Displays

Methane seepage at the seafloor is a source of carbon in the marine environment and has long been recognized as an important window into the deep geo-, hydro-, and bio-spheres. However, the processes and temporal patterns of natural methane emission over multi-million-year time scales are still poorly understood. The microbially-mediated methane oxidation leads to the precipitation of authigenic carbonate minerals within subseafloor sediments, thus providing a potentially extensive record of past methane emission. In this study, we used data on methane-derived authigenic carbonates to build a proxy time series of seafloor methane emission over the last 150 My. We quantitatively demonstrate that variations in sea level and organic carbon burial are the dominant controls on methane leakage since the Early Cretaceous. Sea level controls variations of methane seepage by imposing smooth trends with cyclicities in the order of tens of My. Organic carbon burial shows the same cyclicities and instantaneously controls the volumes of methane released thanks to the rapid generation of biogenic methane. The identified fundamental (26-27 My) cyclicity matches those observed in the carbon cycle associated with plate tectonic processes, the atmospheric CO₂, the oceanic anoxic events, and mass extinction events. A higher (12 My) cyclicity relates to modulations of Milankovitch eccentricity cycles and to variations in global tectonics. These analogies demonstrate that the seafloor methane seepage across the last 150 My relates to a large spectrum of global phenomena and thus has key implications for a better understanding of methane cycling at the present day. Temporal correlation analysis supports the evidence that the modern expansion of hypoxic areas and its effect on organic carbon burial may lead to higher seawater methane concentrations over the coming centuries.

How to cite: Oppo, D., De Siena, L., and Kemp, D.: A record of seafloor methane seepage across the last 150 million years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8396, https://doi.org/10.5194/egusphere-egu2020-8396, 2020.

Gas hydrates exist within a relatively narrow envelope of thermal and pressure conditions,  small changes in which may lead to widespread dissociation and methane release.  During past glacials, extensive ice sheets covered the continental margins of the Arctic Basin yielding ideal high pressure and low temperature conditions for the sequestration of thermogenic and biogenic methane in hydrate-bearing subglacial sediments.  On ice sheet retreat at the end of the last glacial, these hydrate reservoirs experienced major perturbations in thermal and pressure conditions leading to decomposition and methane mobilization over a variety of magnitude, temporal and spatial scales. Using geophysical data to constrain state-of-the-art ice sheet/gas hydrate modelling, we investigate how past Northern Hemisphere ice sheets modulated carbon sequestration and release.  Our results provide the first quantitative assessment of widespread subglacial hydrate formation and mobilization during the last glacial, yields insights into global carbon cycle dynamics and informs potential future atmospheric greenhouse composition and feedbacks associated with shrinkage of the contemporary cryosphere.

How to cite: Hubbard, A., Vadakkepuliyambatta, S., Patton, H., Serov, P., Pau, M., Winsborrow, M., Wadham, J., Mienart, J., and Andreassen, K.: Subglacial gas hydrates: ice sheet modulation of methane, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20809, https://doi.org/10.5194/egusphere-egu2020-20809, 2020.

Large amounts of methane are trapped within gas hydrate in sub-seabed sediments in the Arctic Ocean, and bottom-water warming may induce the release of methane from the seafloor. Yet, the effect of seasonal temperature variations on methane seepage activity remains unknown, as surveys in Arctic seas are mainly conducted in summer. Here, we compare the activity of cold seeps along the gas hydrate stability limit offshore Svalbard during cold (May 2016) and warm (August 2012) seasons. Hydro-acoustic surveys revealed a substantially decreased seepage activity during cold bottom-water conditions, corresponding to a 43 % reduction of total cold seeps and methane release rates compared to warmer conditions. We demonstrate that cold seeps hibernate during cold seasons, when more methane gas becomes trapped in the sub-seabed sediments. Such a greenhouse gas capacitor increases the potential for methane release during summer months. Seasonal bottom-water temperature variations are common on the Arctic continental shelves. We infer that methane-seep hibernation is a widespread phenomenon that is underappreciated in global methane budgets, leading to overestimates in current calculations.

This research is part of the Centre for Arctic Gas Hydrate, Environment and Climate (CAGE) and is supported by the Research Council of Norway through its Centres of Excellence funding scheme grant No. 223259 and UiT.

How to cite: Ferré, B., Jansson, P. G., Mosser, M., Serov, P., Portnov, A., Graves, C., Panieri, G., Gründger, F., Berndt, C., Lehmann, M., and Niemann, H.: Cold seep hibernation in the Arctic sediments during cold bottom water conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4919, https://doi.org/10.5194/egusphere-egu2020-4919, 2020.

Methane is a potent greenhouse gas with strongly increasing atmospheric concentrations since industrialisation. In the ocean, methane is most dominantly produced in sediments and is of microbial and/or thermogenic origin. Uprising methane may escape from the ocean floor to the overlying water column where it can be oxidized by methane oxidizing bacteria. The aerobic methane oxidation (MOx) is thus an important final barrier, which can mitigate methane release from the ocean to the atmosphere where it contributes to global warming. Nevertheless, there is rather little knowledge on the temporal dynamics of the microbial methane filter capacity in the water column. To gain a better understanding of the dynamics, we conducted two 48 hours’ time-series experiments during highly stratified conditions in summer and and mixed water column conditions in autumn above an active methane seep in the North Sea (Doggerbank, 41m water depth). At Doggerbank, dissolved CH₄  δ¹³C-values (<-65 ‰) indicate a microbial CH₄ origin, and seismic data suggest a gas pocket at >50 m sediment depth. Our time series measurement showed that CH₄ concentrations were highly elevated with up to 2100 nM in bottom and 350 nM in surface waters under stratified conditions. The maxima showed a ~12h periodicity, indicating that the flux of CH₄ from the seep was linked to tidal dynamics with the lowest CH₄ concentrations at rising tide and enhanced flux at falling tide. In contrast, during mixed water column conditions we found lower maxima of only up to 450 nM. Yet, during mixed conditions we found that surface water methane concentrations were on average XX-fold higher compared to stratified conditions, suggesting a higher methane efflux to the atmosphere during this time period.  MOx activity showed a similar temporal behaviour suggesting that tidal dynamics are an important control on the efficiency of the microbial CH₄ filter in the water column. Under stratified conditions MOx rates were highest in bottom waters (<5.7 nM/day), however we also found high MOx rates in near-surface waters at times of elevated seep activity during stratified (<3.2 nM/day) and mixed water column conditions (<16.2 nM/day). Our results indicate that the efficiency of the microbial filter is affected by temporal dynamics and seasonality.

How to cite: de Groot, T., Menoud, M., Röckmann, T., Maazallahi, H., Rush, D., Mesdag, C., Meijninger, B., and Niemann, H.: Temporal water column dynamics control microbial methane oxidation above an active cold seep (Doggerbank, North Sea), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5366, https://doi.org/10.5194/egusphere-egu2020-5366, 2020.

Methane is an important greenhouse gas and an energy resource. Methane in sea water can originate from microbially-mediated organic matter (OM) degradation processes at shallow depth  within the sediments, or from thermal cracking of refractory OM at deeper depth. On continental margins, this methane is stored in specific sedimentological bodies or as gas hydrates, or is released at the seafloor as submarine geological seeps followed by its oxidation in the water mass. However, methane released at the seafloor may not entirely be oxidized in the water column and a fraction of it may ultimately reach the atmosphere. The factors that govern the magnitude of methane transfer through the water column to the atmosphere remain poorly known. It has been identified that the amount of methane transferred to the atmosphere is strongly dependent on sites, and the thickness of the water column plays a critical role.

