BG1.8

Today, gas hydrates are predicted to be stable only in the deepest parts of the Barents Sea, however under past glaciations, low pressure, high temperature subglacial conditions would have been an ideal setting for their formation. Multiple studies have documented the storage of methane beneath the Late Weichselian Barents Sea Ice Sheet, and its subsequent release following deglaciation. Furthermore, it has been hypothesised that localised subglacial gas hydrate formation increases frictional resistance at base of the ice and thus may regulate the flow of overlying ice (Winsborrow et al. 2016). This hypothesis has however, never been tested against sedimentological records of paleo-fluid flow and sediment properties.  

Here we present preliminary results on sediment and pore fluid geochemistry from nine gravity cores collected from Ingøydjupet in the SW Barents Sea. These were collected around a hill-hole pair, a glacial landform indicative of variations in subglacial frictional resistance. One of several suggested formation processes is gas hydrate stiffening of subglacial sediments.  

At present, there is a clear difference in methane fluxes between the areas inside the seafloor hole (high fluxes) and the adjacent hill (low fluxes), matching the distribution of a localized subsurface shallow gas accumulation visible in seismic data. Sediment geochemistry revealed a past episode of enhanced upward methane fluxes only recorded in sediments from the hole, resulted in the shoaling of the sulfate-methane transition and precipitation of methane-derived authigenic carbonates (MDAC) with δ¹³C= -35 ‰. Although the oxygen isotopic composition (δ¹⁸O) of MDACs collected from a sediment core in the hole did not show direct evidence for past gas hydrate destabilization, the reconstructed history of methane fluxes as well as the present-day fluxes and subsurface gas distribution support the hypothesis of a differential distribution of subglacial paleo-gas hydrates across the hill-hole pair, possibly controlled by stratigraphic and structural preconditioning.

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

Winsborrow, M., Andreassen, K., Hubbard, A., Plaza-Faverola, A., Gudlaugsson, E. and Patton, H., 2016. Regulation of ice stream flow through subglacial formation of gas hydrates. Nat. Geosci., 9(5), 370-374.

How to cite: Argentino, C., Waghorn, K. A., Winsborrow, M., Bünz, S., and Panieri, G.: Paleo-gas hydrate distribution associated with hill-hole pair formation in the SW Barents Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3932, https://doi.org/10.5194/egusphere-egu22-3932, 2022.

Cold seeps are important biodiversity hotspots, which support unique communities in the deep sea. The occurrence of living or fossil chemosymbiotic molluscs and low oxygen tolerant benthic foraminifera in the sediments, in association with other seepage related features (e.g. aragonite, authigenic carbonate crusts) are important indications of active or past fluid seepage.
The EU Eurofleets2 SEMSEEP Cruise on the R/V Aegaeo along the Israeli coasts (2016) provided sea floor data and sediments for this study.
Three deep-sea cores from representative environments from the Palmahim Disturbance, (coral-transition area, pockmark area and Gal-C channel area) spanning the last 5000 BP were investigated for pteropods, benthic foraminifera and molluscs and cross-analysed with ROV videos and surface samples.
The coral-transition core (AG16-20-BC1b) shows a sharp increase in low-oxygen benthic foraminifera (representing 100% of the faunal assemblage), no agglutinants, pyritized tubes and euhedral gypsum crystals in its bottom part. This evidence together with the low values of δ¹³C of C. oolina give indication that a short-lived advective fluid flux occurred approximatively at 3500 BP. Only few small individuals of the chemosymbiotic bivalve Isorropodon perplexum Sturany, 1896 have been observed above this interval, showing that the chemosynthetic environment was not conducive for the development of a full chemosymbiotic benthic community.
Similarly, evidences of methane emission have been observed in the pockmark core (AG16-23-BC2). Pteropods molds, composed by aragonite needles and High-Mg calcite crystals are present at the base of the core. Aragonite precipitates during advective emissions, when the Sulfate Methane Transition Zone (SMTZ) is located cm to dm below the seafloor, therefore the presence of pteropod molds recrystallized in aragonite is an important evidence that an advective emission occurred. However, these molds co-occur with authigenic carbonate crusts, shrimp claws, low-oxygen tolerant benthic foraminifera and a mature association of chemosymbiotic molluscs (including vesicomyids, lucinids, mytilids and thyasirids). Typically, these organisms are sustained by a moderate, diffusive, pervasive and persistent fluid flow. Therefore, we suggest that this environment was dynamic and supporting advective and diffusive emissions that were able to sustain recruitment and development of mature chemosymbiotic faunal assemblage. 

