Mechanics of methane bubbles in consolidated aquatic muds

Abstract

Methane (CH₄) is a potent greenhouse gas that has a major impact on Earth's climate. CH₄ is accommodated in discrete bubbles in aquatic muds, whose sizes greatly exceed the pore size of the hosting sediment. This critical review examines the mechanics of CH₄ gas in consolidated aquatic muds at the scale of a single bubble and at a macroscale of gassy sediments, obtained from lab experiments, field observations, and numerical and analytical modeling. Linear elastic fracture mechanics (LEFM) theory is shown to control the single bubble shape, size, morphology, and inner pressure evolution over its entire life cycle. Reviewed implications focus on the effects of the inner bubble pressure on its solute exchange with ambient pore waters; on the dynamic water load effect (e.g., waves, tides) on the bubble growth rate and its release from sediment into the water column; and on competitive bubble pair growth in the aquatic muds, the process that presumably shapes the bubble size distribution pattern in muds. Alternatively, gassy sediment effective mechanical and physical characteristics and effective gassy media theories are examined at the macroscale, which makes them suitable for remote sensing acoustic applications. This review indicates, however, that most of the developed macroscale effective medium theories rely on the cumulative sediment gas content. Moreover, no theory for proper upscaling of the entire set of the microscale single bubble descriptors addressed in this review – bubble size distribution, their orientations and spatial locations, and inner bubble pressures – to the effective medium mechanical properties of gassy muds, exists. This review will serve, therefore, as a basis for the improved upscaling, while preserving the basic microscale bubble descriptors, their growth physics, and controls. Laying this foundation will enhance the accuracy of the acoustic applications. Improved assessment of sediment gas retention based on this upscaling will contribute to geohazard prediction and should reduce a long-persisting uncertainty related to CH₄ fluxes from the aquatic sediments.