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Extensive magmatism during the NE Atlantic break-up generated about 6-10 million cubic kilometers of magma in the Paleogene. Magmatic degassing and explosive discharge of thermogenic gases by contact metamorphism is implicated in triggering the Paleocene-Eocene Thermal Maximum (PETM; 56 Ma). In 2022, the CAGE22-5 research cruise collected high-resolution seismic, sub-bottom profiler, and multibeam data across the central and northern Vøring Plateau to tie IODP Expedition 396 Sites U1571 to U1574. One high-resolution P-Cable 3D seismic cube (10.3 by 1.8 km) was collected on the Skoll High, covering both sites U1571 and U1572. The data were fast-track processed with a bin size of 6.1 x 6.1 m. Four Cenozoic sedimentary horizons and the Top Basalt horizon were subsequently interpreted, followed by horizon attribute analyses. The Top Basalt horizon reveals unprecedented details of the nature of the volcanic paleosurface. A faulted and eroded lava horizon characterizes the southeastern part of the cube at 150-250 m below sea floor (bsf) around the U1571 site location. In contrast, in the northwestern part of the cube, the Top Basalt attribute maps reveal a pitted surface and lobate structures with linear ridges with characteristic inter-ridge spacings of a few tens of meters. The pitted basalt surface is mapped in an area of about 5.5 km2 at depths of 250-350 m bsf and drilled by Site U1572. About 270 individual pits have been mapped with radius ranging from about six meters, within the horizontal resolution of the data, to about 50 meters with a mean radius of c. 16 meters. The holes are interpreted as rootless cones, which comprise volcanic craters resulting from the explosive reaction between lava flows which flow over water-saturated sediments. The dimensions of the rootless cones in the Skoll 3D survey are comparable with the field analogue of rootless cones mapped in the Myvatn lake area of NE Iceland. Unlike at Myvatn and at other field analogues, the Skoll3D data allow unique 3D imaging of the rootless cones revealing internal structures and geometries including the base of the cone structures. The cone depths vary from meters to a few tens of meters. The lobate structures are interpreted as separate subaerial lava flows with compressional ridges. These lava flows are similar to outcrop analogue lava flows and compressional ridges imaged by elevation model ISN2016 with a 2x2 m resolution near the Myvatn lake. In conclusion, the interpreted patterns of the basaltic pitted surface within the Skoll3D suggest a wet terrestrial paleoenvironment during effusive volcanism. Unlike in the eastern faulted area of the survey, the extremely well imaged pitted basalt surface of the Skoll3D area did not experience significant erosion, suggesting rapid subsidence and drowning of the paleo-land surface in a low-energy coastal environment. This study demonstrates the unprecedented detail that is possible when high resolution 3D seismic data acquisition is applied to buried volcanic landscapes and opens the potential for unique associated insights in the Northeast Atlantic and further afield.

The Fram Strait opening is associated with a complex stress regime that results from the oblique relation between two ultra-slow spreading mid-ocean ridges, the Molloy ridge (MR) and the Knipovich Ridge (KR), offset by the Molloy Transform Fault (MTF). Gas-charged thick sedimentary deposits developed over both oceanic and continental crust. Sedimentary faulting reveals recent stress transfer into the sub-surface. However, the mechanisms by which stress accommodates across the west Svalbard margin and its effect on fluid flow and seepage dynamics remain poorly understood. An analysis of earthquake occurrence and focal mechanisms can shed light on the present state of tectonic forces in the area, their origin and potential influence on nearby faults. Conventional studies using land instrumentation provide incomplete seismological records even for such comparatively land proximal settings, due to still large distances to the nearest permanent observatories and a poor azimuthal coverage. We deployed 10 ocean bottom seismometers (OBS) for 11 months between 2020-2021 about 10 km north of the northern termination of KR to investigate patterns of stress transfer off the ridge and the influence on the sedimentary system. OBSs are spaced by about 10 km around an area characterized by fault-related seepage and sedimentary slumps visible on the bathymetry. Using partially automated routines we built a catalogue of local earthquakes and computed their epicenters and magnitudes. Earthquake locations roughly follow the plate boundaries and better focus seismicity along their bathymetric imprint versus the land observations. Along the MTF, we observe that the earthquakes are concentrated southwards on the North American plate and seismicity across the west-Svalbard margin is limited. A large number of earthquakes extend beyond the MTF and KR corner and concentrate at a bathymetric depression, adjacent to the recently revised continental-oceanic transition boundary. Focal mechanisms from past observations show a gradual change from strike-slip movement along the MTF to extensional faulting at the corner. The distribution of earthquakes correlates with highly faulted sedimentary overburden interpreted in high resolution seismic data, and with major structures in gravity and magnetic maps. This suggests an efficient stress release at the plate boundary and little to no transfer northward from the KR termination onto the Eurasian plate. We detected only a few events recorded along the Vestnesa contourite drift and on the continental shelf. These earthquakes may indicate reactivation of crustal faults under the weight of thick sedimentary deposits or other processes such as glacial isostacy. The inferred stress distribution in the region has implications for understanding fault-related gas transport and methane seepage at Arctic margins.

