Bubble Emissions Research Articles

Active acoustic measurement of sound speed for quantifying underwater gaseous bubble emissions

Artificial or natural processes can cause underwater gas leakage, which often forms upwelling bubbly plume. The interaction of gaseous plume with acoustic waves would result in the variation of sound speed, which makes it possible to monitor the upwelling process via active acoustics. In this work, we build a numerical model to simulate the transport of gaseous bubbles and the sound speed response to the plume in the acoustic channel. The effectiveness of the model is validated by conducting a tank-based experiment, in which the sound speed variations were measured by varying gas flow rates. Meanwhile, signals at six frequencies (1 to 8 kHz) were transmitted and propagated through the medium part of which was occupied by the gaseous plume. Moreover, the comparison results also give a matched bubble size distribution (BSD), which further confirms the validity of the proposed model.

Focused ultrasound pulse repetition frequency impacts bubble activity and tracer delivery during blood-brain barrier opening

The impact of pulse repetition frequency (PRF) on microbubble activity and blood-brain barrier (BBB) opening, for a fixed total number of pulses, is unclear. We studied bubble response to a range of PRFs (0.125, 0.25, 0.5, 1, and 2 Hz) by monitoring bubble emissions during focused ultrasound (FUS) sonications of the rat brain (274.3 kHz). When a location was treated with a test PRF and a control PRF of 1 Hz, at the same pressure, the change in the harmonic amplitude compared to the control decreased with increasing PRF, with a median change of 73.8% at 0.125 Hz and −38.3% at 2 Hz. Significant changes were not observed with repeated sonications at the same PRF. Furthermore, between PRFs of 0.25 and 1 Hz, no difference was observed in the threshold for broadband emissions, which are linked to the potential for tissue damage. Fluorescence imaging was used to estimate the concentration of Trypan Blue (TB) dye following a fixed-pressure 75-pulse exposure for PRFs of 1 and 0.25 Hz in rats. A 0.25 Hz PRF led to a 68.2% increase in the mean concentration measured at the target, with a 53.9% increase in the mean harmonic sum compared with a 1 Hz PRF. Finally, a harmonic emissions-based controller at a PRF of 0.25 Hz yielded similar TB delivery, with fewer instances of petechiae observed through histology, compared to the same controller at 1 Hz. These results may be adapted to improve the clinical safety margin of FUS-mediated BBB opening and to improve the sensitivity to detecting small harmonic signals from cavitating microbubbles.

Monitoring underwater volcano degassing using fiber-optic sensing

Continuous monitoring of volcanic gas emissions is crucial for understanding volcanic activity and potential eruptions. However, emissions of volcanic gases underwater are infrequently studied or quantified. This study explores the potential of Distributed Acoustic Sensing (DAS) technology to monitor underwater volcanic degassing. DAS converts fiber-optic cables into high-resolution vibration recording arrays, providing measurements at unprecedented spatio-temporal resolution. We conducted an experiment at Laacher See volcano in Germany, immersing a fiber-optic cable in the lake and interrogating it with a DAS system. We detected and analyzed numerous acoustic signals that we associated with bubble emissions in different lake areas. Three types of text-book bubbles exhibiting characteristic waveforms are all found from our detections, indicating different nucleation processes and bubble sizes. Using clustering algorithms, we classified bubble events into four distinct clusters based on their temporal and spectral characteristics. The temporal distribution of the events provided insights into the evolution of gas seepage patterns. This technology has the potential to revolutionize underwater degassing monitoring and provide valuable information for studying volcanic processes and estimating gas emissions. Furthermore, DAS can be applied to other applications, such as monitoring underwater carbon capture and storage operations or methane leaks associated with climate change.

