Coupling and decoupling of DIC, TA and O₂ benthic fluxes along the Rhône River-dominated ocean margin.

Abstract. River-dominated ocean margins (RiOMar) and their benthic compartments play a key role in coastal carbon cycling due to the combined effect of high inputs of particulate organic carbon (POC), benthic mineralization and burial processes. This study quantified benthic fluxes of DIC, TA, and oxygen along a transect influenced by riverine POC inputs, providing new insights in diagenetic processes when switching from anaerobic to aerobic dominated sediment form the river mouth to the adjacent shelf. Fluxes at the sediment-water interface were measured ex-situ with sediment cores incubation during two cruises at the end of spring 2018 and 2022. Stations closest to the river mouth exhibit high benthic fluxes averaging 54 ± 20, 30 ± 21 and −27 ± 14 mmol m⁻² d⁻¹ for DIC_flux, TA_flux and TOU, respectively, reflecting intense anaerobic remineralization and a strong decoupling between oxygen consumption and DIC production (DIC_flux:TOU ratio of 1.5 to 3.5). These large TA fluxes further support that FeS burial limits the reoxidation of reduced compounds in these sediments, allowing the upward diffusion of TA produced at depth towards bottom waters. Offshore, In contrast, shelf stations show lower flux intensities, 8 ± 3 mmol m⁻² d⁻¹ for DIC_flux, −1 ± 2 mmol m⁻² d⁻¹ for TA_flux and −9 ± 2 mmol m⁻² d⁻¹ for TOU, suggesting a greater influence of aerobic mineralization, reoxidation processes and nitrification. Stations located in the Rhône prodelta show intermediate benthic fluxes of 24 ± 11, 6 ± 4 and −17 ± 7 mmol m⁻² d⁻¹ for DIC_flux, TA_flux and TOU, respectively. By substantially expanding the spatial and temporal resolution of benthic DIC_flux, TA_flux and TOU in this system, this study provides a robust quantitative framework for assessing the contribution of RiOMar sediments to coastal alkalinity budgets and emphasizes the important role of TA fluxes released from the sediment in increasing the buffering capacity bottom waters.