Relationship between Density and Biogenic Opal in Sediments from Sites 658 and 660

At Site 658, and especially at Site 660, sediments rich in biogenic opal were recovered. The fractions of biogenic silica, biogenic carbonate, and terrigenous material vary throughout the entire sequence at these sites (see chapters for Sites 658 and 660, this volume). At Site 660, biogenic-opal contents up to 100% are common in Eocene sediments. In studying these opal-rich sediments, a rapid method for estimating biogenic opal published by Mann and Muller (1980) was found useful. These authors applied an X-ray method which measures the height of a broad, diffuse reflection band of opal extending from about 15° to 32° 20, with a maximum at about 22° 20 (i.e., 4.04A) (Fig. 1, IB). Furthermore, this paper describes another method for estimating variations in the biogenic-opal content by using grain density. Grain density (p) can easily be determined by measuring the weight (G) and the volume (V) of the dry sediment, where p = G/P7g/cm3)

Flach- und Tiefenwassergashydrate in Sedimenten polarer Kontinentalränder des Nordatlantiks : geophysikalische Signaturen der Instabilität

Destabilization of hydrates from polar continental margins of the North Atlantic are geophysically detectable within hydrate stabilization zones (HSZ). High-frequency seismic surveys of structures and propagations of compressional wave velocities are changing the classic understanding of the hydrate stability zone to an instable one. The results are important in two respects: first, shallow-water gas hydrates can substantially contribute to the transfer of the intensive greenhouse gas methane to the atmosphere, and second, deep-water gas hydrates indicate destabilization processes. These deep-water gas hydrates paly an important role regarding the instability of the continental margins, whereas their influence on the greenhouse effect is probably secondary

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

Gas hydrate drilling conducted on the European Margin

Since 1996, the Norwegian government has licensed hydrocarbon exploration in seven deep water areas on the continental slope north of the Norwegian Trough. Data acquired in this region, which is of interest to both scientists and the oil industry, provide an opportunity to improve understanding of the geology and development of the area through Quaternary times. Gas hydrates, slope stability, and geohazards are especially important topics for research near the Norwegian Trough

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

Role of subsea permafrost and gas hydrate in postglacial Arctic methane releases

The papers of this thesis are not available in Munin.<br>Paper I: 'Offshore permafrost decay and massive seabed methane escape in water depths > 20 m at the South Kara Sea shelf.' Alexey Portnov, Andrew J. Smith, Jürgen Mienert, Georgy Cherkashov, Pavel Rekant, Peter Semenov, Pavel Serov, Boris Vanshtein. Available in <a href=http://dx.doi.org/10.1002/grl.50735> Geophysical Research Letters, vol. 40, 1–6</a><br>Paper II: 'Modeling the evolution of climate-sensitive Arctic subsea permafrost in regions of extensive gas expulsion at the West Yamal shelf.' Alexey Portnov, Jurgen Mienert, Pavel Serov. Available in <a href=http://dx.doi.org/10.1002/2014JG002685> Journal of Geophysical Research: Biogeosciences, vol. 119, issue 11, 2014</a> <br>Paper III: 'Methane release from pingo-like features across the South Kara Sea shelf, an area of thawing offshore permafrost'. Pavel Serov, Alexey Portnov, Jurgen Mienert, Peter Semenov, Polina Ilatovskaya. (Manuscript). Published version available in <a href=http://dx.doi.org/10.1002/2015JF003467> Journal of Geophysical Research: Earth Surface, vol. 120, issue 8, 2015</a> <br>Paper IV: 'Ice-sheet driven methane storage and release in the Arctic.' Alexey Portnov, Sunil Vadakkepulyambatta, Jurgen Mienert, Alun Hubbard. (Manuscript)Greenhouse gas methane is contained as gas hydrate, an icy structure, under the seabed in enormous amounts of Arctic regions. West Svalbard continental margin, which we investigated here, is one of these regions. Also, in the Russian Kara Sea the subsea permafrost is acting as a cap for the gas to be released in the future. But continuous expulsions of methane have been already observed in both places. This study shows how the subsea permafrost in the Kara Sea, and gas hydrate systems offshore West Svalbard, have evolved from the last ice age to the present day. The conclusions are based on integrated field geophysical and gas-geochemical studies as well as modeling of permafrost, gas hydrate reservoirs and Barents Sea ice sheet dynamics. It shows that continuous permafrost of the Kara Sea is more fragile than previously thought. It is likely to be limited to the shallow water depths of 20 meters on this Arctic shelf region, allowing expulsions of methane from an area of 7500 sq km. Offshore Svalbard almost 2000 active and inactive gas expulsion sites are associated with melting of gas hydrate and thawing of shallow permafrost from past to present. Our research approach shows that natural climate drivers such as methane release can change and that they are connected to the ice sheet retreat since the last ice age. These processes triggered widespread seafloor gas discharge, observed in Arctic shelf and upper continental margins to this day