BG7.1 | Sources and sinks of methane in the aquatic realm: past records, modern examples and future
EDI
Sources and sinks of methane in the aquatic realm: past records, modern examples and future
Convener: Helge Niemann | Co-conveners: Miriam Römer, Claudio ArgentinoECSECS, Alina Stadnitskaia, Tina Treude
Orals
| Mon, 24 Apr, 16:15–17:55 (CEST)
 
Room C
Posters on site
| Attendance Mon, 24 Apr, 10:45–12:30 (CEST)
 
Hall A
Posters virtual
| Attendance Mon, 24 Apr, 10:45–12:30 (CEST)
 
vHall BG
Orals |
Mon, 16:15
Mon, 10:45
Mon, 10:45
Large amounts of methane, one of the most important greenhouse gasses, are produced in marine and lacustrine systems – but the majority is also consumed in sediments and the water column before reaching the atmosphere. Understanding the fate of methane in the aquatic realm is still a major scientific challenge because it is governed by a vast diversity of geological, oceanographic/limnological and biological factors.

In this session we will discuss past, present and future controls on methane dynamics in marine and lacustrine systems. Within this overarching theme we welcome contributions related to the following topics:

- methane formation: from water-rock interactions, to petroleum systems and microbial degradation processes
- methane sources: natural and man-made seepage
- subsurface fluid flow and methane/hydrocarbon transport mechanisms
- gas hydrate and permafrost
- gas/bubble transport: from numerical modelling to (geophysical) imaging
- seasonality, diel variations, and other temporal constrains
- methane sinks: from microbes and biogeochemical pathways to physicochemical processes
- methane-derived carbonates and microbe-mineral interactions
- molecular/micro/macro fossils from paleo systems.
- new methodologies and proxies for the investigation of methane sources and sinks

Orals: Mon, 24 Apr | Room C

Chairpersons: Miriam Römer, Claudio Argentino, Alina Stadnitskaia
16:15–16:25
|
EGU23-6601
|
ECS
|
On-site presentation
Raquel Arasanz, Roger Urgeles, Ricardo León, Lara F. Pérez, Xavier García, and Rafael Bartolomé

The continental margin off the Antarctic Peninsula hosts significant gas hydrates accumulations off the South Shetland Islands. The area has experienced remarkable isostatic rebound due to ice sheet retreat since the Last Glacial Maximum (LGM). Considering heat flow data reported in the area, hydrates could undergone active dissociation. Such dissociation may modify the mechanical properties of hydrate bearing sediments, eventually leading to slope failures and related fluid seepage may also translate in methane emissions to the ocean. Here we use legacy seismic data to map the occurrence of gas hydrates and free gas and their relation to tectonic structures with the aim of determining the nature of fluid emissions (diffuse or focused).

The subduction of the former Phoenix plate beneath the Shetland plate is the main tectonic control of the area. Normal faults are particularly apparent in the upper part of the slope. These faults disrupt the seafloor or the upper subsurface (within 0.25 to 1 s TWTT below seafloor).

Seismic indicators related to the presence of marine gas hydrates, referred to as bottom simulator reflectors (BSRs), have been observed in the continental slope between Snow and Greenwich Islands and among the Shackleton Fracture Zone and Nelson Island. They are located at water depths between 450 and 4800 m and 0.12 to 0.9 s TWTT below seafloor, becoming shallower towards the shelf edge.  The BSRs are commonly affected by normal faults.

In seismic data, free gas is inferred by acoustic blanking and chimneys. However, the same acoustic response could result from intense tectonic activity, particularly at the foot of the slope, where the Shetland block overthrusts the Phoenix plate. Acoustic blanking and seismic chimneys are often found on the slope in water depths between 590 to 5175m and 0 to 2 s TWTT below seafloor. In general the acoustic blanking is identified in the upper part of the sedimentary record, particularly in the accretionary prism. In most seismic profiles, BSRs occur together with faults, chimneys and acoustic blanking.

According to our results, faulting plays a significant role in the migration of fluids trapped in the sedimentary record. In addition, the presence of chimneys and acoustic blanking in replacement of BSRs suggests unstable gas hydrates under the present-day Pressure/Temperature conditions.

How to cite: Arasanz, R., Urgeles, R., León, R., F. Pérez, L., García, X., and Bartolomé, R.: Hydrocarbon plays of the Antarctic Peninsula margin: Determining fluid flow pathways, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6601, https://doi.org/10.5194/egusphere-egu23-6601, 2023.

16:25–16:35
|
EGU23-8398
|
ECS
|
On-site presentation
Maud Fabre, Lies Loncke, Vincent Riboulot, and Nabil Sultan

Understanding and quantifying the migration of free-gas in hydrate-bearing sediments through time is particularly compulsive along continental margins, where gas hydrate dissociation could have triggered some of the largest submarine landslides observed on Earth. Offshore Romania, high-resolution seismic profiles reveal low reflective or low-velocity zones, which are indicative of free gas, beneath vertical stacked Bottom Simulating Reflectors (BSRs). To further understand the occurrence of double BSRs in the area and the possible effect of gas hydrate dynamics on slope instability and free gas releases, we performed a numerical 2D transient modelling of the evolution of the thermodynamic stability of gas hydrates, integrating in-situ measured physical data and indirect assessments of sea-bottom temperature, thermal conductivity, salinity and sea-level variations. We found that the shallowest BSR matches well with the current Base of the Gas Hydrate Stability Zone (BGHSZ) and the deeper one with the Last Glacial Maximum (LGM) base of GHSZ. The reduction of the GHSZ extension subsequently led to widespread gas hydrate dissociation associated with warming conditions and an increase in Black Sea salinity. However, this dissociation is only responsible of some very superficial submarine landslides (< 30 mbsf and 3 m thick in average) that occurred during this same period. These new constraints improve our understanding of the sliding mechanisms on the Romanian slope that have been ongoing since the LGM and support less catastrophic scenarios than those suggested previously in the case of active gas hydrate dissociation. These results also allow solving the mystery of the double BSR, which here corresponds to a relic of the LGM BGHSZ.

How to cite: Fabre, M., Loncke, L., Riboulot, V., and Sultan, N.: Gas Hydrates stability evolution in Black Sea offshore Romania since the Last Glacial Maximum and its impact on seafloor stability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8398, https://doi.org/10.5194/egusphere-egu23-8398, 2023.

