Isotope analysis is a very active area of archaeological research, which can give insight into the geographic origin of artifacts and human remains, support studies into human diet and determine the age of sites. The technique provides a unique fingerprint for the sample.
In 1913, a twenty six year old chemist conducted research at Glasgow University for which he would receive a Nobel Prize for chemistry in 1921. Frederick Soddy discovered isotopes. He found that the elements could exist in different forms which were identical chemically but differed in atomic mass. Francis Aston, at the Cavendish Laboratory in Cambridge identified over 212 naturally occurring isotopes and developed the first mass spectrometer. Francis Aston received the Nobel Prize for chemistry in 1922. For much of the twentieth century Alfred Nicer devoted his life to developing the mass spectrometer. It is a very valuable tool which can precisely count the number of isotope ions that are present in a sample.
So how does this apply to archaeology? The atmosphere contains certain elements such as carbon, oxygen and nitrogen that are essential for line. These bio elements occur in predominantly one isotopic form. Second stable isotopic forms exist in very small proportions. They form in the upper atmosphere through the interaction of cosmic rays through the food chain when animals eat the plants. When a plant or animal dies the isotope ratio is “frozen” into the artefact. The essential point is that although the isotopes are chemically similar different plants take up the isotopes in different proportions. Isotope analysis is a very precise science which can determine the geographic origin of organic specimens. It can also examine remains to determine human and animal diets.
One of the classic used of isotope analysis has been to determine the spread of maize cultivation throughout North America. In the first millennium AD this crop spread among Native Americans from Mexico throughout south east America. In the second millennium the crop spread into the New England states and Canada. In 1977 J.C. Vogel and N.J. van der Merwe published the first ever archaeological study that used isotope analysis. They compared four human samples from different periods at the same site in New England. Using carbon isotope analysis they showed that the diet at the site changed at some time between 100 BC and 1000 AD. Similar comparisons at other sites have mapped the spread of maize cultivation across America.
isotope analysis can be very useful to determine human migrations. The chemical composition of a human body replaces itself every two years. Consequently an isotope analysis contains information about the last two years of life before death. In 2002 an isolated grave was found in Amesbury in Wiltshire close to the Stonehenge site. Archaeologists determined that the body was contemporary with the building of the Stonehenge ancient monument then applied isotope analysis. Research using oxygen isotope analysis from his tooth enamel was not compatible with a local diet. It suggested that he came from an Alpine region in central Europe. Moreover he had severe tooth decay. The isotope analysis suggests that he came to Stonehenge as a pilgrimage to heal or sooth his tooth decay.
A completely different form of isotope analysis is applicable to inorganic objects. For organic materials the isotope ratio is set by cosmic ray activity in the upper atmosphere. For mineral deposits the isotope ratio is set when the mineral is formed. This means that each mine and quarry has a unique isotope signature. This signature can be used to determine the mineral origin of an artefact. It can also be used to determine patterns of trade and exchange.
So far this essay has just considered stable isotopes. Radioactive isotopes can be used to date an archaeological specimen or site. For stable isotopes the isotope for the bio elements is set at the time of death for a plant or animal. However, the ratio of carbon 14 and carbon 12 falls away with time because carbon 14 is radio-active.
When scientists measure the ratio of carbon 14 to carbon 12 they assume that at death the specimen must have had a carbon 14 carbon 12 ratio equal to that of the atmosphere. Knowing the half life of carbon 14, which is 5568 years, scientists can estimate the date when the sample was alive. Radiocarbon becomes less accurate with older samples. A recent technique has been used to date Stonehenge complex by combining radio-carbon data with other evidence, usually in form of logical statements – sample A must predate that of B because it lies lower in the soil – and so forth.
Isotope analysis is a very active field of archaeological research. It is an extremely useful tool to determine human migrations and diet during pre-history. For this period archaeologist have no access to written records.
