In Phase 1, subproject P3 started to study the sources, composition and transformations of organic matter as the main drivers of biogeochemical processes in the deep STE. In Phase 2, P3 will assess the relative importance of organic matter (OM) sources and elucidate abiotic and biotic transformations influenced by morphodynamics and site-specific conditions in the STEs of Spiekeroog and selected validation sites. We will focus on the supply of ancient (millennial) versus recent OM, and evaluate the role of high-energy beach STEs as organic carbon sinks of global relevance. We will challenge the concept of a carbon-limited bioreactor, addressing the paradox of low transformation rates, despite the apparent abundance of organic carbon substrates. Our research will be guided by the overarching hypothesis that the apparent stability of STE-OM is partly an intrinsic property of OM, but also emerges from STE-specific environmental processes. Furthermore, we hypothesize that the processes causing intrinsic and emergent OM stability are universal and can be generalized to other high-energy beach systems. Key areas of our research will be adsorption and desorption of OM to and from minerals, abiotic molecular reactions with reactive oxygen and reduced sulfur species, and the turnover of OM by freeliving and sediment-attached microorganisms. To achieve these goals, we will conduct regular and event-specific field campaigns at Spiekeroog and at the two validation sites, Truc Vert (France) and De Panne (Belgium). We will perform laboratory experiments addressing the mobilization and molecular transformation of dissolved OM (DOM) from organic deposits in the STE, including processes such as iron reduction, Fenton reactions, and the microbial turnover of different DOM sources. We will use state-of-the-art ultrahigh-resolution mass spectrometry and high-field two-dimensional nuclear magnetic resonance spectroscopy to identify characteristic molecular patterns associated with these processes, contributing to an advanced mechanistic biogeochemical understanding of high-energy beach STE dynamics. Subproject P3 will provide essential data and process understanding on OM transformation rates and inventories, complementing insights from inorganic and microbiological studies, and informing reactive transport and global flux models.