Observing the evolution and fate of free methane in the ocean Free

Free methane gas is increasingly observed escaping the seabed in the world’s oceans from sources that are either biogenic, typically in shallow sediments, or from deeper geologic reservoirs. Methane gas bubbles undergo a complicated journey as they rise toward the sea surface. Some or all of the methane may pass through the gas-liquid boundary, into aqueous solution, where it is eventually oxidized and can impact ocean chemistry. Some of the methane, particularly in shallow environments, may reach the atmosphere where it acts as a strong greenhouse gas. One of the key questions regarding the transport of methane upward from the seabed is how much goes where, and this question is being increasingly addressed using acoustic remote sensing techniques. Answering this question begins with seep detection and localization, now routinely performed on data collected with split-beam and multibeam echo sounders. Once located, observations of the bubble-plume backscattering cross section can be used to address questions of flux and vertical gas transport. Both narrow- and broad-band techniques and some of the associated challenges, including wobbly bubbles and multiple scattering in dense plumes, will be discussed.