The characterisation of fossil bone

Abstract

This research presents a multi-disciplinary approach to the analysis of ancient bones, in which many different chemical and physical analytical techniques were applied to a relatively small sample of human and animal bones from different depositional environments. The results of these analyses indicate that the principle mechanisms responsible for diagenetic alteration of buried bones are chemical hydrolysis of bone collagen and microscopic tunnelling by saprophytic micro-organisms. These mechanisms, either independently or together, result in an increase in the porosity of the bone at a microscopic scale from a value of approximately 20 percent in fresh bone up to as much as 65 percent in some archaeological bones. There is no evidence that the hydrolysis of collagen in buried bones directly affects the mineral component of bone, although the breaking of the intimate association between the collagen molecules and the bone apatite crystallites exposes the crystallites to potential dissolution and recrystallization by percolating ground water. Disruption of the collagen-apatite bond has been recognised in optical microscopy of thin sections by loss of the characteristic birefringence seen in unaltered bone when viewed in polarised light. The birefringence in histologically normal bone results from the strongly anisotropic orientation of the bone mineral crystallites imposed by their association with the highly organised collagen fibrils. Loss of birefringence as a result of diagenetic activity is attributed to a randomising of the orientation of crystallites after hydrolytic degradation of the collagen molecule. With progressive loss of collagen the relative calcium and phosphorus contents of fossil bones have been found to increase in proportions close to those of stoichiometrically correct hydroxyapatite. Microscopic and mineralogical studies have suggested that changes in the crystallinity of buried bones may be attributed to the presence of well-ordered crystals of hydroxyapatite in the pore structures of the bones and that these derive from dissolution and re-precipitation of the original bone apatite. However the elemental and isotopic composition of these re-precipitated apatites may not reflect that of the original bio mineral due to the incorporation of strontium, uranium fluoride etc. from the environment. Dissolution of bone mineral can, in most cases, be associated with the action of micro-organisms, many of which are known to favour low pHs and secrete organic acids as a by-product of their metabolism. Although micro-organisms isolated from buried bones produce collagen degrading enzymes (collagenases) these enzymes are too large to enter the spaces between the bone apatite crystallites and are therefore unable to attack the collagenous matrix of undegraded bone. Before micro-organisms can utilise bone collagen, the bone matrix must first be demineralized to expose the collagen fibrils or the collagen must be degraded by hydrolysis into shorter lengths that then escape via disrupted regions of the surrounding crystallites. Analysis of the strengths of modem and fossil bones has demonstrated a near logarithmic relationship between tensile strength and porosity. In addition, plots of strength vs porosity and strength vs nitrogen content are bimodal, indicating that two mechanisms are involved in the degradation of fossil bones. The microscopic and chemical analyses suggest that these mechanisms are chain scissioning of collagen and tunnelling by micro-organisms. Microscopic studies show that surface adsorption of 'humic acids' and metal ions are responsible for the colouration of fossil bones. Analysis of the total lipid extract of fossil bones contain cholesterol and cholesterol degradation products. Fossil cholesterol represents a potentially important and unique resource for palaeodietary studies. Conversely, this research has demonstrated that studies of ancient DNA are compounded by inhibition by compounds from the soil and contamination by modem DNA. Fossil bones in anoxic or wateriogged soils are readily colonised by sulphate-reducing bacteria and these bacteria are responsible for the deposition of iron sulphide in the form of pyrite framboids in pore spaces in the bone. On exposure to atmospheric oxygen, these pyrite framboids oxidise to sulphuric acid which in turn attacks bone apatite, resulting in the formation of vivianite (Fe(_3)(PO(_4))(_2).8H(_2)O) and gypsum (CaSO(_4).2H(_2)O). Crystallization and hydration of these minerals frequently disrupt the physical integrity of the bone specimens. Finally this research indicates potential regimes for the conservation of fossil bone specimens together with the archaeological or environmental evidence preserved within them.