Methane In Seawater Research Articles

Потоки метана на границе вода–атмосфера на акватории российского сектора Восточной Арктики

The average value of methane fluxes from the sea surface in the Chukchi Sea (4±4 mol/(km2‧day)) and parameters of methane supersaturation of the surface water layer (78±39 %) were lower than in the East Siberian Sea (32±24 mol/(km2‧day) and 346±247 %, respectively). In 50 % of cases, the concentrations of dissolved methane in the surface layer of sea waters were two times higher than the equilibrium values with the atmosphere. The heterogeneous distribution of methane in seawater causes a change in the direction and magnitude of methane fluxes at the water–atmosphere boundary under given experimental hydrometeorological conditions. Data analysis showed that the flux was predominantly determined by wind speed (correlation coefficient Q = 0.8), concentration of dissolved methane (Q = 0.6), parameter of methane supersaturation of waters (Q = 0.6), and temperature of the surface water layer (Q = –0.6). A negative correlation coefficient with temperature indicates that as the temperature decreases, the solubility of methane in water increases, the difference in concentrations with the atmosphere decreases, and the intensity of methane flux decreases.

Distribution of dissolved methane in seawater from the East China Sea to the Ryukyu forearc

Methane is a greenhouse gas, and the East China Sea acts as a source of emissions to the atmosphere. On the other hand, the distribution of methane concentration in the Ryukyu Arc shelf and forearc region has not been clarified. Therefore, we investigated the continental slope and continental shelf areas from the Okinawa Trough to the landward side and the island shelf and forearc areas to the seaward side of the East China Sea. The methane concentration in the Kuroshio Current region was very low, and the methane concentration in the seawater just above the seafloor in the area directly above the hydrothermal systems was exceptionally high. In addition, methane concentration in seawater just above the seafloor where cold seeps and mud volcanoes are known is higher than the background, and higher methane concentrations were found in continental slope area and the island shelf area. The results suggest the existence of previously unreported methane sources such as cold seeps and mud volcanoes in the continental slope region and the island shelf region.

Advance in Numerical Simulation Research of Marine Methane Processes

Understanding the modern marine methane processes, which can profoundly affect global climate and have far-reaching impacts on human living environments, is critical for research on the global carbon cycle. Thus, modeling of marine methane processes has attracted increasing attention due to models can accurately simulate and predict the environmental effects of methane on marine and atmospheric ecosystems. In this study, we review the applications of modeling works to marine methane processes, including methanogenesis in sediments, transport and reaction of methane in sediments and seawater, and marine methane emissions to the atmosphere. Compiled a large database of global methanogenesis rates and methane fluxes to the sulfate-methane transition zone, we estimate that the global methanogenesis budget in marine sediments is ∼0.87 Tmol yr−1 and global sedimentary dissolved inorganic carbon produced by anaerobic oxidation of methane is ∼8.9 Tmol yr−1. In addition, although anaerobic oxidation of methane in sediments and aerobic oxidation of methane in seawater act as primary filters to prevent methane leakage from sediments to the hydrosphere as well as the atmosphere, large masses of methane in extreme seafloor environments (e.g., mud volcanic eruptions and hydrate leakage) can still escape microbial oxidation and leakage to seawater or the atmosphere. There is still a lack of models that simulate methane in these extreme marine environments. Therefore, more modeling works are needed to assess the efficiency of marine ecosystems, including sediments and hydrosphere, in filtering methane in the event of large-scale methane leakage from the seafloor. This study provides an interdisciplinary view of methane processes in marine systems and helps identify future directions in the modeling of methane processes in marine system.