Intrusion of magma into sedimentary basins leads to devolatilization of the host rock in contact metamorphic aureoles. Hydrothermal vent complexes are formed by fracturing the overburden sediments if sufficient overpressure is developed in the aureoles, releasing hot fluids and gases into the hydrosphere and atmosphere. We have mapped the structure and distribution of hydrothermal vent complexes using extensive 3D seismic reflection surveys (c. 40,000 km2) in the Møre and Vøring basins offshore mid-Norway by a combination of seismic horizon and attribute mapping. The seismic horizons have been tied to exploration wells to constrain the timing of their formation. A shallowly buried vent complex, the Modgunn Vent, was subsequently imaged by high-resolution P-Cable 3D seismic data collected using the R/V Helmer Hansen. The upper part of this vent complex was recently drilled by five holes during IODP Expedition 396. In total, more than a thousand hydrothermal vent complexes have been identified in the two basins. A typical vent complex has a diameter of between a few hundred meters and five kilometers and extends from the tip of a sill intrusion to the paleosurface. The upper part of the vent complexes are commonly eye-shaped, where the lower surface represents the base of a crater and the upper dome-shaped surface represents the top of the crater infill. Overlying reflections are sub-parallel to the upper vent surface, locally associated with discontinuous high-amplitude reflections and minor faulting. The chimney-shaped lower part of the vent complexes are characterized by disrupted reflections, sometimes including bulbous-shaped transparent bodies with high-amplitude reflections at the top and base. Surrounding reflections are often dipping towards the center of the chimneys. The structure of the vent complexes suggest they were dominantly formed by erupting fluids and sediments during the Paleocene-Eocene Thermal Maximum (PETM), about 56 million years ago. The craters were subsequently rapidly infilled by sediments, and later inverted forming domes above the craters. High-amplitude discontinuous reflections above some vent complexes are interpreted as evidence of long-term fluid flow, sometimes lasting until recent times.

Continental breakup in the NE Atlantic was associated with mafic magmatism recorded by basalt flows, volcanogenic sediments, magmatic underplates, and intrusive sheet complexes in the nearby sedimentary basins and continental crust. The voluminous magmatism is concomitant with the global hot-house climate in the Paleogene, and the injection of magma into organic-rich sedimentary basins is a proposed mechanism for triggering short-term global warming during the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma). IODP Expedition 396 drilled 21 holes along three transects on the mid-Norwegian continental margin to sample 1) Paleogene sediments along the Vøring Transform Margin and in hydrothermal vent complexes, and 2) basalt deposits from the Vøring Marginal High into the oceanic Lofoten Basin. A total of 2 km of core were recovered, including more than 350 m of basalt, 15 m of granite, and 900 m of late Paleocene to early Eocene sediments. Wireline logging data were recorded in eight holes. All the sites were located on industry-standard 2D and 3D seismic data. In addition, high-resolution seismic data were acquired in 2020 and 2022 over all the 21 Expedition 396 boreholes and 5 legacy ODP/DSDP sites using R/V Helmer Hansen. The seismic surveys included three P-Cable 3D cubes covering the 14 boreholes on the Modgunn (5), Mimir (5), and Skoll (4) transects. A comprehensive core-log-seismic integration program is ongoing for each site, based on an integration of high-resolution biostratigraphy, core and log based petrophysical data, and seismic modelling. The expedition recovered the first sub-basalt cores on the mid-Norwegian continental margin, recovering 15 m of granite. It furthermore collected the first samples from an Outer High at Site U1574, recovering both pillow basalts and hyaloclastites. These cores documented a shallow marine depositional environment of the emergent Eldhø volcano located near the foot of the Vøring Plateau. Finally, we drilled five holes through the upper part of a hydrothermal vent complex with a very expanded Paleocene-Eocene Thermal Maximum (PETM) interval dominated by biogenic ooze and volcanic ash deposits, documenting the temporal correlation of intrusive breakup magmatism in the Vøring Basin and a major hypothermal event. Collectively, the Expedition 396 sample archive offers unprecedented insight into tectonomagmatic processes in the NE Atlantic, including links to both rapid and long-term climate variation in the Paleogene.

The overall goal of cruise CAGE 17-3 is to collect seismic, multibeam and water column data in an area along the northern flank of the Bear island trough and on the W-Svalbard margin. The objectives associated with the overall goal are to better understand the occurrence of gas hydrates and fluid leakage, the source of the gas in these systems, and seafloor expressions of fluid seepage. In order to address these objectives we plan to carry out: 2D and 3D P-Cable seismic and ocean-bottom seismic acquisition over a complex leakage and source-to-sink plumbing system in the Leirdjupet Fault complex in the SW Barents Sea;Recovery of one of CAGE’s ocean floor observatory from the crater area at the northern flank of the Bear Island Trough;Repeat of P-Cable 3D seismic acquisition on the eastern segment of the Vestnesa Ridge for time-lapse seismic studies of fluid flow and gas hydrates dynamics;2D multi-channel seismic for reconnaissance and stratigraphic correlation in the Molloy Ridge area;Multibeam mapping to fill in gaps and improve resolution of existing data using the upgraded EM302 system on R/V Helmer Hanssen The cruise may be known as: CAGE17_3

