K-12

Welcome to the K12 section of the Radiocarbon WEBinfo site. The aim here is to provide clear, understandable information relating to radiocarbon dating for the benefit of K12 students, as well as lay people who are not requiring detailed information about the method of radiocarbon dating itself. I have tried here to answer some of the frequently asked questions that I receive from students via email, as well as providing some basic information about scientific dating methods.


"Everything which has come down to us from heathendom is wrapped in a thick fog; it belongs to a space of time we cannot measure. We know that it is older than Christendom, but whether by a couple of years or a couple of centuries, or even by more than a millenium, we can do no more than guess." [Rasmus Nyerup, (Danish antiquarian), 1802 (in Trigger, 1989:71)].

The person who wrote these words lived in the 1800s, many years before archaeologists could accurately date materials from archaeological sites using scientific methods. Rasmus Nyerup's quote reminds us of the tremendous scientific advances which have taken place in the 20th century. In Nyerup's time, archaeologists could date the past only by using recorded histories, which in Europe were based mainly on the Egyptian calendar. They used pottery and other materials in sites to date 'relatively'. They thought that sites which had the same kinds of pots and tools would be the same age. The relative dating method worked very well, but only in sites which were had a connection to the relative scale. Most sites could not be dated. When radiocarbon dating was developed, it revolutionised archaeology, because it enabled them to more confidently date the past, and to build a more accurate picture of the human past. The archaeologist Colin Renfrew (1973) called it the development of this dating method 'the radiocarbon revolution' in describing its great impact upon the human sciences.

How was radiocarbon dating developed?

The radiocarbon method was developed
by a team of scientists led by the late Professor
Willard F. Libby of the University of Chicago after the end of World War 2.
Libby later received the Nobel Prize in Chemistry in 1960 for the radiocarbon discovery.

Today, there are over 130 radiocarbon dating laboratories around the world producing radiocarbon dates for the scientific community. The C14 method has been and continues to be applied and used in many, many different fields including hydrology, atmospheric science, oceanography, geology, palaeoclimatology, archaeology and biomedicine.


How does radiocarbon dating work?

All plants and animals on Earth are made principally of carbon. During the period of a plant's life, the plant is taking in carbon dioxide through photosynthesis, which is how the plant makes energy and grows. Animals eat plants, and some eat other animals in the food chain. Carbon follows this pathway through the food chain on Earth so that all living things are using carbon, building their bodies until they die.

A tiny part of the carbon on the Earth is called Carbon-14 (C14), or radiocarbon. It is called 'radio'-carbon, because it is 'radioactive'. This means that its atomic structure is not stable and there is an uneasy relationship between the particles in the nucleus of the atom itself. Eventually, a particle is emitted from the carbon 14 atom, and carbon 14 disappears. Most of the carbon on Earth exists in a slightly different atomic form, although it is chemically speaking, identical to all carbon.

In the 1940s, scientists succeeded in finding out how long it takes for radiocarbon to disappear, or decay, from a sample of carbon from a dead plant or animal. Willard Libby, the principal scientist, had worked in the team making the nuclear bomb during World War 2, so he was an expert in nuclear and atomic chemistry. After the war he became very interested in peaceful applications of atomic science. He and two students first measured the "half-life" of radiocarbon. The half-life refers to the amount of time it takes for half the radiocarbon in a sample of bone or shell or any carbon sample to disappear. Libby found that it took 5568 years for half the radiocarbon to decay. After twice that time (about 11000 years), another half of that remaining amount will have disappeared. After another 5568 years, again another half will have disappeared. You can work out that after about 50 000 years of time, all the radiocarbon will have gone. Therefore, radiocarbon dating is not able to date anything older than 60 or 70 000 years old. The job of a radiocarbon laboratory is to measure the remaining amounts of radiocarbon in a carbon sample. This is very difficult and requires a lot of careful work to produce reliable dates.

What kind of things can you date using radiocarbon?

Because carbon is very common on Earth, there are alot of different types of material which can be dated by scientists. Below is a list of the different kinds of materials which can be dated:

How did Libby test his method and find out if it worked correctly?

Libby tested the new radiocarbon method on carbon samples from prehistoric Egypt whose age was known. A sample of acacia wood from the tomb of the pharoah Zoser was dated for example. Zoser lived during the 3rd Dynasty in Egypt (2700-2600 BC). Libby figured that since the half-life of C14 was 5568 years, they should obtain a radiocarbon amount of about 50% of that which was found in living wood because Zoser's death was about 5000 years ago. The results they obtained indicated this was the case. Many other radiocarbon dates were conducted on samples of wood of known age. Again, the results were good. In 1949, Libby and his team published their results. By the early 1950s there were 8 new radiocarbon laboratories, and by the end of the decade more than 20.

