The effect of grain size on carbonate contaminant removal from tooth enamel: Towards an improved pretreatment for radiocarbon dating

It is rarely possible to directly radiocarbon date skeletal remains from hot environments as collagen rapidly degrades. Although able to survive in the majority of burial environments for longer, unburnt biological apatites frequently produce inaccurate radiocarbon dates due to contamination from carbonate in the groundwater. The location of this contamination within the skeletal material is rarely investigated, hampering development of improved radiocarbon pretreatment methods. This paper focuses on tooth enamel and aims to test whether carbonate contaminants are sitting at the crystallite boundaries, and from this to test a pretreatment to produce more accurate radiocarbon age estimates. Although the porosity of enamel is low, trace elements are thought to diffuse between enamel prisms and crystallites. Gordon et al. (2015, Science, 347 (6223), 746-750) identified magnesium substituted amorphous calcium phosphate between the apatite crystallites. This phase contains the majority of magnesium within modern rodent enamel, providing an opportunity to monitor its removal, and thus test whether carbonate contaminants are located between or on the surface of the crystallites. Modern Sus scrofa and four ancient Sus scrofa teeth have been used to demonstrate that the more finely ground the enamel, the more magnesium can be removed with an acetic acid leach, and the more accurate the radiocarbon dates. After leaching in acetic acid, teeth dating to beyond the limit of the radiocarbon method (c.50 ka) produce ages of c.20 kBP when hand ground, and c.30 kBP when mechanically ground. This suggests that some contaminants are sitting at the crystallite boundaries. However, although mechanically grinding substantially increases the amount of carbonate contamination removed in an acid leach compared to hand grinding, not all contaminants could be removed from the samples examined in this study, and radiocarbon dates on tooth apatite should still be regarded as minimum ages.