Characteristics of organic mineral and chemical composition of frozen earth materials

The organic-mineral and chemical composition of frozen materials and their granulometry have not been well studied to the present time. Much attention was earlier given to the study of ice (as a mineral and a rock) as a component of big accumulations and deposits. However, seasonal and perennial freeze-thaw bring about not only transformations of ice, but also changes in mineral, chemical and organic composition of the mineral matrix.

Chemical processes that occur in the regions of development of frozen ground have a specific nature since the materials typical of these regions are mainly acidic-neutral and reducing, with increased content of carbon dioxide gas, dissolved carbon dioxide and fulvic acids. The processes of coagulation and peptization are widely developed giving rise to the formation of colloidal and silt-sized particles.

The mineral portion of a frozen soil usually comprises primary water-insoluble minerals, secondary water-insoluble minerals, secondary minerals soluble in water, organic and organo-mineral compounds. The specific feature of the frozen ground is the availability of a new structure-generating mineral, ice, the structure of which is determined by the conditions of formation of the frozen ground and its origin. Ice may fill the pores, form intercalations, lenses and cryogenic conglomerates, or substantial amounts of accumulated ice can form a monomineral rock as represented by ice wedges, injection ice and other types of ground ice.

Primary minerals and their aggregates which arise as a result of physical weathering of igneous and metamorphic rocks in the freezing regions differ greatly from the minerals formed outside these regions. This is demonstrated first of all by the increased content of minerals poorly resistant to weathering. Montmorillonite, hydromicas and beidellite are predominant, among secondary water-insoluble minerals which usually form in the freezing regions as a result of degradation and transformation of laminated and banded silicates and feldspars.

Secondary minerals soluble in water are represented in freezing ground by bicarbonates of calcium and magnesium, calcium and sodium sulphate as well as sodium chloride. Easily soluble salts (chlorides and sulphates) are found in solution, whereas poorly soluble ones (carbonates) are most often in the solid state. With lowering of negative temperature of frozen sediments, according to the degree of solubility, carbonates are the first to precipitate, then follow sulphates. The salts that include crystallization water form the crystalline hydrates which are solid components of the frozen sediments.

The availability of clathrate compounds is typical of the permafrost regions. These usually form at a depth of over 1000 m and constitute useful reserves of methane, ethane, hydrogen sulphide, carbon dioxide, etc. Among mineral deposits of the permafrost regions of importance are cryopegs -thick formations (over 1000 m) with highly saline water that does not freeze at negative temperatures and contains a number of easily soluble salts of calcium, magnesium, sodium, potassium, etc. A significant role in the national economy is played by the ice itself, in its different forms - sheets, veins, icings, etc.

Organic matter in the permafrost regions may take the form of poorly decomposed vegetation and wildlife residues and products of their degradation - humus. The products of degradation migrating in soil form the specific soil horizons of the northern regions. For soil with good drainage in the permafrost regions the development of soil is characterized by podzoliz-ation with 'tialferization' (Ti-Al-Fe) and formation of illuvial humus.

Gley soils formed in poorly draining ground are widely developed in tundra regions. Low permeability of clayey and mixed silty soils in the presence of a shallow table of perennially frozen soils leads to poor differentiation of soil horizons. A gley horizon underlies the peaty layer and gradually grades into the bed material. Sometimes beneath the peaty layer a discontinuous coarse horizon of humus accumulation is distinguished. At the boundary with the perennially frozen soils a higher Fe and humus content is observed.

In the freezing regions peaty soils are widely developed, resulting from degradation of marshy vegetation in conditions of excessive moisture, shortage of oxygen and low temperatures.

Peaty soil is usually subdivided into upper peats that cover elevated sites (watersheds, divides and slopes) and lowland peats that cover low places and depressions. The lowland peats form in more humid conditions with poor drainage and, therefore, their composition differs. The lowland peats contain greater amounts of humic substances (up to 30%) and vivianite, have a higher ash content and less acidic medium. Waterlogging, formation of peat and slow degradation of vegetation and wildlife residues promote formation of different hydrogen-containing compounds: methane CH4, hydrogen sulphide H2S, etc.

