A large-scale evaluation of intraperson isotopic variation within human bone collagen and bioapatite

This study investigated intraperson skeletal (herein referred to as either "intraperson" or "intraskeletal") variation in stable isotope ratios for collagen (C and N) and bioapatite (C and O) extracted from five to six long bones from 27 modern individuals. The maximum intraperson variation observed for collagen was 0.78‰ for δ¹³C_coll values and 1.12‰ for δ¹⁵N_coll values, with a mean variation (± SD) of 0.33 ± 0.18‰ and 0.45 ± 0.27‰, respectively. For bioapatite, the maximum intraperson variation was 1.63‰ for δ¹³C_ap values and 4.80‰ for δ¹⁸O_ap values, with a mean variation (± SD) of 0.81 ± 0.32‰ and 1.00 ± 1.03‰, respectively. These results generally agree with previously reported data on intraskeletal isotopic variation. Using a two- and three-standard-deviations-from-the-mean model with analytical quality control data included, it is proposed that two bones with differing collagen δ¹³C_coll values greater than 0.75‰ are probably from different individuals, and those that have differing values greater than 0.95‰ are from different individuals. Likewise, differing collagen δ¹⁵N_coll values greater than 1.05‰ are probably different, and greater than 1.35‰ are different. For bioapatite, the proposed values change to 1.55‰ and 1.90‰ for δ¹³C_ap, respectively; for δ¹⁸O_ap values no limits were set due to the unexpectedly large variation found in the study population. We highly encourage researchers to use extreme caution when interpreting δ¹⁸O values from bone apatite. We also note that these parameters were evaluated on modern samples and therefore may not reflect intraperson variation in past societies. Finally, we demonstrate application of these interpretative limits to sort commingled human remains cases.