Methane-flow system within the Nyegga pockmark field, offshore mid-Norway

We investigated fluid seepage within the Nyegga pockmark field (600–900 m water depths) off mid-Norway from Remotely Operated Vehicle dives at the so-called CNE sites (CNE01 to CNE17). The seafloor morphology of some of these features corresponds to pockmarks and adjacent ridges, with the latter being the focus of present seepage activity. These structures are underlain by chimneys above a gas-charged zone with, in some cases, a substantial body of hydrate-invaded sediment (down to 1.3 s in two-way travel time at CNE03). Present-day methane-rich fluid seepage through the seabed is indicated by chemosynthetic fauna, in particular Siboglinidae polychaetes (Oligobrachia haakonmobiensis webbi and Sclerolinum contortum), microbial mats and associated Rissoidae gastropod (Alvania sp.) grazers, and confirmed by measured in situ bottom-water methane anomalies, up to 2,130 nL/L. No free-gas bubble emissions were observed or acoustically identified. The presence of authigenic carbonates reveals past seepage with very low δ13C values (down to −58‰) indicating that the major source of carbon was methane carried by the venting fluids. The ages of major periods of methane venting are provided by vesicomyid bivalve shells (Isorropodon nyeggaensis) present in two sedimentary layers, 14,930 and 15,500 14C yr BP (ca. 17,238 and 17,952 cal yr BP), respectively, corresponding to the time of Melt Water Pulse IA. The seafloor morphology and pattern of seepage -chemosynthetic fauna and microbial mat distribution and dissolved methane concentration-are remarkably heterogeneous. Pore-water chemistry profiles in a gravity core taken only 40 m from major seepage sites indicate no seepage and anaerobic methane oxidation at a sub-bottom depth of about 2 m. Present-day seepage from the studied pockmark-chimney fluid-flow system charged with gas hydrate is dominated by the advection of methane solution in pore water. Some of this methane could result from the dissolution of hydrate in the chimney, most of which would have formed during an earlier period (post-LGM times) of history of the chimney, when it was venting free gas. However, the presence of free gas beneath this chimney is probably why the water entering the chimney is already saturated with methane and the process of hydrate formation in the chimney continues today.

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.

The Heliconoides Modified Dissolution Index-HMDI: Do methane seepage environments affect the preservation state of Heliconoides inflatus?

Pteropods are holoplanktonic molluscs presently endangered because their fragile aragonitic shell is very sensitive to ocean acidification. The preservation of pteropods, and in particular of the mesopelagic Heliconoides (Limacina) inflatus, has been used to assess aragonite saturation state in Quaternary sediments (Limacina Dissolution Index, LDX) as an inferred proxy for climatic changes.Three deep-sea cores retrieved in 2016 during the EUROFLEETS2 SEMSEEP cruise offshore Israel are investigated to assess the preservation degree of H. inflatus (d'Orbigny, 1835) in representative environments across the base of the southeastern Mediterranean margin (cold water coral, active seepage pockmark and deep sea channel areas). In some core intervals, yellowish, recrystallized internal molds of this species are present. They have been previously observed in the Eastern Mediterranean, but the nature and origin of their colour was never explained. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD) revealed that the recrystallized molds of the H. inflatus shells are formed by aragonite needles and high-Mg calcite crystals. The formation and preservation of these internal molds is induced by advective methane emissions, typically precipitating aragonite and High-Mg calcite phases close to the sediment–water interface. These molds were never used in previous applications of the LDX or in any existing ranking of pteropod preservation. Therefore, we have categorized these peculiar features combining previous pteropod ranking descriptions with the new observations to develop a modified biotic index (Heliconoides Modified Dissolution Index-HMDI).High values of the HMDI at the base of the pockmark core correspond to the interval where also active seepage-related features – e.g., bubble emissions, low-oxygen benthic foraminifera, chemosymbiotic molluscs, small fragments of authigenic carbonate crusts and slightly depleted δ13C values – occur, showing the applicability of the HMDI to methane seeps environments.

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.

An optical and acoustic investigation of microbubble cavitation in small channels under therapeutic ultrasound conditions.

