大氣所碩一 闕珮羽

The objectives of this paper To discuss the sensitivity of gas hydrate stability in the Storegga Slide complex to changes in sea level and bottom water temperature since the last glacial maximum (LGM). To investigate the timing of slope failure in relation to gas hydrate dissociation forced by the inflow of warmer water masses along the mid-Norwegian margin. To present a key seismic reflection profile illustrating a striking match between the location of the Storegga Slide headwall and the calculated zone of reduced hydrate stability as a result of such post-glacial bottom warming in the North-Atlantic. LGM (22–17.5 ka)

Geological setting N–S trending domes : Hydrocarbon reservoirs While the term ‘Storegga Slide’ used in this paper refers to the enormous retrogressive and multi-phase slide dated at ~8.2 ka. The area also contains numerous fluid escape features (e.g. suspected diapirs and pockmarks). Geothermal gradients in the study area are 50–56 ℃ /km, as measured in deep boreholes.

Geological setting 500m 5 ℃ 900m 0 ℃ ℃

Data and methods Reconstruction of paleo-bottom water temperatures (BWT) is based on existing knowledge of the relationship between specific benthic foraminifera assemblages and associated temperatures. Younger Dryas 12.8~11.5ka Holocene 11.7ka

Data and methods In modelling the evolution of the gas hydrate stability conditions, we use an analytical 3-phase methane–seawater–hydrate equilibrium equation. Similar analytical equations were derived for a variety of gas compositions, deviating from pure methane. Additional parameters for determining the depth of hydrate stability are bathymetry, BWT data, and geothermal data. The modelling incorporates both sea level rise and bottom water warming since the LGM.

Seismic reflection profile JM A key feature along this profile is the presence of an irregular BSR. two-way travel time The time taken for a seismic wave to travel from the shot down to a reflector or refractor and back to a geophone at the surface.

Methane hydrate stability conditions from the LGM to the present At shallower depths where BWT variations occur with higher amplitudes. Model predicts a distinct shoaling of the BHSZ since the LGM Sea level rise Warm inflow Inversion:750–900 m water depths

Correlation of modelling results with seismic reflection profiles Maximum of pore pressure build-up following hydrate decomposition is expected. Small additions of other hydrate-forming gases (e.g. ethane, propane,.) will move the hydrate stability limit deeper A gas mixture of a few percent of ethane (5–10%) fits the observations better. Re-equilibration of hydrate has largely occurred, and that it is not a long-term equilibration process.

Timing of sliding vs. hydrate stability In shallow depths, hydrates could not have been stable over the last 10 ky, except if BWT at glacial conditions was sufficiently low (-1 ℃, green curves in Fig. 7). Main phase of hydrate dissociation, taking place around 12.5–9 ka. At the time of sliding, gas hydrates were less stable compared to the LGM situation.

Conclusions The inflow of warm water masses after the Younger Dryas dominates gas hydrate stability in the upper slope area, resulting in a shoaling of the BHSZ. Lower slope to deep basin area is governed primarily by the eustatic sea level rise, resulting in deepening of the BHSZ.

The presence of a BSR within the Storegga Slide dated at 8.2 ka closely matches the present-day steady-state equilibrium conditions of methane hydrates. Small additions of other hydrate-forming gases cannot be excluded. This close match indicates that gas hydrate formation and their dynamics subsequent to external forcing (e.g. climatic change, slope failure) is a relatively fast process.

The location of the Storegga Slide headwall, as evidenced from seismic reflection profiles, fits exactly the area where hydrate conditions were modelled to shoal either dramatically or disappear, a process controlled by the influx of warm water.

Climatic changes since the LGM may have forced the gas hydrated zone to decrease and partly to dissociate by the time of sliding. Climate-controlled hydrate dissociation may be a secondary process in geohazards.