How much sample material do you need to date using radiocarbon?

A new way of radiocarbon dating was developed in the late 1970s called "AMS Radiocarbon dating". AMS stands for Accelerator Mass Spectrometry. AMS dating is important because using it you can date very small sizes carbon samples. Imagine a grain of rice, this can be dated now with radiocarbon. We can date pollen grains, seeds, tiny pieces of charcoal. What about a hair from someone's head? That too can be dated using AMS! We can now date a variety of very, very small samples, so many more kinds of archaeological and geological samples can be dated than ever before so AMS is a tremendous breakthrough for archaeologists and other researchers.

How much does it cost to date using radiocarbon dating?

The cost varies between different laboratories. On average, a single date will cost about 250 US dollars. The high cost is because it is a big job to date a sample. It takes a long time to change the carbon material into the form it needs to be in to be able to be dated. As well as that, the equipment is very expensive and has ongoing costs associated with it. An AMS dating instrument for example, costs around $2-3 million dollars.

What are the oldest things that can be radiocarbon dated?

Anything that is less than about 50 or 60 000 years can be radiocarbon dated. Beyond 60 000 years there is hardly any radiocarbon left in a sample that is original. Often, in very old material, there is contamination which can significantly affect the accuracy of a date. Dating material from the archaeological or geological record beyond 30 000 years can be very difficult indeed unless the depositional situation of the sample is favourable and scientists can remove any contamination. Even a small amount of c14 from a contaminant can produce an incorrect date in an old sample. Often, radiocarbon daters release dates as being 'greater than 50 000 years' or 'greater than 45 000 years' because of the difficulty in reliably giving a date at this age.

In some places, such as Australia, archaeologists have recognised the problems in dating the oldest sites, which may stretch back over 50 - 60 000 years. Other techniques such as OSL (Optically Stimulated Luminescence dating) which use different methods of determining age, are often used in parallel with radiocarbon to determine the ages of the uppermost parts of the site. The ages derived are compared with another, and usually, there has been good agreement between the methods. At sites in the far north of the continent, the oldest dates have been obtained using OSL, at about 60 000 years. Again, this is really just beyond the c14 limit for sites such as these.

What is the youngest thing that can be radiocarbon dated?

This is a difficult one, because we can date pretty much anything from today or in modern times, but getting an actual 'date' is hard. In the 1950s and 60s, people blew up alot of nuclear bombs, and one thing that happened because of this was that alot of radiocarbon was created in the air artificially. Radiocarbon is a side effect of nuclear bombs. In the early 1960s the amount of radiocarbon produced by bombs was bigger than the amount of radiocarbon naturally present! It sounds bad, and nuclear bombs are not pleasant when they are generated, but for science there have been some spinoffs because we have been able to study the movement of this radiocarbon through the environment and learn alot about how radiocarbon is transported naturally. So this has been beneficial. We can also date things that have happened since 1950 rather well because of the sudden jump in radiocarbon on Earth, so that it is possible to figure out within 2-3 years sometimes, the date of a sample.

Generally, we can date things pretty well over the past 1000 years, it becomes difficult from about 1700 AD to 1900 AD because of natural changes in radiocarbon, and since 1950 AD dating is quite possible.

What kinds of famous things have been radiocarbon dated?

The Dead Sea Scrolls are a very famous archaeological discovery, and many have been dated by using AMS at the Arizona AMS laboratory. They date from the first century BC to the first century AD. There was close agreement between the radiocarbon dates and the dates which had been estimated using the writing styles used on the scrolls, and in some cases the dates recorded on the scrolls themselves.

What about the Iceman? The Iceman is a very famous frozen body found in northern Italy in 1991. Samples of his bones, grass boot, leather and hair were dated, the results showed that he lived almost 5500 years ago (3300-3100 BC), during the age when people first began using copper in Europe. Radiocarbon dating was tremendously important in dating the precise age of the Iceman.

How, in your opinion, did the use of radiocarbon dating change the way scientists are able to interpret and understand history?

Before 1950, when radiocarbon dating was first developed by scientists from the US, archaeologists had no way of knowing precisely how old (in numbers of years) an archaeological site or artefact was. In some parts of the world, where historic records extended back far enough in time, such as in the Mediterranean, archaeologists had dated artefacts by comparison with material from other sites which could be historically dated. This method was called "relative dating" and it is still used today. Radiocarbon dating enabled archaeologists and other scientists to verify the ages of carbon-bearing materials ndependently and almost overnight revolutionised the approach of dating the past. The reason was that now any samples could be dated, so long as they were once living organisms. Radiocarbon dating is one of the critical discoveries in 20th century science and it provided one of the most important tools for archaeologists in their quest to uncover the past. Instead of spending large amounts of time solving the problem of "when" something happened, archaeologists could now concentrate on investigating "how" and "why" things happened.

