BG7.1

Gas hydrate recycling is an important process in natural hydrate systems worldwide. The recycling of hydrates often leads to high hydrate saturation close to the base of the gas hydrate stability zone (GHSZ). However, to date it remains enigmatic how free gas is recycled back into the GHSZ and what the controlling factors are. Here we use a 1D compositional multi-phase flow model to investigate the dominant mechanisms that control natural gas hydrate recycling. As case study, we apply the numerical model to study hydrate recycling at the Green Canyon Site 955 in the Gulf of Mexico, where high sedimentation rates are thought to drive vigorous hydrate dissociation and re-invasion of free gas into the stability zone. Our novel results suggest that hydrate recycling is a highly dynamic process in which hydrates form and dissociate at surprisingly rapid rates with an inherent cyclicity. These cycles can be divided into three phases of 1) gas accumulation phase, 2) gas breakthrough phase and 3) uninhibited hydrate build-up phase. During the first phase hydrates are dissociating and free gas accumulates below. After the free gas saturation reaches a threshold value (given by the mutual effects of entry pressure, bulk permeability, and relative permeability function), gas breaks through the barrier of the remaining hydrate layer. Controlled by permeability and kinetic rate a new hydrate layer forms.  In the absence of external perturbations to the GHSZ, gas migration leads to a distinct hydrate layer with a convex distribution of hydrate saturation. Such a hydrate layer acts like a converging-diverging `nozzle' for the gas flow, when gas enters the hydrate layer, it decelerates until it reaches the peak hydrate saturation (i.e. the nozzle throat), and then accelerates until it exits the hydrate layer on the other side. This nozzling effect, together with the hydrate dissociation kinetics, leads to the cyclic behavior of hydrate recycling. We suggest that the evident cyclicity of burial-driven gas hydrate build-up process provides a new advanced understanding of natural gas hydrate recycling process, and free gas invasion mechanisms into the GHSZ.

How to cite: Schmidt, C., Gupta, S., Burwicz-Galerne, E., Wallmann, K., Hartz, E. H., and Rüpke, L.: Sedimentation-driven cyclic rebuilding of gas hydrates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3687, https://doi.org/10.5194/egusphere-egu22-3687, 2022.

             Methane in anoxic marine sediments comes primarily from microbial methanogenesis. Methanogenesis is facilitated by groups of obligate anaerobic archaea and is the last step in carbon remineralization according to the redox cascade. Before the methane is emitted into the water column and ultimately the atmosphere, where is acts as a potent greenhouse gas, a large portion (~90%) of the methane is consumed by anaerobic oxidation of methane (AOM). In anoxic marine sediments AOM is typically mediated by a consortium of methanotrophic archaea and sulfate-reducing bacteria to oxidize methane to inorganic carbon within a sediment layer classically known as the sulfate-methane transition zone (SMTZ). Organic matter in sediments above the SMTZ is consumed by organoclastic sulfate reduction, which thermodynamically outcompetes methanogenesis for hydrogen and acetate. However, methanogenesis can persist in sulfate-reducing environments with non-competitive substrates such as methylamines, which are produced from the microbial degradation of glycine betaine and dimethylsulfoniopropionate. Methanogenesis from methylamine can directly fuel AOM, now known as the “cryptic methane cycle”, in sulfate-reducing sediments. The cryptic methane cycle above the SMTZ is still poorly understood. Here we will present our preliminary research that shows evidence of cryptic methane cycling in sulfate-reducing sediments of the organic-rich Santa Barbara Basin (SBB).

            We sampled the top 10-15cm of sediments at 5 stations along a depth transect across the basin. Sediment samples were subjected to radioisotope incubations with ¹⁴C-methane, ¹⁴C-mono-methylamine, and ³⁵S-sulfate, gas chromatography, and porewater geochemical and metabolomic analysis. Porewater methane concentrations ranged from 3 to 13 µM. Metabolomic analysis of porewater using nuclear magnetic resonance for mono-methylamine concentrations found evidence of mono-methylamine presence below the quantification limit (< 3 µM). Results from the radiotracer incubations with ¹⁴C-methane detected ex-situ AOM rates at all 5 stations, where the highest rates were found within the top 1 cm. Integrated AOM (0-11cm) activity decreased with station water depth. ¹⁴C-mono-methylamine incubations revealed concurrent methanogenesis and AOM from mono-methylamine in the presence of sulfate reduction at all 5 stations. These results indicate evidence of potential cryptic methane cycling near the sediment-water interface in the Santa Barbara Basin.

