Description: Highlights • Application of mobile underwater in situ gamma-ray spectroscopy. • Localization of pockmarks emanating groundwater. • Radon progeny 214Bi proved an efficient radiotracer for localization purposes. • Potassium 40K is suggested as additional to radon radiotracers to localize fluid emanation areas whenever sediment is in mixture with the fluid or resuspension of sediment occurs. Abstract Eckernförde Bay in the Baltic Sea is well-known for the pockmarks areas which are located in the centre and off the southern shore-line of the bay emanating groundwater in a non-continuous but episodic way. Mobile underwater in situ gamma-ray spectroscopy is exploited proving that both 214Bi and 40K are efficient radiotracers for localization of seepage areas whenever either sediment is in mixture with the emanating fluid or resuspension of surface sediment occurs as a side effect of the fluid emanation.

Description: Seafloor seepage of hydrocarbon-bearing fluids has been identified in a number of marine forearcs. However, temporal variations in seep activity and the structural and tectonic parameters that control the seepage often remain poorly constrained. Subduction-zone earthquakes for example, are often discussed to trigger seafloor seepage but causal links that go beyond theoretical considerations have not yet been fully established. This is mainly due to the inaccessibility of offshore epicentral areas, the infrequent occurrence of large earthquakes, and challenges associated with offshore monitoring of seepage over large areas and sufficient time periods. Here, we report visual, geochemical, geophysical, and modelling results and observations from the Concepción Methane Seep Area (offshore Central Chile) located in the rupture area of the 2010 Mw. 8.8 Maule earthquake. High methane concentrations in the oceanic water column and a shallow sub-bottom depth of sulfate penetration indicate active methane seepage. The stable carbon isotope signature of the methane and hydrocarbon composition of the released gas indicate a mixture of shallow-sourced biogenic gas and a deeper sourced thermogenic component. Pristine fissures and fractures observed at the seafloor together with seismically imaged large faults in the marine forearc may represent effective pathways for methane migration. Upper-plate fault activity with hydraulic fracturing and dilation is in line with increased normal Coulomb stress during large plate-boundary earthquakes, as exemplarily modelled for the 2010 earthquake. On a global perspective our results point out the possible role of recurring large subduction-zone earthquakes in driving hydrocarbon seepage from marine forearcs over long timescales.

Description: Highlights • First study using long-term passive acoustic monitoring of methane seeps at well blowout site 22/4b. • Seep acoustic temporal variations correlated with ocean tides. • Major acoustic transient event recorded on 8 December 2011 with high temporal resolution. Abstract Marine seeps produce underwater sounds as a result of bubble formation and fragmentation upon emission from the seabed. The frequency content and sound levels of these emissions are related to bubble size distribution and emission flux, providing important information on methane release from the seafloor. Long-term passive acoustic monitoring was used to continuously record seep sounds over a 7-month period within the blowout crater at the abandoned well site, 22/4b, in the central North Sea. Also recorded were water column fluid velocities and near-seafloor water conductivity, temperature, and pressure. Acoustic signatures were primarily from ∼1 to 10 kHz. Key features were relatively broad spectral peaks at about 1.0, 1.5, 2.2, 3.1, 3.6 and 5.1 kHz. Temporal variations in spectral levels were apparently associated with tides. The recordings also documented a series of major episodic events including a large and persistent increase (∼10 dB) in overall sound levels and spectral broadening on 8 December 2011. The acoustic temporal pattern of this event was consistent with other recorded large transient events in the literature, and the major event was correlated with dramatic changes in other measurements, including increased water column fluid velocities, increased pressure and decreased salinity, indicating real changes in emission flux. Observed seabed morphology changes reported elsewhere in this special issue, also likely were related to this event. These data demonstrate the dynamic nature of marine seepage systems, show the value of monitoring systems, and provide direct supporting evidence for a violent formation mechanism of many widespread seep-associated seabed features like pockmarks.

