Observations from controlled experiments on the dissolution of free-gas bubbles and hydrate-coated bubbles in water

Methane is released into the ocean from the seafloor in the form of gas bubbles, which transport methane through the water column and potentially into the atmosphere. A rising methane bubble loses methane to the surrounding water via methane dissolution through the bubble surface. When methane gas bubbles are released at certain elevated pressure and reduced temperature conditions, however, they can begin to form a solid gas hydrate that coats the bubble surface. Hydrate on the bubble surface slows down the dissolution process and can extend the lifetime of the bubble within the water column. Researchers have developed mathematical models to track the fate of methane transported in rising bubbles in order to understand how much methane dissolves into the water column as a function of depth. Tracking methane in this way is important because dissolved methane contributes to ocean acidification and reduced dissolved oxygen levels. Controlled laboratory experiments with a counter-flow device were conducted to test existing dissolution models for rising gas bubbles. The pressurized counter-flow device was used to trap gas bubbles (air or xenon) and simulate the dissolution process of free-gas and hydrate-coated gas bubbles in water. Xenon was used as a proxy for methane during this experiment because though both gases form structure I hydrate, xenon hydrate can be formed at relatively lower pressures, allowing the counter-flow device to be made of clear acrylic for multi-directional viewing of the bubble. Two high-speed, high-resolution imaging cameras arranged at 90 degrees to each other were used to obtain information on bubble sizes, shapes and shrinkage rates in order to assess dissolution model predictions for the fate of the bubbles in the ocean.