This article discusses strontium isotope ratios of hair as potential tools to estimate human provenance.
Rationale Methods Strontium isotope ratios (Sr-87 Sr-86) in human fingernail keratin tissues have been underexplored for region of origin and travel history reconstruction studies. Here we investigated Sr-87 Sr-86 ratios in fingernail keratin to establish baseline measurements in a resident group and to examine how Sr-87 Sr-86 ratios changed with relocation. Fingernail clippings were collected from resident (n 10) and non-resident traveler groups (n 4 and n 4) that were part of a larger study in Salt Lake City (UT, USA) from 2015 to 2016. Strontium abundance and Sr-87 Sr-86 ratios were determined via multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). Sr-87 Sr-86 and oxygen (O-18) isotope ratios from the traveler participants were compared to examine temporal patterns. Results Conclusions Strontium abundance and Sr-87 Sr-86 ratios in fingernails from the resident group established a baseline against which we could evaluate potential differences in non-resident traveler groups. Resident Sr-87 Sr-86 ratios remained constant over the study period and were consistent with previously measured tap waters for the area. Sr-87 Sr-86 ratio changes in non-resident traveler groups were rapid and reflected the current location of the individual within 4-5 weeks of arrival. Lastly, O-18 and Sr-87 Sr-86 ratios of the same fingernail clippings did not exhibit similar temporal patterns, since fingernail O-18 ratios required more time to attain values characteristic of the new environment. Our findings suggest that strontium is incorporated into fingernail tissues differently from hair and this could be advantageous to forensic investigations. We found that Sr-87 Sr-86 and O-18 ratios of the same fingernail clippings revealed two different time points reflecting an individual's residence over short- (4-5 weeks, Sr-87 Sr-86 ratios) and long-term (3-5 months, O-18 values) time scales. It is likely that the Sr-87 Sr-86 ratios of fingernail clippings reflect exogenous signals that are incorporated through bathing waters and that these signals change rapidly with movement to a new location. Our results may aid future forensic studies in the determination of region of origin in unidentified remains.