How to cite: Krause, S. J. E., Liu, J., Yousavich, D. J., Robinson, D., Hoyt, D. W., Valentine, D. L., and Treude, T.: Evidence of cryptic methane cycling in sulfate-reducing sediments of the Santa Barbara Basin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6444, https://doi.org/10.5194/egusphere-egu22-6444, 2022.

Recent observations from shallow anoxic lacustrine sediments around the world show an unexpected presence of bacterial methanotrophs, usually typified to be aerobic, together with anaerobic microorganisms, such as methanogens and iron reducers, which may result from undetectable traces of oxygen or a yet to be understood biochemical process producing oxygen. Both mcr gene-bearing archaea and pmoA gene-bearing bacterial methanotrophs were suggested to mediate methane oxidation in Lake Kinneret sediments. In these sediments, iron reduction was shown to be coupled to methane oxidation; however, with an unclear mechanism linked to methanotrophy. Here we show a new set of geochemical and microbial data from slurry experiments that tested the effect of exposure of oxygen on this aerobic and anaerobic activity. Surprisingly, exposure of oxygen levels up to 1% promoted aerobic methanotrophy and increased net iron reduction in anoxic lake sediments. The iron reduction was microbially mediated and performed by either Desulfuromonas or Geobacter or Methylomonas. The experiments provide insight into the complex life and biogeochemical cycles in anoxic lake sediments. 

How to cite: Vigderovich, H., Eckert, W., Elvert, M., and Sivan, O.: Aerobic methanotrophic activity stimulates iron reduction in lake sediments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13456, https://doi.org/10.5194/egusphere-egu22-13456, 2022.

Microbial anaerobic oxidation of methane (AOM) significantly mitigates atmospheric methane emissions on Earth and represents a thermodynamically favorable metabolic strategy for astrobiological targets where methane has been detected. The bulk carbon and hydrogen isotope ratios produced by AOM have been used to probe the thermodynamic drive for intracellular reactions that involve the bi-directional enzymes of the methanogenesis pathway. Recently, measurements of the abundance of doubly-substituted methane isotopologues provide another dimension for assessing kinetic and equilibrium isotope effects and thus the AOM process itself. Towards this end, we measured methane clumped isotope ratios of residual methane in AOM-active microbial incubations using sediment slurry and/or fracture fluid from Svalbard methane seeps, Santa Barbara Channel methane seeps, Nankai Trough, and Beatrix Gold Mine. We also analyzed methane isotopologue abundances in sub-seafloor fluids from a Mariana mud volcano where AOM occurs. Extremely high Δ¹³CH₃D and Δ¹²CH₂D₂ values were found in the Svalbard sediment slurry and the Mariana fluids where minimal reversibility of AOM intracellular reactions preserved signatures of kinetic fractionation of clumped isotopologues. When conditions were consistent with a low thermodynamic drive for AOM, however, methane isotopologues approached intramolecular quasi-equilibrium. This was notably observed in the microbial incubations of the deep biosphere samples from Nankai Trough and Beatrix Mine. This presentation will highlight the environmental controls on the enzymatic activity of intracellular pathways and the reversibility of AOM, and their intrinsic link to methane isotopologue ratios.

How to cite: Liu, J., Harris, R. L., Ash, J. L., Ferry, J. G., Labidi, J., Krause, S. J. E., Prakash, D., Sherwood Lollar, B., Treude, T., Warr, O., and Young, E. D.: Methane clumped isotope signature of anaerobic oxidation of methane, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9032, https://doi.org/10.5194/egusphere-egu22-9032, 2022.

