Sub-seabed fluid-flow systems and gas hydrates of the SW Barents Sea and North Sea margins

The papers of this thesis are not available in Munin: 1. Vadakkepuliyambatta, S., S. Bünz, J. Mienert, and S. Chand: 'Distribution of subsurface fluid-flow systems in the SW Barents Sea', Marine and Petroleum Geology (2013), vol. 43:208-221. Available at http://dx.doi.org/10.1016/j.marpetgeo.2013.02.007 2. Vadakkepuliyambatta, S., S. Chand, and S. Bünz.: 'Can ocean warming destabilize gas hydrate accumulations in the SW Barents Sea?' (manuscript). 3. Vadakkepuliyambatta, S., M. Hornbach, S. Bünz, and B. Phrampus: 'Controls on gas hydrate system evolution in a region of active fluid flow in the SW Barents Sea' (manuscript). 4. Vadakkepuliyambatta, S., S. Planke, and S. Bünz: 'Fluid leakage pathways and shallow gas accumulation in the Peon field, northern North Sea, from high resolution P-Cable 3D seismic data' (manuscript).Denne avhandlingen omfatter studier av utbredelse og evolusjon av fluid migrasjonssystemer og gasshydrater i SV Barentshavet og nordlige Nordsjøen. Avhandlingen er basert på tolkning av seismiske og brønndata og numerisk modelling av hydratstabiliteten. Store fluid lekkasjestrukturer forekommer i SV Barentshavet og viser kobling mot de store tektoniske forkastningssytemene. Det er trolig istidene (heving og erosjon) som har forårsaket fluid lekkasje fra dype reservoarer. Flere steder fører fluid lekkasje til ansamlinger av grunn gass og gasshydrat som er forholdsvis stabil i en gass sammensettning som inneholder termogene hydrokarboner. Men gasshydrat stabiliteten varierer mye i hele SV Barentshavet. Mulige gasshydrat forekomstene over store deler av det SV Barentshavet kan være følsomt for endringer i havtemperatur grunnet den globale oppvarmingen. Dette kan da føre til oppsmelting av store mengder av gasshydrat i de øvre 100 m under havbunnen over relativt korte periode (100-200 år)

Ice-sheet-driven methane storage and release in the Arctic

It is established that late-twentieth and twenty-first century ocean warming has forced dissociation of gas hydrates with concomitant seabed methane release. However, recent dating of methane expulsion sites suggests that gas release has been ongoing over many millennia. Here we synthesize observations of B1,900 fluid escape features—pockmarks and active gas flares—across a previously glaciated Arctic margin with ice-sheet thermomechanical and gas hydrate stability zone modelling. Our results indicate that even under conservative estimates of ice thickness with temperate subglacial conditions, a 500-m thick gas hydrate stability zone—which could serve as a methane sink—existed beneath the ice sheet. Moreover, we reveal that in water depths 150–520 m methane release also per- sisted through a 20-km-wide window between the subsea and subglacial gas hydrate stability zone. This window expanded in response to post-glacial climate warming and deglaciation thereby opening the Arctic shelf for methane release

Impact of tides and sea-level on deep-sea Arctic methane emissions

Sub-sea Arctic methane and gas hydrate reservoirs are expected to be severely impacted by ocean temperature increase and sea-level rise. Our understanding of the gas emission phenomenon in the Arctic is however partial, especially in deep environments where the access is difficult and hydro-acoustic surveys are sporadic. Here, we report on the first continuous pore-pressure and temperature measurements over 4 days in shallow sediments along the west-Svalbard margin. Our data from sites where gas emissions have not been previously identified in hydro-acoustic profiles show that tides significantly affect the intensity and periodicity of gas emissions. These observations imply that the quantification of present-day gas emissions in the Arctic may be underestimated. High tides, however, seem to influence gas emissions by reducing their height and volume. Hence, the question remains as to whether sea-level rise may partially counterbalance the potential threat of submarine gas emissions caused by a warmer Arctic Ocean

Hand-written letters and photo albums linking geoscientists with school classes

Do we miss something about &laquo;traditional&rdquo; media such as handwritten letters and photography before the digital age? Some of the authors remember this age fondly, and we wanted to see if this fondness could be translated into a science dialogue project with school classes. We designed and carried out a communication process with 4 classes at different schools across Europe. During this process, each class would interact with a single scientist primarily via hand-written questions &amp; letters, and a Polaroid photo album. The scientists would make this unique, one-of-a-kind album whilst on board a research expedition in the Barents Sea. We asked the question whether this process might show any benefits to the school students involved. To answer this, we asked the students to write up their thoughts on communicating with a scientist in this way. We analysed the texts and found that most students thought the letters and polaroid albums were a &ldquo;beautiful experience&rdquo;. Others commented on how important it is to actually put pen to paper and write, since they use (almost) only digital media these days. Most importantly, the students learnt different elements of the science connected to the research expedition, but also about the scientific process in general. And, equally important, some of the students were surprised and thankful that the scientists took the time to communicate with them in such a personal way. These results could possibly have been achieved using other media, however the hand-written letters and Polaroids worked very well. They also seemed to conjure up some of the personal memories that we have about communication not so long ago. Maybe there is something to be said for slowing things down with our science communication projects and making them more personal and unique. This is something that snail-mail and making photo albums forces us to do.</p

