Stefan Bünz

Geological controls on the Storegga gas-hydrate system of the mid-Norwegian continental margin

The geologic setting of the formerly glaciated mid-Norwegian continental margin exerts specific controls on the formation of a bottom-simulating reflector (BSR) and the inferred distribution of gas hydrates. On the continental slope the lithology of glacigenic debris flow deposits and pre-glacial basin deposits of the Kai Formation prevent gas-hydrate formation, because of reduced pore size, reduced water content and fine-grained sediment composition. Towards the continental shelf, the shoaling and pinch-out of the gas-hydrate stability zone terminates the area of gas-hydrate growth. These geological controls confine the occurrence of gas hydrates and ensuing formation of a BSR to a small zone along the northern flank of the Storegga submarine slide and the slide area itself. A BSR inside the slide area indicates a dynamically adjusting gas-hydrate system to post-slide pressure–temperature equilibrium conditions. These observations, together with widespread evidence for fluid flow and deep-seated hydrocarbon reservoirs, suggest that the formation of BSR and gas hydrates on the mid-Norwegian continental margin is dominated by an advection of gas from the strata distinctly beneath the gas-hydrate stability zone. Fluids migrate upward within the Naust Formation and are deflected laterally by hydrated sediments and less permeable layers. Gases continually accumulate at the top of the slope, where overpressure eventually results in the formation of blow-out pipes and consequent pockmark development on the seabed.

Polygonal fault systems on the mid-Norwegian margin: A long term source for fluid flow

Abstract 2D and 3D seismic data from the mid-Norwegian margin show that polygonal fault systems are widespread within the fine-grained, Micene sediments of the Kai Formation that overlie the Mesozoic/Early Cenozoic rift basins. Outcropping polygonal faults show that de-watering and development of polygonal faults commenced shortly after burial. On the other hand, the polygonal fault system’s stratigraphic setting, upward decreasing fault throw and the association with fluid flow features that are attributed to de-watering of the polygonal fault systems shows that polygonal faulting and fluid expulsion is an ongoing process since Miocene times.

Thermogenic methane injection via bubble transport into the upper Arctic Ocean from the hydrate‐charged Vestnesa Ridge, Svalbard

We use new gas-hydrate geochemistry analyses, echosounder data, and three-dimensional P-Cable seismic data to study a gas-hydrate and free-gas system in 1200 m water depth at the Vestnesa Ridge offshore NW Svalbard. Geochemical measurements of gas from hydrates collected at the ridge revealed a thermogenic source. The presence of thermogenic gas and temperatures of similar to 3.3 degrees C result in a shallow top of the hydrate stability zone (THSZ) at similar to 340 m below sea level (mbsl). Therefore, hydrate-skinned gas bubbles, which inhibit gas-dissolution processes, are thermodynamically stable to this shallow water depth. This was confirmed by hydroacoustic observations of flares in 2010 and 2012 reaching water depths between 210 and 480 mbsl. At the seafloor, bubbles are released from acoustically transparent zones in the seismic data, which we interpret as regions where free gas is migrating through the hydrate stability zone (HSZ). These intrusions result in vertical variations in the base of the HSZ (BHSZ) of up to similar to 150 m, possibly making the shallow hydrate reservoir more susceptible to warming. Such Arctic gas-hydrate and free-gas systems are important because of their potential role in climate change and in fueling marine life, but remain largely understudied due to limited data coverage in seasonally ice-covered Arctic environments.

Gas hydrates at the Storegga Slide: Constraints from an analysis of multicomponent, wide-angle seismic data

