Forensic Anthropology

Calculation and Interpretation of Inter-laboratory Variation in Isotope Delta (δ) Values Using Real Interpretative Differences

Lesley A. Chesson, Thuan H. Chau, Amelia J. Edwards, Gregory E. Berg — Wed, 16 Apr 2025 00:00:00 -0400

Building on applications in anthropology, the use of stable isotope ratio analysis of human bone collagen to investigate an
individual’s life history is becoming more commonplace in the forensic science community. Careful consideration of the resultant isotope delta (δ) values, particularly in regard to their accuracy and reliability, is paramount when introducing isotope data into the U.S. court system. In this study, we use a simple framework to calculate real interpretative difference (RID) values for collagen (“col”) and assess isotope data comparability for sample analysis (RID_analysis) as well as sample preparation/analysis combined (RID_combined). The RID_combined values of 0.59‰ for δ¹³C_col and 0.91‰ for δ¹⁵N_col are similar to more complex, published calculations of inter-laboratory variability in the stable isotope analysis of skeletal remains, but they are easier to calculate and intuitively elegant. The RID_combined as well as RID_analysis values presented here allow users to examine multiple sources of inter-laboratory isotopic variation (preparation, analysis, and both together) in a two-step process whereby a RID value is constructed and then tested. Implementation of this RID approach will provide surety for the legal and research communities in forensic applications of stable isotope ratio analysis.

Considerations for Stable Isotope Analysis of Human Hair

Tiffany B. Saul, Lesley A. Chesson, Dawnie W. Steadman, Gwyneth W. Gordon — Wed, 16 Apr 2025 00:00:00 -0400

Stable isotope analysis of postmortem hair is performed in order to make inferences about an individual’s diet and geographic
travel history prior to death. During analysis and interpretation, investigators assume that the hair collected from a postmortem
environment has not been altered by exposure conditions and that the isotopic “signatures” of hair prior to exposure are preserved in postmortem samples. In order to confidently make inferences from postmortem hair samples, it is necessary to know whether their isotope ratios undergo postmortem changes. To address this question, post-exposure hair samples (n = 44) were collected from known body donors at the Anthropology Research Facility in Knoxville, Tennessee, USA, at various time points ranging from 22 to 1,140 days of exposure. These samples were analyzed for carbon (δ¹³C), nitrogen (δ¹⁵N), hydrogen (δ²H), and oxygen (δ¹⁸O) isotope ratios, and the results were compared with pre-exposure hair samples collected from the same donors. This study highlights considerations for the interpretation of isotope ratios obtained from postmortem hair samples in forensic contexts. The results indicate that δ¹³C, δ¹⁵N, and δ¹⁸O values from human hair remain relatively consistent over periods up to three years of outdoor exposure, while δ²H values changed significantly between pre-and post-exposure hair samples.

Food Has No Borders

Rhonda L. Quinn, Helen Alesbury, Ligia Ceja, Angela Soler, Linda Godfrey — Wed, 16 Apr 2025 00:00:00 -0400

Isotopic analyses of human remains augment the biological profile with geolocation and dietary information, furthering efforts to identify unknown individuals from a forensic context. Here we test the methodological resolution of geolocation (δ¹⁸O, ⁸⁷Sr/⁸⁶Sr) and dietary (δ¹³C, δ¹⁵N) isotopes of one identified individual who immigrated to New York City from St. Vincent and the Grenadines (SVG),
Lesser Antilles. Isotope-based geolocation estimates did not identify the childhood residency on SVG, but did point to New York City as a possible residence during early adulthood. The individual’s C₃-based diet did not significantly change from childhood to early adulthood, illustrating the maintenance of food traditions after the immigration event. This study illustrates that further development of tissue-specific isoscapes incorporating bioavailable foods, drinking water, and cultural traditions is warranted to refine methodological resolution of isotopic applications in forensic anthropology.

Using Real Interpretative Differences to Assess Inter-laboratory Isotopic Variability Due to Sample Preparation

Wed, 16 Apr 2025 00:00:00 -0400

Here we present a two-part study to assess isotope data comparability between two sample preparation laboratories, based on the concept of Real Interpretative Differences (RID). Inter-laboratory isotopic variability should be evaluated prior to compiling data from different sources. Isotopic variability can result from two main causes: sample preparation and sample analysis. We compared the carbon and nitrogen isotope delta (δ) values of 20 modern human bone collagen sample pairs—representing seven individuals from diverse backgrounds—that were prepared close in time at two laboratories and then analyzed at one facility. From this study, values for RID of preparation, or RID_prep, were calculated for bone collagen as 0.43‰ and 0.75‰ for δ¹³C values and δ¹⁵N values, respectively. These RID_prep values were tested using a set of 30 bone collagen sample pairs—representing 30 modern individuals—that were first prepared over a span of five years (2014–2019) at one laboratory; they were prepared again at the second laboratory in 2019. While the differences in isotope δ values between
the paired samples in the second study were statistically significant, they did not exceed the calculated RID_prep values for either carbon (30 comparisons) or nitrogen (30 comparisons)—with the exception of one carbon difference in a single paired bone sample. The inherent error rate in combining the isotope test results was thus calculated as 1.7%. This study demonstrated that the isotope data generated from human bone collagen prepared at one laboratory can be reliably compared to data generated from samples prepared at the other laboratory.

