Chemical investigation of mineralisation categories used to assess taphonomy

A recent case study from the Geelbek Dunes archaeological site in Western Cape Province, South Africa revealed that, counter to prevailing notions, long-term bone survivorship is not always linked to estimates of bone density. Porosity differences between modern cancellous and compact bones of different animals and skeletal parts provided density estimates for fossil bones. Taphonomic history was inferred from the physical condition and mineral burden of the fossil bones and represented by five “mineralisation categories”. Bones in mineralisation category one had the lowest secondary mineral burden, whereas samples with very high secondary mineral contents were considered category five. Here we examine the proposed link between mineralisation category and taphonomic history by analysing fossils with non-destructive, energy dispersive x-ray fluorescence spectroscopy. We hypothesised that higher mineralisation categories should be evident as higher concentrations of elements associated with secondary minerals. However, chemical measurements could not distinguish fossil bones in mineralisation category one from those in category three. Furthermore, bones in categories four and five could not be distinguished from one another. A control of modern bone specimens and fossils in mineralisation category two formed separate and distinct clusters of principal component score values. Spectra from the Geelbek Dunes specimens were subsequently used to predict mineralisation categories for fossils from a nearby archaeological locality named Anyskop Blowout. Based on the surface chemistry, Anyskop fossils clustered in two principal groups: low (mineralisation categories one, two and three) and high (mineralisation categories four and five). Energy dispersive x-ray fluorescence spectroscopy provided useful taphonomic information for fossils from both the Geelbek Dunes and Anyskop Blowout, although this surface-biased technique was not well correlated with the five mineralisation categories.