Therapeutic focused ultrasound in combination with encapsulated microbubbles is being widely investigated for its ability to elicit bioeffects in the microvasculature, such as transient permeabilization for drug delivery or at higher pressures to achieve 'antivascular' effects. While it is well established that the behaviors of microbubbles are altered when they are situated within sufficiently small vessels, there is a paucity of data examining how the bubble population dynamics and emissions change as a function of channel (vessel) diameter over a size range relevant to therapeutic ultrasound, particularly at pressures relevant to antivascular ultrasound. Here we use acoustic emissions detection and high-speed microscopy (10 kframes/s) to examine the behavior of a polydisperse clinically employed agent (Definity®) in wall-less channels as their diameters are scaled from 1200 to 15µm. Pressures are varied from 0.1 to 3MPa using either a 5ms pulse or a sequence of 0.1ms pulses spaced at 1ms, both of which have been previously employed in an in vivo context. With increasing pressure, the 1200µm channel - on the order of small arteries and veins - exhibited inertial cavitation, 1/2 subharmonics and 3/2 ultraharmonics, consistent with numerous previous reports. The 200 and 100µm channels - in the size range of larger microvessels less affected by therapeutic focused ultrasound - exhibited a distinctly different behavior, having muted development of 1/2 subharmonics and 3/2 ultraharmonics and reduced persistence. These were associated with radiation forces displacing bubbles to the distal wall and inducing clusters that then rapidly dissipated along with emissions. As the diameter transitioned to 50 and then 15µm - a size regime that is most relevant to therapeutic focused ultrasound - there was a higher threshold for the onset of inertial cavitation as well as subharmonics and ultraharmonics, which importantly had more complex orders that are not normally reported. Clusters also occurred in these channels (e.g. at 3MPa, the mean lateral and axial sizes were 23 and 72µm in the 15µm channel; 50 and 90µm in the 50µm channel), however in this case they occupied the entire lumens and displaced the wall boundaries. Damage to the 15µm channel was observed for both pulse types, but at a lower pressure for the long pulse. Experiments conducted with a 'nanobubble' (<0.45µm) subpopulation of Definity followed broadly similar features to 'native' Definity, albeit at a higher pressure threshold for inertial cavitation. These results provide new insights into the behavior of microbubbles in small vessels at higher pressures and have implications for therapeutic focused ultrasound cavitation monitoring and control.

Variability of Marine Methane Bubble Emissions on the Clayoquot Slope, Offshore Vancouver Island, Between 2017 and 2021

Seabed methane gas emissions occur worldwide at cold seeps located along most continental margins. Fluxes of methane gas released from the seabed in the form of bubbles can be extremely variable even over short time intervals. Some factors controlling the variability are still poorly understood. Here, we report on the results of continuous long-term sonar monitoring of bubble emissions at a depth of 1,260 m on the Clayoquot Slope, northern Cascadia margin. With a total monitoring duration of 4 years and a sampling period of 1 h, this is by far the longest high temporal resolution monitoring of seabed methane gas release ever conducted. Our results provide evidence that the diurnal and semi-diurnal tides influence the timing of the onset and cessation of bubble emissions. However, gas emissions within the monitoring area are active more than 84% of the time, indicating that tides alone are not sufficient to make venting pause. We hypothesize that the gas fluxes are transient but generally sufficiently high to maintain ebullition independently of the tidally-induced bottom pressure variations. Results also show that the tides do not seem to modulate the vigor of active gas emissions.

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.

