Oxalate minerals for rock art dating: new developments and applications.

Development of AMS allowed targeting oxalate minerals – whewellite and wedellite as a dating material for rock art. Further studies have demonstrated that carbon in oxalates is not derived from the substrate on which they grow but most probably originate from bacteria and other microbiota residing on rock surfaces utilising atmospheric source. The other rock surface objects which potentially carry oxalates are the remnants of mud dauber nests. Oxalates could form post-construction from bacterial activity utilising favourable conditions in the nest stump. One more recognised source of oxalates associated with rock art is the use of plans sap as a binder for mineral pigments. In all instances oxalates on the rock surface exist in a mixture with other materials which could be carbon bearing contaminants. Hence the study of a bulk sample ends in radiocarbon concentrations of an average of all carbon compounds with unknown source and relationship with the inferred art. Therefore, chemical pretreatmed methods were developed to isolate and target a specific compound, in our case calcium oxalate. We present and discuss some methodological data of studying oxalate crusts from various objects following a compound-specific dating approach and some further applications to date rock surface deposits from Australian and African rock art sites. The presence of whewellite and mineral skins compositions were also investigated with X-ray diffraction, scanning electron microscopy and infrared spectroscopy methods. Selectiveness of the method and planned studies are considered.