Insights into the relationship between luminescence and ESR dating signals from Spanish sedimentary quartz samples of different geologic origins

Abstract

Trapped charge dating of optically bleached quartz deposits that lie beyond the dose-saturation limits of conventional optically stimulated luminescence (OSL) has typically focused on two main techniques over the last decade: electron spin resonance (ESR) and thermally transferred optically stimulated luminescence (TT-OSL). In spite of a clear association between these two dating techniques through their shared dose rate terms, the exact relationships between the various ESR centre (i.e., Al and Ti) and TT-OSL dating signals remain largely unexplored. In this study we undertake ESR and luminescence signal comparisons on paired sedimentary quartz samples from two Middle Pleistocene Spanish archaeological sites (Porto Maior in northwest Spain and Cuesta de la Bajada in northeast Spain) to examine the empirical relationships between quartz dating signals across different geological provinces. For this purpose we undertake 3-dimensional (3D) thermoluminescence (TL) spectral measurements, single-grain TT-OSL dating and signal analyses, normalised multiple centre (MC) ESR signal characterisation (ESR intensities, signal composition, dose-response curve properties), MC ESR De evaluation, and ICP-OES analysis of quartz extracts to evaluate potential correlations between elemental impurity concentrations and Al, Li–Ti and Li–H centre signals. The ESR samples from Cuesta de la Bajada and Porto Maior display similar Al centre signal intensities and dose-response behaviour, but they exhibit major differences in their Ti centre properties. Geochemical analysis reveals that the Cuesta de la Bajada quartz extracts contain higher concentrations of Al and Ti impurities (15 and 21% higher, respectively) when compared with the Porto Maior quartz extracts. These compositional differences appear correlated with the relative Ti–H signal intensities for the two sample datasets but not with the relative Al and Ti–Li signal intensities. The luminescence characterisation comparisons reveal a higher proportion of TT-OSL-producing grains in the Cuesta de la Bajada samples (6.2% versus ~1.6% for Porto Maior), and single-grain TT-OSL signal intensities that are almost an order of magnitude higher than for the Porto Maior samples. The 3D TL spectra for the Cuesta de la Bajada quartz are dominated by two major TL peaks in the red emission band (620 nm) at 265–310 °C and 365 °C (when using a heating rate of 1 °C/s), which are up to seven times more intense than the corresponding signals observed for the Porto Maior quartz. However, no major inter-sample differences were observed in the blue and UV-blue bands of the 3D TL spectra. These contrasting luminescence characteristics may be indicative of variable signal sensitisation histories related to dissimilar sedimentary recycling or geological origins between the two sites. Our results provide a useful baseline dataset for future comparative ESR and luminescence characterisation studies, and indicate a potentially close relationship between the quartz ESR Ti centre and single-grain TT-OSL signals. There is significant potential to refine understanding of these signal relationships in the future by expanding the number, and geographic coverage, of paired ESR–luminescence comparison studies, and by incorporating comparative 3D TL spectral investigations that focus exclusively on TT-OSL-producing grains (as opposed to bulk grain populations) to better elucidate luminescence trapping and recombination characteristics.