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Some of the deepest parts of the Black Sea are still responding to climate change brought on by the last ice age, scientists have discovered – a period that officially ended almost 12,000 years ago.
Analysis of gas hydrate deposits – in this case methane trapped by water molecules, in a solid substance that looks like ice – revealed the delayed response in a known northwestern area of the Black Sea under the Danube fan name.
Along with the temperature readings and other data, drill cores from the gas hydrate fields reveal something quite surprising: Levels of free methane below the seabed have yet to adjust to the warmer conditions that have already been prevalent on the surface for thousands of years.
“This shows that the gas hydrate system in the Danube’s deep-water range is still responding to climate changes initiated at the end of the last ice maximum,” the researchers write in their paper.
Examination of drill core. (Christian Rohleder)
Scientists’ attempts to determine the basis of the gas hydrate stability zone (GHSZ) – the lowest point at which gas hydrates naturally form due to temperature, pressure and a few other factors are at heart of the conclusions. Above and below this area, you will get “ free ” methane not trapped in hydrates.
To find the base of this area, researchers typically turn to a seismic reflection measurement of the sediment known as a bottom simulating reflector, or BSR for short. However, previous work has shown that in this part of the Black Sea there is a curious depth gap between the SSB and the base of the gas hydrate stability zone.
By drilling down to the seabed and taking temperature measurements, the researchers have now concluded that the gas hydrate’s stability zone has adapted to warmer conditions over the past millennia – as indicated by an elevation at a higher level – but the free methane and associated gas BSR is still catching up.
“From our point of view, the limit of gas-hydrate stability has already approached warmer subsoil conditions, but the free methane, which is still at that lower edge, has not yet managed to s ‘raise with it,’ says the geophysicist. Michael Riedel, from GEOMAR Helmholtz-Center for Ocean Research in Germany.
This late response could explain why BSR is not where it should be. Sediment permeability could also play a role, the team believes, and their measurements show that methane has managed to increase in some areas but not in others.
“In summary, we found a very dynamic situation in this region, which also seems to be linked to the development of the Black Sea since the last ice age,” says Riedel.
About 20,000 years ago, the water level was about 100 meters (328 feet) lower in the Black Sea, which means less pressure on the seabed. The water was also much cooler. With regard to free methane gas, these conditions have not yet changed.
As with any study on the effects of climate change, this research will contribute to future climate modeling. There is currently a huge volume of gas hydrate deposits under the Arctic, for example, and it is important to know how they might react to temperature increases in the years to come.
Scientists stress that their results should be interpreted with caution, with many different factors at play and many more possibilities for study – but they also stress the importance of in situ measurements and quality data for an analysis like this – this.
“For our investigations, we used our MARUM-MeBo200 drilling rig and broke all previous depth records with a maximum depth reached of almost 145 meters. [476 feet]Explains geologist Gerhard Bohrmann of the University of Bremen in Germany.
The research was published in Earth and Planetary Science Letters.
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