Atmospheric methane (CH₄) is a potent greenhouse gas with natural and anthropogenic sources. Concentrations have been significantly increasing over the past few decades, which poses a problem for future climate change goals. The contribution of oceans to the global atmospheric CH₄ cycle is largely uncertain. It is accepted that oceans act as a small net source of atmospheric CH₄. As the polar regions are warming faster than the global average, it is important that we can better quantify CH₄ emissions from the polar oceans. 

In this study, we combine various forms of shipborne data (ambient atmospheric methane concentrations, sea-air CH₄ fluxes and isotopic composition of atmospheric CH₄) taken during cruises in the Arctic and Southern Oceans to present a more complete picture of atmospheric CH₄ above polar oceans, including addressing the question of how much the oceanic component is contributing towards the atmospheric budget in these regions. Measurements  were made around the Barents Sea and Greenland Sea in the Arctic, and in the Atlantic sector of the Southern Ocean, including the Scotia Sea.

Sea-air CH₄ fluxes are measured using the eddy covariance method; indeed, this the first study to use this technique to directly measure how much CH₄ is released from the ocean into the atmosphere in both the Southern and Arctic Oceans. Atmospheric CH₄ measurements are then investigated in order to understand the impact that CH₄ released from the ocean has on the atmospheric burden. We also measure the isotopic composition of CH₄ (δ²H and δ¹³C) in air samples taken onboard polar cruises, to understand the sources of atmospheric CH₄ above these oceans. The isotope measurements can indicate if the CH₄ comes from a biogenic or thermogenic source, which can help determine if anthropogenic or natural processes are behind the production.

We investigate the potential sources of CH₄ released by the polar ocean by looking at areas of known seabed CH₄ seepages, investigating phytoplankton abundance, and investigating the isotopic composition of atmospheric CH₄ in areas of elevated CH₄. 

We find that the region of the Arctic Ocean investigated in this study is a slight atmospheric CH₄ source in boreal summer, while the region of the Southern Ocean investigated is a CH₄ source in areas of shallower water/continental shelves and a CH₄ sink in region of open ocean, in austral summer. This finding is consistent with previous studies that have detected seabed CH₄ emission. Seabed CH₄ seepage at shallower depths is more likely to penetrate the sea-air interface, while CH₄ produced at the seabed at deeper depths gets oxidised as it travels through the water column, making it less likely to reach the surface . We also find evidence of localised “hot spots” of methane emission which will be described. 

How to cite: Workman, E., Jones, A., Fisher, R., France, J., Linse, K., Yang, M.-X., Bell, T., and Dong, Y.: Investigating oceanic sources of methane and their influence on ambient concentrations in polar regions., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1297, https://doi.org/10.5194/egusphere-egu23-1297, 2023.

The UNESCO world heritage Wadden Sea is a highly productive coastal system, rich in biodiversity and influenced by physical and biological forcing. Such coastal seas are known to dominate ocean’s methane emissions, but knowledge on variations and controls of methane and the efficiency of the microbial methane filter in these vastly dynamic systems are scarce. We conducted high frequency sampling over a 2-days’ period during four seasons to determine diel and seasonal effects on methane dynamics in the Dutch Wadden Sea. We found that waters were charged with methane throughout the year with maximum concentrations of up to 155 nM in summertime. Methane concentrations were generally lower at high tide and in the colder seasons, whereas MOB activity increased by ~2-fold during low tide compared to high tide. On average, only a minor fraction of the WZ methane budget (~2%) is retained by MOBs in the WZ itself, while ~1/3 escapes to the atmosphere and ~2/3 are flushed out into the open North Sea where it may be consumed by microbes or is liberated to the atmosphere.

How to cite: de Groot, T., Mol, A., Mesdag, K., Witte, H., Ndhlovu, R., Ramond, P., Engelmann, J., Röckmann, T., and Niemann, H.: Diel and seasonal methane dynamics in the shallow and turbulent Wadden Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5981, https://doi.org/10.5194/egusphere-egu23-5981, 2023.

Terrestrial hydrocarbon seeps are widely distributed in oil/gas field. To constrain the sources and post-generation processes occurring in these seeping gases, various geochemical approaches, such as chemical and stable isotope composition (δ¹³C and δD) of hydrocarbons have been extensively used. However, the interpretation can be ambiguous due to the overlap of signatures when using these approaches only¹. Some recently developed analytical techniques, such as methane clumped isotope analysis (Δ¹³CH₃D and Δ¹²CH₂D₂)^2-4 and propane position-specific isotope analysis (PSIA)⁵ may provide new clues to improve our understanding of the origin and fate of hydrocarbons.

