Gas Bubble Emissions Research Articles

Defining that je ne sais QUOI of seep sites: An overview of the Quantitative Ocean-Column Imaging (QUOI) voyage to offshore New Zealand

Detecting and characterizing oceanic seep sites is of considerable interest for the geoscience community due to their influence on global climate, ecological significance, connection to resources of high value, and, in many locations including New Zealand, their cultural importance. Modern acoustic systems provide the means for quantitative analysis of seep systems by collection of backscattered energy by gas bubbles and associated water column phenomena and surrounding seafloor. The July 2018 international, multi-institution Quantitative Ocean-Column Imaging (QUOI) voyage aimed to develop protocols and methodologies for identifying and quantifying seafloor and water-column backscatter data associated with bubble seep sites utilizing a large range of active acoustic systems and direct sampling methods. Here, we will provide an overview of QUOI voyage goals and research activities at several locations offshore of New Zealand, including the Calypso hydrothermal vent field in the Bay of Plenty. Gas bubble and hydrothermal fluid emission were captured in acoustic water column data, video transects, and direct sampling operations. More than 3000 individual seep bases were identified in the Calypso hydrothermal vent field, occupying approximately 9 of 115 km2 of mapped seafloor. Efforts are underway to characterize the bubble size distribution of seep sites and quantify carbon flux to the atmosphere using a combination of broadband acoustic methods and in-situ measurements.

Echo grid integration: A novel method for preprocessing multibeam water column data to quantify underwater gas bubble emissions

AbstractWater column imaging multibeam echo sounder systems (MBESs) are a promising technology for quantitative estimates of the gas bubble volume flow within large gas seepage areas. Considerable progress has been made in recent years toward applicable calibration methods for MBESs as well as developing inversion models to convert acoustically measured backscattering cross sections to gas bubble volume flow. However, MBESs are still not commonly used for quantitative gas flow assessments. A reason for this is the absence of published processing methods that demonstrate how MBES data can be processed to quantitatively represent bubble streams. Here, we present a novel method (echo grid integration) that allows for assessing the aggregated backscattering cross section of targets within horizontal water layers. This derived value enables quantifying bubble stream gas flow rates using existing acoustic inversion methods. The presented method is based on averaging geo‐referenced volume backscattering coefficients onto a high‐resolution 3D voxel‐grid. The results are multiplied with the voxel volume to represent measurements of the total backscattering cross‐section within each voxel cell. Individual gridded values cannot be trusted because the beam pattern effects cause the values of individual targets to “smear” over multiple grid‐cells. The true aggregated backscattering cross‐section is thus estimated as the integral over the grid‐cells affected by this smearing. Numerical simulation of MBES data acquisition over known targets assesses the method's validity and quantify it's uncertainty for different, realistic scenarios. The found low measurement bias (< 1%), and dispersion (< 5%) are promising for application in gas flow quantification methods.

Experimental study on solid particle migration and production behaviors during marine natural gas hydrate dissociation by depressurization

Sand production is one of the main obstacles restricting gas extraction efficiency and safety from marine natural gas hydrate (NGH) reservoirs. Particle migration within the NGH reservoir dominates sand production behaviors, while their relationships were rarely reported, severely constrains quantitative evaluation of sand production risks. This paper reports the optical observations of solid particle migration and production from micrometer to mesoscopic scales conditioned to gravel packing during depressurization-induced NGH dissociation for the first time. Theoretical evolutionary modes of sand migration are established based on experimental observations, and its implications on field NGH are comprehensively discussed. Five particle migration regimes of local borehole failure, continuous collapse, wormhole expansion, extensive slow deformation, and pore-wall fluidization are proved to occur during depressurization. The types of particle migration regimes and their transmission modes during depressurization are predominantly determined by initial hydrate saturation. In contrast, the depressurization mainly dominates the transmission rate of the particle migration regimes. Furthermore, both the cumulative mass and the medium grain size of the produced sand decrease linearly with increasing initial methane hydrate (MH) saturation. Discontinuous gas bubble emission, expansion, and explosion during MH dissociation delay sand migration into the wellbore. At the same time, continuous water flow is a requirement for sand production during hydrate dissociation by depressurization. The experiments enlighten us that a constitutive model that can illustrate visible particle migration regimes and their transmission modes is urgently needed to bridge numerical simulation and field applications. Optimizing wellbore layout positions or special reservoir treatment shall be important for mitigating sand production tendency during NGH exploitation.

