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 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