CAGE 15-4 was a short 3-day cruise with only one major objective which is to repeat a P-cable survey covering the active seeps on the Vestnesa Ridge in order to develop time-lapse 4D seismic studies of gas leakage. Three OBS stations were also deployed during the 3D seismic survey. The cruise may be known as: CAGE15_4

The overall goal of cruise CAGE 16-6 is to collect seismic, multibeam and water column data on the W-Svalbard margin and in the outer parts of the Storfjord Trough. The objectives associated with the overall goal are to better understand the occurrence of gas hydrates and fluid leakage, the source of the gas in these systems, and seafloor expressions of fluid seepage. In order to address these objectives we plan to carry out: P-Cable 3D seismic acquisition over gas hydrate and fluid flow areas with potential abiogenic gas sources;P-Cable 3D seismic acquisition over gas-hydrated pingo structures in the Storfjorden Trough;Recovery of 5 OBS stations that were deployed at the Lunde pockmark in October last year for long-term monitoring of microseismicity;Acquisition of additional OBS seismic data to fill gaps in existing data on the eastern segment of the Vestnesa Ridge;2D multi-channel seismic for reconnaissance and stratigraphic correlation;Multibeam mapping to fill in gaps and improve resolution of existing data using the upgraded EM302 system on R/V Helmer Hanssen;Water column mapping of gas flaring on the Vestnesa Ridge using the upgraded EM302 system on R/V Helmer Hanssen The cruise may be known as: CAGE16_6

Cruise CAGE 14-1 is one of several cruises in 2014 that will be carried out to collect cross-disciplinary data for addressing the objectives of the Norwegian Centre for Arctic Gas Hydrate, Environment and Climate, CAGE. The Norwegian Research Council funds CAGE for a period of 10 years to addressee main scientific questions about gas hydrate environments in Arctic regions. Cruise CAGE 14-1 was also hosting this year’s AMGG research school cruise with 14 PhD students participating, most of them from CAGE. The cruise focused on acquisition of seismic, oceanographic, atmospheric, geochemical and sediment and benthic fauna sampling data from three target areas: the shallow shelf and shelf edge methane seepage sites at Prins Karls Forelanda sediment drift on the western flank of the Knipovich Ridgethe Vestnesa Ridge Research objectives were planned long the goals of CAGE and its individual work packages, and the teaching would be organized along these goals. Scientific problems that are to be addressed in these two key target areas include the periodicity of seepage (i.e. time scales of active, inactive and reactivated systems), quantification of methane concentrations in surface sediments, water column and above the sea surface, gas and hydrates, the nature of gas sources, and the benthic communities occurring at the seep sites. Specific objectives of cruise CAGE 14-1 were as follows: Hydro-acoustic mapping of methane bubble plumes above shallow and deep-water vent fields.Seismic surveying and multibeam mapping across the contours of the Svyatogor Rise.Sediment sampling using gravity, box and multi-corer for sediment and pore-water geochemical analysesSampling of benthic fauna at several seep sites and reference stations in all three target areas.Acquisition of oceanographic data for measuring oceanographic conditions and methane concentrations in the water column, particularly at the shallow shelf sites. The cruise may be known as: CAGE14_1

Tha main goal of CAGE20-5 cruise was to continue with an Ocean Bottom Seismometer (OBS) experiment to address objectives of the SEAMSTRESS project (Tectonic stress effects on Arctic methane seepage[SB1] ). Along the eastern segment of Vestnesa Ridge, the cruise recovered 7 OBS after a one-year deployment and subsequentially redeployed 10 OBS in the larger array on the southern flank of Vestnesa Ridge. The overall aim is to detect local and regional earthquakes that relate to and/or impact local faulting within Vestnesa Ridge and may control fluid flow. Another objective of CAGE20-5 was to collect seismic lines to complete the data base and refine site selections for the IODP985 proposal. We also collected a 3D seismic volume on the elongated depression north of Knipovich Ridge where massive gas hydrates and pore fluid pressure pulses from piezometer data have been observed The cruise may be known as: CAGE20_5

Cruise CAGE19-1 with UiT’s research vessel R/V Helmer Hanssen was the 1st of several planned cruises within the SEAMSTRESS project (Tectonic stress effects on Arctic methane seepage), an early-career starting grant by the Tromsø Research Foundation (TFS) and the Research Council of Norway (RCN-Frinatek) awarded to Andreia Plaza-Faverola. The overall goal of cruise CAGE19-1 was to conduct a large-scale ocean-bottom seismic experiments on the eastern segment of the Vestnesa Ridge. A dense network of 23 ocean-bottom seismometers (OBS) have been deployed over the Lunde pockmark. Seismic acquisition over these OBS will provide data for a tomographic inversion of travel times in order to obtain the velocity structure of the chimney beneath the Lunde pockmark. In a second experiment, 7 OBS have been deployed long-term until summer 2020 and monitor microseismic events related to tectonics and seepage from pockmarks along the eastern segment of Vestnesa Ridge The cruise may be known as: CAGE19_1

