How Things Work: Radiocarbon dating

A year ago, archaeologists made the claim that Stonehenge in England was a part of an ancient religious complex where the remains of cremations were buried. The story, reported in ABC News, goes on to say that the main evidence for this theory came from the discovery of cremated remains of nearly 240 people, which were buried there over the course of 500 years. This astonishing discovery would not have been possible had it not been for a technique that allowed the researchers to correctly calculate the age of the remains.

The technique of radiocarbon dating has contributed immensely to science by figuring out the ages of a variety of specimens ranging from Stone Age men trapped in glaciers to the Dead Sea Scrolls.
The technique of radiocarbon dating was developed by Willard Libby in the 1940s. According to an article in the British newspaper The Times, the technique proved to be such a powerful tool for scientists that Libby received a Nobel Prize for his work in 1960.

Libby’s groundbreaking technique stemmed from the simple discovery that plants absorb a radioisotope of carbon, carbon-14, into their tissues. Carbon-14 is a radioisotope of normal carbon, carbon-12. Both carbon-14 and carbon-12 have six protons, but while carbon-12 has six neutrons, carbon-14 has eight neutrons.
This difference allows carbon-14 to be radioactive while carbon-12 is non-radioactive.

Carbon-14 is present in small quantities in the atmosphere and is constantly being produced by a reaction involving cosmic rays and nitrogen atoms. When cosmic rays enter the earth’s atmosphere, they collide with atoms in the upper layers of the atmosphere; this collision can lead to the production of energized neutrons.
These energized neutrons then collide with nitrogen atoms, which make up the major portion of air. Nitrogen nuclei, which have seven protons and seven neutrons, lose a proton due to the bombardment with a neutron and get converted into carbon-14 nuclei.

Carbon-14 atoms then react with oxygen to produce carbon dioxide containing the carbon-14 isotope. This carbon dioxide (14CO2) is then taken up by plants and enters the life cycle of living organisms.
Carbon-14, being radioactive, emits a beta particle, which is essentially an electron, and gets converted to nitrogen-14.

Thus, there is a constant cycle going on in living organisms: They take up the 14CO2 into their bodies, and the carbon-14 in their bodies decays to nitrogen. Because of this continuous cycle of taking up 14CO2 and then with the decaying of carbon-14, the proportion of carbon-14 in the bodies of all living organisms remains the same.
In living organisms, the ratio of carbon-14 to carbon-12 is approximately 1.3×10-12 to one. However, once the organism dies, it can no longer incorporate new 14CO2 into its body, and the amount of carbon-14 inside the organism gradually goes on decreasing.

Thus, when archaeologists encounter the remains of a plant or animal, they can measure the ratio of carbon-14 to carbon-12 in the organism and compare it to the ratio present in living organisms. By knowing this ratio and the half-life of carbon-14, which is around 5700 years, it is possible to calculate how much time has passed since the organism died.
Ever since it was developed, this technique has been used in a broad range of fields.

The Times article relates the discovery of a frozen Stone Age man found in the Alpine glaciers in 1991.
Carbon dating was used to estimate the ice man’s age to be around 5300 years old. This discovery caused a great stir in the scientific community, as the man’s corpse was the oldest ever to be found.
Radiocarbon dating has also been useful in figuring out the ages of Egyptian mummies, and even the different layers of the Grand Canyon.
Although the technique has proven to be quite helpful, its main drawback is that it assumes that the proportion of carbon-14 in the atmosphere remains constant over time. This proportion actually goes on changing and scientists have been trying to refine the technique’s accuracy by taking into account the changing proportion of carbon-14.
The principle of carbon dating can be used even with different elements. Potassium-40 is a radioactive isotope which is naturally found in the body. Potassium-40, thus, can also be used to determine the age of an organism.

However, carbon dating is still the most popular method of determining the age of fossils and other organic material.
According to an article in Wired, because of an increased level of nuclear activity, the proportion of radioactive isotopes changed drastically in the 1950s and 1960s. Scientists hence need to recalibrate the proportions of carbon-14 for all specimens to be dated after this time.