Geophysical evidence for gas hydrates is widespread along the northern flank of the Storegga Slide on the mid-Norwegian margin. Bottom-simulating reflectors (BSR) at the base of the gas hydrate stability zone cover an area of approximately 4000 km 2 , outside but also inside the Storegga Slide scar area. Traveltime inversion and forward modeling of multicomponent wide-angle seismic data result in detailed P- and S-wave velocities of hydrate- and gas-bearing sediment layers. The relationship between the velocities constrains the background velocity model for a hydrate-free, gas-free case. The seismic velocities indicate that hydrate concentrations in the pore space of sediments range between 3% and 6% in a zone that is as much as 50 m thick overlying the BSR. Hydrates are most likely disseminated, neither cementing the sediment matrix nor affecting the stiffness of the matrix noticeably. Average free-gas concentrations beneath the hydrate stability zone are approximately 0.4% to 0.8% of the pore volume, assuming a homogeneous gas distribution. The free-gas zone underneath the BSR is about 80 m thick. Amplitude and reflectivity analyses suggest a rather complex distribution of gas along specific sedimentary strata rather than along the base of the gas hydrate stability zone (BGHS). This gives rise to enhanced reflections that terminate at the BGHS. The stratigraphic control on gas distribution forces the gas concentration to increase slightly with depth at certain locations. Gas-bearing layers can be as thin as 2 m.

Role of tectonic stress in seepage evolution along the gas hydrate‐charged Vestnesa Ridge, Fram Strait

Methane expulsion from the world ocean floor is a broadly observed phenomenon known to be episodic. Yet the processes that modulate seepage remain elusive. In the Arctic offshore west Svalbard, for instance, seepage at 200–400 m water depth may be explained by ocean temperature-controlled gas hydrate instabilities at the shelf break, but additional processes are required to explain seepage in permanently cold waters at depths >1000 m. We discuss the influence of tectonic stress on seepage evolution along the ~100 km long hydrate-bearing Vestnesa Ridge in Fram Strait. High-resolution P-Cable 3-D seismic data revealed fine-scale (>10 m width) near-vertical faults and fractures controlling seepage distribution. Gas chimneys record multiple seepage events coinciding with glacial intensification and active faulting. The faults document the influence of nearby tectonic stress fields in seepage evolution along this deepwater gas hydrate system for at least the last ~2.7 Ma.

Abiotic methane from ultraslow-spreading ridges can charge Arctic gas hydrates

Biotic gas generation from the degradation of organic carbon in marine sediments supplies and maintains gas hydrates throughout the worlds oceans. In nascent, ultraslow-spreading ocean basins, methane generation can also be abiotic, occurring during the high-temperature (>200 °C) serpentinization of ultramafic rocks. Here, we report on the evolution of a growing Arctic gas- and gas hydrate–charged sediment drift on oceanic crust in eastern Fram Strait, a tectonically controlled, deep-water gateway between the subpolar North Atlantic and Arctic Oceans. Ultraslow-spreading ridges between northwest Svalbard and northeast Greenland permit the sustained interaction of a mid-ocean ridge transform fault and developing sediment drift, on both young ( 10 Ma) oceanic crust, since the late Miocene. Geophysical data image the gas-charged drift and crustal structure and constrain the timing of a major 30 km lateral displacement of the drift across the Molloy transform fault. We describe the buildup of a 2 m.y., long-lived gas hydrate– and free gas–charged drift system on young oceanic crust that may be fed and maintained by a dominantly abiotic methane source. Ultraslow-spreading, sedimented ridge flanks represent a previously unrecognized carbon reservoir for abiotic methane that could supply and maintain deep-water methane hydrate systems throughout the Arctic.

Carbon isotope (d13C) excursions suggest times of major methane release during the last 14 ka in Fram Strait, the deep-water gateway to the Arctic

We present results from a sediment core collected from a pockmark field on the Vestnesa Ridge (∼ 80 N) in the eastern Fram Strait. This is the only deep-water gateway to the Arctic, and one of the northernmost marine gas hydrate provinces in the world. Eight C AMS dates reveal a detailed chronology for the last 14 ka BP. The δC record measured on the benthonic foraminiferal species Cassidulina neoteretis shows two distinct intervals with negative values termed carbon isotope excursion (CIE I and CIE II, respectively). The values were as low as −4.37 ‰ in CIE I, correlating with the Bølling–Allerød interstadials, and as low as −3.41 ‰ in CIE II, correlating with the early Holocene. In the Bølling–Allerød interstadials, the planktonic foraminifera also show negative values, probably indicating secondary methane-derived authigenic precipitation affecting the foraminiferal shells. After a cleaning procedure designed to remove authigenic carbonate coatings on benthonic foraminiferal tests from this event, the C values are still negative (as low as −2.75 ‰). The CIE I and CIE II occurred during periods of ocean warming, sea-level rise and increased concentrations of methane (CH4) in the atmosphere. CIEs with similar timing have been reported from other areas in the North Atlantic, suggesting a regional event. The trigger mechanisms for such regional events remain to be determined. We speculate that sea-level rise and seabed loading due to high sediment supply in combination with increased seismic activity as a result of rapid deglaciation may have triggered the escape of significant amounts of methane to the seafloor and the water column above.

