CYCLING OF IRON AND MANGANESE IN SURFACE SEDIMENTS - A GENERAL-THEORYFOR THE COUPLED TRANSPORT AND REACTION OF CARBON, OXYGEN, NITROGEN, SULFUR, IRON, AND MANGANESE

This paper presents a multicomponent early diagenetic model which expl icitly accounts for the coupling of the redox cycles of Fe and Mn to t hose of oxygen, carbon, sulfur, and nitrogen. Rate expressions are use d to represent the oxidation of organic carbon, the oxidation of secon dary reduced species formed as byproducts of organic matter oxidation, and the precipitation and dissolution of sulfide and carbonate minera ls. The net rate of organic carbon oxidation is broken down into the c ontributions of aerobic respiration, denitrification, dissimilatory Mn (IV) reduction, dissimilatory Fe(III) reduction, sulfate reduction, an d methanogenesis. The computational algorithm is based on a modified M onod kinetics formulation for the organic matter degradation pathways. The non-specific adsorption of ammonia, the surface complexation of F e2+ and Mn2+ cations, and the homogeneous interconversions within the dissolved carbonate-sulfide system are treated as equilibrium reaction s. 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 descri bed by a set df continuity equations characterized by nonlinear reacti on rate terms. The equations are solved by finite-difference. The dist ribution of pore water pH is derived from the calculated profiles of t otal dissolved inorganic carbon, total dissolved sulfide, and alkalini ty. Particulate deposition fluxes and bottom water composition are imp osed as upper boundary conditions. The model is applied to an extensiv e set of data collected in a marine sediment from the Skagerrak (Denma rk). Theoretical depth profiles reproduce the measured pore water conc entrations of O-2, NO3-, NH4+, Mn2+. and Fe2+ ins the solid sediment c oncentrations of Fe(III), Mn(TV), sulfide-bound Fe(II), and non-sulfid e Fe(II). The model also correctly simulates the depth distribution of measured sulfate reduction rates. According to the computations, appr oximately two thirds of the total rate of iron reduction in the sedime nt are utilized directly by bacteria to oxidize organic carbon (dissim ilatory Fe reduction). Manganese reduction, on the other hand, is most ly due to chemical reaction with dissolved Fe2+. Despite the relativel y high rates of iron and manganese reduction, only very small amounts of the Fe(II) and Mn(II) produced during early diagenesis are permanen tly buried in the deeper sediment. Most of the dissolved and solid-bou nd Fe(II) and Mn(II) cations reoxidize in the surface sediment or esca pe to the water column. The main oxidation pathways in the sediment ar e heterogeneous oxygenation of surface complexed Fe2+ and Mn2+ cations . The model also predicts that intense redox cycling of Fe and Mn shou ld cause the appearance of a pH minimum at the base of the aerobic sur face layer.