TRENDS OF PERMAFROST DEVELOPMENT IN THE SELENGE

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PERMAFROST - Seventh International Conference (Proceedings),
Yellowknife (Canada), Collection Nordicana No 55, 1998
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TRENDS OF PERMAFROST DEVELOPMENT IN THE SELENGE RIVER BASIN,
MONGOLIA
N. Sharkhuu
Institute of Geoecology, Mongolian Academy of Sciences, Ulaanbaatar, 210620, Mongolia
Abstract
The Selenge River Basin, Mongolia, is located in the southeastern part of Siberian permafrost. Ground temperature data collected during the past 15-25 years show that mean annual temperatures are increasing at rates
of 0.01-0.02¡C per year. Permafrost is degrading in about 75% of the areas of the basin where it currently exists.
The current annual thaw rate is about 0.5-1.0 cm from the top and about 2-5 cm from the bottom. In the Khentei
taiga, which covers about five per cent of the basin, permafrost is aggrading, especially where forests are dense
and where moss is present. It is projected that most of the permafrost that is less than 15 m thick will disappear
within 50 years which will decrease the entire area currently underlain by permafrost in the basin by 25-35%.
Introduction
Global warming has an impact on permafrost. Factors
aggravating permafrost degradation are deforestation
in the taiga and desertification in the steppe. Therefore,
determination of the trends of permafrost change is of
both practical and scientific importance. Recently,
geocryologists and other scientists from many countries
have published articles about this subject (e.g., Cheng et
al., 1993; Gavrilova, 1993; Nelson et al., 1993; Wang and
Zhang, 1996; Zhou and Wang, 1996).
Even more recently research on permafrost in
Mongolia has been undertaken as exemplified by this
report. The author has been making ground temperature measurements in the Selenge River Basin
(Sharkhuu, 1982, 1993; Sharkhuu and Luvsandagva,
1975; Sharkhuu and Undarmaa, 1986). The data to date
indicate a mean annual increase in temperature at sites
throughout the basin.
Terrain and permafrost
The Selenge River Basin has an area of 282,000 km2. It
is surrounded by mountain ranges which rise up to
4000 m a.s.l. The Selenge delta is at 600-700 m a.s.l. The
majority of the area is forest-steppe with taiga in the
higher mountains and steppe only in the south. In the
mountains (i.e., the taiga), air temperature averages
-3 to -6¡C, precipitation, 40 to 50 cm and snow depth,
20-40 cm. In the basin, the air temperatures are 0 to
-3¡C, precipitation is 20-30 cm, and snow depths are
5-10 cm.
Alluvial and lacustrine deposits include gravels,
sands, sandy loams, loam and clay. On mountain slopes
surface materials are 1 to 5 m thick.
During the past thirty years, small-scale geologic and
geocryologic maps of the basin have been prepared
(Sharkhuu, 1982, 1993). Figure 1 shows the distribution
of permafrost thickness and temperature. It is divided
into five categories: continuous permafrost (>85%), discontinuous (50-85%), widespread isolated (10-50%),
sparsely-spread isolated (1-10%), and sporadic (<1%).
Both continuous and discontinuous permafrost occur in
most surface types and occupy about 29.5% of the
entire basin. In those areas, taliks are found only on
steep, south-facing slopes, under large rivers and deep
lakes, and along tectonic fractures with hydrothermal
activity. Outside the continuous and discontinuous
areas, permafrost is found only on north-facing slopes
and in fine-grained and moist deposits.
It has been found that the thickness of permafrost is
proportional to the (negative) mean annual temperature
and inversely proportional to the temperature gradient.
The majority of permafrost is of the dry variety, with icy
permafrost being confined to lacustrine, alluvial and
some times colluvial deposits. Seasonal thaw to depths
of 2-3 m in silty soils and 4-5 m in coarse materials,
occurs between May and September. In non-permafrost
areas, ground freezing occurs between mid-October
and the end of April.
Climate change
Air temperature and precipitation change affects permafrost development (Gavrilova, 1978). Climatologists
N. Sharkhuu
979
Figure 1. Distribution, temperature and thickness of permafrost in the Selenge River Basin (Sharkhuu, 1993).
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The 7th International Permafrost Conference
whereas Budyko (1988) predicts an increase in precipitation of 50% for the arid lands of central Asia.
Ground temperature change
The earth's surface materials have a "memory" of temperature change (Balobaev and Pavlov, 1983), ground
temperature change that is influenced by atmospheric
temperature change and human activities. To examine
these changes the author has measured temperatures in
boreholes to depths of 25-100 m with thermistors. The
seven boreholes, located in four different areas in the
Basin, which have provided the data for this research
are:
Figure 2. Mean annual, summer and winter air temperature changes in the
Selenge River Basin.
are predicting a temperature rise of 2.5-3.0¡C in the next
50 years along with major changes in precipitation for
circumarctic regions (Budyko, 1988). Both Mongolian
and American scientists have shown that during the
past 50 years temperatures have increased by 1.8¡C in
west Mongolia, 1.0¡C in central Mongolia, and 0.3¡C in
southern and eastern Mongolia. Precipitation has
increased a small amount in the mountains
(Mizhiddorj, 1990).