16:35–16:45
|
EGU23-5037
|
ECS
|
On-site presentation
Toshiki Nagakura, Yuki Morono, Motoo Ito, and Jens Kallmeyer

Guaymas Basin, located in the Gulf of California, Mexico, is a young marginal ocean basin with high sedimentation rates of >1 mm/year, active seafloor spreading, and steep geothermal gradients in its sediment. It hosts a unique subseafloor biosphere as these conditions lead to the thermal cracking of sedimentary organic matter and the production of bioavailable organic carbon compounds and hydrocarbons already at shallow depths. The abundance and diversity of potential microbial substrates raise the question of which substrates are being used for catabolic and anabolic microbial metabolism. We thus analyzed the microbial uptake of hydrocarbons using nanoscale secondary ion mass spectrometry (nano-SIMS) analysis after incubation with stable-isotope labeled substrates. Incubations were carried out with samples from two IODP Exp. 385 drill sites, Site U1545 with undisturbed sedimentary strata and a temperature gradient of 225°C/km, and Site U1546 with a sill intrusion led to temporary heating of the sediment. The temperature gradient of 221°C/km indicates thermal equilibration with the surrounding sediment since sill emplacement. Incubations were carried out with 13C-benzene + 2H-hexadecane or 13C-methane at in-situ temperature (4-62°C) and pressure (25 MPa) for 42 days. Additionally, sulfate reduction rates (SRR) were measured by incubating the samples with four aliphatic hydrocarbons + four aromatic hydrocarbons or methane and radioisotope-labeled 35SO42- at in-situ temperature (4-63°C) and pressure (25 MPa) for 10 days. The nano-SIMS analyses reveal that a few samples showed detectable microbial assimilation of hydrocarbons. Nitrogen (from 15NH4Cl in the medium) was assimilated in some samples incubated with methane. The assimilation mostly occurred in samples from near the seafloor (2 and 44 mbsf). We hypothesize that the relatively short incubation time of 42 days was insufficient to detect extremely small incorporation rates in deep sediments. The results of the SRR measurements indicate that a mixture of hydrocarbons and methane increases the SRR in samples from near the seafloor (2 mbsf) and around the sulfate-methane transition zone (44 and 55 mbsf) but not in samples from greater depths. Our results show that anaerobic microorganisms in Guaymas Basin can use hydrocarbons for anabolic and catabolic metabolism in this extreme environment.

How to cite: Nagakura, T., Morono, Y., Ito, M., and Kallmeyer, J.: Microbial hydrocarbon uptake and the effect of hydrocarbons on microbial sulfate reduction, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5037, https://doi.org/10.5194/egusphere-egu23-5037, 2023.

16:45–16:55
|
EGU23-10382
|
On-site presentation
Jung-Hyun Kim, Dong-Hun Lee, Yung Mi Lee, Germain Bayon, Dahae Kim, Young Jin Joe, Xudong Wang, Kyung-Hoon Shin, and Young Keun Jin

Migration of methane-rich fluids at submarine cold seeps drives intense microbial activity and precipitation of authigenic carbonates. In this study, we investigated authigenic carbonate samples taken from active gas hydrate mounds on the southwestern slope of the Chukchi Borderlands (CB), western Arctic Ocean. Our main objectives were to characterize the distribution patterns of trace elements in carbonate-hosted lipid fractions and to assess metalloenzyme requirements of microbes involved in anaerobic oxidation of methane (AOM). We measured stable isotopes, trace elements, lipid biomarkers, and genomic DNA. Our results indicate the dominance of AOM-related lipid biomarkers in studied carbonate samples, as well as a predominant occurrence of the anaerobic methanotrophic archaea (ANME)-1. We also report evidence for significant preferential enrichments of various trace elements (Li, Ni, Co, Cu, Zn, and Mo) in the total lipid fractions of CB carbonates, relative to elemental compositions determined for corresponding carbonate fractions, which differ from those previously reported for other seep sites. We hypothesize that trace element enrichments in carbonate-hosted lipid fractions could vary depending on the type of AOM microbial assemblage.

How to cite: Kim, J.-H., Lee, D.-H., Lee, Y. M., Bayon, G., Kim, D., Joe, Y. J., Wang, X., Shin, K.-H., and Jin, Y. K.: Geochemical and microbial characteristics of authigenic carbonates from the Chukchi Borderlands in the western Arctic Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10382, https://doi.org/10.5194/egusphere-egu23-10382, 2023.

16:55–17:05
|
EGU23-12133
|
On-site presentation
Daniel Birgel, Alexmar Cordova-Gonzalez, Max Wisshak, Tim Urich, Florian Brinkmann, Gerhard Bohrmann, Yann Marcon, and Jörn Peckmann

Methane seeps are typified by authigenic carbonate formation. Many seep carbonates exhibit corrosion surfaces and secondary porosity, which are believed to be caused by microbial carbonate dissolution. Aerobic methane oxidation and sulfur oxidation are the two most likely processes capable of inducing carbonate corrosion at methane seeps. Although the potential of aerobic methanotrophy to dissolve carbonate was confirmed in laboratory experiments, this process has not been studied in the environment to date. Here, we report on a carbonate corrosion experiment carried out in the REGAB Pockmark, Gabon-Congo-Angola passive margin, in which marble cubes were deployed for 2.5 years at two sites (CAB-B and CAB-C) with apparent active methane seepage and one site (CAB-D) without methane seepage. Marble cubes exposed to active seepage (experiment CAB-C) were found to be affected by a new type of microbioerosion. Based on 16S rRNA gene analysis, the biofilms adhering to the bioeroded marble mostly consisted of aerobic methanotrophic bacteria, predominantly belonging to the uncultured Hyd24-01 clade. The presence of abundant 13C-depleted lipid biomarkers including fatty acids (n-C16:1ω8c, n-C18:1ω8c, n-C16:1ω5t), various 4-mono- and 4,4-dimethyl sterols, and diplopterol agrees with the dominance of aerobic methanotrophs in the CAB-C biofilms. Among the lipids of aerobic methanotrophs, the uncommon 4α-methylcholest-8(14)-en-3β,25-diol is interpreted to be a specific biomarker for the Hyd24-01 clade. The combination of textural, genetic, and organic geochemical evidence suggests that aerobic methanotrophs are the main drivers of carbonate dissolution observed in the CAB-C experiment at the REGAB pockmark.

How to cite: Birgel, D., Cordova-Gonzalez, A., Wisshak, M., Urich, T., Brinkmann, F., Bohrmann, G., Marcon, Y., and Peckmann, J.: Aerobic methanotrophic bacteria cause carbonate corrosion at a marine methane seep, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12133, https://doi.org/10.5194/egusphere-egu23-12133, 2023.

17:05–17:15
|
EGU23-7395
|
On-site presentation
Orit Sivan, André Pellerin, Noam Lotem, Efrat Eilani Russak, Yarden Gerera, and Katey Walter-Anthony

About 40% of the annual methane emissions originate from natural, non-anthropogenic sources. These include mainly freshwater sediments, in which significant increase in methane emissions has been observed throughout the past decades with the ongoing global temperature rise. Thermokarst lakes, formed by abrupt thawing of permafrost, play a significant role in this observed increase in methane emissions. However, methane production rates and natural consumption controls there are not well constrained, as well as their response to global warming.  

We explore the rates and mechanisms of methane production and anaerobic oxidation (AOM) processes several interior Alaska thermokarst lakes, which formed and continue to expand as a result of ice-rich permafrost thaw. This is mainly through geochemical and microbial profiles combined with slurry incubation experiments with labeled isotopes, potential electron acceptors and several inhibitors in different temperatures. Our manipulated experiments shed insight on the controls of methanogenesis onset and the mechanisms of both methanogenesis and AOM. Direct rate measurements using two isotope methods and modeling provide robust rate estimations for methanogenesis and AOM. They indicate that the role of AOM in these lakes is less significant than previous estimations, and that AOM will probably not attenuate the methanogenesis increase in a warmer climate. 

How to cite: Sivan, O., Pellerin, A., Lotem, N., Eilani Russak, E., Gerera, Y., and Walter-Anthony, K.: The importance of anaerobic oxidation of methane in thermokarst lakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7395, https://doi.org/10.5194/egusphere-egu23-7395, 2023.

