Study Finds East Siberian Arctic Shelf Showing Instability and Widespread Venting of the GHG Methane; Releases May Be Much Larger and Faster Than Anticipated
A section of the East Siberian Arctic Shelf (ESAS) seafloor that holds vast stores of frozen methane (CH4) is showing signs of instability and widespread venting of the gas, according to the findings of an international research team led by University of Alaska Fairbanks scientists Natalia Shakhova and Igor Semiletov. Methane is a greenhouse gas more than 30 times more potent than carbon dioxide.
The permafrost under the ESAS—a 2.1 million square kilometer area which encompasses the Laptev, East Siberian and Russian part of the Chuckchi seas—was long thought to be an impermeable barrier sealing in methane. However, the study, published in the 5 March edition of the journal Science, found that the ESAS is perforated and is starting to leak a large amount of methane—currently about 7.98 teragrams, an amount comparable to the amount currently estimated as coming out of the entire World Ocean.
Although the oceanic CH4 flux should be revised, the current estimate is not alarmingly altering the contemporary global CH4 budget. These findings do change our view of the vulnerability of the large sub-sea permafrost carbon reservoir on the ESAS; the permafrost “lid” is clearly perforated, and sedimentary CH4 is escaping to the atmosphere.
To discern whether this extensive CH4 venting over the ESAS is a steadily ongoing phenomenon or signals the start of a more massive CH4 release period, there is an urgent need for expanded multifaceted investigations into these inaccessible but climate-sensitive shelf seas north of Siberia.
—Shakhova et al.
|“Subsea permafrost is losing its ability to be an impermeable cap.”|
Methane is released from previously frozen soils in two ways. When the organic material (which contains carbon) stored in permafrost thaws, it begins to decompose and, under anaerobic conditions, gradually releases methane. Methane can also be stored in the seabed as methane gas or methane hydrates and then released as subsea permafrost thaws. These releases can be larger and more abrupt than those that result from decomposition.
The East Siberian Arctic Shelf is a methane-rich area that is a shallow seaward extension of the Siberian tundra that was flooded during the Holocene transgression 7 to 15 thousand years ago.
The ESAS sub-sea permafrost, which is frozen sediments interlayered with the flooded peatland, not only contains comparable amounts of carbon as still land-fast permafrost in the Siberian tundra but also hosts permafrost-related seabed deposits of CH4. Moreover, ESAS sub-sea permafrost is potentially more vulnerable to thawing than terrestrial permafrost.
In contrast to on-land permafrost, sub-sea permafrost has experienced a drastic change in its thermal regime because of the seawater inundation. The annual average temperature of ESAS bottom seawater (–1.8° to 1°C) is 12° to 17°C warmer than the annual average surface temperature over onland permafrost. A physical implication of combined bottom-up geothermal and topdown seawater heat fluxes is the partial thawing and failure of sub-sea permafrost and thus an increased permeability for gases. We consequently hypothesized that CH4 is released from seabed deposits to vent extensively to the Arctic atmosphere.
—Shakhova et al.
Earlier studies in Siberia focused on methane escaping from thawing terrestrial permafrost. Semiletov’s work during the 1990s showed, among other things, that the amount of methane being emitted from terrestrial sources decreased at higher latitudes. But those studies stopped at the coast. Starting in the fall of 2003, Shakhova, Semiletov and the rest of their team took the studies offshore.
From 2003 through 2008, they took annual research cruises throughout the shelf and sampled seawater at various depths and the air 10 meters above the ocean. In September 2006, they flew a helicopter over the same area, taking air samples at up to 2,000 meters (6,562 feet) in the atmosphere. In April 2007, they conducted a winter expedition on the sea ice.
They found that more than 80% of the deep water and more than 50% of surface water had methane levels more than eight times that of normal seawater. In some areas, the saturation levels reached more than 250 times that of background levels in the summer and 1,400 times higher in the winter. They found corresponding results in the air directly above the ocean surface. Methane levels were elevated overall and the seascape was dotted with more than 100 hotspots. This, combined with winter expedition results that found methane gas trapped under and in the sea ice, showed the team that the methane was not only being dissolved in the water, it was bubbling out into the atmosphere.
These findings were further confirmed when Shakhova and her colleagues sampled methane levels at higher elevations. Methane levels throughout the Arctic are usually 8 to 10 percent higher than the global baseline. When they flew over the shelf, they found methane at levels another 5 to 10 percent higher than the already elevated Arctic levels.
Shakhova notes that the Earth’s geological record indicates that atmospheric methane concentrations have varied between about .3 to .4 parts per million during cold periods to .6 to .7 parts per million during warm periods. Current average methane concentrations in the Arctic average about 1.85 parts per million, the highest in 400,000 years, she said. Concentrations above the East Siberian Arctic Shelf are even higher.
It was thought that seawater kept the East Siberian Arctic Shelf permafrost frozen. Nobody considered this huge area.
The East Siberian Arctic Shelf, in addition to holding large stores of frozen methane, is more of a concern because it is so shallow. In deep water, methane gas oxidizes into carbon dioxide before it reaches the surface. In the shallows of the East Siberian Arctic Shelf, methane simply doesn’t have enough time to oxidize, which means more of it escapes into the atmosphere. That, combined with the sheer amount of methane in the region, could add a previously uncalculated variable to climate models.
The release to the atmosphere of only one percent of the methane assumed to be stored in shallow hydrate deposits might alter the current atmospheric burden of methane up to 3 to 4 times. The climatic consequences of this are hard to predict.
Shakhova, Semiletov and collaborators from 12 institutions in five countries plan to continue their studies in the region, tracking the source of the methane emissions and drilling into the seafloor in an effort to estimate how much methane is stored there.
Shakhova and Semiletov hold joint appointments with the Pacific Oceanological Institute, part of the Far Eastern Branch of the Russian Academy of Sciences. Their collaborators on this paper include Anatoly Salyuk, Vladimir Joussupov and Denis Kosmach, all of the Pacific Oceanological Institute, and Orjan Gustafsson of Stockholm University.
Natalia Shakhova, Igor Semiletov, Anatoly Salyuk, Vladimir Yusupov, Denis Kosmach, and Örjan Gustafsson (2010) Extensive Methane Venting to the Atmosphere from Sediments of the East Siberian Arctic Shelf. Science 327: 1246-1250 doi: 10.1126/science.1182221