Spatial and temporal variability of greenhouse gas ebullition from temperate freshwater fish ponds

Fish ponds with their typically high carbon and nutrient inputs are relevant sources of greenhouse gases. However, not much is known about gas bubble emissions (ebullition) and their high spatiotemporal variability. This is the first study which quantified diffusive and ebullitive greenhouse gas emissions from temperate fish ponds. To improve greenhouse gas estimates, we investigated the diurnal and spatial variability of diffusive and ebullitive fluxes in 12 extensively to semi-intensively managed fish ponds near Bautzen, Germany. Emissions differed greatly between the different ponds but methane was consistently the predominant greenhouse gas. The feeding sites were hotspots with one order of magnitude higher ebullition rates compared to other parts of the ponds. At these hotspots, ebullitive fluxes of up to 38 L/m2d were measured with a mean bubble methane content of 79%, corresponding to a methane flux of 1.24 mol/m2d. Methane accounted for 90% of the global warming potential in one fish pond but carbon dioxide emissions of up to 242 mmol/m2d at the feeding sites were also significant. Nitrous oxide fluxes, in contrast, were low with 5 ± 9 μmol/m2d. Greenhouse gas ebullition decreased exponentially along a transect from the feeding site into the pond and showed some diurnal fluctuations. While diffusion was higher during night, ebullition rates increased in the morning, presumably caused by higher benthivorous fish activity. Our results highlight the potential of temperate fish ponds as significant greenhouse gas sources and ebullition as a significant pathway. For robust quantification, both small scale spatial and temporal variability as well as the hotspot of the feeding area must be considered.

Yam Seep at Four-Way Closure Ridge: a prominent active gas seep system at the accretionary wedge SW offshore Taiwan

Hydroacoustic surveys at the accretionary wedge southwest of Taiwan reveal a confined active hydrocarbon seepage area of ~ 49,000 m2 in ~ 1350 m water depth on the northern crest of the Four-Way Closure Ridge, which we call Yam Seep. In this study, multibeam and side-scan sonar data acquired during surveys with an autonomous underwater vehicle during an expedition with R/V Ocean Researcher I in 2017 showed that the area is characterized by rough topography and high seafloor backscatter. Seafloor observations with a video sled and sediment sampling with gravity corers and the MeBo seafloor drill rig during an expedition with R/V SONNE in 2018 revealed that the area is almost entirely covered by intensely fractured methane-derived carbonates, which indicate that seepage has been ongoing for thousands of years. Hydroacoustic anomalies (‘flares’) in the water column indicated the presence of several gas bubble emission sites mostly at the center and eastern flank of the area in 2019. Drilling through massive carbonates in the northwestern part of Yam Seep induced free gas escape from a depth of ~ 5.1 m. This suggests the presence of gas hydrates in the subsurface as the seep area is located well within the gas hydrate stability zone. The inter-disciplinary investigations of the Yam Seep demonstrate that upward migration of light hydrocarbons and seafloor discharge has a considerable influence on the seabed properties.

P‐Doped MoO2/Porous Ni Self‐Supported Catalyst for Electrocatalytic Hydrogen Evolution in Alkaline Solution

Constructing integrated electrocatalysts free from binder is recognized as a promising approach to promote the long‐term efficiency of hydrogen evolution through water electrolysis. Herein, the in situ growth of MoO2 on a porous Ni substrate followed by phosphorus doping is reported to generate self‐supported P‐doped MoO2/porous Ni electrocatalyst for hydrogen evolution reaction. The porous Ni substrate provides a large specific surface area and highly accessible channels for accommodating the active nanoparticles, which achieves an ultralow overpotential of 81 mV at 10 mA cm−2, a small Tafel slope of 99 mV decade−1, and exceptionally high durability for 24 h. The excellent performance of P‐doped MoO2/porous Ni is mainly attributable to the efficient electrolyte diffusion, fast emission of gas bubbles, and enhanced electrical conductivity brought by porous Ni substrate and the highly active sites. This self‐supported catalyst is distinct and superior to the conventional powdery catalysts in developing new energy conversion technologies.

