BG5.1

Evidences of subsurface fluid flow-driven fractures (from seismic interpretation) are quite common at Vestnesa Ridge (around 79ºN in the Arctic Ocean), W-Svalbard margin. Ultimately, the fractured systems have led to the formation of pockmarks on the seafloor. At present day, the eastern segment of the ridge has active pockmarks with continuous methane seep observations in sonar data. The pockmarks in the western segment are considered inactive or to seep at a rate that is harder to identify. The ridge is at ~1200m water depth with the base of the gas hydrate stability zone (GHSZ) at ~200m below the seafloor. Considerable free gas zone is present below the hydrates. Besides the obvious concern of amount and rates of historic methane seeping into the ocean biosphere and its associated effects, significant gaps exist in the ability to model the processes of flow of methane through this faulted and fractured region. Our aim is to highlight the interactions between physical flow, geomechanics and geological control processes that govern the rates and timing of methane seepage.

For this purpose, we performed numerical fluid flow simulations. We integrate fundamental mass and component conservation equations with a phase equilibrium approach accounting for hydrate phase boundary effects to simulate the transport of gas from the base of the GHSZ through rock matrix and interconnected fractures until the seafloor. The relation between effective stress and fluid pressure is considered and fractures are activated once the effective stress exceeds the tensile limit. We use field data (seismic, oedometer tests on calypso cores, pore fluid pressure and temperature) to constrain the range of validity of various flow and geomechanical parameters in the simulation (such as vertical stress, porosity, permeability, saturations).

Preliminary results indicate fluid overpressure greater than 1.5 MPa is required to initiate fractures at the base of the gas hydrate stability zone for the investigated system. Focused fluid flow occurs through the narrow fracture networks and the gas reaches the seafloor within 1 day. The surrounding regions near the fracture network exhibit slower seepage towards the seafloor, but over a wider area. Advective flux through the less fractured surrounding regions, reaches the seafloor within 15 years and a diffusive flux reaches within 1200 years. These times are controlled by the permeability of the sediments and are retarded further due to considerable hydrate/carbonate formation during vertical migration. Next course of action includes constraining the methane availability at the base of the GHSZ and estimating its impact on seepage behavior.

How to cite: Ramachandran, H., Plaza-Faverola, A., Daigle, H., and Buenz, S.: Time Constraints on Seepage Through Fractured Regions on the Vestnesa Ridge off the W-Svalbard Coast, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11489, https://doi.org/10.5194/egusphere-egu21-11489, 2021.

The Arctic shelf hosts a large, yet poorly quantified reservoir of relic permafrost. It has been suggested that global warming, which is amplified in polar regions, will accelerate the thawing of this subsea permafrost, thus potentially unlocking large stocks of comparably reactive organic matter (OM). The microbial degradation of OM in the thawing and generally anoxic permafrost layer has the potential of producing and, ultimately, releasing important fluxes of  CH₄ to the atmosphere. Because CH₄ is a potent greenhouse gas, such a release would further intensify global warming. However, the potential role of subsea permafrost thaw on microbial CH₄ production and CH₄ emissions from Arctic sediments currently remains unconstrained.
Here, we use a nested model approach to address this critical knowledge gap. We developed a pseudo-three-dimensional reaction-transport model for permafrost bearing sediments on the Arctic shelf to estimate the production, consumption, and, efflux of CH₄ on the Arctic shelf in response to projected subsea permafrost thaw. The model accounts for the most pertinent biogeochemical processes affecting methane and sulfur cycling in permafrost bearing marine sediments.                                                                
It is initialized based on a published submarine permafrost map (SuPerMap, [1]) and forced by a range of projected thawing rate scenarios derived from the Max Planck Institute Earth System Model (MPI-ESM) simulation results for the period 1850-2100. Critical model parameters, such as permafrost OM content and its apparent reactivity are chosen based on a comprehensive analysis of published experimental data. Here, we present the output of this environmental scenario ensemble.                                                
Simulation results reveal that CH₄ production rates are highly sensitive to changes in the apparent reactivity of permafrost OM. Although simulated CH₄ production rates vary over a large range (0.001-130 PgC produced over 250 years), they generally highlight the potential for producing and, thus releasing large amounts of methane from thawing subsea permafrost on the warming Arctic Shelf.

[1] Overduin, P. P., Schneider von Deimling, T., Miesner, F., Grigoriev, M. N., Ruppel, C. D., Vasiliev, A., et al. (2019).
 Submarine permafrost map in the Arctic modeled using 1‐D transient heat flux (SuPerMAP). Journal of Geophysical Research: Oceans, 124, 3490–3507. https://doi.org/10.1029/2018JC014675

How to cite: Ridolfi, E., Wilkenskjeld, S., Miesner, F., Brovkin, V., Overduin, P., and Arndt, S.: Modeling methane production and emission from thawing sub‐sea permafrost on the warming Arctic Shelf, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-176, https://doi.org/10.5194/egusphere-egu21-176, 2021.

