PERMAFROST AND PERIGLACIAL PROCESSES
Permafrost and Periglac. Process. 15: 81–87 (2004)
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/ppp.473
Short Communication
Thermokarst as a Short-term Permafrost Disturbance, Central Yakutia
Anatoli Brouchkov,1* Masami Fukuda,1 Alexander Fedorov,2 Pavel Konstantinov,2 and Go Iwahana3
1
2
3
Hokkaido University, Kita-ku, Sapporo, Hokkaido, Japan
Permafrost Institute, Russian Academy of Sciences Siberian Branch, Yakutsk-10, Sakha (Yakutia) Republic, Russia
Institute of Low Temperature Science, Hokkaido University, Kita-ku, Sapporo, Hokkaido, Japan
ABSTRACT
The widespread occurrence of alas depressions in Central Yakutia is not necessarily evidence of
modern thermokarst activity. Typically, a near-surface ‘shielding layer’, formed as the result of deep
thaw in exceptionally warm years, protects underlying icy sediment from thaw. In spite of current
climatic warming, there is no noticeable increase in regional thermokarst in Central Yakutia. Periodic
forest fires significantly increase soil thermal conductivity and progressive soil salinization is
occurring; however, these are insufficient to cause thaw of the underlying icy sediments. Instead,
active thermokarst should be regarded as a short-term catastrophic process. Use of the Kudryavtsev
algorithm indicates that the depth of thaw beneath a water body in Central Yakutia can reach 8–10 m
within 50 years, and 20þ m within 300 years. Field observations show that current thermokarst has a
tendency for fast (5–10þ cm/year) subsidence. Copyright # 2004 John Wiley & Sons, Ltd.
KEY WORDS:
permafrost; thermokarst; Yakutia
INTRODUCTION
Central Yakutia is a well-known example of thermokarst terrain (Soloviev, 1973; see French, 1996, 113–
116). Thermokarst processes include the thaw of icerich permafrost, and the formation of depressions,
lakes, and other forms of negative relief. A significant
part (up to 40%) of the land surface of Central Yakutia
has been affected by thermokarst. The area is well
suited to thermokarst because the alluvial terraces of
the Lena River consist of silty and sandy loams with
high ice contents (up to 50–80%) with large ice
wedges exceeding 10 m in vertical extent. These
sediments are termed ‘ice-complex’.
According to Soloviev (1973), thermokarst in Central
Siberia develops in a gradual and sequential way
(Figure 1) that corresponds, in general, to the thawlake cycle described in the North America literature
(e.g. Britton, 1967; Everett, 1981; French, 1996, 121–
* Correspondence to: A. Brouchkov, Hokkaido University,
W8 N9, Kita-ku, Sapporo, Hokkaido, 060-0809, Japan.
E-mail: [email protected]
Copyright # 2004 John Wiley & Sons, Ltd.
123). However, Soloviev (1973) concludes that only a
small amount of thermokarst in Central Siberia is
currently forming. Therefore, is thermokarst development sequential and gradual, or is it rapid, and what
might be the rate of development? A second group of
questions relate to the fact that thermokarst is often
related to either forest fire or ecological changes, and
may not be exclusively controlled by climate.
REGIONAL BACKGROUND
The mean annual air temperature in Central Yakutia is
10 to 11 C and the average amplitude of monthly
temperatures is about 62 C. Annual precipitation is
small, 190 to 230 mm only for the summer season.
Snow cover is 30–40 cm, but may reach 60 cm. In
Yakutia, forest plays a key role in keeping permafrost
temperatures low (Table 1). For example, summer soil
temperatures in the active layer are lower in forested
areas than in open areas. Typical permafrost temperatures at 15–20 m depths are 2.0 C in open areas and
3.5 C in forested areas (Table 2).
Received 6 January 2003
Revised 26 September 2003
Accepted 27 November 2003
82
A. Brouchkov et al.
Figure 1 Stages of thermokarst, Yukechi site, right bank of Lena River, Central Yakutia: (a) the first phase—‘bilar’, or wet depression with
hillocks; (b) development of small ponds or ‘dujodas’; (c) the second stage—‘tiimpi’, or formation of lakes with high banks; and (d) the
third phase—alas with lake. (a)–(c) ¼ the ‘active’ phase; (d) ¼ the ‘stable’ phase.
