Isotopes, Diet and Mobility
In order to understand diet and mobility in the past, archaeologists can use isotopic analysis of archaeological skeletal material.
Isotopes and isotopic methods
Isotopes are atoms that have the same number of protons, but a different number of neutrons, resulting in a differing atomic masses. An element can have several isotopes, e.g. nitrogen has two stable isotopes: nitrogen-14 and nitrogen-15. The relative ratio of the different isotopes of an element can be altered through biological and chemical processes. For example, oxygen has three stable isotopes (oxygen-16, -17 and -18) and the ratio of these in water varies throughout the water cycle, as water molecules containing the ‘lighter’ oxygen isotopes are more readily evaporated while those containing the ‘heavier’ isotopes are more readily condensed.
The isotopic ratios of certain elements in our food and drink are incorporated into our bodies, including our bones and teeth. Studying these ratios can provide information about diet and the environment our food and water came from.
Image: Alice Rose
To find out about diet in the past, samples are taken from archaeological remains, usually bones or teeth, for carbon and nitrogen isotope analysis. This can broadly tell us about the types of plants people ate and the proteins they consumed from animals, including fish. In general, the higher someone is in the food chain, the more their bones and teeth are enriched in nitrogen-15, so a person who regularly ate animal proteins would have higher nitrogen isotope values than someone who only ate a plant-based diet. Certain foods, such as marine fish, can result in much higher carbon and nitrogen isotope values in the bones and teeth of the people that ate them regularly.
Because teeth are formed from childhood, tooth samples can tell us about a person’s diet when they were a child. Bone renews constantly throughout a person’s life, and so bone samples tell us about a person’s diet in the last decade or so of their life.
To find out about mobility in the past, samples are taken from archaeological teeth for oxygen and strontium isotope analysis. This can tell us about the geology and climate of the area someone lived in as a child, as these factors affect the isotopic signature of the local water sources that they may have consumed in childhood. This can then be compared to the geology and climate of the area where they are buried to see if they are similar. If not, then it may indicate that a person has not been buried near to where they spent their childhood, which might mean that they died and were buried far from where they originated.
Isotope analysis in archaeology is a broad tool. Carbon and nitrogen isotopes cannot tell us exactly what a person ate and cannot distinguish between different types of animal proteins: cheese, milk and eggs would cause a similar isotopic result to meat consumption. Isotopic values in bones and whole teeth also represent an average over a few years of a person’s diet, so seasonality and more detailed changes in diet cannot be detected. Isotopes that tell us about mobility cannot tell us exactly where someone came from, rather, they are useful for flagging up people who are unlikely to have been born in the area they are buried and for eliminating areas where they are unlikely to have come from.
Diet in medieval Cambridge
To investigate diet in medieval Cambridge, carbon and nitrogen isotope analyses were run for 314 rib bones and 227 teeth from medieval skeletons, mostly adults, from Cambridgeshire. Some data from a previous research project was also included. These skeletons spanned the Neolithic to the post-medieval periods, but most dated to the high/later medieval period.
Overall, results indicate that most people in medieval Cambridge had an omnivorous diet, containing some type of animal protein. The isotope data corresponds well with other studies of later medieval sites and common crops in Britain, indicating they probably ate plants such as wheat, barley, rye and peas. Some individuals, particularly those from the Hospital of St. John and the Augustinian Friary, had values which probably show that they were regularly eating ocean fish. This would fit with religious dietary restrictions at the time as observant Christians, especially members of religious orders, were expected to abstain from meat and eat fish instead on many fast days. There was also a general increase in the availability of marine fish during the later medieval period due to advances in fishing technology and trade routes.
There were differences in the mean and range of the carbon and nitrogen isotope data for each group of skeletons from the main high/later medieval sites (Cherry Hinton, All Saints by the Castle, the Hospital of St. John and the Augustinian Friary). The Friary generally had the highest values, the Hospital having the widest range and the ‘rural’ site of Cherry Hinton generally having slightly lower values. This perhaps indicates that some differences in diet based on social status were present across high and later medieval Cambridge.
The results of the isotope analysis also appear to show some differences in diet between women and men, with men generally showing wider ranges than women at each site. There is evidence of some sex-based differences in both carbon and nitrogen isotope values at the Hospital. This could mean that men and women experienced some dietary differences, with differential access and variety in animal and marine proteins.
The isotopic analysis provided no convincing evidence for large changes in diet after the Black Death or by century when the data was broken down by skeletons from the thirteenth, fourteenth or fifteenth centuries. However, this evidence does not necessarily rule out dietary changes through the high/later medieval period, just that they were not detectable using isotopic methods. There does seem to be a greater range of isotopic values among individuals in later periods, which could indicate increasing social variation in food consumption.
Moving around in Medieval Cambridge
To investigate geographical mobility in high/later medieval Cambridge, samples from 296 teeth from 11 Cambridgeshire sites covering the Iron Age to the post-medieval period were taken for oxygen isotopes. 131 teeth from eight Cambridgeshire sites provided reliable strontium isotope data. Some individuals were targeted for ‘unusual’ characteristics, such as atypical burial practices, but largely the samples were chosen to cover a representative proportion of sex and ages.
Overall, the results of oxygen isotope analysis for the high/later medieval period in Cambridge are inconclusive. The range of the values is very large and there is considerable overlap between sites meaning they are not very helpful for characterising an ‘expected’ signature for Cambridge, or for making comparisons. In contrast, the strontium isotope values for almost all of the high/later medieval individuals sampled form a tightly clustered group within the expected isotopic range for the geology of Cambridge. This means that it is likely that most of the people buried in the various Cambridgeshire sites also spent their childhoods in the local area. However, the particular strontium isotope signature of Cambridge and its environs is found across a wide area of eastern England, as well as some areas such as Northern France and Flanders, and so people who grew up in other areas with similar geology to Cambridge would be indistinguishable from a ‘local’.
However, nine individuals have strontium isotope values that are above the expected range for Cambridge, indicating that they spent their childhoods elsewhere. Some of the values could match the geology of areas not too far from Cambridge, such as Ely and King’s Lynn, while some other values are more commonly found areas such as the Midlands and west of Britain. Of course, values could also match other areas across the world. Several individuals came from somewhere beyond East Anglia, in at least two cases probably either the far west of Britain, Scotland or overseas such as France, Scandinavia or elsewhere. These 'exotic' individuals were buried at a variety of sites: Cherry Hinton, All Saints by the Castle, the Hospital of St. John and Clopton, showing that population mobility wasn’t necessarily more common at one particular site. They were of all different ages, ranging from 18 to 59 years, with a fairly even ratio of women and men. The earliest 'exotic' individual dated to c. AD 940-1100 and the latest to c. AD 1330-1400. As this suggests, even ordinary people in the Middle Ages sometimes moved long distances.