I'll try it a second time, but much quicker
Climate and Permafrost
Climatic Change and Evolution of Permafrost in the Past
Global cooling of climate and perennial freezing of the ground began in the Northern Hemisphere in the late Neogene-early Pleistocene (see above; Baulin and Danilova, 1988; French, 1996). Evolution of the cryolithozone in Russia has been determined not only by climatic changes, but also by cover glaciations and marine transgressions and regressions during the Quaternary (Velichko, 1973; Kudryavtsev et al., 1978b; Baulin and Danilova, 1988). In the western part of the Russian cryolithozone (westwards of the Yenisey), permafrost developed in response to climatic and glacial oscillations in the Quaternary, and marine transgression in the middle Pleistocene and at the beginning of the late Pleistocene. Eastern Siberia has never been glaciated, except for the mountains, which accounts for the most broad extent of permafrost. In the Pleistocene, cooling of climate was accompanied by marine regressions, and arid permafrost conditions in Eastern Siberia were at their extreme (Geocryology of the USSR: Eastern Siberia and the Far East, 1989). The presence of the relict ice wedges confirms a steady permafrost trend. In northeastern Siberia, permafrost has existed uninterruptedly throughout the last 1.5-2 million years, while in Europe and Western Siberia in the Quaternary permafrost developed and thawed repeatedly. There is little evidence on the early stage of permafrost evolution. Only four main stages of Quaternary permafrost variations can be reconstructed: (1) from the early Pleistocene to the beginning of the late Pleistocene; (2) the late Pleistocene; (3) the first half of the Holocene; (4) the late Holocene (Baulin and Danilova, 1988).
The duration of the first stage exceeded 1.5 million years, during which the spread and thickness of permafrost were continuously increasing (Baulin and Danilova, 1988). The first perennial frozen ground was formed in the north of Central and Eastern Siberia and in the Taymyr peninsula. During the early Pleistocene (approximately 2-0.7 millions years ago) permafrost developed across Western Siberia (reaching 54-55° N) and central and western Sakha-Yakutia (Geocryology of the USSR: Western Siberia, 1989). The ice-wedge pseudomorphs and ice-wedge casts of Western Siberia and the relict ice wedges of north-eastern Siberia are testimonies of ancient permafrost. During the glacial period of the middle Pleistocene, transgression of the Arctic Ocean extended to the European north-east and northern Western Siberia, while a continental ice sheet occupied vast areas of Europe (Velichko, 1973; Baulin and Danilova, 1988). Permafrost extended southwards of the glacial sheet and coastline, and the southern limit of permafrost reached 50°N in Europe and Kazakhstan.
At the same time, syngenetic deposits with high ice content and thick ice wedges developed in northeastern Siberia and central Sakha-Yakutia (Regional Cryolithology, 1989). During the interglacials of the middle Pleistocene and of the beginning of the late Pleistocene, ice sheets disappeared and permafrost became more limited in extent in Europe and the coastal sector of the Siberian Seas (Velichko, 1973).
At the second stage of its evolution (from 150-90 to 11-10 thousand years ago), permafrost attained the maximum territorial extent, thickness, and the minimum temperatures in Northern Eurasia. On the European and Siberian plains, the southern limit of permafrost extended to 48-49°N, where its thickness could attain 300-600 m (Velichko, 1975; Kondratjeva et al., 1993). For the last cold period in Northern Eurasia a magnitude of temperature depression of 5-9°C is suggested (Vasilchuk, 1992) while the lowest ground temperatures may have reached -22°C to -25°C in northern Sakha-Yakutia (Kaplina, 1981). Syngenetic moraine sediments containing ice wedges formed in the nearshore areas of Western Siberia and syngenetic alluvial deposits with ice wedges developed in north-eastern Siberia (Kondratjeva et al., 1993). Europe and Siberia experienced cold non-glacial conditions and were the major areas for the development of the late Pleistocene periglacial and permafrost environments.
The third stage or the Holocene climatic optimum (from 10 to 4.5-3 thousand years ago) was the warmest and driest. Global climatic warming and thawing of permafrost in the contemporary mid-latitudes characterized the Holocene optimum in Eurasia (Baulin and Danilova, 1988). Air temperature increased up to 4.4-5.2°C in the high latitudes (Velichko, 1975; Velichko and Nechaev, 1992). The southern limit of permafrost coincided with the Arctic Circle in Western Siberia, ran along 60-61°N in Eastern Siberia, diving south and encirled the Transbaikalian mountains. One of the most distinct manifestations of spatial permafrost decrease is the presence of a thaw layer above the second relict layer of the Pleistocene permafrost in the European north-east and Western Siberia. In Western Siberia, the depth of perennial thaw layer increased from 30-50 m at the Arctic Circle to 50-100 m at 64-66°N and more than 100-300 m further south (Velichko, 1975; Velichko and Nechaev, 1992). There were two stages of permafrost degradation in Western Siberia (Astakhov, 1995). The first stage began when surface water bodies warmed enough to perforate permafrost by numerous sinking thermokarst ponds and to produce local taliks. The second stage, which only took place to the south of the Arctic Circle, began when climate became so warm that direct solar heating of the deposits was capable of producing regional taliks or a perennially thawed layer separating the Pleistocene permafrost from the seasonally frozen layer. The main evidence of this stage is widespread and varying thermokarst formed in the contemporary cryolithozone and ice-wedge casts in the area of ancient cryolithozone. Such thermokarst forms as the alas plains of northern Sakha-Yakutia developed when the mean summer air temperature was 4-5°C higher than at present. Ice-wedge casts formed when the retreat of permafrost was accompanied by melting of ice wedges and infilling of fissures with soil at such a slow rate that the shape of wedges was not destroyed.
The fourth stage, which continued for 3500-4000 years in the late Holocene, is characterized by aggregation of permafrost. The southern boundary of permafrost moved several degrees further south, ground temperatures decreased by 1-2°C, and thermal contraction cracking and frost heave intensified (Baulin and Danilova, 1988). The present-day permafrost environment provides numerous examples of late Holocene periglacial modification.
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