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Science-based archaeology > Methods and maps The application of isotope analysis in tooth enamel to the study of population migration and movement
Strontium isotopes Strontium has four naturally occurring isotopes, of which three are stable, and one (⁸⁷Sr) forms by the decay of ⁸⁷Rb. Strontium is chemically similar to calcium, and enters the biosphere primarily through the uptake of Sr by plants. The isotope composition of Sr in the biosphere is controlled by the isotope composition of the leachable component of the soil on which the plants grow. This, in turn, varies depending upon a combination of factors which include the age of the underlying rocks, the rubidium content, weathering conditions and the susceptibility of different minerals in a heterogeneous rock to weathering processes. Strontium isotopes provide a powerful tool for studying the movement of humans and animals across isotopically different terrains because strontium is absorbed by the body and deposited, with calcium, in teeth and bones. Tooth enamel forms during the early years of life and its composition is not altered subsequently; hence, it constitutes a recoverable archive of childhood diet. The ⁸⁷Sr/⁸⁶Sr biosphere map for Britain to be used in conjunction rainfall map in coastal areas. See Evans, J.A. Montgomery, J. Wildman, G. & Boulton, N (2010) Spatial variations in biosphere ⁸⁷Sr/⁸⁶Sr in Britain. Journal of Geological Society vol 167, pp1-4		Average rainfall, with permission from the Met office. In archaeological skeletal remains, enamel is also the tissue most resistant to post-mortem contamination, thus retaining its isotopic integrity over thousands of years. Once incorporated into plants, Sr passes up the food chain unfractionated. Sr-isotope analysis of tooth enamel provides a tool to differentiate between people or animals based on where they obtained their food and water at the time of tooth mineralisation. Combined with oxygen analysis, which is related to climate zones (see below), these isotopes can provide powerful constraints on the childhood origin of individuals, and can be used to assess population heterogeneity and track migration. Useful links: » 1:5 Million International Geological Map of Europe and Adjacent Areas (Asch, K., 2005. IGME 5000) » BGS geological map of the UK. See also: Voerkelius, S., Lorenz G.D., Rummel, S., Quetel, C.R., Heiss, G., Baxter, M., C. , B.-P., Deters-Itzelsberger, P., Hoelzl, S., Hoogewereff, J., Ponzervera, E., Bocxstaele, M.V. & Ueckermass, H., 2010, Strontium isotopic signatures of natural mineral waters, the reference to a simple geological map and its potential for authentication of food: Food Chemistry, 118, 933-940.

Oxygen isotopes The dominant source of oxygen isotopes within the body is from drinking water and this, in turn, reflects the isotope composition of rainwater which is modified, through fractionation, within the body. There is a general global trend in the oxygen isotope composition of rainwater of increased depleting from the equator to the poles, however this pattern is modified regionally by weather systems. In Europe the main variation in rainwater is from west to east as seen below. The analytical method used at NIGL is to extract the oxygen from the phosphate radical of the bio-apatite that makes up the majority of the tooth. The measured ratio is presented in a standard delta notion given relative to Standard Mean Ocean Water (SMOW). Eg δ¹⁸O =17.84. Oxygen isotope map of drinking water variations across Europe compiled from: Lecolle P, (1985) The oxygen isotope composition of land snail shells as a climatic indicator - applications to hydrogeology and Palaeoclimatology. Chemical Geology, 58 (1-2): 157-181. Forstel H and Henkel C, (1982), Simulation function of the local climate (Julich, Federal-Republic-of-Germany) from a 10-year observation period (1972-1981). Landwirtschaftliche Forschung 35 (3-4): 275-279		Oxygen isotope map of drinking water variations across the UK compiled from data from Darling, W.G. and Talbot, J.C., 2003. The O & H stable isotopic composition of fresh waters in the British Isles: 1, Rainfall. Hydrology and earth System Sciences, 7(2): 163-181. Because of the fractionation of the oxygen within the body, this value has to be recalculated, using experimentally generated calibrations, to determine the composition of the water an individual imbibed, thus making it possible to compare the individual with drinking water datasets. Several calibrations for this conversion exist and it can be a source of error and uncertainty and so must be used with care. At NIGL, in addition to using the drinking water maps, we are developing datasets of δ¹⁸_SMOW values for human tooth enamel both across the country and through time, in order to make direct comparisons of measured population values with the aim of understanding inter-population differences which may reflect such factors, as cultural practices, and water sources, as well as the main effects of climate region.

Lead isotopes Lead is a toxin, but it can become incorporated into the body and the isotope composition can be used to trace the source of lead 'pollution' to which an individual was exposed. The diagram (right) shows a graph of lead isotope composition measured in human tooth enamel vs. the concentration of lead within the tooth enamel of five British populations. The Neolithic and Iron Age tooth enamel tends to show a wide spread of lead isotope compositions associated with low concentration of lead (<1ppm) within the tooth. Later populations show a reduction in the range of lead isotope values but an increase in lead concentrations recorded in the enamel. Populations which pre-date metallurgy, or have little exposure to metalware (i.e. the more ancient populations) have lead concentrations that reflect the natural background levels picked up from the environment and which reflect the variable isotope composition of the geology on which the individuals lived. The advent of trade and the wider use of metalware is reflected in the populations increased Pb concentrations, however, because the exposure is now related to man-made items the natural diversity of the lead isotope compositions is reduced to an average value that reflects the overall composition of the metal sources and reworked material within the particular culture.		Lead focussing diagram - courtesy Janet Montgomery This feature of increased lead concentration with reduced isotope range has been coined 'cultural focussing' (Montgomery et al., 2005). Montgomery, J, Evans, J A, Powlesland, D, and Roberts, C A, 2005. Continuity or colonisation in Anglo-Saxon England? Isotope evidence for mobility, subsistence practice, and status at West Heslerton. American Journal of Physical Anthropology, 126(2), 123-138.

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