BSD from our visual observations (McGovern 2012) 45.26 All bubbles same size (diameter:6 mm, most frequent value of our BSD) 27.60 BSD from Ostrovsky et al. (2008) 27.11 BSD from Sahling et al. (2009) 103.15 BSD from Römer et al. (2011) 34.13 Mean 47.45 Standard deviation 31.99 Relative standard deviation (%) AE67.42

Quantifying marine methane fluxes of free gas (bubbles) from the seafloor into the water column is of importance for climate related studies, for example, in the Arctic, reliable methodologies are also of interest for studying man-made gas and oil leakage systems at hydrocarbon production sites. Hydroacoustic surveys with singlebeam and nowadays also multibeam systems have been proven to be a successful approach to detect bubble release from the seabed. A number of publications used singlebeam echosounder data to indirectly quantify free gas fluxes via empirical correlations between gas fluxes observed at the seafloor and the hydroacoustic response. Others utilize the hydroacoustic information in an inverse modeling approach to derive bubble fluxes. Here, we present an advanced methodology using data from splitbeam echosounder systems for analyzing gas release water depth (> 100 m). We introduce a new MATLAB-based software for processing and interactively editing data and we present how bubble-size distribution, bubble rising speed and the model used for calculating the backscatter response of single bubbles influence the final gas flow rate calculations. As a result, we highlight the need for further investigations on how large, wobbly bubbles, bubble clouds, and multi-scattering influence target strength. The results emphasize that detailed studies of bubble-size distributions and rising speeds need to be performed in parallel to hydroacoustic surveys to achieve realistic mediated methane flow rate and flux quantifications.

Widespread gas venting along the Cascadia margin is investigated from acoustic water column data and reveals a nonuniform regional distribution of over 1100 mapped acoustic flares. The highest number of flares occurs on the shelf, and the highest flare density is seen around the nutrition-rich outflow of the Juan de Fuca Strait. We determine ∼430 flow-rates at ∼340 individual flare locations along the margin with instantaneous in situ values ranging from ∼6 mL min−1 to ∼18 L min−1. Applying a tidal-modulation model, a depth-dependent methane density, and extrapolating these results across the margin using two normalization techniques yields a combined average in situ flow-rate of ∼88 × 106 kg y−1. The average methane flux-rate for the Cascadia margin is thus estimated to ∼0.9 g y−1m−2. Combined uncertainties result in a range of these values between 4.5 and 1800% of the estimated mean values.

We present a comprehensive study showing new results from a shallow gas seep area in ∼40 m water depth located in the North Sea, Netherlands sector B13 that we call “Dutch Dogger Bank seep area.” It has been postulated that methane presumably originating from a gas reservoir in ∼600 m depth below the seafloor is naturally leaking to the seafloor. Our ship‐based subbottom echosounder data indicate that the migrating gas is trapped in numerous gas pockets in the shallow sediments. The gas pockets are located at the boundary between the top of the Late Pliocene section and overlying fine‐grained sediments, which were deposited during the early Holocene marine transgression after the last glaciation. We mapped gas emissions during three R/V Heincke cruises in 2014, 2015, and 2016 and repeatedly observed up to 850 flares in the study area. Most of them (∼80%) were concentrated at five flare clusters. Our repeated analysis revealed spatial similarities of seep clusters, but also heterogeneities in emission intensities. A first calculation of the methane released from these clusters into the water column revealed a flow rate of 277 L/min (SD = 140), with two clusters emitting 132 and 142 L/min representing the most significant seepage sites. Above these two flare clusters, elevated methane concentrations were recorded in atmospheric measurements. Our results illustrate the effective transport of methane via gas bubbles through a ∼40 m water column, and furthermore provide an estimate of the emission rate needed to allow for a contribution to the atmospheric methane concentration.