The grain size of the organic-mineral skeleton of the frozen soils and its mineral composition have a specific nature. In mountain regions the processes of physical-cryogenic weathering (cryoeluviation) occur most intensely, causing fracturing of hard rock giving big blocks, boulders and down to sandy and silty fractions. The lowland deposits, unlike the mountain ones, do not contain big blocks of fragmental material. However, in their fine-grain fractions, silty particles prevail. For the mountain regions it is typical that sedimentary material is poorly sorted and becomes even less homogeneous while grading from one genetic type into another (from eluvium into talus and alluvium). The main reason for such lack of differentiation is intense accumulation of big rock fragments in the upper parts of slopes with a rather intense transfer of fine-grained material to the lower parts of slopes. Another important reason is associated with prevalence of cryogenic degradation processes (physical weathering) in the sedimentary rocks in the wetter lower portions of slopes. Thus, for instance, eluvium of the region adjacent to Kolyma is represented by a narrow range of fractions (10— 30 mm) with the absence of fine earth in the upper part of the section and occurrence of sand and coarse silty particles (Fig. 7.1). As regards talus the fractions occur in the range of 10-30 mm, but there are many fine-grained sandy and silty fractions. In the alluvial deposits the maximum is shifted towards fractions of 5-20 mm with significantly increased content of sand and silt, while in the fine-grained fraction silty particles prevail, in general, the materials become less homogeneous and less sorted, i.e. differentiation of sedimentary material takes place. On plains, where coarse fragmental material is absent, soils are more homogeneous and sorted.

Intense cryogenic breakdown of fragmental rocks conditioned by the processes of cyclic freezing and thawing with poor differentiation of weathering products leads to polydispersion and heterogeneous porosity typical of the northern regions. Differential curves of grain-size distribution of particles (both primary and secondary) usually have a multiple-mode pattern which reflects a high degree of dispersion of the mineral part of soils. The most common modes of fine-grained soils are in the range of colloidal, clayey, sandy, fine- and coarse silty fractions. Polydispersion is characteristic of

Fig. 7.1. Distribution of fractions of fragmental material of various origins (according to Yu.V. Shumilov): 1 - eluvium: 2 - talus; 3 - alluvium.

Fig. 7.1. Distribution of fractions of fragmental material of various origins (according to Yu.V. Shumilov): 1 - eluvium: 2 - talus; 3 - alluvium.

sands, sandy silts, silty clays and clays having, as a rule, a polymineral composition. Monodispersed soils characterized by one distinct mode are found amongst heavy clays and sands and are monomineral for the most part.

Cryogenic breakdown of sandy soils gives rise to the formation of loessi-fied deposits most typical of soils with seasonal thawing and freezing. The granulometric composition of loess-like deposits is rather homogeneous: the coarse silt fraction (0.05-0.01 mm) being 60-90%. Loess-like soils are among the most developed deposits of plains and surrounding piedmonts. They are represented by surficial loessified fine material in Bol'shezemel'skaya Tundra; similar deposits are encountered in West Siberia, Northern and Central Yakutia, etc.

The organic-mineral part of the frozen ground is usually cemented with ice which results in the formation of ice-containing clayey and sandy soils, pebble gravel, ice breccia and ice conglomerates. The pores formed by the mineral skeleton of these soils are completely or partially filled with ice. Therefore, interstitial space and porosity are very important characteristics as is the degree of their filling with ice or unfrozen water. The void volume of the frozen soils is often small, differs in materials of different genesis and diminishes in going from eluvial to talus and alluvial deposits.

Grain-size distribution in the cryolithic zone is greatly influenced by the material's genesis. Thus, coarse fractions are typical of alluvial deposits, while in high mountain areas these deposits are mainly represented by fragmental material cemented with ice. In the cryoeluvial formations of plateaus, a rock waste fraction prevails, while the silty and clayey fractions are as much as 25%. On steep slopes rudaceous rocks are formed without fine-grained infilling owing to washing out of fine products of degradation, whereas on gently sloping hillsides fine material is continuously accumulated.

A distinct sorting is typical of the alluvial deposits of the permafrost regions: in the river bed - rudaceous and coarse-grain deposits; on the shallow places adjacent to the river bed - more fine-grained material; while in the floodplain silty and clayey deposits prevail.

The grain size of marine and lacustrine deposits in the permafrost regions is dictated by specific features of their genesis. Conveyance by river ice and by icebergs is of key importance in the distribution of rudaceous material in sediments of marine origin. The dispersion halo of this material is enormous. Therefore, deep-water microfractions of the bottom marine sediments of the permafrost regions often include gravel, pebbles and boulders.

Lacustrine deposits are represented by silty-clays and clays saturated with organic matter, having a specific, banded lamination. Intercalations of coastal sand and rudaceous inclusions, transported by river and lacustrine ice, are also encountered in these deposits.

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