Stable Isotope Analysis

The Significance of Stable Isotopes

• Isotopes are atoms of an element that vary by mass, meaning greater or fewer neutrons.

• The isotopes of interest to archaeologists are those that are lighter and which figure in organic compounds, principally those of carbon, nitrogen, and sulphur.

• Scientists look at the isotopic chemistry of bone collagen, apatite, and hair. The assumption is that an animal’s diet will leave a specific isotopic signature.

• Plants vary in their isotopic ratios of carbon and nitrogen depending on the way they obtain (“fix”) these elements through photosynthesis.

Plant Isotopic Categories

• C3 Plants: Photosynthesis results in 3-carbon molecule products. These are plants of temperate climates and plentiful groundwater. Examples: wheat, rice, barley, oats, & legumes.

• C4 Plants: Photosynthesis results in 4-carbon molecule products. These are plants of the tropics. Examples: corn, sugarcane, pineapples.

• There are also CAM (Crassulacean acid metabolism) plants. These are plants in arid environments that shut their stomata (pores) during the day to avoid water loss.

Dietary Signatures

• A diet high in C3 plants leaves behind low levels of 13C.

• A diet high in C4 plants leaves high levels of 13C in bone collagen.

• A diet high in marine foods products results in high levels of 13C, 15N, and 34S.

Example from the Tehuacan Valley

Indigenous or foreign born?

• Some isotopes like strontium (87Sr/86Sr) are fixed in tooth enamel childhood and do not change if someone moves to a new locality.

• Bone, however, is subject to isotopic turnover so isotopes of oxygen and strontium in bone will reflect where the person was living before they died.

St. Brice’s Day Massacre 1002 AD