Quaternary and Neogene Reservoirs of the Norwegian Continental Shelf and the Faroe-Shetland Basin

Glaciogenic reservoirs host important hydrocarbon resources across the globe. Examples such as the Peon and Aviat discoveries in the North Sea show that Quaternary and Neogene reservoirs can be prospective in the region. In this study, we interpret 2D and 3D reflection seismic data combined with borehole information to document unconventional play models from the shallow subsurface of the Norwegian Continental Shelf and the Faroe-Shetland Basin. These plays include (i) glacial sands in ice-marginal outwash fans, sealed by stiff subglacial tills (the Peon discovery), (ii) meltwater turbidites, (iii) contouritic fine-grained glacimarine sands sealed by gas hydrates, (iv) remobilized oozes above large evacuation craters which are sealed by megaslides and glacial muds, and (v) Neogene sand injectites. The hydrocarbon reservoirs are characterized by negative-polarity reflections with anomalously high amplitudes in the reflection seismic data as well as density and velocity decreases in the borehole data. Extensive new 3D reflection seismic data are crucial to correctly interpret glacial processes and distinguish shallow reservoirs from shallow seals. These data document a variety of play models with the potential for gas in large quantities and enable the identification of optimal drilling targets at stratigraphic levels which have so far been overlooked

Deglacial bottom water warming intensified Arctic methane seepage in the NW Barents Sea

Funder: M.M.E. is funded by the Research Council of Norway and the Co-funding of Regional, National, and International Programmes (COFUND) – Marie Skłodowska-Curie Actions under the EU Seventh Framework Programme (FP7), project number 274429, and the Tromsø Forskningsstiftelse, project number A31720.AbstractChanges in the Arctic climate-ocean system can rapidly impact carbon cycling and cryosphere. Methane release from the seafloor has been widespread in the Barents Sea since the last deglaciation, being closely linked to changes in pressure and bottom water temperature. Here, we present a post-glacial bottom water temperature record (18,000–0 years before present) based on Mg/Ca in benthic foraminifera from an area where methane seepage occurs and proximal to a former Arctic ice-sheet grounding zone. Coupled ice sheet-hydrate stability modeling shows that phases of extreme bottom water temperature up to 6 °C and associated with inflow of Atlantic Water repeatedly destabilized subsurface hydrates facilitating the release of greenhouse gasses from the seabed. Furthermore, these warming events played an important role in triggering multiple collapses of the marine-based Svalbard-Barents Sea Ice Sheet. Future warming of the Atlantic Water could lead to widespread disappearance of gas hydrates and melting of the remaining marine-terminating glaciers.</jats:p

The Postglacial response of Arctic Ocean gas hydrates to climatic amelioration

Seafloor methane release due to the thermal dissociation of gas hydrates is pervasive across the continental margins of the Arctic Ocean. Furthermore, there is increasing awareness that shallow hydrate-related methane seeps have appeared due to enhanced warming of Arctic Ocean bottom water during the last century. Although it has been argued that a gas hydrate gun could trigger abrupt climate change, the processes and rates of subsurface/atmospheric natural gas exchange remain uncertain. Here we investigate the dynamics between gas hydrate stability and environmental changes from the height of the last glaciation through to the present day. Using geophysical observations from offshore Svalbard to constrain a coupled ice sheet/gas hydrate model, we identify distinct phases of subglacial methane sequestration and subsequent release on ice sheet retreat that led to the formation of a suite of seafloor domes. Reconstructing the evolution of this dome field, we find that incursions of warm Atlantic bottom water forced rapid gas hydrate dissociation and enhanced methane emissions during the penultimate Heinrich event, the B?lling and Aller?d interstadials, and the Holocene optimum. Our results highlight the complex interplay between the cryosphere, geosphere, and atmosphere over the last 30,000 y that led to extensive changes in subseafloor carbon storage that forced distinct episodes of methane release due to natural climate variability well before recent anthropogenic warmingauthorsversionPeer reviewe