Childhood and Adulthood Residency Prediction Using Strontium (87Sr/86Sr) and Oxygen (δ18O) Isotopes for Unidentified Deceased Migrants Recovered in Southern Texas

Robyn T. Kramer, Eric J. Bartelink — Wed, 16 Apr 2025 00:00:00 -0400

Strontium and oxygen isotope values are used in conjunction with a maximum likelihood assignment model to predict the childhood and adulthood geographic residences of 20 deceased migrants recovered along the México-U. S. border in south Texas. The
objectives of this research are to determine (1) if the childhood and adulthood residency assignments predict the same geographic region, (2) if using the predicted residential history/nationality as a filter on the National Missing and Unidentified Persons System (NamUs) reduces the number of potential matching missing person reports, and (3) if we should target teeth or bone as the optimal hard tissue to sample. We found that 12 of the 20 cases (60%) predict similar regions of residency using isotopes derived from bone bioapatite (adulthood residency) and tooth bioapatite (childhood residency). When predicted residential history/nationality data were used as a filter in NamUs, there was a significant reduction in the mean number of case searches (mean = 1568.5 ± 505.3 [1 SD] vs. 107.9 ± 142.6 [1 SD]; t = 12.441, df = 22, p < 0.001, unequal variances assumed). These results demonstrate the utility of isotope data for narrowing down the region of origin and in turn the number of missing persons records to review. Finally, we recommend that death investigators at minimum collect dental samples (premolars or molars) for future unidentified migrant cases to aid in identification efforts utilizing isotope analytical methods. Bone bioapatite samples can provide more recent residential history information but should be used in conjunction with tooth enamel data.

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

Saskia Ammer, Eric J. Bartelink, Jennifer Vollner, Bruce Anderson, Kate Spradley — Wed, 16 Apr 2025 00:00:00 -0400

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.

Using Stable Nitrogen Isotope Ratios from Human Skeletal Muscle Tissue for Postmortem Interval (PMI) Estimation (Part 1)

Melanie Beasley, Julie Lesnik, Hayden McKee-Zech, Anielle Duncan — Fri, 12 Jul 2024 00:00:00 -0400

Human skeletal muscle tissue was analyzed for stable nitrogen isotope analysis (δ¹⁵N) to test the applicability of changes in values (Δ¹⁵N) through the putrefaction process for postmortem interval (PMI) estimation. Seasonal differences in Δ¹⁵N values of decom-posing skeletal muscle tissue were analyzed from 20 human donors at the University of Tennessee Anthropology Research Facility. This study revealed that change in the δ¹⁵N ratios of decomposing muscle tissue has potential as a novel method for PMI estimation. In winter months, when identifiable muscle tissue can be retained on a body for months, the data indicate that there is a systematic increase in Δ¹⁵N values with increasing temperatures when putrefaction progresses faster with values shifting as much as 3.0‰. This initial study indicates there is value in further method development of this quantitative approach to PMI estimation for recently deceased individuals, but further work is needed. While the analysis only requires a small amount of tissue, has a user- friendly collection and analysis protocol, and is less subjective than visual methods, there is still significant research to be done prior to widespread applications as a PMI estimation method. Understanding variability in δ¹⁵N across decomposing muscle tissue groups, individual variation (i.e., muscle mass, disease, body chem-istry), and an accurate approximation for a perimortem δ¹⁵N value necessary to calculate Δ¹⁵N all need to be further researched prior to generation of a reliable regression formula and error rates.

Using Stable Nitrogen Isotope Ratios from Human Skeletal Muscle Tissue for Postmortem Interval (PMI) Estimation (Part 2)

Melanie Beasley, Julie Lesnik, Hayden McKee-Zech, Anielle Duncan — Fri, 13 Sep 2024 00:00:00 -0400

Fly larvae were analyzed for stable nitrogen isotope analysis (δ¹⁵N) to test if values can be used to predict season of death/decomposition as a first step in developing a robust quantitative method for postmortem interval (PMI) estimation. This is the first study to analyze δ¹⁵N values from blow flies (BF) (Ditera, Calliphoridae: Lucilia coeruleiviridis, Calliphora vicina, Calliphora vomitoria, Chrysoma rufifaces) and black soldier flies (BSF) (Diptera, Stratiomyidae: Hermetia illucens) to estimate seasonal PMI in eastern Tennessee. This study is part of a larger project seeking to expand stable isotope methods utilized in forensic anthropology to move beyond questions of geolocation and diet. When BF and BSF δ¹⁵N values are used to predict season of placement at the University of Tennessee Anthropology Research Facility, the BF δ15N values only have an overall accuracy rate of 38.4%, while the BSF δ¹⁵N values perform better with an overall accuracy rate of 72.6%. Based on the seasonal distribution of BSF δ¹⁵N values, it is our recommendation that only δ¹⁵N values from BSF be used as a general indicator of winter versus summer placement when values are above 30‰ versus below 18‰, respectively. Analysis of δ¹⁵N values in fly larvae is promising as a quantitative PMI estimation method, but further research is needed.

Crossing Borders

Eric J. Bartelink, Saskia Ammer — Wed, 16 Apr 2025 00:00:00 -0400

This special issue of Forensic Anthropology focuses on applications of stable isotope analysis in forensic anthropology
and originated from a 2020 American Academy of Forensic Sciences symposium titled “Crossing Borders: International and Multidisciplinary Applications of Isotope Analysis in Forensic Anthropology.” Stable isotope applications are still relatively new in forensic anthropology, and current applications focus on the provenancing (i.e., sourcing) of unidentified human remains, the identification of possible evidence of starvation or undernutrition in potential abuse and neglect cases, and estimations of time since death.