Heterogeneous hydrocarbon seepage at Mictlan asphalt knoll of the southern Gulf of Mexico

Hydrocarbon seepage plays an essential role in defining seafloor morphology and increasing habitat heterogeneity in the deep sea whereby asphalt volcanism ranks among the most complex and proliferous hydrocarbon discharge systems that have been described to date. In this study, seepage of hydrocarbon gas and oil as well as asphalt deposits were investigated at Mictlan Knoll in the southern Gulf of Mexico. A multi-disciplinary approach was used including hydroacoustic surveys and visual seafloor observations to study the seafloor manifestations of hydrocarbon seepage. Mictlan Knoll is an asphalt volcano characterized by a crater-like depression surrounded by an elevated rim. Asphalt deposits are widespread in the depression where a large area of extensive asphalt deposits correlates with a high backscatter area (~75,000 m2). Numerous asphalt deposits appear relatively fresh and probably extruded recently, as oil bubbles were seen to emanate locally within areas covered by extensive asphalt deposits. An area of more irregular seafloor morphology occurring in the northern part of the depression is interpreted to be related to the active extrusion of asphalt below or within older surficial deposits. Additionally, 25 hydroacoustic anomalies indicative for gas bubble emissions were detected. Gas volume quantifications conducted during seafloor inspections with a remotely-operated vehicle (ROV) at a single gas escape site situated above a gas hydrate outcrop revealed up to 0.1 × 106 mol CH4/yr. Gas emission at this site, monitored by an autonomous scanning sonar device, indicated a highly variable bubble release activity. Based on our findings, it is proposed that Mictlan Knoll hosts the most extensive asphalt deposits known to date in the Gulf of Mexico.

Porous sunflower plate-like NiFe2O4/CoNi–S heterostructure as efficient electrocatalyst for overall water splitting

Constructing high-efficient and nonprecious electrocatalysts is of primary importance for improving the efficiency of water splitting. Herein, a novel sunflower plate-like NiFe2O4/CoNi–S nanosheet heterostructure was fabricated via facile hydrothermal and electrodeposition methods. The as-fabricated NiFe2O4/CoNi–S heterostructure array exhibits remarkable bifunctional catalytic activity and stability toward oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline media. It presents a small overpotential of 219 mV and 149 mV for OER and HER, respectively, to produce a current density of 10 mA cm−2. More significantly, when the obtained electrodes are used as both the cathode and anode in an electrolyzer, a voltage of 1.57 V is gained at 10 mA cm−2, with superior stability for 72 h. Such outstanding properties are ascribed to: the 3D porous network structure, which exposes more active sites and accelerates mass transfer and gas bubble emission; the high conductivity of CoNi–S, which provides faster charge transport and thus promotes the electrocatalytic reaction of the composites; and the effective interface engineering between NiFe2O4 (excellent performance for OER) and CoNi–S (high activity for HER), which leads to a shorter transport pathway and thus expedites electron transfer. This work provides a new strategy for designing efficient and inexpensive electrocatalysts for water splitting.

Биогеохимические характеристики мелководных струйных метановых газовыделений в прибрежных районах Крыма в сравнении с глубоководными сипами Чёрного моря

Струйные метановые газовыделения (сипы) — широко распространённый феномен в Мировом океане, в том числе в бассейне Чёрного моря. Актуальность исследований метановых сипов обусловлена их важной ролью в качестве источника метана как парникового и средообразующего газа для водной толщи и атмосферы. В работе представлен сравнительный анализ данных собственных биогеохимических исследований мелководных сипов Крымского полуострова, охватывающих последние десять лет, и материалов, посвящённых глубоководным газовыделениям Чёрного моря. В этот период были проведены поисковые гидроакустические исследования, выявлен компонентный состав пузырькового газа, измерен изотопный состав углерода метана, а также молекулярно-биологическими методами определена структура микробного сообщества бактериальных матов, покрывающих площадки газовыделений, и выполнена оценка газовых потоков от отдельных сипов. В течение многолетнего мониторинга обнаружено и описано 14 отдельных газовыделяющих площадок в прибрежных районах Крыма, которые располагались от мыса Тарханкут на западе полуострова до бухты Двуякорная на юго-востоке. Превалирующая часть прибрежных сипов Крыма имела биогенную природу и сезонный характер газовыделений. К глубинному газу термокаталитического генезиса отнесены сипы в бухте Ласпи. Наблюдался значительный разброс величин изотопного состава углерода метана δ13C-CH4 пузырькового газа прибрежных мелководных районов (−94…−34 ‰), что указывает на разные условия его генерации и созревания в донных отложениях. Так же, как и глубоководным сипам, прибрежным струйным газовыделениям сопутствовали бактериальные маты разной структуры с различными доминирующими видами. Показано, что для формирования устойчивой бактериальной биомассы, основу которой составляли, как правило, сульфид- и сероокисляющие бактерии, необходим флюидный поток восстановленных растворённых газов, тогда как точечная разгрузка пузырькового газа не обеспечивает достаточных градиентов концентрации и может механически разрушать структуру образующегося сообщества. Различными методами сделаны оценки размерных спектров пузырьков и потоков от отдельных струй. Диапазон значений газового потока варьировал от 1,8 л·сут−1 (бухта Мартынова) до 40 л·сут−1 (бухта Ласпи). Проанализированы средообразующие эффекты, связанные с выделением пузырькового газа в прибрежных районах: влияние сипов на кислородный режим в донных осадках и в толще воды́ над точками газовыделений; вертикальное перемешивание вод за счёт газлифтового эффекта; флюидная разгрузка на площадках струйных газовыделений.

