Methane emissions in the East Siberian Arctic Shelf: issues addressed and questions raised

Abstract:

Methane (CH₄) emissions in the East Siberian Arctic Shelf (ESAS) has been studied during the last decade. Our investigation, including observational studies using hydrological, biogeochemical, geophysical, geo-electrical, microbiological, and isotopic methods, and modeling efforts to assess current subsea permafrost state and the ESAS’ contribution to the regional CH₄ budget, have clarified processes driving CH₄ emissions in the ESAS and spilt some light on possible sources involvement. Despite some authors believe that CH₄ fluxes from subsea permafrost in the ESAS will depend on rates of CH₄ production in gradually thawing sediments while subsea permafrost will remain frozen for millennia, results of our investigation showed the opposite. Permafrost failure caused by long-lasting warming by sea water due to sea level rise, deep/open taliks formation due to combined heating effects of seawater, river runoff, geothermal flux, and pre-existing thermokarst and global-change-induced warming, determines destabilization of massive gas reservoirs, leading to large-scale CH₄ releases, including release of pre-formed CH₄ long preserved within/beneath subsea permafrost. Resultant rates of CH₄ emissions over the ESAS vary spatially by 3-5 orders of magnitude. Re-drilling of subsea permafrost performed from the fast ice 30 years after it was first drilled in 1982-1983, revealed modern rates of subsea permafrost degradation. These results contradict previous hypotheses that: 1) taliks beneath thermokarst lakes always freeze after submergence; 2) rates of permafrost degradation after inundation decrease over time; and 3) thousands of years required to form escape paths for permafrost-preserved gas. Involvement of shallow relic hydrates is suggested based on results of experimental work with sediment cores extracted from the near-shore zone of the ESAS. In-situ investigations revealed that dissolved CH₄ could remain in the seawater up to 1000 days, because oxidation rates are low. Storms could release some aqueous CH₄ to atmosphere; dissolved CH₄, captured beneath ice in winter, can spread via currents and escape to atmosphere through breaks in the ice. Progressive subsea permafrost thawing and decreasing ice extent could significantly increase CH₄ emissions from the ESAS.