The Lower Acheulian site of Ambrona, Soria (Spain): ages derived from a combined ESR/U-series model

The Lower Paleolithic sites of Ambrona and Torralba (Soria, Spain) are often associated with each other because of their proximity although they do not represent the same morphostratigraphic unit. Systematic and extensive excavations were conducted at these sites by an American team during the 1960s and 1980s. Recently (1989-2000), a Spanish team has reactivated the research by initiating a new interdisciplinary project. In the central sector and west of the Ambrona site a stratigraphy has been established with six units from AS1 at the base to AS6 at the surface, corresponding to fluvial-lacustrine deposits in which abundant fauna associated with Acheulian artefacts were found. The most abundant species were elephant, horse, deer and aurochs. Other species like carnivores occur in small numbers only. The lithic assemblage is relatively sparse and consists of a few bifaces, choppers and several flake tools distributed throughout levels AS1 to AS5. The fauna and the lithic industry therefore document human presence at Ambrona during the Middle Pleistocene. In order to verify the presumed antiquity of the Ambrona deposits, horse teeth sampled in levels AS1, AS2 and AS6 were analysed using combined ESR/U-series (US) methods. The samples from the lower levels (AS1 and AS2) underwent a very recent uranium uptake as indicated by the U-series dates, ranging between 5 and 18 ka. In contrast, a more conventional postmortem uranium uptake was observed in the upper level samples (AS6) in the enamel and dentine tissues, while a light U-leaching was observed in the cementum. The combined ESR/U-series dates obtained on these samples suggest a minimum age of approximately 350 ka, contemporaneous of OSI 9 or the end of OSI 11, for the Ambrona site.