Chemical Properties of Antarctic Permafrost Soils

Chemical weathering is of very low intensity in these soils, because of the low temperatures and extreme aridity, but soils vary in their chemistry because of environmental variations. The soils contain very small amounts of fine particle size material and even less of clay-sized material, which is the most chemically reactive fraction of the soil. Most of the fine particle size material is produced by physical disintegration of the Beacon Supergroup sandstones, so that the fine fraction of the soil is dominated by rounded quartz grains of fine sand grade, together with smaller amounts of material produced by glacial grinding.

Clay-sized material largely originates from the matrix bonding the sandstones together, and consists of micas and vermiculites of little chemical reactivity. In some instances, these have been altered by soil weathering processes to more hydrous clay minerals, illites, hydrated vermiculites and (in rare instances) smectites. In some old soils, especially those of higher weathering stages, authigenic clay minerals may be formed. The nature of these minerals is dependent on factors such as soil pH and the chemistry of the salts,

Because the climate is extremely arid, salts, mainly derived from precipitation, accumulate in the soils and strongly influence the soil chemistry. Near the coast, where winds from the sea may carry ocean-derived salts some distance inland, the soil salts are largely chlorides and sulphates of sodium, and the soils are alkaline — up to pH 9 in some cases. This may cause the transformation by hydration of some of the micas into illites and more hydrous clays, even forming some smectites (Claridge 1965). Because the buffering capacity of the soil is very low, only small amounts of salts are needed to raise the pH of the soil to high levels. Soils close to the coast are also generally very young, and contain comparatively low amounts of salts.

Further inland, soils are older, and salts have accumulated to a much greater extent than in coastal regions, often forming thick salt horizons. The salts in these soils are considered to have been derived from the oxidation of protein material caught up from the ocean surface and transported through the upper atmosphere, where they become completely oxidised to nitric acid and sulphuric acid (Claridge and Campbell 1977). Other mechanisms are also proposed, such as auroral fixation of nitrogen. Soils of inland regions therefore have low pH values; as low as 6.0 in ultraxerous soils of weathering stage 5 on the inland edge of the Transantarctic Mountains. The pH of the soil can be directly related to distance from the open sea.

Some breakdown of primary minerals takes place in the acid environment of these soils. The ferromagnesians in particular release iron, which causes the reddish staining on grain surfaces as the iron is oxidised in older soils. Cations such as calcium and magnesium are released, so that the soluble salts, which are such a dominant feature of the older soils of inland regions, are nitrates and sulphates of calcium and magnesium. Almost all crystalline phases that can be formed by combinations of calcium, magnesium, sodium, nitrate and sulphate can be identified in the soils (Claridge and Campbell 1977).

Because the salts in solution lower the freezing point, liquid water can be present at very low temperatures, generally as thin films on grain surfaces, and chemical processes can take place at temperatures as low as -50°C. In most of the old, weathering stage 5 soils of the inland edge of the Transantarctic Mountains, the clay-sized fraction of soils formed on till is dominated by clays derived from Beacon Supergroup rocks. However, in some soils formed directly on physically fragmented dolerite, these clays are absent, and it is possible to demonstrate the formation of authigenic clays, such as nontronitic montmorillonite, a consequence of clay mineral formation in an environment rich in iron and magnesium (Claridge and Campbell 1984). In these cases, though, most of the clay-sized material is physically disintegrated fragments of the glassy matrix of the parent rock of the soil, Ferrar Dolerite. In some situations, especially the very old soils of the inland regions, zeolites such as chabazite (Dickinson and Grapes 1997) may form.

Thus, the chemistry of the soil depends on geographic location, which determines the nature of the salts, the weathering processes operating and the secondary mineral that may be formed.

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