Geochemical properties of plant-soil-permafrost systems on landslide slopes , Yamal , Russia

Cryogenic landslides on saline marine sediments are widely distributed in a typical tundra subzone of the Yamal Peninsula. Interrelation between the height and productivity of willow tundra, and the activation of cryogenic processes is discussed. It is supposed that high willow canopies are the indicators of ancient landslide activity and may be used for mapping landslide areas. Various procedures are proposed to evaluate the relative age of the landslides. They include study of the vegetation cover succession, of the ash content in each vegetation group, and of the ground water and sediment chemistry on the landslide-affected slopes. It is shown that the landslide process causes the desalinization of the marine sediments and enriches the active layer with salts. This is the important peculiarity of the cryogenic landslides in the region with saline permafrost distribution. Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 – Study site (station “Vaskiny Dachi” ) – Sites of the observed landslide – Southern boundary of the frozen saline marine deposits (after Dubikov 2002, and Brushkov 1998). Figure 1. Study area in West Siberia. the south, about 1000 m long, and between 22 to 47 m a.s. l. ; II. the “Triangular” site, is oriented to the north, 350 m long, and between 43 to 53 m a.s. l. ; and III. the “Cirque” site, is oriented to the north, 700 m long, and between 23 to 48 m a.s. l. Five or more landslides are noticed on each slope. Some landslides are newly formed and rarely covered with pioneer species. The others are covered by light-green meadow vegetation. The oldest landslides are overgrown with shrub vegetation (Salix glauca dominating). These observations suggest that cryogenic landslides are cyclic in time in various parts of the landslide-affected slope. As a result of the process, the slopes represent a landslide system of various ages with a specific set of morphological elements: (A) tops of the hills and convex stable slopes (not affected by landslides), or a “background”; (B) concave cirque-shaped depression (shearing planes), or denudation zone; and (C) hummocky scarp and terraces (landslide bodies), or accumulation zone. The landslides of three generations are studied: (1) the young, formed 10 to 30 years ago; (2) the old, formed up to 300 years ago; and (3) the ancient, formed 300 to more than 2,000 years ago. The age of some landslides is estimated using radiocarbon dating of buried organic matter (Leibman et al. 2000). In other cases, the serial changes of vegetation or other methods are used as an indicator of landslide age (Table 1). 3 RESULTS AND DISCUSSION 3.1 Change of vegetation on the landslide-affected slopes Willow shrubs are usually considered as the early stage of serial changes of vegetation, which colonize bare surfaces formed due to natural or/and anthropogenic processes (McKendrick 1987, Jumponen et al. 1998, Matsuda et al. 1988 and Andreev 1970). The predominance of high willow canopies (Salix glauca) on landslide-affected slopes is noticed by Rebristaya et al. (1995), Ukraintseva (1997), Ukraintseva et al. (2000). The dominating communities on hilltops and stable slopes (A3) in the study region are the undershrub/grass/moss and moss/lichen tundra (Table 1). They are the background for a subzone of typical tundra. After 10 to 15 years since the landslide event, bare spots alternate with pioneer herbages, such as Puccinellia sibirica, Poa alpigena subsp. alpigena, Deshampsia sp. , Phippsia concinna etc. and forbs Tripleurospermum Hookerii on young surfaces (B1, Table 1). The second stage is colonization by meadow sedge/grass communities (Calamagrostis sp., Poa alpigena subsp. colpodea, Carex concolor, Nardosmia frigida, Equisetum arvense subsp. boreale, Polemonium acutiflorum, Ranunculus borealis etc.) with active willow restoration, as observed on old shearing planes (B2, see Table 1). The next stage is colonization by the high willow/meadow parkland communities (B3). On young landslide bodies (C1) low shrubs (willow, dwarf-birch) are dominating, moss cover degrades, pioneer vegetation (Calamagrostis sp., Poa alpigena subsp. alpigena, Equisetum arvense subsp. boreale, Polemonium acutiflorum, Ranunculus borealis and cotton-grass) is restoring. On C2 high willow/meadow communities with mosses replace them. Dense willow canopies covering moss/herbaceous communities dominate on ancient landslide bodies (C3). The different stages of landslide vegetation provide the criteria to estimate the relative age of landslides. Thus, background surfaces, not disturbed by landslides (A3), young shearing planes (B1) and young landslide bodies (C1) can be clearly distinguished by the herbaceous community. Old and ancient landslides are similar in vegetation cover, but differ in height and density of the shrub layer (Table 1). High willow communities ( 50 cm) occupy the most ancient landslide slopes. According to the structure of their phytomass, landslide-affected slopes differ distinctly from background surfaces. On background surfaces mosses cover 100%, and willow cover less than 10–15% of the area. On the landslide-affected slopes the willow quota increases up to 50–80%, and less than 40–80% is covered by mosses (Ukraintseva et al. 2000). The above-surface phytomass of ancient landslides is higher than that of the background – on average about 1400 g/m2 compared to 850 g/m2 of the background (Fig. 2). Thus, high willow canopies are the indicators of an ancient landslide activity and may be base of the landslide areas mapping. 3.2 Desalinization of the marine sediments due to