Carbon dating: a route to the past

THE recent reports that the remains of a near 11,000-year-old house had been unearthed at the Star Carr archaeological site near Scarborough begs two questions.

How do we know how old it is and how sure can we be of the date?

There are no old newspapers from the time (in fact they would not have been able to make paper let alone print one) but other carbon-based remains do exist including wood, charcoal and some plant and animal remains.

For these, radiocarbon dating can be used to estimate the age. Radiocarbon, or carbon-14, is a rare isotope of carbon found in very small amounts in the carbon dioxide gas in the atmosphere.

It is formed in the upper atmosphere by the interaction of cosmic rays interacting with nitrogen atoms. Carbon-14 is radioactive, so it decays at a fixed and known rate. The rates of formation and decay are balanced so that the amount in the atmosphere is fairly constant.

Spread uniformly throughout the carbon dioxide in the air it is taken up by plants and then by the animals which eat the plants.

Once the animal or tree dies, the proportion of carbon-14 then decays to half its value in a fixed time of just over 5,500 years.

If we find some charcoal or some wood, for example, and measure the proportion of carbon-14 to total carbon, providing we know the proportion of carbon-14 in the carbon dioxide at the time the tree was growing, then we can calculate how long it is since the tree was cut down.

We can make the assumption that the proportion of carbon-14 in the carbon dioxide has been constant for 10,000 years or more to get an approximate date but we know that carbon dioxide in the atmosphere is increasing due to the burning of fossil fuels and, of course, these are so old that they do not include carbon-14.

In fact, the proportion of carbon-14 has decreased since about 1850 due to the burning of coal and later oil and gas, although it did rise in the 1950s due to atmospheric testing of nuclear weapons.

Another problem is that the rate of formation depends on the activity of the sun, which we have even less idea of in the past.

To be certain of our results, we need to calibrate the dating method. This can be done using tree rings which reflect the different growth rates of trees in winter and summer. For a growing tree these date the wood to the nearest year.

Some trees, for example the Bristlecone Pine, grow for over 4,500 years but that is clearly not enough for an 11,000-year-old house.

To continue further back, preserved older pieces of wood have to be used along with matching of growth rates which will vary from year to year. Oak and pine trees preserved in peat bogs in the UK and Germany have been used to calibrate the carbon-14 dating.

To go even further back, marine deposits such as corals are being used to provide at least an improved but still approximate calibration.

Peat bogs, which are acid and devoid of oxygen, are particularly good at preservation. Several bodies found in peat bogs have initially been thought to be recent murders.

A body of a woman found in Cheshire led to the arrest of a man who then confessed to killing his wife several years earlier. He was still convicted even though, after his arrest, the body was carbon dated to 250 AD.

Controversy often does accompany radiocarbon dates. The Turin Shroud is a cloth thought by some to show the image of Jesus Christ at the time of crucifixion.

Small fragments were independently dated in three laboratories about 20 years ago and all the labs agreed that the samples were from around AD 1300.

While there can be little controversy over the dates, there is concern that the samples from the edge of the cloth were in fact a later repair and not representative of the shroud as a whole.

While predicting dates in the distant past is not easy and great care is needed to ensure reliable results, the continued developments are making this a much more reliable process.


Roger Dewsbury is head of the Department of Chemical and Biological Sciences at the University of Huddersfield