Microbial attack of archaeological bones versus high concentrations of heavy metals in the burial environment. A case study of animal bones from a mediaeval copper workshop in Paris

Microbial attack is one of the key processes altering archaeological bone during the early stage of diagenesis. Some burial conditions seem more prone to prevent microbial attack in archaeological bone. It is generally admitted that the presence of certain metals in the burial environment such as copper may have an inhibiting effect on microbial destruction of archaeological bone. This study was aimed at evidencing such effects by the investigation of different archaeological animal bones excavated from several activity zones of one French archaeological urban site corresponding to a mediaeval copper alloys workshop in Paris, at Hôtel de Mongelas. The analytical approach combined morphological observations (SEM), structural investigations (FT-IR, XRD, and TEM) and chemical analyses (SEM/EDX and μPIXE/PIGE) of archaeological bones at different scales down to the nanoscale. It permitted the classification of the bones as a function of their contamination with heavy metals, their conservation state and the presence of signs of microbial attack. Correlations between the observed alteration features and the metallurgical activity on this site as well as the underlying alteration processes are discussed. An inhibiting effect of heavy metals on the microbial attack could be deduced from the available, though restricted, data in agreement with the expectation. However, the relationship between heavy metal uptake and microbial alteration of bone depends on other diagenetic factors and their progress such as bone collagen content, crystallinity of bone mineral, soil geochemistry and hydrology. Significant changes in environmental conditions over time, which can especially occur in urban sites, can also influence alteration phenomena. Generally, simultaneous alterations induced by heavy metal uptake and microbial attack seem to take place in the very early stage of diagenesis and influence one another in sites with soils highly contaminated in heavy metals.