Assessing the Reliability of Mobility Interpretation From a Multi-Isotope Hair Profile on a Traveling Individual

Abstract

Forensic practitioners, archeologists, and ecologists increasingly use hair isotope profiles and isotope databases and maps to reconstruct the life history of unidentified individuals. Relationships between hair isotope profiles with travel history have been primarily investigated through controlled laboratory experiments. However, those controlled studies do not reflect the complex life history of modern individuals who often travel between multiple locations over different periods. Here, we collect one bundle of hairs from a volunteer whose primary residence is in Ottawa (Canada) but who traveled to multiple destinations over a period of 18 months. Those travels include multi-week trips to distant locations and multi-day trips to more local areas. We use multi-isotope profiles on the individual's hairs coupled with isotope databases across the world to reconstruct travel history at sub-monthly temporal resolution. We compare the isotopic interpretation of mobility with the recorded travel history. A prominent shift in δ 2 H values is interpreted as a westward movement toward central Canada, which corresponds to a month-long road-trip to the Prairie. We observed a marked negative excursion in δ 13 C values along the hairs profile, which was related to a multi-week-long trip to several countries in Europe. Except for an exceptionally variable interval likely driven by health issues, δ 15 N values show very little fluctuation across the entire profile, indicating consumption of consistent amounts of animal or marine-based protein at different locations. The isotopic shifts in the proximal part of the hair reveal a clearer picture of traveling destinations than the shifts in the distal part, which have larger uncertainty in terms of timing and amplitude. Except for the last couple of months before collection, 87 Sr/ 86 Sr ratios show little variation throughout the profile likely due to recent exchange with Ottawa tap water during bathing or showering. The different 87 Sr/ 86 Sr ratios in the month preceding collection appear to partially preserve the 87 Sr/ 86 Sr ratio of the volunteer's diet. This study demonstrates the interest of using multi-isotope systems and large isotopic databases to reconstruct individual mobility. This study also underlines the challenges in linking isotope data to mobility, particularly in rapidly growing keratinous tissues.

Multi-isotopes in human hair: A tool to initiate cross-border collaboration in international cold-cases

“…The carbon isotope data further support the proposed dual-isotope geographic assignments. While it is challenging to use carbon isotope for producing probabilistic maps of provenance because of fractionation associated with individual physiology and diet, the δ 13 C trend within a hair profile does record mobility [ 36 ]. In the case of Mr. Halifax, carbon isotope trends validate the dual δ 2 H and δ 34 S geographic assignments.…”

“…Low δ 13 C values (~ −19 ‰) a few months PTD are compatible with regions of Ontario around the Great Lakes (i.e., western Ontario and Manitoba). Even the δ 15 N values, which are usually not sensitive to mobility within a given country [ 36 ], appear to record a shift from higher to lower δ 15 N values around 5 months PTD. The higher δ 15 N values a year PTD possibly reflect a shift in dietary habits between places of residence (e.g., more fish consumption in the southeastern US leading to higher δ 15 N values).…”

“…In 2008, the RCMP Halifax Detachment sent a clump of dreadlock hair to the University of Ottawa from an unidentified individual recovered on October 8, 2004 near the Halifax city airport, Nova Scotia, Canada (hereafter called Mr. Halifax; RCMP Case File #2004–3757)). Rather than analyzing the bulk hair, a chronological isotopic profile was obtained following a procedure that was developed and tested in a previous study to align, combine and sequence multiple hair [ 36 ]. However, the hair of this individual was extremely brittle which made alignment challenging.…”

“…Hydrogen, carbon, nitrogen, oxygen and sulfur are assimilated into hair keratin, and are resistant to post—mortem exposure [ 34 , 35 ], and are thus the ideal candidate for multi-isotope geolocation of humans from their hair. The δ 2 H and oxygen isotope delta ( δ 18 O) values in human hair primarily reflects the isotopic variability in drinking water which follows systematic spatiotemporal patterns along latitude, elevation, and continentality [ 33 , 36 ].When an individual moves through the landscape, their hair incorporates the isotopic composition of the drinking water consumed along the way and can thus be used to reconstruct origin and movements [ 36 ]. Though limited in the geographic resolution of the information it can provide, δ 2 H measurements in human hair has been used for decades in isotope geolocation studies [e.g., 32] and data are available from many countries to develop continental-scale maps of isotope variations (or isoscapes) [ 37 ].…”

Silver Linings at the Dawn of a “Golden Age”

“…Moving forward, it is also clear that strontium isotope research is critically lacking controlled studies that link isoscapes with empirical isotope data to reconstruct individual movements. Although unsuccessful for strontium isotopes, Hu et al (2020) provide an example in this special issue of such a "controlled movement" study. There is a serious need for designing such controlled movement studies linking isoscapes and isotope values from an individual with known travel history.…”