Permafrost environment in the Yari-Hotaka Mountains, southern part of the Northern Japanese Alps

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1 Permafrost, Phillips, Springman & Arenson (eds) 23 Swets & Zeitlinger, Lisse, ISBN 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 28m 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 4 5m below the present-day limit. 1 INTRODUCTION In the Japanese Alps, the present-day lower limit of mountain permafrost is estimated to lie at 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 199s, 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 2), 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 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 2 N, E; Fig. 1). Elevation ranges from about 2 to m ASL. The main ridge of the Yari-Hotaka Mountains runs roughly in a north-south direction, and lies between 27 and 31 m ASL. The highest peak, Mt. Yarigatake, reaches 318 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 2 1 m ASL Nagoya Northern Japanese Alps Central Japanese Alps Southern Japanese Alps Study area Matsumoto Tokyo Mt.Fuji 2 THE STUDY AREA The Yari-Hotaka Mountains are situated in the southern part of the northern Japanese Alps, central Japan Figure o E Location map of the study area. 139 o E 15

2 Mt. Yarigatake Table 1. Summary of air temperature monitoring from October 2 to September 22 at the Minamidake Mountain Hut. Freezing Index Thawing Index Year Mean ( C) ( C days) ( C days) Mt. Nakadake Site 4 25 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 5m 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 236 and 289 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 199). 3 METHODS RG8 25 Yarisawa RG2 Site 1RG3 RG1 RG4 Mt. Minamidake 25 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 2 until September 22, by means of miniature data loggers (Thermo Recorder TR-52, manufactured by T & D corporation, Japan). The loggers recorded temperatures at 1 h C C intervals with a resolution of.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 (281 m ASL) is located on RG1. Site 2 (262 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 (296 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 (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 2 21 and were degree days and degree days, respectively. Thawing indices reached degree days in 2 21 and degree days in The warmest months were July and August in 21 (9.9 C) and July in 22 (1.5 C). January was the coldest month, with mean air temperatures of 15.9 C in 21 and 14.1 C in 22. Autum/n and early winter (October to December) temperatures in 21 were colder than the precedent year. However, winter temperatures (January to March) in 22 were warmer than in 21. Air temperatures stayed below 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 16

3 Temperature (ºC) 2 _ 2 _ 2 _ 2 a b c d e Air temperature Site 1 Site 2 Site 3 Site 4 At site 3, temperature fluctuation in 2 21 and 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 C, and stayed at about C until early July in 21, and the end of July in 22. The mean BTS values for February and March were 4.5 C for each measurement year. MAST values in 2 21 and were 1.2 C and 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.5 2. C during the February through March period. MAST values were.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 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 2 21, the temperatures at site 1 were below C during the period from mid-november to the end of May 21. Thereafter, the temperatures remained at about C until early July. In 21 22, the temperatures were below C during the period from the end of October to early May. Thereafter, it stayed at about C until the end of July. In contrast to the air temperature, winter ground surface temperatures in 22 were colder than in 21. The mean bottom temperatures of winter snow cover (BTS) for February and March were 1.2 C in 21, and 1.9 C in 22. Mean annual surface temperatures (MAST) for 21 and 22 were 1.8 C and.2 C, respectively. At site 2, the temperature profile in 2 21 showed similar general behavior to that of 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 C, and stayed at about C until mid-august in 21, and early August in 22. The mean BTS measurements for February and March were 4.2 C in 21 and 4.6 C in 22. MAST values in 2 21 and were.1 C and.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 275 m ASL and 282 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 25 m ASL and 26 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 2 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 254 m ASL and 264 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 289 m ASL and 296 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. 2 m. A furrow and transverse ridge topography of 1 2 m relief is developed on the 17

4 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 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 2. 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.6 C/1 m, 28 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 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 C. These BTS and MAST values indicate probable permafrost occurrence. In spring, the temperatures at the leeward sites rose towards 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, 28 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 4 ) 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 22). 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 18

5 during the Late Glacial and early Holocene (Aoyama 21). The relict rock glaciers occur above 236 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 4 5 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 28 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 236 m ASL. These rock glaciers may have developed during the Late Glacial and early Holocene. Thus, a depression of 4 5 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. REFERENCES Aoyama, M. 21. Estimating the age of rock glaciers from weathering rind thickness in the Yari-Hotaka Mountain Range, northern Japanese Alps. Geographical reports of Tokyo Metropolitan University 36: Barsch, D Rockglaciers. Indicators for the present and former geoecology in high mountain environments. Berlin: Springer. Fukui, K. & Iwata, S. 2. Result of permafrost investigation in Kuranosuke cirque, Tateyama, the Japanese Alps (in Japanese). Seppyo (Japanese Journal of Snow and Ice) 62: Haeberli, W Die Basis-Temperatur der winterlichen Schneedecke als möglicher Indikator für die Verbreitung von Permafrost in den Alpen. Zeitschrift für Gletscherkunde und Glazialgeologie 9: Haeberli, W Permafrost-glacier relationships in the Swiss Alps-today and in the past. In Proceedings, 4th International Conference on permafrost. National Academy Press, Washington, Haeberli, W Creep of mountain permafrost: internal structure and flow of Alpine rock glaciers. Mitteilungen der Versuchsanstalt für Wasserbau Hydrologie und Glaziologie an der ETH Zürich No. 77: Harayama, S Geology of the Kamikouchi district. With Geological sheet map at 1:5,. Tsukuba: Geological Survey of Japan. Harris, S. A Climatic relationships of permafrost zones in areas of low winter snow-cover. Arctic 34: Harris, S. A. & Pedersen, E Thermal regimes beneath coarse blocky materials. Permafrost and Periglacial Processes 9: Hoelzle, M., Wegmann, M. & Krummenacher, B Miniature temperature dataloggers for mapping and monitoring of permafrost in high mountain areas: first experience from the Swiss Alps. Permafrost and Periglacial Processes 1: Ikeda, A. & Matsuoka, N. 22. Degradation of talus-derived rock glaciers in the Upper Engadin, Swiss Alps. Permafrost and Periglacial Processes 13: Ishikawa, M., Fukui, K., Aoyama, M., Ikeda, A., Sawada, Y. & Matsuoka, N. in press. Mountain permafrost in Japan: distribution, landforms and thermal regimes. Zeitschrift für Geomorphologie, Supplementband. Ito, M. & Vorndran, G Glacial geomorphology and snow-lines of Younger Quaternary around the Yari- Hotaka Mountain Range, Northern Alps, Central Japan. Polarforschung 53: King, L Zonation and ecology of high mountain permafrost in Scandinavia. Geografiska Annaler 68A: Matsuoka, N. & Ikeda, A Some observations regarding mountain permafrost in the Japanese Alps. Annual Report of the Institute of Geoscience, the University of Tsukuba 24: Ono, Y Last glacial paleoclimate reconstructed from glacial and periglacial landforms in Japan. Geographical Review of Japan 57B: Takahashi, N Air and ground temperature conditions in the alpine zone of the Mt. Minamidake, Hida Mountain Range (in Japanese). In Report of grantin-aid for the scientific research, The study of periglacial phenomena and its environment in the alpine zone of Japan

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