The deep ocean methanosphere is defined by the microbial communities that cycle methane, the animals that directly consume or form symbioses with methane-consuming microbes, and the transitional animal communities that gain energy indirectly from methane and/or take advantage of the methane-derived authigenic carbonate. Our research seeks to redefine our understanding of the fate and footprint of methane on Pacific continental margins. By applying molecular, isotopic, geochemical, and radiotracer tools to the seep microbes and fauna we hope to better understand the contribution of methane to deep-sea diversity and ecosystem function. During the 2023 expedition AT50-12 with the RV Atlantis and the submersible Alvin we explored a set of methane seeps located in the Southern California Borderland. Samples were taken from seep carbonates, sediments, and the water column surrounding methane vents to study microbial methanotrophic activity and its relevance for methane removal, habitat engineering, and primary productivity. This poster will provide a first glance into new datasets on methanotrophy generated during the expedition and into the heterogeneity of deep ocean methane seeps off the coast of southern California.

How to cite: Treude, T., Kloniki- Ference, E., Liu, J., Homola, K., Li, Y., Utter, D. R., Wipfler, R. L., Mayr, M. J., Magyar, J. S., Orphan, V. J., Goffredi, S., and Levin, L. A.: The Life Methanic: Microbial Activity in the Deep Ocean Methanosphere of the Southern California Borderland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3678, https://doi.org/10.5194/egusphere-egu24-3678, 2024.

Methane seepage in the Hola area off the coast of Vesterålen (N. Norway) has long been known for its peculiar association with cold-water coral mounds, but only recently it was possible to explore the distribution of seafloor ecosystems using a Remotely Operated Vehicle (ROV) and to conduct microhabitat-specific samplings for biogeochemical investigations. Here, we describe the results from sediment (carbon-nitrogen systematics) and pore fluid geochemistry (sulfate, dissolved inorganic carbon, methane) and interpret them in relation to the seafloor ecosystems. Microbial mats are the dominant seep-related community and form small white patches of a few tens of cm in diameter located at various distances from the coral mounds. Seep carbonates are widespread at this location and form extensive pavements. The seafloor distribution of methane bubbling and chemosynthetic communities seem controlled by fractures in the carbonates. Microbial mats are associated with intense sulfate-driven anaerobic oxidation of methane producing shallow sulfate-methane transitions coupled with highly ¹³C-depleted dissolved inorganic carbon in the pore water.

Acknowledgments: this research was supported by Eman7 project (Research Council of Norway grant No. 320100) and AKMA project (Research Council of Norway grant No. 287869).

How to cite: Argentino, C., Panieri, G., Fallati, L., Savini, A., Barrenechea Angeles, I., Akinselure, A., and Ferré, B.: Biogeochemistry of seep-impacted sediments at a cold-water coral site off the Vesterålen coast, Northern Norway, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6944, https://doi.org/10.5194/egusphere-egu24-6944, 2024.

We investigated water chemical properties (chlrolophyll a, nutrients), particulate/dissolved organic carbon (POC, DOC) and methane in water columns of the semi-closed estuarine system (Masan-Jinhae Bay, South Korea). Together with the overall OM increase in upper boundaries (chlrolophyll a; 13.3±2.3 μg/L, POC and DOC; 0.2-2.1 mg/L) of Masan-Jinhae Bay, methane profiles within water columns showed the distinct predominance (101.2±7.6 nM) near surface layers. Considering OC isotopic compositions (δ¹³C_POC; -19.4±0.6 ‰, δ¹³C_DOC; -22.5±0.4 ‰) may reflect the decomposition of autochthonous OC sources, the methane production near surface seems to be potentially related to chemical reactions (e.g., demethylation and OM aggregates) of biological sources. Specifically, methane concentrations at adjacent terrestrial realm (inner Masan Bay) showed the most increased patterns (401.5±47.3 nM), indicating significant correlations with dissolved nitrogen and DOC. Together with the substantial transport of anthropogenic derived-nitrogen (δ¹⁵N_NO3,δ¹⁸O_NO3; 4.3±0.3 ‰, 6.0±1.0 ‰), our results infer that methanogenic process may be potentially influenced from the predominant OC production/decomposition under increased discharge of domestic wastewater. In near future, the additional analysis of methane isotopes may provide important clues for effectively understanding methane dynamics such as production and removal.

