"Our results suggest that degradation of subsea permafrost in the ESAS currently very likely occurs on a wider scale than was previously thought," Natalia Shakhova told environmentalresearchweb. "Specifically, it was [previously] considered that in the areas of the ESAS shallower than 60–70 metres subsea permafrost is stable, continuous and impermeable for gases. We have shown that areas of the ESAS affected by thermokarst [permafrost melting], submerged taliks and some other processes could serve as migration pathways for methane to escape to the water column and further to the atmosphere."

The permafrost beneath the Laptev Sea in the Dmitry Laptev Strait has experienced many changes in sea level, becoming terrestrial for some periods of its lifetime; it was last inundated about 7000–8000 years ago to a maximum depth of around 15 metres. This resulted in an increase in average temperature of the permafrost from around –17 °C to just below freezing. The team believes that global climate change has caused additional warming to the subsea permafrost by raising the temperature of river run-off entering the ocean. The permafrost is also experiencing geothermal heating from the rift zone below. 

Shakhova and Nicolsky believe that the development of open taliks – unfrozen regions – in the permafrost at sites where thaw lakes and river palaeo valleys were submerged is enabling methane to escape.

"Generally speaking, destabilization of subsea permafrost means that it fails to further prevent methane leakage from seabed deposits of methane stored in the ESAS," said Shakhova. "This provides the global carbon budget with a previously unconsidered and very specific type of methane source."

Unlike other terrestrial and marine sources, which gradually release methane as it forms, the shelf is emitting methane that has accumulated in seabed deposits for hundreds of thousands of years and until now was restricted by permafrost, says Shakhova.

"As methane has been permanently originating in the seabed since it was formed, these deposits are huge and emissions of this ready-to-go methane to the water column only depend on occurence of migration pathways (provided or not provided by permafrost)," she said. "These emissions could be non-gradual, sudden, more or less massive, they could even be abrupt."

The methane released from ESAS does not become oxidized by microbes as it passes through the water column, unlike methane released from the oceanic hydrate deposits found at depths of more than 700 m. "In the ESAS this bio-filter does not work because the water is very shallow – mean depth is less than 50 m – and there is just not enough time for oxidation," said Shakhova.

"Moreover, Arctic shallow hydrate deposits are three times more sensitive to warming than oceanic deposits," said Shakhova. "This means that three times less energy (provided by warming) is required to destabilize them compared to deep oceanic hydrates."

Now the researchers, who reported their work in Environmental Research Letters, plan to model further the current state of subsea permafrost over the entire area of the ESAS using their original multi-year data on sea bottom temperatures and some additional data.

Link:  http://environmentalresearchweb.org/cws/article/news/42221