Investigating intra-bone isotopic variations in bioapatite using IR-laser ablation and micromilling: Implications for identifying diagenesis?

The potential of in situ microsampling (micromilling and IR-laser ablation) to detect intra-sample isotopic variation resulting from diagenesis and/or natural remodeling of bone was examined in this study. Modern sheep bones were exposed to microbial populations to induce diagenesis and the δ13C and δ18O values of bioapatite structural carbonate and phosphate were determined for these samples and for diagenetically modified archaeological samples from three different environments. No significant differences were found between the δ13Claser or δ18Olaser values of microbially colonized and uncolonized areas on the modern bone surfaces, or between well-preserved and poorly preserved regions of the archaeological material. However, high degree of intra-sample variation was observed in δ-values obtained using IR-laser ablation, some of which arose from methodological challenges associated with this method. Further, δ13C values obtained using IR-laser ablation were lower than those acquired by conventional analysis, likely because of contamination from organic matter in the samples. Isotopic compositions of structural carbonate samples obtained by micromilling were less variable. For both methods, however, significant differences in δ13C and δ18O values were observed for different bone surfaces. These variations may indicate differences in the isotopic compositions of food and water incorporated into osteons over a lifetime, and illustrate the relationship between bone remodelling and isotopic heterogeneity in bone. Overall, our results highlight the difficulties of using the IR-laser ablation method for measuring isotopic compositions of bone.