Historical Geology/Fossils and absolute dating

Sandstone with fossil shells.

In this article we shall discuss how fossils can be used for the purposes of absolute dating.

Fossils and dating

We have already discussed the construction of the geological column. If our stratigraphic methods show that fossil A was always deposited below fossil B whenever we are in a position to compare their dates of deposition, then we can conclude that species A is older than species B. We can apply the same sort of reasoning to the stratigraphic relationships of fossils and datable rocks.

For example, suppose that using stratigraphic methods, we can show that a particular fossil is always older than rocks which are 14 million years old or less, and always younger than rocks which are 16 million years old or more, whenever we are in a position to make a comparison.

Now, it is a fundamental principle of science — arguably, the only fundamental principle of science — that a rule that works every time we can test it must be taken as true unless and until we find a counterexample. So in this case we would have to conclude that this fossil species is between 14 and 16 million years old wherever we find it, even in those cases where there are no datable rocks that we can compare it to.

But this means that we can now use the fossil species to date the sedimentary rocks in which it is found; and we can say that those fossils found in the same strata as this species must be the same age; those species which stratigraphy tells us are older than it is must be more than 16 million years old; and those species which stratigraphy tells us are younger than it is must be less than 14 million years old.

Hence we can use datable rocks to put dates on fossil species; and then we can use the fossil species to put dates on other rocks which would otherwise be difficult to date.

Those fossils we have described as "index fossils" are particularly suitable for this purpose, since they have a wide geographical distribution but only inhabit a thin slice of time.

Advantages of the method

There are three main advantages of using fossils for dating in this manner.

First of all, we may want to date a stratum which is a long way up or down from any rocks we can date using radiometric methods. In this case, the use of fossils will be absolutely the best method available.

Second, it is much faster than any more technical method. Why send a rock to a laboratory and wait for a reply when you can just glance at the fossils it contains and say: "Ah yes, Early Ordovician"?

Third, by the same token, it's much cheaper. Radiometric dating requires specialized equipment: lasers, spectrometers, or in the case of Ar-Ar dating a small nuclear reactor. Even the humblest items of equipment come at a price: laboratories that carry out U-Pb dating wash the bottles they use for two years continuously to eliminate contamination. Rather than employ the services of such a laboratory, it is so much cheaper for the geologist to recognize a well-known species of ammonite, trilobite, foraminiferan, or whatever, the age of which is already known.