"This very old, perennially frozen carbon is clearly subject to thawing," Örjan Gustafsson of Stockholm University, Sweden, told environmentalresearchweb. "The thawed soil organic matter appears to be readily available for degradation to carbon dioxide and release to the coastal Arctic Ocean. While the total fluxes from this strip of Siberian Arctic coast are not large compared with global carbon-dioxide releases from fossil-fuel combustion, the process is clearly underway and the total Yedoma carbon stock is large."

Yedoma – or carbon-rich Ice Complex – permafrost, which formed during the last Ice Age, contains a large amount of ice and roughly 1–5% carbon. Yedoma covers around one million square kilometres along the 7000-km coastline of the East Siberian Arctic Shelf.

Gustafsson and colleagues from Sweden, Russia, the US, Denmark and Switzerland studied the eroding permafrost slopes of Muostakh Island in the south-eastern Laptev Sea. They found that Yedoma carbon makes up 57% of the sedimentary carbon budget of the East Siberian Arctic Shelf, overwhelming the components from marine sources and river-carried debris from inland vegetation and soils.

Carbon-isotope studies of the sediments suggested that Ice Complex permafrost releases roughly 44 Tg of old carbon each year, an order of magnitude more than previous studies had suggested. About two-thirds escapes to the atmosphere as carbon dioxide while the remainder is re-buried in shelf sediments, the researchers estimated.

"There are very few mechanisms in the Earth system that can cause a net transfer of carbon from land or ocean to atmosphere over the 100 year timescale," said Gustafsson. "The remobilization of long-term frozen carbon from Arctic permafrost into the atmosphere is recognized as such a key candidate to cause a positive feedback mechanism to a warming climate."

Frozen surficial permafrost in the Arctic contains about half of the carbon held in soils worldwide. Scientists estimate that terrestrial Arctic permafrost holds around 1,000 Pg of carbon, Ice Complex (coastal and inland) permafrost contains around 400 Pg, and subsea permafrost locks up about 1,400 Pg. The atmosphere contains roughly 760 Pg of carbon in the form of carbon dioxide and about 3.5 Pg as methane.

"This study adds to previous reports on extensive methane releases from collapsing subsea permafrost on the East Siberian Arctic Shelf published by the team," said Gustafsson. "In order to project the future trajectory of greenhouse gases in the atmosphere, it is important to study the interaction of a warming climate and releases from the enormous carbon pools held in coastal and subsea permafrost, as well as in methane hydrates, on the East Siberian Arctic Shelf."

The team has now received funding for a 90-day expedition to the East Siberian Arctic Ocean (ESAO) onboard the icebreaker Oden in 2014 as part of the Swedish–Russian–US Investigation of Carbon–Climate–Cryosphere Interarctions in the Arctic Ocean (SWERUS-C3). "Key foci will be carbon-release processes from coastal and subsea permafrost and from methane hydrates in the sediments of the ESAO," said Gustafsson.

The researchers reported their work in Nature.