Soil Science Society of America Journal 67:596-605 (2003)
© 2003 Soil Science Society of America
DIVISION S-5—PEDOLOGY
Mineral Transformations in Permafrost-Affected Soils, North Kolyma Lowland, Russia
A. Alekseev*,a,
T. Alekseevaa,
V. Ostroumova,
C. Siegertb and
B. Gradusovc
a Institute of Physical Chemical and Biological Problems of Soil Science Russian Academy of Sciences, Pushchino, Moscow Region, Russia
b Alfred Wegener Institute for Polar and Marine Research, Potsdam, Germany
c V.V. Dokuchaev Soil Science Institute, Moscow, Russia
* Corresponding author (alekseev{at}issp.serpukhov.su)
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ABSTRACT
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Soils in permafrost sediments were studied on the Kolyma Lowland, North-East Siberia. Mineral transformations were studied by comparing samples from different geochemical positions relative to the permafrost table. The profiles studied are developed from Yedoma deposits (mid to late Pleistocene). Mineralogical composition of study soils is controlled by the illite-chlorite mineralogy of the parent Yedoma deposits, climatic conditions (extremely cold and dry) and the effect of permafrost. Degradation of illite, weathering of Fe-containing minerals (chlorite) with further Fe migration within the active layer and its crystallization at the cryogenic barrier in the form of lepidocrocite are the main processes of mineral transformations in the Kriozems studied. The highest concentration of lepidocrocite is associated with the boundary of permanently frozen ground. Geochemical processes are strongly influenced by the permafrost table. The boundary between the seasonally thawing soil and the permanently frozen ground is an important geochemical barrier.
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INTRODUCTION
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THE PRESENT PAPER deals with mineral transformations in permafrost-affected soils (Kriozems) influenced by permafrost table and its relief. Kriozems are a specific group of cryohydromorphic soils lacking gleyic or stagnic properties. They occur in the arctic, subarctic, and boreal regions under cold continental (semi)humid or semiarid climatic conditions. They support taiga, forest-tundra, or tundra vegetation with a ground cover composed of mosses and lichens. Kriozems develop under the influence of alternate freezing and thawing of the surface layers and permanent subzero temperatures in the subsoil. Intensive cryoturbation, such as cracking and involution, prevents the formation of distinct pedogenic soil horizons except an accumulation of organic matter at the soil surface (weakly and moderately decomposed moss litter). However, incorporation of organic matter into the soil often occurs through cryoturbation. All Kriozems are saturated with water during the summer period. According to Soil Taxonomy (Soil Survey Staff, 1996), Kriozems (Gelisols) correspond to soils, which have a pergelic soil temperature regime, such as Pergelic Cryohemists or Pergelic Cryochrepts (Sokolov et al., 1997).
Perennially frozen ground is an important geochemical soil-forming factor (Makarov, 1993). Because of the presence of ice, frozen grounds differ from thawed soil in composition and properties. At the interface between frozen and thawed soil, an increase in chemical potentials (i.e., possibility of elements to take part in chemical reactions) of the pore-solution components is observed. For example, the diffusion coefficient of ions in a loam, which controls the rate of many reactions in dispersed system, is about 10-6 m2 s-1 under the temperature +20°C and is only 10-9...10-11 m2 s-1 at temperatures below -5°C (Chuvilin et al., 1998). The change in electric potential (so-called freezing potential) may achieve several tens of volts (Ershov et al., 1992). The conditions under which such changes of properties can take place are sufficient to visibly modify the migration of elements and solubility of their compounds (Makarov, 1993). Another specific feature of the cryolithozone is an occasional sharp rise in the mineralization of surface waters related to episodes of deep thawing. Permafrost blocks the infiltration of surface waters and leads to concentration of solutes in the active layer. These and other processes reflect the specific geochemical conditions and special distribution of chemical elements in permafrost-affected soils.
It has been shown that soil formation in Kriozems results in some mineral transformations, as a consequence of their specific properties such as acid pH, humus composition, and contrasting oxidation-reduction conditions. In addition, freeze-drying of soil material results in sharp changes in redox conditions, concentration of pore solutions, coagulation, and crystallization of mineral phases (Naumov and Gradusov, 1964; 1974; Tarnocai et al.,1997; Zvereva and Ignatenko, 1983; Zvereva, 1992). At the same time little is known, however, about mineral Fe behavior in Kriozems. Mössbauer spectroscopy and magnetic measurements data that would confirm morphologically observed mobility of Fe compounds in these soils have not been presented in literature up to now.
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MATERIALS AND METHODS
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Study area and Sampling
The study sites are situated near the Bolshaya Chukochya River (Kolyma Lowland, North-East Siberia, Russia, Fig. 1)
. The study soils are developed from Yedoma deposits. Yedoma deposits are mid- to late-Pleistocene continental loamy sediment with syngenetic permafrost. Yedoma ice-rich sediments are widespread in Beringia and have a relatively low degree of morphological variability and are relatively homogeneous both in the soil profiles and in the upper part of perennially frozen sediments. Deposits are of yellowish, olive, and brownish colors with inclusions of coarse humus and plant roots. Diffuse black, brown, and reddish spots, and microconcretions are observed. The upper layer (0.1–1.5 m) of Yedoma was thawed during Holocene and Late Holocene evolution and secondary frozen layer, so called the transition layer according to Shur (1988), was formed. The transition layer is marked by the ice-rich horizon underneath. This ice-rich horizon is a geochemical boundary between the transition layer and permanently frozen part of the modern Yedoma permafrost. The transition layer was transformed under the impact of positive temperatures, aeration, and active mass transfer within the modern active layer. The active and transition layers have similar morphological and other properties.
