Documenting the degradation of animal-tissue residues on experimental stone tools: a multi-analytical approach

In lithic residue analysis, the identification of degraded animal tissues on stone tools is challenging due to many factors, not least of which is the fact that residues are complex, heterogeneous mixtures of many different kinds of molecules. In order to aid in their identification, a reference library of infrared spectra of residues collected using Fourier-transform infrared microspectroscopy (FTIRM) has recently been published (Monnier et al J Archaeol Sci: Rep 18:806–823, 2018). The goal of the present study is to explore the effects of decomposition on residues. Accordingly, we buried flakes with residues in compost for 1 year, then excavated them and documented both their appearance (using visible-light microscopy (VLM) and scanning electron microscopy (SEM)) and molecular composition (using FTIRM). The results show that while some residues (like meat and blood) disappeared entirely, others (fat and skin) were preserved on the bottoms of flakes buried in deep layers within the compost. Although the residues were damaged by microbial activity, their FTIRM spectra were clearly interpretable. Residues containing hydroxyapatite (bone and fish scales) and keratin (feather barbules, hair, and skin) were relatively well preserved. Their structures were in many cases recognizable, and their FTIRM spectra were entirely consistent with the FTIRM spectra of the standards. The results of the experiment show that the decay of animal tissues in compost proceeds primarily as a result of microbial activity, which appears to remove the tissues before they have a chance to oxidize or experience other biochemical changes. We conclude that if ancient residues have not been removed by microbial action, they can be identified using FTIR standards based upon fresh residues, such as those published in Monnier et al J Archaeol Sci 78:158–178, (2017), Monnier et al. 2018, J Archaeol Method Theory 25(1):1-44.