River-dominated land–sea continuum are not simple transport corridors: they are coupled hydro-sedimentary and biogeochemical systems in which carbon is mobilized, transformed, mineralized and selectively preserved from headwaters to the continental shelf. Here, we synthesize a decade of investigations across the Rhône River–Gulf of Lion continuum, combining hydrological data with dual carbon-isotope measurements (δ¹³C and Δ¹⁴C) in particulate, dissolved and sedimentary carbon pools.
Within the Rhône watershed, carbon sources include modern vegetation and soils, reworked terrestrial organic matter, aquatic primary production, carbonate-derived dissolved inorganic carbon and anthropogenic-derived (nuclear power plant, DIC) and radiocarbon-free organic carbon from outcropping sedimentary rocks (namely ancient organic carbon). Their relative contributions are strongly controlled by hydrology. Floods, particularly those from alpine and mediterranean tributaries such as the Durance, mobilize ¹⁴C-depleted particulate organic carbon (POC) mainly derived from marly badland erosion. Conversely, low-flow conditions favor aquatic production and enhanced incorporation of ¹⁴C-enriched DIC into POC. The Rhône therefore acts as a biogeochemical reactor, where exchanges among DIC, dissolved organic carbon and POC reshape the isotopic signals inherited from upstream sources.
At the river mouth, rapid sedimentation creates a pronounced proximal-to-offshore gradient in carbon cycling. δ¹³C and Δ¹⁴C analyses of sedimentary organic carbon and porewater DIC show that fresh, labile river-derived organic matter is preferentially remineralized in proximal and prodelta sediments. In contrast, older and more refractory terrestrial and ancient organic carbon is selectively preserved and buried. This decoupling produces systematically younger porewater-DIC signatures relative to the sedimentary organic carbon pool. Offshore, marine organic matter increasingly contributes to remineralization, while the sediments store a substantial amount of ancient organic carbon.
Our results demonstrate that evaluating coastal carbon budgets requires explicit distinction between carbon mineralization and carbon storage, as well as between biospheric (more and less fresh) and ancient carbon fractions. The Rhône system provides a model framework for investigating how hydrological extremes, lithological sources, anthropogenic signatures and sedimentary processes jointly regulate carbon fate along land–sea continuum.
 

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