Novel insights into the post-IR IRSL₂₀₀ signal bleachability of single-grain K-feldspars in fluvial modern analogues from the Southern Central Andes, Chile

Abstract. Post-infrared infrared stimulated luminescence (post-IR IRSL) signals from potassium feldspars are gaining prominence in both luminescence dating and luminescence-based sediment tracing techniques. To enhance the accuracy and reliability of these applications, it is essential to develop a comprehensive understanding of how post-IR IRSL signals undergo bleaching. While previous studies have explored post-IR IRSL bleachability using multi-grain approaches, a systematic single-grain investigation on modern analogues has not been conducted. In this study, we examined the bleaching behaviour of the post-IR IRSL₂₀₀ signal at the single-grain level in eleven modern floodplain samples from the tectonically active Southern Central Andes. Our study demonstrated considerable variation in the mean residual doses observed following two days of laboratory solar simulator bleaching across the sample set. This variability was evident not only between different samples but also among individual grains within the same sample. Thus, we evaluated the influence of bleaching duration, grain-specific geochemical composition, catchment-scale lithological variability, and the size of the natural dose on both laboratory-measured residual doses and remnant doses. Our laboratory bleaching experiments showed consistent post-IR IRSL₂₀₀ signal behaviour across four different samples, reaching a plateau based on normalised luminescence signal after two days of exposure to solar simulator light. While individual grains exhibited a wide range of bleaching rates, this variability did not account for the spread in residual dose values. Notably, extended light exposure reduced variability in signal intensity, underscoring its role in dose homogenisation. Geochemical analysis of major oxides showed no significant correlation with either residual dose magnitude or bleaching rate, suggesting that mineral composition (including K-concentration) does not influence bleaching efficiency at the individual grain level. Furthermore, bleaching behaviour remained consistent across samples regardless of catchment lithology, with no discernible relationship between lithological units and remnant or residual dose values. Most importantly, we identified a strong positive exponential correlation (R² = 0.86) between residual dose and natural remnant dose, revealing dose-dependent bleaching efficiency and the presence of a unbleachable component at the time of deposition. By integrating insights on bleachability with the information on the unbleachable component and remnant doses derived from modern analogues, we highlight the limitations of correcting palaeodoses by directly using remanent doses and evaluate three context-sensitive correction strategies. Finally, we discuss how residual dose and the unbleachable component can be leveraged to more reliably identify well-bleached grains, enhancing the accuracy of luminescence-based sediment tracing applications.