BG10

EDI
Hydrocarbon seepage – from past records to modern examples and models to evaluate the future

The Earth’s subsurface hosts enormous methane volumes either trapped in the shallow sediments, gas hydrates and permafrost, or naturally escaping the sediment through methane seepage to enter the hydrosphere/atmosphere. Such environments are highly sensitive to climate change. Despite an increasing awareness about the positive feedback between global warming and methane seepage, the response of these complex and dynamic systems to climate change is still unclear due to complex geo/hydro/atmosphere interactions.
Fossil cold seeps, long-term observatory studies and modern examples form the foundations to understand the mutual dependences between climate and seepage, and to develop robust models to forecast future scenarios at the Earth-system scale. For this session, we welcome geologists, geophysicists, geochemists, biologists, model developers, and any others who have contributed to new case studies in modern and fossil hydrocarbon seeps in the marine and terrestrial environment, gas hydrate and permafrost settings, to describe both new methods/technologies and the scientific outcomes.

Public information:
Pop up networking event
Remember the good, old post-conference beer-in-hand chatting? We recreated a virtual rooftop bar where you will be able to chat and network, exactly as in real life! We want to bring together EGU scientists from the gas hydrate, permafrost, and broader cold seeps community. You will be asked to create your avatar and decide whether you want to keep your camera and microphone active or use the text chat and emotes to communicate.
It’s going to be fun! See you at the party!

Wed, 28 Apr, 17:00–19:00 CEST
Access: https://gather.town/app/wKeswzC2rCkHWNe9/methane_seep_lovers
Co-organized by CL4/SSP1
Convener: Claudio ArgentinoECSECS | Co-conveners: Davide Oppo, Giuliana Panieri, Miriam Römer
vPICO presentations
| Wed, 28 Apr, 11:45–12:30 (CEST)
Public information:
Pop up networking event
Remember the good, old post-conference beer-in-hand chatting? We recreated a virtual rooftop bar where you will be able to chat and network, exactly as in real life! We want to bring together EGU scientists from the gas hydrate, permafrost, and broader cold seeps community. You will be asked to create your avatar and decide whether you want to keep your camera and microphone active or use the text chat and emotes to communicate.
It’s going to be fun! See you at the party!

Wed, 28 Apr, 17:00–19:00 CEST
Access: https://gather.town/app/wKeswzC2rCkHWNe9/methane_seep_lovers

vPICO presentations: Wed, 28 Apr

Chairpersons: Claudio Argentino, Miriam Römer
11:45–11:50
11:50–11:52
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EGU21-480
Aleksandr Sabrekov, Irina Terentieva, Yuriy Litti, Mikhail Glagolev, and Ilya Filippov

Natural gas seepage is considered as a strong methane source influencing future climate predictions and creating local fire and explosion hazards. Recently numerous methane seeps were found in West Siberian middle taiga (WSMT) river floodplains. Seepage occurs in unvegetated spots and areas of saturated quicksand deposits and in the bottom of river stream beds. Based on location and size seeps found in WSMT can be classified in three groups: 1) single seeps and seep puddles away from river stream bed, 2) seep chains along the river streams, including seeps inside stream bed, 3) seep fields with numerous holes, funnels and craters with area up to several thousands m2. Origin of methane in these seeps is still not fully understood and methane emission from them is also not quantified. We aimed to fill these gaps during 2019-2020 August-September field campaigns.

To reveal the origin of seeping gas hydrocarbons concentration, methane carbon and hydrogen stable isotope ratios and methane radiocarbon concentration were measured in a gas sampled throughout the study region (250 km in north-south and 400 km in east-west directions). To compare characteristics of methane from seeps and from wetlands gas dissolved in wetland pore water and peat for incubation studies were sampled on a depth of 1-2 m in three representative bogs near seeps. During anaerobic peat incubation the dynamics of methane (including carbon stable isotope ratios), hydrogen, carbon dioxide and low-molecular fatty acids concentrations were monitored. Methane emission was estimated in the same three bogs using both static chamber method (provided a benchmark) and ecosystem-scale inverse modelling (backward Lagrangian simulation).