During the past two decades, extensive ebullition and elevated methane concentrations in both seawater and atmosphere have been observed in East Siberian Arctic Shelf (ESAS) region. The relative contribution of the potential sources to these enhanced methane concentrations and the related release processes are yet poorly understood. The sources of the observed methane could be recently microbially produced methane from organic material in Holocene sediments or thawing subsea permafrost, or preformed methane released from subsea permafrost, destabilizing methane hydrates or thermogenic gas reservoirs. We here use the ¹³C content of methane toward separating the contribution of thermogenic and microbial sources to methane-enriched bottom waters collected during four expeditions across the East Siberian Arctic Shelf (n = 181).

Our data suggest variability in methane sources between methane hotspots in three different regions of the ESAS, which are separated by large spatial scales (500-900 km). For the outer Laptev Sea, the average, 10^th and 90^th percentile δ¹³C values of near-bottom water methane were -44‰, -54‰ and -35‰, which suggests a dominant thermogenic source (expeditions in 2014, 2016, 2018 and 2020). For the inner Laptev Sea, the average, 10^th and 90^th percentile δ¹³C values of near-bottom water methane were -69‰, -77‰ and -58‰, which suggests a dominant microbial source (expeditions 2016, 2018, 2020). For the East Siberian Sea, samples of the years 2014, 2016 and 2020 have been analysed and the pattern is less consistent in time, where bottom water samples from 2014 are more enriched in ¹³C (δ¹³C average of -41‰) compared to the later years 2016 and 2020 (δ¹³C averages of -65‰ and -57‰). The differences between the three regions suggest that the dominant sources of the methane releases are different in these regions and likely reflect differences both in subsea compartments and processes forcing the current releases. 

How to cite: Brussee, M., Holmstrand, H., Wild, B., Shcherbakova, K., Kosmach, D., Kurilenko, A., Shakhova, N., Semiletov, I., and Gustafsson, Ö.: Stable carbon isotope data of enhanced dissolved methane in the East Siberian Arctic Shelf region, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11636, https://doi.org/10.5194/egusphere-egu22-11636, 2022.

High resolution measurements with acceptable accuracy are crucial to increase our understanding of important environmental processes. The sharp spatial and temporal gradients which characterize seabed seepage environments often means that conventional measuring techniques fall short in representing seepage, content, and the environmental processes of interest. This is especially the case when it comes to measuring dissolved methane, which is still often done using discrete water sampling and subsequent laboratory analysis. This practice is time consuming, resulting in data with poor spatiotemporal resolution which is often unable to represent highly variable environmental processes. The overt solution to this problem is the employment of in situ sensors. Unfortunately, the common sensor design approach in off-the-shelf sensors, where measurements rely on a membrane equilibrium extraction technique and gas detection by some embedded device, are, while theoretically reliable, often plagued by high response times – especially in cold environments, making them unsuitable for applications where rapid changes are expected.

We present a new, easily applicable, lab and field-tested method for recovering fast response data from off-the-shelf methane sensors relying on the principle of membrane separation by using the theoretical framework of statistical inverse theory. This framework allows us to model the uncertainty of the measurements obtained by the internal detector, giving fast response data where measurement uncertainty is explicitly defined – something which has not been possible in the past. The solution is constrained by model sparsity, which in practice gives the user data with the resolution at which the sensor is able to give measurements with a reasonable uncertainty. Furthermore, our method requires no additional input from the user other than what is provided from the manufacturer, such as detector accuracy and response time. Getting reliable fast response data from relatively affordable, off-the-shelf in situ sensors, means that these can be used in new applications such as profiling, towing, or on autonomous platforms such as gliders. This can considerably improve our ability to quantify dissolved methane and resolve and understand related environmental processes in the ocean.

How to cite: Dølven, K. O., Vierinen, J., Grilli, R., Triest, J., and Férré, B.: Response time correction of membrane equilibrium based methane sensor data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12677, https://doi.org/10.5194/egusphere-egu22-12677, 2022.

