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 regions with 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 and sugarcane. • 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. Examples: prickly pear, agave, mesquite and pineapples. 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 The St. Brice’s Day Massacre • King Æthlred “Unræd” (Ill-advised) of Wessex ordered that all Danes in England be killed by the Anglo-Saxon population. This mass killing was timed to happen on Nov. 13th 1002 AD. • The Danish men of Oxford were said to have been killed after they had taken refuge in St. Frideswides’ Church. In 2008 the remains of 38 males were found to have been dumped in the ditch of a Neolithic henge by archaeologists excavating on the campus of St. John’s College, Oxford University. A subsequent bone pit was discovered in 2009 in Weymouth, Dorset. • The remains at the two sites were carbon dated, yielding spans 960-1020 AD & 980-1030 AD. Stable Isotope Analysis There two areas of a skeleton that yield distinct sets of information Bone collagen: Isotope mix reflects diet and isotopes in soil, but these will change if an individual moves so collagen isotopes will reflect the last place a person lived before death. Teeth: Isotope mix is fixed during childhood, and does not change subsequently. Dental isotopes will therefore reflect the place of origin. Analysis • Carbon and nitrogen isotopes were examined in the collagen, and strontium and oxygen isotopes in the enamel. • The isotopes revealed that not only did the men have diets high in seafood, but that they had diets higher in protein than the norm for the Anglo-Saxon population, and had lived in colder climates when young.