17:15–17:25
|
EGU23-5518
|
ECS
|
On-site presentation
Getachew Agmuas Adnew, Moritz Schroll, Sarah Elise Sapper, Thomas Röckmann, Maria Elena Popa, Christian Juncher Jørgensen, Frank Keppler, Carina van der Veen, Malavika Sivan, Thomas Blunier, and Jesper Riis Christiansen

The subglacial environment under the Greenland Ice Sheet (GrIS) is an active zone of methane (CH4) production and consumption (1). Recent studies have shown that the meltwaters are a net source of CH4 to the atmosphere (2), although its global significance remains unquantified. It is unknown how CH4 cycling and net emission is linked to the melting of the GrIS, which is expected to increase (3) as the Artic is warming four times faster than the global average. Evaluating the importance of this poorly known source for the atmospheric CH4 budget and its drivers requires a fundamental understanding of the amounts released, the sources and sinks and its age.

Traditionally, measurements of the isotopic composition (13CH4 and 12CH3D) are used as fingerprints to identify sources and sinks of CH4. However, this method is limited due to the overlap of source signatures. For example, microbial methanogenesis in some environments can produce stable isotope compositions resembling thermogenic methane (4).  Furthermore, substrate isotopic composition, substrate limitation, the kinetics of methane production, transport, and oxidation substantially impact the stable isotope composition of microbially produced CH4. This complicates the interpretation of CH4 cycling and its physicochemical drivers.

Clumped isotopes of methane, i.e. molecules of CH4 with two rare isotopes, (13CH3D and 12CH2DD), and its clumping anomaly (the relative difference between the measured value of 13CH3D and 12CH2DD and its stochastic distribution) provide additional insight to constrain CH4 sources and sinks. From the clumping anomaly, it is possible to calculate the formation temperature of methane (i.e. source of methane) if CH4 was formed in thermodynamic equilibrium. In the case of disequilibrium, the clumped signatures can be used to identify various kinetic gas formation and fractionation processes that are impossible to reconstruct from the bulk isotopic composition alone.

In this study, we present for the first-time isotopic data of clumped CH4 and traditional isotopes of subglacial CH4 together with radiocarbon measurements (14CH4). These data are related to the isotopic composition of subglacial CO2 and mole fractions of the gases in the air and meltwater. Based on this data set, we will discuss the production and consumption pathways of CH4 in the subglacial environment and how it relates to diurnal and seasonal cycles of meltwater discharge.

Reference:

  • Christiansen et al. (2021). DOI: 10.1029/2021JG006308
  • Christiansen, J. R., & Jørgensen, C. J. (2018). DOI: 10.1038/s41598-018-35054-7
  • Ranlanen, et al. (2022), Commun Earth Environ, 2022. DOI: 10.1038/s43247-022-00498-3
  • Valentine et al. (2004). DOI: 10.1016/j.gca.2003.10.012

 

How to cite: Adnew, G. A., Schroll, M., Sapper, S. E., Röckmann, T., Popa, M. E., Jørgensen, C. J., Keppler, F., van der Veen, C., Sivan, M., Blunier, T., and Christiansen, J. R.: Constraining sources and sinks of subglacial methane from the Greenland ice sheet using clumped isotopes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5518, https://doi.org/10.5194/egusphere-egu23-5518, 2023.

17:25–17:35
|
EGU23-15748
|
ECS
|
On-site presentation
Philip Pika, Sandra Arndt, Veronica Tsibulskaya, Ankit Pramanik, Jade Hatton, Jakub Zarsky, Lia Costa Pinto Wentzel, Jakub Trubac, Anna Stehrer-Polášková, Petra Vinšová, Jon R. Hawkings, and Marek Stibal

 

Recent studies have shown the release of methane (CH4) through the melting Greenland Ice Sheet, and have thus identified it to have an additional potential positive climate feedback. This CH4 is thought to originate from biologically active methanogenic ecosystems in subglacial sediments, where microbes produce it by converting overridden organic carbon to CH4, which then accumulates over time. Subsequent CH4 diffusion into the subglacial hydrologic network transports it then to the ice sheet margin, where it is directly emitted to the atmosphere from supersaturated proglacial streams. Methanogenesis is highly dependent on anoxic conditions, which are in turn determined by the seasonally evolving subglacial environment subject to episodic flooding and thereby recharging oxygenated waters from surface melting. The main biogeochemical and hydrological drivers influencing the rate of CH4 production, as well as the magnitude and timing of these subglacial CH4 fluxes remain largely unknown and therefore unconstrained. Addressing these unknowns is essential because CH4 is not only a powerful greenhouse gas, but also because its unaccounted release exacerbates the ongoing climate amplification in the Arctic. The lack of observational data is primarily due to the challenging conditions for accessing the subglacial environment and the shortage of direct measurements of CH4 production, consumption, and export from the Greenland Ice Sheet and the complex nature of the subglacial system. This invites the application of reaction-transport modelling tools in combination with observational data to fill these knowledge gaps by disentangling the complex processes and drivers, and eventually quantifying CH4 cycling processes in Greenland’s subglacial sediments and their impacts on the global CH4 cycle and climate change. However, such modelling tools do not currently exist. 

Here, we develop a coupled subglacial sediment-cavity-stream model to  explore the potential of subglacial environments to produce and accumulate methane beneath the Greenland Ice shield. The model accounts for heterotrophic methane production, methane oxidation, as well as advective and diffusive methane transport. Current field data observations are used to initialize the model, but it will also be forced over a wide range of plausible conditions (i.e. organic matter availability and reactivity, sediment thickness, terminal electron acceptor availability) that have could be found  beneath the Greenland Ice shield. The results of this large model ensemble does not only help identify the most important biogeochemical and hydrological drivers on methane production and accumulation in subglacial environments, but also allows to identify areas beneath the ice sheet that could produce and accumulate important quantities of methane.

These new developments present the first step in the development of a new fully coupled hydrological-biogeochemical model for subglacial environments, which will inform upscaling efforts and guide future field work.

 

How to cite: Pika, P., Arndt, S., Tsibulskaya, V., Pramanik, A., Hatton, J., Zarsky, J., Wentzel, L. C. P., Trubac, J., Stehrer-Polášková, A., Vinšová, P., Hawkings, J. R., and Stibal, M.: The potential for methane production and accumulation in Greenland’s subglacial environment– Steps Towards A New Mechanistic Hydrological-Biogeochemical Model For Subglacial Methane Cycling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15748, https://doi.org/10.5194/egusphere-egu23-15748, 2023.

17:35–17:45
|
EGU23-15853
|
ECS
|
On-site presentation
|
Knut Ola Dølven, Juha Vierinen, Roberto Grilli, Jack Triest, and Bénédicte Férre

To improve our understanding of critical environmental processes, high resolution measurements with acceptable accuracy are essential. Unfortunately, the high spatiotemporal variability often associated with seabed seepage environments is prone to mischaracterization due to limitations in contemporary measurement techniques. This is particularly true for dissolved methane, which are often measured by labor- and time-intensive discrete water sampling and subsequent laboratory analysis. This often yields data with inadequate spatiotemporal resolution. A potential solution to this issue is using in-situ sensors in towing, profiling, mooring/observatory or glider operations. However, typical off-the-shelf sensors with adequate payload and power requirements currently lack the response time necessary for these applications. We offer a new, easy-to-implement, laboratory and field-tested post-processing tool for retrieving fast response data from commercially available methane sensors with slow response times. The tool is based on the framework of statistical inverse theory which in practice enables the user to obtain data with quantified, explicit (modeled) measurement uncertainty and at the resolution (i.e. response time) where the sensor can provide data with a respectable level of accuracy. The user needs no input besides the raw data, sensor accuracy, and response time. In our field experiment, we successfully retrieved data corresponding to a response time of 55 s using a sensor with a stated response time of 29 minutes. Being able to obtain high-resolution data from these types of sensors can considerably enhance the capacity to properly resolve the variability within methane seep sites and comprehend associated environmental processes.

