U-series and radiocarbon cross dating of speleothems from Nerja Cave (Spain): Evidence of open system behavior. Implication for the Spanish rock art chronology. (15th August 2022)

• Record Type: Journal Article
• Authors: Pons-Branchu, E.; Barbarand, J.; Caffy, I.; Dapoigny, A.; Drugat, L.; Dumoulin, J.P.; Medina Alcaide, M.A.; Nouet, J.; Sanchidrián Torti, J.L.; Tisnérat-Laborde, N.; Jiménez de Cisneros, C.; Valladas, H.
• Is Part Of: Quaternary science reviews. Volume 290(2022)
• Publication Date: 2022-08-15
• Subjects: Rock art chronology -- Uranium-thorium ages -- 14C chronology -- FTIR analysis -- Stalagmites; Geology, Stratigraphic -- Quaternary -- Periodicals; Stratigraphie -- Quaternaire -- Périodiques; 551.79
• DOI: 10.1016/j.quascirev.2022.107634
• Languages: English
• ISSNs: 0277-3791

Abstract

Abstract: Two stalagmites from Nerja cave (Andalusia, Spain) were studied. The cave is well known because of its long human occupation from the Upper Palaeolithic to the Chalcolithic and its abundant parietal prehistoric Art. The aims of this study were twofold: i) to compare uranium/thorium ( 230 Th/ 234 U) and Carbon-14 ( 14 C) ages obtained all along the growth axis of the stalagmites in order to understand the consequences of diagenetic processes on the validity of radiometric ages; ii) as one of the stalagmites contains black layers, attributed to combustion soot, to establish when these intense hearths were used and by which culture. 230 Th/ 234 U and 14 C ages were coupled with mineralogical studies using FTIR (Fourier-transform infrared spectroscopy) and thin section observations. The first stalagmite (GN16-9b) displays 230 Th/ 234 U ages in stratigraphic order, and compatible with 14 C ages corrected for a few percent of dead carbon. Homogeneous composition of aragonitic crystals characterized by their needle-like texture is observed throughout this speleothem. For the second stalagmite (GN16-7), in contrast, 230 Th/ 234 U ages display large significant inversions and discordant results on the upper part and at the base of the stalagmite, suggesting a possible open system behavior for this chronometer. Interestingly, 14 C ages are in stratigraphic order all along the stalagmite and are compatible with 230 Th/ 234 U ages only in its central part. Mineralogical studies display evidence of aragonite to calcite transformation at the top and a complex mineralogical assemblage with interlayered silicates (possibly clays) and calcitic mineralogy for the base of GN16-7. In these parts, discordant 230 Th/ 234 U ages were measured. In the middle part of the stalagmite, however, where the fibrous aragonite is well preserved, the 14 C and 230 Th/ 234 U ages agree. Our data suggest that in the case of aragonite to calcite transformation as shown here, 230 Th/ 234 U ages are biased, but 14 C ages seem to remain accurate, as already observed in aragonitic marine bio minerals. 14 C ages obtained are used for the chronology of the soot layer, determined here between 7900 and 5500 years Cal BP, coherent with previous analysis of charcoals in the same sector of the cave. This study highlights the importance of working with at least two chronometers when stratigraphic age verification is not possible, as is the case of some parietal CaCO3 thin layers used for rock art dating. Recent 230 Th/ 234 U ages published for carbonate deposits on Spanish parietal Art are discussed in light of this demonstration. Highlights: We use FTIR and petrology to study mineralogy and diagenesis. We identify aragonite to calcite transformation in speleothem. 14C chronology is not altered by aragonite to calcite transformation. Uranium thorium (U/Th) chronology is altered by diagenetic alteration. Soot layers from prehistoric fire are identified and dated.