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

Abstract

The study of vertical particle dynamics in the highly turbid Gironde Estuary has shown intense cycles of sedimentation and resuspension at both diurnal and neap‐spring time scales. Fluid mud, with suspended particulate matter (SPM) concentrations between 50 and 500 g liter−1, has been observed during neap tides. Vertical profiles of biogeochemical parameters have been measured in the fluid mud. Anoxic conditions have been detected when SPM concentration exceeded 50 g liter−1 in the upstream and 140 g liter−1 in the downstream parts of the maximum turbidity zone (MTZ). At the downstream part of the MTZ, anoxic fluid mud was partitioned into a 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 reworked at the neap spring time scale. Due to the alternation of sedimentation and resuspension periods, most of the sediment experienced oxic/anoxic oscillations throughout the neap‐spring cycle. Fluid mud resuspension occurred without any observable incidence on the surface‐water oxygenation. An increase in total alkalinity was found in the fluid mud, due to both anaerobic respiration and a carbonate dissolution coupled to aerobic respiratory CO2 generation. This phenomenon significantly affected the inorganic carbon budget of the estuary, increasing the HCO3− input to the coastal ocean and reducing 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. This system acts as an efficient oxic/suboxic “fluidized bed reactor,” similar to mobile deltaic muds.