The effect of light exposure on the thermoluminescence signal from calcitic opercula

Abstract

Recent work has suggested that the thermoluminescence (TL) signal of opercula from the gastropod Bithynia tentaculata can be used to date the formation of calcite by the organism when it was alive. The two TL peaks of interest for dating are located at ∼250 °C (Peak 2) and ∼350 °C (Peak 3) when measured at a heating rate of 0.5 °C.s⁻¹. This paper assesses whether these peaks are altered by exposure to visible light, as this is important for how samples are collected in the field, and handled in the laboratory prior to measurement. Neither peak shows systematic change for exposures in a solar simulator of less than 24 h in duration. For longer exposures in the solar simulator the intensity of Peak 2 increases, possibly due to phototransfer. In contrast, the TL signal from Peak 3 is not affected by light exposure in the solar simulator for periods of up to 60 h, or by exposure to natural daylight with the UV-component removed for periods of up to ∼26 d. One experiment which exposed an operculum to natural daylight for ∼5.5 months led to a reduction in the TL signal from Peak 3 by 16 %, but such long exposures are unlikely in sampling and sample preparation. The lack of impact of daylight exposure on Peak 3 indicates that opercula-bearing samples can be collected and processed in normal daylight conditions, and that museum specimens are suitable for TL dating provided an associated sediment sample is available for dose rate calculations. However, as a precaution it is still recommended that light exposure is minimised where possible.