An Exploration of the Effects of Taphonomy on Isotope Ratios of Human Hair

Isotope analyses of human remains have been conducted with growing frequency over the past thirty years in anthropology, in both archaeological and forensic contexts. Analyses of isotope ratios of elements such as carbon, nitrogen, hydrogen, oxygen, and strontium from teeth, bones, and hair have provided information regarding individual diet and geographic movement during different life stages. Hair grows at a predictable rate and provides a serial recording of diet and travel history for the weeks and months just prior to death. What has not been systematically studied is whether postmortem decompositional changes to the body have an effect upon isotope ratios in hair. Hair has been used in conjunction with tap water to create isoscapes, or isotope mapping models, that allow prediction of geographic history from analysis of the isotope ratios of hair. These models have been developed using clean modern samples from salons and do not reflect the typical condition of hair found in archaeological or forensic contexts. If the isotope ratios of human hair are to be used reliably in both archaeological and forensic settings, it is essential to understand the effects of taphonomy on isotope signatures, and whether these signatures persist and reflect those seen during life.

This research was conducted at the Anthropology Research Facility in Knoxville, Tennessee, an outdoor laboratory for the study of human decomposition. Body donors with known residence histories (n=44) plus two additional donors at the Forensic Anthropology Research Facility in San Marcos, Texas, were enrolled in the study, and carbon, nitrogen, hydrogen, oxygen, and strontium isotopes from human hair samples of these donors were analyzed. Postmortem exposure times for the study ranged from 22 days to more than three years. Results of the study revealed that carbon and nitrogen isotope ratios in human hair, commonly used to make dietary inferences, undergo little change over time and are more reliable than hydrogen, oxygen, and strontium isotope ratios, which are impacted by the depositional environment. This study revealed that isotope ratios of human hair can change postmortem and are influenced by geographic placement location, surface or burial placement, and duration of exposure.