Obtained data indicate modern biogenic origin for the methane seeping in WSMT. Similarity in isotope signatures between methane from wetlands and seeps suggests lateral transport of methane through groundwater from raised bogs to seeps in floodplains. Methane produced in the upper layer of raised bogs emits to the atmosphere mainly through the root transport while in deeper layers vertical methane migration is limited. Raised bogs are widely developed in WSMT and cover about half of the total region area. We conclude that methane seeping observed in WSMT floodplains is caused by the lateral groundwater transport downward from the watershed to the floodplain.

Methane emission estimates for three seep fields made by chamber method and inverse modelling were in a good correspondence, although inverse modelling fluxes are 20-40% higher. Methane flux for investigated fields ranges from tens to hundreds mgCH4·m-2·h-1 with a strong difference (up to order of magnitude) between different fields.

This study improves understanding of the insufficiently investigated element of the methane biogeochemical cycle – transport through the groundwater when methane avoids oxidation in the unsaturated surface layer of the wetland. Emission from seeps can make a valuable contribution to the regional methane flux. Potential reaching of a lower explosive limit for methane concentration should be taken into account during planning of groundwater use.

This study was supported by a grant of the Russian Science Foundation (№ 19-77-10074).

How to cite: Sabrekov, A., Terentieva, I., Litti, Y., Glagolev, M., and Filippov, I.: Methane emission from seeps of West Siberian middle taiga river floodplains, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-480, https://doi.org/10.5194/egusphere-egu21-480, 2021.

11:52–11:54
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EGU21-583
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ECS
Abhishek Painuly and Regina Katsman

Gas-charged sediments of shallow water bodies are significant sources of atmospheric methane, an important greenhouse gas. Past accounts of gas bubbles developed in shallow aquatic sediments (and in their surrogates) have reported a controversial occurrence of vertical as well as horizontal bubbles topologies. Within the framework of tensile fracturing of muddy sediment produced by the growing bubbles, the vertical orientation of bubbles is well understood, however factors controlling horizontal bubble growth are largely unclear. This study is conducted by employing a mechanical/reaction–transport numerical model, which couples diffusion-led expansion of gas bubble and elastic-fracture mechanical response of sediment to its growth. Muddy sediment is assumed to exhibit a transverse anisotropy in fracture toughness (a property describing an easiness of breaking the inter particle bonds), attributed to partial or full alignment of plate-like clay particles. Our results demonstrate that bubbles growing in isotropic sediment develop a vertically oriented topology and start their ascent once reaching their mature sizes. Under an increasing measure of anisotropy, the bubbles grow horizontally at the initial stages, however at later stages they start evolving in vertical direction as well, under influence of gravity, and eventually initiate their vertical ascent as well. Our results suggest an explanation of apparent conundrum about preferred orientations of bubbles in muddy sediments. Laterally growing bubbles produced in anisotropic sediment are able to coalesce with neighboring ones and form interconnected permeable horizontal gas networks, as observed in some lab experiments. For the first time, our results reveal that anisotropy-led initial lateral bubble growth can also play a crucial role in accumulating gas reserve from long distances around large and small scale seeps and outlets, at continental margins and inland water bodies sediments. Additionally, horizontal bubbles tend to be stationary (in contrast to the vertical bubbles) thus being responsible for high gas storage (or retention) capability of aquatic sediments.

How to cite: Painuly, A. and Katsman, R.: Influence of anisotropy in sediment mechanical properties on CH4 bubble growth topology and migration pattern in muddy aquatic sediments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-583, https://doi.org/10.5194/egusphere-egu21-583, 2021.

