In a world full of dating apps, how do scientists use “dating” to work out how old something is? When I was growing up I was fascinated with dinosaurs and history, but never took the time to find out how they actually worked out how old all the discoveries were. Let’s learn more about carbon dating.
Carbon dating doesn’t have anything to do with Tinder, Bumble or other dating apps you may be familiar with. Everyone will have seen news articles about ancient finds or archaeological digs which uncover all sorts of treasures, but how? How do scientists know how old these artefacts are? The technique is known as carbon dating and uses radioactivity to work out the age of biological objects (such as bone, wood, plants etc.) up to 60,000 years old.
Every day a large number of cosmic rays enter the earth’s atmosphere. We’re still not 100% sure where cosmic rays come from but they are atom fragments which travel at the speed of light. Every hour, everyone is hit by ~half a million cosmic rays. When a cosmic ray enters our atmosphere it can collide with an atom in the atmosphere and create a secondary cosmic ray (an energetic neutron). This can subsequently collide with nitrogen atoms. When this happens, a nitrogen-14 atom (seven protons, seven neutrons) becomes a carbon-14 atom (six protons, eight neutrons), and a hydrogen atom (one proton, no neutrons). Carbon-14 is radioactive (meaning it is an unstable atom which emits energy overtime) with a half-life of ~5,700 years. In this instance, half-life is the time it takes for the radioactivity to fall to half its original value. Carbon-14 atoms created via cosmic rays can combine with oxygen to form CO2, which plants absorb via photosynthesis. When animals and people eat plants, we take in carbon-14. The ratio of normal carbon (carbon-12) to carbon-14 in the air and in all living things, is nearly constant (~one in a trillion carbon atoms are carbon-14).
You, all plants and animals have the same percentage of carbon-14 atoms in them. However, when a living organism dies it stops taking in new carbon. The ratio of carbon-12 to carbon-14 upon death is the same in every living thing, but carbon-14 decays and is not replaced whilst carbon-12 remains constant. By looking at the ratio of carbon-12 to carbon-14 in a fossil and comparing it to the ratio in a living being, it is possible to determine the age of the artefact. With a 5,700 year half-life, it is only reliable to date objects up to ~60,000 years old. However, we can apply the principle of carbon-14 dating to other isotopes (i.e. forms of the same element with the same number of protons but a different number of neutrons) as well, such as Potassium-40, Uranium-235 and -238, and Rubidium-87; all of which have much longer half-lives so can date much older artefacts.
Although these radioisotopes allow the dating of ancient samples now, this type of dating may not work quite as well in the future. Anything which dies after the 1940’s will be harder to date precisely due to the advent of nuclear bombs, reactors and open-air nuclear tests, all of which will have increased the content of carbon-14 in the atmosphere. These events alone could have doubled the carbon-14 concentration, compared to just that produced from cosmic rays. Fossil fuels could also taint future dating attempts as they have no carbon-14. The burning of fossil fuels over the past century may have reduced the ratio of carbon-14. Both situations will make it much more difficult to use carbon dating to accurately predict the age of an artefact in the future.
Luckily, radioactive carbon dating is just one method scientists have to calculate the age of artefacts and fossils. Other techniques include the relative depth of the finding – Biostratigraphy – (i.e. the further down into the ground it was found, the older it is), Tephrochronolgy which uses knowledge of volcanic eruptions to date layers and finally, using other radioactive elemental decay (such as potassium-40 mentioned above). Overall, the method of radio-carbon dating has provided us with endless data about times gone by which may otherwise have been a mystery. It would be devastating if we lost this technique as we pollute the planet.