Testing the isothermal thermoluminescence dating method to constrain mid-Pleistocene speleothem growth phases in the Bleßberg Cave

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The wide distribution of speleothems (secondary cave carbonates) and their exceptional suitability for high accuracy and precision 230Th/U dating make them extraordinary terrestrial archives of past climatic and environmental changes. With the upper age limit of 230Th/U dating at ca. 600 ka other methods are required to reach deeper into the geological past, including thermoluminescence (TL) dating. The TL signal of calcite saturates at very high dose, thus offering a potential means to extend the age range of speleothems to several millions of years. However, application of TL dating has been hampered by 1) a spurious TL signal from calcite at high temperatures, and 2) the large uncertainty in environmental dose rate estimation. Recently isothermal TL (ITL) dating of calcite has been developed to overcome the influence of the spurious TL signal. However, this ITL dating method has not been tested on samples with independent age control yet. Here, we propose to comprehensively study the characteristics of the ITL signal of speleothems, and develop the most suitable ITL dating protocol. We will use samples from Bleßberg Cave and evaluate critical factors that influence the accuracy of environmental dose rates, such as alpha effectiveness, U-series disequilibrium, and heterogeneous distribution of radioactive elements. MC-ICPMS 230Th/U ages will serve as independent age control points. Bleßberg Cave is ideally suited for the evaluation of the ITL method as 230Th/U-dated speleothems reaching back to ca. 360,000 years are readily available. Sample BB2-1 in particular will be an ideal test case to validate the ITL dating method. Successful evaluation and implementation of this method would open a pathway to establish chronologies for samples that remain inaccessible by U-series dating due to detrital contamination or low uranium concentrations.