By Steven D. Emslie
In 2014, while conducting stable isotope analysis of human bone from Perdigões for variation in diet among individuals, routine analysis of mercury was completed as well. This analysis led to the discovery of unusually high levels of total mercury (THg) in the bone that were too high to have been a result of diet (Emslie et al. 2015). The only likely source was cinnabar or mercury sulfide (HgS) that was present in some of the tombs and used as a bright red pigment in funerary rituals. Subsequent analyses since then have revealed that mercury values are highest in human bones dating to the late Neolithic through Chalcolithic, then become very low or undetectable in burials during the latest Chalcolithic and Bronze Age.
One question that arose early in this research was whether or not the mercury accumulated in the human bones from mining and exposure to cinnabar in life (biogenic source), or if cinnabar placed in burial tombs, pits, and hypogea chemically broke down in the soil with mercury entering the bone after death (diagenetic source). So far, all evidence indicates a biogenic source for this mercury because high values of mercury have been measured in bones from burials with and without cinnabar in association, animal bones from the same sites consistently have very low mercury values, and patterns of mercury deposition by skeletal element are emerging when several bones of the same individual are analyzed. In over 80% of cases, the humerus tends to have more mercury deposited within it than other skeletal elements, perhaps because of greater bone remodeling from heavy use of the arms in life (Emslie et al. 2019).
To address the diagenetic hypothesis in more detail, we are conducting the first-ever controlled experiments using cinnabar powder on animal bone buried in different contexts and environmental conditions. We begin by first analyzing the mercury content in modern animal bone selected for the experiment. The bones are then buried with the soil surrounding them that also is analyzed for mercury prior to the start of the experiment. Then, the cinnabar powder is sprinkled or painted on the bones that are then left buried for a year. After excavating the bones, they are analyzed again for mercury. In this manner, we can determine if the cinnabar broke down chemically and allowed mercury to penetrate and become deposited in the inner bone via diagenetic processes. We currently are planning to expand this study to include bones of both juvenile and adult animals (juvenile bone is undeveloped and more porous, so more susceptible to diagenesis). Currently, bones buried in a previously excavated pit at Perdigões (Pit 50) in May 2019 could not be recovered in 2020 or 2021 due to the pandemic and will be removed and analyzed in 2022. Additional experiments will be initiated at that time as well with animal bones buried with cinnabar at several archaeological sites in Portugal.
Emslie, S. D., Alderman, A., McKenzie, A., Brasso, R., Taylor, A., Molina Moreno, M., Cambra, O., González, A., Maria Silva, A., Valera, A., García Sanjuán, L., & Vijande Vila, E. (2019). Mercury in archaeological human bone: biogenic or diagenetic? Journal of Archaeological Science: DOI: 10.1016/j.jas.2019.05.005.
Emslie, S. D., Brasso, R., Patterson, W., Valera, A. C., McKenzie, A., Silva, A. M., Gleason, J. D., & Blum, J. D. (2015). Chronic mercury exposure in Late Neolithic/Chalcolithic populations in Portugal from the cultural use of cinnabar. Scientific Reports 5: 14679. DOI:10.1038/srep14679.
Emslie, S. (2021), Diagenetic experiment of cinnabar buried with animal bones at Perdigões, Short Notes, www.perdigões.org