How do bones turn into fossils?

Fossilized bones, apart from the role they play as paleontological indicators, can provide information relating to the climatic or ecological environment and the eating habits of men and animals living tens of thousands of years ago. There are many investigative and analysis techniques that can be used on these fossil samples, chosen according to the type of information sought: for example X-ray diffraction, small angle X-ray scattering or electron microscopy for studying the mineral fraction; mass X-ray fluorescence spectrometry for the elemental composition. Fourier transform infrared (FTIR) spectrometry provides access to both the molecular composition of the sample and its structural properties. Until now most studies using FTIR were carried out on powdered bone: all information on variations in chemical composition within the sample was thus lost and analysis of the tissue on the histological scale was impossible. To determine the spatial distribution of components, sections of bone must be studied by coupling infrared spectrometry (FTIRM) to a microscope, an approach previously only available to biomedical research.
 

Close-up of the ultramicrotome, a precision mechanical instrument for cutting semi-thin sections of bone, shown here in resin. The bone sample is held in the jaws, and with a vertical movement of the arcuate segment, it is cut by the glass knife (left), the sections slide down the inclined plane of the knife and are collected dry.

Matthieu Lebon, a post-doc in Christophe Falguères’s group at MNHN and that of Ina Reiche, at C2RMF, is preparing thin new slices of archaeological bone sections in the IPANEMA laboratory.

The composition and mechanical properties of fossil bones are not homogeneous, which makes it more difficult to produce thin sections of this type of sample. A prerequisite to using FTIRM was therefore to develop an experimental protocol for obtaining "routine" thin sections of fossil bone. This long and complex work was initiated by Ina Reiche’s group in the C2RMF Laboratory as part of the ANR "ArBoCo" research program and was conducted in collaboration with Christophe Falguères’ group in the Department of Prehistory at MNHN3 and the IPANEMA research platform at SOLEIL. The aim was to consolidate the microstructures of the sample, by impregnating it with a resin, to replace the collagen initially present in the bone. The resin eventually chosen, with a liquid consistency, is usually used in biomedical studies, because most resins traditionally used in archaeological samples have often proved to be too viscous and unsuitable in such cases.
Sections 1.2 to 1.4 μm thick and over 500 x 500 μm² were cut from two samples of mammalian bones 15,000 and 60,000 years old, respectively. The histological structure of these samples, although fragile at the start, was preserved thanks to the preparation protocol chosen. These sections were then analyzed on the SMIS beamline at SOLEIL using FTIRM.

The maps of bone sections obtained after scanning with a resolution of 12x12 μm² have made it possible to simultaneously characterize changes due to diagenesis in the organic and mineral phases of the bone, and to link them to the state of preservation of the microscopic structure of the bone tissue, thus showing the advantages of the study and the newly developed protocol.
The researchers now want to create a larger database, by multiplying the analyses of "modern" and fossil bones of different ages and at different states of preservation. This is in order to obtain a strong basis for future studies in archeology and paleontology. 
 

Reference :
Imaging fossil bone alterations at the microscale by SR-FTIR microspectroscopy.
M. Lebon, K. Müller, J.-J. Bahain, F. Fröhlich, C. Falguères, L. Bertrand, C. Sandt & I. Reiche
J. Anal. At. Spectrom., 2011, 26, 922-929

¹ - Diagenesis: All the physical and chemical processes that lead to sedimentary rock. This definition can be applied more generally to the process of bone fossilization.

² - C2RMF: Centre de Recherche et de Restauration des Musées de France, UMR 171 du CNRS, http://www.umr171-cnrs.fr et http://www.c2rmf.fr

³ – Département de Préhistoire du Muséum national d'Histoire naturelle, UMR 7194 du CNRS, http://hnhp.cnrs.fr