The Black Sea shelf and margin are known to host a large number of strong methane seepages. It has therefore been identified as a perfect candidate to investigate the fate of methane released from the seafloor to the atmosphere. This area can also act as a proxy for investigating the fate of methane in potential scenarios of hydrate destabilization in a changing climate, which can become a societal problem in the future. In the frame of ENVRIplus H2020 project (www.envriplus.eu) we developed a joint pilot experiment to measure methane transfer from the seafloor to the atmosphere, in a pilot study involving European research infrastructures ICOS, Eurofleets, EMSO and ACTRIS. We investigated the influence of depth by mapping CH4 concentration and bubble distribution at two different sites, at 60m and 100m water depth, respectively. The pilot experiment developed joint monitoring strategy for methane detection at various levels starting from the seafloor and moving across the water column, the water/air interface and the atmosphere. An EK80 echosounder was used to identify emission areas through massive bubble plumes. The methodology applied integrates (1) sampling from the geosphere, hydrosphere and atmosphere for laboratory measurements of methane concentration by well-proven standard methods together with δ13CH4 analysis, (2) in situ measurements of methane concentration into the water column and the atmosphere, and (3) the deployment of a seafloor observatory for a short monitoring period (4-5 days) to evaluate the temporal variability of gas fluxes.

During the cruise we found several occurrences of bubble plumes extending near the surface. Our measurements indicate that dissolved methane concentration drastically decreases from the seafloor to the water surface, highlighting its degradation and dispersion along the pathway to the atmosphere. The atmospheric data suggests a consistent input of marine methane to the atmosphere at the shallower site,. Our study highlights the observational challenges both for the measurement of methane from in situ and laboratory methods, and for the estimation of sea surface fluxes.

How to cite: Paris, J.-D., Ruffine, L., Leau, H., Giunta, T., Donval, J.-P., Guyader, V., Birot, D., Schumacher, M., Greinert, J., Grilli, R., Blouzon, C., Delmotte, M., Longo, M., Scire, S., Italiano, F., Lazzaro, G., Balan, S., Scalabrin, C., and Douillard, T.: Measuring methane from the seafloor to the atmosphere: an integrated experiment in the Black Sea , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16451, https://doi.org/10.5194/egusphere-egu2020-16451, 2020.

Ice sheets and glaciers play an important role for the global carbon cycle through the exchange of their subglacial carbon with the proglacial aquatic environment and the atmosphere in the form of CH₄ and CO₂. However, the subglacial environment below ice sheets and glaciers is largely inaccessible from the surface and hence we know very little about the carbon turnover processes in these extreme habitats that lead to this carbon export.

Biological CH₄ production and oxidation has been found in subglacial sediments across Canada, Antarctica, west Greenland and at the center of the Greenland Ice sheet. This points at a common glacial process for gaseous CH₄ and CO₂ emissions, but this knowledge is backed by very few direct field observations from two locations in Greenland and one in Iceland. The lack of field based studies is the single most-limiting factor for increasing our understanding of the magnitude and extent of subglacial carbon emission to the atmosphere and its relevance for the global carbon budget.

We present new field measurements suggesting that it is possible to quantify the carbon turnover processes in the subglacial environment using high frequency concentration measurements and stable isotope composition of CH₄ and CO₂ in gaseous and dissolved form sampled at a subglacial meltwater outlet.

During three field campaigns in the early, mid and late melt season in 2018 and 2019 we measured significantly elevated CH₄ and CO₂ concentrations in the air and water exiting a subglacial cave system. We devised a field sampling program for retrieval of discrete gas and water samples that allow identification of the original (common) source of the gaseous and dissolved CH₄ and CO₂ by quantifying the d¹³C and dH signature of the source.

Our field measurements are amongst the first to directly quantify the emission of CH₄ and CO₂ to the atmosphere and our isotopic investigations clearly show a biological source of CH₄ and its oxidation to CO₂ in the subglacial environment and point to a hydrological control on the release of both _CH4 and CO₂.

These types of data are instrumental to improve the understanding of subglacial carbon processes and design future field investigations to assess its climatic relevance and to narrow the uncertainty of emission estimates.

How to cite: Riis Christiansen, J., Röckmann, T., Popa, E., Sapart, C., and Juncher Jørgensen, C.: Revealing unknown subglacial carbon processes using high frequency gas measurements and stable isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8412, https://doi.org/10.5194/egusphere-egu2020-8412, 2020.

Increased methane (CH₄) release from a warming Arctic is expected to be a major feedback on the global climate. However, due to the complex effects of climate change on arctic geoecosystems, projections of future CH₄ emissions are highly uncertain. CH₄ emissions from complex tundra landscapes will be controlled not only by direct climatic effects on production, oxidation and transport of CH₄ but, importantly, also by geomorphology and hydrology changes caused by gradual or abrupt permafrost degradation. Therefore, improving our understanding of both the temporal dynamics and the spatial heterogeneity of CH4 fluxes on multiple scales is still necessary.

Here, we present pedon- and landscape-scale CH₄ flux measurements at two widespread tundra landscapes (active floodplains and late-holocene river terraces) of the Lena River Delta in the Siberian Arctic (72.4° N, 126.5° E). The dominating scales of spatial variability of soil, vegetation and CH₄ fluxes differ between the two landscapes of different geological development stage. The active floodplains are characterized by sandy beaches and ridges, and backswamp depressions, forming a mesorelief with height differences of several meters on horizontal scales of 10-1000 m. On the other hand, the river terraces are characterized by the formation of ice-wedge polygons, which lead to a regular microrelief with height differences of several decimeters on horizontal scales of 1 to 10 meters. CH₄ fluxes were investigated on the landscape scale with the eddy covariance method (15 campaigns during 2002-2018 at the river terrace, 2 campaigns 2014-2015 at the floodplain) and on the pedon scale with chamber methods (campaigns at different sites in 2002, 2006, 2013, 2014, 2015).

Average growing season (June-September) CH₄ flux for the floodplain was 166 ± 4 mmol m^-2 (n=2) and for the river terrace 100 ± 25 mmol m^-2 (n=15). There was pronounced spatial variability of CH₄ fluxes within both tundra landscapes types. On the river terrace, growing season CH₄ flux was only 20-40 mmol m^-2 at elevated polygon rims and polygon high centers, respectively, and up to 300 mmol m^-2 at polygon low centers. On the floodplain, CH₄ flux was as low as 5 mmol m^-2 at sandy ridges and above 400 mmol m^-2 in backswamp depressions. Mean growing season CH₄ fluxes at the river terrace were positively linearly correlated (r² = 0.9, n=15) to growing-degree-days (base temperature of 5 °C). Our findings suggest that a warmer climate stimulates the production of CH₄, which is directly reflected in increased CH₄ emissions. On the other hand, warming effects on CH₄ oxidation appear limited because transport processes that bypass the soil oxidation zone, i.e. plant-mediated transport and ebullition, dominate CH₄ emission from wet tundra landscapes. However, since CH₄ emissions strongly vary with (micro-)topographical situation within tundra landscapes, the changes of geomorphology and hydrology due to permafrost degradation will probably be the dominating driver of future CH₄ emissions from arctic tundra landscapes.

How to cite: Kutzbach, L., Rößger, N., Eckhardt, T., Knoblauch, C., Sachs, T., Wille, C., Boike, J., and Pfeiffer, E.-M.: Spatiotemporal variability of methane emissions of tundra landscapes in the Lena River Delta, Siberia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17937, https://doi.org/10.5194/egusphere-egu2020-17937, 2020.

Methane is one of the most potent greenhouse gases affecting climate change. According to different estimates, natural sources contribute 35–50% to global CH₄ emission. Among them, the third-biggest source is lakes emitting to the atmosphere 10–50 TgCH₄ per year [Anderson et al., 2010].

We have discovered two gas seeps during the summer 2019 field campaign within the lake near the Vas’kiny Dachi research station (Central Yamal, Western Siberia). Measurement of the ebullition intensity in tenfold replicate and gas sampling were carried out using a bubble trap of the original design. The concentration of methane in seep gas was determined by a Crystal 5000.2 gas chromatograph with a flame ionization detector; each sample was diluted tenfold with air. We calculated the annual CH₄ flux from seep to the atmosphere with the consideration of the intensity of seep ebullition and the methane concentration in gas equal during the year. To determine the potential source of the gas, we analyzed the isotopic composition of CH₄ (δ¹³C and δD) by a Delta-V mass spectrometer.

