Isotopic analysis in forensic geography

Using isotopic analysis of skeletal remains and forensic evidence, one can reconstruct human profiles (including interpreting ancestry and evaluating social status), analyse the environmental conditions antemortem and estimate the time of death. Other forensics-related investigations using isotope geochemistry include the analysis of trace evidence materials (e.g. paints, soils) as well as provenancing illegal wildlife, bullets, narcotics, and other forensic evidence.

The analysis of isotopes across space, also known as geoisotopic mapping or fingerprinting, can provide vital information about the geographic origin of forensics evidence. In this technique, scientists develop isoscapes, which are detailed maps of isotopic distribution across a region or landscape. This is based on the distribution of distinct isotopic signatures in rock, soil, water, plants, and food sources, which are integrated into bones, hair, bullets, and other forensic evidence.

Identifying a perpetrator

Lead (Pb) is a common component in bullets, which are derived from specific ore deposits depending on the bullet make and model. By measuring a series of lead isotopes (204Pb, 206Pb, 207Pb, 208Pb), one can distinguish between bullets from different manufacturers as well as bullets from different batches of the same manufacturer (Koons et al. 2005). This has been proven to be particularly useful in distinguishing which firearm caused specific damage in shooting incidents when multiple shooters are involved (Zeichner et al. 2006).

Identifying a victim

In cases where the identification of a deceased individual is unknown, one can perform analysis of strontium isotopes as well as radiocarbon dating of bones and hair to reveal personal details. 

Predicted 87Sr/86Sr for bedrock
Predicted 87Sr/86Sr for bedrock of the contiguous United States (Bowen and West, 2019)

Strontium isotopes (87Sr/86Sr) provide a fingerprint of the source of food consumed during an individual’s lifetime, as these isotopes are abundant in water and bedrock and are eventually integrated into soil and plants (Pye et al. 2004; Bartelink et al. 2019). Additionally, oxygen isotopes (𝛿18O) can be analysed to pinpoint water sources based on isotopic signatures in precipitation. As a result, crops and water from a specific region consumed during an individual’s lifetime will have 87Sr/86Sr and 𝛿18O signatures identifiable within human or animal tissue. 

For example, hair from a human skull found in a southern California desert was analysed for oxygen stable isotopes to identify the likely locations where the decedent likely lived in the year before her death. Assuming a hair growth rate (0.39mm/day), the researchers were able to conclude that she moved from a west, southwest or northeast state in the USA to the South during the last 1-3 months of her life (Bowen et al. 2007). In such cases where there is more than one possible geographic origin of a deceased individual, one can utilise multiple isotopes to pinpoint the most likely location. A study of an unidentified individual in Texas utilised oxygen isotopes to estimate the location of drinking water and strontium isotopes to identify food source regions (Bartelink et al. 2018). Individually, these isotopes provided a wide geographic region of potential origin sites for the individual. However, taken together, the overlapping possible sites were greatly reduced, providing a more specific, well evidenced prediction for the region of origin of the unidentified individual.

Based on the assumption that certain bones and tissues develop during distinct periods in one’s lifetime, the 87Sr/86Sr analysis of a specific tissue can provide details on where the individual was living during that particular time period. For example, the analysis of tooth enamel can provide information on early years in an individual’s life, whereas analysis of hair is a good indication of lifestyle in the final months or years of an individual’s life. As a result, tooth enamel can be used to interpret a victim’s ethnic ancestry, whereas hair can be used to interpret living location prior to death (Schwarcz et al. 2007). 

A study by Rauch et al. (2007) demonstrates a case when isotopic analysis was employed while a victim could not be identified by DNA, dental or fingerprint analysis. As a result of studying a series of isotopes (including Pb and Sr), the researchers were able to pinpoint the victim to Romania. Similar to the provenancing studies employed by archaeologists, various isotopes can be employed to investigate forensic evidence to identify the perpetrator and victim when other leads go cold. 

References

Bartelink, E.J. and Chesson, L.A., 2019. Recent applications of isotope analysis to forensic anthropology. Forensic sciences research, 4(1), pp.29-44

Bartelink, E.J., Berg, G.E., Chesson, L.A., Tipple, B.J., Beasley, M.M., Prince-Buitenhuys, J.R., MacInnes, H., MacKinnon, A.T. and Latham, K.E., 2018. Applications of stable isotope forensics for geolocating unidentified human remains from past conflict situations and large-scale humanitarian efforts. In New perspectives in forensic human skeletal identification (pp. 175-184). Academic Press.

Gabriel J. Bowen, Jason B. West,, 2019. Chapter 3 – Isoscapes for Terrestrial Migration Research,

Editor(s): Keith A. Hobson, Leonard I. Wassenaar, Tracking Animal Migration with Stable Isotopes (Second Edition), Academic Press, Pages 53-84, https://doi.org/10.1016/B978-0-12-814723-8.00003-9.

Bowen, G.J., Ehleringer, J.R., Chesson, L.A., Stange, E. and Cerling, T.E., 2007. Stable isotope ratios of tap water in the contiguous United States. Water Resources Research, 43(3).

Koons, R.D. and Buscaglia, J., 2005. Forensic significance of bullet lead compositions. Journal of Forensic Science, 50(2), pp.JFS2004212-11.

Pye, K., 2004. Isotope and trace element analysis of human teeth and bones for forensic purposes. Geological Society, London, Special Publications, 232(1), pp.215-236.

Rauch, E., Rummel, S., Lehn, C. and Büttner, A., 2007. Origin assignment of unidentified corpses by use of stable isotope ratios of light (bio-) and heavy (geo-) elements—a case report. Forensic Science International, 168(2-3), pp.215-218.

Schwarcz, H.P., 2007. Tracing unidentified skeletons using stable isotopes. Forensic Magazine, 4(3), pp.28-31.

Zeichner, A., Ehrlich, S., Shoshani, E. and Halicz, L., 2006. Application of lead isotope analysis in shooting incident investigations. Forensic science international, 158(1), pp.52-64.

Image References

Crime scene tape: https://www.pexels.com/photo/crime-scene-do-not-cross-signage-923681/