Noble gases as tracers for the gas dynamics in methane supersaturated lacustrine sediments

The commonly observed heterogeneous and erratic emissions of methane from lacustrine sediments are difficult to quantify. The combined analysis of methane and noble gas concentrations in the pore water opens a novel route to analyse and quantify such emissions and was used in this work to explore the gas dynamics in the sediments of two lakes with different hydro-geochemical settings. In the hypertrophic Lake Rot the microbial activity in the sediment pore water results in a virtually continuous gas bubble formation and emission. By determining noble gas concentration gradients and the maximum methane supersaturation, the minimum methane emission from the sediment was quantified. The gas evolution in the pore water of the sediments of artificial Lake Lungern is strongly affected by annual lake level variations of up to 20 m. Noble gas concentrations allow past gas dynamics at different lake levels to be traced back. At “hot spots” of bubble emission microbial methane from deeper sediments was released in response to the decreasing hydrostatic pressure, i.e. man-induced lake level drop.Within zones of the sediment that seasonally fall dry, the air supply to the sediment was inferred from the noble gas excess observed in the pore water, which is due to dissolving air bubbles which entered the sediment during the dry period.These case studies demonstrate the potential of the combined analysis of noble gas and CH4 concentrations for exploring the gas dynamics in lacustrine sediments. The feasibility of this method under different conditions calls for an application in other aquatic environments, such as wetlands and oceans.

Methane gas emissions of the Black Sea—mapping from the Crimean continental margin to the Kerch Peninsula slope

This study focuses on gas bubble emissions across ~ 1800 km2 of the southeastern continental slope of the Crimean Peninsula in the Eastern Black Sea basin. In total, about 3500 gas emissions were mapped as so-called flares in single- and multibeam echosounder imaginary. Flares generally emanate from the seafloor, with a few exceptions, where water depths are less than 700 m, which represents the upper limit of the gas hydrate stability zone in this region. Methane in shallow sediments below 700 m is transferred to methane hydrate, and the migration of free gas into the water column is therefore inhibited in most cases. This study illustrates further that flare occurrence is linked to morpho-structural margin characteristics. Two morphologically different margin segments are distinguished, and flare distributions were found to relate to differing morphological structures along these segments. In the western province, where the upper continental margin is steep and incised by gullies, flares occur mainly along downslope-oriented ridge crests. In the eastern province, representing a part of the Don-Kuban paleo-fan, the slope is influenced by extensive mass wasting. Gas emissions in this area were mainly detected aligned along scarps of submarine landslides and related to sediment failures. Almost all flares disappear in the acoustic records of the water column below a water depth of ~ 100 m suggesting that the gas bubbles dissolve within the water body of the Black Sea and contribute to its methane inventory and the anoxic water column.

