Permafrost environment in the Yari-Hotaka ... - IARC Research

Permafrost, Phillips, Springman & Arenson (eds)
© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7
Permafrost environment in the Yari-Hotaka Mountains, southern part of
the Northern Japanese Alps
M. Aoyama
Department of Geography, Tokyo Metropolitan University, Hachioji, Tokyo, Japan
ABSTRACT: The permafrost environment in the Yari-Hotaka Mountains in the southern part of the northern
Japanese Alps is discussed. Temperature monitoring results indicate that the lower limit of the discontinuous
mountain permafrost zone is situated around 2800m ASL. Although the existence of permafrost in debris slopes
with a finer-grained matrix has not been detected, the ground surface temperature data points to the probable existence
of permafrost in several rock glaciers. Thus, permafrost distribution in the Yari-Hotaka Mountains is limited
to certain blocky accumulations, such as rock glaciers. Morphologic features and the vegetation cover on
their surfaces suggest that the rock glaciers in this mountain area are probably inactive or relict. The distribution
and estimated age of the rock glaciers indicate that the lower limit of the discontinuous mountain permafrost zone
during the Late Glacial and early Holocene reached 400–500m below the present-day limit.
1 INTRODUCTION
In the Japanese Alps, the present-day lower limit of
mountain permafrost is estimated to lie at 2500–2800 m
ASL (Ono 1984). Although a number of mountains
exceed this posited lower limit, no study had ever tried
to definitively establish present and past permafrost
distribution, and until recently, only a few rock glaciers
had been identified in the Japanese Alps. Since the late
1990s, a number of studies on permafrost and rock
glaciers were carried out in the Japanese Alps. As
a result, mountain permafrost was discovered in the
Tateyama Mountains, in the northern part of the northern
Japanese Alps (Fukui and Iwata 2000), and a large
number of rock glaciers were identified in the Japanese
Alps (Ishikawa et al. in press). In the Southern Japanese
Alps, the ground temperature data indicated that the
permafrost is virtually confined to shaded rockwalls
above 3000 m ASL (Matsuoka and Ikeda 1998). However,
information on the permafrost environment in
the southern part of the northern Japanese Alps is still
rather scarce. The Yari-Hotaka Mountains are situated
in this area.
The present paper discusses the current permafrost
environment in the Yari-Hotaka Mountains and the
thermal conditions on the surface of rock glaciers
located at the leeward sites and the wind-exposed site,
on the basis of the results of air and ground surface
temperature monitoring. In addition, the lower limit of
the discontinuous mountain permafrost zone during
the Late Glacial and early Holocene in this area was
inferred from the distribution of relict rock glaciers.
(36°20N, 137°40E; Fig. 1). Elevation ranges from
about 2300 to 3000 m ASL. The main ridge of the
Yari-Hotaka Mountains runs roughly in a north-south
direction, and lies between 2700 and 3100 m ASL.
The highest peak, Mt. Yarigatake, reaches 3180 m
ASL. Heavy snowfalls are caused by westerly winter
monsoons. Although no glaciers currently exist, glacial
landforms, such as glacial troughs, cirques and
moraines occur in the area. These glacial landforms
37 o N
Toyama
35 o N
2000
1000
0 m ASL
Nagoya
Northern Japanese Alps
Central Japanese Alps
Southern Japanese Alps
Study area
Matsumoto
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Tokyo
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Mt.Fuji
2 THE STUDY AREA
The Yari-Hotaka Mountains are situated in the southern
part of the northern Japanese Alps, central Japan
Figure 1.
137 o E
Location map of the study area.
139 o E
15

3000
Mt. Yarigatake
Table 1. Summary of air temperature monitoring from
October 2000 to September 2002 at the Minamidake
Mountain Hut.
Freezing Index Thawing Index
Year Mean (°C) (°C days) (°C days)
Mt. Nakadake
Site 4
2500
Tongue-shaped
rock glacier
Lobate Site 2
rock glacier
RG6
Moraine
RG7
Cirque wall Site 3
measurement site
of air temperature
measurement site
of ground surface
temperature
0 500m Mt. Kitahotakadake
were presumed to have been formed during the Late
Pleistocene (Ito and Vorndran 1983). The inherited glacial
landforms and the present-day snow patches concentrate
on east-facing slopes because of snow drift
supplied by predominant westerly wind. Eight rock
glaciers were identified in this area (Ishikawa et al., in
press). The distribution of these rock glaciers (RG1-8)
is presented in Figure 2. The rock glacier fronts occur
between 2360 and 2890 m ASL. RG1-7 is located on
the east-facing side of the main ridge. In contrast, RG8
is located on the west-facing side. The surfaces of these
rock glaciers consist of large angular blocks without
fine-grained soil. The bedrock geology of the area consists
of the Hotaka Andesite (Harayama 1990).
