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

Abstract

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 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 known 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 carbonate contaminants sitting between or on the surface of the crystallites. Modern Sus scrofa enamel 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,

The Spice Islands in Prehistory: Archaeology in the Northern Moluccas, Indonesia

“…# Radiocarbon dates on apatite, whether from enamel or bone, were only undertaken if no collagen was present. These dates very likely underestimate the true age of the sample as contamination cannot be fully removed from these sample types (Wood et al 2016;Zazzo 2014). Pretreatment descriptions where available for ANU and S-ANU samples: 1.…”

“…Enamel apatite: Surface and dentine removed with dental drill. Micronised in a McCrone microniser for 30 minutes, 1M acetic acid for 20 hours under a weak vacuum (Wood et al 2016).…”

“…This tooth was subjected to two pretreatment methods in an attempt to remove as much contamination as possible. Wood et al (2016) suggested that unless enamel was heavily recrystallised, most contaminating carbonate is likely to sit at the crystal boundaries within the enamel, and they found that age increased if the sample was mechanically crushed before cleaning with acetic acid. Radiocarbon dates on finely ground Pleistocene enamel were still too young, but in the late Holocene they were indistinguishable from their known age.…”

DO WEAK OR STRONG ACIDS REMOVE CARBONATE CONTAMINATION FROM ANCIENT TOOTH ENAMEL MORE EFFECTIVELY? THE EFFECT OF ACID PRETREATMENT ON RADIOCARBON AND δ13C ANALYSES

“…Several researchers have attempted to produce accurate dates on carbonate within the hydroxyapatite phase in unburnt bone, which out-survives the organic phase in all but the most acidic of environments, but with little success outside of arid regions (Haynes 1968;Zazzo 2014;Zazzo and Saliège 2011). Carbonate in the mineral phase of tooth enamel may provide an alternative material, but again, attempts to 14 C date the material have met with limited success outside of arid regions (Hedges et al 1995;Zazzo 2014;Wood et al 2016) despite the application of a range of pretreatment methods (Hedges et al 1995;Surovell 2000;Cherkinsky 2009;Hopkins et al 2016). With no pretreatment, more than 10% of the carbonate in tooth enamel from karstic environments in Vietnam was found to be a contaminant (Wood et al 2016), and in some cases tooth enamel appears to contain more carbonate contamination than bone apatite (Zazzo 2014).…”

“…Carbonate in the mineral phase of tooth enamel may provide an alternative material, but again, attempts to 14 C date the material have met with limited success outside of arid regions (Hedges et al 1995;Zazzo 2014;Wood et al 2016) despite the application of a range of pretreatment methods (Hedges et al 1995;Surovell 2000;Cherkinsky 2009;Hopkins et al 2016). With no pretreatment, more than 10% of the carbonate in tooth enamel from karstic environments in Vietnam was found to be a contaminant (Wood et al 2016), and in some cases tooth enamel appears to contain more carbonate contamination than bone apatite (Zazzo 2014). This is somewhat surprising given that, in comparison with bone, enamel porosity is low (Hedges et al 1995;Millard and Hedges 1996), and enamel crystallites are more stable as they are larger (26.3 × 100-1000 nm vs. 5 × 100 nm) (Bottero et al 1992;Cui and Ge 2007) and contain less carbonate (3.5 wt% vs. 6 wt%) (Elliott 2002), but may be related to protection offered by the close relationship between the mineral and protein components in well preserved bone (Zazzo 2014).…”