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

This multi-disciplinary study of the hydrocarbon seepage system at Tsanyao Yang Knoll in the southern Gulf of Mexico illustrates the amount and fate of hydrocarbons (mainly oil and methane) emanating from the seafloor structure and rising through a 3400 m water column. Tsanyao Yang Knoll forms part of the Campeche Knolls and was found to be one of the most active seepage structures at such an exceptional depth. Combining ship-based and AUV-based hydroacoustic mapping with direct seafloor observations and investigations, which used a TV-sled and a ROV with gas and water sampling devices provided an integrated view for the various transport pathways of hydrocarbons from the seafloor to the sea surface. In total, 32 acoustic ‘flares’, indicative of gas bubble emission sites, were detected emanating from depressions on top of the knoll. Most of the emission sites were concentrated in two depressions that comprised a main seep field. An estimated volume of 1359–4900 litres of hydrocarbons per hour are released in the form of gas bubbles, which dissolve almost entirely during their rise in the water column. However, echograms showed gas anomalies to about 500 meters below sea surface and some bubbles were seen to burst at the sea surface. Ex situ concentrations of dissolved methane were highly elevated (~30 000 nmol/L) directly above the seafloor emission site, but decreased to background concentrations (3–5 nmol/L) within the lowermost 100 m. Smaller quantities of oil also escaped from the seafloor, rose to the sea surface and generated natural oil slicks visible from the ship and in satellite images. This study shows that hydrocarbon seepage at ~3400 m water depth can be followed to the sea surface. However, most of the methane dissolves in deeper waters, whereas oil reaches the sea surface.

Anaerobic methane oxidation inducing carbonate precipitation at abiogenic methane seeps in the Tuscan archipelago (Italy).

Seepage of methane (CH4) on land and in the sea may significantly affect Earth’s biogeochemical cycles. However processes of CH4 generation and consumption, both abiotic and microbial, are not always clear. We provide new geochemical and isotope data to evaluate if a recently discovered CH4 seepage from the shallow seafloor close to the Island of Elba (Tuscany) and two small islands nearby are derived from abiogenic or biogenic sources and whether carbonate encrusted vents are the result of microbial or abiotic processes. Emission of gas bubbles (predominantly CH4) from unlithified sands was observed at seven spots in an area of 100 m2 at Pomonte (Island of Elba), with a total rate of 234 ml m-2 d-1. The measured carbon isotope values of CH4 of around -18‰ (VPDB) in combination with the measured δ2H value of -120‰ (VSMOW) and the inverse correlation of δ13C-value with carbon number of hydrocarbon gases are characteristic for sites of CH4 formation through abiogenic processes, specifically abiogenic formation of CH4 via reduction of CO2 by H2. The H2 for methanogenesis likely derives from ophiolitic host rock within the Ligurian accretionary prism. The lack of hydrothermal activity allows CH4 gas to become decoupled from the stagnant aqueous phase. Hence no hyperalkaline fluid is currently released at the vent sites. Within the seep area a decrease in porewater sulphate concentrations by ca. 5 mmol/l relative to seawater and a concomitant increase in sulphide and dissolved inorganic carbon (DIC) indicate substantial activity of sulphate-dependent anaerobic oxidation of methane (AOM). In absence of any other dissimilatory pathway, the δ13C-values between -17 and -5‰ in dissolved inorganic carbon and aragonite cements suggest that the inorganic carbon is largely derived from CH4. The formation of seep carbonates is thus microbially induced via anaerobic oxidation of abiotic CH4.

Morphology and activity of the Helgoland Mud Volcano in the Sorokin Trough, northern Black Sea

Abstract Morphological change and activity of submarine mud volcanoes (MVs) are still poorly understood. In this study, we integrated acoustic subbottom profiles, Autonomous Underwater Vehicle (AUV) high-resolution bathymetric and backscatter data, Remotely Operated Vehicle (ROV) seafloor observations, Bottom Water Temperatures (BWT), and compositions of gas samples collected from three research expeditions, to study the morphology and activity of the Helgoland mud volcano (Helgoland MV) in the Sorokin Trough of the Black Sea. Based on the micro bathymetric and backscatter maps, we conducted a geomorphological analysis which differentiates the Helgoland MV into five regions, which are: the active center, the inner crest, the inner moat, the outer crest, and an outer moat. Special seafloor features, such as gas bubble emissions, mud breccia and mud pools, were observed in different regions by ROV observations. They indicate a heterogeneous distribution of materials and activities at the Helgoland MV. Active mud extrusion from the center was inferred from BWT elevations (up to 1.652 °C) that showed the same orientation as a major fault (NW-SE) at the location. Methane stable carbon isotopes (−58.1∼−63.6‰) and molecular compositions of light hydrocarbons (C1/C2+ = 2836, CH4 = 99.545%, C2H6 = 0.033%, C3H8 = 0.002%, CO2 = 0.417%) suggest a predominance of biogenic hydrocarbons with an admixture of thermogenic gas. Circular faults surrounding the Helgoland MV, as observed in the bathymetric and sub-bottom profiles, are speculated to be formed due to subsidence of the MV caldera and gravity driven sediment movement. A conceptual model is proposed to explain the interrelationship between the seafloor morphology and activity of the Helgoland MV. Combining multidisciplinary data, especially from the application of ROVs and AUVs, broaden the scope in which the activity and morphology of submarine mud volcanoes can be investigated.

