Mogollón, J. M., Dale, Andy W. , L'Heureux, I. and Regnier, P. (2011) Impact of seasonal temperature and pressure changes on methane gas production, dissolution, and transport in unfractured sediments. Journal of Geophysical Research: Biogeosciences, 116 (G3). G03031. DOI 10.1029/2010JG001592.

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Abstract

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.

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