How does Carbon-14 Dating Work?

Carbon-14 dating technique has been successfully used on the Minoan ruins, Dead Sea Scrolls, and tombs of the Pharaohs.

This technique works for objects that are up to 50,000 years old. This method of dating was introduced to the world by Willard Libby in 1949, and for this outstanding work, he was awarded the Nobel Prize in Chemistry in 1960.

To understand how carbon-14 dating works, we need to understand a few things about carbon and its isotopes, how it is formed, and the role it plays in our environment.

All living organisms and our environment are made of carbon. The most common form of carbon in the atmosphere is carbon-12, which is a stable isotope of carbon. An unstable form of carbon isotope, carbon-14, also known as radiocarbon, is formed when cosmic rays hit atoms in the upper atmosphere of the earth.

When cosmic rays collide with the atoms, it creates an energetic neutron, which, upon hitting a nitrogen atom (7 neutrons and 7 protons), creates a carbon-14 atom (6 protons and 8 neutrons) and a hydrogen atom (1 neutron, 0 proton). Carbon-14 is radioactive, and has a half life of about 5700 years.

Plants get their share of carbon atoms from the atmosphere. Animals and humans get it from what they eat. It is seen that at any given point of time, the ratio of stable carbon and radiocarbon in the atmosphere, is almost same as the ratio of stable carbon and radiocarbon in living organisms like plants, animals, and humans.

When a living organism dies, its carbon intake ceases. From that point onwards, the unstable carbon-14 decays with its half life of almost 5700 years, and the amount of carbon-12 remains the same. Hence, gradually, the ratio of stable carbon to radioactive carbon also decreases.

By comparing the ratio of carbon-12 and carbon-14 in a sample to that of a living organism, it is possible to determine the age of an archaeological object, with a fair accuracy. The ratio of carbon-12 and carbon-14 is determined by three techniques, namely gas proportional counting, liquid scintillation counting, and accelerator mass spectrometry.

Formula to calculate the age of a sample by carbon-14 dating is as follows:

t = [ ln (N_f/N_o) / (-0.693) ] x t_1/2

where, ln is the natural logarithm, N_f/N_o is the percentage of carbon-14 in the sample compared to that in a living sample, t_1/2 is the half life of carbon-14, t is the age of the sample

✶ Let's say that the given sample has 15 percent carbon-14, as compared to the living sample. Half life of carbon-14 is 5700 years. Then the age of the given sample would be:

t = [ ln (.15) / (-.693) ] x 5700 years
= [ ln (-1.897) / (-.693) ] x 5700 years
= 1.3146 x 5700 years
= 7493 years old

► Carbon dating can only be used to date objects up to 5700 years. Things older than that have only about 1% carbon-14 left in them and hence, it is not possible to calculate their age accurately.

► Carbon dating cannot be used to date organisms that doesn't intake carbon dioxide form the air. This includes aquatic animals which consumes some carbon from dissolved carbonated rocks. In fact, this rule will include those organisms under its gamut which consumes seafood.

► Carbon dating works only on organic material like cloths, fiber, wood, skeletons, etc. The dating process cannot be applied on things like paintings etc.

► Carbon dating works on the principle that the ratio of carbon-12 to carbon-14 has remained constant over ages. By comparing age of a sample deduced by using carbon-14 dating to results from other dating techniques, discrepancies have been noticed.

Scientists have concluded that the ratio of carbon-12 to carbon-14 might not have been constant over the past 10,000 years.

► With the advancement of nuclear reactors and weapons, it is possible that the amount of carbon-14 has changed in the recent years, specially carbon data from the 1950s and 1960s are questionable. There is also the likelihood of the sample being contaminated with old or new carbon.

For example, logs of woods cut from different trees were burnt in a fireplace. The sample of the end result would be contaminated. Hence, it would not be possible to determine the age of a sample accurately.

To determine if the result deduced by carbon-14 dating is correct, it is often cross-checked by using other dating techniques like Tree-ring counting, Potassium-40 dating (half-life: 1.3 billion years), Uranium -238 dating (half life: 4.5 billion years), etc.

There are calibration charts as well to account for the change in the amount of carbon-14 content in the atmosphere over the years to diminish errors. Although carbon-14 dating technique has its limitations, still it is a very helpful tool for archaeologists.