Methodological considerations for the use of acid based pre-treatment
protocols for carbon and oxygen analysis of tooth enamel
Abstract
Rationale: Chemical pre-treatment is an important
methodological step aimed to remove exogenous materials introduced to
archaeological tooth enamel through various diagenetic pathways prior to
carbon and oxygen isotope analysis. Concerningly, some of these
pre-treatment methods (NaClO, H 2O 2 and
CH 3COOH) have been shown to cause significant changes
to enamel chemical properties and stable isotope values. This study aims
to re-examine the effects of commonly used pre-treatment protocols
applied to the enamel structure on bioapatite δ 13C
and δ 18O values, as well as investigate patterns
related to site burial context and sample preservation, as indicated by
pre-screening using Fourier Transform Infrared (FTIR) spectroscopy.
Methods: Modern and archaeological samples were subjected to
ten commonly used pre-treatment protocols using NaClO, H
2O 2 and CH 3COOH at
various time intervals. Preservation status and diagenetic alteration
prior to and after treatment were investigated using Attenuated Total
Reflectance- Fourier Transform Infrared Spectroscopy (ATR-FTIR). δ
13C and δ 18O values were measured
before and after the treatment to determine if treatments induced
isotopic shifts. Results: The results show that all
pre-treatment protocols caused shifts in δ 13C and δ
18O values in the range of 0‰ to ±1.5‰ in both
archaeological and modern samples. Most treated samples also shown
increased crystallinity, likely indicating sample recrystallisation. We
argue that these changes are indicative of more than just removal of
contamination and diagenetic alterations, but also of dissolution and/or
restructuring of the enamel carbonate leading to changes to the in
vivo isotope signal. Conclusions: We recommend careful
consideration of sample burial context and possible routes of diagenesis
when choosing a pre-treatment protocol. Use of H 2O
2 and NaClO to remove organic matter from samples is
discouraged as it incurs unwanted changes to the enamel chemical
properties and isotope values. We also recommend use of only short
acetic acid treatment protocols to avoid possible recrystallisation
during prolonged contact.