¾«¶«´«Ã½

Murder detectives finally have a reliable way to determine how long a victim has been dead from their bones, as well as glean valuable information about where the person lived.

Stuart Black, an environmental geochemist at the University of Reading, is pioneering a technique similar to carbon dating, but that uses isotopes of other elements. He has just finished his first case.

From the charred remains of a man who was repeatedly stabbed and then set on fire, he determined that the victim was probably from the former Soviet Union and had been dead about a week.

“While that doesn’t give us a name and address, it helps immensely,” says Detective Superintendent David Hankins of the Cambridgeshire police. “Dr Black has given us an incredible amount of information.” Police are so impressed, Black’s lab is already working on two other murder cases, and three more are awaiting analysis.

Decay rate

To date remains, forensic scientists normally rely on studies of how bodies decay in different climates (New ¾«¶«´«Ã½ print edition, 6 January 2001). But the temperature and moisture conditions where the corpse was left are often unknown, making these methods imprecise.

“The error is unquantifiable. It relies on pathologists saying ‘oh, I’ve seen a bone like that before, and it was about this old’,” says Black.

Instead, Black looks at the decay of radioactive isotopes. Carbon dating has long been used by archaeologists looking at bones that are centuries old, but is little use for younger bones. To date more recent remains, Black is using isotopes with shorter half-lives than carbon-14. He has found that the most useful are lead-210 and polonium-210, with half-lives of 22 years and 134 days respectively.

These and other elements entering our bones are primarily from food, so Black can also use his measurements to sketch the victim’s diet. A depletion of certain elements will reveal if the person was a vegetarian, for example.

Regional diet

Black has also refined a way of using various stable isotopes of lead to indicate where the victim lived in the last decade of their life. This lead is breathed in with air, and the amount depends on factors such as local geology and the kind of petrol used by cars in the area.

Knowing where the victim lived is useful for police, but also means Black can take regional differences in diet into account, to pin down the age of the bones even more accurately. In Britain, he consults the Food Standards Agency’s database to check how much radioactive lead and polonium is in different foods.

Black originally tested the technique on a sample of bones from 25 elderly women from a small Portuguese town. He found he could pin down the date of death to within two years for women who had been dead 50 years. That precision probably reflects the fact that they had very similar diets, says Black. To work out how accurate the technique is in other cases, he says we need to find out how variable these radioactive elements are within other populations.

But even with a big margin of error, the method could still sort out the surprisingly common confusion about whether bones are years or centuries old, according to forensic scientist Kenneth Pye, who heads a consultancy in London. “You get heaps of remains found in London building sites. These are treated as crime scenes, but they often turn out to be medieval.”