Cruise CAGE 13-4 focused on the acquisition of acoustic data in two of the main target areas within CAGE, the deep (>1000 m water depth) seeping Vestnesa Ridge area and the shallower (< 400 m) highly active Prins-Karls Foreland (PKF) sites. Scientific problems that are to be addressed in these two key target areas include the periodicity of seepage (i.e. time scales of active, inactive and reactivated systems), quantification of gas and hydrates, the nature of gas sources, trigger mechanisms for seepage and the architecture of the fluid flow structures underlying seafloor seepage sites. Specific objectives of the cruise were as follows: To repeat a P-cable survey covering the active seeps on the Vestnesa Ridge in order to develop time-lapse 4D seismic studies of gas leakage. To acquire a P-cable survey at the active seeping area offshore PKF for planning and re-evaluation of drilling sites during the upcoming MeBo cruise in 2014. To repeat hydro-acoustic surveying for monitoring of flares in PKF active area. The cruise may be known as: CAGE13_7_ECO2_II

The ECO2 project sets out to assess the risks associated with the storage of CO2 below the seabed. Carbon Capture and Storage (CCS) is regarded as a key technology for the reduction of CO2 emissions from power plants and other sources at the European and international level. The proposed project will assess the likelihood of leakage and impact of leakage on marine ecosystems. As part of the ECO2 project, cruise CAGE13-3 intend to acquire a 3D seismic volume (which will act as monitor along with the 3D cube collected in 2011) from the Snøhvit carbon capture area. In addition, we will repeat some of the lines twice and in reversed direction in order to study the repeatability of the P-Cable acquisition for monitoring purposes. The primary objective of the hydrological and biogeochemical sampling in the Snøhvit area is to study any possible effects of a Carbon Capture and Storage (CCS) site on the water column and on the bottom sediment. The cruise may be known as: HH_2013_ECO2_I

Cruise CAGE18-5 was the first research expedition under the helm of UiT The Arctic University of Norway in Tromsø with the new ice-going research vessel R/V Kronprins Håkon. This new vessel provides new opportunities to collect cross-disciplinary data for addressing the objectives of the Norwegian Centre of Excellence for Arctic Gas Hydrate, Environment and Climate, CAGE. CAGE investigates Arctic gas hydrate and methane seepage systems in order to better understand the effects they may have on our oceans, ecosystems and global climate. The new research vessel and its facilities allows CAGE access to a state-of-art remotely-operated vehicle, the ROV Ægir 6000, opening a new domain for experimental work and acquisition of sample material from the seafloor. The overall goal of cruise CAGE18-5 therefore was to utilize the ROV in order to provide guided video imagery and to study active gas seepage systems at the gas-hydrate pingo (GHP) site located in the outer Storfjord Trough, and at Storbanken in the northeastern part of the Barents Sea. The cruise may be known as: CAGE18_5

The CAGE20-4 expedition on RV Helmer Hanssen targeted areas on the outer margins of the Møre and Vøring basins, with emphasis on IODP drilling proposal P944 sites. High-resolution 2D and 3D P-Cable seismic studies focused in particular on the structure of igneous deposits and how tectonic and igneous events have affected the Paleogene and global climate through gas production from organic-rich sediments and release into the atmosphere via hydrothermal venting. This expedition was part of a collaboration between CAGE and the Center for Earth Evolution and Dynamics (CEED) at the University of Oslo. The cruise may be known as: CAGE20_4

AKMA-OceanSenses Research Expedition (11-23 May 2022) aboard the research vessel Kronprins Håkon to the Barents Sea and Arctic Ocean is focused on both science and education. This expedition has received endorsement from the UN Ocean Decade. The cruise may be known as: CAGE22_2

The overall goal of this expedition was the acquisition of geophysical data on the Vøring Plateau and the southernmost part of the Lofoten Basin to improve the understanding of the subsurface structures and geological processes at four sites (U1571-U1574) where scientific drilling was recently successfully completed as a part of the the International Ocean Discovery Program (IODP) Expedition 396 The cruise may be known as: CAGE22_5

The HACON cruise is a major component of the FRINATEK HACON project, which aims at investigating the role of the Gakkel Ridge and Arctic Ocean in biological connectivity amongst ocean basins and global biogeography of chemosynthetic ecosystems. The HACON cruise was designed to conduct a multidisciplinary survey of the Aurora seamount centred around the identified black smokers in 2014. The scientific goal was to obtain visual and physical samples of the different habitats on the seamount to better understand hydrothermal vent communities on the Gakkel ridge and abyssal communities of an Arctic seamount, to assess the sphere of influence of the Aurora vent field across the seamount and adjacent seafloor regions The cruise may be known as: Hacon19