Description: Highlights • High abundance of active anaerobic methanotrophs in sediments of the blowout crater suggests adaptation to methane seepage within at most two decades. • Fast exchange processes in permeable surface sediments prevent sulfate depletion and probably methane-derived carbonate precipitation. • Methane seepage impacts isotopic and assemblage composition of benthic foraminifera. Abstract Methane emissions from marine sediments are partly controlled by microbial anaerobic oxidation of methane (AOM). AOM provides a long-term sink for carbon through precipitation of methane-derived authigenic carbonates (MDAC). Estimates on the adaptation time of this benthic methane filter as well as on the establishment of related processes and communities after an onset of methane seepage are rare. In the North Sea, considerable amounts of methane have been released since 20 years from a man-made gas blowout offering an ideal natural laboratory to study the effects of methane seepage on initially “pristine” sediment. Sediment cores were taken from the blowout crater and a reference site (50 m distance) in 2011 and 2012, respectively, to investigate porewater chemistry, the AOM community and activity, the presence of authigenic carbonates, and benthic foraminiferal assemblages. Potential AOM activity (up to 3060 nmol cm−3 sediment d−1 or 375 mmol m−2 d−1) was detected only in the blowout crater up to the maximum sampling depth of 18 cm. CARD-FISH analyzes suggest that monospecific ANME-2 aggregates were the only type of AOM organisms present, showing densities (up to 2.2*107 aggregates cm−3) similar to established methane seeps. No evidence for recent MDAC formation was found using stable isotope analyzes (δ13C and δ18O). In contrast, the carbon isotopic signature of methane was recorded by the epibenthic foraminifer Cibicides lobatulus (δ13C −0.66‰). Surprisingly, the foraminiferal assemblage in the blowout crater was dominated by Cibicides and other species commonly found in the Norwegian Channel and fjords, indicating that these organisms have responded sensitively to the specific environmental conditions at the blowout. The high activity and abundance of AOM organisms only at the blowout site suggests adaptation to a strong increase in methane flux in the order of at most two decades. High gas discharge dynamics in permeable surface sediments facilitate fast sulfate replenishing and stimulation of AOM. The accompanied prevention of total alkalinity build-up in the porewater thereby appears to inhibit the formation of substantial methane-derived authigenic carbonate at least within the given time window.

Description: Highlights • Mega ebullition of biogenic methane from an abandoned offshore gas well, North Sea. • Evidence for midwater bubble plume intrusion, fallback, and short-circuiting of the plume. • Effective trapping of seabed released methane underneath the thermocline. • First observation of a spiral vortex methane plume and marginal turbulences. • Megaplumes appear less efficient in terms of vertical methane transport than previously thought. Abstract First direct evidence for ongoing gas seepage activity on the abandoned well site 22/4b (Northern North Sea, 57°55′ N, 01°38′ E) and discovery of neighboring seepage activity is provided from observations since 2005. A manned submersible dive in 2006 discovered several extraordinary intense seepage sites within a 60 m wide and 20 m deep crater cut into the flat 96 m deep seafloor. Capture and (isotope) chemical analyses of the gas bubbles near the seafloor revealed in situ concentrations of methane between 88 and 90%Vol. with δ13C–CH4 values around −74‰ VPDB, indicating a biogenic origin. Bulk methane concentrations throughout the water column were assessed by 120 Niskin water samples showing up to 400.000 nM CH4 in the crater at depth. In contrast, concentrations above the thermocline were orders of magnitude lower, with a median value of 20 nM. A dye tracer injection into the gas seeps revealed upwelling bubble and water motion with gas plume rise velocities up to ∼1 ms−1 (determined near the seabed). However, the dissolved dye did not pass the thermocline, but returned down to the seabed. Measurements of direct bubble-mediated atmospheric flux revealed low values of 0.7 ± 0.3 kty−1, much less than current state-of-the-art bubble dissolution models would predict for such a strong and upwelling in situ gas bubble flux at shallow water depths (i.e. ∼100 m). Acoustic multibeam water column imaging data indicate a pronounced 200 m lateral intrusion at the thermocline together with high methane concentration at this layer. A partly downward-orientated bubble plume motion is also visible in the acoustic data with potential short-circuiting in accordance to the dye experiment. This observation could partly explain the observed trapping of most of the released gas below the well-established thermocline in the North Sea. Moreover, 3D analyses of the multibeam water column data reveal that the upwelling plume transforms into a spiral expanding vortex while rising through the water column. Such a spiral vortex motion has never been reported before for marine gas seepage and might represent an important process with strong implication on plume dynamics, dissolution behavior, gas escape to the atmosphere, and is considered very important for respective modeling approaches.