Icy deposits (the ‘ice-complex’) are located at
depths of between 1.2–3.0 m below the surface. These
underlie more than half of Central Yakutia and exceed
20–25 m in thickness. Vegetation is middle-taiga
dominated by larch forest. The greater part of the
primary forest has been cut down or affected by
fires and is now being recolonized by secondary birch
forest.
THE WATER BALANCE
Differences in the thermal properties of water and soil
were understood at an earlier stage of permafrost
studies in the USSR (e.g. Grave, 1944). Shallow water
bodies favour thawing and a rise of soil winter
temperatures. Therefore, once a depression has appeared, water collects, and soil temperatures increase.
Table 1 Heat balance in larch forest and open area of Central Yakutia from May to August, MJ/m2.
Place
Year
Net radiation R
Forest
1963*
2000
2001
1963*
2000
2001
409.6 (on floor)
1187.6
1452.5
927.9
1091.8
1205.0
Open area
Sensible heat H
Latent heat LE
Ground heat G
242.4
585.4
671
539.2
535.7
457
87.8
389.3
222
275.9
338
177
79.4
156.0
79.6
112.8
220.0
264.6
* Data from Gavrilova (1967); on the floor in the forest, below canopy.
Copyright # 2004 John Wiley & Sons, Ltd.
Permafrost and Periglac. Process., 15: 81–87 (2004)
Thermokarst in Central Yakutia
Table 2
83
Annual mean soil temperatures, Neleger site, Central Yakutia.
Mean annual soil temperature (depth 3.2 m), C
Landscapes
Larch forest
Meadow
1998–99*
1999–00*
2000–01*
2001–02*
3.2
2.0
2.5
1.5
3.6
1.8
3.4
1.7
* The annual period starts on 1 October of the current year and ends on 30 September of the next year.
This causes the thaw of icy deposits that continue until
a thermal equilibrium is reached. This promotes what
is known as ‘self-supporting’ or progressive thermokarst. This reflects a positive water balance on the
surface, caused by either water flow from thawing
permafrost or by surface distribution. In the case of a
negative water balance, thermokarst ceases. However,
according to Lubomirov (1987), surface water does
not always lead to increase of permafrost temperatures and active-layer depths; sometimes, the opposite
occurs due to high winter thermal conductivity of
frozen ice-saturated soils.
A peculiarity of Central Yakutia thermokarst is that
many obvious and well-expressed thermokarst forms
are without active development. Many are thought to
have developed during the Holocene climatic optimum (Kachurin, 1961; Fukuda et al., 1995). Today, a
negative surface water balance, with an average annual precipitation of 240–320 mm and evaporation
exceeding precipitation (Gavrilova et al., 1996), is
the major factor preventing current thermokarst.
Even in north-western Siberia, known for its water
abundance, disturbance of the active layer does not
always lead to an increase in surface water and
ponding. In fact, there are many examples of dryingup of soils (e.g. Grigoriev and Baranovsky, 1990).
‘THE SHIELDING LAYER’
Adjacent to the Lena River, on the Abalakh plain, the
total thickness of near-surface icy deposits is as much
as 15 m. Their average ice content is 50–60% or
more. The depth to the top of the large ice wedges
contained in these sediments is about 2.0–2.5 m.
Therefore, if these sediments were to thaw, significant
thermokarst terrain modification would occur. The
reason why this is not happening relates to annual
changes in the thickness of the active layer. For
example, measurements made over a 5-year period
at Neleger, located about 25 km northwest of Yakutsk,
indicate significant changes in pre-winter moisture
Copyright # 2004 John Wiley & Sons, Ltd.
content (Figure 2a), snow accumulation (Figure 2b),
and annual mean ground temperatures (Figure 2c). As
the result of deep thawing in warm years, a layer of
low-ice-content sediment forms in the uppermost part
of permafrost. This layer, in-between the base of the
active layer and the top of ice wedges, is called the
‘transient layer’ or ‘shielding layer’ (Shur, 1988). It
may be as much as 0.6–0.7 m thick. It functions to
protect underlying icy deposits from thaw. In disturbed areas, the thickness of this layer decreases to
0.1–0.3 m. Data on the thickness and water content of
the shielding layer on the Abalakh plain are presented
in Table 3.
