How do we know how old a fossil is? We use carbon, as every living being has carbon. Carbon dating, also known as radiocarbon dating, is a method of estimating the age of carbon-bearing materials up to 60,000 years old.
One of the most frequent uses of radiocarbon dating is to estimate the age of organic remains from archaeological sites. However, it is also used to determine ages of rocks, plants, trees, etc.
How carbon dating works?
There are some carbon particles in the atmosphere. When the sun’s rays reach them, a few of these particles turn into carbon 14 (a radioactive carbon).
The highest rate of carbon-14 production takes place at altitudes of 9 to 15 km (30,000 to 50,000 ft). At high geomagnetic latitudes, the carbon-14 spreads evenly throughout the atmosphere and reacts with oxygen to form carbon dioxide.
Carbon dioxide also permeates the oceans, dissolving in the water. Plants take in atmospheric carbon dioxide by photosynthesis, and are ingested by animals. So, every living thing is constantly exchanging carbon-14 with its environment as long as it lives.
Once a being dies, however, this exchange stops. The carbon in its body will remain until it decomposes or fossilizes. The amount of carbon-14 gradually decreases through radioactive beta decay with a half-life of 5,730 years.
So, scientists can estimate the age of the fossil by looking at the level of decay in its radioactive carbon.
What is half life?
Some chemical elements have more than one type of atom. Isotopes are different atoms of the same element.
Carbon has two stable, nonradioactive isotopes: carbon-12 (12C), and carbon-13 (13C). In addition, there are trace amounts of the unstable isotope carbon-14 (14C) on Earth.
Carbon-14 has a relatively short half-life of 5730 years, meaning that the amount of carbon-14 in a sample is halved over the course of 5730 years due to radioactive decay.
By about ten half-lives, or 58,000 years, the amount of carbon-14 left in the fossil is very little- about 1/1000 of the original number of carbon-14 atoms in the fossil. So, using carbon dating for fossils older than 60,000 years is unreliable.
Carbon dating was developed by American scientist Willard Libby and his team at the University of Chicago. Libby calculated the half-life of carbon-14 as 5568, a figure now known as the Libby half-life.
Following a conference at the University of Cambridge in 1962, a more accurate figure of 5730 years was agreed upon and this figure is now known as the Cambridge half-life.
- If an organism had 100 grams of carbon-14 when it died, after 5730 years, how many grams of carbon-14 would it have?
- In which year did Willard Libby win the Nobel Prize in Chemistry?
- Who were the other two scientists responsible for developing carbon dating?
For more such interesting chemistry articles and videos, visit: http://mocomi.com/learn/science/chemistry/