The case for reprecipitation of human bone as an event in a Bronze Age cist burial, Scotland

A hard whitish precipitate was observed on the lower part of the sandstone sidewalls and as pebble coatings in a Bronze Age cist burial near Forteviot, Strathearn, Scotland. The cist was discovered on excavation of a Neolithic henge in August 2009 during the joint Glasgow and Aberdeen Universities [Strathearn and Environs Royal Forteviot (SERF)] archaeological landscape research project and summer field school. Similar cists have not been found in this area. Scrapings of the precipitate proved on examination by powder X-ray diffraction to be hydroxyapatite. The mamillary material proved on examination by scanning electron microscopy to be calcium carbonate, calcite which had grown as groups of mm-size spheroids consisting of bundles of acicular crystals. Both components of the precipitate were also identified using oil immersion microscopy. Much organic material was preserved in the cist but neither (inorganic) bone nor teeth has been located to date (November 2010). The phosphatic mineral-precipitates provided the first confirmation that there had been bone and therefore an inhumation. Computational aqueous geochemistry using Geochemist’s Workbench confirms that the inorganic calcium phosphate component of human bone is soluble in acidic fluid and demonstrates how it can reprecipitate with change in fluid chemistry. Bone dissolution should be anticipated as being an expected early process when a human body produces an acidic fluid rich in organic molecules as it decays in an essentially closed, but wet, anoxic environment. Any precipitates on grave stonework should be identified as such material could represent human remains and could also provide evidence of environmental processes in the archaeological setting of a burial.