A MULTICOMPONENT REACTIVE TRANSPORT MODEL OF EARLY DIAGENESIS - APPLICATION TO REDOX CYCLING IN COASTAL MARINE-SEDIMENTS

STEADYSED1 is a multicomponent reactive transport code for steady-stat e early diagenesis which fully incorporates the reaction couplings amo ng the elements, C, O, N, S, Fe, and Mn. The model is tested against e xtensive datasets collected by Canfield et al. (1993a,b) at three coas tal marine sites that exhibit high rates of combined iron and manganes e (hydr)oxide reduction. It is shown that the model provides a consist ent explanation of the entire body of multicomponent multisite observa tions. The measured concentration profiles of twenty-eight individual porewater and solid sediment species are satisfactorily reproduced. Fu rthermore, the model predicts the observed distributions of sulfate re duction rates, as well as diagnostic features of the porewater pH prof iles. The coupled nature of the reactive species imposes stringent con straints when fitting model-calculated distributions to the data, beca use a single set of reaction and transport parameters must account for the profiles of all the species at each site. The parameters are furt her separated into site-specific (e.g., deposition fluxes, bottom wate r composition) and reaction-specific parameters (e.g., rate coefficien ts, apparent equilibrium constants, limiting substrate concentrations) . By minimizing the variations of the reaction-specific parameters fro m one site to another, the fitting strategy emphasizes the retrieval f rom field data of reaction parameters that are mechanistically meaning ful. The model reproduces the significant vertical overlap between org anic carbon oxidation pathways observed in the sediments. The overlap is explained by the combination of high rates of organic carbon oxidat ion plus intense bioturbation and irrigation at the sites studied. The model-derived contributions of the various respiratory processes comp are favorably with the incubation results of Canfield et al. (1993a,b) . Aerobic respiration accounts for less than 32% of total organic carb on oxidation at the three sites. From 22 to 46% of the O-2 uptake by t he sediments is directly coupled to organic carbon oxidation, while th e remainder is used for the oxidation of secondary reduced species, in particular dissolved, adsorbed and solid Fe(II) and Mn(II) species. A ccording to the model, the oxidation of Fe2+ by Mn (hydr)oxides is res ponsible for the observed spatial separation of the porewater build-up of Mn2+ and Fe2+. At two of the sites, 75 and 97% of the total rate o f Mn oxide reduction is due to chemical reaction with dissolved Fe2+. In contrast, at the same sites, iron (hydr)oxides are mostly utilized by bacteria for the oxidation of organic matter (dissimilatory iron re duction). As shown also by Canfield et al. (1993a,b), dissimilatory re duction is the principal dissolution pathway for manganese oxides at t he third site. A sensitivity analysis suggests that, for given deposit ion fluxes of reactive Fe and Mn, the competition between dissimilator y and nondissimilatory metal reduction pathways depends primarily on t he total carbon oxidation rate and the intensity of porewater irrigati on. The simulations also highlight the importance of adsorption-desorp tion of Fe(II) and Mn(II) in the redox cycling of the metals, as well as their impact on porewater alkalinity and pH. Based on the calculate d rate distributions, detailed budgets of Fe, Mn, and O-2 in the sedim ents are presented. The model-calculated benthic exchange fluxes of so lutes are dominated by irrigation. For nitrate, molecular diffusion an d irrigation cause fluxes in opposite directions. As a result, there i s a net transfer of nitrate from the water column to the sediments, al though the interfacial porewater gradients predict diffusional fluxes out of the sediments. A significant fraction of the benthic Fe and Mn fluxes to the bottom waters may be due to desorption at the water-sedi ment interface.