How to cite: Lee, D.-H., Jeong, C.-B., Kim, S.-H., and Shin, K.-H.: Potential methanogenic evidence related to the transportation of anthropogenic elements in semi-closed estuarine system (Masan-Jinhae Bay, South Korea) , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11260, https://doi.org/10.5194/egusphere-egu24-11260, 2024.

Methane is widely found on continental margins. It originates from either microbial processes at shallow sedimentary depth or thermal cracking of organic matter at deep depth, and occurs as disolved or free gas, or hydrates. It is the main chemical compound found both in natural gas hydrate deposits and seafloor gas emissions at the cold-seep.

There are extensive methane manifestations both in the sedimentary and water columns of the Black Sea.  This stratified sea is characterized by large quantities of methane bubbles discharged at the seafloor from the very shallow coastal shelf to the deep basin (Riboulot et al., 2017), contributing to the high concentration level measurement in the water column. Hydrate-bearing sediments are also widely distributed within the sediment on the continental slope, and Riboulot et al. (2018) showed that the seawater infiltration make them vulnerable and prone to dissociation since the reconnection of the Atlantic Ocean via the Sea of Marmara.

The expeditions Ghass 2 in September 2021 allowed the investigation of several methane emission sites from the continental shelf to the deep basin in the Romanian sector of the Black Sea, including hydrate-bearing sites. The water column was probed to measure in situ dissolved methane concentration using a commercial methane sensor and the prototype laser spectrometer SubOcean and sampled from CTD-Rosette. A ~6m-length hydrate-bearing core was collected from a long Calypso piston corer from which a high-resolution sampling of hydrates was performed to estimate the influence of geological factors on their cage occupancy.

The presentation aims to provide further background on methane dynamics in the Black Sea.

References

Agnissan Constant Art-Clarie, Guimpier Charlène, Terzariol Marco, Fandino Olivia, Chéron Sandrine, Riboulot Vincent, Desmedt Arnaud, Ruffine Livio (2023). Influence of Clay-Containing Sediments on Methane Hydrate Formation: Impacts on Kinetic Behavior and Gas Storage Capacity . Journal Of Geophysical Research-solid Earth , 128(9).

Riboulot Vincent, Ker Stephan, Sultan Nabil, Thomas Yannick, Marsset Bruno, Scalabrin Carla, Ruffine Livio, Boulart Cedric, Ion Gabriel (2018). Freshwater lake to salt-water sea causing widespread hydrate dissociation in the Black Sea . Nature Communications , 9(117), 1-8

Acknowledgements

The authors thank the different projects and programs for their financial supports DOORS by the EU Project number 101000518, ENVRIPLUS by EC Project number 654182, Blame ANR-18-CE01-0007, ORAGGE by Interdisciplinary graduate School for the Blue planet (ANR-17-EURE-0015 and "Investissements d'Avenir"), SEAMLESS by INSU LEFE Programme 2022

How to cite: Ruffine, L., Agnissan, C. A.-C., Giunta, T., Grilli, R., Lozano, M., Peyronnet, C., Donval, J.-P., Paris, J.-D., Desmedt, A., Riboulot, V., Dupré, S., and Fandino, O.: Methane occurrence in the Black Sea: From hydrates to the water column, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12819, https://doi.org/10.5194/egusphere-egu24-12819, 2024.

Cabled ocean observatories enable permanent continuous observations in situ in addition to regular sampling during observatory maintenance expeditions, which allow for some of the most comprehensive monitoring to understand the fate of methane all the way from below the seafloor (with instrumented boreholes), through the seafloor (with bottom sensors, cameras or cabled vehicles) into the water column (with sonars). Ocean Networks Canada is operating the NEPTUNE observatory off the coast of Vancouver Island since 2009 with two of its instrument nodes at gas hydrate sites. The first site, Clayoquot Slope, is at around 1200 m water depth and is a site of high fluid expulsion including large amounts of methane gas seepage, which has been observed for over a decade with permanent sonar scanning. The second site, Barkley Canyon, is at about 900 m water depth and has hydrate mounds with exposed gas hydrate on the seafloor, and is unique for its thermogenic methane and also oil seepage, and the most important observations have been made by a remotely-operated cabled and instrumented a seafloor crawler called Wally. This presentation will provide a few highlights on gas hydrate observations and invites the research community for new ideas how to expand the use of the permanent and continuous data flow opportunities that stem from the 24/7 presence of power and communication availability at the two different hydrate sights.

