Ninth International Conference on Permafrost ... - IARC Research

Occurrence of Permafrost and Ground Frost Phenomena in MongoliaCh. OppPhilipps-University Marburg, GermanyIntroductionBecause of the climatic peculiarities and of the extremecontinental location, the formation of permafrost andground frost phenomena in Mongolia are widespread(Opp & Barsch 1993). Most of them belong to differentforms of discontinuous and periodic permafrost. Types ofground frost occur in dependence from the zonal climatic,soil, and vegetation conditions. Zones of ground frost aredistinguishable by their location above the sea level, by theirarea, by their spatial share, by their distribution within thezone, and by the thickness of the permafrost (cf. Table 1).On the basis of observations, measurements, andinterpretations during five field trips between 1989 (Barschet al. 1993) and 1998 (Opp 1998, 2005), selected results areimparted.ResultsSeasonal average periods of freezing of discontinuouspermafrost were established between October and May. Thecorresponding periods of thawing were established from lateApril until late September (cf. Fig. 1). The processes andperiods of freezing and thawing can differ in dependencefrom the site conditions. Such examples were proved independence from the exposition (northern and southernslopes), or in river valleys (cf. Fig. 2) Characteristicdifferences of temperatures and pH values between the topand the bottom position of thufurs (earth hummocks) wereanalysed.The degradation of many thufur fields in Mongolia iscaused by climate (warmer winters) and land use changes(higher share of grazing animals) during the last decade.Thermokarst bank erosion is caused by deep thawingduring the summertime and lateral erosion by river water.The erosion rate of bank caving processes is much higherat the natural levee side in southern exposition than at otherriver bank positions. Within the river valley of the OrkhonRiver, south of the city of Darkhan, a maximum value of 1.5m per year of backward erosion of the river bank, causedby permafrost degradation, were measured (Opp 1998,2006). This “fluvio-cryo-thermo-erosion” of the river banksis a characteristic phenomenon of many rivers in northernMongolia.Figure 1. Average seasonal freezing and thawing periods ofdiscontinuous permafrost within the Orchon catchment (northernMongolia).Figure 2. Sequence of seasonal freezing and thawing of a permafrostinfluencedsite near Erdenet (northern Mongolia).Table 1. Selected features of the belts of frozen ground in Mongolia (Source: National Atlas of Mongolia, Ulan Bator, 1990).belts offrozen grounddiscontinuouspermafrostdiscontinuouspermafrostsporadicpermafrostseasonal groundfreezingaltitude a.s.l area portion withinthe permafrostdominant maximum thicknessof permafrosttemperature in 20 mdepthportion within the areacountryin valleys on slopesand onwatersheds(m) (km²) (%) (%) (m) (m) (°C)1200–2800 175,280 11.2 40–80 20–500 (bis 1000) -5 bis +1700–2600 350,560 22.4 1–40 5–50 (bis 100) 5-20 -5 bis +5600–1900 460,110 29.4