3 METHODS
RG8
2500
Yarisawa
RG2
Site 1RG3
RG1 RG4
Mt. Minamidake
2500
RG5
Figure 2. Topographic map showing the location of the
monitoring sites of air and ground surface temperature, and
distribution of rock glaciers.
Air and ground surface temperature monitoring were
conducted from 1 October 2000 until 30 September
2002, by means of miniature data loggers (Thermo
Recorder TR-52, manufactured by T & D corporation,
Japan). The loggers recorded temperatures at 1 h
2000–2001 2.5°C 2013.4 1101.3
2001–2002 2.5°C 1944.9 1033.1
intervals with a resolution of 0.1°C. The monitoring
sites are shown in Figure 2. Air temperature was monitored
at the Minamidake Mountain Hut (2975 m ASL),
which is located on the main ridge of the Yari-Hotaka
Mountains. Ground surface temperature monitoring
was conducted at a single location on each of four rock
glaciers. Site 1 (2810 m ASL) is located on RG1. Site 2
(2620 m ASL) is located on the small mound of RG6.
Site 3 (2625 m ASL) is located at the upper part of RG7.
Site 4 (2960 m ASL) is located at the upper part of RG8.
Morphometric parameters of each rock glacier (e.g.
the frontal angle and relative height of the rock
glacier) were measured by means of an analytical
plotter SD 3000 (manufactured by Leica corporation,
Switzerland). Furthermore, morphologic features of
the rock glacier and vegetation cover on the rock glacier
surface were observed by field survey and airphoto
interpretation. In this paper, rock glaciers RG1,
RG6, RG7 and RG8, where ground surface temperature
was monitored, are described in detail.
4 RESULTS
4.1 Air temperature
Results of air temperature monitoring are summarized
in Table 1. The mean annual air temperature (MAAT)
was 2.5°C for each measurement year. Freezing
indices for 2000–2001 and 2001–2002 were 2013.4
degree days and 1944.9 degree days, respectively.
Thawing indices reached 1101.3 degree days in
2000–2001 and 1033.1 degree days in 2001–2002. The
warmest months were July and August in 2001 (9.9°C)
and July in 2002 (10.5°C). January was the coldest
month, with mean air temperatures of 15.9°C in
2001 and 14.1°C in 2002. Autum/n and early winter
(October to December) temperatures in 2001 were
colder than the precedent year. However, winter temperatures
(January to March) in 2002 were warmer
than in 2001. Air temperatures stayed below 0°C
throughout the entire winter (Fig. 3).
4.2 Ground surface temperature
At sites 1, 2 and 3, located at the leeward site, the temperature
remained nearly constant during the winter
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Temperature (ºC)
30
0
_ 30
30
0
_ 20
30
0
_ 20
30
0
_ 20
30
0
_ 20
a
b
c
d
e
Air temperature
Site 1
Site 2
Site 3
Site 4
At site 3, temperature fluctuation in 2000–2001 and
2001–2002 also showed similar general behavior.
Between February and April, temperature stayed
between about 4°C and 5°C. By end of April, it
rose towards 0°C, and stayed at about 0°C until early
July in 2001, and the end of July in 2002. The mean
BTS values for February and March were 4.5°C for
each measurement year. MAST values in 2000–2001
and 2001–2002 were 1.2°C and 0°C, respectively.
Site 4 is located at the wind-exposed site. In contrast
to sites 1, 2 and 3, which are located at the leeward
site, the temperatures were constantly fluctuating during
winter. Hence, the snow cover at this site must
have been shallow. However, the diurnal range of temperature
in winter was smaller than in the other seasons.
The diurnal range of temperature was about
0.5–2.0°C during the February through March period.
MAST values were 0.8°C for each measurement year.
4.3 Morphology of the rock glaciers and
vegetation cover on the rock glacier surface
1-Oct
1-Jan
1-Apr
1-Jul
1-Oct
1-Jan
2000 2001 2002
1-Apr
1-Jul
Figure 3. Two-years variation in air temperature at the
Mountain Hut Minamidake (a) and ground surface temperatures
on the rock glaciers (b–e).
