Award: PLR-1023600

The second-most important anthropogenic greenhouse gas on a century timescale after carbon dioxide, CO2 is methane, CH4, After nearly stabilizing, atmospheric concentrations are increasing again, although the underlying reasons remain poorly understood. Currently, Arctic global warming is the strongest, where warming Arctic temperatures release CH4 sequestered in and under terrestrial and marine submerged permafrost. Based on sonar data collected and analyzed in this project, the vast CH4 deposits under the East Siberian Arctic Shelf (ESAS) are destabiling over extensive areas, likely covering thousands of square kilometers, yet assessing these emissions is challenged by a lack of robust rapid survey technologies, with multibeam sonar (MBS) the most promising. In this study, we analyzed the results of in situ calibration experiments conducted in the ESAS with respect to the evolution of bubble plume MBS return from rising engineered bubble plumes spanning a broad range of flow rates, and infer the relative importance of small and large bubbles to sonar return signatures. Flows span typical seepage flows and analysis demonstrated that bubble-bubble acoustic interactions are non-negligible for the first 15 m of rise at least. Novel in this study was that the sonar return at different heights above the seabed was considered, This revealed in the data that non-linear bubble-bubble acoustic interactions inside the bubble plume prevent a simple inversion of total sonar signal return signal by assuming a size distribution and summing the individual bubble returns to derive a flux - the linear model, as has been applied by some researchers. For example, a flow doubling from 0.02 to 0.04 L/s increased sonar return by 15 db, or 9 db greater than the linear model prediction of 6 db - a bias of ~300%. Instead of the linear model, a depth dependent calibration was applied to quantify in situ sonar observations of three nearby areas of active natural bubble seepage in the northern Kara Sea in the ESAS. Because the calibration bubble plumes and seep bubble plumes were different gases and from depths with slightly different temperature profiles, bubble dissolution rates are different – i.e., for the same seabed mean volume flux, the depth window averaged volume fluxes are different. We make a first attempt to correct for this factor by applying a numerical bubble-plume model initialized with a typical seep bubble plume size distribution to the two bubble flows (calibration and natural seepage). Applications of the derived seepage-maps to address geologic control questions then were demonstrated by identifying common spatial structural features which likely related to relic-thaw lakes and reiver valleys, Thus, this study demonstrated advances in the use of sonar to map Arctic methane emissions, and also highlighted important challenges. Last Modified: 03/04/2016 Submitted by: Ira S Leifer