The HACON cruise is a major component of the FRINATEK HACON project, which aims at investigating the role of the Gakkel Ridge and Arctic Ocean in biological connectivity amongst ocean basins and global biogeography of chemosynthetic ecosystems. The HACON cruise was designed to conduct a multidisciplinary survey of the Aurora seamount centred around the identified black smokers in 2014. The scientific goal was to obtain visual and physical samples of the different habitats on the seamount to better understand hydrothermal vent communities on the Gakkel ridge and abyssal communities of an Arctic seamount, to assess the sphere of influence of the Aurora vent field across the seamount and adjacent seafloor regions. The cruise may be known as: Hacon21

The research expedition cruise CAGE21-1 is a major component of the project AKMA and it is organized under the helm of the Norwegian Centre of Excellence for Arctic Gas Hydrate, Environment and Climate, CAGE at UiT The Arctic University of Norway in Tromsø. The cruise may be known as: CAGE21_1

The continent–ocean transition along the passive margin off western Svalbard is, in places, only a few kilometers away from the formerly glaciated continental shelf. Strong bottom currents in the Fram Strait have led to the deposition of several kilometers thick sedimentary ridges that extend from continental to oceanic crust all the way onto the flanks of the oblique-spreading Molloy and Knipovich mid-ocean ridges. The sedimentary ridges represent large contourite drifts and are characterized by faults that extend to the present-day seafloor. Generally, it is argued that gravitational forcing or flexure due to fast sedimentation and/or erosion is the main force leading to deformation of Quaternary sediments, and that horizontal forcing is negligible. However, high resolution 2D and 3D seismic data along the western Svalbard margin reveal that sedimentary faults commonly propagate from the termination of rift-related faults in the oceanic crust, and are not always favorably oriented to accommodate gravitational collapse. We suggest that coupling between the slow-spreading oceanic crust and the sedimentary cover results in a transfer of stress and strain that influences near-surface sedimentary deformation. Deep crustal fluids are also transferred into the Quaternary succession utilizing faults as migration pathways. Such faults sustain shallow gas accumulations, wide-spread gas hydrate formation and cold seeps. Simultaneously, glacial isostatic rebound leads to additional lithospheric deformation and also exerts a control on fault kinematics and gas seepage. We discuss seismic examples of deep marine seepage systems and the potential implications of coupling between lithospheric deformation and sedimentation on the spatiotemporal evolution of seafloor seepage at Arctic margins.

The Svyatogor Ridge is the sedimented inside corner high of the Knipovich Ridge – Molloy Transform Fault intersection and despite its mid-ocean ridge system setting, it is host to a well-defined gas hydrate system. There is an extinct transform fault immediately south of the Svyatogor Ridge, which is partially sedimented. Our data suggests that there are distinct locations throughout the area which have elevated heat flow, usually coinciding with evidence of past or present fluid flow.In particular, we show a site along the flank of the extinct transform fault which hosts a seep-like community associated with enhanced anaerobic methane oxidation in the shallow subsurface. While transform faults and fracture zones are important components of mid-ocean ridge systems, their role in fluid exchange between the lithosphere and hydrosphere remains unclear. High resolution seismic data from the inside corner high area at the Knipovich Ridge – Molloy Transform Fault intersection hints towards the important role the Molloy Transform Fault is playing in the fluid flow system in the area. However, integrated seismic, geochemical and heat flow datasets along the extinct transform fault clearly links the fault to elevated heat flow and fluid seepage, and to sites of gas hydrate accumulation. We posit that this site is a type of hybrid seep-vent style of fluid flow system.

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

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.

Large reservoirs of methane present in Arctic marine sediments are susceptible to rapid warming, promoting increasing methane emissions. Gas bubbles in the water column can be detected, and flow rates can be quantified using hydroacoustic survey methods, making it possible to monitor spatiotemporal variability. We present methane (CH4) bubble flow rates derived from hydroacoustic data sets acquired during 11 research expeditions to the western Svalbard continental margin (2008-2014). Three seepage areas emit in total 725-1,125 t CH4/year, and bubble fluxes are up to 2 kg.m(-2).year (-1). Bubble fluxes vary between different surveys, but no clear trend can be identified. Flux variability analyses suggest that two areas are geologically interconnected, displaying alternating flow changes. Spatial migration of bubble seepage was observed to follow seasonal changes in the theoretical landward limit of the hydrate stability zone, suggesting that formation/dissociation of shallow hydrates, modulated by bottom water temperatures, influences seafloor bubble release. Plain Language Summary It has been speculated that the release of methane (a potent greenhouse gas) from the seafloor in some Arctic Ocean regions is triggered by warming seawater. Emissions of gas bubbles from the seafloor can be detected by ship-mounted sonars. In 2008, a methane seepage area west of Svalbard was hydroacoustically detected for the first time. This seepage was hypothesized to be caused by dissociation of hydrates (ice-like crystals consisting of methane and water) due to ocean warming. We present an analysis of sonar data from 11 surveys conducted between 2008 and 2014. This study is the first comparison of methane seepage-related hydroacoustic data over such a long period. The hydroacoustic mapping and quantification method allowed us to assess the locations and intensity of gas bubble release, and how these parameters change over time, providing necessary data for numerical flux and climate models. No trend of increasing gas flow was identified. However, we observed seasonal variations potentially controlled by seasonal formation and dissociation of shallow hydrates. The hydrate formation/dissociation process is likely controlled by changes of bottom water temperatures. Alternating gas emissions between two neighboring areas indicate the existence of fluid pathway networks within the sediments.

