Radiocarbon dating rock art
This often entails taking a piece of the painting out to study it – potentially damaging it."With current dating methods, we need large samples – sometimes hundreds of milligrams of painting – which often means completely destroying these artworks.We also have to consider that in many cases, the art wasn't protected inside caves and rock shelters but created on outdoors rocks exposed to the elements and to human activity, which means that paintings are often in a bad state and cannot be dated", Bonneau explained.Their complete findings are now published in the Journal Antiquity.Hunter-gatherer rock art in Southern Africa is made up both of paintings and engravings, which were produced by ancient communities associated with the present-day San (bushmen) culture.A new technique, developed at ANSTO’s Centre for Accelerator Science, has made it possible to produce some of the first reliable radiocarbon dates for Australian rock art in a study just published online in The approach involved extracting calcium oxalate from a mineral crust growing on the surface of rock art from sites in western Arnhem Land, according to paper co-author research scientist Dr Vladimir Levchenko, an authority on radiocarbon dating using accelerator mass spectrometry.Levchenko, who supervised the radiocarbon dating, collaborated with lead author of the paper, Ms Tristen Jones, a Ph D candidate at the Australian National University (ANU) and co-authors from ANU.They include: (i) the relative superimposition of styles; (ii) the use of diagnostic subject matter (depictions of extinct animals, stone tool technology, introduced European and Asian objects and animals); (iii) the recovery of a ‘painted’ slab from a dated archaeological unit; (iv) radiocarbon dating of beeswax figures, charcoal pigments, organic matter in overlying mineral deposits and ‘accreted paint layers’ (oxalate rich crusts and amorphous silica skin), pollen grains from an overlaying mud-wasp nest; and (v) optically stimulated luminescence (OSL) dating of quartz grains from overlying mud-wasp nests.
Using an innovative approach, researchers have now come up with new dates, which suggest that in south-eastern Botswana, rock art was created as far back as 5723–4420 cal BP – the oldest such evidence found to date in Southern Africa.
They used these fragments to learn about the paintings' overall composition, to select those with a higher likelihood of being successfully dated.
It also allowed them to identify paintings made up of carbon derived from short-lived organic materials rather than from charcoal, to avoid the issue that the charcoal used in paint may be signiﬁcantly older than the painting event itself.
Generally speaking, radiocarbon dating cannot readily be used to date Australian indigenous rock art directly, because it is characterised by the use of ochre, an inorganic mineral pigment that contains no carbon.
The paper authors explain that carbon found in the mineral crusts on the rock surface was most probably was formed by microorganisms.