Drainage of peatlands for intensive and long-term agricultural use leads to higher mineralization rates of the organic material and thus, increased carbon dioxide (CO₂) emissions. However, when degraded peatlands are rewetted, high methane (CH₄) emissions are frequently observed, that may offset the reductions in CO₂ emissions. The created anaerobic conditions are favorable for methanogenic microorganisms and lead to the production of CH₄. The presence of sulfate in marine waters typically inhibits methanogenesis because methanogens are outcompeted by sulfate reducers. Therefore, the rewetting of coastal peatlands with marine waters is assumed to keep CH₄ emissions low. Flooding of coastal wetlands as a consequence of higher sea levels could strengthen the carbon sink function of these systems if the peatlands are able to grow their surface on par with the sea level. We used the January 2019 storm surge in the southern Baltic Sea to investigate the effects of brackish water intrusion on microbial abundance and community data along with CO₂ and CH₄ exchange data on a rewetted minerotrophic fen. Previous studies showed that despite the proximity to the Baltic Sea, the fen’s marine sulfate pool was substantially exhausted, and the microbial community was dominated by acetotrophic methanogens and high CH₄ emission characteristic for freshwater environments. We took parallel soil cores to compare the microbial methane-cycling community to the former freshwater rewetted state from four locations along a brackish water gradient. We used high-throughput sequencing and quantitative polymerase chain reaction (qPCR) on pools of DNA and cDNA targeting total and putatively active bacteria and archaea (16S rRNA gene), methanogens (mcrA), methanotrophs (pmoA) and sulfate-reducing bacteria (dsrB). Greenhouse gas (GHG) fluxes along the salinity transect were measured locally with closed-chambers and in addition on the ecosystem level using the eddy covariance approach. Chamber measurements along the transect imply lower CH₄ emissions at plots with higher salinity post-intrusion. This coincides with a drop in ecosystem CH₄ fluxes and with shifts from methane-cycling to sulfate-reducing microorganisms. We expect that organisms involved in anaerobic CH₄ oxidation with sulfate as terminal electron acceptor will be more prominent after the saltwater intrusion.

Moreover, the effect of rewetting with saltwater on GHG fluxes and microbial communities in degraded fens will be discussed relative to the effects of freshwater inundation and seasonal droughts which were assessed in the same location before.

How to cite: Gutekunst, C., Jenner, A.-K., Jurasinski, G., Böttcher, M. E., Koebsch, F., Kallmeyer, J., Knorr, K.-H., Unger, V., Yang, S., and Liebner, S.: Effects of saltwater intrusion on the methane-cycling microbial community of a freshwater rewetted coastal fen, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3328, https://doi.org/10.5194/egusphere-egu21-3328, 2021.

Shallow groundwater flow from the seasonally thawed active layer is increasingly recognized as an important pathway for delivering methane (CH₄) into Arctic lakes and streams, but its contribution to CH₄ emissions from thaw ponds has not been evaluated. Furthermore, the potential influence of the shallow groundwater-derived CH₄ on the trophic support and nutritional quality of thaw pond food chains remains unexplored. In this study, we used a radon-mass balance approach to quantify the CH₄ transport from the active layer into thaw ponds in a sub-Arctic catchment. We analysed stable isotopes and fatty acids of pond macroinvertebrates to evaluate the potential effects of CH₄ inputs through active layer groundwater flows on the aquatic food chains. Our results indicate that CH₄ fluxes from the active layer can sustain CH₄ emissions from the ponds. Consumers in ponds receiving greater CH₄ inputs from the active layer had lower stable carbon isotope signatures that indicates a greater trophic reliance on methane oxidizing bacteria (MOB), and they had lower nutritional quality as indicated by their lower tissue concentrations of polyunsaturated fatty acids. Accurate predictions of CH₄ release from small thaw ponds will thus require improved knowledge of the contributions from various processes including internal production, flow paths of active layer groundwater, and MOB-consumer interactions.

How to cite: Olid, C., Zannella, A., and Lau, D. C. P.: The role of methane transport from the active layer in sustaining methane emissions and food chains in subarctic ponds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15327, https://doi.org/10.5194/egusphere-egu21-15327, 2021.

Fires and drainage are common disturbance factors in tropical peatlands (TP) in Southeast Asia. These disturbances alter the hydrology, vegetation composition, and peat biogeochemistry; thereby affecting the microbiome where microbial communities reside. Studies from northern peatlands have well established the role of vegetation composition in regulating the labile C, in the form of plant root exudates, and microbial community composition affecting the peat decomposition; however, for tropics, it remains unexplored. Recent studies have also established how these fire-degraded TP areas become a hot spot of sedge-mediated CH₄ emission. To further our understanding of control mechanisms regulating CH₄ dynamics, we investigated the composition of plant root exudates (n=3 per plant species) from sedges (Scleria sumatrensis) and ferns (Blechnum indicum, Nephrolepis hirsutula), the most commonly occurring plant species at our fire-degraded tropical peatland site in Brunei, Northwest Borneo, as well as microbial community composition in plant (n=9 for S. sumatrensis, and B. indicum, and n=5 for N. hirsutula) rhizo-compartments (rhizosphere, rhizoplane, endosphere).

