Refining the chronology of Acheulean deposits at Porto Maior in the River Miño basin (Galicia, Spain) using a comparative luminescence and ESR dating approach

Abstract

Trapped charge dating of optically bleached quartz, which involves techniques such as electron spin resonance (ESR) and thermally transferred optically stimulated luminescence (TT-OSL), is increasingly being used to establish Middle and Early Pleistocene chronologies for archaeological, palaeontological and palaeoenvironmental sedimentary sequences. Comparative dating studies that make use of several independent or semi-independent radiometric techniques in tandem are an invaluable means of assessing the reliability of optically bleached quartz dating methods and ascertaining empirical relationships between different quartz dating signals. In this study, we undertake a comparative dating evaluation of single-grain TT-OSL and various ESR signals that exhibit different optical resetting properties (Al, Ti–Li and, where measurable, Ti–H centres) at the Middle Pleistocene archaeological site of Porto Maior, northwest Spain, which contains a distinctive, regionally significant Acheulean stone tool assemblage. We expand the existing chronological framework for Porto Maior using single-grain TT-OSL dating, derive combined numerical ages for individual depositional units using Bayesian modelling, and further examine the relationship between ESR and TT-OSL signals by reviewing a range of published comparative dating studies. The new single-grain TT-OSL ages obtained for unit PM3 at Porto Maior (225 ± 31 ka and 242 ± 32 ka) are in close agreement with previously published ESR Ti–Li ages for this unit (266 ± 23 ka and 279 ± 26 ka) and significantly younger than those reported for the Al centre (441 ± 75 ka and 452 ± 66 ka). Our broader comparison of published TT-OSL – ESR dating results also shows statistically significant agreement between single-grain TT-OSL and Ti–Li centre ages in other sedimentary settings, suggesting a potentially closer relationship between the TT-OSL-producing traps and the Ti–Li paramagnetic centres in quartz, rather than between the TT-OSL and Al centre signals. The Bayesian modelling results for Porto Maior enable us to refine the existing bracketing depositional age for the archaeological accumulation at Porto Maior and provide a stronger statistical foundation for chronostratigraphic correlations with other Acheulean archaeological deposits found in the Miño River basin. The Bayesian model indicates that the entire Porto Maior sedimentary sequence was deposited between 312.6 ± 32.6 ka and 18.8 ± 7.7 ka, and that the in situ large cutting tool assemblage from level PM4 most likely accumulated during Marine Isotope Stage 7 (mean depositional age = 210.7 ± 24.7 ka). The results of this study reinforce the usefulness of applying the multiple centre (MC) ESR dating approach in many dating contexts, and demonstrate the value of undertaking comparisons between semi-independent quartz dating signals in order to maximise the reliability of ESR and single-grain TT-OSL quartz chronologies.