Luminescence imaging of single grains of sand reveals their sediment transport history

Understanding sediment transport routes is crucial for predicting geomorphic changes driven by natural and anthropogenic processes in coastal and fluvial systems. Sediment tracing methods are vital to gain such understanding, but common sediment tracers are usually limited to following sediment released from a specific point. In our TRacking Ameland Inlet Living lab Sediment (TRAILS) project, we explore the use of natural luminescence signals of minerals to trace nourished sediment grains on an ebb-tidal delta. Towards this, we obtained and analyzed sediment samples from the Dutch Wadden Sea, where a mega-nourishment in the Ameland inlet ebb-tidal delta aims to address the sediment demand of the nearby coast and basin.

Insufficient luminescence signal resetting, e.g. poor bleaching, due to limited light exposure, can serve as a tool for sediment tracing by examining variations in the degree of bleach characteristics of luminescence signals with differing bleaching sensitivities within a single grain. This can inform us about light-exposure of that grain and therefore about sediment transport history, as explored by Reimann et al (2015) for a beach nourishment project at the Dutch coastline. Firstly, we hypothesize that slow-to-bleach signals reveal information about the end-member type: native grains in our ebb-tidal delta will be well-bleached in comparison to nourished grains. Secondly, we hypothesize that fast-to-bleach signals give insight into the transport history of grains: native grains will be more or less fully reset within the dynamic tidal reworking system of the Wadden Sea whilst nourished grains will still inherit part of their original signal. Combining information derived from slow- and fast-to-bleach signals thus provides a promising novel approach for tracing sediment grains in dynamic subaqueous environments, and thereby reveals sediment transport pathways of nourished sand grains.

Luminescence tracing methods rely on quantitative information about the potential and efficiency of subaqueous signal resetting. In a one-day experiment we quantified bleaching potential, that is, the light intensity and spectrum as a function of time, depth and tidal stage, and bleaching efficiency, that is the degree of bleaching of slow- and fast-to-bleach luminescence signals (de Boer et al., 2024a). Strongest subaqueous light attenuation took place during low tide when sediment concentrations are the highest, we also observed stronger attenuation of the ultraviolet part of the light spectrum. Light-sensitive luminescence signals, such as low-temperature feldspar IRSL, bleached more rapidly than less light-sensitive signals, such as high-temperature feldspar post-IR IRSL. None of the investigated signals were fully reset after 13.5 hours of light exposure, even for subaerially exposed samples. We then collected and analyzed over 100 sediment samples from the Ameland ebb-tidal delta. Using an EMCCD camera (de Boer et al., 2024b), we imaged a multitude of single-grain luminescence signals to explore the native or nourished origin of these sand grains. Ultimately, we aim to integrate these findings with Lagrangian sediment transport models to better understand spatial and temporal coastal sediment dynamics and inform coastal nourishment strategies (Pearson et al., 2022).