Spatial Distributions of Isotope Ratios in Tap Water, Hair, and Teeth from Latin America for Region-of- Origin Predictions of Unidentified Border Crossers

Abstract

In this article, we present and summarize reference stable isotope data useful in the identification efforts of deceased undocumented border crossers (UBCs) found in southern Arizona and South Texas. In addition, we discuss the use of reference baseline data to refine predictions of region-of-origin of unidentified UBC remains. Overall, Mexican reference hair samples reflect significant consumption of C₄ resources, indicating a diet dominated by corn and corn products. Even though meat consumption in the United States is almost twofold that of Mexico, the Mexican δ¹⁵N values were elevated over U.S. values, suggesting differences in nitrogen isotope baselines. The comparison of isotope data collected from deceased UBCs recovered from south Texas versus Arizona suggests only small differences in dietary isotopes between the two groups. Geolocation isotopes showed significant differences in δ¹⁸O values of both bone and tooth enamel bioapatite, suggesting differences in the sources of drinking water. However, no significant differences were identified between ⁸⁷Sr/⁸⁶Sr ratios for the two groups. Further, we present a case application of successful region-of-origin
predictions using the δ¹⁸O values and ⁸⁷Sr/⁸⁶Sr ratios and also discuss some of the inherent limitations and challenges of applying reference data sets to large-scale
humanitarian identification projects.