At the Toca do Serrote da Bastiana rock shelter in Brazil, a red, iron ochre pictograph of an anthropomorphic figure had become coated with a ‘calcite’ accretion over time. Using X-ray diffraction and Fourier transform infrared spectroscopy, we determined that the accretion also contains whewellite, the monohydrate of calcium oxalate, in addition to calcium carbonate. An AMS radiocarbon date on the oxalate formed from contemporaneous carbon yielded a minimum date of 2490 ± 30 years BP for the painting. An AMS radiocarbon age of 3730 ± 90 years BP on organic material in the underlying paint layer is consistent with the oxalate result and four direct radiocarbon dates on organic matter extracted from other rock paintings in the same shelter. Our results strongly disagree with a 30,000-40,000-year-old age obtained by electron spin resonance and thermoluminescence dating of the accretion.
Oxalate-rich mineral accretions occur widely on rock surfaces found in many parts of the world including caves [e.g., (1)], ancient buildings and monuments [e.g., (2)], and many important archeological sites, including rock art shelters in Australia (3)(4)(5)(6)(7), North (8) and (9) South America, South Africa (1), and Spain (10). Although the presence of calcium oxalate minerals is the defining feature of these accretions, their mineralogy is complex and variable (11).
Rates of landscape evolution in these competent, durable sandstones are low [~1 m million years −1 , meaning that rock surfaces within the shelters may be very long lived (27). Consequently, many of the oxalate accretions accumulating in Kimberley rock shelters are relatively thick (up to 5 mm) (11) compared to those recorded elsewhere [< 1 mm (5,6,8,9)].