A Franco-Spanish collaboration involving the LSCE (CEA-CNRS-UVSQ) has analysed two stalagmites in the Nerja cave (Andalusia) in order to shed light on the subject for archaeologists. Question: is the black level observed in one of them made of soot, and how old is it?
Discovered in 1959 in Andalusia, near Malaga, the walls of the Nerja cave are decorated with numerous motifs and dots and animal representations, including an ibex, which has become the emblem of the site. In order to pinpoint the chronology of human occupation of the cave, archaeologists are working with experts in the dating of palaeoclimatic archives.
Climatologists from the LSCE studied samples taken from two stalagmites in the cave using different dating methods (uranium-thorium and carbon-14) and compared their results with a stratigraphic study, based on analyses using Fourier transform infrared spectrometry (FTIR).
The latter technique reveals that the first stalagmite is formed from aragonite – a metastable mineral, composed of calcium carbonate CaCO3 and traces of metals, which crystallises in particular environments, notably from water containing a lot of magnesium. The second stalagmite, on the other hand, shows traces of transformation of aragonite into calcite – a mineral that differs from aragonite in the composition of its trace elements.
In the case of the purely aragonitic stalagmite, the U/Th and 14C ages are compatible and in stratigraphic coherence. In contrast, the second stalagmite shows inversions of the U/Th chronometer ages in the zones of recrystallisation of aragonite into calcite, while the 14C ages remain stratigraphically coherent throughout the sample. The 14C chronometer therefore appears to remain valid in the case of the mineralogical transformations observed, while the U/Th ages are distorted by the expulsion of uranium atoms during the crystalline transformation.
Analysis of the black level has confirmed that these are fossilised soot deposits, attributed to fires maintained by the cave’s occupants between 7,900 and 5,500 years ago.
The observations made on these two stalagmites will be very useful for research into the chronology of cave art, based on U/Th and 14C cross-dating and mineralogical analysis of the carbonate veils covering cave paintings, for which the sample size is very limited.
This work was carried out in collaboration with the Geosciences Paris Saclay laboratory (GEOPS), PACEA (De la préhistoire à l’actuel : culture, environnement et anthropologie) in Bordeaux, the Universities of Cordoba and Granada and the Andalusian Institute of Earth Sciences (CSIC-UGR) in Granada (Spain).
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