How to cite: Dølven, K. O., Vierinen, J., Grilli, R., Triest, J., and Férre, B.: High resolution dissolved methane data using a new response time correction technique on slow response methane sensor measurements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15853, https://doi.org/10.5194/egusphere-egu23-15853, 2023.

17:45–17:55
|
EGU23-1866
|
ECS
|
On-site presentation
Tamara Michaelis, Felicitas Kaplar, Anja Wunderlich, Thomas Baumann, and Florian Einsiedl

Ebullition is a major transport pathway of methane from aquatic ecosystems to the atmosphere. Several studies have highlighted that methane ebullition plays an important role in reservoirs, but not much data is available from non-impounded stream sections. Quantifying river methane emissions on a global scale is a challenge due to the high spatiotemporal heterogeneity in these dynamic and variable systems. A better conceptual understanding of riverine methane ebullition is needed for sound carbon budgets, both in terms of flux volumes and predictive factors for hot-spot emission zones.

This study targeted the knowledge gap in riverine methane ebullition with a time-resolved observation-based analysis of the ebullitive transportation pathway in a river cross section. We installed four bubble traps in a small stream in southern Germany and monitored volumes and concentrations of the two greenhouse gases CO2 and CH4 along with carbon stable isotopes (δ13C) of CH4 over the course of a year. The bubble traps were evenly distributed over one cross-section in a curve to represent different sediment-compositions and flow regimes between the undercut slope and slip-off slope. Sediment characterization at each site included grain-size distribution curves, porosity measurements and determination of organic carbon content.

Ebullitive gas fluxes were extremely high at two of the locations centrally in the river: up to 1000 ml m-2 d-1 during summer and autumn, and 100-400 ml m-2 d-1 in December. CH4 concentrations of up to 65% were measured in the gas samples and CH4 exceeded CO2 concentrations (<5%) by far. Each site in the cross section showed relatively stable gas volumes and concentrations during the summer period. As expected, at the undercut slope, only very small gas volumes were detected year-round. Contrary to prior expectations, gas volumes and greenhouse gas concentrations were highest in the central section of the river and not above the fine-grained deposits of the slip-off slope. CH4 isotopes were generally <-60‰, indicating a major contribution of hydrogenotrophic methanogenesis. During July however, δ13C of CH4 at the slip-off slope showed higher values between -60‰ and -45‰, potentially due to a shift in the main methanogenic pathway or connected to increased macrophyte growth observed simultaneously. Overall, this study underlines the importance of ebullitive greenhouse gas fluxes from rivers for the global climate and provides novel insights into the role of different streambed sections for CH4 ebullition during all seasons of the year.

How to cite: Michaelis, T., Kaplar, F., Wunderlich, A., Baumann, T., and Einsiedl, F.: One-year methane ebullition measurements over a cross section of a small stream, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1866, https://doi.org/10.5194/egusphere-egu23-1866, 2023.

Posters on site: Mon, 24 Apr, 10:45–12:30 | Hall A

Chairperson: Helge Niemann
A.244
|
EGU23-1297
|
ECS
Evelyn Workman, Anna Jones, Rebecca Fisher, James France, Katrin Linse, Ming-Xi Yang, Tom Bell, and Yuanxu Dong

Atmospheric methane (CH4) is a potent greenhouse gas with natural and anthropogenic sources. Concentrations have been significantly increasing over the past few decades, which poses a problem for future climate change goals. The contribution of oceans to the global atmospheric CH4 cycle is largely uncertain. It is accepted that oceans act as a small net source of atmospheric CH4. As the polar regions are warming faster than the global average, it is important that we can better quantify CH4 emissions from the polar oceans. 


In this study, we combine various forms of shipborne data (ambient atmospheric methane concentrations, sea-air CH4 fluxes and isotopic composition of atmospheric CH4) taken during cruises in the Arctic and Southern Oceans to present a more complete picture of atmospheric CH4 above polar oceans, including addressing the question of how much the oceanic component is contributing towards the atmospheric budget in these regions. Measurements  were made around the Barents Sea and Greenland Sea in the Arctic, and in the Atlantic sector of the Southern Ocean, including the Scotia Sea.


Sea-air CH4 fluxes are measured using the eddy covariance method; indeed, this the first study to use this technique to directly measure how much CH4 is released from the ocean into the atmosphere in both the Southern and Arctic Oceans. Atmospheric CH4 measurements are then investigated in order to understand the impact that CH4 released from the ocean has on the atmospheric burden. We also measure the isotopic composition of CH42H and δ13C) in air samples taken onboard polar cruises, to understand the sources of atmospheric CH4 above these oceans. The isotope measurements can indicate if the CH4 comes from a biogenic or thermogenic source, which can help determine if anthropogenic or natural processes are behind the production.


We investigate the potential sources of CH4 released by the polar ocean by looking at areas of known seabed CH4 seepages, investigating phytoplankton abundance, and investigating the isotopic composition of atmospheric CH4 in areas of elevated CH4


We find that the region of the Arctic Ocean investigated in this study is a slight atmospheric CH4 source in boreal summer, while the region of the Southern Ocean investigated is a CH4 source in areas of shallower water/continental shelves and a CH4 sink in region of open ocean, in austral summer. This finding is consistent with previous studies that have detected seabed CH4 emission. Seabed CH4 seepage at shallower depths is more likely to penetrate the sea-air interface, while CH4 produced at the seabed at deeper depths gets oxidised as it travels through the water column, making it less likely to reach the surface . We also find evidence of localised “hot spots” of methane emission which will be described. 

How to cite: Workman, E., Jones, A., Fisher, R., France, J., Linse, K., Yang, M.-X., Bell, T., and Dong, Y.: Investigating oceanic sources of methane and their influence on ambient concentrations in polar regions., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1297, https://doi.org/10.5194/egusphere-egu23-1297, 2023.

A.245
|
EGU23-2612
|
Ricardo León, Jesus García-Crespo, Roger Urgeles, Raquel Arasanz, and Xavier García

In the Antarctic Peninsula, a marine gas hydrate system has been identified based on geophysical data (Lodolo et al., 1993; Tinivella et al., 2002). These data suggest gas hydrates average volume concentration of 6.0 ± 1.2% for in the accretionary wedge of the South Shetlands Islands (Tinivella, 2002).

Based on legacy seismic profiles (belonging to 17 oceanographic cruises) retrieved from the Antarctic Seismic Data Library System (SDLS), a continuous Bottom Simulating Reflector (BSR) has been mapped in the accretionary wedge, between Elephant and King George islands. This BSR is located at a sub-bottom depth between ca. 250 ms TWTT in the upper slope and ca. 1s TWTT at the base of the accretionary wedge.

