Isotope analysis in archaeology

Carbon 14 Dating Carbon-14 is radioactive. It has a half life of 5,730 years. This means that after 5,730 years have passed only half of the original amount of C14 will remain. After 2 x 5730 = 11,460 years there will be ½ x ½ = ¼. After 3 x 5730 = years there will be ½ x ½ x ½ = 1/8…..

C14 is made by the action of cosmic rays. Life is based on carbon. Whilst organisms are alive C14 will be absorbed at the same rate as C12. As C14 decays it is continually replaced. But after death no more C14 is absorbed. It is as if a stop clock is started. Archaeologists have only to measure the C14 in bones, wood, hair…. to date them.

The technique can be used for objects up to 48,000 years old. But when the ages of historic artefacts several millennia old were compared to the radiocarbon dates they were found to be too young. It seems that C14 is not always produced at the same rate. So the dates have been calibrated using the wood of the Bristlecone Pine, which lives for over 7,000 years!

Many elements have isotopes that do not radioactively decay. Isotope proportions often vary from place to place. If bone composition matches the contemporary isotope ratii of a site’s soil and water the people are likely to have been local. Stable Isotopes of elements

Bone is being continually broken down and built up… so its isotopic ratii reflect the last 5 – 10 years of life.

Tooth enamel forms during childhood and adolescence, when the milk teeth are replaced. Premolars typically form at 3–6 years, but the third molars are not in place until c9-13 years. A comparison of tooth enamel with bone isotopes allows individual mobility to be traced.

Oxygen Oxygen has two stable isotopes. 16 O 18 O Usually 18 O accounts for only c0.2%. 18 O forms the same compounds as 16 O. H 2 16 OH 2 18 O 8 protons 8 neutrons 8 protons 10 neutrons

Oxygen in bone comes from drinking water, which is ultimately derived from precipitation. H 2 16 O is lighter and easily evaporates. H 2 18 O is heavier, so needs more energy to evaporate H 2 16 OH 2 18 O So drinking water in hotter climates has relatively more 18 O.

The 16 O : 18 O ratio depends on such factors as; 1) Temperature 2) Altitude 3) Amount of precipitation 4) Distance from ocean. In the British Isles it varies from… -4.5% in the west -8.5% in the north east.

Be Mg Ca Sr Ba Strontium is in the same group of the periodic table as calcium. Ca Strontium (Sr) So it can replace Ca in bone minerals. Ca Sr

Strontium has four stable isotopes 84 Sr 0.56% 86 Sr 9.86% 87 Sr 7% 88 Sr 82.58% Living organisms have no preferences between the Sr isotopes in their food and drink.

87 Sr forms from decay of 87 Rb, which has a half life of 4.75 x1010 years. The 86 Sr : 87 Sr ratio depends on the underlying rocks. It is greatest, c0.720, in ancient rocks of highland Britain >100 million years old, eg; granite and gneiss. In the younger rocks of lowland Britain it is c.704. These data can be used to track down people’s origins from their bones.

Limitations Post mortem changes may effect the stable isotope ratii. Although bone can be treated, eg by washing with acid, to minimise these effects. Modern water may not have exactly the same ratii due to climate change.

Case Study; Who were the “Beaker People who had helped build the megalithic stone circles…? …buried their dead under round barrows with their typical pots? And introduced metal into Britain?

The Archer’s enamel isotopes indicated he came from a colder region than Britain… In 2002 Wessex Archaeology excavated the Amesbury Archer near Stonehenge. … probably from central Europe ( dark blue-green).

His isotopic signature was generally consistent with southern England and Ireland (yellow). But the enamel of his wisdom teeth was different, indicative of the Midlands or north east Scotland (mid green) The Archer had a companion…

In 2003 Wessex Archaeology excavated another Beaker Burial at Boscombe, Wiltshire. A mass grave which contained the remains of two adult men, an adolescent male and a child.

The Boscombe burials had very high ratios of 87 Sr: 86 Sr that are not matched in Wessex. But are typical of Cornwall, Man, Lake District and Highland Scotland.

Oxygen isotopes narrow down their origins. Scotland was too cold, with not enough 18 O. Cornwall was too warm, with too much 18 O. So their childhood home is likely to have been in the Lake District, North or South-West Wales.