Fast exchange of strontium between hair and ambient water: Implication for isotopic analysis in provenance and forensic studies

Abstract

Trace elements in hair originate from intake (e.g., diet, inhalation, skin absorption), are transported in the bloodstream, and then incorporated during hair formation. However, the trace element abundance and isotopic compositions may be altered by post-eruption environmental processes. Such alterations must be addressed to obtain a meaningful interpretation of hair analysis for biomonitoring. In this study, we used strontium (Sr) isotopic analysis together with sorption kinetics of ionic Sr to quantify the rate and extent of replacement of endogenous Sr in hair by exogenous Sr from ambient water. We found that with only 10 minutes of exposure at room temperature (22˚C), more than 30% of original endogenous Sr in hair was replaced with exogenous Sr from the solution. After 16 days of exposure to the solution, more than 90% of endogenous Sr was replaced, with a warmer temperature (60˚C) accelerating the exchange substantially. We also found that acid leaching of exposed hair did not remove or isolate the exogenous Sr; therefore, neither the original endogenous nor the exogenous 87 Sr/ 86 Sr signal could be separated. Nonetheless, these findings illustrated that the quantitative correlation between the fraction of exogenous Sr and the soaking time, if established, could be used to estimate the length of water contact time for hair in forensic studies. Even if such time since initial contact cannot be established, the combination of acid leaching and 87 Sr/ 86 Sr analysis of hair samples may still be valuable in provenance studies to identify recent changes in the exogenous Sr pool, including movements or changes in water source.

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

“…A possible explanation for this discrepancy is that Sr in hair has a much long turnover time than hydrogen isotopes used to calibrate the hair age model (Cohn et al, 1962;Kitchings et al, 1976). A more likely reason is that hair incorporates Sr from tap water through surface contamination during bathing or showering instead of drinking (Tipple et al, 2018;Hu et al, 2020). As shown in previous studies, exogenous Sr can be rapidly incorporated into hair keratin after the eruption from the scalp (Chittleborough, 1980;Kempson and Lombi, 2011), and the 87 Sr/ 86 Sr ratios in human keratinous tissues can rapidly inherit the 87 Sr/ 86 Sr of bathing or showering water (Mancuso and Ehleringer, 2018;Tipple et al, 2018;Hu et al, 2020).…”

“…A more likely reason is that hair incorporates Sr from tap water through surface contamination during bathing or showering instead of drinking (Tipple et al, 2018;Hu et al, 2020). As shown in previous studies, exogenous Sr can be rapidly incorporated into hair keratin after the eruption from the scalp (Chittleborough, 1980;Kempson and Lombi, 2011), and the 87 Sr/ 86 Sr ratios in human keratinous tissues can rapidly inherit the 87 Sr/ 86 Sr of bathing or showering water (Mancuso and Ehleringer, 2018;Tipple et al, 2018;Hu et al, 2020). Therefore, we would expect the 87 Sr/ 86 Sr ratios in the hair of the volunteer to incorporate exogenous Sr from the last location where the volunteer resided and/or from the location where the volunteer resided the longest (Hu et al, 2018(Hu et al, , 2020Tipple et al, 2018Tipple et al, , 2019.…”

“…Although studies have been trying to use 87 Sr/ 86 Sr ratios in hair for forensic and archeologic applications (Font et al, 2012;Frei et al, 2015;Vautour et al, 2015), it has been shown that hair can exchange Sr with the ambient water, either in modern environment during bathing or showering (Tipple et al, 2018;Hu et al, 2020), or in post-mortem environment (Gordon et al, 2019). Our result generally agrees that the Sr in modern human hair above 3 cm from scalp is mostly replaced by the exogenous signal.…”

“…Cerling et al (2018) also demonstrated that 87 Sr/ 86 Sr ratios in rhino hair are related to local geological bedrock while the δ 13 C and δ 15 N values are good indicators of food sources for black rhinos (Diceros bicornis) in Kenya. However, unlike major elements that can mostly ignore contamination influence in their isotopic profiles in hair, strontium in hair can be easily contaminated, which will alter its 87 Sr/ 86 Sr ratios (Hu et al, 2020). The co-existent endogenous and exogenous Sr in hair thus make the 87 Sr/ 86 Sr profile in hair complicated to interpret (Chesson et al, 2020).…”

Monthly mobility inferred from isoscapes and laser ablation strontium isotope ratios in caprine tooth enamel

“…The analysis of radiogenic strontium isotope ratios ( 87 Sr/ 86 Sr) in tooth enamel is commonly used to study provenance and mobility of modern-day humans in forensics 1 , 2 , past humans in palaeoanthropology 3 , 4 , and domestic and wild mammals in ecology and archeology 5 – 8 . In all cases, the rationale is the following.…”