Single‐grain optical dating of Quaternary sediments: why aliquot size matters in luminescence dating

TLDR

Optical dating measures the elapsed time since mineral grains were exposed to daylight, and while it is ideal when all grains reset, variable exposure can leave residual charge and cause age overestimation. This article outlines the methods for single‑grain OSL and reviews its use on glacial, fluvial, and aeolian Quaternary sediments. In the past 15 years, controlling the number of grains per measurement—from thousands to one—has become feasible, and single‑grain analysis reveals bleaching variability that is masked when many grains are averaged. Single‑grain OSL expands the environments that can be dated and enhances age reliability by explicitly determining whether samples were bleached at deposition.

Abstract

Optical dating measures the time that has elapsed since mineral grains were exposed to daylight. The technique is ideal for sediments in which all the grains were exposed to sufficient daylight at deposition to reset the optically stimulated luminescence (OSL) signal. However, in many environments the exposure of individual grains to daylight will be variable. Limited exposure to light results in grains retaining a part of any prior trapped charge, and if unaccounted for this causes overestimation of the age. In the past 15 years it has become feasible to control the number of grains used for each luminescence measurement, varying from many thousands to a single grain. Where many grains are measured simultaneously, the luminescence signal is averaged, and any variability in resetting between grains will be obscured. This article describes the methods involved in single‐grain OSL measurements and review the application of the method to glacial, fluvial and aeolian Quaternary sediments. Single‐grain OSL is expanding the range of environments that can be dated and improving the reliability of ages by explicitly assessing whether samples were bleached at deposition or not.