Upward migration of gas in an active tectonic basin: An example from the sea of Marmara

Gases of various sources were collected at the seafloor of the Marmara basin suggesting that the gases expelled have experienced multiple sequences of upward migration, from multiple sources. The pathways of upward migration of gas can be reconstructed by considering the distribution of gas seeps with respect to the near-surface geomorphostructure and the regional stratigraphic architecture of the Marmara Basin. Gas seeps appear to be more favourably localized within a 1–2 km swath around active faults where sediment permeability is probably enhanced by deformation. In the fault zones, fault intersections between sets of transtensive and transpressive subsidiary faults, or between subsidiary faults and main faults, are the preferred gas pathways. These subsidiary structures localize methane seeps, observed as elongated black patches with bacterial mats, gas bubble emission sites, and chemoherms associated with buried mud volcanoes. Gas seeps are, however, rare along active faults segments crossing basin depocenters but focus along basin edges and along topographic highs. Considering the role of sedimentary layers as gas migration pathways can explain this characteristic of the distribution of gas seeps. Fault zones that cross updip gas migration pathways will vent comparatively more gas than fault zones on the downdip side. Moreover, gas accumulation and resulting overpressuring along the western fault segment crossing the Western High may be associated with aseismic fault creep and intense gas emissions at the seafloor. In contrast, the poorly focused seepage along the fault segment crossing the Central High may be linked to the locked state of this fault segment.

Activation of gas bubble emissions indicated by the upward decreasing Lead-210 activity at a submarine mud volcano (TY1) offshore southwestern Taiwan

A submarine mud volcano (MV) known as TY1 is one of the largest conical structures found offshore southwestern Taiwan. Active gas bubble emissions at the flat crest of the mud volcano disperse sediment particles into the water column. Changes in the mud volcanism of TY1 were revealed by radiograph and grain size analysis of the sediment cores. 210Pb geochronology was applied to the near-seafloor sediments to reveal the detailed mud volcanism from the deposition rate. In a 58-cm long sediment core taken from the northern margin of the crest, known as TY1-N.170, the size and frequency of the mud clasts decrease upward. The clasts are smaller than 8mm above a sediment depth of 34cm below the sea floor (cmbsf). Sediments between 0 and 19 cmbsf are massive, and particles larger than 2mm are absent. An enrichment of coarse, silt-sized particles in the massive sediment unit and the restricted distribution of the unit suggest that the massive unit was generated by re-deposition of sediment particles that were dispersed into the water column by gas bubble emission. These characteristics suggest that during the last mud volcanism of TY1, there was a decrease in mud eruption energy, and gas bubble emission became the main activity. In core TY1-S.440, taken from the southern slope, a massive sediment unit enriched with coarse, silt-sized particles, is intercalated between mud breccia structures. This suggests repeated mud breccia flows caused by TY1. The excess 210Pb activity present in the massive sediment unit of TY1-N.170 suggests activation of gas bubble emission. In the massive unit, a decrease in excess 210Pb activity appears upward toward the seafloor. The highest value is 3.1dpm/g at 19.8cmbsf (26.4g/cm2 in cumulative mass); values lower than 1.0dpm/g are distributed 2.8–6.8 cmbsf (3.6–8.8g/cm2). The upwardly decreasing trend is opposite that of the reference core from which a reasonable areal sedimentation rate was obtained using a constant-flux constant-sedimentation model. The anomalous depth profile at the massive unit indicates an increasing rate of sediment deposition, which originated from the activation of gas bubble emission.

