Quantitative spatiotemporal characterization of methane venting from lake sediments

Monday, 15 December 2014
Benjamin Scandella1, Liam Pillsbury2, Thomas Weber3, Carolyn D Ruppel4, Harry Hemond5 and Ruben Juanes1, (1)Massachusetts Institute of Technology, Cambridge, MA, United States, (2)University of New Hampshire Main Campus, Durham, NH, United States, (3)University of New Hampshire Main Campus, Mechanical Engineering, Durham, NH, United States, (4)US Geological Survey, Woods Hole, MA, United States, (5)Mass. Institute of Technology, Cambridge, MA, United States
Methane is a potent greenhouse gas, and the production and emission of methane from sediments in inland waters and shallow oceans both contributes to and may be exacerbated by climate change. In some of these shallow-water settings, methane fluxes are often controlled by episodic free-gas venting. The fraction of the methane released from the sediments that bypasses dissolution in the water column and reaches the atmosphere impacts the magnitude of the climate forcing, and this fraction depends critically on the mode and spatiotemporal characteristics of the bubble releases. The spacing and persistence of the gas vents may be determined by the heterogeneity of the methane source, but within regions of uniform methanogenesis they arise from the competition between mechanisms driving lateral and vertical transport of methane in the sediments. Here, we present measurements of the spacing, persistence and variability in intensity of methane vents within a wide area of lake sediments (~400 m2) and over a multi-month period. The measurements were made using a fixed-location Imagenex DeltaT 837B multibeam sonar, which was calibrated to quantify gas fluxes with unprecedented spatial and temporal resolution (~0.5 m, 6 Hz). Drops in hydrostatic pressure were a characteristic trigger for the sonar-detected ebullition events, and the episodicity of the fluxes is reproduced with a mechanistic numerical model of methane venting through dynamic conduits that dilate in response to hydrostatic unloading. The spatial characteristics of the sonar-detected vents inform conceptual and mathematical models of methane transport and release from deformable sediments, as well as the uncertainty associated with upscaling. Taken together, these results point towards a better understanding of the microscale processes controlling methane venting from deformable sediments, as well as their impact on large-scale methane fluxes from shallow-water bodies. Figure: Top: time series of daily sonar-estimated gas fluxes over the entire observation area (black bars and circles) from March 1 - May 18, 2014, compared with a mechanistic, numerical model (orange) that is driven by fluctuations in hydrostatic pressure (blue, rescaled to fit). Bottom: maps of fluxes at 0.5 m resolution within the sonar fan, for March 13 and 14.