In this study, we focus on gas samples from different gas seeps and mud volcanos in central Japan collected from 2019 to 2022, where hydrocarbons were considered mainly originating from thermal cracking of organic matter⁶. Gas compositions, bulk stable isotopes of hydrocarbons and associated CO₂, clumped isotopes of methane, PSIA of propane and other geochemical parameters have been studied. Coupled methane clumped isotope signatures and propane PSIA information provide direct evidence of secondary microbial methane formation associated with biodegradation of non-methane hydrocarbons. The contribution of secondary microbial methane in different seeps/mud volcanos and its temporal changes are also discussed by a mixing model integrating all these isotope information, which provides valuable constraints on methane sources in terrestrial seeps.

References: [1] Milkov and Etiope, 2018, Org. Geochem.; [2] Stolper et al., 2014, Geochim. Cosmochim. Acta.; [3] Stolper et al., 2014, Science; [4] Young et al., 2017, Geochim. Cosmochim. Acta.; [5] Gilbert et al., 2019, Proc. Natl. Acad. Sci. U.S.A.; [6] Etiope et al., 2011, Appl. Geochem.

How to cite: Zhang, N., Jajalla, M., Nakagawa, M., Taguchi, K., and Gilbert, A.: Coupled methane clumped isotope and propane position-specific isotope analyses indicate significant methane contribution from biodegradation of hydrocarbons in terrestrial methane seeps located at central Japan, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3725, https://doi.org/10.5194/egusphere-egu23-3725, 2023.

Gassy aquatic sediments are abundant over the world. Multiannual CH₄ gas content in shallow sediments of Lake Kinneret, Israel, was evaluated by acoustic applications. Experiments were conducted mainly over the intermediate-deep parts of the lake. Low-to-moderate frequencies wideband acoustic signal was emitted, when sound speed indicating a gas content, was evaluated based on the reflection coefficient. Both frequency dependence of  reflection coefficient and backscattering were analyzed.. The effect of the following factors affecting the dynamics of CH₄ bubbles in aquatic sediments in the lake Kinneret, was investigated statistically: (1) Organic matter flux to sediment controlling CH4 production; (2) Its timing relatively to the date of the acoustic measurements, controlling CH4 bubbles dissolution; (3) Water depth affecting CH4 solubility, mechanical sediment properties, and ebullition from the sediment. Multiple regression analysis indicates that the organic matter supply to the lake sediments due to the crash of phytoplankton bloom in the lake, acts as a major control on the sediment gas content over the multi-annual period. The gas content is least sensitive to water depth, explained probably by the uniform organic matter deposition flux to the medium-deep parts of the lake, from where the ebullitions is unfeasible. [the work is supported by BSF grant 2018150].

How to cite: Katsnelson, B., Uzhansky, E., Katsman, R., Lunkov, A., and Ivakin, A.: Multi-physical controls on gas content in sediments of lake Kinneret, Israel, evaluated by acoustic applications, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7680, https://doi.org/10.5194/egusphere-egu23-7680, 2023.

Since 2012 Ocean Networks Canada (ONC) has acquired high-quality long-term multibeam sonar data from the seafloor at Clayoquot Slope off Vancouver Island which is one of the most active methane vent regions of the Cascadia Margin, and where ONC has one of its nodes of the NEPTUNE cabled seafloor observatory. The sonar was first deployed at a vent field of irregular activity (near Bubbly Gulch), and since 2014 it is located at an extremely active field (Gastown Alley). The data that have been analyzed so far exhibit the strong dependence of gas bubble emissions with tidal pressure, although the tides alone cannot explain all the observed dynamics such as onset cessation of ebullition or periods of strong versus absence of seepage, and other factors need to be considered and ideally monitored to predict future seepage.

Seafloor cabled observatories are ideal to acquire high-resolution data of many ocean and seafloor parameters, including those from high-bandwidth data or power-intensive instruments. This presentation is an opportunity to explore possibilities to modify the existing seepage observatory, adding to the already installed instruments at Clayoquot Slope, or change focus to Barkley Canyon, another ONC node location, where hydrate mounts and outcrops occur and methane vents also exist.

How to cite: Scherwath, M., Römer, M., Marcon, Y., and Riedel, M.: Continuous Seep Monitoring at the Northern Cascadia Margin by Ocean Networks Canada, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16805, https://doi.org/10.5194/egusphere-egu23-16805, 2023.