The values (median ± SD) of the gas ebullition are 175 ± 26 mL/min and 127 ± 10 mL/min for the first and second seeps respectively. The methane concentration in gas is 95–100%. The intensity of CH₄ emission from the first seep is 89.7 thousand L or 64 kg per year; from the second seep is 65.1 thousand L or 46.5 kg per year.

Analysis of the content of δ¹³C and δD isotopes in methane gives the following results.

• For the first seep: δ¹³C vs VPDB, ‰ = −75.73, δD vs VSMOW, ‰ = −226.68.
• For the second seep: δ¹³C vs VPDB, ‰ = −76.97, δD vs VSMOW, ‰ = −222.31.

According to the classification from [Whiticar, 1999], seep methane is of biogenic origin. Potentially, gas could migrate to the lake surface through sub-lake talik from the underlying geological horizon containing methane hydrates in self-preserved form as widely documented for this area [Chuvilin et al., 2000].

To summarize, lake seeps of the Western Siberia tundra zone have been studied as a source of the atmospheric methane for the first time. Considering the occurrence of methane hydrates withing permafrost in the study area, we describe a path of the CH₄ release from decomposing gas hydrates into the atmosphere in the northern part of Western Siberia.

The study was partially supported by the RAS Program no. 20 and the state contract of the IAP RAS no. 075-03-2019-628.

References:

Anderson B., Bartlett K., Frolking S. et al. Methane and nitrous oxide emissions from natural sources. Washington: EPA. 2010. 194 p.

Chuvilin E.M., Yakushev V.S., Perlova E.V. Gas and possible gas hydrates in the permafrost of Bovanenkovo gas field, Yamal Peninsula, West Siberia // Polarforschung. 2000. V. 68. P. 215–219.

Whiticar M. Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane. Chemical Geology. 1999. V. 161. P. 291–314.

How to cite: Krivenok, L., Kazantsev, V., and Dvornikov, Y.: Experimental study of methane emission from lake seeps of Western Siberia permafrost zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-563, https://doi.org/10.5194/egusphere-egu2020-563, 2020.

Lakes are considered an important natural source of methane (CH₄). However, direct measurements of lake-atmosphere gas exchange are still sparse especially in the Alpine region. To overcome this shortcoming, we designed a mobile eddy covariance (EC) station to measure CO₂, CH₄, and energy fluxes at various lakes in the Alps. EC measurements were compared to flux measurements using floating chambers and related to abiotic and biotic factors like temperature, lake morphometry, dissolved components and trophic status.

During the first year, measurements were conducted at 9 lakes at different elevations ranging from 200 to 1900 m.a.s.l. to capture the spatial variability. The following year, measurements were repeated more frequently at three contrasting lakes to capture the seasonal trends of the fluxes.

The results indicate that all lakes were supersaturated with CH₄. However, there was a high variability in the magnitude of CH₄ emissions between lakes with generally higher emissions from warmer lakes at low elevation. In particular, the lake at the lowest elevation, Lake Caldaro, had highest dissolved CH₄ concentrations and emissions and showed a clear seasonal trend with emissions peaking during the hot summer months. In contrast, the lake at the highest elevation, Lake Zoccolo, showed low CH₄ concentrations and emissions with highest concentrations in fall when the water level was low.

How to cite: Scholz, K., Carotenuto, F., Gioli, B., Miglietta, F., Pighini, S., Sommaruga, R., Tomelleri, E., Tonon, G., Zaldei, A., and Wohlfahrt, G.: Methane emissions from lakes in the Alpine region: insights from two years of mobile eddy covariance flux measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10020, https://doi.org/10.5194/egusphere-egu2020-10020, 2020.

Biogeochemical processing of dissolved organic matter, including methane, along sharp salinity gradients in subterranean estuaries greatly alters the composition of submarine groundwater discharge into the marine environment. Along the margins of coastal carbonate (karst) platforms, which account for ~25% of all coastlines, subterranean estuaries extend kilometers inland within porous bedrock, flooding extensive cave networks. This environment harbors a poorly understood, but globally dispersed, anchialine fauna (invertebrates with subterranean adaptations) and characteristic microbial communities. In Mexico’s Yucatan Peninsula, microbial processing of methane and dissolved organic carbon (DOC), originating from overlying tropical soils, is the critical link for shuttling organic matter to higher trophic levels of the food web within the coastal aquifer. To better understand carbon turnover during organic matter transformations in this habitat, we collected samples for stable and radiocarbon analyses targeting the biotic and abiotic components of the carbon cycle. In the freshwater, radiocarbon signatures of terrestrially originated DOC (pMC = 105.1; [DOC] = 517 µM; δ¹³C = ˗27.8 ‰) and methane (pMC = 101.6; [CH₄] = 6460 nM; δ¹³C = ˗71.5 ‰) correspond with modern ¹⁴C ages, suggesting these sources of energy within the habitat are comprised of modern carbon fixed recently by photosynthesizing primary producers at the land surface. By contrast, DOC in the deeper saline groundwater is significantly lower in concentration (21 µM), and substantially older (pMC = 47.3, equates to 6010 ± 95 ¹⁴C yrs). Similarly, dissolved inorganic carbon (DIC) in the freshwater is significantly younger (pMC = 86.5, equates to 1170 ± 15 ¹⁴C yrs) than in the deeper saline water (pMC = 58.4, equates to 4320 ± 25 ¹⁴C yrs). These findings demonstrate that important sources of nutrition for the food web are intimately linked to the overlying subaerial habitat, which suggests these ecosystems are highly vulnerable to nearby land use alterations. Furthermore, this study provides new insights into carbon turnover during the process of methane production/consumption, carbon exchange, and organic matter transformation before the emission of the dissolved constituents into coastal oceans from karst subterranean estuaries. Radiocarbon and stable isotopic analyses of the resident fauna will allow us to evaluate the ecological effects of the rapid top-down transfer mechanism for methane and DOC. Beyond better understanding the sources and fate of these carbon sources, our findings have the potential to support management and conservation efforts aimed at coastal groundwater ecosystems.

How to cite: Brankovits, D., Pohlman, J., Garnett, M., and Dean, J.: Modern methane and dissolved organic matter radiocarbon signatures suggest rapid transfer of organic carbon from a tropical forest to the underlying subterranean estuary ecosystem, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20633, https://doi.org/10.5194/egusphere-egu2020-20633, 2020.

How to cite: Lightfoot, A., Brennwald, M., and Kipfer, R.: The Role of Gases in an Arsenic Contaminated Aquifer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10245, https://doi.org/10.5194/egusphere-egu2020-10245, 2020.

For a long time, biological methane formation was considered to occur only under strictly anaerobic conditions by organisms from the domain Archaea. However, during the past 15 years evidence has been accumulating that Eukaryotes such as plants, fungi, animals and humans produce methane independent of methanogenic Archaea via pathways in oxic environments that have not yet been fully resolved (Keppler et al., 2009, Wang et al. 2013, Liu et al. 2015, Boros & Keppler 2019).

Furthermore, it was recently shown that both marine and freshwater algae (Klintzsch et al. 2019, Hartmann et al. 2020) do produce methane per se and might contribute significantly to the abundance of methane in oxygen-rich surface waters, commonly known as the “methane paradox”.

Finally, very recently it was demonstrated that Cyanobacteria - members of the third domain of life, i.e. Bacteria - that thrive in terrestrial, marine and freshwater environments are also able to directly produce methane (Bižić et al. 2020) and thus revealing that methanogenesis occurs in all three domains of life.

In this presentation, I will give a brief overview of recent observations of biological non-archaeal methane formation from organisms living in terrestrial and marine organisms. Furthermore, I will discuss potential mechanisms and environmental factors that might control formation of methane in Eukaryotes and Cyanobacteria. From these novel results, it becomes clear that it is essential to study methane formation in all three domains of life to fully understand the global biogeochemical cycle of methane.