This research was funded by the Swiss National Science Foundation (SNSF) project Ref. 200021_175587, samples were collected during the EUROFLEETS2 SEMSEEP cruise that was funded by the European Union FP7 Programme under grant agreement n° 312762.

How to cite: Beccari, V., Basso, D., Panieri, G., Almogi-Labin, A., Makovsky, Y., Hajdas, I., and Spezzaferri, S.: Significance of micro-and macrofauna from seeps along the Israeli coast (Palmahim Disturbance)., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13090, https://doi.org/10.5194/egusphere-egu22-13090, 2022.

Marine landslides, which are observed worldwide along continental slopes, constitute the most important processes reworking sedimentary deposits and a major geohazard for marine and coastal domains. They can generate potentially the destruction of marine infrastructures through the formation of turbidity currents and/or hazardous tsunamis. In the Romanian sector of the Black Sea, high amounts of methane are detected in the sediments and at the seafloor through the identification of gas seeps in the water column. They occur on the upper slope, mostly outside the large gas hydrates system occurring in the sediment below -660 m water depth, where methane is trapped in ice cages that act as a buffer zone hampering methane to reach the water column. New geophysical and geotechnical dataset acquired along the Romanian margin reveals that the active seepage zone is associated with numerous slope failures, which incised the continental upper slope. Is there a possible relationship between gas hydrate system and recent slope instabilities? Could intense free gas emissions and/or gas hydrates dissociation have triggered such geohazards? To answer these questions, we present (1) an high resolution mapping and, more important, dating of landslides since the last glacial maximum (35 ka), (2) results of numerical modelling showing the evolution of gas hydrates stability zone inside the sediments since the last 35 ka taking into account the environmental variations that occurred during this time lapse (geothermic gradient, temperature, salinity and sea level).

The models highlight the major effect of environmental changes and particularly the glacial/interglacial cycles and salinity variations on the extent of the gas hydrates. The confrontation of those models with slide extensions, depths and ages allows to better discuss the respective influences of gas hydrates dissociation, stratigraphic overpressure and seepage on slope instability since the last glacial maximum.

How to cite: Fabre, M., Loncke, L., Riboulot, V., Sultan, N., and Ker, S.: Slope instabilities and gas hydrates dissociation in the western Black Sea since the last glacial maximum, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4505, https://doi.org/10.5194/egusphere-egu22-4505, 2022.

The YAM seep area is an active gas seep region among the widespread seep sites offshore south-western Taiwan. The seep area covers a seafloor region of 49,000 m2 at the northern crest of Four-Way Closure Ridge in 1,347 m water depth. During several research cruises (ORI-1163, SO266), shipborne and AUV-related hydro-acoustic investigations revealed that the area of seepage is well documented by high backscatter and a changing micro-bathymetry between rough and flat in the otherwise very flat seabed in the area. 
During expedition SO266, RV Sonne, gas emission sites were observed at the center and eastern flank of the area using ship-borne multi-beam data in the water column. Seawater methane concentrations above the seafloor were collected from the middle west to east of the central transect, revealing concentrations ranging 5.5-18.2 nmol/L with general higher methane concentrations at the rim of the area. Authigenic carbonate samples were collected during gravity coring and MeBo drilling. MeBo drilling was stopped below 5 mbsf because of continuous release of free gas out of the borehole. Carbon isotope measurements of the carbonates showed δ13C values between -38.25 to -52.17 ‰, indicating a mixing of biogenic and thermogenic gas in the methane-derived carbonates. Seismic investigations of the Four-Way Closure Ridge show a well-defined fault below the ridge crest which extends from below the gas hydrate stability zone to the seafloor and acts as a pathway for the methane ascent to the seafloor. 
Seafloor observation using a TV-sled showed a seafloor paved by carbonate rocks consisting of different featured crusts, slabs, and irregular build-ups. Thin-layered crusts were mainly observed at the paved area's rim, while thick-layered slabs and free-standing build-ups were the main features at the elevated region. Chemosynthetic communities, like mussels and clams, mainly were observed within certain carbonate fractures. At the same time, other animals, bacterial mats, and tube worms are presented generally along the whole survey path among the rough seep area. Hydro-acoustic data correlated with visual observation results indicate wide variation through the region of the seep area. Based on our interpretation, the YAM seep area developed over a longer time at the crest of the accretionary ridge, seepage, uplifting of the ridge, tectonic fracturing, and seafloor erosion highly influenced the seafloor manifestation.

