Understanding the Permafrost–Hydrate System and Associated Methane Releases in the East Siberian Arctic Shelf

Abstract

This paper summarizes current understanding of the processes that determine the dynamics of the subsea permafrost-hydrate system existing in the largest, shallowest shelf in the Arctic Ocean; the East Siberian Arctic Shelf (ESAS). We review key environmental factors and mechanisms that determine formation, current dynamics, and thermal state of subsea permafrost, mechanisms of its destabilization, and rates of its thawing; a full section of this paper is devoted to this topic. Another important question regards the possible existence of permafrost-related hydrates at shallow ground depth and in the shallow shelf environment. We review the history of and earlier insights about the topic followed by an extensive review of experimental work to establish the physics of shallow Arctic hydrates. We also provide a principal (simplified) scheme explaining the normal and altered dynamics of the permafrost-hydrate system as glacial-interglacial climate epochs alternate. We also review specific features of methane releases determined by the current state of the subsea-permafrost system and possible future dynamics. This review presents methane results obtained in the ESAS during two periods: 1994-2000 and 2003-2017. A final section is devoted to discussing future work that is required to achieve an improved understanding of the subject.Significant reserves of CH 4 are held in the Arctic seabed [10], but the release of CH 4 to the overlying ocean and, subsequently, to the atmosphere has been believed to be restricted by impermeable subsea permafrost, which has sealed the upper sediment layers for thousands of years [11]. In the regions where permafrost exists, hydrate-bearing sediment deposits can reach a thickness of 400 to 800 m [12,13]. Shallow hydrate deposits are predicted to occupy~57% (1.25 × 10 6 km 2 ) of the East Siberian Arctic Shelf (ESAS) seabed [14]. It has been suggested that destabilization of shelf Arctic hydrates could lead to large-scale enhancement of aqueous CH 4 , but this process was hypothesized to be negligible on a decadal-century time scale [15].Consequently, the continental shelf of the Arctic Ocean (AO) has not been considered as a possible source of CH 4 to the atmosphere until very recently [16][17][18].The key area of the AO for atmospheric venting of CH 4 is the East Siberian Arctic Shelf (ESAS). The ESAS covers greater than two million square kilometers (equal to the areas of Germany, France, Great Britain, Italy, and Japan combined). This vast yet shallow region has recently been shown to be a significant modern source of atmospheric CH 4 , contributing annually no less than terrestrial Arctic ecosystems [19,20]; but unlike terrestrial ecosystems, the ESAS emits CH 4 year-round due to its partial openness during the winter when terrestrial ecosystems are dormant [21]. Emissions are determined by and dependent on the current thermal state of the subsea permafrost and environmental factors controlling permafrost dynamics [21,22]. Releases could potentially increase by 3-5 orders of magnitude,...

Role of Warming in Destabilization of Intrapermafrost Gas Hydrates in the Arctic Shelf: Experimental Modeling

“…According to available field data [39], the Arctic subsea permafrost (e.g., that in the Laptev shelf) has low salinity, and the gas hydrates it stores may dissociate rapidly even upon minor warming. The critical temperature in non-saline sediments was predicted to be as low as t d = −0.3 • C, while the Laptev shelf permafrost may become 0.5 • C warmer already in a few decades [26,62]. Thus, warming of subsea permafrost in the Arctic shelf to −0.5 • C to −1 • C may trigger large-scale gas hydrate dissociation in the near future.…”

“…The gases in the Arctic shelf are often attributed to increasing microbial methane generation, migration of gas through taliks and faults, as well as to decomposition of intrapermafrost and subpermafrost gas hydrates during progressive degradation of subsea permafrost [11,[16][17][18][19][20][21]. The dissociation of hydrates related to subsea permafrost degradation has been largely discussed lately as the main mechanism maintaining the emanation of methane [4,14,18,[22][23][24][25][26].…”

“…The Arctic shelf permafrost has been mapped recently with reference to modeling results, well log and core data, and geophysical surveys [36][37][38][39][40][41]. The modeling predicts that the Arctic shelf permafrost may be as thick as 700 m, which is favorable for the formation and preservation of intra-and subpermafrost gas hydrates [4,26,42,43].…”

“…Thus, the presence of intrapermafrost gas hydrates may account for the persistence of permafrost in the Arctic shelf and for the gradientless temperature distribution observed in some boreholes in the Laptev shelf [4]. The liberated methane passes into sea water, sinks to the bottom, and becomes a source of gas emission [26].…”

Comment on “Understanding the Permafrost–Hydrate System and Associated Methane Releases in the East Siberian Arctic Shelf”

“…The recent 2019 paper in Geosciences, "Understanding the Permafrost-Hydrate System and Associated Methane Releases in the East Siberian Arctic Shelf", by Shakhova, Semiletov, and Chuvilin [1], (henceforth "S2019"), contains a number of false statements about our 2016 paper published in Geophysical Research Letters "Methane fluxes from the sea to the atmosphere across the Siberian shelf seas" [2], (henceforth "T2016"). Here, we address these statements in the order they appear in S2019.…”