Methane Plume Emissions Associated With Puget Sound Faults in the Cascadia Forearc

AbstractMethane gas plumes have been discovered to issue from the seafloor in the Puget Sound estuary. These gas emission sites are co‐located over traces of three major fault zones that fracture the entire forearc crust of the Cascadia Subduction Zone. Multibeam and single‐beam sonar data from cruises conducted in years 2011, 2018, 2019, 2020, and 2021 identified the acoustic signature of 349 individual bubble plumes. Dissolved CH4 gas from the plumes combines to elevate seawater methane concentrations of the entire Puget Sound estuary. Fluid samples from adjacent terrestrial hot springs and deep‐water wells surrounding the estuary contain a helium‐3 isotope signature, suggesting a deep fluid source located near the underlying Cascadia Subduction Zone plate interface. However, limited data from this pilot study suggest that Puget Sound seawater emission sites lack both similar chemical isotope signatures and elevated thermal anomalies that would be expected from association with a deep plate‐interface reservoir. A shallow reservoir within the Holocene sediments that cover the older Puget Sound basement with horizontal transfer to the thinly covered Alki Point and Kingston Arch anticlines is also a possibility, as has been suggested for other methane seep areas. The existence of vigorous marine methane plumes arising from areas of thin sediment cover associated with deeply penetrating forearc fault zones but presenting no thermal or chemical anomalies found in other similar forearc environments, remains an unresolved paradox.

Methane Fluxes at the Water–Atmosphere Interface in the Southern Tatar Strait of the Sea of Japan: Distribution and Variation

Abstract —We present the results of gas-geochemical surveys in the sea surface water layer, water column, and bottom sediments of the Tatar Strait (north of the Sea of Japan) in 2012, 2014, 2015, 2017, and 2018. The distribution of methane fluxes at the water–atmosphere interface is examined, and its relationship with the geologic structure of the Tatar Strait area is discussed. Methane emission has been revealed throughout most of the Tatar Strait area. The most intense methane fluxes at the water–atmosphere interface, up to 482 mol/(km2×day), are observed on the gas-bearing southwestern shelf and on the gas hydrate slope of Sakhalin Island. The high concentrations of methane in seawater and the high contents of methane, hydrogen, and helium in the shelf and slope bottom sediments are probably due to the seismotectonic activity in the region, the presence of gas hydrates, gas concentration zones, gas migration channels, and the regional deep structure. Application of the model for calculation of the flux and impurity transfer fields to the studied water area has shown high methane emission from the sea surface in areas of vertical gas migration from lithospheric sources. The contribution of hydrodynamic factors to the formation of such zones of high methane emission is less than that of geologic factors. The obtained data on methane flux at the water–atmosphere interface for a shallow sea gave a detailed insight into the main gas discharge zones in the southern Tatar Strait.

A record of seafloor methane seepage across the last 150 million years

Seafloor methane seepage is a significant source of carbon in the marine environment. The processes and temporal patterns of seafloor methane seepage over multi-million-year time scales are still poorly understood. The microbial oxidation of methane can store carbon in sediments through precipitation of carbonate minerals, thus providing a record of past methane emission. In this study, we compiled data on methane-derived carbonates to build a proxy time series of methane emission over the last 150 My and statistically compared it with the main hypothesised geological controllers of methane emission. We quantitatively demonstrate that variations in sea level and organic carbon burial are the dominant controls on methane leakage since the Early Cretaceous. Sea level controls methane seepage variations by imposing smooth trends on timescales in the order of tens of My. Organic carbon burial is affected by the same cyclicities, and instantaneously controls methane release because of the geologically rapid generation of biogenic methane. Both the identified fundamental (26–27 My) and higher (12 My) cyclicities relate to global phenomena. Temporal correlation analysis supports the evidence that modern expansion of hypoxic areas and its effect on organic carbon burial may lead to higher seawater methane concentrations over the coming centuries.

Investigation of the kinetics of mixed methane hydrate formation kinetics in saline and seawater

Solidified Natural Gas (SNG) technology for storing methane/natural gas in clathrate hydrates is gaining prominence due to its promising advantages of being extremely safe, relatively greener, more economical and offering a compact mode of long-term reliable storage in comparison to prevalent technologies. Recently, we demonstrated a faster method to store methane in seawater and saline water via mixed sII hydrates in an unstirred reactor configuration. In this study, we investigate in detail the macroscopic kinetics of mixed hydrate formation in presence of saline (3 wt% or 1.1 mol% NaCl) water, artificial and natural seawater. The effect of driving force on the kinetics of mixed hydrate formation was studied by performing hydrate formation experiments at three different pressures of 3.0, 5.0 and 7.2 MPa at 283.2 K. Characteristic difference in the formation kinetics was observed at lower driving force (3.0 MPa and 5.0 MPa) experiments of saline water in comparison to the experiment without salt. Further, effect of amino acids (leucine and arginine) on mixed hydrate formation starting from saline water at moderate pressure of 5.0 MPa have been studied. 200 ppm leucine was found to be enhancing the mixed methane hydrate formation kinetics in saline water under studied experimental conditions. These findings highlight the promise of employing seawater for SNG technology to store methane (natural gas) on a large scale suited for long-term storage.

