Date of Award
Doctor of Philosophy
The positive identification of a decedent is paramount to a forensic investigation in which human remains have been recovered and must be identified. Due to increasing global mobility in the world's populations, it is not inconceivable that an individual might die far away from his or her home. Pinpointing an individual's geographic origin may contribute to definitive forensic identification of contemporary human skeletal remains in cases where dental records and other means of identification are either unavailable to law enforcement personnel or do not yield immediate results. Stable isotope analysis of biogenic tissues such as tooth enamel and bone mineral has become a well‐recognized and increasingly important method for determining the provenance of human remains, and it has been used successfully in bioarchaeological studies as well as forensic investigations. Both 18O and 2H stable isotope signatures are well established proxies as environmental indicators of climate (temperature) and source water and are therefore considered reliable indicators of geographic life trajectories of animals and humans. Similarly, 13C and 15N abundance data have distinguished dietary preferences in ancient human populations, and have been used to qualify 2H and 18O geolocational data that may be consistent with more than one location. Few if any studies have systematically investigated the multi-isotopic signatures in human tooth enamel and dentin from living individuals. Since 18O abundance values obtained from tooth enamel of late-erupting molars are a source of information on geographic origin of an individual during adolescence when crown formation takes place, it was hypothesized that: 1) the stable isotope abundance of 2H, 18O, and 13C in human tooth enamel and dentin is consistent with self-reported residential history and dietary preferences data, and 2) the isotopic variability evident between individuals with shared residential history and nutrient intake is quantifiable, and indicative of intra-individual variability. Two pilot studies were conducted to: 1) evaluate the feasibility of extracting and measuring the 2H composition of human tooth enamel and its suitability as a proxy for human geographic provenance, and 2) compare the isotopic abundance of isotopic abundance of 2H and 13C in human crown dentin collagen obtained from archaeological and modern teeth with the 18O and 13C isotopic composition of the corresponding tooth enamel carbonate. A protocol for preparing tooth enamel for 2H analysis was successfully devised and implemented; however, no correlation was observed between tooth enamel 2H abundance values as measured plotted against source water 2H abundance values. While unexpectedly low rates of hydrogen exchange within mineral hydroxyl groups were documented, and it was possible to analyze tooth enamel for its 2H isotopic composition, the seemingly fixed nature of 2H abundance in tooth enamel means that the 2H isotopic signature of tooth enamel cannot be used as an indicator of geographic provenance. Conversely, positive correlations between collagen 2H abundance values of primary dentin with 2H abundance values for source water and also with enamel 18O abundance values suggests that primary dentin collagen 2H values are linked to the isotopic composition of source water. Third molar tooth enamel was sampled from 10 living volunteers undergoing routine tooth extractions at University of Toronto affiliated dental clinics in Ontario, Canada. The mixed-sex group of patients was given questionnaires in which they provided detailed residential history and answered questions pertaining to dietary preferences (e.g., vegetarian) prior to donating all four third molars. Enamel was drilled from the crown of two third molars from each subject, chemically cleaned, and subjected to an acid digest before being analyzed for its 18O and 13C composition using Isotope Ratio Mass Spectrometry. Herewith, isotope abundance values are presented using the delta notation as delta values in per mil (‰). Mean d13CVPDB values for all samples ranged from -9.47 ‰ to -11.31 ‰ (pooled mean = -10.37 ‰), which suggested a persistent C4 plant dietary influence at the time the sampled tooth enamel was forming and is consistent with the typical North American diet. While inter-subject variation contributed the largest proportion of total d13C variability, differences were not significant. The pooled mean d18OVSMOW value for enamel samples was 24.39 ‰, while individual subject mean d18OVSMOW values ranged from 23.76 ‰ to 25.18 ‰. Marked offsets (0.01 ‰ - 0.51 ‰) in mean d18OVSMOW values for each pair of third molars were observed. While subject variation was significant (p=0.0034), neither diet nor sex significantly influenced the oxygen (or carbon) isotope data. Following conversion of d18OVSMOW values to d18OPhosphate values, drinking water values were calculated using the Daux et al. (2008) equation, and compared to their corresponding regional estimated annual average d18O values in precipitation retrieved from the Online Isotopes in Precipitation Calculator (OIPC) (Bowen 2014). Despite observed correlations between the drinking water d18OWater values and d18OOIPC values of four subjects, no statistically significant correlations were evident between the two limited data sets. However, when the isotopically similar d18O values of Toronto area residents were averaged and combined with the remaining Canadian data and the 18O data from 5 enamel samples analyzed in the 2H enamel pilot study, strong positive correlations were evident between d18OPhosphate valuesand d18OOIPC values (R2 = 0.87). Moreover, an equally strong linear relationship was observed between modeled annual precipitation 18O and calculated source water 18O (R2 = 0.87). Based on the d2H, d18O, and d13C values measured in tooth enamel and dentin, it is possible to infer and confirm geographic provenance and dietary intake. However, while enamel carbonate d13C values were consistent with self-reported dietary intake information and residential history, as could be expected vegetarians could not be distinguished from those who consumed meat without determining C/N isotopic ratios in dentin collagen. Further, the presence of strong linear relationships between the 18O composition of enamel carbonate and modeled source water 18O in the merged dataset illustrates the importance of considering site-specific isotopic complexities and using multi-isotope data obtained from multiple tissues when investigating the geographic origins of humans in an archaeological or forensic context. It is not possible to quantify intra-individual isotopic variability without sampling from larger, geographically diverse populations and controlling for as many variables as possible. The construction of regional databases containing 18O and 2H isotopic data obtained from a variety of environmental and human and faunal tissue samples, and the application of such data to individual cases in which geographic origins are desired, is recommended provided the following caveats are considered: 1) whenever possible, d18OPhosphate values should be used when comparing d18O values in tooth enamel with those in precipitation in order to avoid the unmitigated error associated with the application of carbonate conversion equations to enamel phosphate d18O data, 2) the continuous consumption of food sourced elsewhere is liable to affect the overall enamel carbonate d18O values, 3) d18O values in precipitation are not inclusive of the variety of postprecipitation and hydrological processes unique to a particular location, nor are they an accurate representation of the isotopically mixed nature of tap water sourced from distant reservoirs. Hence, extrapolations should be made with caution. Enamel sequential microsampling methods may be advisable for narrowing down a geographic timeline if the overlapping of isotopic signals at various points along the tooth’s longitudinal plane can be quantified and validated. While standardization of analytical methodology is critical to appropriate interpretations of the data, stable isotope profiling is not a standalone method and should be used in conjunction with other lines of evidence in determinations of human provenance.
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