Assessing the survival to subaerial conditions of disinterred human bones in Southern Patagonia (Argentina): Archaeological and forensic implications

MIOTTI, L.; BARRIENTOS, G.; MAGNIN, L.; GARCÍA, R.

Preston

Simposio; Taphos Nomos. Taphonomy in archaeology, forensic science & paleontology; 2018

School of Forensic and Applied Sciences, University of Central Lancashire

The estimation of the above-ground survival time of human bones under different environmental conditions is of paramount importance from both an archaeological and a forensic point of view. This is particularly the case of previously buried bone elements that become disinterred by different mechanisms like erosion, fossorial mammal activity, and grave looting. Because the time between the exposure of a bone and its complete destruction by agents and processes that operate on the ground surface (e.g. weathering, abrasion, trampling, vegetation) can be very extensive, its quantification is usually very difficult or even impossible to carry out. However, such an estimate can be made, at least in an approximate way, by taking advantage of certain types of information already present in the bones. In this connection, the objective of this presentation is to describe and discuss the results of a suite of observations performed on a weathered and lichen-colonized bone assemblage?exposed on the ground surface after the looting of an ancient aboriginal tomb in southern Patagonia (Argentina)?, aimed at establishing its minimal time of permanence under subaerial conditions. The case study corresponds to the bones from a despoiled burial cairn?Chenque El Sargento, Deseado Massif, Santa Cruz province; 717-555 years BP (calibrated range, 2D). The assemblage consists of 14 identified, almost complete bone elements and 13 unidentified fragments, all belonging to a single individual. Each specimen was thoroughly inspected both macroscopically and microscopically, recording the weathering stage at previously defined points and the presence and type of lichens growing on the bone surface. All this information was entered into a GIS for spatial modelling purposes. A total number of 63 lichen specimens were recorded, 48 of which (76%) could be taxonomically identified. They belong to eight species of the genera Acarospora, Caloplaca, Candelariella, Lecanora, Psiloparmelia, and Xanthoria. No local growth curves are available for any of them; however, the size of the thallus of some specimens?particularly that of the species Psiloparmelia distincta?reaches several millimetres, thus indicating decades of growth. If we take into account that a large proportion of the lichens have developed on highly weathered surfaces (WS4) (Figure 3), then the time necessary for a bone to reach such a state of alteration must be added to calculate the total time of exposure to subaerial conditions. Although there are no regional data for human bones, information available for other species of Patagonian mammals suggests that the time necessary for bones to reach WS4 is around eight years or more. In any case, our results suggest that in depositional environments in which abrasion does not seem to be an important destructive force, the survival time of the bones seems to be relatively long (several decades). The opposite condition has been registered in dunes, where most of the information available for bone assemblages disinterred by erosion comes from. Future studies should calibrate the weathering and lichen ?clocks? in order to make a better estimation of time elapsed since bone subaerial exposure.