Detailed dynamics and seasonal persistence of methane venting from lakes

Lake-bottom sediments emit methane, a potent greenhouse gas, into the overlying water column and atmosphere. A large fraction of the methane is released as bubbles, but constraining the magnitude of this methane flux is challenging because ebullition is patchy in space and episodic in time. Extrapolating observations from individual methane seeps to a larger scale in time or space can result in severe over- or under-estimation of the methane flux, yet to date observations have not combined large, complete spatial coverage with multiple-season deployment periods. We present methane ebullition data from a fixed-location multibeam sonar, which observes a large area (420 m2) over a deployment period of over 6 months and with sufficient spatiotemporal resolution to detect individual bubbles. The large amount of data generated by the system presents a challenge to identify bubble signals that are infrequent, short in duration, and spatially compact. Addressing this challenge yields processed ebullition signals, which are compared against vents detected in the water column and near-surface sediment during geophysical surveys that utilize a commercial fishfinder sonar and a 4-24 kHz chirp seismic towfish. The ebullition signals are then used to develop conceptual models relating distributed methanogenesis to ebullition at localized sites. In particular, the spacing and persistence of vents implies potential mechanisms for their creation and maintenance, while the ebullitive response to hydrostatic pressure variations is used to validate a conduit dilation model of methane venting. Finally, the level of synchronicity in activity between distant venting sites suggests the relative importance of the external hydrostatic forcing over internal dynamics of methane generation. The mechanistic understanding provided by this work is critical to upscaling gas flow measurements from individual vents to infer lake-wide fluxes to the water column and atmosphere. Map of maximum sonar intensity values observed over a 10-minute period in April 2012, where high-intensity regions indicate potential methane ebullition. Axes show lateral distance from the sonar head in meters.