References:

Bižić, M.,  Klintzsch, T., Ionescu, D., Hindiyeh, M.Y., Gunthel, M., Muro-Pastor, A. M., Eckert, W., Urich, T., Keppler, F., Grossart, H.-P., Aquatic and terrestrial cyanobacteria produce methane. Science Advances, 6, eaax5343, 2020.

Boros, M., Keppler, F., Methane Production and Bioactivity-A Link to Oxido-Reductive Stress. Frontiers in Physiology,  10, 2019.

Hartmann, J. F., Gunthel, M., Klintzsch, T., Kirillin, G., Grossart, H.-P., Keppler, F., Isenbeck-Schröter, M., High Spatio-Temporal Dynamics of Methane Production and Emission in Oxic Surface Water. Environ. Sci. Technol. 2020.

Keppler, F., Boros, M., Frankenberg, C., Lelieveld, J., McLeod, A., Pirttilä, A. M., Röckmann, T., Schnitzler, J., Methane formation in aerobic environments, Environmental Chemistry, 6, 459-465, 2009.

Klintzsch, T., Langer, G., Nehrke, G., Wieland, A., Lenhart, K., Keppler, F., Methane production by three widespread marine phytoplankton species: release rates, precursor compounds, and potential relevance for the environment. Biogeosciences, 16, 4129-4144, 2019.

Liu, J.; Chen, H.; Zhu, Q.; Shen, Y.; Wang, X.; Wang, M.; Peng, C., A novel pathway of direct methane production and emission by eukaryotes including plants, animals and fungi: An overview. Atmospheric Environment, 115, 26-35, 2015.

Wang, Z.-P., Chang, S. X., Chen, H., Han, X.-G.: Widespread non-microbial methane production by organic compounds and the impact of environmental stresses, Earth-Science Reviews, 127, 193-202, 2013.

How to cite: Keppler, F.: Methanogenesis 2020: An update, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4193, https://doi.org/10.5194/egusphere-egu2020-4193, 2020.

Cold seeps are areas of the seafloor where hydrocarbon-rich fluids, primarily composed of methane (CH₄), migrate from below reservoirs through the sediments to reach the seafloor surface. This CH₄ is an important energy source for biological communities at cold seeps and it is taken up by specialized archaeal and bacterial methane oxidizers in anaerobic and aerobic environments. Reaction products, such as sulphide, are thereafter cycled into the microbial food web, by other microbial functional groups, underlining the importance of microorganisms in supporting biological production at cold seeps. However, large gaps of knowledge on total microbial biodiversity at these methane seeps and their spatial distribution remain, especially at high latitudes. South of Svalbard, five geological mounds shaped by the formation of CH₄ gas hydrates (gas hydrate pingos GHPs), have been described recently. While one GHP was inactive, four of them showed CH₄ seeping activity with flares primarily concentrated at the summits. This suggest that the environmental conditions gradually change from the rim of the GHP toward the summit. We hypothesized that the microbial biodiversity varies along that gradient, where the summits would harbor the highest abundances of methane oxidizers. In order to test this hypothesis, we investigated the microbial community structure at two active GHPs, an inactive GHP and a reference site. Porewater chemistry and sequencing-based community analyses of Archaea, Bacteria and Eukaryotes were investigated at several depths of the sediment along a distance gradient from the summit to the rim of each GHP. We show that local environmental conditions, such as the presence of CH₄, do affect the microbial community structure and composition. The anaerobic methane oxidizing ANME-1 dominates the archaeal libraries and are detected various types of sulphate-reducing bacteria, although none demonstrated a clear co-occurrence with the predominance of ANME-1. Additional common taxa observed in these CH₄-rich sediments that likely benefited from the metabolites of CH₄ oxidation were sulphide oxidizing Epsilonproteobactaerota, as well as organic matter degraders, such as Bathyarchaeota, Woesearchaeota or thermoplasmatales MBG-D, and heterotrophic ciliates and Cercozoa. Beyond our expectations, the distribution of the different community types were not separated in concentric zones around the GHPs and similar methane oxidizing communities could be retrieved at different location over a GHP.

How to cite: Carrier, V., Kalenitchenko, D., Gründger, F., and Svenning, M. M.: Changes in microbial community structure by methane fluxes at arctic cold seeps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16669, https://doi.org/10.5194/egusphere-egu2020-16669, 2020.

Methane is a potent greenhouse gas that is produced naturally via microbial processes in anoxic environments (i.e. marine and lake sediments). The release of methane to the atmosphere from sediments is controlled by its aerobic and anaerobic oxidation. Anaerobic oxidation of methane (AOM) consumes up to 90% of the produced methane in marine sediments and over half of the produced methane in freshwater sediments. The most common electron acceptor in marine sediments for AOM is sulfate, however, in freshwater lake sediments, where sulfate concentrations are low, other electron acceptors can take its place (i.e. iron/manganese/nitrate). In lake Kinneret (Israel), iron-coupled AOM was evident by in-situ sedimentary profiles and in fresh sediment slurry incubations. Here we present geochemical and molecular analyses results of slurry experiments of long-term incubated lake Kinneret sediments with labeled ¹³C-methane, different potential electron acceptors and a few inhibitors. These experiments are part of an ongoing research to characterize the AOM processes in lake sediments, and indicate another possible type of AOM that has evolved in the long-term incubated lake sediments.

How to cite: Vigderovich, H., Eckert, W., and Sivan, O.: The evolvement of anaerobic oxidation of methane in fresh water sediments , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20020, https://doi.org/10.5194/egusphere-egu2020-20020, 2020.

The investigation of the gas hydrate system and hydrocarbon distribution were targets of IODP expeditions 372 and 375 on the Hikurangi Margin offshore New Zealand. Isotopic and molecular signatures clearly indicate a biogenic signature of methane at all sites drilled along a section crossing the accretionary wedge and basin sediments. The gas void and headspace samples from depth of a few meters up to 600 m below the seafloor have varying amounts of light hydrocarbons with high amounts of methane and changing ratios of C₂:C₃. The best example is the high-resolution profile gained from gas voids collected at Site U1517. Drilling at U1517 reached through the creeping part of the Tuaheni Landslide Complex (TLC), the base of the slide mass, and the Bottom Simulation Reflector (BSR) just above the base of the hole. Whereas gas hydrates could not be observed macroscopically, the distribution of gas hydrates was determined by logging while drilling (LWD) and pore water data revealing the occurrence of gas hydrates at roughly 105 – 160 mbsf with elevated saturations in thin coarse-grained sediments. The application of cryo-Scanning Electric Microscopy (cryo-SEM) on samples preserved in liquid nitrogen enabled the visualization of gas hydrates.

At Site U1517 the high-resolution void sampling reveals molecular and isotopic fractionation of hydrocarbons in close relation to the gas hydrate occurrences and allows for drawing conclusions on the recent history of the gas hydrate system and absence of free gas transport from below at the site. The molecular and isotopic composition further indicates ongoing propanogenesis.

How to cite: Heeschen, K., Schloemer, S., Torres, M., Cook, A. E., Screation, L., Georgiopoulou, A., Pecher, I., Mayanna, S., Barnes, P., Solomon, E., and LeVay, L.: Distribution and fractionation of light hydrocarbons related to gas hydrate occurrence and biogenic production at Hikurangi Margin (IODP Site U1517), New Zealand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15046, https://doi.org/10.5194/egusphere-egu2020-15046, 2020.

