Isotopes are found everywhere in the environment, in plants via sediments and water, and in animal tissues (and therefore in human tissues) through eating, drinking and breathing. The analysis of stable isotopes, such as those of carbon, nitrogen, strontium, etc., makes it possible to study the diets of an individual or group of individuals, and to determine where an individual grew up or lived for the last twenty to twenty-five years of his or her life.
Strontium is abundant in nature, found mainly in rocks and sediments. As sediments are eroded and dispersed in water and food resources, it is absorbed by the body and incorporated into bone tissue. The isotopic ratio of strontium varies from one geographical region to another. Consequently, the analysis of strontium isotope ratios in bones or teeth can be used to determine the geographical origin of an individual, or to measure the homogeneity of a group of individuals.
Bone and teeth are the most frequently analyzed tissues, as they are hard and can be preserved for a long time in archaeological contexts. Bone is made up of two components: an organic matrix composed mainly of collagen, and an inorganic mineral matrix composed mainly of calcium phosphates. Bone is a living tissue that constantly renews itself as we grow and age. However, this process is very slow, and dense cortical bone reflects approximately the last ten to fifteen years of an individual's life. Teeth are also composed of organic and mineral materials, but tooth enamel does not renew itself. Teeth are therefore very useful in determining the environment of an individual's early years. Moreover, by comparing the teeth and bones of the same individual, it is possible to determine whether he or she has migrated from one region to another since childhood. Teeth show where a person lived during childhood, and bones show where they lived in the years leading up to death.
The principle is to compare the 87Sr/86Sr ratio of bone and/or dental enamel with that of the environment (sediments) around the archaeological site and in neighbouring regions (with different geological substrates). This will enable us to discuss the geographical origin of the food ingested by the individuals.
As far as possible, the selection of material should respect the preservation of the archaeological remains, while satisfying the needs of the study. Bone fragments should preferably be sampled from the cortical bone. To be relevant, the study should be carried out on series of several subjects, and we will always try to select the same anatomical part. For dental tissue studies, depending on the inter- or intra-individual study issues, we will either carry out an analysis per tooth, or a multi-analysis per stratum. Analysis will be carried out using a multi-collector mass spectrometer , with or without laser ablation.
Animal tooth samples are cleaned with ethanol and placed on a glass slide to remove the outer enamel surface. For the specific case of herbivore teeth, we will choose analysis zones at the base, middle and top of each tooth, in order to assess the enamel formed at different times in the individual's life. The spectrometer is coupled to a laser system, equipped with an ablation cell (LA-ICP-MS). For laser ablation analysis, 500 µm long line scans are applied, with a circular spot size of 100 µm and a translation speed of 5 µm/s. We use several internal reference materials of bioapatite, as well as apatite and carbonate.
Strontium analysis is invaluable for studying the movement of populations. Thanks to our experienced scientists and state-of-the-art equipment, strontium analysis is a specialty of CIRAM laboratories.
