Investigating the low temperature thermoluminescence peak from calcite for monitoring thermal lag.

Calcitic snail opercula are a promising new material for thermoluminescence (TL) dating with the potential to cover the last ~3 Ma [1]. The ERC-funded Bridging Europe: a Quaternary Timescale for the Expansion and Evolution of Humans (EQuaTe) project aims to produce a secure dating framework for the earliest human occupation and expansion across Europe using this TL signal. Thermal lag is a key consideration for TL measurements and assessing the reproducibility of heating samples during TL measurement is an important but challenging issue. The potential magnitude of the problem is illustrated by Schmidt et al. [2] who reported a variation of ~60°C in the temperature of the "110 °C" TL peak from quartz grains when heating at 5 °C/s. It is unclear what proportion of this variability is due to thermal lag or incorrect calibration of hotplates. Duller et al. [3] showed that across eight different readers it was possible to reduce the variability in the "110 °C" TL peak from quartz to 2°C, and they suggested the temperature at which the TL peak was observed could be used to routinely assess the reproducibility of sample heating during measurement protocols. This relatively simple procedure involves measuring the TL glow curve and fitting the 110 °C TL peak with a Weibull function [see 4] thereby monitoring the peak position during a measurement sequence. The TL signal from calcitic opercula displays one low temperature peak (~100 °C, peak I) and two high temperature peaks (~275 °C and 350 °C, peak II and peak III respectively). Lifetimes of 7.4 x 107 and 1.4 x 1011 years have been reported for peaks II and III [1], and these are the signals that are of value for dating [e.g. 5, 6]. Little work has been published for peak I (~100°C). Here we present our investigation of the low temperature TL peak produced by Bithynia tentaculata opercula, and whether it can be used to investigate thermal lag and monitor temperature during TL measurements. The approach is used to assess the reproducibility of heating between different TL measurements during a single aliquot regenerative dose (SAR) procedure, and also to examine variability in heating between different opercula from a single sample. [ABSTRACT FROM AUTHOR]