Radiocarbon dating is probably the most important scientific method used by archaeologists to date objects. It is also an indispensable tool to researchers in other fields such as geology, geophysics and environmental science.

Radiocarbon Dating

Carbon usually exists as ¹²C atoms. Radiocarbon is the unstable isotope ¹⁴C. It is produced by cosmic rays in the upper atmosphere, and quickly diffuses through the atmosphere as carbon dioxide, dissolves in the oceans and enters all living matter through photosynthesis and the food chain.

As ¹⁴C is unstable it will eventually decay by emitting an electron or beta particle. It reaches an equilibrium concentration in all living matter and gives it a small natural radioactivity. The principle behind radiocarbon dating is that when a plant or animal dies, there is no more exchange of carbon with the atmosphere. As the ¹⁴C will decay exponentially, we can find the age of the lifeform from the amount of ¹⁴C it contains. This method was developed by the American scientist Willard Libby in the 1940s, and soon attracted the attention of scientists from many different subject areas.

Exponential decay means that the amount of ¹⁴C, and the radioactivity it produces will drop by 50% every 'half life'. For ¹⁴C this is 5730 years. Thus the age of any material containing carbon which was once part of the biosphere can be determined by measuring its ¹⁴C content.

To date a sample, it is necessary to determine the amount of ¹⁴C present. One method is to measure the activity of the sample, or the number of beta particles emitted per second. This is proportional to the number of ¹⁴C atoms and can be measured using various methods. Another method is accelerator mass spectrometry (AMS), which counts a proportion of the number of ¹⁴C and ¹²C atoms.

Applications of Radiocarbon Dating

The discovery of radiocarbon dating probably had a greater influence on modern archaeology than any other technological advance, especially on prehistoric periods where without written records archaeologists could previously only speculate the age of artefacts and sites. Before it was developed artefacts were dated largely by guesswork and assuming connections with other objects, the discovery of radiocarbon dating showed that many of these assumptions were wrong. Many radiocarbon results were so unexpected that archaeologists initially questioned the accuracy of the method, however, with time, its reliability was established.

The vast majority of prehistoric sites could not be dated before radiocarbon dating. There was much uncertainty over the age of Stonehenge and the many burial mounds throughout Europe. It had generally been assumed that they were younger than Mycenae¹ as the technology had diffused from the Near East. Radiocarbon dating showed that they were actually several centuries older.

There were many other radical results. Traditionally, the ages of prehistorical sites were estimated by studying the geology of the surroundings; sometimes archaeologists made a wild guess based of the depth a sample had been buried. Radiocarbon dating changed all this. It was shown that humans arrived in North America earlier than had been previously thought. Agriculture began later than had been supposed in the Near East, but earlier than predicted in Europe. This questioned the established idea that farming had developed in the Middle East and spread westwards.

Radiocarbon dating also changed the nature of archaeology as a subject. Previously archaeologists spent a great deal of time debating the age of sites and objects, trying to develop chronologies and showing which discoveries predated others. Their work was largely collecting objects, identifying and dating them. As radiocarbon dating allowed chronologies to be established relatively easily, archaeologists started to spend their time developing theories about the culture and society of early people.

The value of radiocarbon dating to geology and other earth sciences is at least as great as it is to archaeology. In the 1950s, when the accuracy of the technique had still to be proved, it was used to date organic matter in moraines (rock and sediment deposited by glaciers), showing that the ice sheets had reached their maximum extent 18,000 years ago.

Radiocarbon dating was quickly established as an invaluable tool to researchers studying the Quaternary period. It was used by palaeontologists to discovery the age of plant and animal matter. For example, it was shown that the woolly mammoth became extinct in Europe 12,000 years ago, and in Siberia 10,000 years ago, but recent radiocarbon dating on tusks found frozen in the arctic ice on a remote island were found to be only 4000 years old.

¹⁴C dating is also widely used in other earth sciences including hydrology, oceanography, climatology and environmental science. Deep sea sediments can be dated from calcite shells, and groundwater from dissolved carbonates. Carbon dioxide trapped in ice cores can be dated providing atmosphere samples for various ages.

