COMBINING STRONTIUM AND OXYGEN ISOTOPE RATIOS OF HAIR FOR HUMAN PROVENANCING

Understanding an individual’s travel-history can be an important piece of information in a forensic investigation. The analysis of stable oxygen (O) isotopes of human hair has assisted in the reconstruction of the geographic-movement histories of individuals in criminal investigations. While oxygen isotope values (δ¹⁸O) of human hair provide geographic origin information, the estimated regions can be spatially broad. Strontium (Sr) isotope ratios (⁸⁷Sr/⁸⁶Sr) of human hair can provide a complementary isotopic approach to measurement of ¹⁸O in hair for human provenancing as ⁸⁷Sr/⁸⁶Sr ratios record independent geographic information and thus further constrain the estimated regions predicted by δ¹⁸O values. To understand the linkages between the Sr and O isotope systems in keratinous tissues and an individual’s environment, human hair and tap waters were collected throughout the Salt Lake Valley of Utah and the ⁸⁷Sr/⁸⁶Sr ratios and δ¹⁸O values were measured on paired samples. We found that different communities were provided municipal water with distinct ⁸⁷Sr/⁸⁶Sr ratios. Hairs with higher ⁸⁷Sr/⁸⁶Sr ratios were from individuals that resided in neighborhoods that had higher tap water ⁸⁷Sr/⁸⁶Sr ratios. Furthermore, we observed the majority of paired tap water and hair ⁸⁷Sr/⁸⁶Sr ratios had a 1:1 relationship, indicating little to no fractionation between Sr isotope ratios in water and hair. We found the δ¹⁸O values of Salt Lake City water and hair samples had an average value of -16.2 ‰ and 9.8 ‰, respectively. We used the hair model of Ehleringer et al. (2008) to predict tap water δ¹⁸O values of an individual’s drinking water source and observed that the predicted tap water δ¹⁸O values agreed with the measured δ¹⁸O values of tap water available in the community. Our findings indicated that different municipalities within the Salt Lake Valley used different combinations of water resources and these combinations of water resources provided a unique isotopic marker for a municipality. In turn, these community-specific isotopic signals were reflected in the isotope ratios of the hair of individuals that resided within that particular neighborhood. We propose that the paired analyses of both O and Sr isotope ratios in hair may allow for greater refinement in reconstructing the travel-movement history of an individual.