Oxic/anoxic oscillations and organic carbon mineralization in an estuarinemaximum turbidity zone (The Gironde, France)

The study of vertical particle dynamics in the highly turbid Gironde Estuar y has shown intense cycles of sedimentation and resuspension at both diurna l and neap-spring time scales. Fluid mud, with suspended particulate matter (SPM) concentrations between 50 and 500 g liter(-1), has been observed dur ing neap tides. Vertical profiles of biogeochemical parameters have been me asured in the fluid mud. Anoxic conditions have been detected when SPM conc entration exceeded 50 g liter(-1) in the upstream and 140 g liter(-1) in th e downstream parts of the maximum turbidity zone (MTZ). At the downstream p arr of the MTZ, anoxic fluid mud was partitioned into denitrification layer (SPM = 140-250 g liter(-1)), intensively reworked at the tidal time scale, and by an Mn(IV)-reduction layer (SPM > 250 g liter(-1)) preferentially re worked at the neap-spring time scale. Due to the alternation of sedimentati on and resuspension periods, most of the sediment experienced oxic/anoxic o scillations throughout the neap-spring cycle. Fluid mud resuspension occurr ed without any observable incidence on the surface-water oxygenation. An in crease in total alkalinity was found in the fluid mud, due to both anaerobi c respiration and a carbonate dissolution coupled to aerobic respiratory CO 2 generation. This phenomenon significantly affected the inorganic carbon b udget of the estuary, increasing the HCO3- input to the coastal ocean and r educing the CO2 flux to the atmosphere. An accumulation of labile-dissolved organic carbon observed in the fluid mud suggests that these oscillations result in an acceleration of particulate organic matter (POM) decomposition . in the Gironde MTZ, a net loss of refractory land-derived POM occurs. Thi s system acts as an efficient oxic/suboxic "'fluidized bed reactor," simila r to mobile deltaic muds.