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Methane gas bubbles emitted from the seafloor transport methane through the water column. Methane transport is important to track because of the substantial role methane plays in biological processes and in the Earth’s climate system. Existing models used to predict methane transport based on methane dissolution rates from rising bubbles generally estimate bubble rise velocity from the bubble’s volume. This standard theoretical approach relies on equations that parameterize the dynamic interaction between volume, shape, and rise velocity for flexible-walled bubbles. These equations hold for bubbles rising in pure water, but when gas bubbles are emitted from sediment at water depths exceeding ~300-500 meters, the combination of elevated pressure and low bottom water temperatures can lead to a solid gas hydrate forming on the bubble surface. The solid gas-hydrate coating can create a rigid bubble surface, which prevents the usual dynamic interaction between bubble volume, shape and rise velocity and reduces the bubble’s rise velocity. To better understand how a hydrate coating on the bubble surface affects the bubble’s rise velocity, the U.S. Geological Survey conducted controlled laboratory experiments in which calibrated, high-speed imagery was used to measure the rise velocity of individual hydrate-free and hydrate-coated gas bubbles. Xenon was the hydrate-forming gas used because xenon hydrate has the same structure as methane hydrate, but forms at low enough pressures that rise-velocity experiments can be run in transparent acrylic cylinders. Estimates of the equivalent diameter, de, which is the diameter of a spherical bubble with the same volume as the observed bubble, ranged from 0.8 to 12 millimeters (mm), whereas the aspect ratio (major axis length divided by minor axis length) of the generated gas bubbles ranged from 1.04 to 2.17 mm. The rise-velocity data show that hydrate-coated gas bubbles generally rise slower than hydrate-free bubbles of the same volume. For hydrate-coated bubbles with de between 4–7 mm, the average rise velocity is 19.8 centimeters per second (cm/s) or 10 percent slower than hydrate-free bubbles of the same volume. Rise velocities for hydrate-free and hydrate-coated bubbles also diverge as the aspect ratio increases. For hydrate-free bubbles, the rise velocity increases with increasing aspect ratio, as is anticipated for bubbles in clean systems (bubbles with flexible interfaces). In contrast, hydrate-coated bubble rise velocities decrease as their aspect ratio increases. Measurable differences between hydrate-free and hydrate-coated bubble rise velocities indicate that the existing clean-bubble rise velocity parameterizations currently used in methane dissolution modelling need to be carefully examined before they are used to predict the bubble rise behavior of rigid, hydrate-coated bubbles. [More]
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