How to cite: Scherwath, M., Riedel, M., Marcon, Y., Römer, M., Thomsen, L., Purser, A., Chatzievangelou, D., and Du Preez, C.: Observing the fate of methane utilizing Ocean Network Canada's cabled seafloor NEPTUNE Observatory, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14307, https://doi.org/10.5194/egusphere-egu24-14307, 2024.

Particles generating Maximum Turbidity Zones (MTZ), in estuaries undergo several cycles of deposition/resuspension cycles before definitive burial or expelling towards the continental shelf. Positioning and spatial coverage depend on its morphology, riverine discharge and tidal dynamics. In the Loire estuary, the decadal flood of February 2021 displaced a lot of material from the upper to the lower estuary. Consequently, cores sampled at four sites upstream Paimboeuf (15 km from estuarine mouth) showed very dark cohesive sediments while those taken at two stations downstream, within the MTZ, showed a thick (10-50 cm), unconsolidated layer of a light-brown sediment over a darker-cohesive one. More striking, these four upstream stations, covering a river line of some 40 km, showed gas ebullition generating numerous cracks on the first decimetres of interface cores.

A few months later (June 2021), interface and long cores were sampled and methane analysed at three stations along the salinity gradient (two upstream and one downstream of Paimboeuf). Upstream, the entire interface and long cores showed methane saturated samples (about 2 mmol L^-1) and cracks remained ebullitive. Near Paimboeuf, the cores were no longer ebullitive and a clear sulphate-methane transition zone (SMTZ) was observed at 50 cm depth. Downstream, the SMTZ was located at 30 cm depth. These results suggest that the important currents induced by the 2021 winter flood eroded a significant layer of sediment to generate depressurization and allow methane to escape in the upper estuary while the lower estuary remained capped by enough sediment to maintain the SMTZ. They also showed that the decrease of riverine discharge allowed the MTZ to migrate upstream stopping methane to escape in the mid estuary while the upper estuary continued to release methane through ebullition four months after the erosion event. Crack formation and methane release also affected benthic fluxes, increasing total oxygen uptake by a factor of ten (94 mmol m^-2 d^-1) compared to diffusive flux, for example.

The rare opportunity to document such processes because of extreme navigation conditions, particularly for the deployment of the corer, allows us to emphasize that winter flooding can be an important source of methane that is immediately transferred to the atmosphere due to shallow depth of estuaries and must be taken into consideration for budgets and fluxes between reservoirs. This process must also be taken into account for a better understanding of other estuarine biogeochemical cycles, such as oxygen and nutrient cycles, as crack formation and methane release significantly increase their benthic fluxes.

How to cite: Metzger, E., Bombled, B., Hulot, V., Maillet, G., Mouret, A., Fleurant, C., Deflandre, B., Rigaud, S., Thibault de Chanvalon, A., Sanchez, S., Beneteau, E., Poprawski, Y., and Rabouille, C.: The exceptional winter flood of Loire river 2021: an unexpected source of methane in the inner estuary, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19144, https://doi.org/10.5194/egusphere-egu24-19144, 2024.

Recent research underscores a potential, yet overlooked, positive climate feedback mechanism: the transport of subglacially produced methane (CH₄) to the atmosphere via meltwater. While the majority of research focused on release from beneath the Greenland Ice Sheet, mountain glaciers have been largely understudied, creating a gap in our understanding of the spatial distribution of subglacial CH₄ emissions. Emerging research from glaciers in Iceland, Canada, Alaska, and China suggests the presence of CH₄ release also from glaciers other than the Greenland Ice Sheet.

Here, we explore the potential of CH₄ release from outlet glaciers of the Jostedalsbreen ice cap and Midtdalsbreen in central Norway. We investigated whether meltwaters from outlet glaciers of Jostedalsbreen (Tuftebreen, Fåbergstølsbreen, Bøyabreen, Supphelebreen, Austerdalsbreen and Nigardsbreen), along with Midtdalsbreen in Finse, act as a source of CH₄ to the atmosphere. We collected discrete samples for dissolved CH₄ (dCH₄) and CO₂ concentrations at all glacier outlets multiple times throughout the melt season. Additionally, we conducted longer time-series measurements of dCH₄ at Tuftebreen, Fåbergstølsbreen and Midtdalsbreen, utilizing custom-made dCH₄ sensors. Accompanying these measurements were samples analysed for water chemistry and stable isotopes of dCO₂ (δ¹³C-CO₂) in samples where concentrations were elevated compared to atmospheric equilibrium.

