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Archives of Early Human Occupation in Western Europe: OSL Chronologies beyond the Middle Pleistocene in the Iberian Peninsula

Final Report Summary - ARCHOSL (Archives of Early Human Occupation in Western Europe: OSL Chronologies beyond the Middle Pleistocene in the Iberian Peninsula)

This interdisciplinary project entails the application of newly developed techniques and protocols in the field of luminescence dating (e.g. single-grain TT-OSL on quartz and pIR-IR on K-feldspar) to obtain robust absolute chronologies on key Lower Palaeolithic archaeological sites in Western Europe. The main aim of the research is to improve and expand on the current chronological framework of important late Early Pleistocene (1.2 Ma - 780 ka) and Middle Pleistocene (780 ka - 125 ka) archaeological records in the Iberian Peninsula in order to accurately reconstruct and interpret the earliest human (Homo species) expansions into Europe. An integral aspect of this research is the pursuit of methodological advancements and refinements of the latest protocols in quartz luminescence dating to push back the existing age range of the technique. This project will focus on several important early human occupation sites in the Iberian Peninsula, namely the Atapuerca karstic complex (northern Spain), Orce sites (Fuente Nueva III and Barranco León, Guadix-Baza Basin, southeast Spain), and La Boella (south Barcelona, northeast Spain).

In total, 89 sediment samples have been collected from 16 archaeological sites at Atapuerca, Orce and La Boella. The sites (and number of samples) are as follows: Atapuerca: Galería (12), Cueva de Zarpazos (4), Sima del Elefante (15), Gran Dolina (3), Sima de los Huesos (10), Cueva mayor – Sala del Ciclopes (6) Hotel California (4), Pico Terrace (1) – Orce: Fuente Nueva III (5), Barranco León (8), Barranco del Paso (2), Huéscar-1 (3), Cúllar de Baza-1 (3), Cortes de Baza (3) – Tarragona: La Boella (7) and La Canseladeta (3). In each case, the samples cover the entire stratigraphic sequence. Samples were collected from levels that bracket the main archaeological or palaeontological horizons, as well as from sections that have independent chronologies (where applicable). For each sediment sample collected for luminescence dating, a separate sample was also collected for estimation of the environmental dose rate.

The results of this project have provided invaluable insights into the inherent luminescence characteristics of silicate sediments from the Iberian Peninsula and their suitability for luminescence dating applications. Specifically, it has been found that sedimentary quartz from the Atapuerca karst-infill, and the northern Iberia meseta in general, is characterised by high-sensitivity (bright) luminescence signals that are rapidly bleaching and are suitable for the application of the SAR equivalent dose (De) estimation protocol over very high dose ranges. At Orce, the luminescence characteristics vary widely between sites because of local-scale differences in sediment sourcing. Sites such as (Huescar-1, <780 ka) show highly suitable luminescence characteristics, permitting extended-range dating applications using OSL, TT-OSL and post-IR IRSL signals. Other sites within the basin (Barranco Leon, >780 ka old) display inherently unsuitable quartz OSL signals that are devoid of rapidly bleaching components. Such findings are rare and provide a unique opportunity for ascertaining the types of crystal lattice defects that give rise to the most widely used OSL dating signal.

One of the most important results derived from this project, and which has never been reported before, is the successful application of TT-OSL dating at the single grain level. This methodological advancement has been made possible by the inherently bright luminescence characteristics of quartz from this geological province. The results of this single-grain research have opened the possibility of applying TT-OSL to a wider range of deposits, particularly those that are most prone to reduced daylight exposure prior to burial (i.e. cave sediments). The ages obtained using this novel approach span 40 ka to 800 ka. The TT-OSL ages obtained for deposits spanning the late Early Pleistocene (Sima del Elefante) to the late Middle Pleistocene (Galería) have been tested against independent age control and display a high degree of stratigraphic consistency. The TT-OSL chronologies are also found to be in excellent agreement with replicate ages derived using different luminescence signals, such as post-IR IRSL and quartz ‘supergrain’ OSL. Together, the results of this project demonstrate the reliability of using extended-age optical dating methods on samples from the Iberian Peninsula, and highlight the potential for obtaining accurate chronologies as far back as the Early/Middle Pleistocene boundary (~780 ka). Significantly, the chronologies obtained as part of this study are among the oldest produced so far using newly emerging luminescence techniques. These methodological refinements and novel applications have the potential to set a new benchmark in extended-range luminescence dating and place Spain and Europe at the forefront of geochronology research. They also have the potential to increase the current demand for luminescence dating within the archaeology consulting and research sectors by demonstrating the applicability of new numerical dating techniques over Early to Middle Pleistocene timescales.

In addition to the aforementioned technical developments, the chronologies developed for the Atapuerca sites as part of this project have contributed significantly to our understanding of Middle Pleistocene hominin evolution in Europe, as follows:
(1) Using these improved techniques, new chronologies have been developed for the largest Middle Pleistocene hominin bone accumulation worldwide at the Sima de los Huesos site, Atapuerca. These are the first radiometric ages to be produced for the site that are unequivocally associated with the human bone bed and provide a firm minimum age of ~427 ka for the hominin remains. The results from this study were published in the Journal of Human Evolution in 2014. The new luminescence chronologies have major implications for unravelling the H. neanderthalensis – H. sapiens divergence histories. The palaeoanthropological significance of these chronologies are presented in a major research article that has been submitted to the journal Science.
(2) TT-OSL and post-IR IRSL chronologies have been developed for the entire sequence at the Middle Pleistocene archaeological site of Galería (Atapuerca). Here the first appearance of lithic tools (Acheulean/Mode II) has been dated to ~313 ka (MIS 9). The broad-scale implication of these results include: (i) an improved temporal resolution of human occupation at Atapuerca during the Middle Pleistocene, which allowed us to establish a firm relationship with other dated sites within the same system (i.e. Sima de los Huesos); (ii) an improved understanding of sediment transport and deposition processes in these complex karstic settings; and (iii) resolution of previous chronological discrepancies that existed for the Galería sequence. The results from this study will shortly be submitted to Plos One for publication.
(3) Single-grain TT-OSL chronologies established on unit TD6 of the Gran Dolina site have provided improved age constraint on one of the most important accumulations of Homo antecessor remains globally. A new weighted mean age of ~850 ka has been obtained for the fossil-bearing horizon of this site and the results have recently been submitted for publication in Quaternary International.