Assessing whether a thermoluminescence peak at ∼100 °C in calcitic opercula can be used to monitor thermal reproducibility

The opercula of the gastropod Bithynia tentaculata is composed of calcite and exhibits three main thermoluminescence peaks at ∼100, 280 and 360 °C when heated at 0.5 °C.s-1. The two higher temperature peaks can be used for geological and archaeological dating, whilst the lower temperature peak has a lifetime of hours and is only seen after laboratory irradiation. This study explores whether this lower temperature TL peak can be used to assess the reproducibility of laboratory heating of opercula. As found in previous studies, the TL peak at ∼100 °C is seen to consist of at least two signals which have different lifetimes at room temperature. By fixing the interval between irradiation and measurement of the TL signal, the impact of these different lifetimes on the temperature at which the TL peak occurs (Tm) can be reduced, and replicate measurements on a single operculum using the same radiation dose yield values of Tm within 1°C. Comparison of different opercula shows variation in Tm of 9 °C when heating at 5 °C.s-1, interpreted as primarily arising from variation in thermal lag between the hotplate and the different samples assessed. Reducing the heating rate to 0.5 °C.s-1 reduces the variation in Tm between opercula to 2 °C. Imaging the TL emission from opercula using an EMCCD shows spatial variation in heating, and also demonstrates the reduction in variability that can be achieved by using a slower heating rate. Thus, monitoring the position of the TL peak at ∼100 °C can be used to assess thermal reproducibility of TL measurements on calcitic opercula.