CAUSES OF THERMOKARST TODAY
Possible causes of present-day thermokarst in Central
Yakutia include (a) forest fires and (b) the progressive
salinization of soils. These are discussed below.
In the northern forests of both North America and
Eurasia, it is well known that forest fire alters the
surface albedo, and changes soil properties such as
density, infiltration and evaporation rates, thermal
conductivity and heat capacity (Viereck, 1982; Hinzman
et al., 2001). In Siberia, field observations show that
the Yakutian forest cover makes the soil cooler in
summer (Pavlov, 1984). For example, the active layer
is about 1.2–1.5 m in forested areas near Yakutsk, but
1.8–1.9 m in open areas.
Using Kudryavtsev’s method (Kudryavtsev et al.,
1974), it was concluded that disturbance might cause
the active layer to increase in thickness by about 30–
40 cm and for permafrost temperatures to increase by
approximately 2 C. However, whether this difference
is sufficient to cause icy deposits to thaw will depend
on the properties and thickness of the shielding layer.
Moreover, the tops of most ice wedges in Central
Yakutia are too deep to be affected by a 30–40 cm
increase in active-layer depths.
The history of forest fires is also important. The
fire-return interval is 25 to 70 years in Central Yakutia,
Permafrost and Periglac. Process., 15: 81–87 (2004)
84
A. Brouchkov et al.
Figure 2 Summary of typical field data collected at the Neleger site, Central Yakutia, 1996–2001. (a) Variations in the pre-winter moisture
content of the active layer beneath a larch forest; (b) snow accumulation depth in the forest; (c) variations in mean annual permafrost
temperature in larch and birch forests, and average annual air temperature.
however, the variation is large; the upper limit is 250
to 300 years for wet sites and the lower limit is 7 to
15 years (Global Forest Fire Assessment 1990–2000,
2001). If it is assumed that (a) a maximum of about
40% of the land surface was affected by thermokarst
during the Holocene, and (b) the frequency of fires is
at least every 200 years, the calculated probability of
Copyright # 2004 John Wiley & Sons, Ltd.
thermokarst appearance after fire in Central Yakutia is
about 1%. Should fires occur more often, or a smaller
area is affected by thermokarst, the probability becomes even lower.
Frozen saline soils are widely distributed in Central
Yakutia, where ‘continental’ salinization is caused by
the predominance of evaporation above precipitation.
Permafrost and Periglac. Process., 15: 81–87 (2004)
Thermokarst in Central Yakutia
Table 3
85
Characteristics of the shielding layer on Abalah plane of the right bank of Lena River.
Landscape
Depth of
ice wedges, cm
Active-layer
depth, cm
200
195
185
200
210
225
140
130
155
135
190
220
Larch forest 130–150 years old
Larch forest 80 years old
Larch forest 50 years old
Larch forest 15–20 years old
Grass land between alases
Thermokarst depression
These soils are defined as those containing 0.05%
by weight of soluble salt compared to dry soil
(Brouchkov, 1998). Forested sites are characterized
by a minimum salt content, while salinization increases in alases. Together with a water content
increase, salinization causes vegetation changes, creates a positive feed-back, and accelerates thermokarst.
However, the effect of salinization is slow in comparison with forest fires and takes tens or hundreds of
years.
SPEED OF THERMOKARST
The possibility of catastrophic lake appearance in
permafrost terrain was first demonstrated by Grave
(1944). The time of talik formation can be calculated
using the Kudryavtsev method (Kudryavtsev et al.,
1974). It can be demonstrated that if lake levels are
stable or increasing, thaw reaches 8–10 m over a
period of 50 years, and about 20 m over a period of
300 years. This is sufficient to completely melt the
ice-complex.
