Abstract River deltas play a crucial role in the global carbon cycle, acting both as significant carbon sinks and as sources of CO₂ to the atmosphere. The Rhône River delta is a key site for studying these processes due to its high sedimentation rates and its unique radiocarbon signatures influenced by anthropogenic activities. This study compiles over 10 years of research to assess the sources and fate of organic matter (OM) in the Rhône River delta, using stable (δ 13 C) and radiogenic (Δ 14 C) carbon isotopes. New data from the MissRhoDia II campaign (2018) are compared with previous datasets (CarboRhone 2012; DICASE 2014) to analyze the interactions between terrestrial, riverine, and marine OM sources. The study examines sedimentary processes along a transect from the river mouth to the continental shelf, considering both sediment burial and porewater analyses. Results indicate that OM mineralized in proximal sediments is primarily of terrestrial origin, freshly supplied by the river, and potentially influenced by nuclear-related activity. In contrast, on the continental shelf, remineralization occurs at a slower rate due to the limited availability of organic substrate and isotopic signatures reveal a predominantly marine origin with a minimal anthropogenic influence. In the Rhône pro-delta sediments, the burial of radiocarbon-free organic carbon (aOC) underscores the role of sediment transport in long-term carbon storage, with contributions from both petrogenic and aged terrestrial organic carbon sources. The mineralization of young, labile carbon and the burial of more refractory carbon create significant differences between the 14 C signatures measured in porewaters and sediments, highlighting the need to analyze both compartments. This study improves our understanding of carbon dynamics in the Rhône delta and provides valuable perspectives to better understand coastal carbon budgets, carbon sources, as well as the anthropogenic influence on marine ecosystems.
Dumoulin et al. (2026) studied this question.