Abstract

Numerous marine hydrocarbon seeps have been discovered in the past three decades, the majority of which are dominated by methane-rich fluids. However, an increasing number of modern oil seeps and a few ancient oil-seep deposits have been recognized in recent years. Oil seepage exerts significant control on the composition of the seep-dwelling fauna and may have impacted the marine carbon cycle through geological time to a greater extent than previously recognized. Yet, distinguishing oil-seep from methane-seep deposits is difficult in cases where δ¹³C_carb values are higher than approximately -30‰ due to mixing of different carbon sources. Here, we present a comparative study of authigenic carbonates from oil-dominated (site GC232) and methane-dominated (site GC852) seep environments of the northern Gulf of Mexico, aiming to determine the geochemical characteristics of the two types of seep carbonates. We analyzed (1) Major and trace element compositions of carbonates, (2) total organic carbon (TOC), total nitrogen (TN) and carbon isotope (δ¹³C_TOC) of residue after decalcification, (3) sulfur isotope signatures of chromium reducible sulfur (CRS, δ³⁴S_CRS) and residue after CRS extraction (δ³⁴S_TOS ), as well as (4) sulfur contents (TOS) of residue after CRS extraction. Carbonates from the studied oil seep are dominated by aragonite and exhibit lower δ³⁴S_CRS values, suggesting carbonate precipitation close to the sediment surface. In addition, oil-seep carbonates are characterized by higher TOC and TOS contents and higher TOC/TN ratios, as well as less negative δ¹³C_TOC values compared to methane-seep carbonates, probably reflecting a contribution of residual crude oil enclosed in oil-seep carbonates. Very low δ¹³C_TOC values (as low as −68.7‰, VPDB) and low TOC/TN ratios of methane-seep carbonates indicate that the enclosed organic matter is derived mainly from the biomass of methanotrophic biota. This study presents new geochemical data that will allow the discrimination of oil-seep from methane-seep deposits. Although some of the geochemical patterns are likely to be affected by late diagenesis, if applied with caution, such patterns can be used to discern the two end-member types of seepage – oil seeps and methane seeps – in the geological record.

How to cite: Sun, Y., Gong, S., Li, N., Peckmann, J., Jin, M., H. Roberts, H., Chen, D., and Feng, D.: A method to differentiate hydrocarbon source (oil vs methane) in authigenic carbonate rock from seeps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1038, https://doi.org/10.5194/egusphere-egu2020-1038, 2020.

During the past ~2.6 Ma, some 30 glaciations have caused episodic high pressure and low temperature conditions and forced growth and decay of extensive subglacial methane hydrate accumulations globally. Research on Arctic methane release has primarily focused on warm, interglacial episodes when hydrates became unstable across territories either abandoned by former ice sheets or affected by permafrost degradation. Here we present a new mechanism – the subglacial erosion of gas hydrate-bearing sediments – that actively mobilizes methane in hydrate and dissolved form and delivers it to the ice sheet margin. We investigate this mechanism using geophysical imaging and ice sheet/gas hydrate modeling focused on a study site in Storfjordrenna, that hosted large ice stream draining the Barents Sea ice sheet. During the last glacial, we find that this ice stream overrode an extensive cluster of conduits that supplied a continuous methane flux from a deep, thermogenic source and delivered it to the subglacial environment. Our analysis reveals that 15,000 to 44,000 m³ of gas hydrates were subglacially eroded from the 17 km² study site and transported to the shelf-edge. Given the abundance of natural gas reservoirs across the Barents Sea and marine-based glaciated petroleum provinces elsewhere, we propose that this mechanism had the potential to mobilize a substantial flux of subglacial methane throughout multiple Quaternary glacial episodes.

How to cite: Serov, P., Patton, H., Waage, M., Shackleton, C., Mienert, J., Andreassen, K., and Hubbard, A.: Methane hydrate mobilization by ice stream erosion during the last glacial, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21868, https://doi.org/10.5194/egusphere-egu2020-21868, 2020.

The transition from gas hydrate to gas-bearing sediments at the base of the hydrate stability zone (BHSZ) is commonly identified on seismic data as a bottom-simulating reflection (BSR). At this boundary, phase transitions driven by thermal effects, pressure alternations, and gas and water flux exist. Sedimentation, erosion, subsidence, uplift, variations in bottom water temperature or heat flow cause changes in marine gas hydrate stability leading to expansion or reduction of gas hydrate accumulations and associated free gas accumulations. Pressure build-up in gas accumulations trapped beneath the hydrate layer may eventually lead to fracturing of hydrate-bearing sediments that enables advection of fluids into the hydrate layer and potentially seabed seepage. Depletion of gas along zones of weakness creates hydraulic gradients in the free gas zone where gas is forced to migrate along the lower hydrate boundary towards these weakness zones. However, due to lack of “real time” data, the magnitude and timescales of processes at the gas hydrate – gas contact zone remains largely unknown. Here we show results of high resolution 4D seismic surveys at a prominent Arctic gas hydrate accumulation – Vestnesa ridge - capturing dynamics of the gas hydrate and free gas accumulations over 5 years. The 4D time-lapse seismic method has the potential to identify and monitor fluid movement in the subsurface over certain time intervals. Although conventional 4D seismic has a long history of application to monitor fluid changes in petroleum reservoirs, high-resolution seismic data (20-300 Hz) as a tool for 4D fluid monitoring of natural geological processes has been recently identified.

Our 4D data set consists of four high-resolution P-Cable 3D seismic surveys acquired between 2012 and 2017 in the eastern segment of Vestnesa Ridge. Vestnesa Ridge has an active fluid and gas hydrate system in a contourite drift setting near the Knipovich Ridge offshore W-Svalbard. Large gas flares, ~800 m tall rise from seafloor pockmarks (~700 m diameter) at the ridge axis. Beneath the pockmarks, gas chimneys pierce the hydrate stability zone, and a strong, widespread BSR occurs at depth of 160-180 m bsf. 4D seismic datasets reveal changes in subsurface fluid distribution near the BHSZ on Vestnesa Ridge. In particular, the amplitude along the BSR reflection appears to change across surveys. Disappearance of bright reflections suggest that gas-rich fluids have escaped the free gas zone and possibly migrated into the hydrate stability zone and contributed to a gas hydrate accumulation, or alternatively, migrated laterally along the BSR. Appearance of bright reflection might also indicate lateral migration, ongoing microbial or thermogenic gas supply or be related to other phase transitions. We document that faults, chimneys and lithology constrain these anomalies imposing yet another control on vertical and lateral gas migration and accumulation. These time-lapse differences suggest that (1) we can resolve fluid changes on a year-year timescale in this natural seepage system using high-resolution P-Cable data and (2) that fluids accumulate at, migrate to and migrate from the BHSZ over the same time scale.

How to cite: Waage, M., Bünz, S., Waghorn, K., Singhorha, S., and Serov, P.: Fluid dynamics at the base of hydrate-bearing sediments at the Vestnesa Ridge inferred from 5 years of high-resolution 4D seismic surveying, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21949, https://doi.org/10.5194/egusphere-egu2020-21949, 2020.

Natural methane seepage from the seafloor to the hydrosphere occurs worldwide in marine environments, from continental shelves to deep-sea basins. Depending on water depth, methane fluxes from the sediment to the water column and mixing rate of the seawater, methane may partially reach the atmosphere where it could contribute to the global greenhouse effect. This can be observed from hydro-acoustic systems during research surveys. However, natural gas emission is not a continuous process and may vary in intensity and frequency. It is therefore necessary to study the temporal variability of methane seeps using long-term observation methods. One sensitive, accurate and reliable way to do this is by hydro-acoustic systems mounted on ocean observatories.

Here we present new long-term hydro-acoustic monitoring data from a known highly active seepage site offshore Prins Karls Forland, Svalbard. The data were acquired by a horizontally looking M3 multibeam echosounder system that was mounted on a benthic ocean observatory from October 2016 to July 2017. Our preliminary results show the presence of several individual seeps in the vicinity (<40 m) of the observatory throughout the observation period. Their activity patterns vary from non-existent to constant phases. We present the frequency of appearance and changes of the observed seeps over time. The first results confirm that methane seepage is not a constant process and emphasize the importance of long-term monitoring of methane seeps with regard to reliable flux rates estimates for a more accurate impact assessment on the climate.

