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Methane gas-phase dynamics in marine sediments: A model study

José M. Mogollón^*,, Ivan L'Heureux^**, Andrew W. Dale^* and Pierre Regnier^*,***

^* Department of Earth Sciences-Geochemistry, Faculty of Geosciences, Utrecht University, P.O. Box 80021, 3508TA Utrecht, The Netherlands
^** Department of Physics, University of Ottawa, Macdonald Hall, 150 Louis Pasteur, Ottawa, Ontario, Canada K1N 6N5
^*** Department of Earth and Environmental Sciences, CP 160/02, Université Libre de Bruxelles, 50 Av. F. D. Roosevelt, B-1050 Brussels, Belgium

Corresponding author: mogollon{at}geo.uu.nl

A one-dimensional reactive transport model including mass, momentum and volume conservation for the solid, aqueous, and gaseous phases is developed to explore the fate of free methane gas in marine sediments. The model assumes steady–state compaction for the solid phase in addition to decoupled gas and aqueous phase transport, instigated by processes such as buoyancy, externally impressed flows and compaction. Chemical species distributions are governed by gas advection, dissolved advection and diffusion as well as by reaction processes, which include organoclastic sulfate reduction, methanogenesis and anaerobic oxidation of methane (AOM). The model is applied to Eckernförde Bay, a shallow-water environment where acoustic profiles confirm a widespread occurrence of year-round biogenic free methane gas within the muddy regions of the sediment, and where subsurface methanogenesis, overlaid by a zone of AOM has been reported. The model results reveal that, under steady-state conditions, upward gas migration is an effective methane transport mechanism from oversaturated to undersaturated intervals of the sediment. Furthermore, sensitivity tests show that when methanogenesis rates increase, the gas flux to the AOM zone becomes progressively more important and may reach values comparable to those of the aqueous methane diffusive flux. Nevertheless, the model also proves that the gas transport rates always remain smaller than the removal rates by combined gaseous methane dissolution and oxidation. Consequently, for the range of environmental conditions investigated here, the AOM zone acts as an efficient subsurface barrier for both aqueous and gaseous methane, preventing methane escape from the sediments to the water column.