New U-series results for the speleogenesis and the Palaeolithic archaeology of the Almonda karstic system (Torres Novas, Portugal)

Abstract

The Almonda karstic system is an extensive network of cavities associated with the spring of the Almonda River. Among those of archaeological interest, the lowermost, 5–15 m above the current spring, contain deposits of Upper Palaeolithic and Holocene age. Higher up in a 70 m high escarpment, the labyrinth of passages features several collapsed cave entrances, two of which have been cleared for archaeological excavation: the Gruta da Aroeira/Galerias Pesadas/Brecha das Lascas complex of Acheulean localities, and the Middle Palaeolithic site of Gruta da Oliveira. U-series dating of speleothems associated with these deposits places the Acheulean of the Almonda beyond 390 ka, dates the palaeophreatic level of ∼40 m above the extant outlet back to a time before 200 ka, and constrains the accumulation of the Gruta da Oliveira sediment fill to between 23 ka and at least 102 ka. Bones from Gruta da Oliveira layer 8, which contains the uppermost human occupation of the site, were dated to the 34–40 ka range by U-series (Diffusion/Adsorption), in good agreement with available radiocarbon results. Combined with evidence from other sections of the system, these results set at half a million years ago or more the earliest human settlement of the Atlantic façade of Iberia, provide the first secure chronometric evidence for the emergence of the Acheulean in the peninsula (during Marine Isotope Stage 12 at the latest), and support the regional persistence of a Neanderthal-associated Middle Palaeolithic beyond 41.5 ka (the time of emergence elsewhere in western and central Europe of the Protoaurignacian, widely assumed to be modern human-related). U-series dating of speleothems collected in the inner passages of the system suggests uninterrupted speleothem formation through the Middle and Upper Pleistocene, enabling the construction of a high-resolution continental palaeonvironmental record for comparison with the evidence from the deep sea cores obtained off the Portuguese coast.