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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.

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.

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 3^rd 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.

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.

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.