Description: The presence of a seasonal thermocline likely plays a key role in restraining methane released from a seabed source in the deeper water column, thereby inhibiting exchange to the atmosphere. The bubble plume itself, however, generates an upward motion of fluid, e.g. upwelling and may thereby be partially responsible for an early breakdown of the seasonal thermocline. Measurements at site 22/4b, located at (57°550N, 1°380E) in the UK Central North Sea, 200 km east of the Scottish mainland, where gas is still being released since a blow out in 1990, have been used to identify the generation of the seasonal thermocline, and thus, the depth of the upper mixed layer and its breakdown in autumn. Data derived from two landers, containing an Acoustic Doppler Current Profiler and a Conductivity Temperature Depth recorder, were used to determine the mixed layer depth and the breakdown of the thermocline. Mixing of upper layer fluid into the lower layer has been inferred from large amplitude variations in the nearbottom temperature. The ADCPs estimate velocity profiles in four beam directions using Doppler shifted frequency from acoustic pings sent out and received by four different transducers in a specific configuration. Besides that, the intensity of the backscattered sound per transducer is also recorded. Bubbles from the nearby plume contaminate the signal during part of the tidal cycle, but in bubble free periods, the mixed layer depth can be estimated using the acoustic backscatter signal as local maxima. Results show that the thermocline broke down between mid-October and early November, several weeks earlier than the breakdown of the thermocline in nearby/comparable areas, likely caused by bubble-induced downwelling at the site. The early breakdown of the thermocline was accompanied by multiple occurrence of a strong jet-like structure, associated with the seasonal tidal mixing front.

Description: Extremely intense bubble plumes, like the North Sea 22/4b blowout megaplume (defined as more than 10(6) L day(-1)), create very strong upwelling flows (〉1 m s(-1)), which lead to detrainment of methane-enriched water, but leave direct bubble-mediated transport unaffected. Dissolved CH4 depth profiles and atmospheric measurements during a fall 2011 survey of the 22/4b site suggest strong constraint of seabed CH4 below the thermocline. Seabed bubbles were nearly pure CH4. The effect of the upwelling flow on the fate of bubble plume CH4 was investigated with a numerical bubble-propagation model. The model considered different representative bubble plume size distributions, phi, and a global (total) megaplume bubble size distribution, Phi, synthesized from video survey data and phi from the literature. Simulations showed that none of the literature plumes or variations in the upwelling flow could constrain CH4 sufficiently below the thermocline. Two new bubble megaplume processes were simulated, vortical bubble trapping (slow rise) and a hypothesized, enhanced bubble gas exchange, k(BE), an enhancement factor applied to the normal bubble gas exchange rate, k(B). The latter could arise from plume turbulence increasing bubble boundary-layer turbulence and thus its gas exchange. Observations could not be reproduced solely by slow rise, however, simulations with k(BE)similar to 6 reproduced observational constraints, as could weaker k(BE) in conjunction with slow rise. Field validation of k(BE) is needed given its implications for the fate of megaplume CH4 emissions (anthropogenic or natural) for stratified and unstratified conditions. kBE suggests marine CH4 geologic contributions to the atmosphere from all but shallow waters primarily arises from bubble plumes that are less than megaplume size.

