Chemical and mineralogical characterization and C-14 dating of white and red pigments in the rock paintings from Nyero (Uganda)

摘要

Petro-mineralogical, micro-chemical and isotopic (delta S-34-CaSO4 and C-14 dating) analyses were carried out in selected samples from the rock paintings of Nyero (Upper Lake Victoria Region, Uganda) aiming to characterize for the first time both the composition of the red and white pigments of the drawings (mainly representing concentric circles, and shapes resembling vegetables, boats and other stylized forms) and determine their age. The rock panels were classified as metamorphic rocks (gneiss), containing quartz, plagioclase, orthoclase, microcline and muscovite. Such minerals were also found in the paints, though the red drawings also showed the presence of Fe-oxides, which were practically absent in the white ones where kaolinite was recognized. A consistent amount of Ca-oxalates was found to be well distributed through the white and red pigments, suggesting its origin in the organic binder used to properly spread the pigments onto the rock panels. Thus, the 14C ages on the Ca-oxalates are to be considered as the age of the paint. The 68% probability time intervals of calibrated ages for the white and red pigments were in cal 2880-2280 BCE and 1390-1010, respectively, demonstrating 1000 years time gap between them. This difference was also pronounced in motifs of complexity. The red drawings (attributable to settled or semi-settled societies) are richer in styles, forms and patterns than the white ones, the latter being likely related to a nomadic population as also suggested by the simpler drawing techniques. Our results have important implications for integrating semiotics and anthropologic studies, to delimit the boundaries of a homogeneous cultural area and shedding new light on human settlement dynamics and change from nomadic lifestyle to settling during the Late Stone Age in East Africa.