Methane (CH₄) is a potent greenhouse gas with a global warming potential far higher than that of carbon dioxide (CO₂). Near-shore marine ecosystems often emit less methane than freshwater wetlands due to higher sediment concentrations of sulfate (the second most abundant anion in seawater). Sulfate-reducing bacteria can outcompete methane-producing microorganisms and mediate the anaerobic oxidation of methane, curtailing methane emissions when sulfate is present. Sea level rise is one of the most significant global changes affecting estuaries. Although sea level rise poses a threat to their stability, estuaries may emit less methane and sequester more carbon as they experience greater sulfate availability through seawater incursion. To assess the impacts of increasing sea levels and salinity - aka sulfate - on methane production, we characterize the geochemistry (concentrations and stable isotopes of CH₄, CO₂, DIC, and SO₄^2-, anions, cations, alkalinity, [HS^-], and [Fe²⁺]) of four sites across a salinity gradient from marine to freshwater of the Medway Estuary in November 2021 and January 2022. We also manipulated salinity in sediment incubations with cores from the freshwater end of the estuary to characterize methane production when nominally freshwater sediments are exposed to higher sulfate concentrations. As hypothesized, freshwater sites (salinity 0.3 ppt) have the greatest concentrations of pore fluid dissolved methane (up to 1.5 mM), two orders of magnitude greater than brackish or marine sites (salinities 6 and 32 ppt). Lower δ¹³C_CH4 (< -65‰) characterizes freshwater and marine sites, while deeper in the brackish sites there is higher δ¹³C_CH4 (-18 to -30‰). We use the carbon isotopic composition of CO₂ and dissolved inorganic carbon (δ¹³C_CO2 and δ¹³C_DIC) to understand the depth distribution of methane production. These isotopic compositions increase with depth at the freshwater site, hinting at in situ methane production, but decrease at the other sites, possibly due to organic carbon or methane oxidation. Our freshwater endmember is dominated by iron reduction and methanogenesis, while the brackish sediments have greater rates of nitrate, iron, and sulfate reduction. The most seaward sediments have geochemical evidence of nitrate and iron reduction, with the sulfate reduction zone likely below 40 cm depth. Incubation results will be presented, illustrating how the addition of sulfate impacts methane production pathways in otherwise freshwater sediments.

How to cite: Dietrich, Z. A., Hawthorne, S. E. G., Prudence, S. M. M., Tsola, S. L., Sanders, I. A., Eyice, Ö., and Turchyn, A. V.: Potential impacts of sea level rise on methane production in a UK estuary, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10507, https://doi.org/10.5194/egusphere-egu22-10507, 2022.

Methane (CH₄) is the most abundant hydrocarbon in the atmosphere, largely originating from biogenic sources that recently have been linked to an increasing number of organisms living in both oxic and anoxic environments. Traditionally, biogenic CH₄ has been regarded as the final product of the anoxic decomposition of organic matter by methanogenic Archaea. However, plants, fungi, algae, lichens and cyanobacteria have recently been shown to produce CH₄ in the presence of oxygen. While methanogens produce CH₄ enzymatically during anaerobic energy metabolism, the requirements and pathways for CH₄ production by “non-methanogenic” cells are poorly understood. Here, we present a CH₄ formation mechanism that most likely occurs in all living organisms (Ernst et al. 2022). Firstly, we show results from two bacterial species (Bacillus subtilis and Escherichia coli) demonstrating that CH₄ formation is triggered by free iron and reactive oxygen species (ROS), which are generated by metabolic activity and enhanced by oxidative stress. ROS-induced methyl radicals, derived from organic compounds containing sulfur- or nitrogen-bonded methyl groups, are key intermediates that ultimately lead to CH₄.