(February and March; Fig. 3). This thermal condition
suggests that a thick insulating snow cover developed
at these monitoring sites during that period.
In 2000–2001, the temperatures at site 1 were below
0°C during the period from mid-November to the end
of May 2001. Thereafter, the temperatures remained
at about 0°C until early July. In 2001–2002, the temperatures
were below 0°C during the period from the
end of October to early May. Thereafter, it stayed at
about 0°C until the end of July. In contrast to the air
temperature, winter ground surface temperatures in
2002 were colder than in 2001. The mean bottom temperatures
of winter snow cover (BTS) for February and
March were 1.2°C in 2001, and 1.9°C in 2002.
Mean annual surface temperatures (MAST) for 2001
and 2002 were 1.8°C and 0.2°C, respectively.
At site 2, the temperature profile in 2000–2001
showed similar general behavior to that of 2001–2002.
During the period from mid-December to mid-April,
temperatures remained nearly constant at around
4.5°C. By mid-April, the temperature rose towards
0°C, and stayed at about 0°C until mid-August in 2001,
and early August in 2002. The mean BTS measurements
for February and March were 4.2°C in 2001 and
4.6°C in 2002. MAST values in 2000–2001 and
2001–2002 were 0.1°C and 0.2°C, respectively.
RG1 is located at the foot of the east-facing talus below
a cirque wall. The front and head altitudes of RG1 are
2750 m ASL and 2820 m ASL, respectively. An
enclosed hollow exists in the central portion of the
rock glacier, while the outer part of the rock glacier
consists of a continuous ridge. The ridge crest tends to
be rounded. The frontal slope of RG1 is 33 m high,
with an angle of 33°. The majority of the rock glacier
surface is not covered with vegetation.
RG6 is located at the foot of the southeast facing
talus below a cirque wall. The front and head altitudes
of RG6 are 2500 m ASL and 2630 m ASL, respectively.
The lowest part of the rock glacier consists of a
continuous ridge with dense vegetation. There is a
small, subdued mound in the central part of the rock
glacier. The height of the frontal slope of RG6 is 20 m,
with an angle of 36°.
RG7 is located at the foot of the north-facing talus
below a cirque wall. The front and head altitudes of
RG7 are 2540 m ASL and 2640 m ASL, respectively.
The upper part of the rock glacier consists of distinct
multiple-lobes and transverse ridges with patchy vegetation,
while the lower part consists of a subdued longitudinal
ridge, with many boulders are covered with
lichen. The height of the frontal slope is 17 m, and the
angle is 25°.
RG8 is adjacent to the cirque wall on the west side.
The front and head altitudes of RG8 are 2890 m ASL
and 2960 m ASL, respectively. RG8 is tongue-shaped,
and characterized by a distinct hummocky topography
(Fig. 4). At the central part of the rock glacier, the
height of RG8 is ca. 20 m. A furrow and transverse
ridge topography of 1–2 m relief is developed on the
17

Mt. Nakadake
rock glacier, and patches of vegetation were observed
on its surface. The frontal slope is convex upward,
joining the upper surface in a smooth curve. The
frontal slope has a height of 23 m, and an angle of 29°.
5 DISCUSSION
Mt.Yarigatake
Figure 4. View of the rock glacier RG8. The dashed line
shows the rock glacier margin. Patches of vegetation are
observed on the rock glacier surface. Photograph taken in
October 2000.
As has been elucidated in previous studies (e.g.
Haeberli 1983, King 1986), the lower limit of discontinuous
mountain permafrost zone is defined by a mean
annual air temperature (MAAT) of 1 to 2°C. In
the present study, the MAAT at the monitoring site
(2975 m ASL) was 2.5°C for each measurement
year. According to the diagram by Harris (1981), the
freezing and thawing indices indicate that the monitoring
site belongs to the discontinuous permafrost
zone. Hence, the air temperature conditions at the monitoring
site correspond to those in the discontinuous
permafrost zone. Assuming a regional lapse rate of
0.6°C/100 m, 2800 m ASL represents the maximal
value for the regional lower limit of the discontinuous
permafrost zone.
Snow has a low heat transfer capacity. A sufficiently
thick snow cover of around 1 m therefore insulates
the soil from short-term variations in
atmospheric conditions (Hoelzle et al. 1999). At sites
1, 2 and 3, which are located at the leeward site, the
temperatures remain nearly constant during winter
(February and March). In light of the insulation effect,
this temperature constancy can be explained by thick
snow cover (ca. 1 m) insulating those sites from
short-term variations in atmospheric conditions. Thus,
BTS is free of atmospheric influences and the winter
period BTS measurement is adequate at the leeward
site of this mountain area.
