You are what you eat: exploring questions of migration, diet and status through the use of isotope analysis

Isotope analysis involves taking samples from bone or teeth (of humans or animals) and measuring the ratios of different isotopes to provide information about an individual. The technique can be used to answer questions about the mobility of people and animals, and their diet.  Differing isotope ratios provide information about the chemistry of the food and drink consumed by an individual at the time their body tissues are formed. The chemical composition of ingested material is tied to the source location of where plants were grown (geology) or where the water originated from (related to both geology and climate).

Strontium isotope ratios are one of the main indicators used for examining childhood origin, i.e., the geology of the area where plants that were eaten grew when an individual’s adult teeth were forming. If the geology of the area where an individual died differs from that revealed by the isotope analysis, this can provide us with information about the way that people moved around. Using samples of tooth enamel that developed during childhood, oxygen isotope ratio data is also used in conjunction with strontium ratios to examine where the drinking water ingested during an individual’s formative years originated.

Carbon and nitrogen isotope ratios are examined to provide dietary information, which comes from the collagen in bone. As the food chain ultimately leads back to plants, it is important to examine local animal bone ratios alongside those of the human population, in order to understand the local background isotope ratios of the animals consumed by people. Interpreting the results of carbon and nitrogen ratios allows us to explore variation in levels of consumption of animal protein, aquatic resources (particularly marine fish) and plants.

We often conduct these analyses on excavated human skeletons in order to explore particular research questions, such as whether or not the individuals were local to the area in which they had been buried, or whether we can ascertain a person’s social status from the type of food they had consumed.

These types of questions produced some very interesting answers at our site at Weyhill, Andover, which was the site of a late Anglo-Saxon and Norman period execution cemetery. Twelve of the skeletons from the cemetery were examined for oxygen isotope ratios, while strontium ratios were analysed for eight of the same group. Carbon and nitrogen isotope ratios were analysed for 20 of the skeletons, along with two sheep (the latter in order to provide information about local background values).

The skeletons were selected for isotope analysis in order to explore whether they were likely to have been local people, or whether they had travelled from further afield. The earlier part of the date range of the cemetery coincided with a time when Viking raids on Britain were taking place, for instance, and the analysis had the potential to provide insights into the origin of some of those executed. We were also interested in the diet of the individuals, to try to determine whether there was a difference between the method of execution used (hanging or decapitation) according to the social status of the individuals. This is based on the presumption that those of higher status are likely to have had a diet higher in meat (and thus a higher ratio of nitrogen to carbon) than those occupying the lower echelons of society.  

The results of the analysis did not disappoint. Two of the individuals had oxygen ratios which indicated that they were of likely Scandinavian origin. A further two of the skeletons had strontium ratios suggesting that they had not come from Andover, and may therefore also have been immigrants; another had oxygen and nitrogen ratios that were indicative of an origin from elsewhere in Britain.

The nitrogen ratios of the group overall were lower than would be expected for a diet which included marine resources, suggesting that sea fish did not form a large part of their diet. The carbon and nitrogen isotope ratios were ambiguous, however, and provided no link between the diet of individuals and the method used during their execution.

Cotswold Archaeology is grateful to Mandy Jay for undertaking the isotopic analysis, and for providing the above information.

If you would like to learn more about the exciting site at Weyhill, Andover, further details can be found in the monograph about the site, which was published in 2020.

Sharon Clough