Using a 3-stage burning categorization to assess post-depositional degradation of archaeofaunal assemblages: Some observations based on multiple prehistoric sites in Australasia

The role of degradational processes that act on archaeological faunal assemblages after burial has attracted less attention than it warrants, in part because many of the changes take place over time scales that are not amenable to experimental investigation. We propose a straightforward method for evaluating the level of post-depositional degradation in a stratified archaeological faunal assemblage based primarily on trends in the burning composition of the assemblage. The twin foundations of the method are: 1) the experimentally verified changes in physical properties that occur in bone as it is progressively burned through carbonized to calcined stages: and 2) postulated contrasts between burning stages in susceptibility to a suite of degradational processes that operate in the burial context, including a range of microbial, fungal, thermal and chemical processes. We present data from seven archaeological sites from Australasia and three ethnographic assemblages from Papua New Guinea, including sites in open-air, rockshelter and cave contexts, and sites with both acidic and alkaline soil conditions. Each of the archaeological sites is treated as a temporally-seriated assemblage that documents the potential impacts of post-depositional degradation on the faunal assemblage. We compare the relative abundance of unburned, burned/carbonized or calcined bone through each sequence and identify commonalities of pattern that are highly suggestive of post-depositional degradation. For each assemblage, these patterns are verified by visual inspection and identification of classic signs of surface degradation such as pitting and root channelling. In each of the acidic contexts there was a clear trend of increasing degradation of bone with excavation depth (= age). Assemblages from alkaline deposits were more complex, but some also provided more subtle evidence of degradation within the profile. Investigation of burning profiles in combination with careful examination of bone macro-damage provides a useful tool for identifying the pattern of post-depositional degradation within a stratified sequence and for distinguishing this from other various kinds of taphonomic patterning, and from signals produced by human behaviour.