Field measurement, started in 1992 in an area of
active thermokarst near Yukechi caused by logging of
trees and changing agricultural land use, indicates that
thermokarst depressions evolve rapidly as ground
temperatures increase (Table 4). For example, in the
central parts of wet depressions with depths of 2–
2.5 m, the rate of subsidence was 5–10 cm/year and
Thickness of the Water content of the
shielding layer, cm
shielding layer
60
65
25
65
10
5
0.19–0.39
0.18–0.37
0.18–0.35
0.20–0.35
0.17–0.33
0.66
adjacent inter-depression sites averaged 2.6–5.4 cm/
year. Elsewhere, on well-drained flat inter-alas surfaces, the rate of subsidence averaged only 0.5–
0.8 cm/year. The magnitude of these values agrees
with earlier rates of thermokarst activity reported
from various localities in Arctic Canada (e.g. French,
1975, 1978, 1984).
The drying-up of the site appears to stop the thermokarst process which then becomes latent. Examples of
this phase of thermokarst are old shallow lakes and
small dry depressions. A stable phase may also occur
if there is no increase in surface temperature, water
collection or drastic vegetation change. Examples of
this phase are flat, treeless areas of agricultural land
with a thick ‘shielding layer’ with a tendency for
vegetation and forest development, and alases that
have finished their growth. In Yakutian terms, the
initial depression is called a ‘bilar’ but this quickly
changes to a ‘dujoda’ or small thermokarst depressions, 10–15 m wide and 1.0–1.3 m deep. Dujodas can
form within 40 years or less, and early stages can be
observed in 10–15 years (Gavrilova et al., 1996).
Bosikov (1998) reports that expansion of thermokarst
lakes may exceed 0.8–4.0 m/year, thus thermokarst
can reach a mature stage within 100–200 years.
The period of alas formation depends on the depth
and thickness of the ‘ice-complex’. Depth of alas
depressions varies from 2–3 m to > 40 m. The time
required for complete thaw of the ice-complex does
not usually exceed 200–300 years. According to
Table 4 Temperature of soils in a thermokarst depression, Yukechi site. The surface was dry with adjacent, lakes
1.0–1.3 m deep underlain by up to silt 6–7 m.
Depth, m
2.0
3.0
5.0
7.0
26.08.01
0.4
1.0
1.1
1.1
26.06.02
29.09.02
11.12.02
20.02.03
09.04.03
0.9
0.8
1.1
1.1
0.1
0.7
1.1
1.1
0.3
0.6
1.0
1.1
3.6
0.6
1.0
1.1
5.1
3.3
1.6
1.1
Copyright # 2004 John Wiley & Sons, Ltd.
Permafrost and Periglac. Process., 15: 81–87 (2004)
86
A. Brouchkov et al.
Bosikov (1998), the development of thermokarst in
Central Yakutia has a cyclic character, with 150- to
180-year cycles being most prominent. These cycles
are sufficiently long to cause the complete thaw of the
ice-complex. Thus, the active phase of thermokarst is
short in comparison to the Holocene.
CONCLUSION
A positive annual water balance is necessary for the
development of thermokarst lakes; that is why modern
thermokarst processes are rarely observed in the dry
climate of Central Yakutia. Another reason is that the
layer of low-ice-content sediments in the upper part
of permafrost (the ‘shielding layer’) protects the ‘icecomplex’ from thaw.
Present-day thermokarst in Central Yakutia occurs
in response to either forest fire, human impact on the
environment, or climatic change. Fire can cause thaw
of the ‘shielding layer’ and, in some cases, of the icy
deposits. However, fires and vegetation disturbances
do not always cause thermokarst; as a rule, disturbance must be combined with climatic change and
salinization of soils.
Calculations of the thermal conditions beneath
thermokarst lakes in Central Yakutia show that, if
the water level is stable or increasing, thaw may reach
10–15 m over a period of 50 years, and 30 m over a
period of 200 years. These values are enough to melt
the ‘ice-complex’ completely. Observations at the
Yukechi site show that thermokarst depressions have
a tendency for rapid (5–10 cm/year and above) subsidence. Therefore, the active phase of thermokarst is
short and its development should be considered a
short-term catastrophic event.
ACKNOWLEDGEMENTS
The author acknowledges the assistance of Professor
Hugh French, Editor-in-Chief, for substantial manuscript modification and review, and for subsequent
language polishing.
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