Description: Highlights: • MIMS used to quantify the dissolved CH4 inventory around a bubble emission site. • Conservative estimate of well 22/4b seabed CH4 emission was 1.8 ktons yr−1. • Stratification impedes immediate CH4 release into the atmosphere. The dissolved methane (CH4) plume rising from the crater of the blowout well 22/4b in the Central North Sea was mapped during stratified water column conditions. Geochemical surveys were conducted close to the seafloor at 80.3 m water depth, below the thermocline (61.1 m), and in the mixed surface layer (13.2 m) using membrane inlet mass spectrometry (MIMS) in combination with a towed CTD. Seawater was continuously transferred from the respective depth levels of the CTD to the MIMS by using an inline submersible pump. Close to the seafloor a well-defined CH4 plume extended from the bubble release site ∼460 m towards the southwest. Along this distance CH4 concentrations decreased from a maximum of 7872 nmol l−1 to less than 250 nmol l−1. Below the thermocline the well-defined CH4 plume shape encountered at the seafloor was distorted and filaments were observed that extended towards the west and southwest in relation to current direction. Where the core of the bubble plume intersected this depth layer, footprints of high CH4 concentrations of up to 17,900 nmol l−1 were observed. In the mixed surface layer the CH4 distribution with a maximum of up to 3654 nmol l−1 was confined to a small patch of ∼60 m in diameter. The determination of the water column CH4 inventories revealed that CH4 transfer across the thermocline was strongly impeded as only ∼3% of the total water column inventory was located in the mixed surface layer. Best estimate of the CH4 seabed release from the blowout was 1751 tons yr−1. The fate of the trapped CH4 (∼97%) that does not immediately reach the atmosphere remains speculative. In wintertime, when the water column becomes well mixed as well as during storm events newly released CH4 and the trapped CH4 pool can be transported rapidly to the sea surface and emitted into the atmosphere.

Description: As a result of extensive hydrocarbon exploration, the North Sea hosts several thousand abandoned wells; many believed to be leaking methane. However, how much of this greenhouse gas is emitted into the water column and ultimately reaches the atmosphere is not known. Here, we investigate three abandoned wells at 81-93m water depth in the Norwegian sector of the North Sea, all of which show gas seepage into the bottom water. The isotopic signature of the emanating gas points towards a biogenic origin and hence to gas pockets in the sedimentary overburden above the gas reservoirs that the wells were drilled into. Video-analysis of the seeping gas bubbles and direct gas flow measurements resolved initial bubble sizes ranging between 3.2 and 7.4mm in diameter with a total seabed gas flow between 1 and 19 tons of CH4 per year per well. Estimated total annual seabed emissions from all three wells of ~24 tons are similar to the natural seepage rates at Tommeliten, suggesting that leaky abandoned wells represent a significant source of methane into North Sea bottom waters. However, the bubble-driven direct methane transport into the atmosphere was found to be negligible (〈2%) due to the small bubble sizes and the water depth at which they are released.

Description: While the distribution patterns of cold-water corals, such as Paragorgia arborea, have received increasing attention in recent studies, little is known about their in situ activity patterns. In this paper, we examine polyp activity in P. arborea using machine learning techniques to analyze high-resolution time series data and photographs obtained from an autonomous lander cluster deployed in the Stjernsund, Norway. An interactive illustration of the models derived in this paper is provided online as supplementary material. We find that the best predictor of the degree of extension of the coral polyps is current direction with a lag of 3h. Other variables that are not directly associated with water currents, such as temperature and salinity, offer much less information concerning polyp activity. Interestingly, the degree of polyp extension can be predicted more reliably by sampling the laminar flows in the water column above the measurement site than by sampling the more turbulent flows in the direct vicinity of the corals. Our results show that the activity patterns of the P. arborea polyps are governed by the strong tidal current regime of the Stjernsund. It appears that P. arborea does not react to shorter changes in the ambient current regime but instead adjusts its behavior in accordance with the large-scale pattern of the tidal cycle itself in order to optimize nutrient uptake.