Nicht neu: Es blubbert Methan aus dem Meeresboden

Gashydrate in der Arktis haben der Fachwelt lange Sorge bereitet. Im Zuge der Erderwärmung glaubte man, dass hier riesige Mengen des Treibhausgases Methan in die Atmosphäre gelangen könnten und die Erwärmung weiter anheizen. Die Wissenschaftler machten sich an die Arbeit und fanden glücklicherweise, dass es wohl doch nicht so schlimm kommen wird, wie befürchtet. Wir haben bereits in der Vergangenheit an dieser Stelle berichtet:

Im Folgenden wollen wir weitere neue Literatur zum Thema vorstellen. Das National Oceanography Centre (NOC) in Southampton gab am 19. Oktober 2015 bekannt, dass aus dem Meeresboden vor Spitzbergen Methan austritt, dieses jedoch nur in geringen Mengen an die Meeresoberfläche gelangt, da es schnell von Bakterien zersetzt bzw. von Strömungen forttransportiert wird. Dieser „Deckel“ verhindert letztendlich das Austreten des Methans in die Atmosphäre über Spitzbergen. Hier die Pressemitteilung (via Science Daily):

Methane bubbling off Svalbard is not a source of atmospheric greenhouse gas

Methane seeps from seafloor deposits near Svalbard release less ‚greenhouse gas‘ into the atmosphere than other Arctic sites because ocean currents there form an effective barrier. This research, by scientists at the National Oceanography Centre (NOC) and the University of Southampton, was published this week in the Journal of Geophysical Research: Oceans.

Dr Carolyn Graves, the lead author of the study from the NOC, said „Strong ocean bottom currents carry the dissolved methane northwards, while bacteria consume it to produce carbon dioxide. This slows and reduces the volume of methane rising up through the ocean towards the atmosphere.“

The Arctic contains large volumes of methane stored in forms that turn into gas if temperatures rise or the pressure they are subject to decreases. These forms include methane trapped in marine sediment beneath permafrost as hydrate- a form of methane ice. If methane gas escapes from these deposits, as well as from seafloor reservoirs, it could add to atmospheric warming, causing a positive climate forcing feedback.

More than 250 plumes of methane gas were discovered rising from the seafloor near Svalbard in 2008. This gas is likely released from methane hydrate destabilized in seafloor sediments beneath about 400 metres water depth. Very high dissolved methane concentrations were observed near the seafloor, while much less methane was present in the surface waters that are in contact with the atmosphere. Researchers used these observations to set up a simple model of physical and biochemical processes. This model shows that strong ocean bottom currents prevent the direct release of the greenhouse gas into the atmosphere.

Dr Doug Connelly from the NOC, and a co-author of this paper, said „While this research shows that methane coming from the seafloor is not reaching the atmosphere in this particular location near Svalbard, there are areas that have more methane hydrate deposits. These may still pose a high risk of increasing global environmental change if released.“

This research involved scientists from seven institutes in the UK, Germany, and Switzerland and forms part of the NOC’s ongoing investigation into the relationship between processes happening at the sea floor, the ocean and the atmosphere. This study received funding from: The Natural Environment Research Council (NERC), the Graduate School of Ocean and Earth Science (University of Southampton, UK), and a Natural Sciences and Engineering Research Council of Canada Postgraduate Scholarship.

Carolyn A. Graves, Lea Steinle, Gregor Rehder, Helge Niemann, Douglas P. Connelly, David Lowry, Rebecca E. Fisher, Andrew W. Stott, Heiko Sahling, Rachael H. James. Fluxes and fate of dissolved methane released at the seafloor at the landward limit of the gas hydrate stability zone offshore western Svalbard. Journal of Geophysical Research: Oceans, 2015; DOI: 10.1002/2015JC011084

Das norwegische Centre for Arctic Gas Hydrate, Environment and Climate (CAGE) erinnerte im Februar 2015 daran, dass der Austritt von Methan aus dem arktische Meeresboden kein neues Phänomen ist, sondern seit vielen Millionen Jahren fester Bestandteil des Kohlenstoffzyklus in Arktis darstellt:

Methane seepage from Arctic seabed occurring for millions of years

Natural seepage of methane offshore the Arctic archipelago Svalbard has been occurring periodically for at least 2,7 million years. Major events of methane emissions happened at least twice during this period, according to a new study. We worry about greenhouse gas methane. Its lifetime in the atmosphere is much shorter than CO2´s, but the impact of methane on climate change is over 20 times greater than CO2 over a 100-year period.  60 percent of the methane in the atmosphere comes from emissions from human activities. But methane is a natural gas, gigatonnes of it trapped under the ocean floor in the Arctic.

