Gas hydrate systems respond slowly to seafloor warming

In a marine environment, gas hydrates are stable at certain pressure (sea level) and temperature (bottom water temperature) conditions. Changes in these conditions may result in the destabilization of the gas hydrates. In this study we investigate the temporal response of a continental margin gas hydrate reservoir to changes in the pressure and/or temperature regime, considering the latent heat of hydrate dissociation and the long response times to conductive heat transport in submarine sediments. Gas hydrates and the surrounding sediments do not instantly respond to changing environmental conditions. A vertical subsoil column without gas hydrates needs more than 5,000 years to adapt its temperature profile to an increase in seafloor temperature. A vertical subsoil column containing gas hydrates has the same response time if the stability of the hydrates is not affected. Although, when gas hydrates stability is affected due to changes in their environment, the response time to these changes is extended. Destabilized gas hydrates will dissociate into methane gas and water. The dissociation process happens at a constant temperature and requires a lot of energy (heat). Dissociation of gas hydrates thus delays the response time of the surrounding subsoil; up to 100,000 years may pass before the temperature profile completely adapted to the changed environmental parameters. Because of this slow response to changes in environmental parameters, gas hydrate dissociation cannot be regarded as the trigger to global warming at the end of glacial and stadial periods and gas hydrate dissociation cannot be responsible for the high observed atmospheric methane concentrations in ice core records, as has been postulated in a number of high-profile publications

A new methodology for quantifying bubble flow rates in deep water using splitbeam echosounders: Examples from the Arctic offshore NW-Svalbard

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 (> 100m). 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

The use of acoustic seafloor backscatter measurements for quantitative and qualitative characterization of methane seep areas

During the 2003 and 2004 cruises of the EC project CRIMEA almost 3000 active methane seeps were detected with an adapted scientific split-beam echosounder in the Dnepr paleo-delta area in the NW Black Sea (Naudts et al., in press). The seeps are widely, but not randomly, distributed over the transition zone between the continental shelf and slope, in water depths of 66 to 825 m. The highest concentration of seeps occurs on the shelf, in water depths of 80 to 95 m. Here, the location of the seeps is controlled by the underlying geology (filled channels) and seepage is characterized by the presence of pockmarks and high acoustic seafloor backscatter, visible on both multibeam and side-scan sonar data.Since seep detection during the CRIMEA cruises was performed independently but simultaneously with the multibeam and side-scan sonar recordings, these datasets possess a great potential for quantitative and qualitative analyses of acoustic seafloor backscatter in relation to the seep locations. Our analyses are further sustained by visual observations, high-resolution 5 kHz seismic data and sediment samples from gravity and multi-coring.For this study we selected an area of 37 km2 on the shelf.Within this area the normalized multibeam backscatter values ranges from -28.32 dB to 20.42 dB. After eliminating high-backscatter values caused by high topographic gradients, all seep positions within this area correspond to backscatter values of more than -2.89 dB and have a standard normal distribution. Furthermore, no seeps occur at locations characterized by the highest backscatter values. Within the area, 99.3 % of the seeps correspond to backscatter values ranging between -1.39 and 4.60 dB.These data indicate that actively bubbling seeps do not necessarily correspond to the highest backscatter values as would be expected; they rather surround the highbackscatter areas. This is also clear from visual observations in which bubbles are seen to emanate at the perimeter of white Beggiatoa mats. Since Beggiatoa mats are commonly associated with the precipitation of authigenic carbonates formed via AOM, these carbonates are very likely to be the cause of the higher backscatter values. Sediment samples and visual observation also indicated that areas corresponding to higher backscatter values are characterised by more shell material in the first 5-10 cm of the seabed.Also pockmarks are characterised by typical backscatter patterns. Better evolved, deeper, pockmarks are characterised by higher backscatter values and the seep activity is lower than at shallow pockmarks, which are often active bubbling. This could be explained by some sort of self-sealing of these seeps, as postulated by Hovland (2002).All these observations at the seafloor are clearly a result of the underlying geology where fluid migration is focussed to the sides of filled paleo-channels. The seismic data show the presence of a distinct “gas front” that locally domes up to the seafloor. These areas of gas front updoming on the shelf are characterised by seeps, higher backscatter values, Beggiatoa mats and pockmarks

A northeast trending structural deformation zone near North Hinder

A northeast trending sequence of structural deformations east on North Hinder on the Belgian continental shelf and adjacent areas seems to be the surface expression of deeper faults, cutting across the whole width of the London-Brabant Massif in the axial zone of the eastern Channel. These fractures have probably been reactivated in a wrench-fault style in tertiary time