The origin of modern seafloor methane emissions in the Barents Sea is tightly connected to the glacio-tectonic and oceanographic transformations following the last ice age. Despite the increasing number of new active seep discoveries, their accurate geochronology and paleo-dynamic is still poorly resolved, thus hindering precise identification of triggering factors and mechanisms controlling past and future seafloor emissions. Here, we report the distribution, petrographic (thin section, electron backscatter diffraction), isotopic (δ¹³C, δ¹⁸O) and lipid biomarker composition of methane-derived carbonates collected from Leirdjupet Fault Complex, SW Barents Sea, at 300 m depth during an ROV survey in 2021. The integration of phase-specific isotopic analysis and U/Th dating enabled us to track carbonate mineral precipitation over the last 8 ka.  Our results indicate that methane and petroleum seepage in this area followed a similar evolution as in other southernmost Barents Sea sites controlled by the asynchronous deglaciation of the Barents Sea shelf, and that methane-derived carbonate precipitation is still an active process at many Arctic locations.  

This study was supported by AKMA project (Research Council of Norway grant No. 287869) within the frame of the Centre for Arctic Gas Hydrate, Environment and Climate (CAGE) (Research Council of Norway grant No. 223259), and by Erasmus+ Programme of the European Union.

How to cite: Argentino, C., Lee, A., Fallati, L., Sahy, D., Birgel, D., Peckmann, J., Bünz, S., and Panieri, G.: Biogeochemistry and timing of methane-derived carbonate formation at Leirdjupet fault complex, SW Barents Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1917, https://doi.org/10.5194/egusphere-egu23-1917, 2023.

Coastal systems including river deltas are the major sources of methane from the ocean to the atmosphere; however, large uncertainties exist on the actual source strength. Abiotic and biotic factors controlling methanogenesis, methanotrophy and methane efflux to the atmosphere thus need better understanding. In this presentation, we will show data from a recent cruise along the Sheldt Estuary from Antwerp (salinity: 2 psu) towards the open North Sea (salinity: 31 psu). Methane concentrations were elevated in Antwerp with up to 110 nM, decreased downriver to values ~50nM, increased to 180 nM at the inflow of a large canal (Ghent–Terneuzen Canal) and then decreased to ~70nM in the open North Sea. Methane oxidation rates were highest in the city of Atnwerp (~29 nM/d), only slightly elevated at the inlet of the Ghent–Terneuzen Canal (~10 nM/d) and lower in the open North Sea (5nM/d). Differently to previous findings in other river systems, we could not find a clear dependency of methane oxidation to salinity. In this presentation, we will also present data on CH₄ fluxes from the river to the atmosphere, CH₄ stable isotope systematics and the composition of the methanotrophic community to further constrain methane dynamics in the estuary.

How to cite: Delre, A., de Groot, T., Röckmann, T., Engelmann, J., Reichart, G.-J., and Niemann, H.: "Methane oxidation in the Estuary of a medium-sized European river, the Scheldt", EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11959, https://doi.org/10.5194/egusphere-egu23-11959, 2023.

Methane (CH₄) bubbles residing in shallow aquatic muds pose a significant threat to the environment. Impeded by the muddy sediment opacity and insufficient resolution for their characterization, past studies overlooked bubble interactions during their growth. The competitive growth of CH₄ bubble pairs with different initial sizes is simulated, using a mechanical/reaction-transport numerical model. Mechanical and solute transport interactions were found to dominate at the different stages of the bubble growth, both retarding the smaller competitive bubble growth. Stress from the large competitive bubble affects the inner pressure and diffusive flux to the smaller bubble, producing its slower initial growth. The large competitive bubble diverts CH₄ from the smaller one at the later stages, thus inhibiting its growth even more. Bubble stress interactions may produce more laterally oriented smaller bubbles and significant deformations of the larger ones. Competitive bubble growth may shape a bubble size distribution pattern, promote muddy sediment CH₄ gas retention, and produce gas domes. The latter acts as pockmark precursors whose formation induces a violent gas release to the water column and potentially to the atmosphere. Our study presents a basis for proper upscaling to various effective gassy muddy sediment characteristics and gas retention models. It contributes to the evaluation and even reduction of a long-persisting uncertainty related to the CH₄ fluxes from the shallow aquatic sediments.

How to cite: Katsman, R. and Zhou, X.: Competitive CH4 bubble growth in aquatic muds, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4257, https://doi.org/10.5194/egusphere-egu23-4257, 2023.

The estimations of the diffusive methane flux from the water phase into the atmosphere in coastal waters is relevant for a better estimate of the atmospheric greenhouse-gas budget. Unfortunately, so far, the numerical determination of the fluxes has a high level of uncertainty in coastal waters.

