Investigating Population Movement by Stable Isotope Analysis: A Report from Britain

Article excerpt

Introduction

Isotopic methods, originally developed in geological and environmental science, and now applied to archaeological material, allow us to comment directly on the place of childhood residence of individual people from the study of their skeletal remains. We therefore now have the potential to test inferences about immigration suggested by the study of the material record and particularly from burial practice and grave goods. The results reviewed here show that analyses of oxygen, and to a lesser extent strontium isotope are now capable of identifying first generation immigrants among burial groups and, in some cases, of suggesting their probable place of childhood residence. However, fundamental problems remain regarding the geographic interpretation of some of the data so that determination of the place of origin remains elusive as a consistent and reliable tool for archaeological study. Here we discuss recent results in the context of the problems which remain and the steps needed to unlock the full potential of the technique.

Principles and potential

The link between skeletal composition and place of residence arises from natural systematic variations in the isotopes of particular elements between localities (Faure, 1986). Strontium isotopes have been used to study archaeological residential mobility since the mid 1980s (e.g. Ericson, 1985). One isotope, [sup.87]Sr, is produced by the radioactive decay of rubidium; an element occurring in many rocks and minerals. The abundance of [sup.87]Sr (measured as a ratio to the stable isotope [sup.86]Sr) therefore depends on the initial Rb/Sr ratio and the age of the rock or mineral in which it is found. Strontium is taken up by organisms, but the relative proportions of its isotopes are unaltered in the processes, so that soil, plant and animal strontium isotope ratios have all been shown to be related to those of the underlying geological formations and local hydrology (Blum et al. 2000). The characteristic Sr-isotope ratios of particular geographical areas and their persistence in local foods and the tissues of feeding animals (including humans) provides the basis for the reconstruction of the place of residence at the time of the tissue formation. One difficulty which may arise in the use of strontium isotopes is the potential for post-mortem contamination, or diagenesis, of tissues in the burial environment. However, this may be successfully addressed by careful sample selection and in particular by selecting dental enamel, rather than bone, as the tissue of choice for analysis (Budd et al. 2000a; Trickett et al. 2003).

Oxygen isotopes also vary in the environment in a systematic way, but in this case as a result of differences in climate and geography rather than geology. In the body, the oxygen isotope composition of skeletal tissue is directly linked to that of oxygen consumed and this is controlled by drinking water (Longinelli 1984). Biological processes alter (or 'fractionate') oxygen isotopes (unlike those of strontium) quite readily. However, mammalian skeletal tissues form at a relatively constant body temperature so that the fractionation that does take place is very similar, both within and between species (Bryant & Froelich 1996; D'Angela & Longinelli 1990; Longinelli 1984; Luz et al. 1990; Luz & Kolodny 1985; Luz et al. 1984). Although there is some inter-species variation due to body mass, diet and metabolism, a number of researchers have developed calibrations that relate skeletal oxygen isotope ratios to those of drinking water. For humans, the calibration developed by Levinson et al. (1987) is most widely used.

In antiquity, drinking water was drawn from surface waters, near-surface groundwaters and collected from precipitation. Precipitation falling across Great Britain, as elsewhere, is not isotopically uniform, but varies in a systematic way with geographical location. This provides the basis to link tissue oxygen isotope composition to a person's place of residence when a particular tissue was formed. …