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 proces ses occurring in gassy, anoxic sediments dominated by sulfate reduction (SR ), methane production (MP), and methane oxidation (MO). The model is compos ed of mass conservation equations in which reaction rates are balanced by d iffusive and advective transport. It directly couples biogeochemical zones using error functions that serve as a toggle to simulate cessation of sulfa te reduction, initiation of methane production and oxidation, and productio n of gaseous methane when in situ solubility is exceeded. Model-derived sul fate and methane concentration distributions combined with kinetic rate exp ressions are used to calculate rates of SR, MP and MO. Application of the m odel to gassy coastal and estuarine sediments reveals the extreme sensitivi ty of predicted methane distributions to the flux (F-G) and degradation rat e constant (k(G)) of reactive organic matter. Sulfate and methane concentra tions from Eckernforde Bay in the Kiel Eight of the German Baltic Sea, and Cape Lookout Eight and the White Oak River Estuary of North Carolina, USA, can be predicted accurately from independently determined (F-G) values. Com parison of model-predicted results with a complete set of measured summerti me concentration and rate data from the Cape Lookout site shows that introd uction of 10-40% variations in individual rate parameters produce readily o bservable discrepancies in model results. In general, increases in the magn itude of F-G and decreases in k(G) at the same total sediment accumulation rate increase the relative importance of methanogenesis in total organic ma tter remineralization as a result of more rapid depletion of dissolved pore water sulfate closer to the sediment-water interface. The predictive capab ilities of the model should prove useful when concentration, rate, or flux measurements are not available. (C) 1998 Elsevier Science Ltd. All rights r eserved.