Our results indicate that dCH₄ concentrations in the meltwater of all studied glaciers remained below atmospheric equilibrium concentrations throughout the melt season. In contrast, dCO₂ concentrations surpassed atmospheric equilibrium levels, suggesting that the studied glacial runoffs do not act as CH₄ source to the atmosphere but might contribute as a small source of CO₂. This dataset of dissolved greenhouse gases enhances our understanding of the spatial distribution of subglacial CH₄ emissions, fostering discussions on carbon cycling beneath glaciers and the factors influencing the presence or absence of CH₄ emissions from the subglacial domain.

How to cite: Stehrer Polášková, A., Hatton, J., Yde, J. C., Engen, S. H., Trubač, J., Costa Pinto Wentzel, L., Lamarche-Gagnon, G., Tingey, S., Wadham, J., Stibal, M., and Sapper, S. E.: Is methane being released from Norwegian glaciers? , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19278, https://doi.org/10.5194/egusphere-egu24-19278, 2024.

Naproxen and caffeine may enter marine environments through discharge of wastewater, subsequently accumulating in marine sediments at nanogram per gram (ng/g) levels. These compounds, containing methyl groups linked to heteroatoms, may serve as potential substrates for microbial communities, including methanogens.  Methanogens could utilize these methyl groups directly for methylotrophic methanogenesis, or indirectly for hydrogenotrophic or acetoclastic methanogenesis. Alternatively, some other microorganisms are capable solely of demethylation. Our study investigates the capacity of marine sedimentary communities to demethylate naproxen and caffeine, potentially leading to methane production in marine sediments.

To elucidate the biotransformation pathways and potential methane production from these compounds, we employed a multiple line of evidence approach, including microbial incubation with surface marine sediments, stable isotope analysis, concentration analysis, and 16S rRNA amplicon sequencing. Sediments were collected from Hakefjorden, near the town of Stenungsund on the Swedish west coast, adjacent to the Strävliden WWTP's discharge outflow. Sediments were characterized to contain 4 and 7 ng/g of caffeine and naproxen, respectively. The top sediment layer (2-10 cm) was used for microbial incubation experiments with ¹³C-labeled methyl group substrates, including naproxen, caffeine, and standard methanogenesis substrates such as acetate, methylamine, and carbonate. This enabled tracing of the microbial transformation of the methyl groups within these compounds. Results indicate that both naproxen and caffeine act as precursors to ¹³C-methane production, with naproxen additionally leading to ¹³C-carbon dioxide formation. The presence of these compounds enriched specific microbial populations, including methanogens (Methanomicrobiaceae), other Archaeal groups (Lokiarchaeia), various fermentative bacteria, and sulfate-reducing bacteria.

The transformation of naproxen and caffeine stemming from wastewater into methane and carbon dioxide highlights alternative substrates for greenhouse gas production in marine sediments, an area of concern considering these and other anthropogenic compounds' presence in the sediment. This underscores the need for detailed research into the interactions between marine microbes and methylated pharmaceuticals, given their potential impact on greenhouse gas emissions.

How to cite: Gilevska, T., Rotaru, A., Fonseca, A., Kümmel, S., Krauss, M., Inostroza, P. I., and Bonaglia, S.: Demethylation of Naproxen and Caffeine by Marine Sedimentary communities, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19350, https://doi.org/10.5194/egusphere-egu24-19350, 2024.

