Abiotic methane forms through a process called serpentinization, which 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, said Joel Johnson, an associate professor at the University of New Hampshire, lead author, and visiting scholar at CAGE. The optimal temperature range for serpentinization of ocean crust is 200 – 350 ˚C.

He noted that current geophysical data from the flank of the ultraslow-spreading Knipovich ridge in the deep Fram Strait of the Arctic Ocean, described in their paper, shows that the Arctic environment is ideal for this type of methane production.

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.

—co-author and CAGE director Jürgen Mienert

Methane produced by serpentinization can escape through cracks and faults, and end up at the ocean floor. In the Knipovich Ridge in the Arctic, it is trapped as gas hydrate in the sediments.

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.

—Joel Johnson

Ultra-slow spreading ocean ridges were discovered in the Arctic in 2003 by scientists at Woods Hole Oceanographic Institution. They found that for large regions 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. Illustration: Dr. Henry J.B. Dick, WHOI/nsf.gov. Click to enlarge.

Another peculiarity about the Knipovich Ridge is that because it is so slowly spreading, it is covered in sediments deposited by the 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.

—Stefan Bünz of CAGE, co-author

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.

Our geophysical results suggest that abiotic-dominated gas–gas hydrate systems can initiate, develop, and survive on tectonic time scales near young, sedimented, ultraslow- spreading mid-ocean ridge transform intersections. These active tectonic environments may not only provide an additional, serpentinized crustal 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. Future scientific ocean drilling and isotopic characterization of the recovered gases is necessary to quantify the proportion of biotic and abiotic gases stored in these deep-water reservoirs throughout the ultraslow-spreading Arctic Ocean ridges.

—Johnson et al.

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

—Jürgen Mienert

Resources

• Joel E. Johnson, Jürgen Mienert, Andreia Plaza-Faverola, Sunil Vadakkepuliyambatta, Jochen Knies, Stefan Bünz, Karin Andreassen, and Bénédicte Ferré (2015) “Abiotic methane from ultraslow-spreading ridges can charge Arctic gas hydrates,” Geology doi: 10.1130/G36440.1