11:54–11:56
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EGU21-963
Valentina Yanko, Anna Kravchuk, Irina Kulakova, and Tatiana Kondariuk

This presentation represents a case study that reviews research into the relationship between meiobenthos distribution and concentrations of hydrocarbon gases (HG), primarily methane, in the sediments of the northwestern part of the Black Sea, including gases released by mud volcanoes and gas seeps. Evidence forming the basis of this research comes from meiobenthos here represented by 29 species of benthic foraminifers, 7 species of ostracods, and 44 species of nematodes. The potential use of these meiobenthic organisms as indicators of gaseous hydrocarbons reservoirs existing under the seabed is evaluated according to two linked axes, namely the dual analysis of abiotic factors (physical and chemical parameters of the water column, gasmetrical, geochemical, lithological, and mineralogical properties of the sediments) and biotic characteristics (quantitative and taxonomic composition of foraminifers, nematodes, and ostracods). Studies of this kind have been directed toward developing interdisciplinary methods to improve the search for HG accumulations, especially methane, under the seabed. Development of such methods might have substantial socio-economic importance for the economy of Ukraine as well as that of other Black Sea countries, and such methods might also contribute to the sustainable development of Black Sea ecosystems.

How to cite: Yanko, V., Kravchuk, A., Kulakova, I., and Kondariuk, T.: Meiobenthos as indicator of gaseous hydrocarbons reservoirs existing under the seabed of the Black Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-963, https://doi.org/10.5194/egusphere-egu21-963, 2021.

11:56–11:58
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EGU21-1807
Alexey L. Sobisevich, Elena I. Suetnova, and Ruslan A. Zhostkov

Large amounts of methane hydrate locked up within marine sediments are associated to mud volcanoes. We have investigated by means of mathematical modeling the unsteady process of accumulation of gas hydrates associated with the processes of mud volcanism. A mathematical model has been developed. The system of equations of the model describes the interrelated processes of filtration of gas-saturated fluid, thermal regime and pressure, and accumulation of gas hydrates in the seabed in the zone of thermobaric stability of gas hydrates. The numerical simulation of the accumulation of gas hydrates in the seabed in the deep structures of underwater mud volcanoes has been carried out using the realistic physical parameters values. The influence of the depth of the feeding reservoir and the pressure in it on the evolution of gas hydrate accumulations associated with deep-sea mud volcanoes is quantitatively analyzed. Modeling quantitatively showed that the hydrate saturation in the zones of underwater mud volcanoes is variable and its evolution depends on the geophysical properties of the bottom environment (temperature gradient, porosity, permeability, physical properties of sediments) and the depth of the mud reservoir and pressure in it. The volume of accumulated gas hydrates depends on the duration of the non-stationary process of accumulation between eruptions of a mud volcano. The rate of hydrate accumulation is tens and hundreds times the rate of hydrate accumulation in sedimentary basins of passive continental margins.

How to cite: Sobisevich, A. L., Suetnova, E. I., and Zhostkov, R. A.: Mathematical model of a non-stationary process of accumulation of gas hydrates, confined to deep-water mud volcanoes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1807, https://doi.org/10.5194/egusphere-egu21-1807, 2021.

11:58–12:00
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EGU21-5628
Leonid Yurganov, Dustin Carroll, Andrey Pnyushkov, Igor Polyakov, and Hong Zhang

Existence of strong seabed sources of methane, including gas hydrates, in the Arctic and sub-Arctic seas with proven oil/gas deposits is well documented. Enhanced concentrations of dissolved methane in deep layers are widely observed. Many of marine sources are highly sensitive to climate change; however, the Arctic methane sea-to-air flux remains poorly understood: harsh natural conditions prevent in-situ measurements during winter. Satellite remote sensing, based on terrestrial outgoing Thermal IR radiation measurements, provides a novel alternative to those efforts. We present year-round methane data from 3 orbital sounders since 2002. Those data confirm that negligible amounts of methane are fluxed from the seabed to the atmosphere during summer. In summer, the water column is strongly stratified from sea-ice melt and solar warming. As a result, ~90% of dissolved methane is oxidized by bacteria. Conversely, some marine areas are characterized by positive atmospheric methane anomalies that begin in November. During winter, ocean stratification weakens, convection and winter storms mix the water column efficiently. We also find that the amplitudes of the seasonal cycles over Kara and Okhotsk Seas have increased during last 18 years due to winter concentration growth. There may be several factors responsible for sea-air flux: growing emission from clathrates due to warming, changes in methane transport from the seabed to the surface, changes in microbial oxidation, ice cover, etc. Finally, methane remote sensing results are compared to available observations of temperature in deep ocean layers, estimates of Mixed Layer Depth, and satellite microwave sea-ice cover measurements.

