Strontium isotope ratios (87 Sr/86 Sr) of human fingernail clippings reveal multiple location signals.

Abstract

Strontium isotope ratios (87 Sr/86 Sr) in human fingernail keratin tissues have been underexplored for region of origin and travel history reconstruction studies. Here we investigated 87 Sr/86 Sr ratios in fingernail keratin to establish baseline measurements in a resident group and to examine how 87 Sr/86 Sr 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 87 Sr/86 Sr ratios were determined via multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). 87 Sr/86 Sr and oxygen (δ18 O) isotope ratios from the traveler participants were compared to examine temporal patterns. Strontium abundance and 87 Sr/86 Sr ratios in fingernails from the resident group established a baseline against which we could evaluate potential differences in non-resident/traveler groups. Resident 87 Sr/86 Sr ratios remained constant over the study period and were consistent with previously measured tap waters for the area. 87 Sr/86 Sr ratio changes in non-resident/traveler groups were rapid and reflected the current location of the individual within 4-5 weeks of arrival. Lastly, δ18 O and 87 Sr/86 Sr ratios of the same fingernail clippings did not exhibit similar temporal patterns, since fingernail δ18 O 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 87 Sr/86 Sr and δ18 O ratios of the same fingernail clippings revealed two different time points reflecting an individual's residence over short- (4-5 weeks, 87 Sr/86 Sr ratios) and long-term (3-5 months, δ18 O values) time scales. It is likely that the 87 Sr/86 Sr 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.