Tidally controlled gas bubble emissions: A comprehensive study using long-term monitoring data from the NEPTUNE cabled observatory offshore Vancouver Island

Long-term monitoring over one year revealed high temporal variability of gas emissions at a cold seep in 1250 m water depth offshore Vancouver Island, British Columbia. Data from the North East Pacific Time series Underwater Networked Experiment observatory operated by Ocean Networks Canada were used. The site is equipped with a 260 kHz Imagenex sonar collecting hourly data, conductivity-temperature-depth sensors, bottom pressure recorders, current meter, and an ocean bottom seismograph. This enables correlation of the data and analyzing trigger mechanisms and regulating criteria of gas discharge activity. Three periods of gas emission activity were observed: (a) short activity phases of few hours lasting several months, (b) alternating activity and inactivity of up to several day-long phases each, and (c) a period of several weeks of permanent activity. These periods can neither be explained by oceanographic conditions nor initiated by earthquakes. However, we found a clear correlation of gas emission with bottom pressure changes controlled by tides. Gas bubbles start emanating during decreasing tidal pressure. Tidally induced pressure changes also influence the subbottom fluid system by shifting the methane solubility resulting in exsolution of gas during falling tides. These pressure changes affect the equilibrium of forces allowing free gas in sediments to emanate into the water column at decreased hydrostatic load. We propose a model for the fluid system at the seep, fueled by a constant sub-surface methane flux and a frequent tidally controlled discharge of gas bubbles into the ocean, transferable to other gas emission sites in the world's oceans.

Peculiarities of the distribution of methane concentration and methane fluxes at the water-air interface in the Tatar Strait of the Sea of Japan

The spatial distribution ofthe methane content in the sea water in the Tatar Strait ofthe Sea of Japan is investigated using the data of expeditionary studies in 2012-2013. The developed model enabled computing methane fluxes at the water-air interface. Identified are the water areas of active methane emission in the area of underwater gas blowouts. Revealed is the abnormal concentration of methane in the areas of emission of natural gas bubbles on the western shelf and on the slope of Sakhalin Island. These bubbles give rise to the upwelling of bottom water to the overlying layers. It is discovered that the formation of the cold subsurface layer (CSL) of sea water being especially active in the area of gas blowouts is caused by phase transitions as a result of the hydrophobic hydration of natural gas molecules.

Gas bubble transport and emissions for shallow peat from a northern peatland: The role of pressure changes and peat structure

Gas bubbles are an important pathway for methane release from peatlands. The mechanisms controlling gas bubble transport and emissions in peat remain uncertain. The effects of hydrostatic pressure and peat structure on the dynamics of gas bubbles in shallow peat were therefore tested in laboratory experiments. A peat monolith was retrieved from a raised bog and maintained in a saturated state. Three distinct layers were identified from noninvasive permittivity measurements supported by soil physical properties (porosity, bulk density). Phase I of the experiment involved monitoring for the accumulation of gas bubbles under steady pressure and temperature conditions. The data showed evidence for gas bubbles being impeded by a shallow semiconfining layer at depths between 10 and 15 cm. Visible gas bubbles observed on the side of the sample box were recorded over time to estimate changes in the vertical distribution of volumetric gas content. Porosity estimates derived using the Complex Refraction Index Model (CRIM) suggest that gas bubbles enlarge the pore space when the exerted pressure is high enough. Phase II involved triggering release of trapped bubbles by repeatedly increasing and decreasing hydrostatic pressure in an oversaturated condition. Comparison of changes in pressure head and methane density in the head space confirmed that the increasing buoyancy force during drops in pressure is more important for triggering ebullition than increasing mobility during increases in pressure. Our findings demonstrate the importance of changes in hydrostatic pressure on bubble size and variations in resistance of the peat fabric in regulating methane releases from peatlands.

First evidence of widespread active methane seepage in the Southern Ocean, off the sub-Antarctic island of South Georgia

An extensive submarine cold-seep area was discovered on the northern shelf of South Georgia during R/V Polarstern cruise ANT-XXIX/4 in spring 2013. Hydroacoustic surveys documented the presence of 133 gas bubble emissions, which were restricted to glacially-formed fjords and troughs. Video-based sea floor observations confirmed the sea floor origin of the gas emissions and spatially related microbial mats. Effective methane transport from these emissions into the hydrosphere was proven by relative enrichments of dissolved methane in near-bottom waters. Stable carbon isotopic signatures pointed to a predominant microbial methane formation, presumably based on high organic matter sedimentation in this region. Although known from many continental margins in the world's oceans, this is the first report of an active area of methane seepage in the Southern Ocean. Our finding of substantial methane emission related to a trough and fjord system, a topographical setting that exists commonly in glacially-affected areas, opens up the possibility that methane seepage is a more widespread phenomenon in polar and sub-polar regions than previously thought.