The theoretical Base of Gas Hydrate Stability Zone (BGHSZ) calculated with a static model (León et al., 2009) for the present oceanographic conditions (pressure/bathymetry, seafloor temperature, geothermal gradient and salinity) is located 100 to 400 m shallower than this BSR level, considering available geothermal data for the area. The BSR-BGHSZ mismatch points that gas hydrates in the area seem to be in a transient state with respect to their theoretical location calculated from both pure methane and thermogenic compositions.

Dynamic models developed with TOUGH+HYDRATE in the frame of ICEFLAME project (PID2020-114856RB-I00, Spanish Ministry of Science and Innovation), reveal two possible scenarios for the above mismatch between BSR and BSGHZ: isostatic rebound and/or tectonic uplift.

References

León, R., Somoza, L., Giménez-Moreno, C.J., Dabrio, C.J., Ercilla, G., Praeg, D., Díaz-del-Río, V., Gómez-Delgado, M., 2009. A predictive numerical model for potential mapping of the gas hydrate stability zone in the Gulf of Cadiz. Mar. Pet. Geol. 26, 1564–1579. https://doi.org/10/czq8vq

Lodolo, E., Camerlenghi, A., Brancolini, G., 1993. A bottom simulating reflector on the South Shetland margin, Antarctic Peninsula. Antarct. Sci. 5. https://doi.org/10/bfcb22

Tinivella, U., 2002. The seismic response to overpressure versus gas hydrate and free gas concentration. J. Seism. Explor. 11, 283–305.

Tinivella, U., Accaino, F., Camerlenghi, A., 2002. Gas hydrate and free gas distribution from inversion of seismic data on the South Shetland margin (Antarctica). Mar. Geophys. Res. 23, 109–123. https://doi.org/10/fcgq3q

 

How to cite: León, R., García-Crespo, J., Urgeles, R., Arasanz, R., and García, X.: Dynamic modelling of marine gas hydrates north of the South Shetland Islands (Antarctic Peninsula), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2612, https://doi.org/10.5194/egusphere-egu23-2612, 2023.

A.246
|
EGU23-5981
|
ECS
Tim de Groot, Anne Mol, Katherine Mesdag, Harry Witte, Rachel Ndhlovu, Pierre Ramond, Julia Engelmann, Thomas Röckmann, and Helge Niemann

The UNESCO world heritage Wadden Sea is a highly productive coastal system, rich in biodiversity and influenced by physical and biological forcing. Such coastal seas are known to dominate ocean’s methane emissions, but knowledge on variations and controls of methane and the efficiency of the microbial methane filter in these vastly dynamic systems are scarce. We conducted high frequency sampling over a 2-days’ period during four seasons to determine diel and seasonal effects on methane dynamics in the Dutch Wadden Sea. We found that waters were charged with methane throughout the year with maximum concentrations of up to 155 nM in summertime. Methane concentrations were generally lower at high tide and in the colder seasons, whereas MOB activity increased by ~2-fold during low tide compared to high tide. On average, only a minor fraction of the WZ methane budget (~2%) is retained by MOBs in the WZ itself, while ~1/3 escapes to the atmosphere and ~2/3 are flushed out into the open North Sea where it may be consumed by microbes or is liberated to the atmosphere.

How to cite: de Groot, T., Mol, A., Mesdag, K., Witte, H., Ndhlovu, R., Ramond, P., Engelmann, J., Röckmann, T., and Niemann, H.: Diel and seasonal methane dynamics in the shallow and turbulent Wadden Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5981, https://doi.org/10.5194/egusphere-egu23-5981, 2023.

A.247
|
EGU23-3725
Naizhong Zhang, Mellinda Jajalla, Mayuko Nakagawa, Koudai Taguchi, and Alexis Gilbert

Terrestrial hydrocarbon seeps are widely distributed in oil/gas field. To constrain the sources and post-generation processes occurring in these seeping gases, various geochemical approaches, such as chemical and stable isotope composition (δ13C and δD) of hydrocarbons have been extensively used. However, the interpretation can be ambiguous due to the overlap of signatures when using these approaches only1. Some recently developed analytical techniques, such as methane clumped isotope analysis (Δ13CH3D and Δ12CH2D2)2-4 and propane position-specific isotope analysis (PSIA)5 may provide new clues to improve our understanding of the origin and fate of hydrocarbons.

In this study, we focus on gas samples from different gas seeps and mud volcanos in central Japan collected from 2019 to 2022, where hydrocarbons were considered mainly originating from thermal cracking of organic matter6. Gas compositions, bulk stable isotopes of hydrocarbons and associated CO2, clumped isotopes of methane, PSIA of propane and other geochemical parameters have been studied. Coupled methane clumped isotope signatures and propane PSIA information provide direct evidence of secondary microbial methane formation associated with biodegradation of non-methane hydrocarbons. The contribution of secondary microbial methane in different seeps/mud volcanos and its temporal changes are also discussed by a mixing model integrating all these isotope information, which provides valuable constraints on methane sources in terrestrial seeps.

References: [1] Milkov and Etiope, 2018, Org. Geochem.; [2] Stolper et al., 2014, Geochim. Cosmochim. Acta.; [3] Stolper et al., 2014, Science; [4] Young et al., 2017, Geochim. Cosmochim. Acta.; [5] Gilbert et al., 2019, Proc. Natl. Acad. Sci. U.S.A.; [6] Etiope et al., 2011, Appl. Geochem.

How to cite: Zhang, N., Jajalla, M., Nakagawa, M., Taguchi, K., and Gilbert, A.: Coupled methane clumped isotope and propane position-specific isotope analyses indicate significant methane contribution from biodegradation of hydrocarbons in terrestrial methane seeps located at central Japan, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3725, https://doi.org/10.5194/egusphere-egu23-3725, 2023.

A.248
|
EGU23-4058
|
ECS
|
Guiyuan Dai, Xiaogang Chen, Lijun Cui, Guangchao Zhuang, Feng Ju, and Ling Li

Methanogenesis is important to the net carbon burial capacity in seagrass sediments. This microbially driven biogeochemical process can be fulfilled via three main pathways: hydrogenotrophic, acetoclastic and methylotrophic methanogenesis. However, the relative importance of each methanogenic pathway in seagrass meadows is poorly reported. In marine sediments where sulfate is abundant (15-43 mmol/L), hydrogenotrophic and acetoclastic methanogenesis are usually inhibited because of the competition for methanogenic substrates including hydrogen and acetate by sulfate-reducing bacteria. Thus, methylotrophic methanogenesis is hypothesized to play a crucial but yet-underappreciated role in the carbon cycle. In this study, culture-independent metagenomic approaches were used to profile the methanogenic pathways in seagrass sediments. Based on the functional potential analysis, methylotrophic methanogenesis is revealed as the dominant pathway in the seagrass sediments. Based on the metagenome-assembled genome analysis, Methanococcoides, which harbors known methylotrophic methanogens, is the only detected genus of methanogens in the seagrass meadow metagenomes. In the bare sediment, the abundance of Methanococcoides in the bottom was 41% higher than that in the surface due to the low oxygen in the bottom. While in the sediment covered by seagrasses, the abundance of Methanococcoides in the surface was 43-82% higher because of the higher fresh organic carbon content, which provides abundant substrates for methanogens. These findings reveal the methylotrophic methanogenesis is the main methanogenic pathway in both bare sediments and sediments covered by seagrasses. The hydrochemical analysis further suggested that in bare sediments, the methanogenesis was mainly controlled by oxygen content. However, in seagrass sediments, the availability of substrates was the dominated factor.