 

How to cite: Yurganov, L., Carroll, D., Pnyushkov, A., Polyakov, I., and Zhang, H.: Wintertime Methane Emission From the Barents and Kara Seas and Sea of Okhotsk: Satellite Evidence., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5628, https://doi.org/10.5194/egusphere-egu21-5628, 2021.

12:00–12:02
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EGU21-2545
Davide Oppo, Sian Evans, Christopher A-L Jackson, David Iacopini, SM Mainul Kabir, and Vittorio Maselli

Hydrocarbon escape systems can be regionally active on multi-million-year timescales. However, reconstructing the timing and evolution of repeated escape events can be challenging because their expression may overlap in time and space. In the northern Levant Basin, eastern Mediterranean, distinct fluid escape episodes from common leakage points formed discrete, cross-evaporite fluid escape pipes, which are preserved in the stratigraphic record due to the coeval Messinian salt tectonics.

The pipes consistently originate at the crest of prominent sub-salt anticlines, where thinning and hydrofracturing of overlying salt permitted focused fluid flow. Sequential pipes are arranged in several kilometers-long trails that were progressively deformed due to basinward gravity-gliding of salt and its overburden. The correlation of the oldest pipes within 12 trails suggests that margin-wide fluid escape started in the Late Pliocene/Early Pleistocene, coincident with a major phase of uplift of the Levant margin. We interpret that the consequent transfer of overpressure from the deeper basin areas triggered seal failure and cross-evaporite fluid flow. We infer that other triggers, mainly associated with the Messinian Salinity Crisis and compressive tectonics, played a secondary role in the northern Levant Basin. Further phases of fluid escape are unique to each anticline and, despite a common initial cause, long-term fluid escape proceeded independently according to structure-specific characteristics, such as the local dynamics of fluid migration and anticline geometry.

Whereas cross-evaporite fluid escape in the southern Levant Basin is mainly attributed to the Messinian Salinity Crisis and compaction disequilibrium, we argue that these mechanisms do not apply to the northern Levant Basin; here, fluid escape was mainly driven by the tectonic evolution of the margin. Within this context, our study shows that the causes of cross-evaporite fluid escape can vary over time, act in synergy, and have different impacts in different areas of large salt basins.

How to cite: Oppo, D., Evans, S., Jackson, C. A.-L., Iacopini, D., Kabir, S. M., and Maselli, V.: Leaky salt: pipe trails record the history of cross-evaporite fluid escape in the northern Levant Basin, Eastern Mediterranean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2545, https://doi.org/10.5194/egusphere-egu21-2545, 2021.

12:02–12:04
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EGU21-8529
Daniela Fontana, Stefano Conti, Chiara Fioroni, and Claudio Argentino

