| 09 Sep 2026
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 DICflux, TAflux and TOU, respectively, reflecting intense anaerobic remineralization and a strong decoupling between oxygen consumption and DIC production (DICflux: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 DICflux, −1 ± 2 mmol m−2 d−1 for TAflux 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 DICflux, TAflux and TOU, respectively. By substantially expanding the spatial and temporal resolution of benthic DICflux, TAflux 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.
The authors examined benthic fluxes of total alkalinity (TA), total dissolved inorganic carbon (DIC), and dissolved oxygen (DO) using core incubation methods at three areas of the Rhone River dominated margin, proximal domain near the river mouth, prodeltaic domain, and the shelf that are away from the immediate river influence. They found that fluxes generally decrease as one moves away from the river mouth, along with decreasing extent of anaerobic respiration as witnessed by increasing coupling between DIC and DO fluxes. Further, the authors discussed extensively how TA fluxes are produced via anaerobic processes, for example, burial of reduced compounds and the lack of reoxidation of upward diffused reduced compounds. The study used two separate trips post major flood stage of the river and authors justified the choice of timing as representing “quasi-stable sediment conditions”.
The manuscript reads well and the authors put the measured fluxes in the context of a number of prior studies. I have some comments and would like to see them addressed.