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 (delta C-13 and delta O-18) 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 CH3COOH), and analyzed for delta C-13 and delta O-18 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 delta C-13 and delta O-18, with oxygen isotope values being more dramatically affected. Specifically, it is possible to produce isotopic shifts of as much as 1.0 parts per thousand and 4.0 parts per thousand for delta C-13 and delta O-18, 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.