Cruise MSM57-1/-2 was funded by the German Research Foundation ( DFG ), the Research Center/Excellence Cluster “The Ocean in the Earth System” at MARUM–Center for Marine and Environmental Sciences, University of Bremen and funds from CAGE. This work is partly supported by the Research Council of Norway (RCN) through Petromaks2-NORCRUST (project number 255150 ) and the Centre of Excellence funding scheme for CAGE (project number 223259 ). APF’s contribution was part of the SEAMSTRESS project supported by starting grants from the Tromsø Research Foundation and the Research Council of Norway (grant nr. 2878659 ). Seafloor seepage sites along the Vestnesa Ridge off west-Svalbard have been, for decades, a natural laboratory for the studies of fluid flow and gas hydrate dynamics along passive continental margins. The lack of ground truth evidence for fluid composition and gas hydrate abundance deep in the sediment sequence however prohibits us from further assessing the current model of pockmark evolution from the region. A MARUM-MeBo 70 drilling cruise in 2016 aims to advance our understanding of the system by recovering sediments tens of meters below seafloor from two active pockmarks along Vestnesa Ridge. We report pore fluid composition data focusing on dissolved chloride, stable isotopes of water (δ18O and δD), and the isotopic composition of dissolved boron (δ11B). From one of the seepage sites, we detect a saline formation water with two layers where gas hydrates were recovered. This saline formation pore fluid is characterized by elevated chloride concentrations (up to 616 mM), high B/Cl ratios (9 × 10−4 mol/mol), high δ18O and δD isotopic signatures (+0.6‰ and +3.8‰, respectively) and low δ11B signatures (+35.0‰), which collectively hint to a high temperature modification at great depths. Based on the dissolved chloride concentration profiles, we estimated up to 47% of pore space occupied by gas hydrate in the sediments shallower than 11.5 mbsf. The observation of bubble fabric in the recovered gas hydrates suggests formation during past periods of intensive gaseous methane seepage. The presence of these gas hydrates without associated positive anomalies in dissolved chloride concentrations however suggests that the decomposition of gas hydrate is as fast as its formation. Such a state of gas hydrates can be attributed to a relatively low methane supply transported by the saline formation water at present. Our findings based on pore fluid composition corroborate previous inferences along Vestnesa Ridge that fluids sustaining seepage have migrated from great depths and that the variable gaseous and aqueous phases through the gas hydrate stability zone control the distributions of authigenic carbonates and gas hydrates. Publisher PDF

Anaerobic oxidation of methane (AOM) in marine sediments strongly limits the amount of gas reaching the water column and the atmosphere but its efficiency in counteracting future methane emissions at continental margins remains unclear. Small shifts in methane fluxes due to gas hydrate and submarine permafrost destabilization or enhanced methanogenesis in warming Arctic continental shelves may cause the redox boundary in which AOM occurs, known as Sulfate-Methane Transition Zone (SMTZ), to move closer to seafloor, with potential gas release to bottom waters. Here, we investigated the geochemical composition of pore water (SO42− and DIC concentration, δ13CDIC) and gas (CH4, δ13CCH4) in eight gravity cores collected from Ingøydjupet trough, South-Western Barents Sea. Our results show a remarkable variability in SMTZ depth, ranging from 3.5 m to 29.2 m, and that all methane is efficiently consumed by AOM within the sediment. From linear fitting of the sulfate concentration profiles, we calculated diffusive sulfate fluxes ranging from 1.5 nmol cm−2 d−1 to 12.0 nmol cm−2 d−1. AOM rates obtained for two cores using mixing models are 6.5 nmol cm−2 d−1 and 6.7 nmol cm−2 d−1 and account for only 64 % and 56 % of total sulfate reduction at the SMTZ (SRRtot), respectively. The remaining 36 % and 44 % SRRtot correspond to organoclastic sulfate reduction with rates of 3.7 nmol cm−2 d−1 and 5.3 nmol cm−2 d−1. The shallowest SMTZs (tot rates are associated with a shallow subsurface accumulation of gas visible in seismic data, highlighting how small changes in sulfate reduction rates linked to subsurface methane gradients resulted in vertical shifts in SMTZ position of > 20 m. This study provides new insights into the dynamic and biogeochemistry of the SMTZ in marine sediments of continental margins and may help predict the response of the microbial methane filter to future increase in methane fluxes due to ocean warming.