The research is part of the Centre for Arctic Gas Hydrate, Environment and Climate (CAGE) and is supported by the Research Council of Norway through its Centres of Excellence funding scheme grant No. 223259 and UiT.

How to cite: Moser, M., Bergès, B., Hanssen, A., and Ferré, B.: Long-term multibeam monitoring of natural methane seepage offshore Prins Karls Forland, Svalbard, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7945, https://doi.org/10.5194/egusphere-egu2020-7945, 2020.

Serving as an indicator or fluid seepage from seabed sediments, cold seeps are ubiquitous along continental margins worldwide. In this study, a 14 m long sediment core (# 973-4) from the Dongsha Area on the northern continental slope of the South China Sea, was investigated to trace the cold seep activity and sedimentary paleo-environmental changes and its consequence for sediment mineralogy, contents of major and trace elements, total organic and inorganic carbon and total TRIS (total reducible inorganic sulfur) and δ³⁴S of sulfide minerals. In addition, planktonic foraminifera were selected for accelerator mass spectrometer carbon 14 (AMS¹⁴C) dating [1]. Furthermore, we identified the strength and effects of cold seep activity and its impact on the underlying seawater redox condition, and finally elucidated the derived force and paleoenvironment constraints of cold seep activity. C-S-Fe geochemistry, δ³⁴S of sulfide minerals and major and trace elements suggest that anaerobic oxidation of methane (AOM) occurred at 619-900 cmbsf (centimeters below seafloor). The ³⁴S enrichments (up to 23.6 ‰), abundant TRIS contents, high S/C ratios close to the seawater, together with high enrichments of Mo indicate temporal sulfidic methane seep events. Lithological distribution and AMS¹⁴C dating of planktonic foraminifera show that a turbidite (~35ka) is related to a foram-rich interval (440-619 cm) and increased carbonate productivity during the Last Glacial Maximum (LGM). An enrichment of Mo and U was observed accompanied by low contents of other trace and major (Al, Ti, V, Ni, Fe, Mn and Cu) in this interval. The foram-rich interval of cold seep sediments was probably linked to the phenomenon of inconsecutive sedimentary sequence due to the turbidites, which resulted in the lack of Fe, Mn and Ba. Based on the new results, it can be speculated that this area has experienced several episodes of methane seep activity and aerobic oxidation occurring alternatively in the last glacial period which may have been caused by fluctuating non-steady conditions. Further exploration of AOM should focus on the impact of rapid deposition, especially the impact of turbidites on sedimentary biogeochemical processes.

^[1] Zhang Bidong, Pan Mengdi, Wu Daidai etc. Distribution and isotopic composition of foraminifera at cold-seep Site 973-4 in the Dongsha area, northeastern South China Sea. J. Asian Earth Sciences.

The research supported by the Institution of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences (no. ISEE2018YB03) and the special project for marine economy development of Guangdong Province (no. GDME-2018D002).

How to cite: Sun, T., Wu, D., and Ye, Y.: Biogeochemical processes in continental slope sediments of the Dongsha Area, South China Sea , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4602, https://doi.org/10.5194/egusphere-egu2020-4602, 2020.

The East Siberian Arctic Shelf (ESAS) had high methane concentrations in the seawater of the inner shelf over the decades, which was regarded as significant methane source for global warming. The source information of elevated dissolved methane at the inner ESAS has so far been reported, however, the characterizations (i.e., formation and transport) of enriched ones in the outer ESAS remain to date still unclear. To unravel this, we have reported methane properties along south-north transects of the outer ESAS (73.7°-77.1°N and 164.3°-178.0°E, water depths; 41-370m) performed from 2016, 2018 and 2019 ARAON Expeditions. The dissolved methane concentrations in surface seawater were mostly higher than those of the atmospheric equilibrium concentration and its maximum value in the water column of the outer ESAS hotspots had ca. 204 nM. Based on principal component analysis including CTD profiles (i.e., temperature, salinity, dissolved oxygen and fluorescence) and methane concentrations, elevated methane concentrations (88 to 204 nM) were close to fluorescence concentrations (0.1 to 0.4 mg/m³). Furthermore, the isotopic signatures of dissolved methane (δ¹³C; -66.6 to -26.6‰ and δD; -218.8 to -34.0‰) and dissolved inorganic carbon (δ¹³C; -10.1 to -4.4‰) showed large isotopic variations, indicating the methane production in the study area is likely to be complicated using carbon dioxide and methyl substrates. In this regard, organic matter preserved in the submerged permafrost and/or methyl compound produced by phytoplankton might be also potential substrates for elevated methane at some locations. In the near future, mass balance model via end-member approach will be applied for determining the discriminative contributions of possible methane sources in the outer ESAS.

How to cite: Lee, D., Kim, J.-H., Lee, Y. M., Jin, Y. K., and Shin, K.-H.: Biogeochemical signature of elevated methane in water column of the outer East Siberian Arctic Shelf, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7898, https://doi.org/10.5194/egusphere-egu2020-7898, 2020.

Methane is the second most important greenhouse gas and, considering a period of 100 years, has a more than 30 times higher “global warming potential” than carbon dioxide. Emissions from the production, storage, distribution and use of fossil energy resources in recent years sum up to about 15 % of global methane emissions with numbers still being under discussion and topic of numerous research programs.

Abandoned oil and gas wells are one of the sources of methane from the oil and gas sector. Recent studies found escaping methane at selected abandoned drill holes in the central North Sea. Assuming this would hold for one third of the ~11.000 wells in the region, the process would introduce significant amounts of methane at shallow water depth. Interestingly, the collected methane was of biogenic rather than thermogenic origin, potentially escaping from shallow gas pockets. Likely, this methane was mobilized by mechanical disturbance of the sediments through the drilling operation and the well section has served as a pathway thereafter. However, little is known about the number of wells affected and the relevance for the amounts of methane realeased.

During a research cruise with the German research vessel Heincke in July, 2019, we studied seafloor characteristics, water column anomalies and sediment methane geochemistry and further inspected visually nine abandoned well sites at ~40 m water depth in the German sector of the central North Sea (Dogger Bank). The cruise targeted different situations, including known seeps in the Dutch part of the Dogger Bank, well sites of different ages and an area where abandoned wells penetrate shallow gas pockets. First data demonstrate that at none of the studied sites concentrations of dissolved methane were enriched in the upper water column. For most sites, sediment and deep water methane data demonstrate concentrations in the range known as background for that area (i.e., deep water methane close to ~ 10 nM). At one site with high indications for the presence of shallow gas pockets, we observed methane abundances several times enriched compared to background. However, the enrichments also occurred 500 m away from the drill site and did not increase towards the center. Based on our data we argue for an active natural seep situation rather than a leaking well and underline that natural seeps may challenge the identification of potentially leaking wells.

How to cite: Blumenberg, M., Schlömer, S., Römer, M., Heeschen, K., Müller, H., Barckhausen, U., Müller, S., and Schwalenberg, K.: Methane emissions from abandoned offshore wells– First data from a 2019 research cruise to the Dogger Bank, German North Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6792, https://doi.org/10.5194/egusphere-egu2020-6792, 2020.

