Ice Free Areas and Terrestrial Habitats

During the austral summer only about 330,000 km2 of the Antarctic surface is free of ice and snow. These ice-free areas are mainly located in the western Antarctic Peninsula; on the continent they occur only in scattered coastal areas, on the steep slopes of the Transantarctic Mountains or in nunataks. Although the term oasis originated in hot deserts to describe areas with groundwater and vegetation, it is also used to indicate ice-free areas of continental Antarctica. Pickard (1986) distinguished Antarctic oases (minimal surface area 10 km2) from smaller ice-free areas (i.e. nunataks, beaches and moraines), and identified the Victoria Land Dry Valleys (mountain oasis) and six low coastal oases: Bunger Hills,Vestfold Hills, Windmill Islands, Schirmacher Oasis, Soya Coast and Thala Hills. Most of these areas are cold deserts,with very sparse biota and a small number of cryptogamic and (terrestrial and aquatic) invertebrate species. They therefore play a minor role in the global carbon cycle and in driving global climate trends. Nevertheless, Antarctic ice-free areas are a unique laboratory for understanding processes of cold adaptation, the spread and development of pre-adapted organisms, and colonisation processes in extreme environments very far from propagule sources. These areas play an important role as breeding grounds for seabirds and as sites in which to study the geological and glacial history of the continent. Besides contributing to ecological and geological science as a whole, Antarctic ice-free areas offer a unique opportunity for detecting and predicting the effects of global climate changes on terrestrial ecosystems (Bargagli 2000,2001).

By virtue of their reduced complexity and largely unpolluted and pristine conditions, Antarctic terrestrial ecosystems allow the assessment of fluxes of nutrients and other elements between biotic and abiotic components of ecosystems, and the detection of persistent pollutants derived from human activity in Antarctica and elsewhere in the Southern Hemisphere.

Weather fluctuations and the physical environment exert a determining influence on the survival and development of Antarctic organisms. Life history strategies are often based on long lifecycles, slow growth rates, low reproductive output and high energy investment in the ability to respond rapidly to adverse changes. They are adapted to extreme environmental conditions but may be highly sensitive or intolerant to changes exceeding pre-existing thresholds. The biological colonisation of old or newly exposed substrata is easily recognisable in desert environments, and knowledge of the response of terrestrial organisms to climate and environmental change is essential for the management and protection of these sensitive ecosystems. Such knowledge is also useful for a better understanding of climate-induced changes in more complex ecosystems elsewhere (Block 1994).

Many ice-free areas have emerged during the past few thousand years from the retreating ice in Antarctica, and glacial erosion is the dominant land-

forming factor. Several coastal areas show evidence of isostatic uplift, which has produced raised beaches and inland cliffs and, sometimes, freshwater or brackish small lakes. The surface of these periglacial environments is characterised by scattered erratic boulders and suites of glacial till and unsorted rock rubble. In general, moraines are rare and limited in size compared to those in the Alps or other mid-latitude regions. On the contrary, sediment cores from continental shelves around Antarctica show a widespread occurrence of glacio-marine deposits (Anderson JB 1991; Barrett et al. 1991).

Most ice-free areas in East Antarctica are typical cold desert environments. The term cold desert was first applied to largely unvegetated lands in the Russian High Arctic and to the western and northern portions of the Canadian Arctic Archipelago, with annual precipitation ranging from 100 to 250 mm and the warmest monthly mean air temperatures<5 °C (Aleksan-drova 1988; Vincent 1997). Antarctic cold deserts are much smaller than Arctic ones, but their annual precipitation is often well below 100 mm and the warmest mean air temperatures are <0 °C. The Antarctic cold desert undoubtedly represents a climatic and environmental extreme relative to the rest of the biosphere.

Due to low temperatures and arid conditions, chemical weathering and many other water-based rock decay processes are ineffective. Most snowfalls ablate, and in summer even snow meltwaters (especially near or over large, dark, north-facing boulders sheltered from the wind) may suddenly evaporate. Thus, processes such as frost wedging, frost cracking and the transport of products of disintegration play a marginal role in continental Antarctica; features such as talus or scree slopes are therefore rather uncommon. The main processes involved in rock disintegration and the formation of regolith are glacial and wind action, salt weathering, insolation, ice formation, and biotic exfoliation by endolithic communities. These weathering processes determine very slow transformations. Glacial deposits from the Miocene or earlier may show well-preserved landscapes and, for the distinctive features of soil and landscape,Antarctica can be regarded as a distinct morphogenetic region (Campbell and Claridge 1987).

As shown in Fig. 14, the wind armed with fine sand, snow and ice causes faceting and polishing of rocks (ventifacts), and the redistribution of fine materials with the formation of coarse armoured lag (desert pavements). Strong evaporation determines the accumulation of soluble salts on the surface of regolith, or encrustations and efflorescences just beneath surface boulders, cobbles and pebbles (Ugolini and Anderson 1973; Keys and Williams 1981; Gore et al. 1996). These salts consist largely of chlorides, nitrates and sulphates of sodium, potassium, calcium and magnesium. Although salts may originate from different sources, most ions come from the marine environment through marine aerosols, snowfall and seabirds (Bockheim and Wilson 1992; Rankin and Wolff 2000; Bargagli et al. 2001). The crystallisation of salt within rock pores and cracks contributes to rock decay and the formation of

Fig. 14. The effect of wind on ice-free areas. Above The faceting and polishing of rocks. Below Salt encrustations (northern Victoria Land)

heavily pockmarked surfaces. The cold desert soils of continental Antarctica contain large amounts of soluble salts, and soils are underlain by permanently frozen ground (permafrost) even in coastal fringes. Permafrost occurs in icefree areas with an average annual temperature lower than -1 °C (Bockheim 1995). Although an active surface layer melts above the permafrost during summer, it seldom results in the formation of liquid water. The depth of the active layer (usually a few decimetres) mainly depends on temperature and the characteristics of surface materials; freezing and melting cycles determine the formation of ground patterns such as block fields and cracks.

In maritime Antarctica, climatic and environmental conditions are much less extreme, and ice crystallisation and chemical weathering are more important in rock disintegration. The increased leaching allows the formation of clay-sized materials and reduced amounts of salt encrustations. The organic content of soils may be low, but pads of humus can develop beneath isolated plant cushions. At poorly drained sites, peat-like material can accumulate under moss turf. This material is often cemented by ice crystals, and humic compounds only penetrate slightly into the underlying mineral soil (Fogg 1998).

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