Observations of Shallow Methane Bubble Emissions From Cascadia Margin

Open questions exist about whether methane emitted from active seafloor seeps reaches the surface ocean to be subsequently ventilated to the atmosphere. Water depth variability, coupled with the transient nature of methane bubble plumes, adds complexity to examining these questions. Little data exist which trace methane transport from release at a seep into the water column. Here, we demonstrate a coupled technological approach for examining methane transport, combining multibeam sonar, a field-portable laser-based spectrometer, and the ChemYak, a robotic surface kayak, at two shallow (&amp;lt;75 m depth) seep sites on the Cascadia Margin. We demonstrate the presence of elevated methane (above the methane equilibration concentration with the atmosphere) throughout the water column. We observe areas of elevated dissolved methane at the surface, suggesting that at these shallow seep sites, methane is reaching the air-sea interface and is being emitted to the atmosphere.

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.

Surface wettability change on TF nanocoated surfaces during pool boiling heat transfer of refrigerant R-141b

Experiments were performed to demonstrate the impact of surface wettability on the nucleate boiling heat transfer of Silicon Dioxide (SiO2) Thin Film (TF) nanocoated surfaces using the saturated refrigerant R-141b at atmospheric pressure. Six numbers of circular flat type test sections of copper material having thickness of 0 nm (plain surface), 125 nm, 250 nm 375 nm, 500 nm and 625 nm surface coating thicknesses were fabricated with the Sol-Gel method followed by spin coating process and characterized through atomic force microscope (AFM), field emission scanning electron microscopy (FE-SEM), Telescope Micro-Goniometer (TMG), and Energy – Dispersive X-Ray spectroscopy (EDX) etc. The experimental results from plain and nanocoated copper surfaces were validated with well-established correlations to predict the pool boiling curve. In comparisons with plain surface, results obtained from other surfaces show that the reduction of wall superheat and additional improvement of heat transfer coefficient (HTC), for all TF nanocoated surfaces at atmospheric pressure. It has been revealed that surface wettability improves the vapor bubble departure radius for hydrophilic surfaces and decreases the frequency of bubble emissions.

Subharmonic spherical bubble oscillations induced by parametric surface modes.

A potential source of subharmonic bubble emissions is revealed experimentally by high-speed imaging. When an acoustic bubble is driven at sufficiently large pressure amplitudes, energy transfer from surface to volume oscillations can lead to the triggering of subharmonic spherical oscillations. This experimental evidence is in agreement with recent theoretical modeling of nonspherical bubble dynamics accounting for nonlinear mode coupling. Implications for the monitoring of stable cavitation activity are discussed.

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.

Enhanced Detection of Bubble Emissions Through the Intact Spine for Monitoring Ultrasound-Mediated Blood-Spinal Cord Barrier Opening.

We previously developed short burst, phase keying (SBPK) focused ultrasound (FUS) to mitigate standing waves in the human vertebral canal. Here, we show microbubble emissions from these pulses can be detected through the human vertebral arch and that these pulses are effective for blood-spinal cord barrier (BSCB) opening. At f0 = 514kHz, circulating microbubbles were sonicated through ex vivo human vertebrae (60kPa-1MPa) using a dual-aperture approach and SBPK exposures engineered to incorporate pulse inversion (PI). Signals from a 250kHz receiver were analyzed using PI, short-time Fourier analysis and the maximum projection over the pulse train. In rats (n = 14), SBPK FUS+microbubbles was applied to 3 locations/spinal cord at fixed pressures (∼0.20-0.47MPa). MRI and histology were used to assess opening and tissue damage. In human vertebrae between 0.2-0.4MPa, PI amplified the microbubble/baseline ratio at f0/2 and 2f0 by 202 ± 40% (132-291%). This was maximal at 0.4MPa, coinciding with the onset of broadband emissions. In vivo, opening was achieved at 40/42 locations, with mean MRI enhancement of 46 ± 32%(16%-178%). Using PI, f0/2 was detected at 14/40 opening locations. At the highest pressures (f0/2 present) histology showed widespread bleeding throughout the focal region. At the lowest pressures, opening was achieved without bleeding. This study confirmed that PI can increase sensitivity to transvertebral detection of microbubble signals. Preliminary in vivo investigations show that SBPK FUS can increase BSCB permeability without tissue damage. SBPK is a clinically relevant pulse scheme and, in combination with PI, provides a means of mediating and monitoring BSCB opening noninvasively.

