Cycling of iron and manganese in surface sediments: A general theory for the coupled transport and reaction of carbon, oxygen, nitrogen, sulfur, iron, and manganese

This paper presents a multicomponent early diagenetic model which explicitly accounts for the coupling of the redox cycles of Fe and Mn to those of oxygen, carbon, sulfur, and nitrogen. Rate expressions are used to represent the oxidation of organic carbon, the oxidation of secondary reduced species formed as byproducts of organic matter oxidation, and the precipitation and dissolution of sulfide and carbonate minerals. The net rate of organic carbon oxidation is broken down into the contributions of aerobic respiration, denitrification, dissimilatory Mn(IV) reduction, dissimilatory Fe(III) reduction, sulfate reduction, and methanogenesis. The computational algorithm is based on a modified Monod kinetics formulation for the organic matter degradation pathways. The non-specific adsorption of ammonia, the surface complexation of Fe2+ and Mn2+ cations, and the homogeneous interconversions within the dissolved carbonate-sulfide system are treated as equilibrium reactions. The transport processes included are sediment advection, pore water diffusion, and particle mixing and irrigation by benthic macrofauna. Chemical component concentrations and pore water alkalinity are described by a set df continuity equations characterized by nonlinear reaction rate terms. The equations are solved by finite-difference. The distribution of pore water pH is derived from the calculated profiles of total dissolved inorganic carbon, total dissolved sulfide, and alkalinity. Particulate deposition fluxes and bottom water composition are imposed as upper boundary conditions. The model is applied to an extensive set of data collected in a marine sediment from the Skagerrak (Denmark). Theoretical depth profiles reproduce the measured pore water concentrations of O-2, NO3-, NH4+, Mn2+. and Fe2+ ins the solid sediment concentrations of Fe(III), Mn(TV), sulfide-bound Fe(II), and non-sulfide Fe(II). The model also correctly simulates the depth distribution of measured sulfate reduction rates. According to the computations, approximately two thirds of the total rate of iron reduction in the sediment are utilized directly by bacteria to oxidize organic carbon (dissimilatory Fe reduction). Manganese reduction, on the other hand, is mostly due to chemical reaction with dissolved Fe2+. Despite the relatively high rates of iron and manganese reduction, only very small amounts of the Fe(II) and Mn(II) produced during early diagenesis are permanently buried in the deeper sediment. Most of the dissolved and solid-bound Fe(II) and Mn(II) cations reoxidize in the surface sediment or escape to the water column. The main oxidation pathways in the sediment are heterogeneous oxygenation of surface complexed Fe2+ and Mn2+ cations. The model also predicts that intense redox cycling of Fe and Mn should cause the appearance of a pH minimum at the base of the aerobic surface layer.