Probing atomic scale transformation of fossil dental enamel using Fourier transform infrared and nuclear magnetic resonance spectroscopy: A case study from the Tugen Hills (Rift Gregory, Kenya)
Résumé
A series of fossil tooth enamel samples was investigated by Fourier transform infrared (FTIR) spectroscopy,13C and 19F magic-angle spinning nuclear magnetic resonance (MAS NMR) and scanning electronmicroscopy (SEM). Tooth remains were collected in Mio-Pliocene deposits of the Tugen Hills in Kenya.Significant transformations were observed in fossil enamel as a function of increasing fluorine content(up to 2.8 wt.%). FTIR spectroscopy revealed a shift of the m1 PO4 stretching band to higher frequency.The m2 CO3 vibrational band showed a decrease in the intensity of the primary B-type carbonate signal,which was replaced by a specific band at 864 cm1. This last band was ascribed to a specific carbonateenvironment in which the carbonate group is closely associated to a fluoride ion. The occurrence of thiscarbonate defect was consistently attested by the observation of two different fluoride signals in the 19FNMR spectra. One main signal, at 100 ppm, is related to structural F ions in the apatite channel and theother, at 88 ppm, corresponds to the composite defect. These spectroscopic observations can be understoodas resulting from the mixture of two phases: biogenic hydroxylapatite (bioapatite) and secondaryfluorapatite. SEM observations of the most altered sample confirmed the extensive replacement of thebioapatite by fluorapatite, resulting from the dissolution of the primary bioapatite followed by the precipitationof carbonate-fluorapatite. The m2 CO3 IR bands can be efficiently used to monitor the extent ofthis type of bioapatite transformation during fossilization.