INFRARED SPECTROSCOPY OF BIOAPATITE FROM TAXONOMICALLY DIVERSE MODERN TEETH – IMPLICATIONS FOR DIAGENESIS

Structural carbonate in mammalian bioapatite is commonly analyzed for both carbon (δ¹³C) and oxygen (δ¹³C) isotopes to infer paleoecology and paleodiet. Within the bioapatite structure, carbonate substitutes into two crystallographic sites – the OH-site, forming type-A, and the PO₄-site, forming type-B carbonated-bioapatite. Infrared (IR) spectroscopy is frequently used to characterize the carbonate environment within bioapatites, but no study has yet investigated inter-taxon variations in bioapatite carbonate. Because variations in the proportion of type-A to type-B carbonate might influence the δ¹³C and δ¹⁸O values of enamel, understanding distribution of bioapatite carbonate in modern taxa is crucial.

We collected paired samples of tooth enamel and dentine from a range of modern taxa including Artiodactyla, Perissodactyla, Proboscidea, Rodentia, Mustelidae and Marsupialia. The samples were powdered, bleached, and pretreated in buffered weak acid to remove labile carbonates and organic material. After pretreatment we collected IR spectra for each sample pair.

We used proposed API and BPI peak height indices to characterize the proportion of CO₃ in the A and B sites, respectively, relative to PO₄. API and BPI for modern enamel vs. dentine reveals significant differences between these two tissues (MANOVA, p < 0.005). Modern enamel had a mean API of 0.140±0.070 and mean BPI of 0.261±0.924, whereas dentine had means of 0.323±0.070 and 0.417±0.097. We attribute these differences to a 2-3% higher carbonate content in dentine. No consistent variation in the distribution of A- vs. B-type carbonate was observed among taxa.

Pilot data from fossil enamel and dentine both show significant reductions in type-A carbonate (ANOVA, p < 0.0005), but no statistically significant change in type-B carbonate (ANOVA, p > 0.1). This suggests that although the carbonate content of bioapatite may be diagenetically altered, alteration predominately affects type-A carbonate.