What if any arguments were provoked because of the use of radio-carbon dating?

One of the most controversial examples of the use of radiocarbon dating was the analysis of the Turin Shroud, the supposed burial cloth of Jesus. The shroud itself appears to show a person who was crucified and is an object of some veneration because of its supposed association with Christ. Its history dates back at least as far as the mid 14th century AD. The first photograph of the shroud showed the man as a negative image, a kind of three dimensional picture. This, along with other discoveries, such as the supposed presence of pollen spores from Israel on the cloth have suggested the shroud might be an important and genuine relic. In the 1980s, the Archbishop of Turin gave permission to a group of scientists to date small pieces of fabric sampled from the shroud. Radiocarbon laboratories at Tucson (US), Oxford (England) and Zurich (Switzerland) dated the samples, along with 3 control samples of varying ages. The results were very consistent and showed the shroud dated between 1260-1390 AD. This fits closely with its first appearance in the historical record and suggests strongly that it is a medieval artefact, rather than a genuine 2000-year-old burial cloth.

(You can read the original scientific paper on the age of the Shroud here).

Can you find the age of rocks by using radiocarbon dating or are they generally too old? If a rock was shot from a volcano and isn't that old, can we use radiocarbon dating?

Samples of rock are not able to be dated using radiocarbon, because rocks contain no organic carbon from living organisms that are of recent enough age. Most rocks formed hundreds of thousands if not millions of years ago. Geologic deposits of coal and lignite formed from the compressed remains of plants contain no remaining radiocarbon so they cannot be dated. Radiocarbon dating is limited to the period 0 - 60 000 years, because the 'half-life' of radiocarbon is about 5700 years, so to date rocks scientists must use other methods. There is a number of different techniques available. We can date volcanic rocks using a method called argon-argon dating for instance. This method uses principles of isotopic decay like radiocarbon, but different isotopes (argon-39 and argon 40) which have a longer halflife (1250 million years). This means scientists can date rock which is many millions of years old. The technique can date materials the size of one grain of volcanic ash, using a laser. There are other methods which can be used as well which operate using different radiochemistries. The only way to date a volcanic ash layer using radiocarbon dating is to find ash within a lake sediment or peat layer and then date the organic carbon from above and below it, and therefore fix an age for the ash event. This is a commonly used approach to date volcanic events over the past 60 000 years around the world.

How do you know that radiocarbon really works?

It is possible to test radiocarbon dates in different ways. One way is to date things that you already know the age of. Libby did this when he first developed the method, by dating artefacts of Egyptian sites, which were already dated historically. Another way is to use tree rings. Every year a tree leaves a ring, the rings increase in number over time until a pattern of rings is formed. Sometimes the tree has many hundreds of rings. Scientists can date the age of the tree by counting and measuring the rings. Radiocarbon daters can then date the tree rings and compare the dates with the real age of the tree. This is a very good way of testing radiocarbon, and we now know that there are some differences in radiocarbon dates and real time. Most of the time radiocarbon dating is accurate, but sometimes it is different from the real age by a small amount. Using a calibration curve, which is based on radiocarbon dates of tree rings over the last 10000 years, radiocarbon daters can correct for this problem.

We can also test radiocarbon by comparing the results with the dates produced by other dating methods, and there are many of those. These methods are completely different to radiocarbon dating and use different methods to provide dates. Some of the dating methods include Uranium/Thorium dating (dating coral etc), Thermoluminescence (pottery, sediments), Obsidian Hydration (obsidian), Electron Spin Resonance (teeth), Amino Acid Racemisation dating (eggshell, bones), and many others.


If you have any other questions or comments, contact Dr Tom Higham (Email: thomas.higham@archaeology-research.oxford.ac.uk).

Books which might interest for younger readers.

Mummies, Dinosaurs, Moon Rocks : How We Know How Old Things Are
by James Jespersen, Jane Fitz-Randolph, Bruce Hiscock (Illustrator). ISBN: 0689318480.
You can buy this Book at Amazon.com for only US12.00. Recommended for young adults 10-14 years. You can read reviews of this book as well.

Online articles of interest:

Try this: Geolabs on-line Virtual Dating -an interactive quiz for High School students. For other interesting topics visit the Geolabs on-line Home Page.

For students with questions about the reliability of radiometric dating vs creationist arguments visit Tim Thompson's web page which includes a list of resources on this topic.

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