Determining the structure of a large tilted block between two major boundary faults in a continental rift (central Lake Baikal): a reflection seismic study

Between the major boundary faults of the central part of Lake Baikal (ie. the Ol’khon fault and the Primorsky fault), a structurally complex tilted area exists that is strongly influenced by the interaction between these two faults. This area, that is about 30 kilometer wide and a 100 kilometers long, consists of three main parts: Pri-Ol’khon, Ol’khon-island and the submerged Maloe More depression. It is believed that the area formed by the gradual propagation of the Primorsky fault in a southeast direction towards the Ol’khon fault.During the summer of 2001 a large amount of high resolution reflection seismic profiles were shot in Maloe More (>600 km), that could be used to get a better insight in the structural development of the area, and in the geometry of its different sub-blocks and basins. In a first stage we have investigated the morphology of the basement underneath the sedimentary cover, and we determined which structures were fault related and which not. Age constraints on the subsequent evolution came from the correlation of the sedimentary units in Maloe More with deposits on Ol’khon-island, and with data from the long BDP-cores in a nearby area (Academician Ridge).The depth of the basement gradually increases from the southwest towards the northeast, and its morphology is characterised by several ridge structures and faults that strike at high-angle to the main faults. Several of these ridges border basins that contain relatively old sediments (Miocene age; Unit A) later overlain by younger units. Therefore the main basement structures of the Maloe More area should be older than the general believed age for the southward propagation of the Primorsky fault (1 Ma according to earlier models). Moreover the occurrence of relatively thick deposits of unit A in the southwestern extremity of Maloe More and in Ol’khon-gate contradicts the idea that these parts of the area are the youngest, being submerged only recently.Instead, older (isolated) sedimentary traps and lacustrine environments must have existed in this area. Faulting in the younger sediments however shows that the presentday activity of the major boundary faults, still has a pronounced effect on the local structure between them. Some of the formed basins are still determined by displacements on the older structures.For this study we have tried to determine the evolution of the Maloe More area, based on its interpreted structure and the relation with overlying sedimentary deposits, and we have tried to link our observations with existing models for the development of the Primorsky and Ol’khon faults

Dispersed methane flux to the water column from natural gas bubble streams at the Black Sea shelf

Gas bubble streams are detected in the water column by the presence of strong, flare shaped backscatter signals recorded during hydroacoustic single beam echosounder surveys (flares). Some of these flares even reach the sea surface. In motion bubbles get into an evolutionary process caused by a variety of effects, including gaseous exchange with surrounding water. Simplistically, kinetics of such a gas exchange can be described by the Fick’s law; the direction of the transfer of any given gas through the bubble depends on partial pressure of respective the gas in bubble, Henry’s law constants and the concentration of dissolved gas in the water. In general, methane gradually dissolves during the lifetime of a bubble, while other gases enter the bubble. Consequently, bubbles cause a vertical transfer of methane from the sea bottom to upper water layers and can be considered as sources of dispersed methane flux to the water column. In present work an attempt is made to trace the methane gas phase exhaustion in flares trough the water column at the Black Sea shelf.Our approach is based upon acoustic observations and measurements carried out in 2003 and 2004 with the scientific echosounder EK-500 onboard RV Vodianitskiy as part of the EU funded project CRIMEA. For the estimation of bubble size distributions our data from direct measurements of acoustic cross-section of single bubbles were used. Data for the relation between rising speed and shrinking rate vs. bubble size were obtained by tracking of single bubbles. Modelling was used to evaluate features of the gas transfer process induced by rising bubbles. Having initial bubble size, gas composition and water depth as starting conditions the model produces series of time based values of bubble size, gas composition, rising depth and rising speed. Acoustic observations were utilized to verify the chosen model parameters.For seeps detected at 90 - 95 m water depth hydroacoustically measured bubble sizes ranged from 1.3 to 11.3 mm in diameter. This bubble size range was confirmed by visual observations during video and submersible inspections.We assumed a gas content of 99% methane and small amounts of nitrogen and oxygen as initial gas composition according to geochemical analyses of gas bubbles sampled by submersible just above the sea floor. To determine the entire free methane flux from the sea floor into the water column and maybe into the atmosphere we run our model for several bubble sizes classes. Then simulation data were summed up with weighting coefficients according to the respective amount of bubbles per class. As a result, vertical profiles of molar content (mkmol) and methane flux (mkmol/s m) per average statistic bubble vs. depth were obtained. To get methane flux from the whole seepage simple multiplication is required by average statistic initial number of bubbles above the bottom per unit height. Depending on the spatial extension of the seep area, point or volume backscattering methods were used to quantify the bubble amount. Of great importance for both methods is the averaging of a high amount of data in space and time. We detected an average of 400 bubbles at high intensity seep sites within a water volume of 1m thickness above the bottom. The hydroacoustically determined amount of bubbles is again in very good agreement with direct visual observations. With an average initial rising speed of 0.25 m/s, 400 bubbles escaping from the sea floor cause a methane flux of 3.45 mmol/s using average bubble methane content of 34.5 mkmol (400 x 0.25 x 34.5 = 3.45 mmol/s). As final methane content of average bubble at the sea surface is 6.5 mkmol, only 6.5/34.5*100 = 18.8 % of methane can reach the atmosphere due to the methane flux into the water column on the way up to the sea surface

