The region of Dordogne in southern France is home to more than 200 caves decorated with colorful Paleolithic drawings, but little is known about its age. Radiocarbon dating of the prehistoric art is impossible due to its coloration with iron- or manganese-oxide-based material. It was assumed that the cave art had been created during the Magdalenian Period from 12,000 to 17,000 years ago.
Carbon-Based Prehistoric Art
A team of researchers from the Center de Recherche et de Restauration des Musées de France reported the first discovery of black charcoal-based art in Dordogne's Font-de-Gaume cave. Their findings are discussed in the study "First discovery of charcoal-based prehistoric cave art in Dordogne."
The drawings were discovered in late February 2020 in the cave's leading galleries. The cave is also called "Bison Cave" due to its 80 depictions of bison and other animals, such as mammoths, horses, and deer. About two-thirds of all cave art consists of animal figures, while the remaining one-third comprises tectiforms.
Some bison are shown in two or more shades, while others are primarily in black and red or only black. In 1902, Nobel Prize in Chemistry winner Henri Moissan determined the components of iron and manganese oxide by using coloring matter samples from the art. However, the cave is currently protected by UNESCO World Heritage, so the authorities permit sporadic sampling cases.
A New Look at Radiocarbon Dating
To address this challenge, the researchers turned to non-invasive analytical techniques. They used visible-light and infrared photography, superimposition of the visible-light and infrared images, portable X-ray fluorescence (pXRF), and portable micro-Raman spectroscopy to generate their data. These methods unveiled carbon-based drawings under the others known to contain iron and manganese oxide pigments.
Through superimposition of visible-light and infrared images obtained from the same vantage point, infrared false color photography (FCIR) images were created. The camera and filter used to take the infrared images allowed radiation wavelengths of only above 900 nanometers. Then, the software was used to combine them with green and red layers of visible-light images to generate FCIR images and enable differentiation of several materials used in creating the images.
Meanwhile, micro-Raman spectroscopy allowed them to detect carbon-based compounds within the images. It enabled them to identify specific mineralogical stages of the iron and manganese oxide-based pigments.
This spectroscopy technique made it possible to identify images created with charcoal-based compounds, unlike those made from other carbon-based materials. Finally, pXRF enabled the experts to differentiate among different manganese-oxide compounds in the black figures.
It was also observed that the different kinds of pigment materials could represent various creation phases according to observations in other comparable caves. For instance, the well-known Lascaux cave shows prehistoric depictions with cryptomelane as the basis of manganese-oxide pigment. Meanwhile, the manganese-oxide colorations of Font-de-Gaume are based on pyrolusite and romanechite.
In the future, scientists hope that the discovery of carbon-based figures in the cave will open new opportunities for radiocarbon dating, enhancement of the creation phase, and even precise inter-regional comparison.
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