Coupled anoxic nitrification/manganese reduction in marine sediments

Pore water and solid phase distributions of oxygen, manganese, and nitrogen from hemipelagic and shelf sediments sometimes indicate a close coupling between the manganese and nitrogen redox cycles. Reaction coupling must be sustained in part by biological reworking of Mn-oxide-rich surface sediments into underlying anoxic zones. Surface sediment from Long Island Sound (USA) was used in laboratory experiments to simulate such intermittent natural mixing processes and subsequent reaction evolution. Mixed sediment was incubated anoxically under either diffusively open (plugs) or closed conditions (jars). In closed anoxic incubations, pore water NO3 2 increased regularly to a maximum (up to 17 mM) after one to several days, and was subsequently depleted. Mn21 was produced simultaneously with NO3 2. NO2 2 was also clearly produced and subsequently reduced, with a formation-depletion pattern consistent with coupled nitrificationdenitrification in the anoxic sediment. Manipulative additions of Mn-oxides (5–10 mmol g21 net) demonstrated that net anoxic NO3 2 production correlated directly with initial Mn-oxide content. During initial net NO3 2 production there was no evidence for SO4 22 reduction. A direct correlation was also observed between anoxic nitrification rates and estimated sulfate reduction rates; the larger nitrification rates, the larger the eventual net sulfate reduction rates. Diffusively-open incubations using sediment plugs of four different thicknesses (2, 5, 10 and 20 mm) exposed to anoxic overlying water, also showed net production of pore water NO3 2 (;15–20 mM) despite the absence of NO3 2 in the overlying water for at least five days. In general, higher nitrate concentrations were maintained in the open relative to the closed incubations, due most likely to lower concentrations of dissolved reductants for NO3 2 in the open system. These experiments imply simultaneous coupling between the benthic nitrogen, manganese, and sulfur redox cycles, involving anoxic nitrification and sulfide oxidation to SO4 22. Anoxic nitrate production during Mn reduction indicates that nitrification and denitrification can occur simultaneously in subsurface sediments, without vertical stratification. The existence of anoxic nitrification implies new reaction pathways capable of increasing coupled sedimentary nitrificationdenitrification, particularly in bioturbated or physically mixed deposits