And it is leaking. And it has been leaking for longer time than the humans have roamed the Earth

” Our planet is leaking methane gas all the time. If you go snorkeling in the Caribbean you can see bubbles raising from the ocean floor at 25 meters depth. We studied this type of release, only in a much deeper, colder and darker environment. And found out that it has been going on, periodically, for as far back as 2,7 million years.” says Andreia Plaza Faverola, researcher at CAGE and the primary author behind a new paper in Geophysical Research Letters. She is talking about Vestnesa Ridge in Fram Strait,  a thousand meters under the Arctic Ocean surface offshore West-Svalbard. Here, enormous – 800 meters high – gas flares rise from the seabed today. That’s the size of the tallest manmade structure in the world – Burj Khalifa in Dubai. “Half of Vestnesa Ridge is showing very active seepage of methane. The other half is not. But there are obvious pockmarks on the inactive half, cavities and dents in the ocean floor, that we recognized as old seepage features. So we were wondering what activates, or deactivates, the seepage in this area.,” says Plaza Faverola.

Why 2,7 million years?

She, and a team of marine geophysicists from CAGE, used the P-Cable technology , to figure it out. It is a seismic instrument that is towed behind a research vessel. It recorded the sediments beneath these pockmarks. P-Cable renders images that look like layers of a cake. It also enables scientists to visualize deep sediments in 3D. ” We know from other studies in the region that the sediments we are looking at in our seismic data are at least 2.7 million years old. This is the period of increase of glaciations in the Northern Hemisphere, which influences the sediment..  The P-Cable enabled us to see features in this sediment, associated with gas release in the past.“ “These features can be buried pinnacles or cavities that form what we call gas chimneys in the seismic data. Gas chimneys appear like vertical disturbances in the layers of our sedimentary cake.  This enables us to reconstruct the evolution of gas expulsion from this area for at least 2,7 million years.” says Andreia Plaza Faverola. The seismic signal penetrated into 400 to 500 meters of sediment to map this timescale.

How is the methane released?

By using this method, scientists were able to identify two major events of gas emission throughout this time period: One 1,8 million years ago, the other 200 000 years ago. This means that there is something that activated and deactivated the emissions several times. Plaza Faverola´s paper gives a plausible explanation: It is the movement of the tectonic plates that influences the gas release. Vestnesa is not like California though, riddled with earthquakes because of the moving plates. The ridge is on a so-called passive margin. But as it turns out, it doesn´t take a huge tectonic shift to release the methane stored under the ocean floor. “Even though Vestnesa Ridge is on a passive margin, it is between two oceanic ridges that are slowly spreading. These spreading ridges resulted in separation of Svalbard from Greenland and opening of the Fram Strait. The spreading influences the passive margin of West-Svalbard, and even small mechanical collapse in the sediment can trigger seepage.” says Faverola.

Where does the methane come from?

The methane is stored as gas hydrates, chunks of frozen gas and water, up to hundreds of meters under the ocean floor. Vestnesa hosts a large gas hydrate system. There is some concern that global warming of the oceans may melt this icy gas and release it into the atmosphere. That is not very likely in this area, according to Andreia Plaza Faverola. ” This is a deep water gas hydrate system, which means that it is in permanently cold waters and under a lot of pressure. This pressure keeps the hydrates stable and the whole system is not vulnerable to global temperature changes. But under the stable hydrates there is gas that is not frozen. The amount of this gas may increase if hydrates melt at the base of this stability zone, or if gas from deeper in the sediments arrives into the system.  This could increase the pressure in this part of the system, and the free gas may escape the seafloor through chimneys. Hydrates would still remain stable in this scenario .”

 Historical methane peaks coincide with increase in temperature

Throughout Earth´s history there have been several short periods of significant increase in temperature. And these periods often coincide with peaks of methane in the atmosphere , as recorded by ice cores. Scientists such as Plaza Faverola are still debating about the cause of this methane release in the past. ” One hypotheses is that massive gas release from geological sources, such as volcanos or ocean sediments may have influenced global climate..  What we know is that there is a lot of methane released at present time from the ocean floor. What we need to find out is if it reaches the atmosphere, or if it ever did.” Historical events of methane release, such as the ones in the Vestnesa Ridge, provide crucial information that can be used in future climate modeling. Knowing if these events repeat, and identifying what makes them happen, may help us to better predict the potential influence of methane from the oceans on future climate.

Reference: Role of tectonic stress in seepage evolution along the gas hydrate-charged Vestnesa Ridge, Fram Strait. A.Plaza Faverola, S.Bünz, J.E.Johnson, S. Chand, J. Knies, J. Mienert and P. Franek. Geophysical Research Letters. 2015.

Vier Monate später, im Juni 2015, berichte CAGE über neueste Expeditionsergebnisse zur Methanforschung in der Arktis.

