In vitro simulation of oxic/suboxic diagenesis in an estuarine fluid mud subjected to redox oscillations

Estuarine turbidity maxima (ETMs) are sites of intense mineralisation of land-derived particulate organic matter (OM), which occurs under oxic/suboxic oscillating conditions owing to repetitive sedimentation and resuspension cycles at tidal and neap-spring time scales. To investigate the biogeochemical processes involved in OM mineralisation in ETMs, an experimental set up was developed to simulate in vitro oxic/anoxic oscillations in turbid waters and to follow the short timescale changes in oxygen, carbon, nitrogen, and manganese concentration and speciation. We present here the results of a 27-day experiment (three oxic periods and two anoxic periods) with an estuarine fluid mud from the Gironde estuary. Time courses of chemical species throughout the experiment evidenced the occurrence of four distinct characteristic periods with very different properties. Steady oxic conditions were characterised by oxygen consumption rates between 10 and 40 mu mol L-1 h(-1), dissolved inorganic carbon (DIC) production of 9-12 mu mol L-1 h(-1), very low NE4+ and Mn2+ concentrations, and constant NO3 production rates (0.4 - 0.7 mu mol L-1 h(-1)) due to coupled ammonification and nitrification. The beginning of anoxic periods (24 h following oxic to anoxic switches) showed DIC production rates of 2.5-8.6 mu mol L-1 h(-1) and very fast NO consumption (5.6-6.3 mu mol L-1 h(-1)) and NH4+ production (1.4-1.5 mu mol L-1 h(-1)). The latter rates were positively correlated to NO concentration and were apparently caused by the predominance of denitrification and dissimilatory nitrate reduction to ammonia. Steady anoxic periods were characterised by constant and low NO3- concentrations and DIG and NH4+ productions of less than 1.3 and 0.1 mu mol L-1 h(-1), respectively. Mn2+ and CH4 were produced at constant rates (respectively 0.3 and 0.015 mu mol L-1 h(-1)) throughout the whole anoxic periods and in the presence of nitrate. Finally, reoxidation periods (24-36 h following anoxic to oxic switches) showed rapid NH4+ and Mn2+ decreases to zero (1.6 and 0.8-2 mu mol L-1 h(-1), respectively) and very fast NO production (3 mu mol L-1 h(-1)). This NO3- production, together with marked transient peaks of dissolved organic carbon a few hours after anoxic to oxic switches, suggested that particulate OM mineralisation was enhanced during these transient reoxidation periods. An analysis based on C and N mass balance suggested that redox oscillation on short time scales (day to week) enhanced OM mineralisation relative to both steady oxic and steady anoxic conditions, making ETMs efficient biogeochemical reactors for the mineralisation of refractory terrestrial OM at the land-sea interface