Methane hydrates, also known as 'burning ice', occur at all ocean margins. The compound of gas and water occurs in the seafloor and it is only stable under relatively high pressures and low temperatures. If the pressure is too low or the temperature too high, the hydrates dissociate (break down), the methane is released and the gas can seep from the seafloor into the ocean. Thus, scientists fear that warming of global water temperatures could destabilize gas hydrates on a large scale. At the same time, it has not been fully understood which other factors influence the stability of gas hydrates.
Bathymetric map of the Nyegga region off the coast of Central Norway [Credit: Jens Karstens/GEOMAR] |
For their study, the team had investigated the history of gas hydrates in the Nyegga area. "This region off middle Norway is quite interesting if you want to study the dynamics of gases and liquids in the seafloor. There are large gas hydrate deposits, and many crater-like structures, so-called 'pockmarks', on the seabed. They are generally associated with gas leaks from deeper gas reservoirs, but their exact origin in this area is still unclear."
Map of the Northern Atlantic Ocean with the largest extent of the northern ice shield during the last ice age [Credit: Jens Karstens/GEOMAR] |
Despite the rising sea level and therefore increasing pressure, the simulation showed that towards the end of the ice age large amounts of gas hydrate became unstable and the released gas escaped through the sediment to the seawater. "Gas hydrates are only stable at a certain depth below the actual seafloor. When dozens of meters of new sediment settle on the seafloor, the solid compounds dissociate at the base of the hydrate stability zone, while new hydrates can form at the upper end of the stability zone. However, if the seafloor is already saturated with gas and the process takes place very quickly, the released gases make their way to the seafloor, without forming new hydrates," says Dr. Karstens.
Structure of gas hydrate [Credit: J. Greinert/GEOMAR] |
"We show that rapid changes in sedimentation can have a pronounced impact on the gas hydrate system and thus the entire carbon cycle", Dr. Karstens concludes. To date, this aspect has hardly been considered. However, further studies on other ocean margins are needed to obtain a more global picture, emphasizes the Kiel geophysicist.
The findings are published in Nature Communications.
Source: GEOMAR [February 12, 2018]