 |
| Titel |
Timing of methane release from hydrate dissociation on the west Svalbard margin |
| VerfasserIn |
Kate Thatcher, Graham Westbrook, Anne Chabert, Sudipta Sarkar, Tim Minshull, Christian Berndt |
| Konferenz |
EGU General Assembly 2011
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250053488
|
|
|
|
|
|
| Zusammenfassung |
| The release of methane from methane hydrate has been invoked as a contributing agent to
rapid climate change. In 2008, plumes of methane bubbles were observed emanating from the
seabed in an area of the West Svalbard margin that has undergone about 1°C of warming of
the bottom water at about 400-m depth, over the last 30 years. The locations of the bubble
plumes, spreading upslope from the present upper limit of the methane hydrate stability zone,
indicate that the gas in the plumes could be methane from warming-induced hydrate
dissociation in the sediment beneath the area from which the hydrate stability zone has
retreated.
Through numerical modelling, we investigated the controls on the time lag between the
increase in seabed temperature and gas flow from the seabed. The lag results from the time
taken for heat diffusion, hydrate dissociation and gas migration. These are dependent on the
distribution and concentration of hydrate and on sediment permeability, among
other parameters. We investigated possible scenarios in which plumes could be
explained by recent warming. If the top of the hydrate was initially 5 m below
seabed, in sediment with permeability 10-14 m2, we would expect the time lag to
be 40 years, which is too long to explain current gas emission. The temperature
history of the area is key. A 60-year time series, constructed from extrapolation of
more southerly oceanic temperature records, shows that prior to the last 30 years
warming, the area was cooling from a warm period in the early 1960s. So, a period of
warming longer than 30 years is not the sole explanation for the present-day methane
release.
When hydrate is initially less than 2 m below the seabed, gas from hydrate dissociation
can reach the seabed in less than 30 years from the onset of warming. Generally, hydrate
would not be found as shallow as this, because diffusion acts to transport methane from the
sediments to the ocean, removing hydrate close to the seabed. Under conditions of
locally high background methane flux, greater than 100 mol.m-2.yr-1, however,
hydrate can exist close to the seabed and thus be a source of the gas in the observed
bubble plumes. The warm period in the early 1960s could have brought gas close
to the seabed, which, during the subsequent cooling, formed hydrate close to the
seabed. This process could have occurred many times in the past, although we have
no direct instrumental record of the temperature change at the seabed to confirm
this conjecture. Cyclic variation of the temperature of the seabed over periods of
tens of years, driven by periodic changes in the North Atlantic Current has the
potential to influence the distribution of hydrate within the sediment, leading to hydrate
formation at depths shallower than predicted from models with linear or no change in
temperature with time. At locations where there is significant input of gas beneath
the hydrate stability zone, this temperature cycling creates conditions for a ‘rapid’
response to increases in the temperature of the water in the depth range 300-500 m. |
|
|
|
|
|
|