The Distribution of Dissolved Methane and Its Air-Sea Flux in the Plume of a Seep Field, Lingtou Promontory, South China Sea

Methane (CH4), the most abundant hydrocarbon gas in the atmosphere, plays an important role in global climate change. Quantifying the dissolved methane and its air-sea flux from hydrocarbon seeps is therefore of great importance. Large quantities of natural gas are emitted from the seafloor to the coastal ocean near the Lingtou Promontory, South China Sea. We quantified concentrations of methane in surface and bottom waters at 48 stations in a 56 km2 study area. High spatial variability in dissolved methane concentrations was observed in the surface mixed layer (0.5 m water depth) and bottom water (water-sediment interface), with values ranging from 2.90 nmol L−1 to 13570.02 nmol L−1 and from 4.98 nmol L−1 to 31740.02 nmol L−1, respectively. The significant difference between concentrations of dissolved methane in surface and bottom waters suggests that most of the methane emitted from the seafloor is dissolved in the water column. The dissolution of methane in seawater may result in local oxygen depletion that may lead to ecological effects. The δ13C values of dissolved methane ranging from −59.76‰ to −48.59‰ indicate a mixture of biogenic and thermogenic gas sources. The average air-sea methane flux of Yinggehai Basin was 672.57 μmol m−2 d−1, which cannot be ignored in environment assessment. Coastal regions, especially with hydrocarbon seeps in shallow waters of the continental margin, may therefore be an important source of methane to the atmosphere.

Sub-Ocean: Subsea Dissolved Methane Measurements Using an Embedded Laser Spectrometer Technology

We present a novel instrument, the Sub-Ocean probe, allowing in situ and continuous measurements of dissolved methane in seawater. It relies on an optical feedback cavity enhanced absorption technique designed for trace gas measurements and coupled to a patent-pending sample extraction method. The considerable advantage of the instrument compared with existing ones lies in its fast response time of the order of 30 s, that makes this probe ideal for fast and continuous 3D-mapping of dissolved methane in water. It could work up to 40 MPa of external pressure, and it provides a large dynamic range, from subnmol of CH4 per liter of seawater to mmol L-1. In this work, we present laboratory calibration of the instrument, intercomparison with standard method and field results on methane detection. The good agreement with the headspace equilibration technique followed by gas-chromatography analysis supports the utility and accuracy of the instrument. A continuous 620-m depth vertical profile in the Mediterranean Sea was obtained within only 10 min, and it indicates background dissolved CH4 values between 1 and 2 nmol L-1 below the pycnocline, similar to previous observations conducted in different ocean settings. It also reveals a methane maximum at around 6 m of depth, that may reflect local production from bacterial transformation of dissolved organic matter.

동해 울릉분지 해수의 현장 용존 메탄 분석에 사용된 심해용 수중 질량분석기

심해용 수중 질량분석기(Deep-Ocean Mass Spectrometer; DOMS)는 해수에 포함된 용존 가스(dissolved gases)를 현장(in situ) 분석할 수 있는 장비이다. 이 심해용 수중 질량분석기는 수심 4,000 m까지 사용할 수 있도록 제작되었는데, 동해 울릉분지에서 가스하이드레이트 해리(dissociation)에 따른 해수의 용존 가스 분포를 분석하기 위하여 사용되었다. 현장 분석은 수중에 심해용 수중 질량분석기를 투입하여 멤브레인 주입 장치(polydimethylsiloxane-based membrane inlet system)에 해수를 유입시킨 후에 이온화하여 고진공 상태에서 자동공진 이온트랩 질량분석기에서 질량 대 전하비(mass-to-charge)를 측정하는 방식으로 용존 가스를 분석한다. 본 연구에서는 가스하이드레이트 침니가 발달된 울릉분지 UBGH1-09 정점(수심 2,106 m)의 해수의 용존가스를 현장 분석하였으며, 분석결과 질소, 수소, 아르곤, 산소, 이산화탄소와 메탄이 검출되었다. UBGH1-09 정점의 해수에서는 열기원 가스하이드레이트에 포함되는 에탄, 프로판, 부탄은 검출되지 않았다. 앞으로 심해용 수중 질량분석기는 한반도 전 해역에서 용존 가스의 배경 값 조사와 가스 방출 지역에서의 장기간 용존 가스모니터링을 위한 현장 분석에 사용될 것이다.

