Single-grain OSL dating of the Middle Palaeolithic site of Galeria de las Estatuas, Atapuerca (Burgos, Spain)

This study presents single-grain optically stimulated luminescence (OSL) chronologies for the archaeological site of Galeria de las Estatuas - the first systematically excavated Middle Palaeolithic site within the karst system of the Sierra de Atapuerca archaeological complex, northern Spain. The single-grain OSL ages are compared with paired single-grain thermally transferred OSL (TT-OSL) dating results for a selection of samples in order to better assess quartz signal bleaching characteristics of endokarstic deposits preserved at Atapuerca. In total, seven luminescence dating samples were collected from four lithostratigraphic units exposed in two excavation pits (GE-I and GE-H). The single-grain OSL equivalent dose (D-e) distributions are characterised by generally low overdispersion (20-30%), suggesting appropriate bleaching at deposition. The resultant single-grain OSL ages reveal that the sediment sequence and archaeological remains excavated in pit GE-I accumulated 80-112 ka, while the upper layers of excavation area GE-II were deposited 70-79 ka. The replicate single-grain TT-OSL ages are in agreement with the OSL chronologies at 2 sigma for three of the four samples investigated; although in all cases the TT-OSL ages were systematically older than their single-grain counterparts. Apparent TT-OSL residual doses (i.e., TT-OSL De values in excess of their corresponding OSL De values) of 9-65 Gy were observed for all samples. These excess TT-OSL De values are generally low in comparison to the natural dose ranges of TT-OSL dating applications undertaken elsewhere in the Atapuerca karst system. The single-grain TT-OSL and OSL dating comparisons build on daylight bleaching experiments and modern analogue studies performed on other Atapuerca exogeneous infill deposits and suggest reasonable potential for TT-OSL signal resetting down to relatively low levels for at least some sediments preserved in the Atapuerca karstic cavities. The quartz single-grain OSL chronologies obtained in this study place the Middle Palaeolithic sequence of Galeria de las Estatuas within marine isotope stage (MIS) 5 and the beginning of MIS 4, and provide firm evidence for human occupation of the Sierra de Atapuerca during a previously unreported time period.