Submarine gas seepage in a mixed contractional and shear deformation regime: Cases from the Hikurangi oblique‐subduction margin

Gas seepage from marine sediments has implications for understanding feedbacks between the global carbon reservoir, seabed ecology and climate change. Although the relationship between hydrates, gas chimneys and seafloor seepage is well established, the nature of fluid sources and plumbing mechanisms controlling fluid escape into the hydrate zone and up to the seafloor remain one of the least understood components of fluid migration systems. In this study we present the analysis of new three-dimensional high-resolution seismic data acquired to investigate fluid migration systems sustaining active seafloor seepage at Omakere Ridge, on the Hikurangi subduction margin, New Zealand. The analysis reveals at high resolution, complex overprinting fault structures (i.e. protothrusts, normal faults from flexural extension, and shallow (<1 km) arrays of oblique shear structures) implicated in fluid migration within the gas hydrate stability zone in an area of 2x7 km. In addition to fluid migration systems sustaining seafloor seepage on both sides of a central thrust fault, the data show seismic evidence for sub-seafloor gas-rich fluid accumulation associated with proto-thrusts and extensional faults. In these latter systems fluid pressure dissipation through time has been favored, hindering the development of gas chimneys. We discuss the elements of the distinct fluid migration systems and the influence that a complex partitioning of stress may have on the evolution of fluid flow systems in active subduction margins.

Massive blow-out craters formed by hydrate-controlled methane expulsion from the Arctic seafloor

Massive methane blow-outs may be responsible for clusters of kilometer-wide craters in the Barents Sea. Methane takes the quick way out Accounting for all the sources and sinks of methane is important for determining its concentration in the atmosphere. Andreassen et al. found evidence of large craters embedded within methane-leaking subglacial sediments in the Barents Sea, Norway. They propose that the thinning of the ice sheet at the end of recent glacial cycles decreased the pressure on pockets of hydrates buried in the seafloor, resulting in explosive blow-outs. This created the giant craters and released large quantities of methane into the water above. Science, this issue p. 948 Widespread methane release from thawing Arctic gas hydrates is a major concern, yet the processes, sources, and fluxes involved remain unconstrained. We present geophysical data documenting a cluster of kilometer-wide craters and mounds from the Barents Sea floor associated with large-scale methane expulsion. Combined with ice sheet/gas hydrate modeling, our results indicate that during glaciation, natural gas migrated from underlying hydrocarbon reservoirs and was sequestered extensively as subglacial gas hydrates. Upon ice sheet retreat, methane from this hydrate reservoir concentrated in massive mounds before being abruptly released to form craters. We propose that these processes were likely widespread across past glaciated petroleum provinces and that they also provide an analog for the potential future destabilization of subglacial gas hydrate reservoirs beneath contemporary ice sheets.

Gas hydrate dissociation and sea-floor collapse in the wake of the Storegga Slide, Norway

Two-dimensional seismic data from the Mid-Norwegian margin provide evidence for sediment liquefaction and fluid mobilisation within the sediments that were located at the base of the hydrate stability zone before the Storegga Slide occurred. The disturbed subsurface sediments are overlain by a prominent roll-over structure and sea-floor collapse. This indicates fluid escape from the formerly hydrated sediment and suggests that the landslide caused a pressure drop strong enough to dissociate, the gas hydrates. We calculate that this fluid escape must have taken place within less than 250 years after the slide, as the effect of pressure decrease on hydrate stability was later compensated by a temperature decrease, related to the slumping process. The volume of expelled fluids from the collapse structure exceeds the volume of the gas hydrate dissociation products, implying that gas hydrate dissociation significantly affected the surrounding sediments.