Infrared spectral library of tooth enamel from African ungulates for accurate electron spin resonance dating

Abstract

Electron spin resonance coupled with uranium-series dating (ESR/U-series) of carbonate hydroxyapatite in tooth enamel is the main technique used to obtain age determinations from Pleistocene fossils beyond the range of radiocarbon dating. This chronological information allows to better understand diachronic change in the palaeontological record, especially with regard to the evolution of the genus Homo. Given the relative paucity of human teeth at palaeontological and archaeological localities, ESR/U-series is widely applied to the teeth of ungulate species. However, the accuracy of ESR/U-series ages is greatly affected by the incorporation of uranium in the enamel during burial in sediments. It has been shown that uranium content is positively correlated with an increased degree of atomic order in carbonate hydroxyapatite crystals, the latter determined using infrared spectroscopy. Here we present a reference infrared spectral library of tooth enamel from African ungulates, based on the grinding curve method, which serves as baseline to track the diagenetic history of carbonate hydroxyapatite in different species and thus select the best-preserved specimens for dating.

Fig. 1

Workflow followed to compile the reference spectral library and grinding curve plot.

Fig. 2

Grinding curve plot including all the species in the database. Note the large range of offsets among curves.

Fig. 3

Grinding curves of fossil teeth from Lovedale (South Africa) compared to a reference curve of springbok (dashed line). The three most recrystallised specimens, which exhibit greater curve offset compared to the springbok curve, are also the richest in uranium (ranging from 9.7 to 29.6 ppm). All other specimens are characterised by amounts of uranium ranging from 1.7 to 5.8 ppm (data from Richard et al.^,).