In a second step, we made numerous experiments and collected data from many other model organisms (over 30 species) from the three domains of life (Bacteria, Archaea and Eukarya), including several human cell lines and a non-methanogenic archaeal species. All of the selected species clearly showed CH₄ formation under sterile growth conditions. As the mechanism described for CH₄ formation depends on several factors such as the availability of methylated precursor compounds, free iron, cellular stress factors and antioxidants, production rates can vary by several orders of magnitude. For terrestrial plants and cyanobateria, measured CH₄ emission rates have been reported to vary by almost four orders of magnitude. In both cases, rates were measured for many species and under varying environmental conditions and stressors, although the formation mechanism(s) were unknown. Our proposed ROS-driven pathway not only provides a mechanistic explanation for the observed CH₄ emissions under oxic conditions but also for the large variability of emission rates observed for terrestrial plants, marine and freshwater algae, fungi, lichens and cyanobacteria, which have caused many controversial discussions since their publication. Furthermore, now it is very clear that any global upscaling will be highly challenging given the complex variables that control emissions from specific organisms.

In summary, the observed and experimental validated process of CH₄ formation across all living organisms is a major step to better understand biological CH₄ (in addition to the well-described archaeal methanogenesis) formation and cycling on Earth.

Reference:

Ernst, L., Steinfeld, B., Barayeu, U., Klintzsch, T., Kurth, M., Grimm, D., Dick, T.P., Rebelein, J.G., Bischofs, I.B., Keppler, F. (2022). ROS-driven methane formation across living organisms. Nature, in press.

How to cite: Keppler, F., Ernst, L., and Bischofs, I.: Methane formation across living organisms driven by ROS: new perspectives for understanding of biochemical methane formation and cycling on Earth, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10255, https://doi.org/10.5194/egusphere-egu22-10255, 2022.

The surface waters of the open ocean are mostly oversaturated with methane and thus act as a net source to the atmosphere. In situ production from methylated compounds or inorganic carbon has been proposed to act as a potential source of methane in the oxygenated surface waters. However, the distribution and importance of aerobic methane production in different marine regions remains poorly constrained. We investigated the processes and microorganisms involved in aerobic methane production in the surface waters of the western Tropical North Atlantic off Barbados. Using stable isotope incubation experiments, we showed that within 24 hours methane was readily produced from methylphosphonate (MPn) but not from dissolved inorganic carbon. MPn-derived methane production reached up to ca. 8 nmol l^-1 d^-1, with the highest rates measured in surface waters above the deep chlorophyll maximum (DCM). Additions of inorganic phosphate resulted in the suppression of methane production in the surface waters but not below the DCM. Additional controlling factors of MPn-derived methane production, both physicochemical (depth, nutrients) as well as biological (primary production, nitrogen fixation), were also investigated. Our metagenomic and metatranscriptomic analyses revealed that various microbial groups, including Trichodesmium and Alphaproteobacteria, had the capacity to utilise MPn in situ, making them potential contributors to methane production in this region. Overall, our results highlight the importance of MPn-derived methane production in the phosphate-limited western Tropical North Atlantic and identify the controlling factors that may regulate aerobic methane production and thereby exert control over marine methane emissions.

How to cite: von Arx, J., Kidane, A., Ahmerkamp, S., Lavik, G., Philippi, M., Schorn, S., Kuypers, M., Mohr, W., and Milucka, J.: Aerobic methane production in the western Tropical North Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10167, https://doi.org/10.5194/egusphere-egu22-10167, 2022.

The global ocean is a net source of CH₄ to the atmosphere. Among the natural processes, marine emissions are significant contributors with large uncertainties that deserves effort to improve current estimates, and eventually predict their trajectories in a changing climate. Oceanic CH₄ emissions to the atmosphere can either be transported from seafloor or in situ produced in surface waters. Seafloor emissions include both CH₄ emanating from CH₄ hydrate degradation and from free gas in the sediment. Ultimately, CH₄ enters the atmosphere across the sea-air interface either from bubbles rising from the seafloor or by diffusion of dissolved gas. Estimates of global marine emissions diverge widely due to very large uncertainties linked to limited data coverage, seasonal and methodological differences and the difficulty to capture the environmental factors that lead to high variability of the emissions.

