Assessing marine gas emission activity and contribution to the atmospheric methane inventory: A multidisciplinary approach from the Dutch Dogger Bank seep area (North Sea)

Abstract

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.

Nonseismic Signals in the Ocean: Indicators of Deep Sea and Seafloor Processes on Ocean‐Bottom Seismometer Data

“…Depending on the gas injection rate, Varas et al () also described two modes of fluid emission: a bubbling regime where large, individual bubbles are formed and an open‐channel regime, where small bubbles are continuously growing. In time, a cold seep can also change from a mode of intense emission to a more quiet mode (Römer et al, ), under the influence of an earthquake (Tsang‐Hin‐Sun et al, ) or tides (Hsu et al, ; Römer et al, ). This temporal pattern in the mode of occurrence of SDEs was also reported offshore Taiwan (Hsu et al, ), in Svalbard (Franek et al, ), or in the Galicia Margin (Diaz et al, ).…”

“…During the 3 weeks of the experiments, although many earthquakes occurred, none could be associated with a significant increase in the number of SDEs (see supporting information figure). The relationship between seismicity and gas released is not systematic and depends on many factors, including the seismic energy received, the connection to the source of gas, and local azimuthal effects (e.g., Römer et al, ; Figure ).…”

Amount and Fate of Gas and Oil Discharged at 3400 m Water Depth From a Natural Seep Site in the Southern Gulf of Mexico

“…The gas volume emission rates calculated above indicate that TYK is an important source of methane to the hydrosphere, as its mass output is on the upper limit of other natural seep environments quantified worldwide. For example, Hydrate Ridge was found to emit 2.2 × 10 7 mol CH 4 /yr (Torres et al, 2002), the Haakon Mosby mud volcano ∼1.9 × 10 7 mol CH 4 /yr (Sauter et al, 2006) or the Dutch Dogger Bank seep area 1.7-3.7 × 10 7 mol CH 4 /yr (Römer et al, 2017). Anthropogenic disasters such as the Deepwater Horizon blowout released oil as well as up to 1 × 10 10 moles of methane within 83 days (Kessler et al, 2011) and the UK22/4b blowout in the North Sea still releases 5400 L/min (or 2 × 10 9 mol/yr), even though the accident happened already 20 years before this estimation in 1990 (Leifer and Judd, 2015).…”

Modern Assessment of Natural Hydrocarbon Gas Flux at the Coal Oil Point Seep Field, Santa Barbara, California

“…Incomplete spatial coverage is a limitation for many direct measurement instruments (i.e., Flux Buoy and BCD) and for narrow beam acoustic transducers due to their small cross‐sectional area or small beam widths, respectively. Recent studies on gas flow in the ocean have attempted to address the limited coverage in acoustic surveys by using multibeam echo sounders, which can cover large spatial areas compared to narrow beam echo sounders and direct measurement devices (Römer et al, ).…”

“…Previous studies have exploited acoustic techniques to observe and map natural hydrocarbon seeps in Coal Oil Point and other ocean basins (Greinert et al, ; Heeschen et al, ; Hornafius et al, ; Leifer et al, ; Quigley et al, ; Römer et al, ; Weber et al, ). The acoustic volume backscattering strength, S v , which is the decibel equivalent of the volume backscatter coefficient in a unit volume, has been used previously to estimate gas and oil flux emissions (Nikolovska et al, ; Weber et al, ).…”