Particle size matters: The effect of particle size on carbon and oxygen isotope composition of bone hydroxyapatite.

OBJECTIVES: Stable isotope studies often focus on hydroxyapatite (bioapatite) to answer questions of paleodiet, paleomobility, and paeloenvironment. This study seeks to determine the effect that sample particle size (in particular SA:V, or surface area to volume ratios) has on measured carbon and oxygen stable isotope values (δ13 C and δ18 O) in bone hydroxyapatite. MATERIALS AND METHODS: Previously ground Homo sapiens sapiens cortical bone samples were subdivided using geological screens to obtain three separate sub-samples, differing only in their particle size. These aliquots (n = 60) were then treated using established protocols to remove any exogenous organic material (2.5% NaOH) and adsorbed carbonates (0.1 M CH3 COOH), and analyzed for δ13 C and δ18 O using a Kiel-IV Carbonate Device coupled to a Thermo-Finnigan DeltaPlus IRMS. RESULTS: Data obtained indicate that decreased particle size leads to increases in both δ13 C and δ18 O, with oxygen isotope values being more dramatically affected. Specifically, it is possible to produce isotopic shifts of as much as 1.0‰ and 4.0‰ for δ13 C and δ18 O, respectively, solely by analyzing different sized particles from the same individual, bone, and sample. DISCUSSION: Based upon the variability seen between different size fractions from the same sample, it is clear that particle size has a meaningful impact on carbon and oxygen isotope composition. We attribute these shifts to the differential adsorption or precipitation of environmental carbon and oxygen during pretreatment. We recommend that particle size be added to the list of potential variables affecting isotope composition, alongside other factors including diagenesis, reagent concentration, and treatment time. We would also note that while most individuals exhibit consistent changes, some do not, and thus further investigation into these phenomena is warranted.

An isotopic study of dietary diversity in formative period Ancachi/Quillagua, Atacama Desert, northern Chile.

OBJECTIVES: To characterize the paleodiet of individuals from Formative Period (1500 B.C.-A.D. 400) Atacama Desert sites of Ancachi and Quillagua as a means of understanding the dietary and cultural impacts of regional systems of exchange. MATERIALS AND METHODS: Thirty-one bone samples recovered from the cemetery of Ancachi (02QU175) and in/around the nearby town of Quillagua were the subject of carbon and nitrogen stable isotope analysis of bone collagen and hydroxyapatite and multisource mixture modeling (FRUITS, food reconstruction using isotopic transferred signals) of paleodiet. These individuals were compared with nearly 200 other Formative Period individuals from throughout the region to identify differences in dietary behaviors. RESULTS: 80.6% (25/31) of the samples yielded sufficient well-preserved collagen and were included in the multisource mixture model. The FRUITS model, which compared individuals with a robust database of available foods from the region, identified a wide diversity of diets in the Ancachi/Quillagua area (including both coastal and interior individuals), and, most notably, thirteen individuals who consumed an average of 11.2 ± 1.9% terrestrial animals, 19.8 ± 1.9% legumes, and 22.5 ± 3.1% marine fauna, a balanced pattern of protein consumption distinct from both the coastal and inland individuals in our larger regional sample. CONCLUSIONS: The combination of stable isotope analysis and multisource mixture modeling permitted the characterization of dietary behavior of 25 individuals from nodal sites in the Atacama Desert, thus enhancing our understanding of the economic and social relationships that bound together Formative Period sites, populations, and individuals in this hyperarid region.