As the world’s largest natural anoxic waterbody, the semi-enclosed Black Sea (BS) is very sensitive to human and climate perturbations. It is characterized by widespread seafloor CH₄ emissions from the shallow coast to the deep basin. One of the major issues that arises on the BS methane dynamics is the determine to what extent and in which quantity part of the urge amount of dissolved methane stored in the anoxic bottom water layer is transferred to the atmosphere.

During the GHASS2 (Gas Hydrates, fluid Activities and Sediment deformations in the black Sea) cruise in September 2021, CH₄transfer to the atmosphere has been investigated in the Western sector of the BS at sites with water depth ranging from 60 m to 1200m. CH₄ partial pressures were measured in the surface water and in the atmosphere using optical spectrometers, respectively the SubOcean membrane inlet laser spectrometer (Grilli et al., 2021, https://doi.org/10.3389/feart.2021.626372) and an ICOS-calibrated commercial analyzer (Picarro G2401). We have also developed an open-path setup dedicated to shipborne measurement composed by an open-path CH₄ analyzer Li-7700, a H₂O-CO₂ analyzer 7200RS from LiCor, a Gill 3D sonic anemometer, and an inertial navigation sensor (Lord).  An inox structure was specifically designed to protrude by 1m the front mast of the R/V Pourquoi Pas? to install the open-path sensor.

We present preliminary flux estimates comparison obtained from partial pressure gradient by the diffusive method with the experimental eddy covariance set-up. We also discuss our preliminary results in comparison with previous reports for the area and conclude on the respective challenges and relative basin-scale representativity of the various measurement techniques.

How to cite: Paris, J.-D., Lozano, M., Grilli, R., Ruffine, L., Delmotte, M., Bermell, S., Riboulot, V., and Dupré, S.: Preliminary results of marine methane flux measurement to the atmosphere from the Western Black Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11812, https://doi.org/10.5194/egusphere-egu22-11812, 2022.

Coastal waters with depths less than 40 m contribute at least 90% to the marine methane flux to the atmosphere and these emissions partially offset the carbon burial efficiency of shallow-water inshore environments. Northern high-latitude shallow-water environments have summer productivity on a similar scale to many well-investigated tropical shallow-water ecosystems, but reliable spatial and temporal assessments of methane emissions have been difficult due to the high habitat diversity and the large seasonal variability in temperature and productivity. Here we report on methane fluxes from shallow bays in the archipelago seas of the central Baltic with a focus on both environmental and methodological factors controlling methane emissions for the period August 2020 to May 2021. Three methods were used to determine methane fluxes: floating chambers (FC), eddy covariance (EC), and thin-film boundary layer models (BL). We present 263 repeated FC flux measurements with corresponding BL calculations, and 3013 EC 30-minute flux periods and related these to environmental controlling factors in three different shallow-water ecotypes. The results showed that vegetation density and sediment type were poor predictors for methane fluxes during the period of our study, while eutrophication influences were clearly detectable. Water depth and distance to shore at the scale of <50 meters were not found to be statistically significant when determining methane flux, whereas the day hour of sampling influenced the results. Wind velocity and temperature have commonly been used to predict methane fluxes, but our results showed that wind was only influential for exposed bays and temperature did not appear to have a direct relationship with methane fluxes. The BL method underestimated the gas transfer at low wind speeds and the EC method showed a low signal to noise ratio, with the majority of the methane fluxes below the detection limit. Overall the three methods showed relatively good agreements, but in terms of sensitivity and correlation with environmental factors the FC method was the most suitable method for spatiotemporal scaling of methane fluxes in these complex inshore habitats.

How to cite: Brüchert, V., Bisander, T., and Prytherch, J.: Sea-to-air methane fluxes in heterogeneous shallow-water environments – how should we assess the integrated flux?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6395, https://doi.org/10.5194/egusphere-egu22-6395, 2022.