Mithilfe von Tauchrobotern hat man heute eine recht gutes Bild, wie es um die Methanaustrittsstellen herum am Meeresboden aussieht. Fasziniert zeigten sich Forscher der University of Oregon, als sie hier komplexe Lebensgemeinschaften fanden, die das Methan als Antrieb nutzten und damit zum Teil entschärften. Pressemitteilung der Uni vom 31. Mai 2016:

Hydrothermal vents, methane seeps play enormous role in marine life, global climate

The hydrothermal vents and methane seeps on the ocean floor that were once thought to be geologic and biological oddities are now emerging as a major force in ocean ecosystems, marine life and global climate. However, even as researchers learn more about their role in sustaining a healthy Earth, these habitats are being threatened by a wide range of human activities, including deep-sea mining, bottom trawling and energy harvesting, scientists say in a report published in Frontiers in Marine Science.

Researchers from Oregon State University first discovered these strange, isolated worlds on the ocean bottom 40 years ago. These habitats surprised the scientific world with reports of hot oozing gases, sulfide chimneys, bizarre tube worms and giant crabs and mussels – life forms that were later found to eat methane and toxic sulfide. “It was immediately apparent that these hydrothermal vents were incredibly cool,” said Andrew Thurber, an assistant professor in the OSU College of Earth, Ocean and Atmospheric Sciences, and co-author on the new report. “Since then we’ve learned that these vents and seeps are much more than just some weird fauna, unique biology and strange little ecosystems. Rather than being an anomaly, they are prevalent around the world, both in the deep ocean and shallower areas. They provide an estimated 13 percent of the energy entering the deep sea, make a wide range of marine life possible, and are major players in global climate.”

As fountains of marine life, the vents pour out gases and minerals, including sulfide, methane, hydrogen and iron – one of the limiting nutrients in the growth of plankton in large areas of the ocean. In an even more important role, the life forms in these vents and seeps consume 90 percent of the released methane and keep it from entering the atmosphere, where as a greenhouse gas it’s 25 times more potent than carbon dioxide. “We had no idea at first how important this ecological process was to global climate,” Thurber said. “Through methane consumption, these life forms are literally saving the planet. There is more methane on the ocean floor than there are other forms of fossil fuels left in the oceans, and if it were all released it would be a doomsday climatic event.”

In reviewing the status of these marine geological structures and the life that lives around them, a group of researchers from 14 international universities and organizations have outlined what’s been learned in the past four decades and what forces threaten these ecosystems today. The synthesis was supported by the J.M. Kaplan fund. These vents and seeps, and the marine life that lives there, create rocks and habitat, which in some settings can last tens of thousands of years. They release heat and energy, and form biological hot spots of diversity. They host extensive mussel and clam beds, mounds of shrimp and crab, create some prime fishing habitat and literally fertilize the ocean as zooplankton biomass and abundance increases. While the fluid flows from only a small section of the seafloor, the impact on the ocean is global.

Some of the microorganisms found at these sites are being explored for their potential to help degrade oil spills, or act as a biocatalytic agent for industrial scrubbing of carbon dioxide. These systems, however, have already been damaged by human exploitation, and others are being targeted, the scientists said. Efforts are beginning to mine them for copper, zinc, lead, gold and silver. Bottom trawling is a special concern, causing physical disturbance that could interfere with seeps, affect habitat and damage other biologic linkages. Oil, gas or hydrate exploitation may damage seeps. Whaling and logging may interfere with organic matter falling to the ocean floor, which serves as habitat or stepping stones for species reliant on chemosynthetic energy sources. Waste disposal of munitions, sewage and debris may affect seeps.

The range of ecosystem services these vents and seeps provide is just barely beginning to be understood, researchers said in their report. As many of these habitats fall outside of territorial waters, vent and seep conservation will require international collaboration and cooperation if they are going to continue to provide ecosystem benefits. Contributors to this report included researchers from the Scripps Institution of Oceanography, Florida State University, the National Institute of Water and Atmospheric Research in New Zealand, University of the Azores, Temple University, Universidade de Aveiro, the U.S. Geological Survey, University of the West Indies, Dalhousie University, University of Victoria, Duke University, Ghent University and the University of Hawaii at Manoa.

Siehe hierzu auch Artikel in Spiegel Online aus dem Oktober 2016:

Küste der USA: Es blubbert aus dem Meeresboden
Vor den USA strömen gewaltige Mengen Gas aus dem Meeresgrund. Ein Tauchroboter spürte 500 Quellen auf – daneben erstaunliche Lebewesen.

„Es scheint, dass die gesamte Küste vor Washington, Oregon und Kalifornien eine gigantische Methanquelle ist“, sagt der Meeresforscher Robert Ballard. Er und seine Kollegen haben vor der US-Westküste 500 sprudelnde Methanquellen im Meeresboden entdeckt. Das Team um Ballard, der durch die Entdeckung des „Titanic“-Wracks bekannt wurde, hatte den Fund im Sommer vom Forschungsschiff „Nautilus“ aus mithilfe zweier ferngesteuerter Unterwasser-Rover gemacht.

Weiterlesen auf Spiegel Online