Mean summer temperatures in the Selenge River
Basin have increased by only 0.5¡C whereas winter temperatures have increased by 4.0¡C in the past 50 years.
The mean change for the year is 1.4¡C. Precipitation
increased slightly in the Khentei taiga area while
remaining nearly constant elsewhere (Natsagdorj,
1988).
It is predicted statistically that the basin's temperature
will increase by 2.5¡C by 2050 (Mizhiddorj, 1994)
Figure 3. Ground temperature changes in the boreholes in the Burenkhaan
(20;21;206), Terkh (15), Erdenet (34) and Baganuur (12;142) areas.
N. Sharkhuu
981
Table 1. Ground temperature, permafrost and seasonal thawing changes in boreholes under natural conditions.
N-20 - south-facing slope at 1665 m a.s.l.
N-21 - watershed area at 1715 m a.s.l.
N-206 - north-facing slope at 1700 m a.s.l.
N-15 - high flood plain at 2050 m a.s.l.
N-34 - south-facing step at 1440 m a.s.l.
N-12 - the top of a frost mound at 1342 m a.s.l.
N-142 - gently sloping surface at 1350 m a.s.l.
Boreholes N-20, N-21 and N-206 are in Cambrian
limestone with a 1-2 m overburden of sand in the
Burenkhaan area. N-15, located in the Terkh area and
drilled in 1969 to a 90 m depth, has an upper half (45 m)
of gravel and sand with ice totalling 17%. This layer is
underlain by 45 m of interbedded clays and silty sand
with an ice content of 32%. N-34, in the Erdenet area, is
in Permian granite. N-12 has 3.1 m of gravel and loamy
sands underlain by lacustrine clay with 40-80% ice content by volume to a depth of 14.3 m. Below this is
Cretaceous siltstones and coals. N-142 is in Cretaceous
sandstone with 15% ice. Both N-12 and N-142 are in the
Baganuur area.
Figure 3 and Table 1 depict the ground temperatures
recorded in the boreholes. The measurements were
made under different conditions: 1969 and 1970 were
cold; 1976, 1983, and 1986 were normal, and 1996 was
warm. So that decadal rates of warming would not be
influenced by the conditions in the year of measurement, correction factors were calculated by correlating
the ground temperatures with the linear trend of air
temperatures. A correction factor of 0.8 was used for the
decadal rate of ground temperature change, whereas
the correction factor for the rate of change of seasonal
thawing was 0.5 (Table 1).
Conclusions based on comparisons of borehole data
(Figure 3 and Table 1) include:
(1) during the last 15 to 25 years, mean annual ground
temperatures in the Selenge River Basin have risen at a
rate of 0.01- 0.02¡C per year;
(2) the rate of temperature change is relatively high on
south-facing slopes, relatively low on north-facing
slopes, and moderate in the watershed;
(3) the temperature increases in perennially frozen
ground is less than in thawed ground;
(4) the temperature increase in ice-rich permafrost is
less than in ice-poor permafrost;
(5) the average geothermal gradient is about 2¡C/100
m but it decreases in the near-surface due to the recent
warming; and
(6) at a depth of 50 m, temperatures increased by
0.05¡C in 13 years in borehole N-206 (bedrock) and 17
years in borehole N-15 (gravels and sand). Using these
data with Balobaev and Pavlov's (1983) calculations, we
find that the depth of penetration of surface temperature is 8-15 m in unconsolidated deposits and 12-25 m
in rock in one year; it is 35-45 m in unconsolidated
deposits and 40-60 m in rock in ten years and 120-145 m
in unconsolidated deposits and 140-170 m in rock in 100
years.
Present development of permafrost
Degradation, aggradation and stability of permafrost
development all occur in the Selenge River Basin.
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The 7th International Permafrost Conference
Figure 4. Development of thermokarst subsidence in high ice-content lacustrine clays, Chuluut River valley, Khangai Mountains. Photo taken in 1969.
Degradation predominates in about 75% of the permafrost area of the basin. The factors accounting for
this proportion are the increase in mean annual permafrost temperature (see above) and seasonal thaw.
The latter is increasing at rates of 0.5-1.0 cm per year.
This seasonal thaw rate is larger than that calculated by
Nakayama et al. (1996) for the Arctic, a difference that
can be explained by the more southern location of
Mongolia and by a series of especially warm winters.
