An FTIR-based model for the diagenetic alteration of archaeological bones

Bones and teeth from archaeological records are direct evidence of past individuals and they represent valuable archives for palaeo-anthropological and palaeoenvironmental studies. However, pristine information may be obliterated by the diagenetic alteration of bone specimens. Thus, defining in detail their preservation state is fundamental to assess the potential of extracting information about the past life of the individual, as well as to investigate the palaeo-environment at the burial site. For this reason, we have selected a set of archaeological samples (petrous bones and tooth roots) of different origin and chronology that have experienced diverse environmental and burial conditions, from arid and semi-arid to temperate regions in a time span from prehistoric to fresh bones. Thus, the selected samples underwent a variety of diagenetic processes, resulting in different types and extent of alteration patterns. Here we have applied a minimally-invasive sampling strategy and an analytical protocol based on Fourier transform infrared spectroscopy (FTIR) in order to ensure the investigation of a statistically significant set of samples. We provide a method to describe and quantify the changes in the physico-chemical properties of both the organic and inorganic constituents of bones, by establishing a set of parameters calculated from the FTIR spectra and evaluating their sensitivity in describing the alteration types and extent in bone specimens. Through FTIR spectral analysis, we have defined the most suitable parameters to effectively describe the alteration degree of bone specimens. Notably, the results have shown that, despite the complexity of multifactorial diagenetic processes, their measurable effects on differently altered bones can be described by a common mechanism. This indicates a progressive loss of collagen which is paralleled to the recrystallization of bioapatite and the loss of structural carbonate, regardless of the origin and chronology of the specimens. Thus, here we propose an FTIR-based model for bone diagenesis of general validity, highlighting that distinct diagenetic processes affecting the archaeological bones have acted according to common alteration mechanisms.