Permafrost

116
Present-day Thermokarst Development in Central Yakutia
Alexander Fedorov 1 , Masami Fukuda 2 , Pavel Konstantinov 1 , Nikolai Bosikov 1 , Go Iwahana 2
(1.Melnikov Permafrost Institute SB RAS, Yakutsk, Russia; 2.Institute of Low Temperature Science,
Hokkaido University, Sapporo, Japan)
Abstract: As part of the joint Russian-Japanese projects, investigations on post-disturbance
permafrost dynamics are being conducted. Thermokarst research is a major component in these
investigations. Thermokarst studies are carried out at the Yukechi, Neleger, Kerdyugen and
Umaibyt sites near Yakutsk. The sites are heavily underlain by ground ice and contain
numerous alases.
Geographically, thermokarst development occurs primarily on level surfaces with
gradients up to 1-2 о and on gentle slopes of 2-3 о at the border between open sites and forest
stands where a frozen barrier resulting from the difference in thaw depth between these
landscapes promotes the accumulation of surface water.
In Central Yakutia, high thermokarst activity is observed at present in previously disturbed
and treeless landscapes related to the regional increase in air temperature. The development of
young thermokarst features is especially intensive. Dry thaw depressions are becoming wet,
young thermokarst lakes are enlarging, taliks are growing below the lakes, and ground
subsidence is occurring in the surrounding terrain.
The results of monitoring observations of thermokarst development indicate that the
largest rates of surface subsidence over the period of 1992 to 2005 occurred in depressions with
incipient thermokarst lakes (up to 25 cm/yr). In the central parts of wet thaw depressions with
relative depths of 2 to 2.5 m, the average rates of subsidence were 5-10 cm/yr, in some of which
incipient lakes tend to develop. No or very little surface subsidence was observed in dry gently
sloped depressions with relative depths of up to 1m.
At present, conditions for thermokarst development do exist. In treeless landscapes with
the dry active layer, the depth of seasonal thaw frequently reaches the top of ice wedges,
resulting in ground subsidence. Ground subsidence is significant where hummocky polygonal
topography has begun to develop, having the average rates of up to 5 cm/yr. Flat areas that were
not previously subject to thermokarst are also subsiding with the rates of up to 1 cm/yr.
Current disturbances, primarily tree cutting and fire, are the major causes of thermokarst
development. Our observation at the Neleger site indicate, however, that the rapid thawing of
ground ice and ensuing ground subsidence up to 10-15cm may cease during the first few years
after disturbance. This is related to the active layer properties and winter meteorological
conditions. This is a common phenomenon, since polygonal features, evidence of the early
stages of thermokarst, are frequently encountered in the recovered landscapes.
Observations on the dynamics of initial thermokarst forms in Central Yakutia indicate that
the process has intensified during the last decades when the increase in air temperature has been
the greatest. However, the initiation of thermokarst under present-day conditions depends on
many factors, in addition to geocryological ones. The self-restoration mechanism in permafrost
landscapes is strong, so thermokarst development is limited in area and is confined to sensitive
sites.
Key words: Thermokarst, surface subsidence, depression, permafrost.