ALBERT

Description: 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.

Description: A one‐dimensional reaction‐transport model is used to investigate the dynamics of methane gas in coastal sediments in response to intra‐annual variations in temperature and pressure. The model is applied to data from two shallow water sites in Eckernförde Bay (Germany) characterized by low and high rates of upward fluid advection. At both sites, organic matter is buried below the sulfate‐reducing zone to the methanogenic zone at sufficiently high rates to allow supersaturation of the pore water with dissolved methane and to form a free methane gas phase. The methane solubility concentration varies by similar magnitudes at both study sites in response to bottom water temperature changes and leads to pronounced peaks in the gas volume fraction in autumn when the methanic zone temperature is at a maximum. Yearly hydrostatic pressure variations have comparatively negligible effects on methane solubility. Field data suggest that no free gas escapes to the water column at any time of the year. Although the existence of gas migration cannot be substantiated by direct observation, a speculative mechanism for slow moving gas is proposed here. The model results reveal that free gas migrating upward into the undersaturated pore water will completely dissolve and subsequently be consumed above the free gas depth (FGD) by anaerobic oxidation of methane (AOM). This microbially mediated process maintains methane undersaturation above the FGD. Although the complexities introduced by seasonal changes in temperature lead to different seasonal trends for the depth‐integrated AOM rates and the FGD, both sites adhere to previously developed prognostic indicators for methane fluxes based on the FGD.