Methane is a potent greenhouse gas whose emission from sediments in inland waters and shallow oceans may both contribute to global warming and be exacerbated by it. The fraction of methane emitted by sediments that bypasses dissolution in the water column and reaches the atmosphere as bubbles depends on the mode and spatiotemporal characteristics of venting from the sediments. Earlier studies have concluded that hot spots—persistent, high‐flux vents—dominate the regional ebullitive flux from submerged sediments. Here the spatial structure, persistence, and variability in the intensity of methane venting are analyzed using a high‐resolution multibeam sonar record acquired at the bottom of a lake during multiple deployments over a 9 month period. We confirm that ebullition is strongly episodic, with distinct regimes of high flux and low flux largely controlled by changes in hydrostatic pressure. Our analysis shows that the spatial pattern of ebullition becomes homogeneous at the sonar's resolution over time scales of hours (for high‐flux periods) or days (for low‐flux periods), demonstrating that vents are ephemeral rather than persistent, and suggesting that long‐term, lake‐wide ebullition dynamics may be modeled without resolving the fine‐scale spatial structure of venting.
A method for characterizing and quantifying marine methane gas seeps along the Western Atlantic Margin (U.S. East Coast) was developed and applied to 70 free-gas seeps observed by the R/V Okeanos Explorer in 2012 and 2013, in water depths ranging from 300–2000 m. Acoustic backscatter from an 18 kHz split-beam echo sounder and a 30 kHz multi-beam echo sounder provided information on the height to which the gas seeps rose from the seabed. Profiles of the depth-dependent target strength and scattering strength per unit depth were generated from the acoustic data. These profiles were compared to models of the evolution of rising bubbles in order to help constrain the ultimate fate of the bubbles. Of particular interest are comparisons of profiles for seeps originating below, at, and above the gas hydrate stability zone.
Naturally occurring methane bubbles in the ocean are often observed to have ellipsoidal or otherwise deformed shapes. Models of acoustic scattering from non-spherical gas bubbles suggest changes in resonance frequency and scattering strength relative to spherical gas bubbles with identical volumes. These changes potentially confound our ability to easily relate measurements of acoustic backscatter from gas bubbles to their size and quantity. To help quantify the magnitude of this effect, we have conducted a series of laboratory measurements of acoustic backscatter from non-spherical air bubbles rising from the bottom of a 6 m deep test tank. The acoustic measurements were made at frequencies between 10 kHz to 150 kHz, well above the frequency of bubble resonance (as is often the case for measurements of methane bubbles in shallow coastal environments). Laboratory measurements of bubbles with different sizes and deformations are compared with models for spherical bubbles with identical volumes.
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