Helena Alexanderson

Resetting or bleaching of the luminescence signal is a fundamental factor in luminescence dating. It must occur in nature during the event or process to be dated for an accurate age, but if it happens during sample processing in the laboratory it destroys the sample for dating purposes. In this study, we look into bleaching of quartz optically stimulated luminescence by light in nature and in the laboratory. Unsieved quartz-rich extracts and 180–250 µm quartz grains with known doses wereexposed to outdoor light and laboratory light sources, respectively, and the change in dose with exposure time was measured. The outdoor conditions included direct sunlight, diffuse light from a cloud-covered sky and weak twilight, while indoor light sources were white fluorescent light, light from a computer screen and red darkroom light. Complete resetting took place only in daylight and was faster during sunny than cloudy conditions, and with bleaching rates that changed with exposure time. For all other light sources, including the darkroom lights, bleaching occurred to various degrees but was not complete after the longest exposure, which ranged from 15 min to 24 hours. The results show that some bleaching occurs even by low-intensity light with a limited spectrum. This implies that care should be taken in the laboratory not to expose samples to any light unnecessarily, but at the same time gives hope for bleaching in nature even in settings with limited or variable light conditions.