Large Methane Plumes Discovered on Laptev Continental Slope Boundary: Evidence of Possible Methane Hydrate Release


(The Swedish Icebreaker Oden — now home to the 80 scientists and tons of equipment of the SWERUS 2014 research expedition aimed at measuring sea floor methane release throughout the Arctic this summer. Among the scientists leading the expedition is Igor Semiletov whose 2011 expedition discovered 1 kilometer wide plumes of methane issuing from the floor of the East Siberian Arctic Shelf. Image source: Commons.)

SWERUS-C3 researchers have on earlier expeditions documented extensive venting of methane from the subsea system to the atmosphere over the East Siberian Arctic Shelf. On this Oden expedition we have gathered a strong team to assess these methane releases in greater detail than ever before to substantially improve our collective understanding of the methane sources and the functioning of the system. This is information that is crucial if we are to be able to provide scientific estimations of how these methane releases may develop in the future (emphasis added). — Örjan Gustafsson

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Over the past few years, the Arctic has been experiencing an invasion.

Emerging from the Gulf Stream, a pulse of warmer than normal water propagated north past Iceland and into the Barents Sea. There, it dove beneath the surface fresh water and retreating sea ice, plunging to a depth of around 200-500 meters where it concentrated, lending heat to the entire water column. Taking a right hand turn along the Siberian Continental Shelf, it crossed through the mid water zones of the Kara. Finally, it entered the Laptev and there it abutted against the downward facing slopes of the submarine continental region.

As the water temperatures at these depths warmed, researchers began to wonder if they would trigger the destabilization of methane hydrate stores locked  in deeper waters along the shelf boundary. And, now, a new expedition may have uncovered evidence that just such an event is ongoing.

Methane Hydrates and Troubling Releases from the East Siberian Arctic Shelf

Oceanic methane hydrates form when methane upon or beneath the sea bed freeze into a crystalline ice lattice. It is a hybrid water-methane mixture that only remains stable at higher sub-sea pressures and lower temperatures. Normally, oceanic hydrates form at great depth (about 600 meters or deeper) where a combination of high pressure and low temperature are the prevailing environmental factor. But the colder Arctic is a sometimes exception to this general rule.

In recent years, deep ocean warming due to human caused climate change has accelerated. It is feared that this warming may unlock vast stores of methane laying frozen along the deep sea bed or in more vulnerable continental shelf slope zones.

This warming is also feared to have begun a process of methane release along a unique submarine feature called the East Siberian Arctic Shelf (ESAS). There rising temperatures are hypothesized to have sped the thaw of submarine permafrost.

Frozen permafrost stores biologically generate gaseous methane at depths of 10-80 meters. Methane hydrate stores are locked away at depths starting at around 100 meters. Submerged beneath only a couple hundred feet of water, these methane stores are much shallower and, therefore, are in a naturally unstable zone.

The East Siberian Sea zone is unique due to the fact that it was only recently flooded, in geological terms. The frozen permafrost has only rested beneath the Arctic Ocean waters since the end of the last ice age and much of it remained frozen due to chill Arctic conditions. But now, human-caused climate change is driving warmer and warmer waters into the Arctic environment.

Elevated Methane ESAS

(Elevated atmospheric methane levels over East Siberian and Laptev Seas during October of 2013. Image source: Arctic News via Methane Tracker)

As the warming progressed during the first decade of the 21st Century, researchers observed what appeared to be an increasing release of methane from these thawing permafrost stores. In 2011, plumes from the sea bed stretching 1 kilometer across were observed by an Arctic expedition headed by Igor Similetov and Natalia Shakova. It appeared that the 250 to 500 gigatons of carbon locked in the ice in that shallow ocean was destabilizing and releasing from the sea floor as methane.

Now it is estimated that about 17 megatons of methane from this store vents through the shallow waters into the atmosphere each year. But this may just be the start of a far larger emission.

Methane Hydrate Release During Past Hothouse Events

Though the ESAS carbon and methane store is arguably one of the most vulnerable to human-caused warming, a far greater store of methane hydrate is estimated to be locked in crystalline ice lattice structures along the world’s continental slope systems and in the world’s deep ocean environments. Since the Earth has been cooling for the better part of 55 million years, a huge store of carbon as methane is now thought to have accumulated there. In total, between 3,000 and 10,000 gigatons of carbon are estimated to be captured in this vast store.

methane bubbles near the Laptev sea surface

(Methane bubbles near the Laptev Sea surface as observed by the SWERUS expedition last week. These bubbles were issuing from what are thought to be destabilizing methane hydrates along the Outer Laptev Continental Slope zone. Image source: Stockholm University.)

Global warming science, especially the science related to paleoclimate, indicates that Earth Systems warming tends to dump a lot of heat into the deep ocean. The atmosphere ocean-interface along the equator warms and becomes salty due to enhanced evaporation. The warmer, saltier water sinks, driving heat into the deep ocean. At the poles, ice sheet melt sends out a wave of fresh water along the ocean surface. The fresh water acts as an insulator between atmosphere and water, locking the warm water beneath the surface and pushing it toward the bottom. This process, called ocean stratification, is, among other things, an ocean heat exchange machine that turns the ocean bottom into a warming-induced house of horrors.

We would expect a similar process to be set in motion through human warming.

Ultimately, this combination of forces results in a collision of warm water with frozen methane stores and serves as a mechanism for their destabilization. If even a portion of this deep ocean methane hits the air, it can further accelerate already rampant warming.

Today, we may be at the start of just this kind of process.

Large Methane Plumes Discovered Along The Laptev Slope Boundary

Last week, large plumes of methane were found to be issuing from the outer Laptev Sea floor at the border zone where the bottom climbs up to meet the East Siberian Arctic Shelf. Researchers on the scientific study vessel Oden found:

elevated methane levels, about ten times higher than in background seawater, [that] were documented … as we climbed up the steep continental slope at stations in 500 and 250 m depth.

Expedition researchers noted:

This was somewhat of a surprise. While there has been much speculation of the vulnerability of regular marine hydrates (frozen methane formed due to high p [pressure] and low T [temperature]) along the Arctic rim, very few actual observations of methane releases due to collapsing Arctic upper slope marine hydrates have been made.

An Ice-Free Laptev Sea

(An ice-free Laptev Sea on July 28, 2014. Last week, researchers discovered a kilometers wide plume of methane bubbling up from the Continental Shelf sea bed in these typically-frozen waters. Image source: LANCE-MODIS.)

Overall the size of the release zone was quite large, covering several kilometers of sea bed and including over 100 methane seepage sites:

Using the mid-water sonar, we mapped out an area of several kilometers where bubbles were filling the water column from depths of 200 to 500 m. During the preceding 48 h we have performed station work in two areas on the shallow shelf with depths of 60-70m where we discovered over 100 new methane seep sites.

Due to the depth and location of the methane above the continental slope zone, researchers hypothesize that the source of the methane is from hydrate stores in the region.

It is worth noting that though it is rare to observe methane releases from the upper slope zone, current science has found destabilizing hydrates in deep water off the US East Coast along the continental shelf slope zone and in deep waters off Svalbard among other places. In addition, satellite observation of the Arctic Ocean has recently shown periods of high and above normal methane readings in the Laptev, Kara and East Siberian Seas. Elevated atmospheric readings have also appeared over the Nares Strait near Greenland. These are all zones that have experienced substantial deep ocean warming over the past few decades.

SWERUS 2014 is now heading toward ESAS waters where so many large methane plumes were discovered in 2011. There, the expedition hopes to use its impressive array of sensors and expertise to better define and understand what appear to be large-scale but not yet catastrophic methane releases underway there.





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