The oxygen isotope relationship between the phosphate and structural carbonate fractions of human bioapatite

Rationale: Oxygen isotope analysis of archaeological human dental enamel is widely used as a proxy for the drinking water composition (δ(18)O(DW)) of the individual and thus can be used as an indicator of their childhood place of origin. In this paper we demonstrate the robustness of structural carbonate oxygen isotope values (δ(18)O(C)) in bioapatite to preserve the life signal of human tooth enamel by comparing it with phosphate oxygen isotope values (δ(18)O(P)) derived from the same archaeological human tooth enamel samples.

Methods: δ(18)O(C) analysis was undertaken on 51 archaeological tooth enamel samples previously analysed for δ(18)O(P) values and strontium isotopes. δ(18)O(C) values were determined on a GV IsoPrime dual inlet mass spectrometer, following a series of methodological tests to assess: (1) The reaction time needed to ensure complete release of CO(2) from structural carbonate in the enamel; (2) The effect of an early pre-treatment with dilute acetic acid to remove diagenetic carbonate; (3) Analytical error; (4) Intra-tooth variation; and (5) Diagenetic alteration.

Results: This study establishes a direct relationship between δ(18)O(C) and δ(18)O(P) values from human tooth enamel (δ(18)O(P) = 1.0322 × δ(18)O(C) - 9.6849). We have combined this equation with the drinking water equation of Daux et al. (J. Hum. Evol. 2008, 55, 1138) to allow direct calculation of δ(18)O(DW) values from human bioapatite δ(18)O(C) (δ(18)O(DW) = 1.590 × δ(18)O(C) - 48.634).

Conclusions: This is the first comprehensive study of the relationship between the ionic forms of oxygen (phosphate oxygen and structural carbonate) in archaeological human dental enamel. The new equation will allow direct comparison of data produced by the different methods and allow drinking water values to be calculated from structural carbonate data with confidence.