Introduction Various types of stability coefficients, expressed as the ratio of stable to unstable minerals, are widely used as palaeo-geographical indicators (e.g., Gaigalas et al., 1975). From the palaeo-geographical point of view, this ratio reflects the general relation between the processes of physical and chemical weathering. Cryogenic weathering of polymictic source rocks and unconsolidated deposits is associated with strong physico-chemical disintegration of major rock-forming minerals and the accumulation of the latter in definite granulometric fractions. The accumulation of a) quartz particles within the 0.05-0.01 mm grain-size fraction, b) recent feldspar par- ticles within the 0.1-0.05 mm fraction and c) biotite par- ticles within the 0.25-0.1 mm fraction, due to freeze- thaw cycles has been confirmed earlier (e.g., Konishchev, 1982; Konishchev and Rogov, 1993). Correlation between the quartz and feldspar mineral distribution in specific grain-size fractions of unconsoli- dated fine-grained sediments in cold (cryogenic) regions was shown to be exactly opposite to the distrib- ution of these minerals in deposits formed under tem- perate and warm-climate conditions. The reason for this is the protective role of a stable film of unfrozen water. This is highest with biotite and muscovite, less with feldspar, and lowest with quartz (Konishchev, 1982). Cryogenic disintegration occurs when the thickness of the protective unfrozen water film becomes less than the dimensions of the microfractures and defects that characterize the surface of mineral particles. Detailed study of cryogenic disintegration allows one to differentiate globally the physical weathering processes. The special index for cold regions which characterizes the distribution of major rock-forming minerals over the granulometric spectrum is called the cryogenic weathering index (CWI). [1] where Q 1 is quartz content (%) in the 0.05-0.01 mm fraction; F 1 - feldspar content (%) in the 0.05-0.01 mm fraction; Q 2 - quartz content (%) in the 0.1-0.05 mm fraction; F 2 - feldspar content (%) in the 0.1-0.05 mm fraction. Laboratory results Previous work (e.g., Konishchev, 1982) upon soils and sediments in areas of seasonal and perennial freezing now allows a more rigorous substantiation of the rela- tion between CWI and mean annual ground tempera- ture (MAGT). Laboratory experiments reveal the dependence of the cryogenic disintegration rate of vari- ous minerals upon the temperature regime associated with freezing and thawing (Konishchev and Rogov, 1983; Konishchev et al., 1983). Abstract The accummulation of quartz particles within the 0.05-0.01 mm grain size fraction and of the feldspar parti- cles within the 0.1-0.05 mm fraction due to freeze-thaw was confirmed by experimental data and laboratory investigations of cryogenic soils. A cryogenic weathering index (CWI) is proposed to estimate the role of cryo- genic weathering in soil formation. The general zonality of the CWI has already been defined. This permits one to express more precisely the relation between CWI values and mean annual ground temperature. This is obtained for different geocryologi- cal conditions. V.N. Konishchev 591 RELATIONSHIP BETWEEN THE LITHOLOGY OFACTIVE-LAYER MATERIALS AND MEAN ANNUAL GROUND TEMPERATURE IN THE FORMER USSR V.N. Konishchev Department of Cryolithology and Glaciology, Faculty of Geography, Moscow State University, Moscow 119899, Vorobyovy Gory, Russia. e-mail: [email protected]CWI Q F Q F = 1 1 2 2 / /
Transcript
Introduction
Various types of stability coefficients, expressed as theratio of stable to unstable minerals, are widely used aspalaeo-geographical indicators (e.g., Gaigalas et al.,1975). From the palaeo-geographical point of view, thisratio reflects the general relation between the processesof physical and chemical weathering.
Cryogenic weathering of polymictic source rocks andunconsolidated deposits is associated with strongphysico-chemical disintegration of major rock-formingminerals and the accumulation of the latter in definitegranulometric fractions.
The accumulation of a) quartz particles within the0.05-0.01 mm grain-size fraction, b) recent feldspar par-ticles within the 0.1-0.05 mm fraction and c) biotite par-ticles within the 0.25-0.1 mm fraction, due to freeze-thaw cycles has been confirmed earlier (e.g.,Konishchev, 1982; Konishchev and Rogov, 1993).Correlation between the quartz and feldspar mineraldistribution in specific grain-size fractions of unconsoli-dated fine-grained sediments in cold (cryogenic)regions was shown to be exactly opposite to the distrib-ution of these minerals in deposits formed under tem-perate and warm-climate conditions. The reason for thisis the protective role of a stable film of unfrozen water.This is highest with biotite and muscovite, less withfeldspar, and lowest with quartz (Konishchev, 1982).Cryogenic disintegration occurs when the thickness ofthe protective unfrozen water film becomes less than
the dimensions of the microfractures and defects thatcharacterize the surface of mineral particles.
Detailed study of cryogenic disintegration allows oneto differentiate globally the physical weatheringprocesses. The special index for cold regions whichcharacterizes the distribution of major rock-formingminerals over the granulometric spectrum is called thecryogenic weathering index (CWI).
