Deep-sea methane remains in the ocean

Researchers examine methane sources in the Indian Ocean

The seabed in the study area off Pakistan is partially covered with shells and crabs. © MARUM, University of Bremen
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Methane, which escapes from the seabed, usually does not escape into the atmosphere. It dissolves in seawater and therefore does not affect the global climate. This is what Bremen researchers discovered during an expedition to the northeastern Indian Ocean. They also found that alone on the Makran Ridge there, about 640, 000 kilograms of methane from the subsurface rise into the water every year, as reported in the journal "Journal of Geophysical Research".

Methane bubbles out of the seabed in many places. The gas also reaches the surface in the Black Sea, the Gulf of Mexico and the North Atlantic. During an expedition with the German research vessel METEOR, a research team now investigated methane emissions on the Makran continental margin off Pakistan. The 400- to 500-kilometer-wide marine slope stretches over 1, 000 kilometers along the Iranian-Pakistani coast and consists for the most part of parallel mountain ridges. "The underwater ridge system covers an area as big as Sweden, " says Miriam Römer of the MARUM Center for Marine Environmental Sciences. "At its center, we have systematically recorded the seabed in a long, 50-kilometer wide strip with our ship's echos and our diving robot MARUM-QUEST [...]. We found a total of 18 sources of methane; Twelve of them were active. "

Hardly affect the climate

From the sonar data, it can be seen that the methane emanating from the seabed can form up to 2, 000 meters high gas flags. Although the gas bubbles rise towards the surface, the measurements also show that the gas plumes evaporate above 700 meters sea depth: "The methane dissolves in the seawater. So it does not escape into the atmosphere and has no impact on the global climate, "summarizes Miriam Römer.

The results are due to many oceanic methane leaks, but not all transferable stresses the geoscientist. For example, there are indications that gas bubbles encased in an oil film can transport methane from deep water to the atmosphere. The researchers want to investigate this in a further study in the Gulf of Mexico. Because there are known natural oil and methane leaks on the seabed.

1.6 liters of methane per minute

The gripper arm of the MARUM-QUEST (right) releases a funnel or the "Bubblmeter", with which the escaping gas bubbles are captured and measured. MARUM, University of Bremen

The Bremen scientists also examined the total amount of gas emitted. Using the HD cameras installed on the dive robot, they found that the gas bubbles averaged about half a centimeter in diameter. In addition, the springs are bubbling differently. Some give only 90 milliliters, others up to 1.6 liters of methane per minute into the sea, as the researchers report. Estimating how much methane is exiting the region altogether or making forecasts is difficult, as the sources are very unstable. If one dies, another is already forming elsewhere. "According to conservative calculations, around 640, 000 kilograms of methane are emitted annually at the Makran Ridge, " says Miriam R mer. display

The cameras also provided images of the unusual habitat: "In some places, the deep-sea floor was covered in tons of shells, crabs and tube warmers, " says R mer. These deep-sea dwellers could only survive because of methane, which feeds many microorganisms. These microorganisms form the basis for a fascinating ecosystem without any light (doi: 10.1029 / 2011JC007424).

(JOURNAL OF GEOPHYSICAL RESEARCH, 09.11.2012 - NPO)