Methane emissions from Arctic continental margins may increase due to global warming. Present-day ocean fluxes seem to provide a minor contribution to the atmosphere methane pool, but large uncertainties still remain on the magnitude of future emissions from methane seeps and gas hydrate-bearing sediments. The Barents Sea is a natural laboratory to study the evolution of methane seeps in relation to climate change, as it recorded several phases of ice-sheet advance and retreat during the Pleistocene. Glaciations and its concurrent denudation of the Barents Sea influenced the subsurface, causing reservoir expansion and fracturing, thereby driving hydrocarbon (mostly gas) migration which resulted in a sustained regional fluid flow system. New data from this area can shed light on future response of other high-latitude continental shelves worldwide. Here, we present reconstructed methane emission dynamics at Leirdjupet Fault Complex (LFC), SW Barents Sea, since last deglaciation (occurred after ~19 cal Ka BP). The geochemical composition of sediment cores indicate prolonged methane emissions, which started after 14.5 cal Ka BP. Geochemical proxies for anaerobic oxidation of methane in the sediment (barium, calcium and sulfur enrichments, isotopic composition of foraminifera) indicate an overall decrease in seepage intensity over the Holocene toward present-day conditions. Methane-derived authigenic carbonates with aragonite mineralogy and heavy δ18O signature recorded an episode of gas hydrate destabilization in this region. Paleo-hydrate stability models suggest that this event was triggered by the influx of warm Atlantic water and isostatic uplift linked to the retreat of the Barents Sea Ice Sheet. Present-day distribution of methane seeps at LFC is strongly linked to underlying faults. Methane hydrates are stable in the southern part of the investigated seepage area and might respond to a future increase in bottom water temperatures.

Methane seepage off the coast of Svalbard is demonstrated by active gas flares in the water column today and through precipitation of methane-derived authigenic carbonates (MDAC) over the past 160 000 years. Though submarine discharge of thermogenic methane is well documented, the geological history of past leakage is still debated, largely due to unconstrained free gas sources and seepage trigger mechanisms. We have assembled a high-resolution 3D petroleum systems model to evaluate the impact and charge of potential thermogenic hydrocarbons on the Vestnesa Ridge, NW Svalbard. We show that gaseous hydrocarbons, originating from Miocene age terrigenous organic matter, accumulates largely in ~2 million-year-old (Ma) sedimentary sequences underneath the pockmark system on the ridge's crest. These traps are constantly charged until present day. The supply of free gas to the gas hydrate stability zone (GHSZ) initiated the hydrate formation at ~3.0 Ma. We also show that gas leakage to the seafloor is governed by fault corruption of the hydrocarbon traps and not by excess pore fluid pressure. The onset of episodic seafloor seepage on Vestnesa Ridge can be associated with the first shelf edge glaciation of the Svalbard-Barents Sea ice sheet (SBIS), ~1.5 Ma ago. The results of the modelled petroleum system are consistent with the notion that repeated forebulge uplift and subsidence, due to cyclic SBIS build-up and decay, can be advocated as a mechanism that repeatedly caused extensional fracturing of the eastern Vestnesa Ridge segment. This fault damage affected modelled gas accumulations and led to the formation of hydrocarbon migration pathways to the seafloor. Repeated tapping into hydrocarbon reservoirs due to (a) fracture formation promoted by glacial isostatic adjustments (GIA) and (b) fracture re-activation could explain recent observations of multiple seepage events on Vestnesa Ridge during episodes of intense cooling over the past 160.000 years.

Accepted manuscript version, licensed CC BY-NC-ND 4.0. The Barents Sea has experienced intense erosion throughout the Cenozoic due to uplift and repeated episodes of glaciation. This, in turn, has driven large pressure and temperature fluctuations in the sediment substrate along with rearrangement of thermogenic oil and gas accumulations. As a result, some hydrocarbon fields have relatively shallow depths, and natural gas release is widespread. This study focuses on the process of hydrocarbon leakage from the Realgrunnen reservoir - encompassing the Hanssen and Wisting discoveries - to the shallow subsurface caused by repeated cycles of glacial erosion in the central Barents Sea throughout the Quaternary. We apply 2D basin and petroleum system modeling to two seismic sections using data from two wells and run ten different scenarios that test model sensitivity to key parameters. We find that the primary factors governing gas leakage are the erosion amount, its distribution between glacial and preglacial stages, and the timing of the glaciations. Our results demonstrate that intense oil and gas leakage from the Realgrunnen reservoir occurs primarily through widespread faults activated during the first deglaciation episode. Further considerable gas leakage occurs by the seal breach after a critical overburden thickness is eroded and pressure on the reservoir decreases to ca. 9 MPa. Modeling reveals that the first deglaciation episode causes up to ca. 20% loss of oil and gas from the reservoir, whereas leakage after the seal breach yields a further ca. 15% decrease in gas. Our results are supported by seismic analyses that demonstrate hydrocarbon leakage in the study area

