Researchers investigating prehistoric cave art have refined the methods used to determine the age of mineral deposits that form over ancient paintings, providing more reliable minimum age estimates for some of the world’s earliest artistic expressions.
The team employed a laser-ablation uranium–thorium (U–Th) dating technique to analyse thin carbonate crusts that developed on top of rock art. These crusts contain trace amounts of uranium that slowly decay into thorium over time, allowing scientists to calculate when the mineral layer formed. Because the crust forms after the artwork is created, the resulting date provides a minimum age for the painting beneath it.
To ensure measurement accuracy, scientists used a strict calibration sequence involving multiple international reference standards. Each sample analysis began with three measurements each of the NIST610 and NIST612 glass standards, followed by two coral-based standards, CoralSTD1 and CoralSTD2.
The sequence was repeated at the end of each sample in reverse order, with additional standard checks every two hours. This approach allowed the researchers to maintain consistent instrument calibration throughout the analytical process.
The calibration method relied on coral standards to account for differences in mineral composition during laser ablation. Previous research has shown that ages obtained from laser-ablation U–Th analysis match those produced through traditional solution-based methods, indicating that potential “matrix effects” between coral (aragonite) and calcite samples are negligible when measurements are conducted under comparable conditions.
One challenge in dating calcium carbonate deposits is contamination by detrital material such as wind-blown dust or waterborne sediment. These particles can contain pre-existing thorium-230, which may cause samples to appear older than they actually are. Since scientists cannot directly separate detrital thorium from the radiogenic thorium produced by uranium decay, they apply corrections based on the ratio of thorium-230 to thorium-232 in the sample.
Higher ratios—above about 20—indicate little contamination, while lower values suggest that significant corrections may be required. In this study, most samples showed extremely high ratios, indicating that the carbonate deposits were relatively pure. Researchers therefore applied corrections using standard bulk-Earth assumptions for the detrital component.
Another potential complication arises if uranium has been leached from carbonate deposits over time. Such “open-system” behaviour would make calculated ages appear artificially old. To avoid this issue, researchers selected dense, non-porous samples and analysed multiple regions within each deposit. Consistent chronological patterns across subsamples confirmed that the mineral layers remained chemically stable.
Age calculations were performed using the geochronology software IsoplotR, with results reported at two standard errors. Because the carbonate crusts formed after the paintings, the dates represent conservative minimum estimates rather than the exact age of the artwork itself.
Sources : NATURE – https://doi.org/10.1038/s41586-025-09968-y