Analysis of hydrogen and methane in seawater by “Headspace” method: Determination at trace level with an automatic headspace sampler

“Headspace” technique is one of the methods for the onboard measurement of hydrogen (H2) and methane (CH4) in deep seawater. Based on the principle of an automatic headspace commercial sampler, a specific device has been developed to automatically inject gas samples from 300ml syringes (gas phase in equilibrium with seawater). As valves, micro pump, oven and detector are independent, a gas chromatograph is not necessary allowing a reduction of the weight and dimensions of the analytical system. The different steps from seawater sampling to gas injection are described. Accuracy of the method is checked by a comparison with the “purge and trap” technique. The detection limit is estimated to 0.3nM for hydrogen and 0.1nM for methane which is close to the background value in deep seawater. It is also shown that this system can be used to analyze other gases such as Nitrogen (N2), carbon monoxide (CO), carbon dioxide (CO2) and light hydrocarbons.

Fluxes and fate of dissolved methane released at the seafloor at the landward limit of the gas hydrate stability zone offshore western Svalbard

AbstractWidespread seepage of methane from seafloor sediments offshore Svalbard close to the landward limit of the gas hydrate stability zone (GHSZ) may, in part, be driven by hydrate destabilization due to bottom water warming. To assess whether this methane reaches the atmosphere where it may contribute to further warming, we have undertaken comprehensive surveys of methane in seawater and air on the upper slope and shelf region. Near the GHSZ limit at ∼400 m water depth, methane concentrations are highest close to the seabed, reaching 825 nM. A simple box model of dissolved methane removal from bottom waters by horizontal and vertical mixing and microbially mediated oxidation indicates that ∼60% of methane released at the seafloor is oxidized at depth before it mixes with overlying surface waters. Deep waters are therefore not a significant source of methane to intermediate and surface waters; rather, relatively high methane concentrations in these waters (up to 50 nM) are attributed to isopycnal turbulent mixing with shelf waters. On the shelf, extensive seafloor seepage at <100 m water depth produces methane concentrations of up to 615 nM. The diffusive flux of methane from sea to air in the vicinity of the landward limit of the GHSZ is ∼4–20 μmol m−2 d−1, which is small relative to other Arctic sources. In support of this, analyses of mole fractions and the carbon isotope signature of atmospheric methane above the seeps do not indicate a significant local contribution from the seafloor source.

Pore size controls on the base of the methane hydrate stability zone in the Kumano Basin, offshore Japan

AbstractThe base of the methane hydrate stability zone (MHSZ) in the Kumano Basin, offshore Japan, is marked by a bottom‐simulating reflection (BSR) on seismic data. At Integrated Ocean Drilling Program Site C0002, which penetrates this BSR, the in situ temperature profile combined with bulk seawater methane equilibrium conditions suggest that the base of the MHSZ is 428 m below seafloor (bsf), which is 28 m deeper than the observed BSR (400 m bsf). We found that submicron pore sizes determined by mercury injection capillary pressure are sufficiently small to cause 64% of the observed uplift of the base of the MHSZ by the Gibbs‐Thomson effect. This is the most thorough characterization of pore sizes within the MHSZ performed to date and illustrates the extent to which pore size can influence MHSZ thickness. Our results demonstrate the importance of considering lithology and pore structure when assessing methane hydrate stability conditions in marine sediments.