Variability of Acoustically Evidenced Methane Bubble Emissions Offshore Western Svalbard

AbstractLarge reservoirs of methane present in Arctic marine sediments are susceptible to rapid warming, promoting increasing methane emissions. Gas bubbles in the water column can be detected, and flow rates can be quantified using hydroacoustic survey methods, making it possible to monitor spatiotemporal variability. We present methane (CH4) bubble flow rates derived from hydroacoustic data sets acquired during 11 research expeditions to the western Svalbard continental margin (2008–2014). Three seepage areas emit in total 725–1,125 t CH4/year, and bubble fluxes are up to 2 kg·m−2·year−1. Bubble fluxes vary between different surveys, but no clear trend can be identified. Flux variability analyses suggest that two areas are geologically interconnected, displaying alternating flow changes. Spatial migration of bubble seepage was observed to follow seasonal changes in the theoretical landward limit of the hydrate stability zone, suggesting that formation/dissociation of shallow hydrates, modulated by bottom water temperatures, influences seafloor bubble release.

Contrasting methane emissions from upstream and downstream rivers and their associated subtropical reservoir in eastern China

Subtropical reservoirs are an important source of atmospheric methane (CH4). This study investigated the spatiotemporal variability of bubble and diffusive CH4 emissions from a subtropical reservoir, including its upstream and downstream rivers, in eastern China. There was no obvious seasonal variation in CH4 emissions from the main reservoir, which increased slightly from the first half year to the next half year. In the upstream river, CH4 emissions were low from February to June and fluctuated widely from July to January due to bubble activity. In the downstream river, CH4 emissions were lowest in February, which was possibly influenced by the low streamflow rate from the reservoir (275 m3 s−1) and a short period of mixing. There was spatial variability in CH4 emissions, where fluxes were highest from the upstream river (3.65 ± 3.24 mg CH4 m−2 h−1) and lowest from the main reservoir (0.082 ± 0.061 mg CH4 m−2 h−1), and emissions from the downstream river were 0.49 ± 0.20 mg CH4 m−2 h−1. Inflow rivers are hot spots in bubble CH4 emissions that should be examined using field-sampling strategies. This study will improve the accuracy of current and future estimations of CH4 emissions from hydroelectric systems and will help guide mitigation strategies for greenhouse gas emissions.

The importance of small artificial water bodies as sources of methane emissions in Queensland, Australia

Abstract. Emissions from flooded land represent a direct source of anthropogenic greenhouse gas (GHG) emissions. Methane emissions from large, artificial water bodies have previously been considered, with numerous studies assessing emission rates and relatively simple procedures available to determine their surface area and generate upscaled emissions estimates. In contrast, the role of small artificial water bodies (ponds) is very poorly quantified, and estimation of emissions is constrained both by a lack of data on their spatial extent and a scarcity of direct flux measurements. In this study, we quantified the total surface area of water bodies &lt; 105 m2 across Queensland, Australia, and emission rates from a variety of water body types and size classes. We found that the omission of small ponds from current official land use data has led to an underestimate of total flooded land area by 24 %, of small artificial water body surface area by 57 % and of the total number of artificial water bodies by 1 order of magnitude. All studied ponds were significant hotspots of methane production, dominated by ebullition (bubble) emissions. Two scaling approaches were developed with one based on pond primary use (stock watering, irrigation and urban lakes) and the other using size class. Both approaches indicated that ponds in Queensland alone emit over 1.6 Mt CO2 eq. yr−1, equivalent to 10 % of the state's entire land use, land use change and forestry sector emissions. With limited data from other regions suggesting similarly large numbers of ponds, high emissions per unit area and under-reporting of spatial extent, we conclude that small artificial water bodies may be a globally important missing source of anthropogenic greenhouse gas emissions.

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.