The pattern of human bone dissolution—A histological study of Iron Age warriors from a Danish wetland site

Abstract

Bioerosion of archaeological bone is well documented histologically. However, although several studies have examined the physicochemical properties of dissolved archaeological bone, few studies have focused on the specific histological pattern of dissolution, or generalised destruction. Hence, the primary aim of this study is to characterise the histomorphological pattern and chronology of in situ dissolution of bone caused by precipitation and oxidation of pyrite and other exogenous minerals. We studied cortical bone from 24 human tibiae of one single event of mass deposition 2,000 years ago. Bones from 3 groups with contrasting degrees of macroscopic preservation were included: (a) bones excavated at shallow depth (approximately 60- to 70-cm subsurface); (b) bones excavated in 2012–2013 (1- to 2-m subsurface); and (c) bones excavated in 1954–1959 from the same level as (b). Bones are at present found even in the plough layer (i.e., <30 cm below surface) in this organic soil, meaning that parts of the find are extremely endangered or already lost to decomposition. Bone histology and identification of precipitations were performed by transmitted and reflected light microscopy and scanning electron microscopy–energy-dispersive X-ray spectroscopy. Quantitative analysis of inclusions and infiltrations of Fe (iron) and Mn (manganese) compounds from the environment was determined by inductively coupled plasma excitation with optical emission spectrometry. Field measurement results of present-day in situ soil and water quality are discussed in relation to the bone preservation. A postdeposition uptake of Fe and Mn has occurred in the bone structure, and a direct link between areas of generalised destruction and precipitation/oxidation of pyrite is suggested. Moreover, a distinction is made between physical and chemical generalised destruction (microfragmentation vs. mineral dissolution) of the bone matrix; however, both processes seem to be initiated in the periphery of the osteons between the resting line and a “secondary” resting line within the osteon. Also, the process seems to start at the canaliculi adjacent to the osteocytic lacunae. Understanding this initial deterioration region may provide knowledge on best ancient DNA sampling area.