CHARACTERIZING AND QUANTIFYING MARINE METHANE GAS SEEPS USING ACOUSTIC OBSERVATIONS AND BUBBLE DISSOLUTION MODELS

A method for characterizing and quantifying marine methane gas seeps along the U.S. Western Atlantic Margin 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 meters. 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 seafloor. Profiles of the depth-dependent target strength and scattering strength were compared to models of the evolution of rising bubbles to help constrain the ultimate fate of the methane gas. To do so, a refined methodology was developed that decoupled the target strength of a bubble plume from the inherent background noise and reverberation in the ocean. This methodology was particularly useful for acoustically weak (i.e. low signal-to-noise ratio) seeps, and for examining the acoustic trends of seeps as their echo signature approached background noise levels. Comparisons of target strength profiles to models of bubble dissolution demonstrated that the parameters used in the model (e.g. gas transfer rate) are consistent with empirical observations

Ephemerality of discrete methane vents in lake sediments

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.National Science Foundation (U.S.) (1045193)United States. Department of Energy (DE-FE001399