Gas hydrate dynamics and the fluid flow systems from two active pockmarks along Vestnesa Ridge (offshore west Svalbard) were investigated through the pore fluid geochemistry obtained during the 2016 MARUM-MeBo 70 drilling cruise. Based on the pore water chloride concentration profiles from Lunde and Lomvi pockmarks, we estimated up to 47% pore space occupied by gas hydrate in the sediments shallower than 11.5 mbsf. These gas hydrates were formed during periods of gaseous methane seepage, but are now in a state of dynamic equilibrium sustained by a relatively low methane supply at present. We detect a saline formation pore fluid around nine meters below seafloor from one of the seepage sites in Lunde pockmark. This formation pore fluid has elevated dissolved chloride concentrations and B/Cl ratios, higher δO and δD isotopic signatures of water and lower δB signatures, which collectively hint to a high temperature modification of this fluid at great depths. By integrating our findings with the previous work from Vestnesa Ridge, we show that the variable fluid phases (gaseous vs. aqueous fluid) and migration pathways are controlled by the sediment properties, such as buried carbonate crusts, and the state of fluid reservoirs.

The Svyatogor Ridge and surroundings, located on the sediment-covered western flank of the Northern Knipovich Ridge, host extensive gas hydrate and related fluid flow systems. The fluid flow system here manifests in the upper sedimentary sequence as gas hydrates and free gas, indicated by bottom simulating reflections (BSRs) and amplitude anomalies. Using 2D seismic lines and bathymetric data, we map tectonic features such as faults, crustal highs, and indicators of fluid flow processes. Results indicate a strong correlation between crustal faults, crustal highs and fluid accumulations in the overlying sediments, as well as an increase in geothermal gradient over crustal faults. We conclude here that gas generated during the serpentinization of exhumed mantle rocks drive the extensive occurrence of gas hydrate and fluid flow systems in the region and transform faults act as an additional major pathway for fluid circulation.

The transition from gas hydrate to gas-bearing sediments at the base of the hydrate stability zone (BHSZ) is commonly identified on seismic data as a bottom-simulating reflection (BSR). At this boundary, phase transitions driven by thermal effects, pressure alternations, and gas and water flux exist. Sedimentation, erosion, subsidence, uplift, variations in bottom water temperature or heat flow cause changes in marine gas hydrate stability leading to expansion or reduction of gas hydrate accumulations and associated free gas accumulations. Pressure build-up in gas accumulations trapped beneath the hydrate layer may eventually lead to fracturing of hydrate-bearing sediments that enables advection of fluids into the hydrate layer and potentially seabed seepage. Depletion of gas along zones of weakness creates hydraulic gradients in the free gas zone where gas is forced to migrate along the lower hydrate boundary towards these weakness zones. However, due to lack of “real time” data, the magnitude and timescales of processes at the gas hydrate – gas contact zone remains largely unknown. Here we show results of high resolution 4D seismic surveys at a prominent Arctic gas hydrate accumulation – Vestnesa ridge - capturing dynamics of the gas hydrate and free gas accumulations over 5 years. The 4D time-lapse seismic method has the potential to identify and monitor fluid movement in the subsurface over certain time intervals. Although conventional 4D seismic has a long history of application to monitor fluid changes in petroleum reservoirs, high-resolution seismic data (20-300 Hz) as a tool for 4D fluid monitoring of natural geological processes has been recently identified.Our 4D data set consists of four high-resolution P-Cable 3D seismic surveys acquired between 2012 and 2017 in the eastern segment of Vestnesa Ridge. Vestnesa Ridge has an active fluid and gas hydrate system in a contourite drift setting near the Knipovich Ridge offshore W-Svalbard. Large gas flares, ~800 m tall rise from seafloor pockmarks (~700 m diameter) at the ridge axis. Beneath the pockmarks, gas chimneys pierce the hydrate stability zone, and a strong, widespread BSR occurs at depth of 160-180 m bsf. 4D seismic datasets reveal changes in subsurface fluid distribution near the BHSZ on Vestnesa Ridge. In particular, the amplitude along the BSR reflection appears to change across surveys. Disappearance of bright reflections suggest that gas-rich fluids have escaped the free gas zone and possibly migrated into the hydrate stability zone and contributed to a gas hydrate accumulation, or alternatively, migrated laterally along the BSR. Appearance of bright reflection might also indicate lateral migration, ongoing microbial or thermogenic gas supply or be related to other phase transitions. We document that faults, chimneys and lithology constrain these anomalies imposing yet another control on vertical and lateral gas migration and accumulation. These time-lapse differences suggest that (1) we can resolve fluid changes on a year-year timescale in this natural seepage system using high-resolution P-Cable data and (2) that fluids accumulate at, migrate to and migrate from the BHSZ over the same time scale.

Methane seepage on continental margins derived from shallow gas reserves and gas hydrate stores is significant globally, with initial observations most commonly derived from pockmark expressions on the seafloor. The processes driving fluid flow (liquids and gases) through upper (200m) marine sediments is not well understood. Pockmarks signify present or past seepage events, and are prominent across Vestnesa Ridge. Not all pockmarks are active (venting), suggesting that the mechanism behind fluid flow varies across the ridge. The main structures observed in the seismic are gas chimneys, faults and fractures. Here we study the characteristics of the observed features through attribute analysis at three significant horizons (age estimates