How to cite: Tseng, Y., Römer, M., Pape, T., Chen, T.-T., Lin, S., Berndt, C., and Bohrmann, G.: The YAM seep area – an active carbonate-paved gas seep field at the accretionary margin SW offshore Taiwan, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1532, https://doi.org/10.5194/egusphere-egu22-1532, 2022.

Gas flares from natural sources of hydrocarbon gases escaping through the seafloor to the water column is a phenomenon observed in many parts of the World’s Oceans. The occurrence of these acoustically and visually observed seepages have been recorded by using various sensors onboard different platforms. But the use of multibeam echosounder systems, with the capability of recording the whole water column acoustic backscattering, in recent years have given the opportunity to cover large areas in a short time span along with bathymetric mapping in a cost-effective way. Even though the data sizes are multiple orders of magnitude larger, the use of dedicated software’s and high-performance processing systems have given the opportunity to find acoustic anomalies resulting from the streaming of gas bubbles in water column.

The MAREANO programme which is aimed at mapping habitats through various methods has surveyed large areas of the Norwegian Arctic using multibeam systems. This has resulted in the acquisition of water column acoustic data covering a large area of Arctic Norway over in the last decade. These data have been interpreted and analysed together with other geological and ancillary data from other sensors such as photo/video observations, backscatter data, etc. leading us to relate these anomalies to various structural and geological formations. The database also gave us an opportunity to compare the differences between some of the multibeam systems in capturing these acoustic anomalies. More than approximately 5000 flares of varying magnitude and sizes were detected based on MAREANO water column data, in an area of about 139000 km². We present the results from these comprehensive surveys and discuss various possibilities that such a database can provide for present and future understanding in the development of Arctic. 

How to cite: Chand, S., Thorsnes, T., Bellec, V., and Bjarnadóttir, L. R.: Gas Flares of the Norwegian Arctic - Sources and distribution: A comprehensive mapping using MAREANO multibeam data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9510, https://doi.org/10.5194/egusphere-egu22-9510, 2022.

Cold seeps are locations on the seafloor where reduced compounds from subsurface hydrocarbon reserves enrich sediment fluids or emanate freely as gas from the seabed. Associated with these spots, numerous underwater landscapes and various chemosynthetic communities were uncovered during the last decades of seafloor exploration. 

Arctic cold seeps offshore Svalbard were explored using Ægir6000, a work-class ROV (Remotely Operated Vehicle) equipped with three HD video cameras that filmed the ocean floor at different angles. The ROV, moving at a constant speed of 1 knot, followed predefined routes to guarantee optimal lateral overlap between adjacent transects. From the videos of the nadiral camera, a photogram every two seconds was automatically extracted. Then, the images were processed in Agisoft Metashape® following a well-established photogrammetry workflow. As final outputs, we obtained 3D mesh, orthomosaics and DTMs at ultra-high-resolution (mm) allowing us to obtain detailed morphometric maps.

These data allowed us to reconstruct accurate georeferenced 3D models representing a variety of small-scale (sub-cm) seabed features and provide essential information for a better understanding of the spatial pattern associated with submarine biogeochemical and physical processes at the seafloor. Moreover, the realized models present the locations where push corers were collected. This correspondence will allow us to integrate fine-scale habitat mapping and pore fluid datasets to quantify the areal methane fluxes.

This work was supported by the Research Council of Norway, for AKMA - Advancing Knowledge on Methane in the Arctic, project number 287869.

Keywords: Submarine geomorphology, ROV, Underwater 3D Photogrammetry, optical 3D models, Cold Seeps, Arctic Ocean

How to cite: Fallati, L., Savini, A., Argentino, C., Bünz, S., and Panieri, G.: Using ROV video photogrammetry to reconstruct seafloor landforms of Arctic cold seep environments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13267, https://doi.org/10.5194/egusphere-egu22-13267, 2022.

Methane recently became the focus of attention as the target to slow global warming in the near future. Hence, measures to reduce anthropogenic methane emission are globally discussed and researchers test methods to actively reduce atmospheric methane levels. Complicating advancement in this field, there are still high uncertainties associated with methane sources and sinks. One example is the methane emission from abandoned oil and gas wells. The USA, with about 4,000,000 abandoned wells, is the only country worldwide to include emissions from these wells in their yearly greenhouse gas emissions inventory. Studies estimated that these emissions account for about 1–13% of the U.S. energy sector (Williams et al. 2021). In addition to the USA, only a few countries like Canada, the United Kingdom and the Netherlands collected data on methane emissions from abandoned gas wells. Currently Germany has about 20,000 abandoned wells (formerly productive and dry wells) of different ages, which were generally filled and buried and since 1950s officially have to be plugged, cut, and buried at the end of their lifecycle. It is unclear, whether they are emitting methane or not.