The effects of global warming on marine gas hydrate stability along continental margins is still unclear and discussed within the scientific community. Long-term datasets can be obtained from the geological record and might help us better understand how gas-hydrate reservoirs responds to climate changes. Present-day gas hydrates are frequently associated or interlayered with authigenic carbonates, called clathrites, which have been sampled from many continental margins worldwide. These carbonates show peculiar structures, such as vacuolar or vuggy-like fabrics, and are marked by light δ13C and heavy δ18O isotopic values. Evidences of paleo-gas hydrate occurrence are recorded in paleo-clathrites hosted in Miocene deposits of the Apennine chain, Italy, and formed in different positions of the paleo foreland system: in wedge-top basins, along the outer slope of the accretionary prism, and at the leading edge of the deformational front. The accurate nannofossil biostratigraphy of sediment hosting paleo-clathrites in the northern Apennines allowed us to ascribe them to different Miocene nannofossil zones, concentrated in three main intervals: in the Langhian (MNN5a), in the upper Serravallian-lower Tortonian (MNN6b-MNN7) and the upper Tortonian-lowermost Messinian (MNN10-MNN11). By comparing paleo-clathrite distributions with 3rd order eustatic curves, they seem to match phases of sea-level lowering associated with cold periods. Therefore, we suggest that the drop in the hydraulic pressure on the plumbing system during sea-level lowering shifted the bottom of the gas hydrate stability zone to shallower depths, inducing paleo gas-hydrate destabilization. The uplift of the different sectors of the wedge-top foredeep system during tectonic migration might have amplified the effect of the concomitant eustatic sea-level drop, reducing the hydrostatic load on the seafloor and triggering gas-hydrate decomposition. We suggest that Miocene climate-induced sea-level changes played a role in controlling gas hydrate stability and methane emissions along the northern Apennine paleo-wedge, with hydrate destabilization roughly matching with sea-level drops and cooling events.

 

How to cite: Fontana, D., Conti, S., Fioroni, C., and Argentino, C.: Gas hydrate destabilization and sea-level changes: insights from Miocene seep carbonate deposits of the northern Apennines (Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8529, https://doi.org/10.5194/egusphere-egu21-8529, 2021.

12:04–12:06
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EGU21-9321
Kun Zhang, Haibin Song, Hongbin Wang, Yi Gong, Wenhao Fan, Yongxian Guan, Jun Tao, and Minghui Geng

Cold seep is a widespread geological process mainly caused by hydrocarbon fluid migration. Methane bubble plumes released from cold seeps are often observed at the seafloor. These methane bubbles might be released into the atmosphere and have a huge effect on climate changes. It is of great significance for understanding the fate of these methane bubble plumes.

Many kinds of methods have been used to observe the methane bubble plumes, e.g., acoustical, geochemical, and optical methods. Video imaging is a kind of optical methods widely used in methane bubble plume studies. Compared to other methods, video imaging is a non-intrusive, high-resolution, and quick-collected method. Many studies have estimated bubbles' size, rise velocities, behavior, and the fate of bubbles by analyzing video images manually. However, manual analysis is time-consuming, one dimension, and has not been able to determine temporospatial changes in a two-dimension profile perspective.

In this study, we applied the manual analysis method and the particle image velocimetry (PIV) method to analyze in-situ video image sequences of Haima cold seep bubble plumes, a newly discovered, active cold seep in the Qiongdongnan Basin of the northern South China Sea during 2019. Quantitative and temporospatial change information about the plume flow filed is obtained. The results show that the sizes of bubbles in the plume range from 2.556 ~ 4.624 mm, with a rising velocity of ~ 0.26 m/s. The flux for an individual bubble stream is ~ 94.8 ml/min. The flow velocity field of the bubble plume is consistent with the manual analysis, and it reveals that the bubble plume's flow field is a multiscale turbulent flow field. The bubble plumes are usually V-shaped. Through carrying the adjacent water column, the bubble plumes swell and change rapidly. The direction and velocity of the bubble plume flow change with time, and its streamlines are sinuous. The max velocity of the bubble plume flow field changes at a 6.6 s period cycle.

Although there is some indetermination, our results show that the PIV method is feasible for calculating the bubble plume flow field and that it has some unique advantages, e.g., it is fast, non-invasive, it provides two-dimension temporospatial change images, and it has a high resolution. The images of the bubble plume flow field provide a new perspective to observe the cold seep systems. We hope that this method can be improved and widely applied in cold seep plume studies in the future.

How to cite: Zhang, K., Song, H., Wang, H., Gong, Y., Fan, W., Guan, Y., Tao, J., and Geng, M.: Using video images to study Haima cold seep bubble plumes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9321, https://doi.org/10.5194/egusphere-egu21-9321, 2021.