Spatial Changes in Gas Transport and Sediment Stiffness Influenced by Regional Stress: Observations From Piezometer Data Along Vestnesa Ridge, Eastern Fram Strait

Gas transport through sediments to the seabed and seepage occurs via advection through pores, faults, and fractures, and as solubility driven gas diffusion. The pore pressure gradient is a key factor in these processes. Yet, in situ measurements for quantitative studies of fluid dynamics and sediment deformation in deep ocean environments remain scarce. In this study, we integrate piezometer data, geotechnical tests, and sediment core analyses to study the pressure regime that controls gas transport along the Vestnesa Ridge in the eastern Fram Strait. The data show a progressive westward decrease in induced pore pressure (i.e., from c. 180 to c. 50 kPa) upon piezometer penetration and undrained shear strength of the sediments, interpreted as a decrease in sediment stiffness. In addition, the data suggest that the upper c. 6 m of sediments may be mechanically damaged due to variations in gas diffusion rates and exsolution. Background pore pressures are mostly at hydrostatic conditions, but localized excess pore pressures (i.e., up to 10 kPa) exist and point toward external controls. When analyzed in conjunction with observations from geophysical data and sediment core analyses, the pore pressure data suggest a spatial change from an advection dominated to a diffusion dominated fluid flow system, influenced by the behavior of sedimentary faults. Understanding gas transport mechanisms and their effect on fine-grained sediments of deep ocean settings is critical for constraining gas hydrate inventories, seepage phenomena and sub-seabed sediment deformations and instabilities

Lack of detectable chemosynthesis at a sponge dominated subarctic methane seep

We used high-resolution imagery within a Geographic Information System (GIS), free gas and porewater analyses and animal bulk stable isotope measurements to characterize the biotic and abiotic aspects of the newly discovered Vestbrona Carbonate Field (VCF) seep site on the Norwegian shelf (63°28′N, 6° 31′E, ∿270 m water depth). Free gas was mainly composed of microbial methane. Sediment porewater sulfide concentrations were in the millimolar range and thus high enough to sustain seep chemosymbiotrophic animals. Nonetheless, the VCF lacked chemosymbiotrophic animals despite an abundance of methane-derived carbonate crusts which are formed by the same anaerobic processes that sustain chemosymbiotrophic animals at seeps. Furthermore, none of the sampled taxa, across various trophic guilds exhibited a detectable contribution of chemosynthetically fixed carbon to their diets based on bulk stable isotope values, suggesting a predominantly photosynthetic source of carbon to the VCF seep food web. We link the absence of chemosymbiotrophic animals to highly localized methane flow pathways, which may act as a “shunt-bypass” of the anaerobic oxidation of methane (AOM) and by extension sulfide generation, thus leading to sediment sulfide concentrations that are highly heterogeneous over very short lateral distances, inhibiting the successful colonization of chemosymbiotrophic animals at the VCF seep. Instead, the seep hosted diverse biological communities, consisting of heterotrophic benthic fauna, including long lived taxa, such as soft corals (e.g., Paragorgia arborea) and stony corals (i.e., Desmophyllum pertusum, formerly known as Lophelia pertusa). Compared to the surrounding non-seep seafloor, we measured heightened megafaunal density at the seep, which we attribute to increased habitat heterogeneity and the presence of a variety of hard substrates (i.e., methane-derived authigenic carbonates, dropstones and coral rubble), particularly since the most abundant taxa all belonged to the phylum Porifera. Compared to the surrounding non-seep seafloor, marine litter was denser within the VCF seep, which we link to the more variable local topography due to authigenic carbonates, which can rip off parts of bottom trawling nets thereby making the seep act as catchment area for marine litter

Diverse gas composition controls the Moby-Dick gas hydrate system in the Gulf of Mexico

In marine basins, gas hydrate systems are usually identified by a bottom simulating reflection (BSR) that parallels the seafloor and coincides with the base of the gas hydrate stability zone (GHSZ). We present a newly discovered gas hydrate system, Moby-Dick, located in the Ship Basin in the northern Gulf of Mexico. In the seismic data, we observe a channel-levee complex with a consistent phase reversal and a BSR extending over an area of ∼14.2 km2 , strongly suggesting the presence of gas hydrate. In contrast to classical observations, the Moby-Dick BSR abnormally shoals 150 m toward the seafloor from west to east, which contradicts the northward-shallowing seafloor. We argue that the likely cause of the shoaling BSR is a gradually changing gas mix across the basin, with gas containing heavier hydrocarbons in the west transitioning to methane gas in the east. Our study indicates that such abnormal BSRs can be controlled by gradual changes in the gas mix influencing the shape of the GHSZ over kilometers on a basin scale