Here, we present our project to fill this knowledge gap for Germany and focus on onshore-abandoned oil & gas wells and their potential to emit methane. Therefore, we will measure soil-atmosphere methane fluxes at several abandoned wells exemplary in four regions of Lower Saxony (Federal State in Northern Germany). In case of methane emission to the atmosphere, we will determine the origin of the methane in soil gas at 1 m depth by isotopic analyses. In addition to these analyses in the direct vicinity of the boreholes, we will investigate the surrounding natural methane situation in groundwater and soil with the help of molecular-geomicrobiological methods and determinations of methane oxidation rates.

Williams, J. P., Regehr, A., & Kang, M. (2021). Methane Emissions from Abandoned Oil and Gas Wells in Canada and the United States. Environ Sci Technol, 55(1), 563-570. https://doi.org/10.1021/acs.est.0c04265

How to cite: Jordan, S. F. A., Schlömer, S., Krüger, M., and Blumenberg, M.: Methane emissions from abandoned wells? A German case study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7382, https://doi.org/10.5194/egusphere-egu22-7382, 2022.

In recent years, Nd isotopes have seen increasingly common use in studies of hydrocarbon seeps. Given the distinct Nd isotope signature of mafic igneous rocks, particular emphasis in these investigations has been on reconstructing former interactions between the seeping fluids and volcanic materials. The results of our case studies on ancient seeps underlain by mafic volcanic bodies, including Cretaceous seep carbonates of the Outer Carpathians (Czech Republic) and Basque-Cantabrian Basin (Spain), as well as Eocene seeps of the Cascadia convergent margin (Washington) consistently document their significant enrichment in volcanogenic Nd, reflected in their increased Nd isotope ratios (ε_Nd values). The extent of this ¹⁴³Nd-enrichment varies depending on the geological context of given seeps, most notably the ε_Nd signatures and thickness of the volcanics and overlying sedimentary piles, and the Nd isotope signal of background local pore waters. The highest ε_Nd values are observed in seep carbonates very shallowly underlain by thick mafic volcanics: the Cretaceous seep of the Carpathians and Eocene seeps of Cascadia. For the former, the ε_Nd values are up to 7.5 units higher than the signature of coeval non-seep pore water, whereas for the latter the ε_Nd values are as high as +1.9, close to the highest value ever recorded for seawater. More moderate ¹⁴³Nd-enrichment typifies the Cretaceous seeps of Spain, for which the volcanic intrusions were emplaced at considerable depths below the seafloor. In such cases, the Nd isotope signature of the fluid-volcanic interactions was partially obscured by subsequent interactions between the fluids and the overlying sediments. Rather than focusing solely on exploring the new geochemical tool, the primary aim of our studies was to address broader questions regarding the tectonic architecture and geological history of the sedimentary basins that host given seep deposits. For the Eocene seeps of Washington, the Nd isotope data served to document interactions between the methane-rich fluids and the volcanic terrane of Siletzia, which underlies the Cascadia forearc; these results placed important stratigraphic and structural constraints on the activation and earliest history of convergence in Cascadia, following Siletzia accretion. For the studied Cretaceous seeps, all hosted by early, sediment-covered rifts, the studies demonstrated that Nd isotopes offer a valuable new tool of deconvolving methane fluxes from different organic matter alteration pathways for the very complex, sedimentary-magmatic systems of incipient rifts. At the same time, these studies emphasized important limitations of Sr isotopes, the system most commonly used to document interactions between the seeping fluids and igneous rocks. Because of the much higher Sr/Nd ratios observed in pore waters than in igneous rocks, the potential of Sr isotopes to record fluid-volcanic interactions is considerably lower than that of Nd isotopes. Thus, broader use of Nd isotopes can assist in identifying potential volcanogenic fluid endmembers for the numerous sedimented rifts for which evidence for magmatic involvement in the fluid expulsion remains equivocal.

This work was supported by the National Science Centre, Poland, grant No. 2016/23/D/ST10/00444

How to cite: Jakubowicz, M., Agirrezabala, L., Kiel, S., Goedert, J., Dopieralska, J., and Belka, Z.: The potential of Nd isotopes in disentangling fluid sources at hydrocarbon seeps: a recent perspective, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6546, https://doi.org/10.5194/egusphere-egu22-6546, 2022.