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. Estuarine regions are generally considered a major source of atmospheric CO2 , as a result of the high organic carbon (OC) mineralization rates intheir water column and sediments. Despite this, the intensity of anaerobicrespiration processes in the sediments tempered by the reoxidation ofreduced metabolites near the sediment–water interface controls the flux ofbenthic alkalinity. This alkalinity may partially buffer metabolic CO2 generated by benthic OC respiration in sediments. Thus, sediments with highanaerobic respiration rates could contribute less to local acidificationthan previously thought. In this study, a benthic chamber was deployed inthe Rhone River prodelta and the adjacent continental shelf (Gulf ofLion, northwestern Mediterranean) in late summer to assess the fluxes of totalalkalinity (TA) and dissolved inorganic carbon (DIC) from the sediment.Concurrently, in situ O2 and pH micro-profiles, voltammetric profiles and porewater composition were measured in surface sediments to identify the mainbiogeochemical processes controlling the net production of alkalinity inthese sediments. Benthic TA and DIC fluxes to the water column, rangingbetween 14 and 74 and 18 and 78 mmol m −2  d −1 , respectively, were up to 8 times higher than dissolved oxygen uptake (DOU) rates ( 10.4±0.9  mmol m −2  d −1 ) close to the river mouth, but theirintensity decreased offshore, as a result of the decline in OC inputs. Inthe zone close to the river mouth, pore water redox species indicated thatTA and DIC were mainly produced by microbial sulfate and iron reduction.Despite the complete removal of sulfate from pore waters, dissolved sulfideconcentrations were low and significant concentrations of FeS were found,indicating the precipitation and burial of iron sulfide minerals with anestimated burial flux of 12.5 mmol m −2  d −1 near the river mouth.By preventing reduced iron and sulfide reoxidation, the precipitation andburial of iron sulfide increases the alkalinity release from the sedimentsduring the spring and summer months. Under these conditions, the sedimentprovides a net source of alkalinity to the bottom waters which mitigates theeffect of the benthic DIC flux on the carbonate chemistry of coastal watersand weakens the partial pressure of CO2 increase in the bottom watersthat would occur if only DIC was produced.

Benthic alkalinity and dissolved inorganic carbon fluxes in the Rhône River prodelta generated by decoupled aerobic and anaerobic processes