Dissolved CH₄ concentrations in the Belgian coastal zone (BCZ) (North Sea) ranged between 1607 nmol L^-1 near-shore and 4 nmol L^-1 off-shore during field cruises in 2016, 2017, 2018 and 2019. Spatial variations of CH₄ were related to sediment organic matter (OM) content and gassy sediments. In near-shore stations with fine sand or muddy sediments, the CH₄ seasonal cycle followed water temperature, suggesting methanogenesis control by temperature in these OM rich sediments. In off-shore stations with permeable sediments, the CH₄ seasonal cycle showed a yearly peak following the Chlorophyll-a spring peak, suggesting that in these OM poor sediments, methanogenesis depended on freshly produced OM delivery. The annual average CH₄ emission was 126 mmol m^-2 yr^-1 in the most near-shore stations (~4 km from the coast) and 28 mmol m^-2 yr^-1 in the most off-shore stations (~23 km from the coast), 1,260 to 280 times higher than the open ocean average value (0.1 mmol m^-2 yr^-1). The strong control of CH₄ by sediment OM content and by temperature suggests that marine coastal CH₄ emissions, in particular in shallow areas, should respond to future eutrophication and warming of climate. This is supported by the comparison of CH₄ concentrations at five stations obtained in March 1990 and 2016, showing a decreasing trend consistent with alleviation of eutrophication in the area. This is also supported by the response to the European heatwave of 2018 that led to record-breaking temperatures in many countries across northern and central Europe. Average seawater temperature in July was 2.5°C higher than the mean from 2004 to 2017 for same month in the BCZ. The mean dissolved CH₄ concentration in surface waters in July 2018 (338 nmol L^-1) was three times higher than in July 2016 (110 nmol L^-1), and an extremely high dissolved CH₄ concentration in surface waters (1,607 nmol L^-1) was observed at one near-shore station. The high dissolved CH₄ concentrations in surface waters in the BCZ in July 2018 seemed to be due to a combination of enhancement of methanogenesis and of release of CH₄ from gassy sediments, both most likely related to warmer conditions. The emission of CH₄ from the BCZ to the atmosphere was higher in 2018 compared to 2016 by 57% in July (599 versus 382 µmol m^-2 d^-1) and by 37% at annual scale (221 versus 161 µmol m^-2 d^-1). The European heatwave of 2018 seems to have led to a major increase of CH₄ concentrations in surface waters and CH₄ emissions to the atmosphere in the BCZ.

How to cite: Borges, A. V., Royer, C., Lapeyra Martin, J., Scranton, M. I., Champenois, W., and Gypens, N.: Productivity and temperature as drivers of seasonal and spatial variations of dissolved methane in the Southern Bight of the North Sea, leading to a response from eutrophication and heatwaves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8425, https://doi.org/10.5194/egusphere-egu2020-8425, 2020.

Rivers are suspected to be a main suppliers of greenhouse gases (methane and carbon dioxide) to coastal seas, while the role of the interjacent tidal flats is still ambiguous. In this study we investigated the role of the Elbe and Weser estuaries as source of methane to the North Sea. We used high spatially resolved methane measurements from an underway degassing system and subsequent analysis with cavity ring down spectroscopy. Thus, a high-resolution representation of the methane distribution in surface waters as well as of hydrographic parameters was obtained for several cruises with two ships in 2019. For most areas, riverine methane was simply diluted by seawater, overlain by a strong tidal signal. However, on several occasions unexpectedly high methane concentrations were observed. Further detailed analysis will elucidate the role of riverine versus tidal impact on coastal North Sea methane fluxes.

How to cite: Bussmann, I., Brix, H., Fischer, P., and Flöser, G.: Methane distribution at high spatial resolution in North Sea estuaries, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5241, https://doi.org/10.5194/egusphere-egu2020-5241, 2020.

Arctic ponds are significant sources of methane, but their overall contribution to pan-Arctic methane emissions is still uncertain. Ponds come in different sizes and shapes, which are associated with different stages of permafrost degradation. Methane concentrations and fluxes show large spatiotemporal variability. To better understand this variability, as a first step towards upscaling pond methane emissions, we studied 41 ponds in the Lena River Delta, northeast Siberia. We collected water samples at different locations and depths in each pond and determined methane concentrations using gas chromatography. Additionally, we collected information on the geomorphology, vegetation cover as well as on key physical and chemical properties of the ponds and combined them with meteorological data.

The ponds are divided into three geomorphological types with distinct differences in methane concentrations: water-filled degraded polygon centers, water-filled interpolygonal troughs and larger collapsed and merged polygons. These ponds exhibit mean surface methane concentrations (with standard deviation) of 1.2 ± 1.3 μmol L-1, 4.3 ± 4.9 μmol L-1 and 0.9 ± 0.7 μmol L-1 respectively, while mean bottom methane concentrations amount to 102.6 ± 145.4 μmol L-1, 263.3 ± 275.6 μmol L-1 and 17.0 ± 34.1 μmol L-1. Using principle components and multiple linear regressions, we show that a large portion of spatial variability can be explained by the ponds’ shape and vegetation. Merged ponds have the least relative vegetation cover, and they also tend to be better mixed, both of which explains the lowest methane concentrations and the lowest variability in these ponds. Vegetation covers larger fractions in polygon centers and troughs, leading to a larger methane variability. Finally, troughs, as they are underlain by ice wedges, exhibit more pronounced stratification and the highest methane concentrations. More results will be presented at the conference.

How to cite: Rehder, Z., Zaplavnova, A., and Kutzbach, L.: Drivers of methane variability in Arctic ponds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9670, https://doi.org/10.5194/egusphere-egu2020-9670, 2020.

Greenhouse gases (GHGs), especially, methane (CH₄) emissions from the littoral zones of the lakes play an important role in regional biogeochemical budgets. Only a few studies are available in literature highlighting the direct flux measurements of CH₄   from the aquatic systems. In the present study, an attempt has been made to quantify the spatio-temporal variations of CH₄ efflux and the key physical factors controlling the emission rate, from the vegetated littoral zones of lake Vellayani (5.55Km²), located in the urbanized area of Thiruvananthapuram city, Kerala, South-West India. CH₄ efflux were collected from different vegetations in littoral zones, using a static chamber, during the peak growing seasons from March to October in 2016 and further analyses were carried out by using Gas Chromatograph (PE Clarus 500, PerkinElmer, Inc.). The mean efflux rate of CH₄   from the emergent plant species (Phragmites australis and Typha spp.) was 114.4 mg CH₄ m^-2h^-1; while, in the floating leaved species (Nymphaea spp. and Nelumbo Spp.), it   was   observed to be 32.6 mgCH₄ m^-2h^-1. The results reveal that CH₄ efflux in the zone of emergent vegetation was significantly higher than the floating-leaved zone indicating the importance of plant biomass and standing water depths for the spatial variations of CH₄ efflux. However, no significant temporal variations were noticed in the physical factors during the peak growing seasons. These results indicate that the vegetated littoral zones of lake, especially the emergent plant zones were supersaturated with CH₄, facilitating the production of carbon for CH₄ emission_, but also enable the release of CH₄ by the diffusion from the intercellular gas lacunas. We conclude that the atmospheric CH₄ emissions will be affected by the growth of exotic species in the lake systems and may be the reason for enhancing the climate warming in local/regional scale.

How to cite: Das, R., Krishnakumar, A. P., AnoopKrishnan, K., and Nandakumar, V.: Implications of methane emissions in biogeochemical budgeting: A study from a eutrophic tropical lake of South India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1509, https://doi.org/10.5194/egusphere-egu2020-1509, 2020.

Anaerobic oxidation of methane (AOM) is a globally important CH₄ sink that is offsetting potential CH₄ emission into the atmosphere. The AOM depends on the availability of the alternative to oxygen electron acceptors (AEAs) which can be of inorganic (e.g. NO₃^-, Fe³⁺, SO₄^2-), and organic (e.g. humic acids) origin. Flooded paddy soils are among the ecosystems with pronounced AOM. Due to a variety of fertilization practices, including combinations of mineral (NPK) and organic (pig manure, biochar) fertilizers, there is a range of AEAs available in paddy soil under anaerobic conditions. However, it remains unclear whether (i) AOM has a preferential pathway in paddy soil, and (ii) how do AEAs and fertilization type affect anaerobic microbial interactions. Therefore, we tested the effects of key AEAs – NO₃^-, Fe³⁺, SO₄^2-, and humic acids – on bacterial community structure (by 16s rRNA gene sequencing) in paddy soil with ongoing AOM experiment under mineral and organic fertilization. We hypothesized that incorporation of labeled ¹³C-CH₄ during AOM into CO₂ and phospholipid fatty acid biomarkers (PLFA) along with co-occurrence bacterial network analysis will reveal the preferential AOM pathway as related to a type of fertilization.