12:06–12:08
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EGU21-10216
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ECS
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Cristina Corradin, Angelo Camerlenghi, Michela Giustiniani, Umberta Tinivella, and Claudia Bertoni

In the Mediterranean Basin, gas hydrate bottom simulating reflectors (BSR) are absent, with very few and spatially limited exceptions occurring in Eastern Mediterranean mud volcanoes and in the Nile deep sea fan. This is in spite of widespread occurrence of hydrocarbon gases in the subsurface, mainly biogenic methane, from a wide range of stratigraphic intervals.
In this study we model the methane hydrate stability field using all available information on DSDP and ODP boreholes in the Western Mediterranean and in the Levant Basin, including the downhole changes of pore water salinity. The models take into account the consequent pore water density changes and use known estimates of geothermal gradient. None of the drilled sites were located on seismic profiles in which a BSR is present.
The modelled base of the stability field of methane hydrates is located variably within, below, or even above the drilled sedimentary section (the latter case implies that it is located in the water column). We discuss the results in terms of geodynamic environments, areal distribution of Messinian evaporites, upward ion diffusion from Messinian evaporites, organic carbon content, and the peculiar thermal structure of the Mediterranean water column.
We conclude that the cumulative effects of geological and geochemical environments make the Mediterranean Basin a region that is unfavorable to the existence of BSRs in the seismic record, and most likely to the existence of natural gas hydrates below the seabed.

How to cite: Corradin, C., Camerlenghi, A., Giustiniani, M., Tinivella, U., and Bertoni, C.: On the lack of widespread bottom simulating reflectors in the Mediterranean Basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10216, https://doi.org/10.5194/egusphere-egu21-10216, 2021.

12:08–12:10
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EGU21-10621
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ECS
Claudio Argentino, Kate Waghorn, Sunil Vadakkepuliyambatta, Stéphane Polteau, Stefan Bünz, and Giuliana Panieri

Methane emissions from Arctic continental margins may increase due to global warming. Present-day ocean fluxes seem to provide a minor contribution to the atmosphere methane pool, but large uncertainties still remain on the magnitude of future emissions from methane seeps and gas hydrate-bearing sediments. The Barents Sea is a natural laboratory to study the evolution of methane seeps in relation to climate change, as it recorded several phases of ice-sheet advance and retreat during the Pleistocene. Glaciations and its concurrent denudation of the Barents Sea influenced the subsurface, causing reservoir expansion and fracturing, thereby driving hydrocarbon (mostly gas) migration which resulted in a sustained regional fluid flow system. New data from this area can shed light on future response of other high-latitude continental shelves worldwide. Here, we present reconstructed methane emission dynamics at Leirdjupet Fault Complex (LFC), SW Barents Sea, since last deglaciation (occurred after ~19 cal Ka BP). The geochemical composition of sediment cores indicate prolonged methane emissions, which started after 14.5 cal Ka BP. Geochemical proxies for anaerobic oxidation of methane in the sediment (barium, calcium and sulfur enrichments, isotopic composition of foraminifera) indicate an overall decrease in seepage intensity over the Holocene toward present-day conditions. Methane-derived authigenic carbonates with aragonite mineralogy and heavy δ18O signature recorded an episode of gas hydrate destabilization in this region. Paleo-hydrate stability models suggest that this event was triggered by the influx of warm Atlantic water and isostatic uplift linked to the retreat of the Barents Sea Ice Sheet. Present-day distribution of methane seeps at LFC is strongly linked to underlying faults. Methane hydrates are stable in the southern part of the investigated seepage area and might respond to a future increase in bottom water temperatures.

How to cite: Argentino, C., Waghorn, K., Vadakkepuliyambatta, S., Polteau, S., Bünz, S., and Panieri, G.: Dynamics of methane seepage at Leirdjupet Fault Complex (SW Barents Sea) since last deglaciation and possible future scenarios, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10621, https://doi.org/10.5194/egusphere-egu21-10621, 2021.

12:10–12:12
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EGU21-13179
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ECS
Seasonal variability of natural methane seepage offshore Western Svalbard
(withdrawn)
Manuel Moser, Knut Ola Dølven, and Bénédicte Ferré
12:12–12:30