Luminescence dating of alluvial sediments from the Quaternary fan–terrace sequence of the lower Bruche valley, Upper Rhine Graben, France

Abstract. Geochronological constraints on fluvial terraces provide valuable insights into channel migration and adjustment involved within actively meandering river systems. However, numerical age estimation of fluvial deposits using luminescence dating is often challenged by inherent methodological limitations. Pre-depositional partial bleaching of grains can lead to overestimation, while anomalous fading in feldspar grains often results in underestimation of the luminescence ages. The current study applies both optically stimulated luminescence and multi-elevated temperature post-infrared infrared stimulated luminescence dating to fine sand and sandy matrix deposits collected from three distinct fluvial terrace levels across the lower reach of the Bruche River, in the Upper Rhine Graben (URG, north-eastern France). To evaluate the extent of partial bleaching, modern alluvial sand from the active riverbed is analysed for residual luminescence signals. Additionally, fading rates are measured, and quartz and feldspar ages are compared to evaluate the effectiveness of the applied measurement protocol in targeting signals less susceptible to fading. Overall, our study provides (i) a methodological focus, highlighting the effect of partial bleaching and fading on age estimation, and (ii) new insights into landscape evolution of the lower Bruche valley through numerical dating of the terrace deposits. The luminescence ages of the three fluvial terraces reveal distinct periods of aggradation in the URG. The uppermost sequences of the youngest (i.e. lowest) and middle terraces were deposited during the Younger Dryas (∼ 12–14 ka) and Marine Isotope Stage 3 (∼ 27–35 ka), respectively, while the oldest (i.e. highest) alluvial terrace has a minimum age of ∼ 200 ka.