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| Titel |
Methane release from hydrate dissociation on the West Svalbard continental margin |
| VerfasserIn |
Kate Thatcher, Graham Westbrook |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250039414
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| Zusammenfassung |
| Sonar data from the West Svalbard continental margin in August-September 2008 have
shown the presence of plumes of methane bubbles emanating from the seabed up-slope from
the upper limit of the methane hydrate stability zone. In the same area, there is evidence for
an increase in bottom water temperatures of 1oC over the last 30 years. Numerical
modelling has been used to evaluate the effect of such warming of the seabed on the
release of methane from dissociation of hydrate within the gas hydrate stability
zone.
The amount of methane released at the seabed, derived from dissociating hydrate,
depends on a number of often poorly constrained variables. The time history of seabed
temperature, the concentration of hydrate and its distribution in the sediment beneath the
seabed and the effective permeability of the sediments affect the amount of gas produced and
the time it takes for the gas to flow to the seabed. Using a numerical model, many different
scenarios have been examined, derived from the uncertainty in the data describing the
system.
Over the 30-year warming period, numerical models show that gas reaches the seabed
within this period if the hydrate is less than 5 metres below the seabed with a permeability of
>9x10-14 m2 and an initial hydrate concentration of >10% of pore space. Deeper
hydrate layers undergo less dissociation as it takes time for seabed warming to move
down through the sediments. Small amounts of dissociation and less concentrated
hydrate layers will produce less gas and gas flow will be slower due to reduced
relative permeability. Lower absolute permeability also slows the gas flow to the
seabed.
The time history of temperature change at the seabed over the last 1000 years could have
redistributed hydrate within the hydrate stability zone. Warming associated with the Medieval
warm period would shoal the base of the hydrate stability zone (BHSZ) and increase hydrate
concentration at this depth. The subsequent cooling would deepen the BHSZ leaving a layer
of high hydrate concentration above the BHSZ. Such a process could be responsible for
moving hydrate close enough to the seabed that it has released gas to the seabed in the last 30
years. Our results show a time lag between the seabed warming and the release of
gas at the seabed caused by the time taken for the hydrate to dissociate and the
time for gas to flow to the seabed. The numerical models have been run into the
future and the duration of gas release due to 30 years of warming is presented. |
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