Simulation of sulfur diagenesis in anoxic marine sediments using rickard kinetics for FeS and FeS 2 formation

Abstract

Steady-state diagenetic profiles of reduced sulfur species (H 2S, FeS, FeS 2, and S°) have been modeled using Rickard's kinetic rate laws for goethite sulfidation and pyrite formation. The system of coupled, nonlinear differential equations that describe the diagenesis of sulfur is solved by numerical techniques in conjunction with a constrained, trial and error procedure for determining the correct gradient of H 2S at the sediment-water interface. The behavior of the model is sensitive to the values selected for the initial concentration of goethite and the particle sizes of goethite, FeS, and S°. Using trial and error we have been able to select values for these parameters that produce simulations similar to the profiles measured by the FOAM group in Long Island Sound sediments. Good fits between the model and the FOAM data, however, require a strong decrease with depth in the specific molar surface area of goethite, especially if available iron exceeds the limit to pyrite formation imposed by exhaustion of sulfate or organic matter. These model experiments suggest that Rickard's rate laws are an adequate base upon which to build a model for sulfur diagenesis. However, a truly comprehensive model for sulfur diagenesis will require incorporation of effects due to particle mixing, interaction of reduced sulfur with oxygen and variation of iron reactivity with depth. Calibration and validation of the present model or more complex models of the future will require data sets that include measurements of the molar surface areas of goethite, FeS, and S°. Without this information it is unlikely that a thorough and quantitative understanding of the vertical profiles of H 2S and FeS in marine sediments can be achieved.