What is stable isotope analysis?

Isotopes are atoms that have an equal number of protons and unequal number of neutrons, and can be divided into two categories—radioactive and stable. While radioactive isotopes have unstable nuclei that breakdown and release potentially dangerous radioactive emission, stable isotopes have a stable nucleus that doesn’t degrade and produces no harmful effects.

Stable isotopes surround the world in which we live and breathe. As we grow, they are incorporated into our bone and teeth through the food we eat and the water we drink, ready to be extracted, identified and measured. In archaeology, the most popular stable isotopes for study are:

• carbon (C¹²and C¹³)
• nitrogen (N¹⁴ and N¹⁵)
• oxygen (O¹⁶ and O¹⁸)
• strontium (⁸⁶Sr and ⁸⁷Sr)

These atoms are extracted using an analytical instrument known as a mass spectrometer. Archaeologists use the balance (ratio) of these stable isotopes to explore a wide range of questions surrounding diet (meat and fish consumption; weaning age), and migration (see Step 2.11).

Dietary analysis

In the 1980s scholars realised that different ratios of strontium, nitrogen and carbon can provide information on the type of diet individuals consumed. For example, high nitrogen: low carbon ratios suggest an individual consumes a terrestrial (agricultural) diet made up of meat and grains, while high nitrogen: high carbon ratios suggest a marine (fishing) diet of fish and shellfish. Individuals whose ratios fall between these two, would have consumed a diet comprising both meat and fish (see Figure 1). For example, studies of our eating habits in the past have revealed that Neolithic populations consumed practically no fish, preferring dairy rich foods, while high status medieval Bishops consumed a great deal of fish in comparison to their meat eating congregation.

Figure 1. Summary of isotopic ratios used to determine diet
© Müldner, University of Reading

Weaning

Weaning is a term to describe the gradual transition of a child’s diet from breast milk to solid food. The study of weaning age in past populations relies on the fact that stable isotope ratios of nitrogen and oxygen, will vary according to the ‘trophic’ level (or point on the food chain) of an individual. The nitrogen content (or δ¹⁵N) of a newborn infant is similar to its mother. Through breastfeeding the child is effectively consuming its mother’s tissues, placing it higher on the food chain and increasing the nitrogen levels. When weaning begins and the mother’s milk is gradually replaced by other foods, tissue δ¹⁵N will decrease and once again will fall in line with the values of the mother and other adults in that population.

Similarly, oxygen isotopes are taken up by the body from the water in the environment. When breastfeeding, a child consumes high levels of O¹⁸, which drop gradually as water begins to be consumed from other less oxygen-rich sources. Tracking levels of nitrogen and/or oxygen in teeth (relative to the age of the child when that tooth formed) helps us to determine the age, at which the child was weaned off breast milk and on to solid foods.