Accelerator Radiocarbon Dating of Plant-Fibre Binders in Rock Paintings from Northeastern Australia

Article excerpt

During the late Holocene, Aboriginal rock painters in north Queensland selected and combined various natural inorganic and organic materials in paint recipes -- possibly to increase the longevity of their paintings. The organic materials make direct radiocarbon dating possible.

The Cape York Peninsula of north Queensland contains a remarkable body of richly coloured Aboriginal rock paintings (Trezise 1971; Flood & Horsfall 1986; Cole & David 1992). The antiquity of rock paintings on Cape York Peninsula was previously not known, although estimates were made that they might be mid Holocene (Flood 1987; Rosenfeld et al. 1981).

Our research was initially aimed at finding the mineralogical and chemical compositions of Aboriginal rock pictures in the Laura region. It was thought that identifying sources of paints by using their mineral contents would provide information about prehistoric ochre trading patterns, degrees of sophistication in paint preparations and applications and developments of painting technologies through time.

But that aim was superseded by the prospect of accurately dating two paintings for the first time because, much to our surprise, we discovered that not only are the prehistoric paints composed of readily available natural earth pigments (Watchman et al. in press), but many also contain plant fibres possibly used as binding agents (Cole & Watchman 1992).

Plant binders

Organic binders in prehistoric rock paintings are reported for North America (Grant 1967; Dewdney & Kidd 1967), Africa (Biesele 1974; Rudner 1982) and Australia (Severin 1973; Loy et al. 1990), but few organic binders have actually been found (Weisbrod 1978; Conard et al. 1988; Pepe et al. 1991). It is suspected that many prehistoric paints were mixed solely with water (Wainwright 1985) because trace organic components cannot be found in paint samples.

Information about organic binders in paints on Cape York Peninsula was previously not known because the oral traditions of rock painting practices were lost to the present Aboriginal people after early European settlement devastated the pre-existing culture, art and religion. Aborigines are known to have used a wide variety of Australian plants for artefacts and utensils, medicines, narcotics and decorative purposes (Roth 1904). However, the use of plants as fixatives in the preparation of paints for Australian rock art is not widely documented (Mountford 1956).

It was therefore surprising for us to find relatively large quantities of plant fibres in two paintings at the Yam Camp site (YC), 35 km southwest of Laura. We subsequently found single fibrous strands and splinters of wood, probably mainly from brushes rather than binders, in 21% of paints sampled in the region. The extent and distribution of plant fibres in two YC paintings led us to believe that inclusion of fibres was a deliberate action by the artists and not an accidental isolated occurrence.

Radiocarbon dating

Apart from charcoal in France (Valladas et al. 1990) and Spain (Valladas et al. 1992) and unidentified organic matter from the Lower Pecos, USA, (Russ et al. 1990), few attempts have been made worldwide to use radiocarbon AMS to date prehistoric rock pictures, mainly because of the scarcity of relict organic matter. Direct dates for Australian Aboriginal rock paintings and drawings are gradually being obtained (Watchman 1987; 1992; in preparation; Loy et al. 1990; McDonald et al. 1990; Nelson et al. 1992; Campbell & Mardaga-Campbell in press).

The heavily engraved sandstone surface at YC also contains monochrome and bichrome images in white, yellow-brown and red (Cole 1988). Samples of paint mixed with binder were hand-picked for AMS dating from two female figures painted with a similar thick, white, quartz-kaolinite paint. This coarse-grained paint was bonded by a sticky, tangled mat of fibres, possibly derived from crushing some nearby wild orchid tubers. …