78 research outputs found
Norwegian margin outer shelf cracking: a consequence of climate-induced gas hydrate dissociation?
A series of en echelon cracks run nearly parallel to the outer shelf edge of the mid-Norwegian margin. The features can be followed in a *60-km-long and *5-km-wide zone in which up to 10-m-deep cracks developed in the seabed at 400â550 m water depth. The time of the seabed cracking has been dated to 7350 14C years BP (8180 cal years BP), which corresponds with the main Storegga Slide event (8100 Âą 250 cal. years BP). Reflection seismic data suggest that the cracks do not appear to result from deep-seated faults, but it cannot be ruled out completely that tension crevices were created in relation to past movements on the headwall of the Storegga slide. The cracking zone corresponds well to the zone where the base of the hydrate stability zone (BHSZ) outcrops. Evidence of fluid release in the BHSZ outcrop zone comes from an extensive pockmark field. We suggest that post-glacial ocean warming triggered the dissociation of gas hydrates while the interplay between dissociation, overpressure, and sediment fracturing on the outer shelf remains to be understood.publishedVersio
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
Thermal Characterization of Pockmarks Across Vestnesa and Svyatogor Ridges, Offshore Svalbard
The Svalbard margin represents one of the northernmost gas hydrate provinces worldwide. Vestnesa Ridge (VR) and Svyatogor Ridge (SR) west of Svalbard are two prominent sediment drifts showing abundant pockmarks and sites of seismic chimney structures. Some of these sites at VR are associated with active gas venting and were the focus of drilling and coring with the seafloorâdeployed MARUMâMeBo70 rig. Understanding the nature of fluid migration and gas hydrate distribution requires (among other parameters) knowledge of the thermal regime and in situ gas and pore fluid composition. In situ temperature data were obtained downhole at a reference site at VR defining a geothermal gradient of ~78 mK mâ1 (heat flow ~95 mW mâ2). Additional heat probe data were obtained to describe the thermal regime of the pockmarks. The highest heat flow values were systematically seen within pockmark depressions and were uncorrelated to gas venting occurrences. Heat flow within pockmarks is typically ~20 mW mâ2 higher than outside pockmarks. Using the downhole temperature data and gas compositions from drilling we model the regional base of the gas hydrate stability zone (BGHSZ). Thermal modeling including topographic effects suggest a BGHSZ up to 40 m deeper than estimated from seismic data. Uncertainties in sediment properties (velocity and thermal conductivity) are only partially explaining the mismatch. Capillary effects due to small sediment grain sizes may shift the free gas occurrence above the equilibrium BGHSZ. Changes in gas composition or pore fluid salinity at greater depth may also explain the discrepancy in observed and modeled BGHSZ
Microseismicity linked to gas migration and leakage on the western Svalbard shelf
Source at: http://doi.org/10.1002/2017GC007107The continental margin off Prins Karls Forland, western Svalbard, is characterized by widespread
natural gas seepage into the water column at and upslope of the gas hydrate stability zone. We
deployed an ocean bottom seismometer integrated into the MASOX (Monitoring Arctic Seafloor-Ocean
Exchange) automated seabed observatory at the pinch-out of this zone at 389 m water depth to investigate
passive seismicity over a continuous 297 day period from 13 October 2010. An automated triggering algorithm
was applied to detect over 220,000 short duration events (SDEs) defined as having a duration of less
than 1 s. The analysis reveals two different types of SDEs, each with a distinctive characteristic seismic signature.
We infer that the first type consists of vocal signals generated by moving mammals, likely finback
whales. The second type corresponds to signals with a source within a few hundred meters of the seismometer,
either due east or west, that vary on short (tens of days) and seasonal time scales. Based on evidence
of prevalent seafloor seepage and subseafloor gas accumulations, we hypothesize that the second type of
SDEs is related to subseafloor fluid migration and gas seepage. Furthermore, we postulate that the observed
temporal variations in microseismicity are driven by transient fluid release and due to the dynamics of thermally
forced, seasonal gas hydrate decomposition. Our analysis presents a novel technique for monitoring
the duration, intensity, and periodicity of fluid migration and seepage at the seabed and can help elucidate
the environmental controls on gas hydrate decomposition and release
Enhanced CO2 uptake at a shallow Arctic Ocean seep field overwhelms the positive warming potential of emitted methane
Source at https://doi.org/10.1073/pnas.1618926114.Continued warming of the Arctic Ocean in coming decades is projected to trigger the release of teragrams (1 Tg = 106 tons) of methane from thawing subsea permafrost on shallow continental shelves and dissociation of methane hydrate on upper continental slopes. On the shallow shelves (<100 m water depth), methane released from the seafloor may reach the atmosphere and potentially amplify global warming. On the other hand, biological uptake of carbon dioxide (CO2) has the potential to offset the positive warming potential of emitted methane, a process that has not received detailed consideration for these settings. Continuous seaâair gas flux data collected over a shallow ebullitive methane seep field on the Svalbard margin reveal atmospheric CO2 uptake rates (â33,300 Âą 7,900 Îźmol mâ2â
dâ1) twice that of surrounding waters and âź1,900 times greater than the diffusive seaâair methane efflux (17.3 Âą 4.8 Îźmol mâ2â
dâ1). The negative radiative forcing expected from this CO2 uptake is up to 231 times greater than the positive radiative forcing from the methane emissions. Surface water characteristics (e.g., high dissolved oxygen, high pH, and enrichment of 13C in CO2) indicate that upwelling of cold, nutrient-rich water from near the seafloor accompanies methane emissions and stimulates CO2 consumption by photosynthesizing phytoplankton. These findings challenge the widely held perception that areas characterized by shallow-water methane seeps and/or strongly elevated seaâair methane flux always increase the global atmospheric greenhouse gas burden
Dating submarine landslides using the transient response of gas hydrate stability
Submarine landslides are prevalent on the modern-day seafloor, yet an elusive problem
is constraining the timing of past slope failure. We present a novel age-dating technique
based on perturbations to underlying gas hydrate stability caused by slide-impacted seafloor changes. Using three-dimensional (3-D) seismic data, we mapped an irregular bottom
simulating reflection (BSR) underneath a submarine landslide in the Orca Basin, Gulf of
Mexico. The irregular BSR mimics the pre-slide seafloor geometry rather than the modern
bathymetry. Therefore, we suggest that the gas hydrate stability zone (GHSZ) is still adjusting
to the post-slide sediment temperature. We applied transient conductive heat-flow modeling
to constrain the response of the GHSZ to the slope failure, which yielded a most likely age of
ca. 8 ka, demonstrating that gas hydrate can respond to landslides even on multimillennial
time scales. We further provide a generalized analytical solution that can be used to remotely
date submarine slides in the absence of traditional dating technique
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