Brent Cornell

Key Knowledge: Relative and absolute dating of fossils

Dating techniques are used to predict the ages and trace the evolutionary history of fossilised remains

• The two types of dating methods employed by scientists are relative dating and absolute dating

Relative Dating

Relative dating is an inexact measurement system that uses rock layers (formed by stratification) to assign comparative ages to fossils

• The earth is arranged into sedimentary layers (strata), with younger stratum sitting on top of older stratum (law of superposition)
• Fossils are identified as older or younger depending on the strata in which they are found (relative dating does not offer specific dates)
• If an igneous intrusion or fault cuts through existing rock, the intrusion is younger than the horizontal rock layers it cuts through

Occasionally, a strata representing a particular time period may be missing due to environmental conditions (e.g. flooding, erosion, etc.)

• The presence of index fossils can be used to synchronise the age of rock layers between two different regions

Absolute Dating

Absolute dating uses the rate of radioisotope decay to determine the exact age of a fossil sample or rock strata

• Radioactive isotopes decay at a constant rate and the time taken for half the original radioisotope to decay is known as the half life
• Scientists determine how long ago fossils formed by comparing the ratio of radioisotopes in the fossils to that found in the atmosphere
  
• Different radioisotopes have different half lives and are thus useful for dating different types of fossilised remains

Short Range Dating

• All living things contain carbon and this carbon exists as a mix of two isotopes – ¹²C (stable) and ¹⁴C (radioactive)
• While alive, the proportion of the two isotopes will mirror environmental levels (as carbon is constantly being cycled)
• When an organism dies, the ratio no longer remains fixed but changes as ¹⁴C breaks down into ¹⁴N (beta decay)
• Scientists can measure the amount of ¹⁴C remaining in a sample to determine how long ago it died
• ¹⁴C has a half life of only 5,730 years and so can only effectively date samples less than ~60,000 years old

Long Range Dating

• Long range dating require the use of radioisotopes that are not present in fossils (or the sedimentary rock in which they are found)
• Long range dating can only be undertaken on igneous rock around the fossil and is used to determine the age range of a rock strata
• The age of the fossil is then approximated according to the strata in which it is located (i.e. relative dating must be utilised)
• Some examples of possible radioactive isotopes that may be used for long range dating include:
  • Uranium-238 breaks down into Lead-207  (half life = 704 million years)
  • Potassium-40 breaks down into Argon-40  (half life = 1,300 million years)

⇒  Click on the diagram to show the proportion of radioisotope remaining