To improve the estimation of coastal methane fluxes, not only a high temporal and spatial sampling resolution of the dissolved methane in the water are required. Besides, also the atmospheric methane concentration and the wind speed and wind direction above the surface is important. In most cases, these atmospheric data are obtained from near-by atmospheric and meteorologic monitoring stations. In this study, we measured wind speed, direction and atmospheric methane local directly on board of three research vessel cruising in the southern North Sea within the MOSES project and compared the effects of local versus remote measurements of these data on the flux data. In addition, using the wind direction and speed, we try to assess the origin of the atmospheric methane measured in the study area. Using these “improved” data sets, we discuss if local measurements of auxiliary data provide better insights in the determining factors of the methane flux, and thus also improve the regional aquatic methane budget.

How to cite: Bussmann, I., Achterberg, E., Brix, H., Brüggemann, N., Fischer, P., Flöser, G., Greinert, J., Ködel, U., and Schütze, C.: Influence of wind speed and wind direction above the sea surface on the diffusive methane flux and the atmospheric methane concentration at the North Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8623, https://doi.org/10.5194/egusphere-egu23-8623, 2023.

As societies gradually shift from oil and gas to renewable energy, many offshore wells will be plugged and abandoned, while some will be transformed to facilitate carbon storage (CCS). In order to mitigate any methane leakage associated with abandonment, it is necessary to understand whether the leakage has a natural or anthropogenic origin. During exploration and production, emphasis is almost exclusively on the reservoir, while post abandonment monitoring programs generally focus on the water column. Thus, only little attention has been given to the shallow subsurface, how it has been influenced by hydrocarbon production, and to which degree the geology facilitates or inhibits fluid migration. In order to monitor and evaluate future leakage of hydrocarbons to the marine environment, it is crucial to understand the natural seepage through the seabed both locally at platforms and regionally. For CCS, understanding the shallow subsurface is equally important as monitoring cannot be confined to the water column alone. Mapping potential migration paths is thus necessary in order to mitigate any leakage.

 The aim of the SEEP project is to develop a Danish North Sea baseline for methane seepage in the shallow subsurface, near oil and gas platforms and in areas without any hydrocarbon production. Applying such a baseline will facilitate identification of anthropogenic seepage and thus help recognize the potential local environmental impact associated with abandonment.

Using newly collected geophysical data, we have categorized various types of shallow methane seeps, and placed them in a geological context. By combining the shallow seismic data with deep industry seismic data, we have identified potential sources and paths for thermogenic methane migrating from reservoir depths.  The geophysical data is then integrated with results from sediment core analysis. These include Facies analysis of cores, dating of sediments, benthic faunal variations between core sites, geochemistry of bivalves and foraminifera, studies of the chemical and isotopic composition of the dissolved gas in the pore water, as well as the community-composition of gas-degrading bacteria.

This integrated approach will provide a solid model for gas distribution, frequency and origin, as well as impact on the environment in the shallow subsurface.    

Here we present the different tools that are the foundation of such a baseline, including results and examples from geophysical mapping (both multibeam echosounder and seismic data), biostratigraphy, geochemistry and geomicrobiology.  

How to cite: Prins, L. T., Lauridsen, B. W., Clausen, O. R., Røy, H., Kjeldsen, K. U., and Knutz, P.: SEEP - Building a SEabed Environmental baseline for Platform abandonment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12876, https://doi.org/10.5194/egusphere-egu23-12876, 2023.

Permafrost and glaciers in the high Arctic form an impermeable ‘cryospheric cap’ that traps a large reservoir of sub-surface methane and hinders it from reaching the atmosphere. The vulnerability of the cryosphere to climate warming is making releases of this methane possible, but uncertainty in the magnitude and timing makes future predictions of Arctic greenhouse gas emissions difficult. In Svalbard, where air temperatures are rising more than twice as fast as the average for the Arctic, glaciers are retreating and leaving behind an exposed forefield that enables rapid methane escape.

We undertook a field survey of unprecedented spatial coverage across central Svalbard to identify methane emission hotspots in glacial forefields, a previously unknown emission source. Here we document how methane-rich groundwater springs that have formed in recently revealed forefields of 78 land-terminating glaciers are bringing deep-seated methane gas to the surface. Waters collected from these springs are supersaturated with methane up to 600,000-times greater than atmospheric equilibration, with strong isotopic evidence of a thermogenic source. We estimate annual emissions of methane degassing from such groundwater springs to be up to 2.31 kt across the Svalbard archipelago. Our findings reveal that climate-driven glacial retreat is facilitating widespread release of methane, a positive feedback loop that is likely to be prevalent across other regions of the rapidly warming Arctic.