Bacterial alpha-diversity was significantly increased after 84-day anaerobic incubation. Pig manure significantly increased the microbial biomass as compared with NPK and Biochar, but the AEAs amendment did not affect the biomass. Anaerobic incubation, fertilization treatments specific biochar and NPK, and AEAs amendments specific SO₄^2- and humic acids were factors contributing to microbiome variation. Network analysis indicated that microbial communities involved in CH₄ cycling (i.e. NC10, sulfate-reducing bacteria, Geobacter, syntrophic bacteria with methanogens and ANME-2) had non-random co-occurrence patterns and was modularized. There were 16 ¹³C-enriched PLFA biomarkers confirming the incorporation of C-CH₄ into bacteria. AOM and ¹³C-PLFA were significantly higher under Pig manure relative to other fertilizations. AOM was more intensive under NO₃^- than Fe³⁺ and humic acids, but was close to zero under SO₄^2- amendment. However, the relative abundance of NC10 phylum which includes organisms performing AOM, and sulfate-reducing bacteria were higher under SO₄^2-. The relative abundance of Geobacter was highest under biochar and NPK fertilization with SO₄^2- and humic acids amendments. Taken together, NO₃^--driven AOM is the most potent AOM pathway in paddy soil, which however co-exists with the AOM pathways via reduction of NO₂^- by NC10 bacteria and reduction of Fe³⁺ and humic acids by consortia of ANME with Geobacter. Consequently, the co-occurrence network and evidence from ¹³C incorporation into CO₂ and PLFAs indicate the multiple competitive pathways of AOM in paddy soil.

How to cite: Fan, L.: Paddy soil fertilization, organic and inorganic alternative electron acceptors shape microbial community network and determine competitive pathways of Anaerobic Oxidation of Methane, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-861, https://doi.org/10.5194/egusphere-egu2020-861, 2020.

Microbially enhanced coal-bed methane (MECBM) production is an innovative idea to stimulate biogenic coal-bed methane production by providing nutrients to the native microbial community. Through additional substrate in the subsurface, a stimulation of microbes occurs, which leads to an increased methane production. Experimental studies, performed at Montana State University, provide the basis for modelling MECBM production with two-phase multi-component transport processes using the numerical simulator DuMuX [1].

We will present the calibrated and validated numerical batch model. The conceptual model comprises a food-web that includes two types of bacteria and three types of archaea representing substrate-specific members of the community with the corresponding biogeochemical reactions. These are derived from experimental studies [2]. The model is able to capture the interactions between different microbial groups, coal bioavailability, biofilm growth and decay as well as CH4 production.

The numerical batch model is extended to simulate column studies [3]. The model is being used to test hypotheses on different processes e.g. coal bioavailability and retardation or filtering effects when adding substrate. The numerical model provides a more detailed understanding of the relevant processes involved in MECBM production as well as a general understanding of biogeochemical reactions coupled with possibly changing flow and transport conditions in the subsurface. This model will be an instrumental tool in further development of a more sustainable method of harvesting methane from unmineable coal-beds.

[1] Koch, Timo, et al. "DuMu^X3--an open-source simulator for solving flow and transport problems in porous media with a focus on model coupling." arXiv preprint arXiv:1909.05052 (2019).
[2] Davis, Katherine J., et al. "Biogenic Coal-to-Methane Conversion Efficiency Decreases after Repeated Organic Amendment." Energy & fuels 32.3 (2018): 2916-2925.
[3] Davis, Katherine J., et al. "Biogenic coal-to-methane conversion can be enhanced with small additions of algal amendment in field-relevant upflow column reactors." Fuel 256 (2019): 115905.

How to cite: Emmert, S., Davis, K., Gerlach, R., and Class, H.: Calibrating and validating a numerical model concept for microbially enhanced coal bed methane production with batch and column data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9011, https://doi.org/10.5194/egusphere-egu2020-9011, 2020.

One of the principal theories about the origin of life is based on the abiotic reduction of carbon oxides to various organic molecules in hydrothermal systems. This synthesis is most favored in ultramafic environments undergoing hydrothermal alteration where the serpentinization reaction efficiently produces H₂. Nevertheless, decades of hydrothermal experiments have hardly succeeded in producing abundant organic volatiles such as CH₄ and short-chain hydrocarbons. On another hand, natural observations have shown the occurrence of other abiotic compounds such as organic acids in fluids and carbonaceous matter (CM) within serpentinized rocks. But organic acids as carbon source and CM as product have not been investigated so far in experiments reproducing hydrothermal peridotite alteration. Here, we explored the effect of formic acid (HCOOH) on the serpentinization reaction and possible feedback effects on carbon speciation in both fluid and solid. We performed reactions at 300°C and 250 bar using peridotite powder (<40 microns) in the presence of  0.1 M formic acid. A temperature of 300°C has been shown to be optimal for olivine serpentinization, while formic acid should partly decomposed into H₂, CO, and CO₂. After 4 months, H₂, CO, CO₂, CH₄ and short-chain alkanes (mainly ethane) were measured in the fluid, and the powder was completely indurated. The solidified powder displayed a black and white layering perpendicular to fluid diffusion. Its analysis showed the advancement of the serpentinization reaction, and the incorporation of carbon compounds into the solid phase. XRD analysis indicated 70% of serpentinization. SEM-EDX observations showed peculiar texture with large and localized euhedral magnetite grains alternating with larger magnetite grains mixed with C-enriched areas of long chrysotile fibers. FT-IR measurement attested of the widespread formation of carbonaceous material in the solid. Liquid analyses are under progress. Those first results suggest that serpentine formation not only provides additional H₂ to the system, but also mineral surfaces that could play a role in the precipitation of carbonaceous material and carbon speciation in natural systems. The nature and formation mechanisms of this latter remain to be addressed but this opens new paths for abiotic organic synthesis under hydrothermal conditions. In addition to their implications as an abiotic carbon source for deep hydrocarbon degraders ecosystems, it could have important implications for the total carbon cycle.

How to cite: Barbier, S., Andreani, M., Gaucher, E. C., Daniel, I., Ménez, B., Grossi, V., Antheaume, I., Albalat, E., Fellah, C., Cardon, H., Jame, P., Saupin, X., and Bonjour, E.: Effect of serpentinization on carbon speciation: an experiment with formic acid , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20924, https://doi.org/10.5194/egusphere-egu2020-20924, 2020.

The lack of reliable low-cost greenhouse gas flux measurement approaches limit our ability quantify regulation and verify mitigation efforts at the local level.   Methane (CH4), one of the most important greenhouse gases, is particularly dependent on local measurements because levels are regulated by a complex combination of sources, sinks and environmental conditions. There are still major gaps in the global methane budget and the reasons for the irregular development over time remains unclear. Facilitation of local flux measurements in all parts of the world therefore seem important to constrain large-scale assessments. As the high cost of gas analysers is a limiting factor for flux measurements, we here present how low-cost CH4 sensors can be used outside their specified range to yield reasonably accurate chamber-based flux measurements. By using a two-step calibration approach, testing multiple alternatives on how to model interference from temperature and humidity, an R2 ≥ 0.99 was achieved over a CH4 concentration range of 2 – 700 ppm under variable temperature and relative humidity. We also demonstrate ways to reach such calibration results without complicated calibration experiments and instead using in the order of 20 in situ reference measurements at different environmental conditions. Finally we, constructed and described a make-it-yourself Arduino based logger with the tested sensors for CH₄, temperature, humidity and carbon dioxide (CO₂) intended for flux chamber use with a material cost of approximately 200 Euro. We hope that this can contribute to more widespread greenhouse gas flux measurements in many environments and countries.

How to cite: Bastviken, D., Nygren, J., Schenk, J., Massana, R. P., and Duc, N. T.: Low-cost methane (CH4) sensors for use in in flux chambers , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9070, https://doi.org/10.5194/egusphere-egu2020-9070, 2020.