Some warm winters have resulted in a thin suprapermafrost layer that did not freeze back after the summer
thaw (e.g., between 3.5-3.9 m in borehold N142 in
1995-1996).
Thaw at the base of permafrost is more important
than active layer deepening. By using Balobaev and
Pavlov's (1983) procedure, the rate of thaw from below
in the Selenge Basin can be up to 2-5 cm/year and eventually complete thaw will occur. This has happened in
borehole N21 and the temperature conditions of the
Table 2.Permafrost temperature and thickness changes in
boreholes N-23 and N-196 in the Nalaikh and Baganuur
areas as a result of mining
permafrost in boreholes N-12 and N-142 indicate that
they could thaw completely within 10 years.
In the Khangai and Khubsugal mountains
thermokarst and thermal erosion processes are active.
Thermokarst lakes and sinks are found in ice-rich sediments (Figure 4), permafrost river banks are eroding,
and frost mounds are cracking. Thermokarst development and permafrost degradation are more intense in
the west and north than in the east of the basin.
Locally, degradation is caused by human activities,
especially near cities and mines, and because of forest
fires. An example is the degradation caused by mining
of the Nalaikh and Baganuur coal deposits. Table 2
shows the ground temperature changes at Nalaikh
(N-23) and Baganuur (N-196) at the start, middle and
end of mining. N-23, at an elevation of 1475 m a.s.l., is
on a gentle slope. It has a 4 m thick surficial cover of
sands and silt over interbedded sandstone, mudstone
and coal of Cretaceous age. Mine shafts were dug to
depths of 150 m 50 years ago. Permafrost with a thickness of 50 m has been thawing from below at a rate of
70 cm/year. Its temperature has increased by
0.04¡C/year at a depth of 50 m and 0.02¡C/year at a
depth of 15 m.
Borehole N-196 is in a swamp at an altitude of 1350 m
a.s.l. near a spring. It is composed of a fine-grained
sandstone topped by 10 m of gravelly sand and one
metre of loam. Mining caused the spring to dry up and
the swamp to drain. The permafrost, which was 25 m
thick, completely thawed in 8 to 10 years (Table 2).
Aggradation of permafrost has only been observed in
the Khentei taiga area. It is believed that this aggradation results from increased precipitation. Since the
1940Õs, bare slopes have become forested and valley
bottoms have new growth of shrubs and moss.
N. Sharkhuu
983
Cryogenic processes have increased as shown on maps
and photographs taken in 1942. Further, historical documents record that many of these areas were pasture 50
to 100 years ago. Research also suggests that ground
temperatures in the Khentei area can decrease by 1-3¡C
due to the growth of vegetation.
(3) if Budyko's (1988) prediction of a major increase in
precipitation actually occurs, permafrost degradation
will be decreased; and
(4) frequent forest fires and increasing gold mining
will increase the degradation rates, as will expansion of
settlements, industrial areas, and agriculture.
Ground temperature data show that the most stable
permafrost areas (about 20% of those underlain by permafrost) lie between the areas where degradation and
aggradation are occurring. This especially true in the
Khentei foothills and in the eastern Khubaugul
mountains.
Chinese researchers (Zhou and Gao, 1996) have
shown similar results for northeast China. This is not
surprising for both areas are located at about the same
altitude in the southern part of Northern Hemisphere
permafrost.
Future development of permafrost
Conclusions
Assuming that the mean annual temperature in the
Selenge River Basin will be 0.5-1.0¡C greater than now
(as predicted by current trends for the middle of the
21st century), we can expect the following
developments:
Under the influence of climate warming permafrost
degradation is to be expected as is the fact that it will
degrade more rapidly toward its southern margin.
Further degradation will locally vary, being more rapid
in the western Selenge River Basin than in the east, in
sporadic and isolated permafrost areas more than in
continuous and discontinuous zones, in bedrock more
than in unconsolidated rocks, in ice-poor substrates
more than ice-rich ones, on south-facing more than on
north-facing slopes, and in areas with human activity
more than in those without it.
(1) present-day permafrost in unconsolidated materials with thicknesses of 10 to 15 m and in solid rock with
thicknesses of 15 to 20 m will disappear. In contrast,
permafrost with thicknesses of more than 25 m will
degrade only slightly. Permafrost of more than 50 m
will only thaw from its base by 1.0 to 2.5 m;
Acknowledgments
(2) whereas sporadic and isolated permafrost will
degrade considerably or will thaw completely, areas of
discontinuous and continuous permafrost should
change little. By 2050, permafrost in the Selenge Basin
should underlie about 20% of the total area, and this
change represents a decrease from the present of about
25-35%;
The assistance of Professor J. Walker, Louisiana State
University, in rendering the text suitable for the
Permafrost Conference is gratefully acknowledged.
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