Addressing data comparability in the creation of combined data sets of bioapatite carbon and oxygen isotopic compositions

Before amalgamating published isotope data, comparability should be demonstrated. This paper compares carbon and oxygen isotopic compositions of 30 enamel samples measured by two laboratories. The aims were to see what, if any, isotopic variation was observed, to determine the causes as needed and to correct if possible. Bioapatite was acidified at 90 degrees C in 2006 and at 26 degrees C in 2017, while delta values were corrected via one-point normalization in 2006 and by two-point normalization in 2017. One case (of the 30) produced different delta values between the analysis dates, suggesting contamination. Repeated carbon isotope ratio measurements were not meaningfully different. Repeated oxygen isotope ratio measurements were significantly different, even following correction for acid-carbonate fractionation at different temperatures and the renormalization of 2017 delta values using one point; however, differences were not meaningful for interpretations. Results were used to calculate real interpretative differences (RIDs) for comparing enamel bioapatite as 0.6 parts per thousand for delta C-13 values and as 1.6 parts per thousand for delta O-18 values.