Ebullition and storm-induced methane release from the East Siberian Arctic Shelf

Vast quantities of carbon are stored in shallow Arctic reservoirs, such as submarine and terrestrial permafrost. Submarine permafrost on the East Siberian Arctic Shelf started warming in the early Holocene, several thousand years ago. However, the present state of the permafrost in this region is uncertain. Here, we present data on the temperature of submarine permafrost on the East Siberian Arctic Shelf using measurements collected from a sediment core, together with sonar-derived observations of bubble flux and measurements of seawater methane levels taken from the same region. The temperature of the sediment core ranged from -1.8 to 0 degrees C. Although the surface layer exhibited the lowest temperatures, it was entirely unfrozen, owing to significant concentrations of salt. On the basis of the sonar data, we estimate that bubbles escaping the partially thawed permafrost inject 100-630 mg methane m(-2) d(-1) into the overlying water column. We further show that water-column methane levels had dropped significantly following the passage of two storms. We suggest that significant quantities of methane are escaping the East Siberian Shelf as a result of the degradation of submarine permafrost over thousands of years. We suggest that bubbles and storms facilitate the flux of this methane to the overlying ocean and atmosphere, respectively.

Recent Development in IR Sensor Technology for Monitoring Subsea Methane Discharge

AbstractRecently developed methane sensors, based on infrared (IR) absorption technology, were successfully utilized for subsea methane release measurements. Long-term investigation of methane emissions (fluid flux determination) from natural methane seeps in the Hikurangi Margin offshore New Zealand were performed by using seafloor lander technology. Small centimeter-sized seep areas could be sampled at the seafloor by video-guided lander deployment. In situ sensor measurements of dissolved methane in seawater could be correlated with methane concentrations measured in discrete water samples after lander recovery. High backscatter flares determined by lander-based Acoustic Doppler Current Profiler (ADCP) measurement indicate bubble release from the seafloor. Highest methane concentrations determined by the IR sensor coincided with periods of high ADCP backscatter signals. The high fluid release cannot be correlated with tidal changes only. However, this correlation is possible with variability in spatial bubble release, sudden outbursts, and tidal changes in more quiescent seepage phases.A recently developed IR sensor (2,000 m depth-rated) with a detection limit for methane of about 1 ppm showed good linearity in the tested concentration range and an acceptable equilibration time of 10 min. The sensor was successfully operated offshore Santa Barbara by a small work-class ROV at a natural methane seep (Farrar Seep). High background methane concentration of 50 nmol L−1 was observed in the coastal water, which increases up to 560 nmol L−1 in dissolved methane plumes south of the seepage area. ROV- and lander-based sensor deployments have proven the applicability of IR sensor technology for the determination of subsea methane release rates and plume distribution. The wide concentration range, low detection limit, and its robust detection unit enable this technology for both subsea leak detection and oceanographic trace gas investigations.

Methane in the Sevastopol coastal area, Black Sea

Concentrations of dissolved methane in seawater and bottom sediments, as well as of methane emanating from gas seeps were measured at 18 stations including several small bays in the Sevastopol coastal area (Black Sea) during 2007–2008. Methane concentrations in surface waters ranged from 10 to 2,970 nmol l−1, and correlated well with values recorded for sediments. Methane concentrations in the water column were influenced by water depth, as well as by air and water temperatures. In the spring and summer of 2008, in situ CH4 saturation relative to air was in the range of 970–71,900%. Maximum saturation was in summer. CH4 fluxes to the atmosphere from the Sevastopol coastal area were estimated to vary from 190 to 1,550 μmol m−2 day−1. Gas bubbles escaping from the seepages contained about 57 vol% methane. Radiocarbon dating of the methane revealed an age not exceeding 150 years, implying a biogenic origin.

Episodic methane concentrations at seep sites on the upper slope Opouawe Bank, southern Hikurangi Margin, New Zealand

