Dienstag, 11. November 2014

More methane from the deep sea: Mud-volcanoes as methane source

he mud-volcano Haakon Mosby in the Barents sea near Norway emits yearly several hundred tons of methane, a potent greenhouse gas. A research team, coordinated by the Max-Planck-Institute in Bremen, reports on the results of a long-term observatory in NATURE Communications. They collected during 431 days data on temperature, pressure and pH and documented using a camera 25 eruptions of mud and gas. Some of these eruptions were so violent that the seabed topography was profoundly changed. They calculated that the mud-volcano may emit 10 times more methane than previously assumed.

There are over 1000 terrestrial mud-volcanos known, and a growing number is found in the sea, between 200 and 4000 m depth, such as the Haakon Mosby Mud Volcano. Scientists estimated that the submarine volcanos emit 27 Million tons of methane, which is about 5% of the global emission. This may be an underestimation, as not all volcanoes are found and because they are not monitored by long term observatories.

The rhythm of mud-volcanoes
The question is whether submarine mud-volcanoes emit gas and mud continuously or if they have now and then an outburst? In a steady state a continuous of gaseous mud flows through the chimney from large depth. A part of the gas goes into the seawater, and a part is microbially converted in the sediments. While assuming a steady flow scientists have calculated the emission and conversion rates. Eruptions have never been observed during visits with research vessels. To see such rare events, permanent observations are needed. Such an instrument was developed and used by the team of scientist led by Dr. Dirk de Beer.

A biological methane filter
It is believed that a large part of the methane does not reach the atmosphere, because it is mostly microbially converted to CO2 in the seabottom. This oxidation is driven by sulfate, by very slow growing bacteria that divide every 3-6 months. This microbial filter is therefore very sensitive for disturbances by eruptions. The microbial filter works only efficiently when the gas flow is slow and steady. When the gas flows fast, it will simply pass the filter into the water column. An eruption would perturb the sediment and the filter gets totally destroyed.

The long-term observatory
To see the occurrence and frequency of eruptions, the scientists placed a platform with a variety of sensors on the Haakon Mosby Mud Volcano at 1200 meter depth. The volcano has a diameter of 1 kilometer and an elevation of only 10 m. While the bottom water is ice-cold, inside the sediment the temperature increases rapidly. Dr. Tom Feseker (MARUM center for marine environmental sciences, University of Bremen): „We measured in the center of the volcano at 1 meter depth a temperature of 25oC. The heat is carried from large depth by gas-rich fluids. “
The scientists aimed with the LOOME observatory to find out if the gas hydrates in the seabottom could be disintegrated by this heat. In 2009 LOOME was deployed by the deep sea robot Quest (MARUM). The sensors were placed on the most active area, connected by cables to the observatory. The thermometers showed several heat pulses, gas was liberated and elevated the seabottom in pulses by over 1 meter. The sediments slipped sideways over several hunderds of meters. After the series of eruptions, the seabottom subsided gradually to its original elevation.

10 times more methane than thought
Dr. Dirk de Beer from the Max-Planck-Institute for Marine Microbiology and scientific leader of the LOOME project explains: „The eruptions are driven by gas that originates from large depths. Also the heat destabilizes the gas hydrates in the surface of the volcano and the methane gets liberated. The gas reaches finally the water column. We calculated that to drive these eruptions at least 10 times more gas is emitted that previously assumed. Most of the gas may not reach the atmosphere, but gets diluted in the seawater where it can be oxidized by aerobic bacteria.“

A study of the bathymetric maps made since 2003 showed indeed drastic changes of the seabottom topography. Remarkably, no signs were observed of mud flowing out of the volcano. That means that the moving mud is recycled inside the volcano. The horizontal movement of the mud could precisely be reconstructed, as a 20 m long and 2 ton heavy temperature lance moved in the year a distance of 165 m.
An important conclusion of the study is that the microbial filter is destroyed regularly by the eruptions, and has no time to grow back in between. Prof. Dr. Antje Boetius, cruise leader during deployment and recovery, says: „We have learned a lot from the year-long observations. As it was the first in its kind, we got unique data on the volcano and its influence on the environment. Since eruptions of such volcanoes can cause large avalanches and form significant sources of methane, we should deploy more long-term observatories.“

Eruption of a deep-sea mud volcano triggers rapid sediment movement
Tomas Feseker, Antje Boetius, Frank Wenzhöfer, Jerome Blandin, Karine Olu, Dana R. Yoerger, Richard Camilli, Christopher R. German, Dirk de Beer.
Nature Communications, November 2014, DOI: NCOMMS6385

Dr. Manfred Schloesser Presse- und ÖffentlichkeitsarbeitMax-Planck-Institut für marine Mikrobiologie

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