Estimating Production and Consumption of Solid Reactive Fe Phases in Marine Sediments from Concentration Profiles

1D diffusion models may be used to estimate rates of production and consumption of dissolved metabolites in marine sediments but are applied less often to the solid phase. Here we used a numerical inverse method assuming steady state conditions to estimate solid phase Fe(III) and Fe(II) consumption and production rates, respectively, from sediment concentration profiles. First, porewater concentration profiles of dissolved inorganic carbon (DIC) and ammonium (NH₄⁺) in muddy northern Gulf of Mexico sediments were analyzed by the model. The runs were used to fit model concentration outputs to sediment sample data, identify zones of production and consumption, and obtain modeled fluxes across the sediment-water interface that best matched experimentally-obtained flux rates. Modeled fluxes were sensitive to coefficients of sediment irrigation and bioturbation in addition to boundary concentrations. Assuming diffusion coefficients = 0 for solid phase minerals, drivers for estimating production and consumption rates from solid phase concentrations are then the rates of sedimentation and porewater advection, and coefficients of bioturbation and irrigation. Thus, applying the model with sediment irrigation and bioturbation terms obtained in the DIC and NH₄⁺ runs, and replacing values of porosity with 1-porosity, rates of Fe(II) production closely matched rates of Fe(III) consumption at about 10^-3 nmole cm^-1 sec^-1. High modeled Fe transformations occurred in the upper 3.5 cm of sediment. In addition, porewater Fe²⁺ and dissolved Mn concentration profiles both yielded modeled production in the top 0.5 cm of sediment with consumption below that suggesting a zone of precipitation as sulfide or carbonate minerals. These results demonstrate that simple 1D models can be applied to the solid phase of marine sediments and may additionally be helpful in estimating coefficients of sediment irrigation and bioturbation.