Radiocarbon also has many less obvious applications. Much research has been done to see if there is any evidence for past intense cosmic ray activity in ¹⁴C levels. This could provide a record of past supernovae and other astronomical phenomena. ¹⁴C is also used as a biomedical tracer as nearly all biochemicals contain carbon.

Accuracy of Radiocarbon Dating

Radiocarbon dating is a reasonably reliable method for dating objects between 300 and 30,000 years old. However it is not 100% accurate, and there are many factors limiting its accuracy.

Samples can be contaminated by calcium carbonate (limestone) from groundwater, and humic acids from organic matter in soil. These must be removed by pre-treatment techniques before a sample is dated. Sometimes the level of ¹⁴C in a sample when it died, is not the same as the equilibrium level in the atmosphere. For example, marine samples show lower ¹⁴C levels, as some has decayed by the time it dissolves in the sea.

Also the level of ¹⁴C in the biosphere is not constant but has changed in the past, so it is necessary to calibrate radiocarbon dates to produce accurate results. This is done by comparing the dendrochronology (tree ring) and radiocarbon dates of wood samples from the bristlecone pine tree, which can live for more than 4000 years. As there is no carbon exchange between the rings, the ¹⁴C content of the centre of a tree will be less than the younger wood on the outside.

Accelerator Mass Spectrometry

Traditional methods of measuring the ¹⁴C content of a sample work involve measuring its activity. A typical sample, however, only contains one ¹⁴C atom for every 10^{12 12}C atoms, and only a tiny fraction of these will decay each day. Therefore in order to detect a sufficient number of disintegrations to produce a reliable result it is necessary to either use a large sample or to wait for a considerable time period. This means that the smallest samples that can be dated using these methods are about 1-10g.

An alternative way of measuring the ¹⁴C content of a sample which avoids this problem, is to directly count the number of ¹⁴C atoms, or a proportion of them. This is done in accelerator mass spectrometry (AMS).

Mass spectrometry is a technique which is widely used in chemistry to measure the masses of atoms and molecules in a material. A sample is ionised and the resulting ions are accelerated into a magnetic field, which deflects their path. Lighter particles are deflected more than heavier particles, detectors positioned at different angles will detect particles of different masses, so a spectrum of the number of particles of each mass detected can be determined.

Ordinary mass spectrometry cannot, however, be used to measure radiocarbon concentrations, as it cannot distinguish between ¹⁴C and ¹⁴N atoms, nor any other particles of mass 14. Therefore accelerator mass spectrometry is used, in which the ions are accelerated to great speeds allowing the carbon and nitrogen atoms to be separated using various methods.

The best radiation counters can still achieve a higher precision than AMS, however as the sensitivity of AMS measurements is much greater, the minimum size of sample needed is much smaller, only 1mg or smaller for some materials. This massively increases the number of applications of radiocarbon dating. Very valuable artefacts or works of art can be dated without being damaged (eg, the Turin Shroud). Tiny samples of pigment from paintings, single microfossils in rocks or pollen grains can be dated. The development of AMS led to many discoveries in archaeology which could be done using conventional radiocarbon dating; the dating of charcoal samples confirmed that the Vikings had settled in Newfoundland, and dating Australian rock art proved humans had reached the continent 50,000 years ago.

AMS also allows the study of other radionuclides - radioactive elements produced by cosmic rays. As early as 1978, it was realised that AMS could be used to detect ³⁶Cl, as this isotope has a half life of 301,000 years it is not possible to detect it using decay counting. However, this half life is ideal for dating ground water. Hydrologists now use several different radioisotopes including ¹⁴C and ³⁶Cl for dating water samples.

Although much time and effort is spent researching these ideas, radiocarbon laboratories spend the majority of time simply dating archaeological or geological samples. Radiocarbon dating is now over 50 years old, yet it remains the single most useful dating technique available to archaeologists and it still produces fascinating and sometimes startling results.