Étude des pigments rouges du panneau du « Genyornis » Terre d’Arnhem, Australie : origines de l’hématite ?

Western Arnhem Land’s rock art is world famous yet very poorly dated. Understanding its history over tens of thousands of years has major implications for understanding Aboriginal cultural history in Australia. In particular, very little is known about the composition of paints and the techniques used to make Rock Art. Here we investigate the pigments and rock surfaces of an undated rock painting that has been argued in the literature to represent the extinct megafaunal bird Genyornis newtoni, thought to have become extinct across Australia 40-45 000 years ago. Small flakes of pigmented and unpigmented rock were sampled from the so-called Genyornis panel in order to address three major questions concerning : i) the constituents of the pigments used and their modes of preparation (mixing with extender(s) or binder(s) ; grinding ; heat treatment) ; (ii) the geological formations which provided such pigments ; and (iii) the antiquity of paintings on the rock walls. Following macroscopic observations, pigment compositions were analyzed by scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM-EDX) and Raman microspectroscopy. Complementary structural information was obtained using X-ray diffraction (XRD). Both unprepared samples and cross-sections of resin-embedded samples were analyzed to obtain morphological, chemical and structural characterizations.

Iron oxides (hematite Fe₂O₃ and goethite FeOOH) were identified in the paint layer. A white alumino-phosphate-rich matrix was identified as a weathering, microstratigraphically basal bedrock layer on the rock wall. The punctual presence of gypsum (CaSO₄, 2H₂0) was observed as an underlayer below the pigment and could be explained by the weathering process of the quartzite support.

This study highlights the difficulty of obtaining information on ancient pigments significantly modified through time in a relatively open site and distinguishing poorly crystallized iron oxides, which result from the weathering of rocks and the sub-micronic iron oxide crystals in the leached paint layer.