Lakes are responsible for substantial emissions of methane (CH₄) to the atmosphere globally, but their contribution is poorly constrained and current estimates vary widely. One of the reasons for this large uncertainty lies in the practical challenges involved in collecting CH₄ concentration and flux data at the suitable temporal and spatial scales to capture their natural variability. Here, we present the results of an attempt to account for the spatial and temporal variability of CH₄ concentrations and fluxes when deriving whole-lake and yearly average values. We used these average values to investigate the main environmental drivers of CH₄ concentrations in the surface water of lakes and CH₄ emissions from lakes. We made surface water CH₄ concentration and CH₄ fluxes measurements using headspace equilibration and floating chambers, respectively, in a set of boreal lakes located in different parts of Sweden and with various morphological and biogeochemical properties. Measurements covered different periods of the open-water season at multiple locations covering various depths/distances to shore in each lake. Individual CH₄ flux measurements (150-300 measurements per lake) were interpolated based on relationships with local spatial and temporal variables. Relationships between mean open water season CH₄ concentrations and emissions, and one to three independent environmental variables were tested as different models. The results suggest that the frequency and spatial coverage of the measurements is critical for identifying reliable quantitative empirical models of CH₄ concentrations and emissions from lakes.

How to cite: Schenk, J., Sieczko, A. K., Rudberg, D., Pajala, G., Sawakuchi, H. O., and Bastviken, D.: Evaluating empirical models of lake methane emissions and concentrations in hemiboreal and subarctic regions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1794, https://doi.org/10.5194/egusphere-egu22-1794, 2022.

Lakes are major global sources of atmospheric methane (CH₄), representing an important greenhouse gas. Dissolved molecular Oxygen (DO) in lakes hinders production of CH₄ while promoting CH₄ oxidation. Consequently, it has been suggested as an important regulator of CH₄ emissions from lakes. Presence or absence of DO at the sediment-water interface could therefore influence the extent of CH₄ production in top sediment layers, and the amount of CH₄ that is stored in the anoxic layer of the water column and potentially emitted during water column mixing events. However, the quantitative importance of DO on CH₄ fluxes remains unknown. We studied CH₄ fluxes from two lake basins in northern boreal Sweden of which DO was experimentally added to the deep waters in one of the basins (experimental basin) while the other basin was left in a natural state (reference basin). We used spatial and temporally distributed flux chambers to measure CH₄ fluxes while the lake basins were stratified (from June to October) and found that there was no significant difference in CH₄ fluxes between the two lake basins attributable to the water column experiment. Moreover, we found that the oxygenation of the hypolimnion resulted in a large decrease in CH₄ concentration in the experimental basin, in contrast to the reference basin. However, our monthly lake profile measurements indicated that only a small amount of this CH₄ may have been emitted during the open-water season. First, the two lake basins were subject to incomplete spring and fall mixing events. Second, our CH₄ emission and oxidation model, based on bathymetry, CH₄ concentrations, depth distributed ¹³C/¹²C measurements, and gas transfer velocities, indicated that 0 – 24 % of the stored CH₄ may be emitted on a yearly basis. This shows that the overall impact on CH₄ emissions from boreal forest lakes, due to CH₄ storage and DO, may be smaller than previously believed. However, Fluxes during the mixing events still represent a large uncertainty for the total yearly lake CH₄ emissions.

How to cite: Pajala, G., Sawakuchi, H., Gålfalk, M., Schenk, J., Rudberg, D., Sieczko, A., Karlsson, J., and Bastviken, D.: How important is water column dissolved oxygen (O2) for lake methane emissions?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8863, https://doi.org/10.5194/egusphere-egu22-8863, 2022.

Glaciers and ice sheets cover around 10% of the Earth’s surface and the Greenland Ice Sheet (GrIS) is the largest ice mass in the Northern hemisphere, but is melting at an increasing rate, losing ~400 km³ annually. There have been recent studies linking subglacial environments of the GrIS with methane (CH₄) production and release, presenting a possible positive climate feedback. Previous work has linked organic carbon in subglacial environments with significant CH₄ export via methanogenesis. It has been hypothesised that the GrIS overlies a methanogenically active wetland environment, and thus needs to be included in the global CH₄ budget.

However, the subglacial system of the GrIS is complex and highly heterogenous, hosting oxic and anoxic ecosystems, which have developed over a range of timescales. There are still questions outstanding surrounding the ubiquity of CH₄ release from the GrIS, mainly because of the limited understanding of subglacial carbon cycling and the potential sources of CH₄ in these environments.  