How to cite: Dai, G., Chen, X., Cui, L., Zhuang, G., Ju, F., and Li, L.: Metagenomic insight into methanogenic pathways in seagrass sediments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4058, https://doi.org/10.5194/egusphere-egu23-4058, 2023.

A.249
|
EGU23-7680
Boris Katsnelson, Ernst Uzhansky, Regina Katsman, Andrey Lunkov, and Anatoliy Ivakin

Gassy aquatic sediments are abundant over the world. Multiannual CH4 gas content in shallow sediments of Lake Kinneret, Israel, was evaluated by acoustic applications. Experiments were conducted mainly over the intermediate-deep parts of the lake. Low-to-moderate frequencies wideband acoustic signal was emitted, when sound speed indicating a gas content, was evaluated based on the reflection coefficient. Both frequency dependence of  reflection coefficient and backscattering were analyzed.. The effect of the following factors affecting the dynamics of CH4 bubbles in aquatic sediments in the lake Kinneret, was investigated statistically: (1) Organic matter flux to sediment controlling CH4 production; (2) Its timing relatively to the date of the acoustic measurements, controlling CH4 bubbles dissolution; (3) Water depth affecting CH4 solubility, mechanical sediment properties, and ebullition from the sediment. Multiple regression analysis indicates that the organic matter supply to the lake sediments due to the crash of phytoplankton bloom in the lake, acts as a major control on the sediment gas content over the multi-annual period. The gas content is least sensitive to water depth, explained probably by the uniform organic matter deposition flux to the medium-deep parts of the lake, from where the ebullitions is unfeasible. [the work is supported by BSF grant 2018150].

How to cite: Katsnelson, B., Uzhansky, E., Katsman, R., Lunkov, A., and Ivakin, A.: Multi-physical controls on gas content in sediments of lake Kinneret, Israel, evaluated by acoustic applications, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7680, https://doi.org/10.5194/egusphere-egu23-7680, 2023.

A.250
|
EGU23-16805
Martin Scherwath, Miriam Römer, Yann Marcon, and Michael Riedel

Since 2012 Ocean Networks Canada (ONC) has acquired high-quality long-term multibeam sonar data from the seafloor at Clayoquot Slope off Vancouver Island which is one of the most active methane vent regions of the Cascadia Margin, and where ONC has one of its nodes of the NEPTUNE cabled seafloor observatory. The sonar was first deployed at a vent field of irregular activity (near Bubbly Gulch), and since 2014 it is located at an extremely active field (Gastown Alley). The data that have been analyzed so far exhibit the strong dependence of gas bubble emissions with tidal pressure, although the tides alone cannot explain all the observed dynamics such as onset cessation of ebullition or periods of strong versus absence of seepage, and other factors need to be considered and ideally monitored to predict future seepage.

Seafloor cabled observatories are ideal to acquire high-resolution data of many ocean and seafloor parameters, including those from high-bandwidth data or power-intensive instruments. This presentation is an opportunity to explore possibilities to modify the existing seepage observatory, adding to the already installed instruments at Clayoquot Slope, or change focus to Barkley Canyon, another ONC node location, where hydrate mounts and outcrops occur and methane vents also exist.

How to cite: Scherwath, M., Römer, M., Marcon, Y., and Riedel, M.: Continuous Seep Monitoring at the Northern Cascadia Margin by Ocean Networks Canada, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16805, https://doi.org/10.5194/egusphere-egu23-16805, 2023.

A.251
|
EGU23-1917
|
ECS
Claudio Argentino, Amicia Lee, Luca Fallati, Diana Sahy, Daniel Birgel, Jörn Peckmann, Stefan Bünz, and Giuliana Panieri

The origin of modern seafloor methane emissions in the Barents Sea is tightly connected to the glacio-tectonic and oceanographic transformations following the last ice age. Despite the increasing number of new active seep discoveries, their accurate geochronology and paleo-dynamic is still poorly resolved, thus hindering precise identification of triggering factors and mechanisms controlling past and future seafloor emissions. Here, we report the distribution, petrographic (thin section, electron backscatter diffraction), isotopic (δ13C, δ18O) and lipid biomarker composition of methane-derived carbonates collected from Leirdjupet Fault Complex, SW Barents Sea, at 300 m depth during an ROV survey in 2021. The integration of phase-specific isotopic analysis and U/Th dating enabled us to track carbonate mineral precipitation over the last 8 ka.  Our results indicate that methane and petroleum seepage in this area followed a similar evolution as in other southernmost Barents Sea sites controlled by the asynchronous deglaciation of the Barents Sea shelf, and that methane-derived carbonate precipitation is still an active process at many Arctic locations.  

This study was supported by AKMA project (Research Council of Norway grant No. 287869) within the frame of the Centre for Arctic Gas Hydrate, Environment and Climate (CAGE) (Research Council of Norway grant No. 223259), and by Erasmus+ Programme of the European Union.

How to cite: Argentino, C., Lee, A., Fallati, L., Sahy, D., Birgel, D., Peckmann, J., Bünz, S., and Panieri, G.: Biogeochemistry and timing of methane-derived carbonate formation at Leirdjupet fault complex, SW Barents Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1917, https://doi.org/10.5194/egusphere-egu23-1917, 2023.

A.252
|
EGU23-11959
|
ECS
Annalisa Delre, Tim de Groot, Thomas Röckmann, Julia Engelmann, Gert-Jan Reichart, and Helge Niemann

Coastal systems including river deltas are the major sources of methane from the ocean to the atmosphere; however, large uncertainties exist on the actual source strength. Abiotic and biotic factors controlling methanogenesis, methanotrophy and methane efflux to the atmosphere thus need better understanding. In this presentation, we will show data from a recent cruise along the Sheldt Estuary from Antwerp (salinity: 2 psu) towards the open North Sea (salinity: 31 psu). Methane concentrations were elevated in Antwerp with up to 110 nM, decreased downriver to values ~50nM, increased to 180 nM at the inflow of a large canal (Ghent–Terneuzen Canal) and then decreased to ~70nM in the open North Sea. Methane oxidation rates were highest in the city of Atnwerp (~29 nM/d), only slightly elevated at the inlet of the Ghent–Terneuzen Canal (~10 nM/d) and lower in the open North Sea (5nM/d). Differently to previous findings in other river systems, we could not find a clear dependency of methane oxidation to salinity. In this presentation, we will also present data on CH4 fluxes from the river to the atmosphere, CH4 stable isotope systematics and the composition of the methanotrophic community to further constrain methane dynamics in the estuary.

How to cite: Delre, A., de Groot, T., Röckmann, T., Engelmann, J., Reichart, G.-J., and Niemann, H.: "Methane oxidation in the Estuary of a medium-sized European river, the Scheldt", EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11959, https://doi.org/10.5194/egusphere-egu23-11959, 2023.