BTS values are grouped into three categories in
relation to the likelihood of permafrost occurrence
(Haeberli 1973): (a) Permafrost probable (3°C), (b)
permafrost possible (2 to 3°C), and (c) permafrost
improbable (2°C). At site 1, the mean BTS for
February and March was above 2°C, and MAST was
above 0°C. These BTS and MAST values indicate the
absence of permafrost in this location. At sites 2 and 3,
the mean BTS for February and March was
below 3°C, and MAST was below 0°C. These BTS
and MAST values indicate probable permafrost occurrence.
In spring, the temperatures at the leeward sites
rose towards 0°C, and the zero curtain was observed
until July and August. This thermal condition corresponds
to the period of snow melting.
In contrast to these sites, at site 4, a wind-exposed
location, temperature was constantly fluctuating,
exhibiting a pattern similar to that of the air temperature
during the winter. This can be explained by the
thinner snow cover on this site due to the strong winter
monsoon winds that sweep snow away. Hence, the
temperature was influenced by atmospheric variations
throughout the entire winter. Thus, the site 4 BTS
should not be considered an indicator of the presence
of permafrost.
Results of air temperature monitoring suggest that
the alpine zone of the Yari-Hotaka Mountains belongs
to the discontinuous mountain permafrost zone, while
the results of ground surface temperature monitoring
indicate probable permafrost occurrence in several
rock glaciers located below the regional lower limit
for a discontinuous mountain permafrost zone, 2800 m
ASL. However, the existence of permafrost was not
detected in a debris slope with finer-grained matrix,
located close to the air temperature monitoring site
(Takahashi 1999). The open blocky active layer permits
intensive inflow and storage of cold air in winter,
which favors the preservation of permafrost (Harris &
Pedersen 1998). Therefore, permafrost distribution in
the Yari-Hotaka Mountains is restricted to certain
blocky accumulations such as rock glaciers, where
permafrost exists below the regional lower limit of
discontinuous permafrost.
Active rock glaciers have a steep (approximately
40°) vegetation-free frontal slope, while many inactive
and relict rock glaciers have a large depression
within the highest outer ridges and a more gentle
frontal slope, with a partial or full vegetation cover
(Ikeda and Matsuoka 2002). The rock glaciers in the
study area has to be considered inactive and relict,
given the existence of enclosed hollows on the rock
glacier, and the gentle frontal slope (36°), partially
or fully covered with vegetation. The lower limit of
relict rock glacier can be used as an indicator for variations
of the distribution of discontinuous mountain
permafrost (Haeberli 1985, Barsch 1996). In the study
area, the results of weathering-rind measurements
suggest that the rock glaciers may have been formed
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during the Late Glacial and early Holocene (Aoyama
2001). The relict rock glaciers occur above 2360 m
ASL (RG5 in Fig. 2). As can be deduced from the
occurrence of these relict rock glaciers, the lower limit
of the discontinuous mountain permafrost zone during
these periods was 400–500 m lower than the present-day
limit in the study area.
6 CONCLUSIONS
In the Yari-Hotaka Mountains, in the southern part of
the northern Japanese Alps, air temperature conditions
at the monitoring site (2975 m ASL) suggest that
the regional lower limit of discontinuous mountain
permafrost lies at about 2800 m ASL. Although the
existence of permafrost was not detected in debris
slopes with a finer-grained matrix, ground surface
temperature data point to probable permafrost occurrence
within several rock glaciers. Hence, permafrost
distribution in the Yari-Hotaka Mountains is restricted
to certain block accumulations such as rock glaciers.
The morphologic features of the rock glaciers and the
vegetation cover on their surfaces suggest that the
rock glaciers in the Yari-Hotaka Mountains are probably
inactive or relict. Relict rock glaciers occur above
2360 m ASL. These rock glaciers may have developed
during the Late Glacial and early Holocene. Thus, a
depression of 400 –500 m below the lower limit of the
present-day discontinuous mountain permafrost zone
is likely to have occurred during those periods.
ACKNOWLEDGEMENTS
I would like to thank Professor S. Iwata of Tokyo Metropolitan
University for continuous support and helpful
comments during the course of this work. Thanks are
due to Professor H. Fukusawa and Dr. S. Tsukamoto of
Tokyo Metropolitan University for helpful advice.
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