We present the first data from two new, complimentary projects investigating CH₄ release from the GrIS margin, where we aim to quantify the production and release of CH₄ into the atmosphere from the GrIS. We have developed an ambitious temporal and spatial sampling regime to evaluate the CH₄ release along the western margin of the GrIS. We present the first radiocarbon (¹⁴C) dated CH₄ samples from Greenland, helping to shed light on the carbon cycling processes occurring under the ice sheet. We analyse a mixture of atmospheric CH₄ exported from subglacial ice caves and dissolved CH₄ from proglacial rivers draining subglacial portals to explore the age of subglacially sourced CH₄.

We can combine the carbon age of exported CH₄ with microbial analysis and stable isotope data to improve our understanding of the environmental controls on and microbial sources of subglacial CH₄ production and export. Understanding the mechanisms behind subglacial CH₄ export is crucial when attempting to upscale the point source data that is available currently and we consider whether the GrIS could be a potentially important source of CH₄, leading to a substantial, yet currently understudied climatic feedback.

How to cite: Hatton, J., Polášková, A., Garnett, M., Trubac, J., Christiansen, J., Jørgensen, C., Sapper, S., Vinšová, P., Blunier, T., Zarsky, J., Dyonisius, M., Znamínko, M., and Stibal, M.: Subglacial methane cycling under the Greenland Ice Sheet, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4202, https://doi.org/10.5194/egusphere-egu22-4202, 2022.

Authigenic seep carbonates, which are found globally at continental margins, can serve to characterise the seepage of hydrocarbon-enriched fluids into the oceans. This study aims to identify past seepage activity and gas migration pathways on the south-eastern margin of the Mediterranean, based on the analysis of authigenic seep carbonates collected during the 2016 EUROFLEETS 2 SEMSEEP expedition aboard the RV AEGEO. Seep carbonates with three different morphologies (chimneys, crusts and pavements), are studied using standard sediment petrography (fluorescence, CL and standard optical microscopy), as well as X-ray diffraction, Raman spectroscopy and stable isotope analyses. Recurrent cement and replacement phases identified contain different amounts of aragonite, low-magnesium calcite (LMC), high-magnesium calcite (HMC) and dolomite. Carbonate chimneys consist of micrite (δ¹³C_VPDB of -10 to +5 ‰) with dispersed barite and dolomite crystals and fan-shaped aragonite (δ¹³C_VPDB of -52 to -30 ‰). Locally, aragonite fans are replaced by LMC spherulites and blocky HMC. Botryoidal LMC cements are forming in small cavities. Carbonate crusts consist mainly of micrite rich in fossils and detrital grains with LMC breccias, HMC nodules (δ¹³C_VPDB of -35 to -20 ‰) and cements and fan-shaped aragonite cement. These are partly replaced by LMC microsparite and show several growth stages. Carbonate pavements consist mainly of micritic dolomite and HMC. LMC microsparite can be identified as well. Fan-shaped aragonites are locally present as pore-lining cement. Fe-oxides are coating the low- and high-Mg calcitic and dolomitic cements. Raman spectroscopic analyses confirm the presence of aragonite, dolomite and specific organic compounds associated to different crystals.

Sediment petrography, XRD and stable isotope analysis reveal several phases of methane seepage through time. Distinct mineralogies (dolomite and aragonite) within the seep carbonate morphologies, result from different formation mechanisms (anaerobic oxidation of methane during aragonite formation and predominately sulphate reduction during dolomite formation). Raman spectroscopy highlights the presence of organic compounds within specific carbonate phases, which might play an important role in the carbonate formation.

How to cite: Weidlich, R., Bialik, O., Rüggeberg, A., Grobéty, B., Vennemann, T., Makovsky, Y., and Foubert, A.: Insights into the Formation of Southeastern Mediterranean Seep Carbonates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8133, https://doi.org/10.5194/egusphere-egu22-8133, 2022.