[1] An understanding of the mechanics of bubble rise in sediments is essential because of the role of bubbles in releasing methane to the atmosphere and the formation and melting of gas hydrates. Past models to describe and predict the rise of other buoyant geological bodies through a surrounding solid (e.g., magmas and hydrofractures) appear not to be applicable to bubbles in soft sediments, and this paper presents a new model for gas bubble rise in soft, fine-grained, cohesive sediments. Bubbles in such sediments are essentially "dry" (little if any free water) and grow through a process of elastic expansion and fracture that can be described using the principles of linear elastic fracture mechanics, which assume the existence of a spectrum of flaws within the sediment fabric. By extending this theory, we predict that bubbles initially rise by preferential propagation of a fracture in a (sub) vertical direction. We present a criterion for initial bubble rise. Once rise is initiated, the speed of rise is controlled by the viscoelastic response of the sediments to stress. Using this new bubble rise model, we estimate rise velocities to be of the order of centimeters per second. We again show that capillary pressure plays no substantive role in controlling bubble growth or rise.

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[1] Methane is a strong greenhouse gas, and marine and wetland sediments constitute significant sources to the atmosphere. This flux is dominated by the release of bubbles, and quantitative prediction of this bubble flux has been elusive because of the lack of a mechanistic model. Our previous work has shown that sediments behave as elastic fracturing solids during bubble growth and rise. We now further argue that bubbles can open previously formed, partially annealed, rise tracts (fractures) and that this mechanism can account for the observed preferential release at low tides in marine settings. When this mechanical model is applied to data from Cape Lookout Bight, NC (USA), the results indicate that methanogenic bubbles released at this site do indeed follow previously formed rise tracts and that the calculated release rates are entirely consistent with the rise of multiple bubbles on tidal time scales. Our model forms a basis for making predictions of future bubble fluxes from warming sediments under the influence of climate change. Citation:

[1] Bubbles in sediments, imaged via Computed Tomography (CT) scanning, and in surrogate transparent material (gelatin), are well-described geometrically as eccentric oblate spheroids. While sediments are undoubtedly visco-elasto-plastic solids, only part of that complex behavior appears to influence significantly the formation and shape of gas bubbles. Specifically, the shape of these bubbles can be explained if the mechanical response of fine-grained sediment is approximated by Linear Elastic Fracture Mechanics (LEFM). To determine the adequacy of the LEFM approximation for gas bubble growth in fine-grained sediments, a number of gas bubbles were injected and grown in natural sediments, while monitoring the size and shape using an industrial CT scanner. A comparison of measured inverse aspect ratios (IARs) of the injected bubbles with calculated IARs from pressure records provides support for the LEFM theory. Deviations from LEFM are observable in the data, but as bubbles grow larger they trend more closely toward the theory. The use of LEFM has been shown to describe gas bubble growth in shallow coastal sediments to first order.

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