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Only one sample is required for this method as both the argon-39 and argon-40 can be extracted from the same sample.
In special cases, bones can be compared by measuring chemicals within them.
Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.
These include radiometric dating of volcanic layers above or below the fossils or by comparisons to similar rocks and fossils of known ages.
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 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.