In the North Sea a number of surveys have turned their attention to the extent of methane emissions from abandoned or decommissioned wells that may provide gas migration pathways for shallow biogenic gas accumulations. As part of the diverse and partly contradicting investigations, the research cruise MSM98 was to study sedimentary gas ebullition and the consequent distribution and fate of methane (CH₄) within the water column in the German EEZ, particularly at the eastern Dogger Bank in the German EEZ, where a number of abandoned wells and natural gas ebullition sites are present. During MSM98 in January 2021, sedimentary gases were sampled from two different ebullition sites using an ROV. Their main component was biogenic methane, isotopically light ethane with concentrations of 50 - 125 ppm and traces of propane. The area of the Berta salt dome on the eastern Dogger Bank was the only study site with significant elevations of methane in the water column. Four additional abandoned well sites in the German EEZ had methane concentrations close to the background values, with methane profiles at two sites showing a slightly irregular pattern. Methane maxima at known ebullition sites were significantly lower compared to an earlier study in summer 2019, when the water column was stratified with a clear thermocline. During the first three days of sampling at the eastern Dogger Bank, trace amounts of ethane and propane were detected in water samples throughout the water column showing decreasing concentrations with time. For these thermogenic gases, the analysis of currents and hydrographic properties indicated a source outside the working area. In conjunction with ventilated offshore waters coming into the working area these allochthonous light hydrocarbons disappeared except for background concentrations of methane. Short-term changes, likely horizontal input of light hydrocarbons, and rapid mixing of the complete water column did not allow for any quantitative statement with regard to sedimentary input of methane into the water column or export of methane into the atmosphere.

How to cite: Heeschen, K., Schlömer, S., Kopte, R., Römer, M., and Blumenberg, M.: Sedimentary gas ebullition in shallow waters of the Central North Sea (German Dogger Bank), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21466, https://doi.org/10.5194/egusphere-egu24-21466, 2024.

Recent studies have shown the release of methane (CH₄) from the melting Greenland Ice Sheet (GrIS) and identified it as having an additional potential positive climate feedback. This methane originates mainly from acetoclastic methanogenesis in subglacial sediments, accumulates over time, and subsequently diffuses into the subglacial hydrologic network which transports it to the ice sheet margin. The rates of methane production and emission from GrIS subglacial sediments likely depend on a number of factors, including sediment depth and distribution, organic matter content in the sediment and its reactivity, the redox conditions, and downstream methanotrophic activity; however, their relative significance remains unquantified.

Here, we use a reaction-transport model that accounts for heterotrophic methane production, methane oxidation, as well as advective and diffusive methane transport to quantitatively assess the potential for biogenic methane production and emissions from subglacial sediments underneath the Greenland Ice Sheet. The model is run over a large environmental condition model ensemble (n=3840) covering the entire range of plausible subglacial sediment thickness, subglacial organic matter availability and reactivity, oxygen concentration and methanotrophic activity as constrained by available field observations from subglacial and/or similar environments and/or laboratory experiments. Model results are discussed in the context of available field observations.

Results show that methanogenic activity in subglacial sediments can produce large quantities of methane (10^-5 -7.9⋅10¹ mmol m-² yr-¹). Subglacial methane production rates compare well with observations from laboratory studies. They are strongly controlled by organic matter availability and subglacial sediment depth, but are less sensitive to the availability of oxygen in overlying waters. Only for low organic carbon contents, low methanotrophic rate constants and/or high oxygen concentrations does methane production become more sensitive to oxygen concentration in overlying waters. Simulated methane effluxes vary four orders of magnitude and again strongly depend on organic matter availability and subglacial sediment depths. However, in contrast to methane production, methane efflux is also sensitive to oxygen concentration and methanotrophic activity. Methane effluxes generally decrease with increasing oxygen concentration and their sensitivity to oxygen concentration increases with increasing methanotrophic activity. Model results show that subglacial sediments can support methane effluxes that are up to 100 times higher than the flux required to sustain observed subglacial methane fluxes at the outflow (0.653 mmol m-² yr-¹ Lamarche-Gagnon et al., 2019) for realistic organic carbon contents (0.06 - 0.5 wt%), reactivity (0.013-1.1 yr^-1), subglacial sediment depths (100-500 cm) and methanotrophic rate constants (10¹⁰-10¹² mol cm-3yr-¹) under both anoxic and partly oxic conditions (<100 µM).

How to cite: Pika, P., Arndt, S., Lamarche-Gagnon, G., Hatton, J., Žárský, J., Wentzel, L. C. P., Trubač, J., Stehrer Polášková, A., Klímová, P., Hawkings, J. R., and Stibal, M.: The potential for methane production and release from subglacial sediments underneath the Greenland Ice Sheet – a model sensitivity study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17526, https://doi.org/10.5194/egusphere-egu24-17526, 2024.