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

洞穴碳酸盐(次生洞穴碳酸盐)的广泛分布及其对高精度和高精度230Th/U测年的特殊适用性，使它们成为过去气候和环境变化的非凡陆地档案。由于年龄上限为230Th/U，测年约为600ka，需要其他方法来深入了解地质过去，包括热释光(TL)测年。方解石的热释光信号在非常高的剂量下饱和，因此提供了一种潜在的手段，将洞穴化石的年龄范围扩大到数百万年。然而，热释光测年的应用一直受到以下因素的阻碍：1)方解石在高温下产生的虚假热释光信号；2)环境剂量率估算的巨大不确定性。近年来，方解石的等温热释光(ITL)测年技术已经发展起来，以克服虚假热释光信号的影响。然而，这种ITL测年方法还没有在独立年龄控制的样品上进行测试。在这里，我们建议全面研究洞穴沉积物ITL信号的特征，并开发出最适合的ITL测年方案。我们将使用Bleçberg Cave的样本，评估影响环境剂量率准确性的关键因素，如阿尔法有效性、U系列不平衡和放射性元素的不均匀分布。MC-ICPMS 230/U年龄将作为独立的年龄控制点。Bbleüberg Cave非常适合于评估ITL方法，因为可以很容易地找到可以追溯到大约360,000年前的230年代/U年代的洞穴。特别是样品BB2-1将是验证ITL测年方法的理想测试案例。这一方法的成功评估和实施将为建立由于碎屑污染或铀浓度低而无法通过U系列测年获得的样品的年表开辟一条途径。

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.