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THE WILTSHIRE ARCHAEOLOGICAL AND NATURAL HISTORY MAGAZINE 



Principles 



The reconstruction of residential mobility from the 

 analysis of dental enamel is based on systematic 

 natural variations between localities of the isotopes 

 of a number of elements. Lead, strontium and 

 oxygen all have isotopes which vary in this way and 

 can be used for this purpose (Budd er al. 1999; in 

 press a; in press c; Montgomery er al. 2000). 

 Elements with isotope ratios characteristic of 

 specific environments become incorporated into 

 enamel during tooth formation in childhood. The 

 enamel is highly resistant to change after death and 

 hence retains this early life isotopic 'signature' 

 (Budd etal. 2000a). 



Strontium has four isotopes, one of which, 87Sr, 

 is derived from the radioactive decay of rubidium 

 over geological time. The concentration of this 

 isotope, measured as a ratio to its non-radiogenic 

 sister 86Sr, depends on both the rubidium content 

 and age of the rock in which it is found. Strontium 

 is taken up by biological systems, but the relative 

 proportions of its isotopes remain unaltered in the 

 process (Blum er al. 2000). As a result, soil, plant 

 and ultimately human enamel strontium isotope 

 ratios all remain closely related to (although not 

 necessarily exactly the same as) those of the 

 hydrology and underlying geology of the region in 

 which the individual lived when the tissue was 

 formed: early childhood in the case of permanent 

 human teeth. 



Lead has four stable isotopes, but in this case 

 three (206Pb, 207Pb and 208Pb) are formed by 

 radioactive decay (of uranium and thorium). 

 Therefore geological concentrations of these three 

 isotopes, expressed as ratios to the only non- 

 radiogenic lead isotope, 204Pb, depend on both 

 the parent uranium and thorium contents of the 

 rock or mineralising fluid, and the time since 

 deposition. In pre-metallurgical societies the main 

 source of lead in the diet, like strontium, was from 

 the underlying geology via the food chain. In such 

 cases it is possible to use the lead isotope 

 composition of tooth enamel to comment on place 

 of origin in a manner directly analogous to that of 

 strontium. Later however, and especially in the 

 Roman and medieval periods, ore-derived lead 

 becomes dominant as the source of human 

 exposure as a result of the use of lead metal, its 

 alloys and products (Budd er al. 2000b). 



Oxygen isotopes are highly complementary in 

 producing information related to place of childhood 

 residence, but by virtue of climatic rather than 



geological variation. Unlike lead and strontium, the 

 much lighter isotopes of oxygen are readily altered 

 by biological processes. Fortunately however, 

 mammalian tooth and bone are composed of 

 biological apatite and organic material formed at 

 constant temperature (37 C) so that the oxygen 

 isotope ratio of skeletal phosphate directly relates 

 to that of body fluids and local, meteoric, drinking 

 water (Fricke er al. 1995; Levinson er al. 1987). A 

 simple calibration is all that is required. 



Analysis 



The Natural History Museum removed the upper 

 left first premolar and replaced it with a cast. A 

 clean core enamel sample was then extracted for 

 analysis using the methods described by Budd er 

 al. (in press a; c). Lead and strontium isotope ratio 

 analyses and concentration analysis using the 

 isotope dilution method were performed at NIGL 

 by Thermal Ionization Mass Spectrometry (TIMS) 

 using a Finnegan Mat 262 multi-collector mass 

 spectrometer. Errors (all 2s) were calculated from 

 repeat measurements of the international standard 

 for strontium (NBS 987, n=10) and lead (NBS 981, 

 n= 1 6) during the period of analysis. Oxygen isotope 

 sample preparation was carried out at NIGL using 

 the laser fluorination method described by Budd 

 er al. (in press b; c). A V. G. Isotech Optima dual 

 inlet isotope ratio mass spectrometer operating 

 Micromass DI2.47 software was used to determine 

 the enamel oxygen isotope composition dl80. 

 Errors (2s) were calculated by reference to repeat 

 measurements of phosphate mineral standards, 

 NBS 120b (n=6) and NBS 120c (n=2). O-isotope 

 data were calibrated using Levinson er al. (1987). 

 Results appear in Table 2. 



Table 2. Analysis of tooth from skeleton 4.10.4 



Tooth enamel 206Pb/204Pb isotope ratio: 18.62 ± 0.02 

 Tooth enamel 207Pb/204Pb isotope ratio: 15.82 ± 0.02 

 Tooth enamel 208Pb/204Pb isotope ratio: 39.06 ± 0.05 

 Lead concentration of enamel: 2.2 ± 0.3 ppm 

 Tooth enamel 87Sr/86Sr isotope ratio: 0.70837 ± 0.00003 

 Strontium concentration: 55 ± 5 ppm 



Aqueous leachate of soil from near burial site 87Sr/86Sr 



isotope ratio: 0.70794 

 Childhood drinking water dl80 value: -7.8 to -7.3%o 