How to cite: Kleber, G., Hodson, A., Magerl, L., Schytt Mannerfelt, E., Bradbury, H., Zhu, Y., Trimmer, M., and Turchyn, A.: Glacial retreat driving enhanced methane emissions in the high Arctic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15412, https://doi.org/10.5194/egusphere-egu23-15412, 2023.

Many chemosynthesis-based communities prospering in deep-sea environments rely on the metabolic activity of sulfur-oxidizing bacteria. This is also the case for vestimentiferan siboglinid tubeworms, whose demand for nutrition is entirely satisfied by their endosymbiotic bacteria harbored in the trophosome. Such chemosymbiosis leads to a significantly lower nitrogen isotope composition of the trophosome than in other types of soft tissue. However, the specific process of nitrogen utilization by siboglinids remains unclear. As a key element in the relevant enzymes (nitrogenase, nitrate reductase), molybdenum (Mo) is indispensable in the biogeochemical cycling of nitrogen. The Mo isotope composition (δ⁹⁸Mo) of siboglinids is thus a potential proxy to decode the mode of nitrogen utilization. In this study, we found that δ⁹⁸Mo along the chitinous tube of the vestimentiferan siboglinid Paraescarpia echinospica from the Haima seep of the South China Sea yields values as negative as -4.59‰ (-1.13 ± 1.75‰, n = 19) – the lowest δ⁹⁸Mo signature ever reported for any kind of natural material. It is suggested that this extremely negative Mo isotope composition is caused by preferential utilization of isotopically light Mo by the tubeworm symbionts during nitrate reduction. Such Mo isotope signature could provide a means to identify siboglinid tubeworms in the rock record, a group of annelids that has previously escaped unambiguous identification due to the lack of mineralized skeleton.

How to cite: Wang, X., Peckmann, J., Bayon, G., Jia, Z., Gong, S., Li, J., and Feng, D.: The molybdenum isotope signature of microbial nitrogen utilization, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10760, https://doi.org/10.5194/egusphere-egu23-10760, 2023.

The West‐Svalbard continental slope represents one of the northernmost gas hydrate provinces in the world. It is located on the western Svalbard margin in the eastern Fram Strait at ~79°N, north of the Knipovich Ridge and Molloy transform fault, situated on a hot and relatively young oceanic crust. The project Advancing Knowledge of Methane in the Arctic (AKMA), funded by the Norwegian Research Council, explores Arctic methane sources, processes, ecosystems and geological history in part of this province. During a dedicated research expedition (CAGE21-1-AKMA) a spectacular pinnacle-like structure has been identified at 914 m of water depth in the area of the north Kniponich Ridge, during the exploration by remotely operated vehicle. 

The structure is located at the base of a small-scale escarpment, typified by the presence of carbonate slabs. It is more than 1 m in height, has a diameter of at least 50 cm, and appears isolated on a flat seafloor with muddy-dominated heterogeneous sediment. From the video, the structure was apparently composed of dead bivalves cemented in a substrate of finer particles. Many bivalves were still articulated and with their valves closed, although no sign of living bivalves could be detected. Living regular echinoids, one large crinoid, several sponges and a few soft corals colonized the surface at the time of observation. Small samples have been collected from the surface of this fossil bivalve pinnacle. These samples supported the video-based interpretation of a fossil structure composed of cemented and carbonate-encrusted dead valves, among which the most abundant and the largest specimens belong to Thyasira cf capitanea, both juvenile and adult. The valves, either articulated or disarticulated, are often cemented by a white thick (3 mm) authigenic carbonate crust that binds together the mollusk shells, but also encrusts some of them internally. This means that these crusts continued to grow also after the death of the mollusks, when some of the valves were open. Additional interesting benthic fauna recognized in the cement includes small gastropods and foraminifers. The genus Thyasira has already been described as a typically chemosymbiotic group of species and as such, it has been reported from active cold seeps in the Arctic, as well as in other geographic areas and in the geologic record. We interpret the pinnacle as fossil evidence of a site of past methane emission, possibly exhumed by recent erosional or gravity-driven resedimentation processes.

How to cite: Riva, M., Bracchi, V. A., Argentino, C., Savini, A., Fallati, L., Panieri, G., and Basso, D.: A pinnacle-like structure dominated by the chemosymbiotic bivalve Thyasira from the Arctic Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14499, https://doi.org/10.5194/egusphere-egu23-14499, 2023.