Comparing bioapatite carbonate pre-treatments for isotopic measurements: Part 2 – Impact on carbon and oxygen isotope compositions

Carbon and oxygen isotope measurements (δ13C, δ18O) of bioapatite carbonate fractions in archaeological and fossil remains are essential tools to investigate past diets and climates. The reliability of these reconstructions is however dependent on the recovery of the biological signal and the removal of competing sources of carbon and oxygen isotopes, such as remaining organic material and secondary carbonates. Modern, archaeological and fossil bone and tooth specimens have been treated with different chemicals commonly used to pre-treat the samples with the aim of removing these competing sources of isotopes. Results highlight significant shifts in the isotope values of the samples tested, with different isotopic effects according to which pre-treatment was used. This has a clear impact on the data recovered, which may lead to misinterpretations of the results and inferred past diets and climates. The results of this study suggest that both hydrogen peroxide and sodium hypochlorite, the most commonly used oxidising agents employed to remove organic substance, have major problems: hydrogen peroxide is only partially efficient as organic removal agent and causes unwanted bioapatite dissolutions and isotopic shift, while sodium hypochlorite induces the incorporation of exogenous carbonates into the bioapatite, likely from the dissolution of atmospheric CO2 into the test tube. In both cases, there is an impact on the recovered isotope composition. Hydrazine hydrate, a third chemical for organic removal, has the lowest influence on the bioapatite isotope composition, being also relatively efficient at removing organic matter from bioapatites. The latter is recommended as an organic removal agent if such pre-treatment is deemed necessary. The use of acetic acid also causes isotopic shifts that seem independent on the removal of secondary/diagenetic carbonate, as also occurring in modern samples. Overall, our study clearly shows that any chemical treatment commonly used to remove unwanted contaminants from the bioapatite samples has also unpredictable, often significant effects on the pristine isotopic composition recovered, and corroborates the need for consistency in the pre-treatment of bone and teeth specimens. It also questions the necessity of removing organic matter at all from most skeletal samples prior to isotopic analysis.