A.253
|
EGU23-4257
Regina Katsman and Xiongjie Zhou

Methane (CH4) bubbles residing in shallow aquatic muds pose a significant threat to the environment. Impeded by the muddy sediment opacity and insufficient resolution for their characterization, past studies overlooked bubble interactions during their growth. The competitive growth of CH4 bubble pairs with different initial sizes is simulated, using a mechanical/reaction-transport numerical model. Mechanical and solute transport interactions were found to dominate at the different stages of the bubble growth, both retarding the smaller competitive bubble growth. Stress from the large competitive bubble affects the inner pressure and diffusive flux to the smaller bubble, producing its slower initial growth. The large competitive bubble diverts CH4 from the smaller one at the later stages, thus inhibiting its growth even more. Bubble stress interactions may produce more laterally oriented smaller bubbles and significant deformations of the larger ones. Competitive bubble growth may shape a bubble size distribution pattern, promote muddy sediment CH4 gas retention, and produce gas domes. The latter acts as pockmark precursors whose formation induces a violent gas release to the water column and potentially to the atmosphere. Our study presents a basis for proper upscaling to various effective gassy muddy sediment characteristics and gas retention models. It contributes to the evaluation and even reduction of a long-persisting uncertainty related to the CH4 fluxes from the shallow aquatic sediments.

How to cite: Katsman, R. and Zhou, X.: Competitive CH4 bubble growth in aquatic muds, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4257, https://doi.org/10.5194/egusphere-egu23-4257, 2023.

A.254
|
EGU23-5453
|
ECS
|
Ellen Schnabel and Jens Kallmeyer

All hydrocarbon (HC) reservoirs experience some degree of leakage, so HCs will enter the overlying sediment. While strong leakage causes surface manifestations, minor leakage can remain undetected as the hydrocarbons are completely metabolized during their ascent to the sediment surface. However, even minor seepage affects the sediment’s geochemistry and microbiology as it adds electron donors. The PROSPECTOMICS project aims to use these sometimes minute microbiological and geochemical changes as a tool for HC prospecting.

We recovered fifty 2-3 m long sediment cores in the Barents Sea from three potential HC seepage zones (HC zones) and two zones without seepage (REF zones) and sampled sediment and pore water with high spatial resolution.

We measured sulfate reduction rates and quantified microbial cell abundance, and characterized the organic matter via FT-ICR-MS. We also quantified anions and cations in the pore water via ion chromatography and ICP-MS and determined alkalinity via titration.

FT-ICR-MS and cell counts did not show any differences between HC zones and REF zones. Sulfate concentration profiles decrease linearly with depth and show a much steeper decline and greater variability in the HC zones than in the REF zones. The linear profiles imply the absence of active sulfate reduction within the cored depth intervals and a sink for sulfate at greater depth, most probably sulfate-driven anaerobic oxidation of methane (AOM). This would also explain the correlating linear increase in alkalinity. At some sites in the HC zones pore water sulfide profiles also increase linearly with depth whereas at other HC sites and at all REF sites, sulfide concentrations remain below our detection limit throughout the entire core.

Using highly sensitive 35SO42- radiotracer incubations, we were able to detect low rates of microbial sulfate reduction in the pmol*cm-3*d-1 range in some single samples from deeper layers in HC cores. Thus, despite apparently linear pore water sulfate gradients indicating no net sulfate reduction, we observed minor but detectable microbial turnover of sulfate. Modeling of the sulfate reduction rate based on pore water concentration data also yielded rates in the same order of magnitude as the radiotracer measurements, confirming microbial activity.

Looking at the pore water cation concentration profiles, manganese and calcium show different linear trends in the HC zones compared to REF zones. In HC zones, manganese decreases with depth, while in the REF zones, manganese concentrations increase. Calcium concentrations decrease at HC sites while they remain constant at REF sites. These findings can partly be explained by microbial activity and associated alteration of clays, potentially due to microbial reduction of structural metals, ion exchange processes and mineral dissolution and formation. Barium was only detected in the pore water of some cores originating from HC zones where it might have been released during sulfate reduction accompanied with destabilization of baryte.

In summary, relative differences in pore water ion concentration trends and the occurrence of sulfate reduction may be indicators of HC seepage.

How to cite: Schnabel, E. and Kallmeyer, J.: Geochemical detection of minor hydrocarbon seepage in marine sediment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5453, https://doi.org/10.5194/egusphere-egu23-5453, 2023.

A.255
|
EGU23-8623
Ingeborg Bussmann, Erich Achterberg, Holger Brix, Nicolas Brüggemann, Philipp Fischer, Götz Flöser, Jens Greinert, Uta Ködel, and Claudia Schütze

The estimations of the diffusive methane flux from the water phase into the atmosphere in coastal waters is relevant for a better estimate of the atmospheric greenhouse-gas budget. Unfortunately, so far, the numerical determination of the fluxes has a high level of uncertainty in coastal waters.

To improve the estimation of coastal methane fluxes, not only a high temporal and spatial sampling resolution of the dissolved methane in the water are required. Besides, also the atmospheric methane concentration and the wind speed and wind direction above the surface is important. In most cases, these atmospheric data are obtained from near-by atmospheric and meteorologic monitoring stations. In this study, we measured wind speed, direction and atmospheric methane local directly on board of three research vessel cruising in the southern North Sea within the MOSES project and compared the effects of local versus remote measurements of these data on the flux data. In addition, using the wind direction and speed, we try to assess the origin of the atmospheric methane measured in the study area. Using these “improved” data sets, we discuss if local measurements of auxiliary data provide better insights in the determining factors of the methane flux, and thus also improve the regional aquatic methane budget.

How to cite: Bussmann, I., Achterberg, E., Brix, H., Brüggemann, N., Fischer, P., Flöser, G., Greinert, J., Ködel, U., and Schütze, C.: Influence of wind speed and wind direction above the sea surface on the diffusive methane flux and the atmospheric methane concentration at the North Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8623, https://doi.org/10.5194/egusphere-egu23-8623, 2023.

A.256
|
EGU23-12876
|
ECS
Lasse Tésik Prins, Bodil Wesenberg Lauridsen, Ole Rønø Clausen, Hans Røy, Kasper Urup Kjeldsen, and Paul Knutz

As societies gradually shift from oil and gas to renewable energy, many offshore wells will be plugged and abandoned, while some will be transformed to facilitate carbon storage (CCS). In order to mitigate any methane leakage associated with abandonment, it is necessary to understand whether the leakage has a natural or anthropogenic origin. During exploration and production, emphasis is almost exclusively on the reservoir, while post abandonment monitoring programs generally focus on the water column. Thus, only little attention has been given to the shallow subsurface, how it has been influenced by hydrocarbon production, and to which degree the geology facilitates or inhibits fluid migration. In order to monitor and evaluate future leakage of hydrocarbons to the marine environment, it is crucial to understand the natural seepage through the seabed both locally at platforms and regionally. For CCS, understanding the shallow subsurface is equally important as monitoring cannot be confined to the water column alone. Mapping potential migration paths is thus necessary in order to mitigate any leakage.

 The aim of the SEEP project is to develop a Danish North Sea baseline for methane seepage in the shallow subsurface, near oil and gas platforms and in areas without any hydrocarbon production. Applying such a baseline will facilitate identification of anthropogenic seepage and thus help recognize the potential local environmental impact associated with abandonment.

