies like gas proportional counters and liquid scintillation counters were developed. These methods detect the tiny flashes of light or ionization caused by the decay of carbon-14 atoms. They require shielding to block background radiation and other safety measures to ensure accurate results.

Accelerator mass spectrometry

AMS counts carbon-14 and carbon-12 atoms directly. The sample is prepared, often as graphite, and then ions are created and accelerated through a machine. This process separates the different carbon isotopes, allowing scientists to count them accurately. AMS is particularly useful for distinguishing carbon isotopes from other similar atoms, ensuring precise measurements.

Calculations

The calculations for determining a sample's age depend on the measurement method used. For beta counting, scientists compare the sample's radioactivity to a standard. For AMS, they use the ratio of different carbon isotopes. These measurements are corrected for various factors to ensure accuracy, and a specific formula is used to calculate the sample's age in years.

Errors and reliability

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

Calibration

To get accurate calendar dates, radiocarbon dates need to be adjusted using calibration curves. These curves are created by comparing radiocarbon dates with dates from tree rings and other known records. Over time, scientists have developed improved calibration curves, such as IntCal, which help convert radiocarbon years to calendar years more accurately. These curves account for variations in carbon-14 levels over time, providing better date estimates.

Reporting dates

Scientists report radiocarbon dates in specific formats. Uncalibrated dates are given as the laboratory's determination of the age, along with an error range. Calibrated dates show the probable calendar years, often with a confidence level. These reports include details about the laboratory, sample, and methods used to ensure transparency and accuracy in the dating process.

Use in archaeology

A key idea in using radiocarbon dating in archaeology is understanding the relationship between objects found together at a site. Sometimes, scientists can take a sample directly from the object they want to date. Other times, they cannot—for example, with metal grave goods. In such cases, they might date something found with the object, like charcoal or a coffin, assuming it was deposited at the same time. This gives an estimate for when the grave goods were placed there.

Care is needed when dating very old materials, as contamination from newer carbon can make objects seem younger than they are. Also, wood samples can be tricky because only the outermost ring exchanges carbon with the environment. This means a piece of wood might appear older than when the tree was cut down, especially if it was reused for different purposes over time. Similar issues can arise with other materials like bitumen, which can make objects seem older than they truly 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 palaeobotanists and palaeoclimatologists 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 greenhouse gases.