Characterizing and dating authigenic phosphates from the sedimentary infill of Atapuerca archaeo-paleoanthropological cave sites (Spain)

Abstract

We present the results of a characterization and dating study of eight authigenic apatite samples collected from various stratigraphic units of three cave sites across the Atapuerca complex, namely Gran Dolina, Galería Complex and Sima del Elefante. Characterization analyses using X-ray diffraction, X-ray Fluorescence and Electron Spin Resonance (ESR) spectroscopy have been used to determine the nature and composition of the samples: seven of them are mostly made of hydroxyapatite, while one is composed of crandallite, and all show variable amounts of contamination by calcite and/or sediment. The timing of authigenic apatite formation, which results from the weathering of the limestone in a karstic environment, has been tentatively constrained through a combination of bulk and spatially resolved analyses using ESR and U-series methods. The dating results obtained enable the identification of various formation events during the Middle and Late Pleistocene at each site, around 170-180 ka and 110-120 ka in Gran Dolina, ∼250 ka and ∼40 ka in Galería Complex, and potentially ∼360 ka in Sima del Elefante. Importantly, all these ages are significantly younger than the depositional age of the host sediment, thus confirming the post-depositional nature of authigenic apatite. Additionally, the absence of age consistency across sites suggests that they have all experienced independent diagenetic events, which cannot be attributed to the overall karst dynamics of the Sierra de Atapuerca, but should rather be regarded as local processes. Beyond the dating results, this study provides an overview of the potential and current limitations of ESR and U-series methods applied to Quaternary authigenic apatite. The main complication regarding the ESR method lies in the evaluation of the internal dose rate given the high uranium concentrations measured in the samples, and the absence of a directly related alpha efficiency value for this type of material. Laser ablation U-series analyses also highlight the methodological challenges caused by significant spatial heterogeneity of the U-series data across the apatite samples, which reflects the complexity, non-uniformity and long duration of apatite formation processes around the limestone blocks.