Using newly collected geophysical data, we have categorized various types of shallow methane seeps, and placed them in a geological context. By combining the shallow seismic data with deep industry seismic data, we have identified potential sources and paths for thermogenic methane migrating from reservoir depths.  The geophysical data is then integrated with results from sediment core analysis. These include Facies analysis of cores, dating of sediments, benthic faunal variations between core sites, geochemistry of bivalves and foraminifera, studies of the chemical and isotopic composition of the dissolved gas in the pore water, as well as the community-composition of gas-degrading bacteria.

This integrated approach will provide a solid model for gas distribution, frequency and origin, as well as impact on the environment in the shallow subsurface.    

Here we present the different tools that are the foundation of such a baseline, including results and examples from geophysical mapping (both multibeam echosounder and seismic data), biostratigraphy, geochemistry and geomicrobiology.  

How to cite: Prins, L. T., Lauridsen, B. W., Clausen, O. R., Røy, H., Kjeldsen, K. U., and Knutz, P.: SEEP - Building a SEabed Environmental baseline for Platform abandonment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12876, https://doi.org/10.5194/egusphere-egu23-12876, 2023.

A.257
|
EGU23-15412
|
ECS
Gabrielle Kleber, Andrew Hodson, Leonard Magerl, Erik Schytt Mannerfelt, Harold Bradbury, Yizhu Zhu, Mark Trimmer, and Alexandra Turchyn

Permafrost and glaciers in the high Arctic form an impermeable ‘cryospheric cap’ that traps a large reservoir of sub-surface methane and hinders it from reaching the atmosphere. The vulnerability of the cryosphere to climate warming is making releases of this methane possible, but uncertainty in the magnitude and timing makes future predictions of Arctic greenhouse gas emissions difficult. In Svalbard, where air temperatures are rising more than twice as fast as the average for the Arctic, glaciers are retreating and leaving behind an exposed forefield that enables rapid methane escape.

We undertook a field survey of unprecedented spatial coverage across central Svalbard to identify methane emission hotspots in glacial forefields, a previously unknown emission source. Here we document how methane-rich groundwater springs that have formed in recently revealed forefields of 78 land-terminating glaciers are bringing deep-seated methane gas to the surface. Waters collected from these springs are supersaturated with methane up to 600,000-times greater than atmospheric equilibration, with strong isotopic evidence of a thermogenic source. We estimate annual emissions of methane degassing from such groundwater springs to be up to 2.31 kt across the Svalbard archipelago. Our findings reveal that climate-driven glacial retreat is facilitating widespread release of methane, a positive feedback loop that is likely to be prevalent across other regions of the rapidly warming Arctic.

How to cite: Kleber, G., Hodson, A., Magerl, L., Schytt Mannerfelt, E., Bradbury, H., Zhu, Y., Trimmer, M., and Turchyn, A.: Glacial retreat driving enhanced methane emissions in the high Arctic, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15412, https://doi.org/10.5194/egusphere-egu23-15412, 2023.

Posters virtual: Mon, 24 Apr, 10:45–12:30 | vHall BG

Chairpersons: Helge Niemann, Tina Treude
vBG.15
|
EGU23-10760
|
ECS
Xudong Wang, Jörn Peckmann, Germain Bayon, Zice Jia, Shanggui Gong, Jie Li, and Dong Feng

Many chemosynthesis-based communities prospering in deep-sea environments rely on the metabolic activity of sulfur-oxidizing bacteria. This is also the case for vestimentiferan siboglinid tubeworms, whose demand for nutrition is entirely satisfied by their endosymbiotic bacteria harbored in the trophosome. Such chemosymbiosis leads to a significantly lower nitrogen isotope composition of the trophosome than in other types of soft tissue. However, the specific process of nitrogen utilization by siboglinids remains unclear. As a key element in the relevant enzymes (nitrogenase, nitrate reductase), molybdenum (Mo) is indispensable in the biogeochemical cycling of nitrogen. The Mo isotope composition (δ98Mo) of siboglinids is thus a potential proxy to decode the mode of nitrogen utilization. In this study, we found that δ98Mo along the chitinous tube of the vestimentiferan siboglinid Paraescarpia echinospica from the Haima seep of the South China Sea yields values as negative as -4.59‰ (-1.13 ± 1.75‰, n = 19) – the lowest δ98Mo signature ever reported for any kind of natural material. It is suggested that this extremely negative Mo isotope composition is caused by preferential utilization of isotopically light Mo by the tubeworm symbionts during nitrate reduction. Such Mo isotope signature could provide a means to identify siboglinid tubeworms in the rock record, a group of annelids that has previously escaped unambiguous identification due to the lack of mineralized skeleton.

How to cite: Wang, X., Peckmann, J., Bayon, G., Jia, Z., Gong, S., Li, J., and Feng, D.: The molybdenum isotope signature of microbial nitrogen utilization, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10760, https://doi.org/10.5194/egusphere-egu23-10760, 2023.

vBG.16
|
EGU23-14499
|
ECS
Marta Riva, Valentina Alice Bracchi, Claudio Argentino, Alessandra Savini, Luca Fallati, Giuliana Panieri, and Daniela Basso

The West‐Svalbard continental slope represents one of the northernmost gas hydrate provinces in the world. It is located on the western Svalbard margin in the eastern Fram Strait at ~79°N, north of the Knipovich Ridge and Molloy transform fault, situated on a hot and relatively young oceanic crust. The project Advancing Knowledge of Methane in the Arctic (AKMA), funded by the Norwegian Research Council, explores Arctic methane sources, processes, ecosystems and geological history in part of this province. During a dedicated research expedition (CAGE21-1-AKMA) a spectacular pinnacle-like structure has been identified at 914 m of water depth in the area of the north Kniponich Ridge, during the exploration by remotely operated vehicle. 

The structure is located at the base of a small-scale escarpment, typified by the presence of carbonate slabs. It is more than 1 m in height, has a diameter of at least 50 cm, and appears isolated on a flat seafloor with muddy-dominated heterogeneous sediment. From the video, the structure was apparently composed of dead bivalves cemented in a substrate of finer particles. Many bivalves were still articulated and with their valves closed, although no sign of living bivalves could be detected. Living regular echinoids, one large crinoid, several sponges and a few soft corals colonized the surface at the time of observation. Small samples have been collected from the surface of this fossil bivalve pinnacle. These samples supported the video-based interpretation of a fossil structure composed of cemented and carbonate-encrusted dead valves, among which the most abundant and the largest specimens belong to Thyasira cf capitanea, both juvenile and adult. The valves, either articulated or disarticulated, are often cemented by a white thick (3 mm) authigenic carbonate crust that binds together the mollusk shells, but also encrusts some of them internally. This means that these crusts continued to grow also after the death of the mollusks, when some of the valves were open. Additional interesting benthic fauna recognized in the cement includes small gastropods and foraminifers. The genus Thyasira has already been described as a typically chemosymbiotic group of species and as such, it has been reported from active cold seeps in the Arctic, as well as in other geographic areas and in the geologic record. We interpret the pinnacle as fossil evidence of a site of past methane emission, possibly exhumed by recent erosional or gravity-driven resedimentation processes.

How to cite: Riva, M., Bracchi, V. A., Argentino, C., Savini, A., Fallati, L., Panieri, G., and Basso, D.: A pinnacle-like structure dominated by the chemosymbiotic bivalve Thyasira from the Arctic Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14499, https://doi.org/10.5194/egusphere-egu23-14499, 2023.