[1]
where Q1 is quartz content (%) in the 0.05-0.01 mmfraction; F1 - feldspar content (%) in the 0.05-0.01 mmfraction; Q2 - quartz content (%) in the 0.1-0.05 mm fraction; F2 - feldspar content (%) in the 0.1-0.05 mm fraction.
Laboratory results
Previous work (e.g., Konishchev, 1982) upon soils andsediments in areas of seasonal and perennial freezingnow allows a more rigorous substantiation of the rela-tion between CWI and mean annual ground tempera-ture (MAGT). Laboratory experiments reveal thedependence of the cryogenic disintegration rate of vari-ous minerals upon the temperature regime associatedwith freezing and thawing (Konishchev and Rogov,1983; Konishchev et al., 1983).
Abstract
The accummulation of quartz particles within the 0.05-0.01 mm grain size fraction and of the feldspar parti-cles within the 0.1-0.05 mm fraction due to freeze-thaw was confirmed by experimental data and laboratoryinvestigations of cryogenic soils. A cryogenic weathering index (CWI) is proposed to estimate the role of cryo-genic weathering in soil formation.
The general zonality of the CWI has already been defined. This permits one to express more precisely therelation between CWI values and mean annual ground temperature. This is obtained for different geocryologi-cal conditions.
V.N. Konishchev 591
RELATIONSHIP BETWEEN THE LITHOLOGY OF ACTIVE-LAYER MATERIALS AND MEAN ANNUAL GROUND TEMPERATURE
IN THE FORMER USSR
V.N. Konishchev
Department of Cryolithology and Glaciology, Faculty of Geography, Moscow State University, Moscow 119899, Vorobyovy Gory, Russia.
Under natural conditions, cryogenic disintegrationproceeds in accordance with different ground freezingand thawing regimes, and with temperature variationamplitudes. Two series of experiments were carried outto study the influence of temperature conditions on thecryogenic transformation of unconsolidated sedimentsof different mineral compositions.
In the first experiments, granulometric mono-mineralfractions of standard sizes and of different mineralswere subject to cyclic freezing and thawing under fourregimes (t¡ = -5¡, +50¡C; t¡ = -10¡, +20¡C; t¡ = -20¡,+20¡C; t¡ = -40¡, +20¡C). The common factor for allthese regimes was the temperature transition through0¡ C.
Minerals are clearly divided into two groups withrespect to these freezing-thawing regimes. The firstgroup is composed of minerals with high surface ener-gy (e.g., biotite and limonite). The cryogenic resistanceof these minerals is minimal under the most rigorousregime (i.e., t¡ = -40¡, +20¡C). The second group com-prises minerals with relatively low surface energy (e.g.,quartz, magnetite and apatite). The maximum destruc-tion of grains of this group occurs under conditions ofcomplete moisture saturation and under probably themost optimum regime of freezing and thawing(t¡ = -10¡ to -20¡ , +20¡C). The minerals that differ in
their cryogenic resistance with respect to freeze-thawcorrespond to the mineral groups identified above.These changes are influenced by the degree of humidity.
In the second experiments, a spectrum of granulome-tric fractions of different minerals in a humid state wasfrozen and then subject to cyclic temperature variations(t¡ = -1¡, -20¡ C). These investigations aimed to establishwhether cryogenic transformation took place or not,and what degree of phase translation occurred inunfrozen water which was associated with negativetemperature variations. Such experiments help toreduce to a minimum the influence of the adjoining iceon particle destruction. The experiments simulate con-ditions of cryogenic transformation of material duringthe cold period of the year, after the freezing of theactive layer when the near-surface permafrost layer canalso change sharply in temperature due to surface tem-perature variations.
The most important and general result of this series ofexperiments is the inference that the cryogenic disinte-gration of all minerals, with the exceptions of horn-blende and magnetite, is considerably lower with nega-tive temperature variations than with temperature vari-ations which involve a transition through 0¡ C.
Relationship between CWI and temperatureIN MODERN soils
The results of the experiments suggest that, undernatural conditions, there is a consistent relationshipbetween the CWI of sediments in the seasonally thawedand perennially frozen layers, and the mean annualground temperature. To quantify this relationship, CWIvalues and corresponding mean annual ground tempe-ratures were determined in different regions of seasonalfreezing and in the permafrost zone (Figure 1).
CWI values and corresponding mean annual groundtemperatures (at depth of 0.4-0.5 m) for 15 regions within Northern Eurasia are calculated and plotted onFigure 2.
The CWI values characterize structurally differentiat-ed taiga soils developed on surface loams in variouswatersheds within the zone of seasonal freezing:Byelorussia (Minsk region, Poozyorye), Klin-Dmitrovsite, Syktyvkar, Troitsko-Pechersk and Laryak (WestSiberia) Ð Figure 1, Numbers 1, 2, 3, 4, 5, 6.