Abnormally high acoustic sea-floor backscatter patterns in active methane venting areas, Dnepr paleo-delta, northwestern Black Sea

During the 58th and 60th cruise of R.V. Vodyanitskiy, conducted in the framework of the EU-funded CRIMEA project, almost 3000 active bubble-releasing seeps were detected with an adapted split-beam echosounder within the 1540 km2 of the studied Dnepr paleo-delta area. The distribution of these active seeps is not random, but is controlled by morphology, by underlying stratigraphy and sediment properties, and by the presence of gas hydrates acting as a seal and preventing upward migrating gas to be released as bubbles in the water column (Naudts et al., 2006).Here we present the relation between acoustic sea-floor backscatter and the distribution of more than 600 active methane seeps detected within a small area on the continental shelf. This study is further sustained by visual sea-floor observations, highresolution seismic data, pore-water data and grain-size analysis.The backscatter data indicate that seeps are generally not located within highbackscatter areas, but rather surround them. Most seeps are located within shallow pockmarks which are characterized by medium-backscatter values, whereas deeper pockmarks have high-backscatter values with much lower seep densities. The seismic data show the presence of a distinct gas front (free gas); shallow gas fronts correspond to high- and medium-backscatter areas, which are associated with gas seeps, whereas deep gas fronts correspond to low-backscatter areas without seeps. The presence of shallow gas is also confirmed by the pore-water data, showing higher amounts of dissolved-methane concentrations for areas with medium- to high-backscatter values.Visual observations showed that the high-backscatter areas correspond to white Beggiatoa mats. These thiotrophic bacterial mats are indicators for the anaerobic oxidation of methane (AOM) which results in the formation of methane-derived carbonates (MDAC’s). AOM was also confirmed by the pore-water data. No clear correlation with grain-size distribution could be established.Based on the integration of all datasets, we conclude that the observed highbackscatter anomalies are a result of methane-derived authigenic carbonates (MDAC’s). The carbonate formation appears to lead to a gradual (self)-sealing of the seeps (Hovland, 2002), followed by a relocation of the bubble-releasing holes. Furthermore, the degree of MDAC-formation is directly linked to the backscatter intensity and seep activity which makes it possible to use the backscatter strength as a proxy for the seep activity and distribution

Stratigraphic and structural controls on the location of active methane seeps on Posolsky Bank, Lake Baikal

The distribution and origin of shallow gas seeps occurring at the crest of the Posolsky Bank in Lake Baikal have been studied based on the integration of detailed seismic, multibeam and hydro-acoustic water-column investigations. In total 65 acoustic flares, indicating gas-bubble release at the lake floor (seepage), have been detected within the 630 km² area of the Posolsky Bank. All seeps are located on the Posolsky Fault scarp near the crest of the Posolsky Bank or on similar locations in water depths of -43 m to -332 m. Lake Baikal is the only fresh-water basin in the world where gas hydrates have been inferred from BSRs on seismic data and have been sampled. Our seismic data also portray BSRs occurring up to water depths of -300 m, which is much shallower than the previously reported -500 m water depth. Calculations for hydrate stability, heat flow and topographic effect based on the BSR occurrence and multibeam bathymetry allowed the determination of a methane-ethane gas mixture and heat-flow values wherefore gas hydrates could be stable in the lake sediments at the given ambient conditions. None of the seeps associated with the Posolsky Bank have been detected within this newly established gas-hydrate stability zone. Our observations and data integration suggest that the seeps at the crest of Posolsky Bank occur where gas-bearing strata are cut off by the Posolsky Fault. These gas-bearing layers could be traced down the Posolsky Bank to below the base of the gas-hydrate stability zone (BGHSZ), suggesting that the detected seeps on the crest of the Posolsky Bank are mainly fed by gas coming from below the BGHSZ