Recently a team from Russia, the US, and Sweden found that the East Siberian Arctic Shelf (ESAS) is releasing around 8 teragrams of methane from subsea sediments each year. Now team member Natalia Shakhova and colleague Dmitry Nicolsky have come up with a new model for the Dmitry Laptev Strait region of the shelf to explain exactly how the methane is escaping through the permafrost layer above it.
"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
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 , 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.
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http://environmentalresearchweb.org/cws/article/news/42221
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