Methods of Dating Fossils

Methods of Dating Fossils

We have learnt how fossils are formed over geologic timescales. In this section we will learn how we determine the age of a fossil. There are two methods of dating fossils:

1. Radiometric dating
2. Relative Dating

1. Radiometric Dating

In order to understand radiometric dating, it is necessary to revise our understanding of the atom. The nucleus of an atom is made up of protons and neutrons. The number of protons in the nucleus define what type of element it is. However, the number of neutrons of an element may vary. Atoms with the same number of protons, but different numbers of neutrons are called isotopes.

Some isotopes are stable, while others are unstable. Unstable isotopes undergo a process called radioactive decay, whereby they spontaneously change to elements of a different type. We can never predict when a specific atom will undergo radioactive decay. However, when considering many atoms, we observe that the decay occurs at an exponential decay rate. Exponential decay means that over a certain period of time, called a half life, half of the unstable isotopes in a sample will undergo radioactive decay.

One of the most useful radiometric dating methods is radiocarbon dating.

Radiocarbon dating

• The carbon atom exists as three different isotopes. These are carbon-12 (C-12), carbon-13 (C-13) and carbon-14 (C-14).
• All living organisms maintain an equilibrium of carbon-14 with the atmospheric carbon-14.
• After an organism dies, it no longer incorporates new carbon into its body.
• The carbon-14 present within them undergoes radioactive decay to nitrogen-14, and decreases from the original equilibrium with carbon-12.
• The half-life of carbon-14 is 5730 years. This means that if we start out with say 10 units of carbon-14, in 5730 years' time only 5 units will remain. In another 5730 years' time only 2.5 units will remain and so on.

• Thus, using radiocarbon dating, scientists can determine how much carbon-14 remains within a particular fossil, and thereby infer the age of the fossil.

The isotopes of different elements undergo decay at different rates; some decay much more rapidly than others. This makes radiometric dating a useful tool as scientists can use different elements to date longer and shorter time-scales. Carbon has a relatively short half-life, and therefore it is not of much use when dating fossils that are millions of years old.

Uranium-lead and potassium-argon dating

In order to date older fossils, scientists use uranium-lead dating and potassium-argon dating. The half-life of uranium-235 is 700 million years, and the half-life of potassium-40 is 1.3 billion years. In order to estimate the age of incredibly old fossils, scientists date the age of the igneous (volcanic) rock in which the fossils are buried. This provides them with an estimate of the age of the fossils contained within them.

2. Relative Dating

• Relative dating is the method of determining the order of events from the fossil record.
• By studying the order in which fossils occur in the fossil record, geologists can determine the order of events as they occurred but not when exactly they occurred.
• Fossils found at the lowest layer of rock would be the oldest, as these would have been buried for the longest time, whereas fossils found closer to the surface would be buried more recently and therefore be younger.
• The geological time-scale you studied earlier was almost entirely developed by relative dating methods. It is a useful method of dating when fossil materials lack radioactive isotopes.