Results of isotope analysis on human tooth enamel from Winterbourne, Dorset
By Jane Evans, Doris Wagner and Kotryna Savickaite 

Contact details: Dr Martin Smith, Principal Academic, Forensic Anthropology, Department of Archaeology and Anthropology, Bournemouth University, UK
mjsmith@bournemouth.ac.uk

Introduction
A human tooth (Lower M2) was submitted, in May 2022, by Martin Smith (Bournemouth University) for Sr and oxygen isotope analysis.

Sample preparation. 
The enamel tooth surface was cleaned by abrasion, using a diamond burr, to a depth of about 100 microns, to remove surface contamination on the outer side of the enamel. A sample of c. 30mg was then cut from the tooth and residual dentine was completely remove from the inner enamel surface. This enamel was rough crushed, using an impact mortar, and split into two subsamples: c.5 milligrams for O isotope the remaining material (c. 25 milligrams) for Sr isotope analysis. The samples for oxygen analysis were further reduced to a powder in an agate mortar and pestle. 

The preparation and isotope analysis for Sr in tooth enamel. 
The enamel samples for Sr isotope analysis were transferred to a class 100 clean suite where they were further cleaned in Teflon© distilled 2M HNO3 acid, for ~5 minutes and then rinsed twice in >18 megaohm-cm resistivity Milli-Q water and left to soak in water for an hour on a hot plate set at 60° C. Following this, the sample was again rinsed twice in de-ionised water, and then dried and weighed into a pre-cleaned Teflon© beaker. 

Sample chemistry for Sr isotope and concentration analysis.
The sample was mixed with 84Sr tracer solution and dissolved in Teflon© distilled 16M HNO3. After evaporation to dryness, the sample was converted to chloride form by addition of Teflon© distilled 6MCl and taken up in 2.5MHCl. Sr was collected using Eichrom© AG50 X8 resin. Strontium was loaded onto a single Re Filament following the method of Birck and the isotope composition and strontium concentrations were determined by Thermal Ionisation Mass spectroscopy (TIMS) using a Thermo Triton multi-collector mass spectrometer. Samples were run of a peak jumping programme for 100 scans with a target precision of < ± 0.00001 (2SE) which all samples achieved. The international standard for 87Sr/86Sr, NBS987, gave a value of 0.710259 ± .000020 (2SD, n=8) during the analysis of these samples and the data are corrected to the accepted value of 0.710250. 

The isotope analysis of oxygen and carbon in tooth enamel.
Approximately 1 mg of carbonate material was used for isotope analysis using an IsoPrime 100 dual inlet mass spectrometer plus Multiprep device. Samples are loaded into glass vials and sealed with septa. The automated system evacuates vials and delivers anhydrous phosphoric acid to the carbonate at 90oC. The evolved CO2 is collected for 15 minutes, cryogenically cleaned and passed to the mass spectrometer. Isotope values (δ13C, δ18O) are reported as per mille (‰) deviations of the isotopic ratios (13C/12C, 18O/16O) calculated to the VPDB scale using a within-run laboratory standard calibrated against NBS-19. δ18O values are then converted to the VSMOW scale using the following formula δ18OVSMOW = 1.03091×δ18OVPDB +30.92 ‰. The calcite-acid fractionation factor applied to the gas values is 1.00813. The Craig correction is also applied to account for δ17O. The average analytical reproducibility of the standard calcite (KCM) is <0.1‰ for δ13C and δ18O. Analytical reproducibility for this run of laboratory standard calcite (KCM) for δ18OVPDB were ± 0.05‰ (1σ, n=43) and δ13CVPDB is ± 0.05‰ (1σ , n=43). 
Typical reproducibility for enamel analysis, based on duplicate pairs is δ18OVPDB ± 0.04‰ (1σ, n=4) and δ13CVPDB is ± 0.08‰ (1σ , n=4). 

Data are presented below. 

Batch	N	sample	description	Sr ppm	87Sr/86Sr_N	δ13C ‰ VPDB	δ18O ‰ VSMOW	
P988	16	DB 10	LM2	129	0.70888	-13.5	27.7	

Table 1 Strontium concentration and strontium, carbon, and oxygen isotope compositions for human tooth enamel sample.

The sample has a relatively common Sr isotope composition for Britain of 0.70888 and is thus not strongly diagnostic of geographic origin. The oxygen isotope composition is relatively high and just outside the 1SD range of west and south cost populations. Figure 1 shows, in orange the areas of Britain that cannot be excluded as the childhood origin of this individual based on the 1SD range of oxygen isotope composition of British human enamel (using the upper limit for the 1 SD range of 27.5 as a proxy for individual) and the interquartile range of Sr isotope composition from plants data, which is considered a reasonable approximation for human Sr uptake. The data indicate that there is no evidence to exclude the individual having spent a childhood close to the area of Winterbourne, Dorset. However, values are not exclusive and it is possible that areas in Europe could supply a similar combination of values. 

Figure 1. Areas that cannot be excluded as a childhood origin for the human tooth sample DB10 based on Sr and oxygen isotope compositions. (Winterbourne marked for reference). 

References
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