The CWI values for soils in the permafrost zone werecalculated for tundra-gley and taiga soils of the follow-ing areas: (a) mainland tundra-(Vorkuta), Kular moun-tain ridge (Yana-Omoloy interfluve) (see Figure 1,Numbers 7, 8, 9); (b) areas of seasonal thaw-layer:Vorontsovsky; Chukochy Yars (see Figure 1, Numbers10, 11); (c) eluvium of sandstone and siltstone- Sovinayahill, Chukotka; lower reaches of Kolyma River (seeFigure 1, Numbers 13, 14, 15); and (d) soil - eluviumdeveloped in the Pamir highlands (see Figure 1,Number 12).
CWI values were calculated as an average throughoutthe soil profile. The soil thickness does not exceed 50cm. In all cases, samples for CWI are located within theactive layer (i.e., the layer of seasonal freezing andthawing). It is important to emphasize that all sampleswere taken either from mature soils or from the cryo-genic weathering crust of stable interfluve surfaces.
The soils analyzed represent mostly homogeneoussilty loams but sometimes include coarse materials(gravel, pebbles). The correlation between CWI and themean annual ground temperatures (at depth of 0.5 m) isstrong (r = 0.94): the lower the temperature, the higherare CWI values (Figure 2).
Conclusions
The increasing severity of cryogenic conditionschanges the distribution pattern of the quartz-feldsparratio within the sand-silt component of soils. This is theresult of two processes: (1) cryogenic quartz disintegra-tion and (2) chemical weathering of feldspars.
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A lowering of the mean annual ground temperaturetends to increase the cryogenic weathering intensity.This leads to the accumulation of quartz particles in the0.05-0.01 mm grain-size fraction. The mechanism ofcryogenic disintegration is based on the wedging effectwhen ice forms in micro-cracks and induces volumewidening. This effect is very active in tiny fissures andchannels and leads to the disintegration of particles onsmall blocks 10-100 mm in size. Other factors involvedin the cryogenic disintegration of particles are: a) cryo-hydration weathering and the freezing of water in gas-liquid inclusions and b) the breakage of particlesbecause of volume increase. These factors are controlledby the layer of unfrozen water, which is smallest on thesurfaces of quartz particles compared with other miner-als. Cryogenic disintegration reaches its maximumwhen the thickness of the protective unfrozen waterfilm becomes less than the dimensions of the fracturesand defects that characterize the mineral particle sur-
face. For quartz particles, this limit is 0.05-0.01 mm, forfeldspars the limit is 0.1-0.05 mm. This is because thethickness of the protective unfrozen water film onfeldspar particles is higher than on quartz particles.
An increase in mean annual ground temperate leadsto an increase in the duration of summer and the corres-ponding intensity of chemical weathering processes inthe active layer. The processes of chemical weatheringdo not affect the distribution of quartz particles in soilsbecause of the high chemical resistance of quartz. Bycontrast, the feldspar particles are subject to chemicalweathering (mostly hydrolysis) in the active layer. Inparticular, chemically-weathered feldspars disintegrateby cryogenic processes very actively and reach thesmallest grain-size dimensions (Konishchev, 1982). Thecombination of these processes tends to increase thequantity of feldspar particles in the 0.1-0.05 mm fraction.
Thus, the lower the temperature, the higher is the con-centration of quartz and feldspar grains in the 0.05-0.01and 0.1-0.05 mm size fractions, respectively.
This basic relationship between CWI and mean annu-al ground temperature is true for all polymict sedi-ments characterized by a predominance (up to 80-90 %)of quartz and feldspars within their mineral composi-tion. This conclusion is supported by observations inthe northern part of the zone of seasonal freezing and inthe permafrost zone, both areas where chemical wea-thering of minerals is not strong and feldspars do not
lose their mineral individuality. This relationship is alsovalid for non-saline soils and sediments.
Acknowledgements
The author gratefully acknowledges financial supportfrom Russian Science Foundation (grant 97-05-64283)and state support of Russian leading scientific schools(grant 96-15-98457). The author also thanks ProfessorHugh French (University of Ottawa) for substantialhelp in rewriting and editing this paper.
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Figure 2. Relationship between cryogenic weathering index and mean annual ground temperature (tgm). The numbers are identical to those in Figure 1.
Konishchev, V.N. (1982). Characteristics of Cryogenic weath-ering in the Permafrost zone of the European USSR. Arcticand Alpine Research, 3, 261-265.
Konishchev, V.N. and Rogov, V.V. (1983). The cryogenic evo-lution of mineral matter (an experimental model). InProceedings of the Fourth International Conference onPermafrost. National Academy Press Washington, D.C. pp.656-659.
Konishchev, V.N. and Rogov, V.V. (1993). Investigation ofCryogenic Weathering in Europe and Northern Asia.Permafrost and Periglacial Processes, 1, 49-64.
Konishchev, V.N., Rogov,V.V. and Kolesnikov, S.F. (1983).Investigation of main factors and mechanisms of cryogenictransformation of minerals. In Problems of Geocryology.Nauka Press, Moscow, pp. 56-65 (In Russian).
