Where do the people buried on Hare Pie Hill come from? HFWG have just received a grant to undertake Isotope analysis of the skeletons associated with the medieval chapel, thought to be a site of Pilgrimage.
Isotope analysis uses elements such as oxygen, carbon, nitrogen and sulphur as well as strontium and lead found in living organisms to provide information about their place of origin, diet and mobility.
Oxygen in human tissue comes primarily from water in ingested fluids and reflects the isotopic value of available drinking water. Direct comparison with reference datasets can be used to provenance samples and help to determine an individuals place of origin using climate based maps.
Oxygen isotopes (d18O) in tooth enamel and dentine is fixed at the time of formation and not replaced during an individual’s life time. The isotopic composition of tooth enamel reflects different periods in a child’s life, for example the second premolar forms between the age of 3 and 7 while the third adult molar forms between 9 and 13 years of age. Bone, on the other hand, remodels during life and the rate of turnover varies with the type of bone, age and health of an individual. For example, in rib the turnover rate is approximately 2-3 years while for femur the rate is between 10-15 years. If that material is available, it is possible to examine at four stages of life: early childhood, adolescence, mid-life and the last few years of life.
Strontium is chemically similar to calcium, and enters the biosphere primarily via plants. This, in turn, relates to the underlying geology and varies depending upon a combination of factors which include the age of the rocks and weathering conditions. In essence strontium isotope values represent the geology of the area where an individual obtained their food and water. The distribution of strontium isotopes in the UK is well understood based on measurements of bio-available materials and geological maps and similar data is available for the rest of Europe. Combined strontium and oxygen analysis provide a powerful tool for determining an individual’s geological and climatic place of origin. As bone is susceptible to post burial alteration, tooth enamel is the material of choice for this analysis.
Dietary Isotopes – Carbon, nitrogen and sulphur
Carbon stable isotope ratios vary between foods from different ecosystems (e.g. land or water), while nitrogen stable isotope ratios give an indication of the trophic level an organism is feeding at. Carbon and nitrogen isotopes can provide evidence for sources of dietary intake and can also helpful in determining food related culture, status and lifestyle.
This analysis is usually conducted on bone or dentine collagen but can also be measured in hair and skin. Different bones have different collagen turnover times, and reflect diet at different stages in an individual’s life, with dentine preserving the isotopic signature of diet at the time of tooth formation in childhood.
Carbon isotopes in humans and animals are derived from proteins and carbohydrates found in animal protein, grains, vegetables, and fruits. Carbon isotopes also provide information on food sources dependent on the local climate. Additionally, compared to terrestrial feeders, individuals consuming significant amounts of marine foods will have higher values and a high intake of freshwater fish will result in slightly lower values.
Nitrogen isotopes come from dietary proteins primarily meat, fish, eggs and milk products but also from vegetable protein sources such as legumes and mushrooms. They provide information about an individual’s position in the food chain. In general, the higher the nitrogen isotope values the higher up the food chain an individual is feeding. High consumers of herbivores and dairy products will have lower nitrogen isotope values than consumers of terrestrial and aquatic carnivores. Consumers of significant amounts of marine and freshwater foods tend have very high values as aquatic food chains are longer.
Sulphur isotopes (in the diet are related to the local geology, soil environment and proximity to the sea and consumption marine food sources. Sulphur is found in bone and dentine collagen, and keratin in soft tissues such as skin, hair and finger nails. Conditions such as waterlogging can affect the isotopic composition but in general, away from the coast, sulphur isotopes in humans, animals and plants is related to regional geology. With diets that include significant marine resources and or where food is sourced close to the coast, sulphur isotopes are conspicuously high.
When combined carbon, nitrogen and sulphur isotope values become a powerful tool for use in understanding both diet composition and identification of food sources.
Lead is a toxin, but it can become incorporated into the body and used to trace the amount and source of lead ‘pollution’ to which an individual was exposed. Populations which have little exposure to metal-ware (i.e. the more ancient populations) have lead concentrations that reflect the natural background levels picked up from the environment and the isotope composition of the local geology where the individuals lived. The advent of trade and the wider use of metal-ware is reflected in the populations with increased lead concentrations.