The evolution of permafrost during the last climatic cycle in the coastal lowlands and shelf areas of eastern Eurasia




Thick continuous permafrost developed on the large Siberian shelf areas during the Weichselian period due to the up to 120 m lower sea level and the exposure of these areas to the cold temperatures.With the transgression at the beginning of the Holocene warming complex processes of the interaction of sea water with the permafrost landscape occurred.The transformation of terrestrial to offshore permafrost in the Laptev Sea region has been studied during the last years within the framework of the Russian-German program "System Laptev Sea".Field andlaboratory studies as well as mathematical simulations of permafrost evolution and the gas hydrate stability zone were carried out.Ice-bearing and ice-bonded permafrost exists presently within coastal lowlands and under the shallow shelf.Open taliks,giving pathway to methane from unstable former gas hydrates,can develop from modern and palaeo-river taliks in active fault zones and from lake taliks over fault zones or lithospheric blocks with a higher geothermal heat flux.Ice-bearing and ice-bonded permafrost,as well as the zone of gas hydrate stability in the northern regions of lowlands and in the inner shelf zone,have been preserved during at least four Pleistocene climatic and glacial-eustatic cycles.Presently,they are subjected to degradation from the bottom under the impact of geothermal heat flux.In addition to the development of permafrost during the last glacial cycles,and the development of the gas hydrate stability zone with time,we will show the processes causing coastal erosion and changing the permafrost topography before and during the Holocene transgression of the large shelfes.About 13 Kyr BP thermokarst started to destroy the so called Ice Complex(IC) both on the shelf and coastal lowlands.Thermokarst lakes and depressions were formed(11-11.5)-(9.5-8.5)Kyr BP when the position of the shoreline was at the actual isobathes-60-45 m.Thermokarst processes started before the submergence of the shelf by sea water with negative temperature.Due to the transgression thermokarst lakes and alases with a bottom level below the actual sea level particulary on the shallow part of the shelf(between isobath- 20 m and the recent shoreline) were transformed into "thermokarst lagoons".On the Bykovsky Peninsula and the vast areas eastward from the Yana river delta this process still occurs today and the resulting relief types can be found in the field.The formation of thermokarst lagoons resulted in the formation of an irregular coastline,an increase of shores length subjected by thermoerosion,especially after 7.5 kyr BP.


Evolution of permafrost;;climatic cycle;;coastal lowlands;;shelf areas


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