Along many active and some passive margins cold seeps are abundant and play an important role in the mechanisms of methane supply from the subsurface into seawater and atmosphere. With numerous cold seeps already known, the convergent Hikurangi Margin east of North Island, New Zealand, was selected as a target area for further detailed, multidisciplinary investigation of cold seeps within the New Vents and associated projects. Methane and temperature sensors (METS) were deployed at selected seep sites on the Opouawe Bank off the southeastern tip of North Island and near the southern end of the imbricate-thrust Hikurangi Margin, together with seismic ocean bottom stations. They remained in place for about 48 h while seismic data were collected. The seeps were associated with seep-related seismic structures. Methane concentrations were differing by an order of magnitude between neighbouring stations. The large differences at sites only 300 m apart, demonstrate that the seeps were small scale structures, and that plumes of discharged methane were very localised within the bottom water. High methane concentrations recorded at active seep sites at anticlinal structures indicate focused fluid flow. Methane discharge from the seafloor was episodic, which may result from enhanced fluid flow facilitated by reduced hydrostatic load at low tides. The strong semi-diurnal tidal currents also contribute to the fast dilution and mixing of the discharged methane in the seawater. Despite dispersal by currents, fluid flow through fissures, fractures, and faults close to the METS positions and tidal fluctuations are believed to explain most of the elevated methane concentrations registered by the METS. Small earthquakes do not appear to be correlated with seawater methane anomalies.

Methane seepage and its relation to slumping and gas hydrate at the Hikurangi margin, New Zealand

Dissolved methane and high resolution bathymetry surveys were conducted over the Rock Garden region of Ritchie Ridge, along the Hikurangi margin, eastern New Zealand. Multibeam bathymetry reveals two prominent, northeast trending ridges, parallel to subduction along the margin, that are steep sided and extensively slumped. Elevated concentrations of methane (up to 10 nM, 10× background) within the water column are associated with a slump structure at the southern end of Eastern Rock Garden. The anomalous methane concentrations were detected by a methane sensor (METS) attached to a conductivity‐temperature‐depth‐optical backscatter device (CTDO) and are associated with elevated light scattering and flare‐shaped backscatter signals revealed by the ship's echo sounder. Increased particulate matter in the water column, possibly related to the seepage and/or higher rates of erosion near slump structures, is considered to be the cause of the increased light scattering, rather than bubbles in the water column. Methane concentrations calculated from the METS are in good agreement with concentrations measured by gas chromatography in water samples collected at the same time. However, there is a c. 20 min (c. 900 m) delay in the METS signal reaching maximum CH4 concentrations. The maximum methane concentration occurs near the plateau of Eastern Rock Garden close to the edge of a slump, at 610 m below sea level (mbsl). This is close to the depth (c. 630 mbsl) where a bottom simulating reflector (BSR) pinches out at the seafloor. Fluctuating water temperatures observed in previous studies indicate that the stability zone for pure methane hydrate in the ocean varies between 630 and 710 mbsl. However, based on calculations of the geothermal gradients from BSRs, we suggest gas hydrate in the study area to be more stable than hydrate from pure methane in sea water, moving the phase boundary in the ocean upward. Small fractions of additional higher order hydrocarbon gases are the most likely cause for increased hydrate stability. Relatively high methane concentrations have been measured down to c. 1000 mbsl, most likely in response to sediment slumping caused by gas hydrate destabilisation of the sediments and/or marking seepage through the gas hydrate zone.

Methane-driven oceanic eruptions and mass extinctions: Comment and Reply

Dickens raises an important question. Phase diagrams do not tell the whole story, however. (If they did, diamonds would have turned into graphite long ago; instead, they seem to last forever.) For methane in seawater, [Handa (1990)][1] calculated the metastable gas-liquid solubility (saturation

Hydrate dissociation in chemical potential gradients: theory and simulations

Kinetic stability limits of hydrate reservoirs partially exposed to the ocean floor were studied with respect to methane concentration in surrounding seawater. The implications of low chemical potential of seawater-dissolved methane and the corresponding lack of thermodynamic stability of hydrates were analyzed. The minimum concentrations of methane in seawater necessary to achieve equilibrium between methane-containing seawater and methane hydrate were calculated. Nucleation theory was used to estimate the critical size and shape of the dissociating front as a function of methane concentration in the seawater, for concentration regions outside the stability limits for hydrate. The critical shape of the dissociating front was predicted to be flat and thin. Estimated induction times to reach critical size for dissociated layer vary from in the order of seconds for mole-fraction methane as low as 1.0×10 −8 to 3 days for a seawater mole-fraction of methane a magnitude lower than the necessary seawater methane concentration for equilibrium with hydrate. The theoretical predictions were compared with molecular dynamics investigations of model methane hydrate–free water and methane hydrate–brine systems under different temperatures and pressures.