The skeleton key to putting meat on the bones...

Date published: 13 August 2010

DR ROGER JEWSBURY head of department of chemical and biological sciences, University of Huddersfield, reveals how decay can tell us a lot more than how long something has been dead...
One of the marvels of modern science is the ability of analysts to measure very small amounts, and very small differences.

Isotope ratios are all about detecting differences: — between what we might expect, and what we find. Those tiny differences can tell us plenty about the recent or distant past.

So, what is an isotope ratio?

Elements are generally stable: the protons and neutrons at their core balance in various combinations or isotopes.

In elements with two or more stable isotopes, the ratio of the isotopes changes, depending on the compounds they are in and their physical locations.

Take water — H2O. There are two isotopes of the oxygen atoms in water, and water molecules containing the heavier oxygen will be less volatile. Clouds will be depleted in the heavier oxygen. Fresh water from rain will also be depleted in the heavier oxygen compared to the oceans. Though these differences are tiny, they also change with average temperature and vary with latitude and altitude, which is why they become useful.

This process, called fractionation, occurs in other atoms too. The two stable isotopes of carbon, for example, vary slightly in the carbon dioxide in the air with temperature, but are also affected by their absorption by plants.

And so to investigation. Teeth and bones are sources of information about our environment as a child. Nearly all of the oxygen that goes into the formation of tooth and bone comes from the water we drink and that water is derived from rain or snow. The mineral tooth enamel takes up the oxygen and so the isotope ratio will give information about the environment in which the teeth were formed.

For instance, last year, while excavating a new road in Dorset, the remains of over 50 decapitated bodies were found in a mass grave. Radio-carbon dating of artefacts found in the pit indicated that they were buried about 1,000 years ago, so this was an ancient crime, not a recent one.

But archaeologists were also interested in who the bodies, apparently young men, were. Isotope ratio analysis provided the answer. Tooth enamel showed they had grown up in a much colder climate than southern England, and one even appeared to come from within the Arctic Circle. So they were probably Vikings — perhaps a captured raiding party — rather than locals.

Another isotope ratio used in teeth and bones is that of the element strontium. Found in rocks, the ratio of two stable isotopes varies significantly by location and, because it enters the diet and replaces calcium in the tooth enamel and the bones, is another indication of location.

In 2001, the torso (no head, arms or legs were found) of a young African boy was found floating in the River Thames. HIs DNA didn’t match anything on the databases so the scientist turned to the bones.

The rather high strontium isotope ratio was typical of rocks older than 2,500 million years. There was a suspicion that the murder was a ritual African killing and the isotope data indicated West Africa, around Nigeria.

The next step was for the investigation to collect samples from across Nigeria to develop a map of the variation within the country. This identified a region of about 5,000 square miles around Benin.

Though the case is still open, one man is now in jail for human trafficking as a result.

These are just two examples of this powerful technique, which is finding increasing uses including drug analysis and food authentication.

It is a method which we can expect to see much more of in the future.