A dual-aliquot regenerative-dose protocol (DAP) for thermoluminescence (TL) dating of quartz sediments using the light-sensitive and isothermally stimulated red emissions

Abstract

Quartz sediments, collected from a cave deposit in eastern Indonesia, display very weak optically stimulated luminescence (OSL) ultraviolet emissions, which we attribute to their volcanic provenance. They do, however, emit at longer (‘red’) wavelengths. Here we provide details of a new method of using a light-sensitive red thermoluminescence (TL) signal to date the last time of exposure of quartz grains to natural sunlight, which we have used previously to constrain the burial age of Homo floresiensis remains found on the Indonesian island of Flores. The samples examined typically contained a rapidly bleaching (‘bleachable’) signal, a slowly bleaching signal and a light-insensitive (heat-reset) signal. We isolated the bleachable TL signal from the other (‘unbleachable’) TL signals by means of a dual-aliquot regenerative-dose protocol (DAP), and the bleachable dose was estimated by subtracting the unbleachable dose from the total dose, taking into account the dose–response differences between these signals. The bleachable TL signal was found to be stable over geological timescales (∼17.5 million years), to display minimal sensitivity change during successive cycles of dosing, bleaching and heating, and to exhibit reproducible dose-response behaviour. Red TL measurements are commonly plagued by poor signal-to-noise ratios due to incandescence, and possibly thermal quenching, at high temperatures. Such difficulties have been minimised in this study by measuring the red TL isothermally (at 260 °C) for an extended duration (1000 s). Red TL dating results are presented for eight samples of quartz from diverse sedimentary environments, to illustrate the potential of this dating procedure, in particular but not exclusively, for quartz that has been heated in the past (e.g., due to volcanic activity).