         Using a targeted analysis, we found that the root exudates compounds secreted from sedge (Scleria sumatrensis) and one species of fern (Blechnum indicum) were significantly different (p<0.05) and showed a similar ratio of 2:1 for sugars (glucose, fructose) and organic acids (acetate, formate, lactate, malate, oxalate, succinate, tartrate), which is in contrast to that secreted from trees in intact tropical peatlands (1:2). Further, using 16S rRNA gene amplicon sequencing, we found that the microbial community composition in rhizo-compartments of plant species showed significant differences (p<0.001). Interestingly, the sedge species harboured a relatively higher abundance of methanogens (Thermoplasmata) and lesser methanotrophs (Alphaproteobacteria, Gammaproteobacteria) across all three compartments compared to fern species, which further supports the higher sedge-mediated CH₄ emissions from fire-degraded TP.

            Our results provide fresh insights into the effects of post-fire vegetation composition in regulating the labile C and microbial community composition, and hence affecting CH₄ emissions from fire-degraded TP. Further, our results can form an important basis for future CH₄ dynamics studies as the emissions might increase with the expansion of degraded TPs as a consequence of frequent fire episodes and flooding

How to cite: Lupascu, M., Akhtar, H., Bandla, A., Sukri, R. S., and Swarup, S.: Root exudates compounds and microbial community composition regulates CH4 dynamics in fire degraded tropical peatland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10461, https://doi.org/10.5194/egusphere-egu21-10461, 2021.

Methane is one of the main greenhouse gases in the atmosphere. Lakes are the third-largest natural source of methane on a global scale [Kirschke et al., 2013]. Currently, the chamber method is quite often applied in the measurements of diffusive GHG emissions from natural ecosystems, especially in remote areas, due to low cost and mobility. In lakes, methane can be transported to the atmosphere not only by diffusion but also by bubbling, so during measurements, it is important to divide these two pathways. We have complemented customary floating chambers with plastic shields located underside not blocking diffusive transfer but preventing gas bubbles from entering into the chamber.

The study was conducted on August 19–20, 2019 in the vicinity of Vaskiny Dachi field station (68.8663° N, 70.3040° E, Central Yamal, Western Siberia, Russia). Measurements were carried out in the central part of the thermokarst lake with a depth of 1.6 m. To compare results of customary and modified chambers, samples were taken in parallel from chambers with and without shields (two chambers of each type) every two hours during the day. Sampling and flux calculations were conducted according to [Bastviken et al., 2010]. The methane concentrations in samples were determined in the laboratory by a Crystal 5000.2 gas chromatograph with a flame ionization detector.

According to the sign test for the 0.05 p-level, methane fluxes measured using chambers with and without shields differ statistically significant considering their diurnal dynamics. At the same time, within the group of fluxes measured by the same type of chambers, no statistically significant differences were found, and mean and median flux values are higher for chambers without shields. It appears that observed differences are not due to natural variability, but due to the contribution of bubble component to the fluxes measured by chambers without shields.

References

Bastviken D., Santoro A.L., Marotta H. et al. 2010. Methane emissions from Pantanal, South America, during the low water season: toward more comprehensive sampling. – Environmental Science & Technology, vol. 44, No. 14, pp. 5450–5455.

Kirschke S., Bousquet P., Ciais P. et al. 2013. Three decades of global methane sources and sinks. – Nature Geoscience, vol. 6, No. 10, pp. 813–823.

How to cite: Krivenok, L. and Kazantsev, V.: Features of the diffusive methane emission measurements on lakes by chamber method, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12685, https://doi.org/10.5194/egusphere-egu21-12685, 2021.

The results of passive seismic studies of subsurface fluid transport systems associated with mud volcanic phenomena in Northwestern Caucasus and the Taman mud-volcanic province are presented. Comparative analysis of results of geophysical cross-sections featuring the deep subsurface structures of several mud volcanoes obtained by means of passive microseismic sounding approach with respect to previous studies has demonstrated advantages of the ambient noise seismic prospecting. It has been shown that subvertical pathways of hydrocarbon migration and so feeding systems of mud volcanoes represent nearly-ideal case of local geological heterogeneities affecting the amplitudes of low-frequency microseismic noise. The analysis of the results was performed with respect to available geological as well as geomorphological data. At the same tine, results of past active seismic experiments with controlled vibroseismic sources were reanalyzed and followed by mathematical modeling of processes of hydrodynamic outflow under various mechanisms of mud volcanic eruptions. For several mud volcanoes there were outlined three-dimensional subvertical feeding structures in sedimentary layers and deeper in the crust, responsible for fluid migration and eruptive activity. Specific features of volcanic products (gas components and mineral inclusions in breccia) were analyzed with respect to the new geophysical data obtained.

How to cite: Puzich, I., Sobisevich, A., and Dudarov, Z.: Subsurface fluid migration associated with feeding systems of mud volcanoes in Northwestern Caucasus, results of passive seismic studies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15054, https://doi.org/10.5194/egusphere-egu21-15054, 2021.