How can stable isotope analysis help identify migrants to medieval cities like Westoby?
In Step 2.8, we learned that stable isotopes are incorporated into our bones and teeth as we grow. Isotopes such as strontium are contained with the geology of the environment (or rock types). Any plant that grows on a specific geological environment contains a specific strontium signature. Any animals that eat the plants, or humans that eat both the plants and the animals, also contain this signature.
As the distribution of rocks is well mapped in Britain and around the world, the geology provides the key to geographic location (Figure 1). In a similar way, oxygen, contained in the water can suggest a geographical location when it is taken up by plants, animals and humans through their diet (Figure 2). In order to determine whether someone migrated, we measure oxygen and strontium isotopes in the teeth and bones.
As teeth do not remodel throughout life, they contain the childhood signature. But bone turns over almost continually, and reflects the geological signature where the person spent the last years of their life. By comparing the teeth with the bones, we can identify whether someone moved from one location to another during adulthood. Oxygen isotopes also allow us to determine whether someone moved from a hotter or colder climate than the one they died in.
Lead isotope and concentration analysis is a relatively new method for tracing migration. We know teeth are an accurate recorder of childhood lead exposure, and that lead levels differ between pre- and post-Roman populations, rural and urban dwellers, and modern environments with different levels of industrial pollution. Lead analysis is ideally suited to discriminate adolescents migrating from rural areas to medieval towns such as York. Selecting teeth that develop early in life (ie the first molar) allows us to explore the movement of teenagers such as William Westoby.
Figure 1: Geological map of England with strontium isotope values
Figure 2: Oxygen isotope values for modern European drinking water
© NERC Isotope Geoscience Laboratory
In later medieval England, we have growing evidence that young people moved from rural to urban areas, and from north to south seeking work. We have evidence that London attracted immigrants from across Britain (eg Devon, Cornwall, York, Wales and Scotland), Europe and the world, and by the mid-sixteenth century, around 90% of London’s apprentices were migrants. But migration was also local, for example, after the Black Death (AD 1348), York attracted 42% of migrants from a 20-mile radius. Using stable isotope analysis and lead concentrations, we have been successful in tracing the movement of people from near and far in the past.
Exciting as this new area of research is, there are some limitations. Some areas of the world share similar geology, such as the tip of Cornwall and areas in North Africa, making it difficult to always tell whether someone was a local or a migrant. We know that many individuals in medieval England travelled from parts of Europe, but as much of Europe shares our geology, these individuals’ movements become invisible to us.
We are far from completing our geological map of the world, and there are still areas that are likely to have produced immigrants to medieval England (such as Poland), where the geological signature is still largely unknown. Environmental lead concentrations can tell us if an individual moved from the North to the South of England, but it is harder for us to identify individuals who moved from one Northern town to another, or from the east to London as they share similar lead signatures.
To learn more, take a look at the video by Dr Janet Montgomery (University of Durham) one of the team members working on the medieval Teenagers project.