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Fossils themselves, and the sedimentary rocks they are found in, are very difficult to date directly.
Instead, other methods are used to work out a fossil’s age. These include radiometric dating of volcanic layers above or below the fossils or by comparisons to similar rocks and fossils of known ages.
Why date a fossil?
Knowing when a dinosaur or other animal lived is important because it helps us place them on the evolutionary family tree. Accurate dates also allow us to create sequences of evolutionary change and work out when species appeared or became extinct.
How to date a fossil
There are two main methods to date a fossil. These are:
- absolute dating methods that tell us the actual age (in years) of an object. There are many absolute dating methods. Nearly all of these methods make use of radioactive elements that occur naturally in various types of minerals and organic matter.
- relative dating methods that can only tell us whether one object is older or younger than another – they cannot pinpoint an actual age in years. Relative dating methods are used to work out the chronological sequence of fossils. They can be applied to fossils found at a particular site and can also be used to make comparisons between sites.
Where possible, several different methods are used and each method is repeated to confirm the results obtained and improve accuracy. Different methods have their own limitations, especially with regard to the age range they can measure and the substances they can date. A common problem with any dating method is that a sample may be contaminated with older or younger material and give a false age. This problem is now reduced by the careful collection of samples, rigorous crosschecking and the use of newer techniques that can date minute samples.
|Dating methods||Useful age ranges (years)||Materials dated|
|Absolute dating methods:|
|Fission track||5000 to 100 million||volcanic minerals, teeth|
|Potassium-argon and argon-argon||200,000 to > 4 billion||volcanic rocks and minerals|
|Relative dating methods:|
|Chemical analysis||0 to > 4 billion||bone and fossilised bone|
|Stratigraphy||0 to > 4 billion||fossils and other objects found in identifiable layers of sediment or sedimentary rock|
|Biostratigraphy||0 up to 2 billion||similar fossils from different sites|
|Palaeomagnetic stratigraphy||0 up to 80 million||fossils found in layers of identified magnetic orientation|
What kind of rocks are most useful in dating fossils?
Volcanic rocks – such as tuff and basalt – can be used in dating because they are formed at a particular moment in time, during an eruption. Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.
Absolute dating methods
Uranium is present in many different rocks and minerals, usually in the form of uranium-238. This form of uranium usually decays into a stable lead isotope but the uranium atoms can also split – a process known as fission. During this process the pieces of the atom move apart at high speed, causing damage to the rock or mineral. This damage is in the form of tiny marks called fission tracks. When volcanic rocks and minerals are formed, they do not contain fission tracks. The number of tracks increases over time at a rate that depends on the uranium content. It is possible to calculate the age of a sample by measuring the uranium content and the density of the fission tracks.
The age of volcanic rocks and ash can be determined by measuring the proportions of argon (in the form of argon-40) and radioactive potassium within them. Each volcanic eruption produces a new deposit of ash and rock. Fossils and other objects that accumulate between these eruptions lie between two different layers of volcanic ash and rock. An object can be given an approximate date by dating the volcanic layers occurring above and below the object.
Argon is gas that gradually builds up within rocks from the decay of radioactive potassium. It is initially formed in the molten rock that lies beneath the Earth’s crust. The heat from a volcanic eruption releases all the argon from the molten rock and disperses it into the atmosphere. Argon then starts to re-accumulate at a constant rate in the newly formed rock that is created after the eruption.
This relatively new technique was developed in order to achieve more accurate dates than those obtained from the potassium-argon method. The older method required two samples for dating and could produce imprecise dates if the argon was not fully extracted. This newer method converts a stable form of potassium (potassium-39) into argon-39. Measuring the proportions of argon-39 and argon-40 within a sample allows the age of the sample to be determined. Only one sample is required for this method as both the argon-39 and argon-40 can be extracted from the same sample.
Relative dating methods
In special cases, bones can be compared by measuring chemicals within them. Buried bones absorb chemicals, such as uranium and fluorine, from the surrounding ground and absorb more of these chemicals the longer they remain buried. The rates of absorption depend on a number of factors which are too variable to provide absolute dates. This technique is, however, useful for providing relative dates for objects found at the same site.
Another useful chemical analysis technique involves calculating the amount of nitrogen within a bone. The level of nitrogen gradually reduces as the bone decays. Absolute dating is not possible with this method because the rate at which the nitrogen content declines depends on the surrounding temperature, moisture, soil chemicals and bacteria. The technique can, however, provide the relative ages of bones from the same site.
Most fossils are found in sedimentary rocks deposited in layers. Where the rocks are not strongly folded or tilted it is possible to work out the order in which the layers were formed. The oldest rocks and fossils are at the bottom and the youngest are on top.
Scientists are able to recognise fossils that are characteristic of various rock layers. With this knowledge, they can place the fossils into detailed chronological sequences. These known sequences can be compared with the layers of rock and fossils uncovered at other sites to provide relative dating. Some fossils are particularly useful for these comparisons as they show distinct changes over time.
This method of dating is based on the changes in the direction of the Earth’s magnetic field. Today this field is centred on magnetic north. Prior to 780,000 years ago it was centred near the South Pole and before that it was centred north and so on. These changes in direction are known as reversals. Scientists work out the direction of the Earth’s magnetic field in the past by looking for traces of iron-oxide minerals that are found in many rocks. Because iron oxide is magnetic, the minerals tend to be oriented in the direction of the Earth’s magnetic field at the time the rock was formed. This technique has established a known sequence of reversals from dated layers found all around the world. If a sequence of reversals is found at a particular site then it can be compared with this known sequence in order to establish an approximate date.