U-series in situ dating of secondary carbonates associated with rock paintings using LA-MC-ICPMS

Abstract

Rock art is found in many different regions of the world, with ages dated from late Palaeolithic period to the present. However, determining the precise ages for such artworks with direct chronological methods is challenging. U-series dating of secondary carbonates can constrain the age of rock art if these carbonates stratigraphically connected to the art. Hitherto, U-series isotope-dilution analyses with multi-collector inductively-coupled plasma mass spectrometry (ID-MC-ICPMS) has been increasingly used for rock art dating. This approach can achieve U–Th isotope-ratio quantification at the per-mille or submille level, but requires chemical pretreatment using isotope spikes and results in a relatively low sampling resolution. While in situ U-series dating using laser ablation-MC-ICPMS (LA-MC-ICPMS) is significantly less precise than ID-MC-ICPMS analysis, it allows to collect spatially resolved data sequences on very thin samples (<1 mm). This is important for identification of open systems and the chronological integrity of the analyses. Therefore, we developed a new method for high spatial resolution U-series in situ analysis. A piece of homogeneous stalagmite was selected as an in-house standard (RM-C1) for U-series in situ dating analysis. RM-C1 contains high U (17.3 ± 1.0 μg/g) and low Th concentrations (<5 ng/g). The ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U activity ratios of 1.0116 ± 0.0006 and 0.9525 ± 0.0013, respectively, corresponding to a U-series age of 303.5 ± 1.5 ka, were determined by the ID-MC-ICPMS analyses (n = 10). Using the RM-C1 standard, we were able to date the Dansgaard–Oeschger (D/O) 19 event (with an expected age of 68.9 to 70.3 ka) in a stalagmite to a range from 68.0 ± 2.0 to 71.2 ± 2.5 ka, with an average age of 69.6 ± 1.5 ka (n = 15). This demonstrates the accuracy and reproducibility of our dating protocol. Subsequently, we conducted U-series in situ dating of secondary carbonates associated with rock paintings with two samples from the Cangyuan shelter (CY2-4 and CY2-7) and two samples from the Wanrendong Cave (SL3-A and SL5-A) in Yunnan Province, southwestern China. These samples had high U-concentration (>4 μg/g) and negligible environmental Th. The carbonate layers immediately overlying the pigments of CY2-4 and CY2-7 were dated to 3.7 ± 0.3 and 3.2 ± 0.2 ka, respectively, suggesting that the Canyuan rock paintings belongs to a late Neolithic culture of this area. The pigment layer sandwiched in the sample SL3-A was bracketed to between 8.7 ± 0.3 and 10.1 ± 0.3 ka and in SL5-A to between 10.4 ± 0.3 and 10.8 ± 0.4 ka, implying that the Wanrendong rock paintings probably were created by a hunting-gathering population during the early Holocene.