Carbon dating and the Jersey case

Paul Rincon

Science reporter, BBC News 

The police chief leading the Jersey abuse inquiry says attempts to carbon date remains from at least five children are unlikely to yield results.

So far, police have found 65 milk teeth and more than 100 bone fragments during their search of the former Haut de la Garenne children's home.

Radiocarbon dating can be a useful tool in police investigations when combined with other forensic information.

The technique relies on a simple natural phenomenon: living organisms contain both stable forms of the element carbon and a radioactive form, called carbon-14 (C14).

Humans take this radioactive carbon into their bodies by eating plants and animals. But when an organism dies, the C14 inside it begins to disappear.

Scientists can use this fact to measure how much radioactive carbon is left and how much has disappeared.

By comparing this against modern levels, they can calculate a date for the death of the organism. This is done by testing C14 in organic matter such as bone, teeth or seeds.

Nuclear contamination

However, attempts to date organic remains from after the late 1950s are affected by one of the consequences of 20th Century politics.

"Nuclear bomb testing that started in the late 1950s significantly contaminated the whole of the atmosphere. So you have very high levels of carbon-14 through that period that didn't exist prior to that," said Professor Gerry McCormac, an expert on radiocarbon dating from Queen's University Belfast.

"When we're going back in time, you don't have that effect, it's really the natural radiocarbon you're using to get the measurements from."

This nuclear "enrichment" can be very useful in forensic cases dealing with remains from after the 1950s. If the remains from Jersey were from after this time, the signal of nuclear tests would be obvious.

But even before the "bomb carbon" period, humans were already disrupting the natural radiocarbon signal by burning fossil fuels such as oil, coal and gas.

Fossil fuels contain no carbon-14; the organic matter is so old - millions of years old - that all the radioactive C14 has decayed away.

Misleading signal

Fossil fuel burning releases carbon dioxide (CO2) into the atmosphere. This CO2 is composed only of the stable forms of carbon, but no radioactive C14.

This then mixes with existing atmospheric CO2, diluting the overall concentration of radioactive carbon.

Dr Gordon Cook, from the radiocarbon lab at the Scottish Universities Environmental Research Centre (SUERC) in Glasgow, explained: "A plant takes up the C14, an animal eats the plant, the animal looks as though it has a lower C14 activity for a living organism than it should do.

"It dies, you measure it, it looks as though it died some time ago when, in fact, it only just died."

Deputy police chief Lenny Harper, who is leading the Jersey abuse investigation, told the BBC that one bone from the Haut de la Garenne site, which had been radiocarbon dated yielded "a probability the person had died in 1650, but also a smaller probability they had died in 1960".

In addition, the natural production of C14 fluctuated between 1670 and 1950, making it very difficult to date material from this period anyway.

All of this means that if the Haut de la Garenne remains are pre-1950s, or just on the cusp of the nuclear era, it might be difficult to distinguish them from much older remains - those, say, from the 19th or 18th Centuries.

Burial context

"It's unlikely that carbon dating will be able to shed much light on the dates of these individuals," said Professor McCormac.

"What it could do is rule out a prehistoric origin - for example, if this was an old burial site of 200 years ago or beyond that, carbon dating could tell you that. What it couldn't tell you, to within a few years, is when that individual died."

Radiocarbon dating can also be difficult if the bones are very small or fragmentary. This is because scientists rely on the bone retaining significant amounts of the protein collagen to test. Extensively burned remains are also unsuitable for dating.

Professor McCormac explained: "The context of where they found [the human material] and the buildings around them and the strata in which they were found will typically give them more information than a carbon date would."

Deputy chief officer Harper explained: "We have the evidence that the bones were placed where we found them no earlier than the late 60s/early 70s. We have the evidence that they were burned.

"We have the evidence they were deliberately concealed. And we seem to have evidence - we think - that they were moved from one part of the building to another."

He told the BBC's Today programme that it was always possible the human remains were much older.

"Then you have to ask, why would people go to all the trouble of moving the bones, of burning them at some stage, of hiding them in a different place and then of covering them up."

Paul.Rincon-INTERNET@bbc.co.uk