13/04/2015 New source of methane discovered in the Arctic Ocean

Svalbard_Fram Strait_Knipovich

A reservoir of abiotic methane has been discovered in the Arctic Ocean. This means that there is more of the greenhouse gas trapped under the seabed than previously thought.

By: Maja Sojtaric

Methane, a highly effective greenhouse gas, is usually produced by decomposition of organic material, a complex process involving bacteria and microbes.

But there is another type of methane that can appear under specific circumstances: Abiotic methane is formed by chemical reactions in the oceanic crust beneath the seafloor.

New findings by a team of CAGE scientists show that deep water gas hydrates, icy substances in the sediments that trap huge amounts of the methane, can be a reservoir for abiotic methane. One such reservoir was recently discovered on the ultraslow spreading Knipovich ridge, in the deep Fram Strait of the Arctic Ocean. The study suggests that abiotic methane could supply vast systems of methane hydrate throughout the Arctic.

The results were recently published in Geology online and will be featured in the journal´s May issue.

Previously undescribed

Joel Johnson

Associate professor Joel Johnson, visiting scholar at CAGE.

“Current geophysical data from the flank of this ultraslow spreading ridge shows that the Arctic environment is ideal for this type of methane production. “ says Joel Johnson associate professor at the University of New Hampshire (USA), lead author, and visiting scholar at CAGE.

This is a previously undescribed process of hydrate formation; most of the known methane hydrates in the world are fueled by methane from the decomposition of organic matter.

“It is estimated that up to 15 000 gigatonnes of carbon may be stored in the form of hydrates in the ocean floor, but this estimate is not accounting for abiotic methane. So there is probably much more.” says co-author and CAGE director Jürgen Mienert.

Life on Mars?

NASA has recently discovered traces of methane on the surface of Mars, which led to speculations that there once was life on our neighboring planet. But an abiotic origin cannot be ruled out yet.

On Earth it forms through a process called serpentinization.

“Serpentinization occurs when seawater reacts with hot mantle rocks exhumed along large faults within the seafloor. These only form in slow to ultraslow spreading seafloor crust. The optimal temperature range for serpentinization of ocean crust is 200 – 350 degrees Celsius.” says Johnson.

Methane produced by serpentinization can escape through cracks and faults, and end up at the ocean floor. But in the Knipovich Ridge it is trapped as gas hydrate in the sediments. How is it possible that relatively warm gas becomes this icy substance?

“In other known settings the abiotic methane escapes into the ocean, where it potentially influences ocean chemistry. But if the pressure is high enough, and the subseafloor temperature is cold enough, the gas gets trapped in a hydrate structure below the sea floor. This is the case at Knipovich Ridge, where sediments cap the ocean crust at water depths up to 2000 meters. “ says Johnson.

 

Stable for two million years

Scientists have discovered a new "ultra-slow" class of ocean ridge involved in seafloor spreading. Investigations in the remote regions of the planet—in the far south Atlantic and Indian Oceans and the sea floor beneath the Arctic icecap—found that for large regions there, the sea floor splits apart by pulling up solid rock from deep within the earth. These rocks, known as peridotites (after the gemstone peridot) come from the deep layer of the earth known as the mantle.Dr. Henry J.B. Dick, Woods Hole Oceanographic Inst.

At ultra slow spreading ridges plates spread away from each other very slowly so that the mantle is exposed. Knipovich Ridge has a spread rate of less than 1.5 cm per year. A new class of these ridges was discovered as late as 2003 by scientists at Woods Hole Oceanographic Institution. Illustration: Dr. Henry J. B. Dick , WHOI/nsf.gov

Another peculiarity about this ridge is that because it is so slowly spreading, it is covered in sediments deposited by fast moving ocean currents of the Fram Strait. The sediments contain the hydrate reservoir, and have been doing so for about 2 million years.

“ This is a relatively young ocean ridge, close to the continental margin. It is covered with sediments that were deposited in a geologically speaking short time period –during the last two to three million years. These sediments help keep the methane trapped in the sea floor.” says Stefan Bünz of CAGE, also a co-author on the paper.

Bünz says that there are many places in the Arctic Ocean with a similar tectonic setting as the Knipovich ridge, suggesting that similar gas hydrate systems may be trapping this type of methane along the more than 1000 km long Gakkel Ridge of the central Arctic Ocean.

The Geology paper states that such active tectonic environments may not only provide an additional source of methane for gas hydrate, but serve as a newly identified and stable tectonic setting for the long-term storage of methane carbon in deep-marine sediments.

Need to drill

Wikimedia commons

Samples of the gas hydrates will provide more knowledge on abiotic methane. But they need to be drilled, as they are 140 meters under the ocean floor. Photo: Wikimedia Commons.

The reservoir was identified using CAGE’s high resolution 3D seismic technology aboard research ressel Helmer Hanssen. Now the authors of the paper wish to sample the hydrates 140 meters below the ocean floor, and decipher their gas composition.

Knipovich Ridge is the most promising location on the planet where such samples can be taken, and one of the two locations where sampling of gas hydrates from abiotic methane is possible.

“ We think that the processes that created this abiotic methane have been very active in the past. It is however not a very active site for methane release today. But hydrates under the sediment, enable us to take a closer look at the creation of abiotic methane through the gas composition of previously formed hydrate.” says Jürgen Mienert who is exploring possibilities for a drilling campaign along ultra-slow spreading Arctic ridges in the future.

 

Methane is second most prevalent greenhouse gas, 28 times as potent as CO2.

Living organisms produce more than 90 percent of known methane on Earth. Man made emissions account for 50–65% of the atmospheric methane on a global scale.
Barack Obama, announced in January that USA is planning to cut methane emissions from its booming oil and gas industry by as much as 45% over the next decade.
Atmospheric concentrations of methane are currently at over 2000 parts per billion (ppb), four times as high as the preindustrial levels. The methane in the atmosphere increased rapidly in the 18th century and paused in the 1990s. Methane levels began to rise again more recently, potentially from a combination of increased anthropogenic and natural emissions.
Large amounts of methane are stored in the form of hydrates in continental margins worldwide, particularly in the Arctic. Gas hydrate consists of ice-like crystalline solids of water molecules encaging gas molecules, and is often referred to as ‘the ice that burns’.
Many view gas hydrates as a potential energy source. But warming oceans can easily destabilize methane hydrates, causing the gas to escape from the ocean floor, into the water column and potentially into the atmosphere.

 

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