Biogeochemical processes controlling methane in gassy coastal sediments—Part 1. A model coupling organic matter flux to gas production, oxidation and transport

A new kinetic model has been developed for predicting biogeochemical processes occurring in gassy, anoxic sediments dominated by sulfate reduction (SR), methane production (MP), and methane oxidation (MO). The model is composed of mass conservation equations in which reaction rates are balanced by diffusive and advective transport. It directly couples bio-geochemical zones using error functions that serve as a toggle to simulate cessation of sulfate reduction, initiation of methane production and oxidation, and production of gaseous methane when in situ solubility is exceeded. Model-derived sulfate and methane concentration distributions combined with kinetic rate expressions are used to calculate rates of SR, MP and MO. Application of the model to gassy coastal and estuarine sediments reveals the extreme sensitivity of predicted methane distributions to the flux (FG) and degradation rate constant (kG) of reactive organic matter. Sulfate and methane concentrations from Eckernförde Bay in the Kiel Bight of the German Baltic Sea, and Cape Lookout Bight and the White Oak River Estuary of North Carolina, USA, can be predicted accurately from independently determined (FG) values. Comparison of model-predicted results with a complete set of measured summertime concentration and rate data from the Cape Lookout site shows that introduction of 10–40% variations in individual rate parameters produce readily observable discrepancies in model results. In general, increases in the magnitude of FG and decreases in kG at the same total sediment accumulation rate increase the relative importance of methanogenesis in total organic matter remineralization as a result of more rapid depletion of dissolved pore water sulfate closer to the sediment–water interface. The predictive capabilities of the model should prove useful when concentration, rate, or flux measurements are not available.