The actual thickness of the thawed layer at the date of its measurement (5 Aug. 1994) was up to 32 cm. According to the results of direct 1-wk observations it was increasing with a mean rate of about 2 mm d-1. The depth of thawing was least at sites with a thick peat or moss cover, where a portion of this cover was preserved in the frozen state. The maximal thawing of soil occurred under the bare sites on the surface of hillocks. As a result of irregular thawing with depth, the surface of the thawed layer (permafrost table) is usually bowl shaped. Figure 2
shows the morphology of the upper part of Yedoma deposit and hillocky-depression microrelief.
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Fig. 2. "Hillocky-depression" microrelief and sampling scheme: 1–moss; 2–peat; 3–yellowish, olive, and brownish dusty loam; 4–bottom of modern active layer, loam with the thick-schlieren cryostructure; 5–bottom of transition layer, loam with the large-scale layered or basal-layered cryostructure; 6–permanently frozen loam with horizontal layered cryostructure; 7–ice wedge; 8–boundary of modern active layer; 9–boundary of transition layer; 10–sampling point.
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The bottom surface of the transition layer has a very complicated relief. Generally it is an alternation of depressions and hillocks of irregular shape with the characteristic size from 2 to 5 m in a diameter and a 20- to 35-cm height. The surface of the hillocks is predominantly partly covered with moss vegetation typical for tundra overlying weakly decomposed peat up to 25 cm thick. Such typical microrelief was selected for sampling. Sixteen samples, which characterize zones distinctly different from neighboring ones by their cryogenic structure and other morphological properties, were taken for investigation (Fig. 2).
The soil profile developed under the depression of permafrost table relief (hummock profile) is characterized by the presence of second organic horizon in the bottom of an active layer. According to Gubin (1994)(1997) such dark layers enriched in organic material are buried late Pleistocene soils, age of which is about 13 000 yr. There are, however, other opinions about this commonly observed phenomenon in permafrost-affected soils. According to Sletten (1997), cryoturbations could lead also to organic matter accumulation on the top and within the permafrost zone.
The most significant morphological features of a similar pedon can be seen from the figure and the description given by Tarnocai et al. (1997).
Methods
Ammonium acetate extract (pH = 4.8) was used to determine the content of exchangeable cations. Sodium, K, Ca, Mg concentrations in extracts were determined by atomic absorption spectroscopic (AAS) techniques (Table 1). Soil moisture levels were measured by gravimetric techniques, whereas the ice content was determined as the difference between the total moisture and the quantity of unfrozen water which is typically about 3% at -8°C for the Yedoma deposits (Ershov et al., 1979).
The mineralogical composition of the clay fraction (<2 µm) of samples was determined by x-ray diffractometry. Diffraction patterns were obtained using CuK
radiation and a DRON-3.0 diffractometer (Burevestnik, St. Petersbourg, Russia). The samples were step-scanned between 2 and 35° 2
, using 0.1 and 0.05° 2 increments with a 5-s counting time per increment. The clay fraction was separated by sedimentation in distilled water. Clay was extracted from the soil samples by crumbling them in a wet paste followed by dispersal. This process was repeated three times (Gorbunov, 1971). No additional chemical dispersion of soils before the clay separation has been used. To remove organic matter from clay fractions they were pretreated with 10% H2O2 solution on the boiling water-bath (three to four times depending on organic matter content). Magnesium-saturated samples were prepared using 0.5 M solution of MgCl2. The procedure included a 48-h saturation under stirring of organic-matter free samples (200 mg of solid/50 mL of solution) followed by centrifugation and several washings till Cl-free supernatants. Parallel-oriented specimens were prepared by sedimentation of 20 mg aliquots of monocationic clay onto standard (25 x 25 mm) glass slides. Magnesium-saturated clays were then examined at room temperature, after ethylene glycol solvation and after heating to 300 and 550°C (Thorez, 1976). Ethylene glycol solvation has been done under vapors in the exicator for 48 h.
Diffraction patterns were computer processed to get peak position, its intensity and area. The content of clay minerals was determined by the Biscaye method (Biscaye, 1965) using peak areas on the diffractograms of ethylene glycol solvated specimens. Each stage of the investigation has been done in duplicate.
Iron compounds were studied by Mössbauer spectroscopy and magnetic measurements. The Mössbauer spectra were recorded with a MS1101-E spectrometer (Rostov Uni Instrument, Russia) with a constant-acceleration drive system (57Co/Cr source with an activity of about 1.18 GBq [32 mCi]). The velocity scale was calibrated with reference to sodium nitroprusside and metallic Fe. Spectra were collected on 512 channels of multichannel analyzer until about 1 x 106 to 2 x 106 counts per channel had been accumulated. The spectra were computer-fitted with a number of overlapping Lorentzian peak line shapes using a nonlinear regression 
2 minimization procedure. The relative content of total and divalent Fe in samples was established from the numerical analysis of the Mössbauer spectra using UNIV programs (Dubachev et al., 1990).
The specific magnetic susceptibility was measured with a Kappabridge KLY-2 instrument (AGICO, Czech Republic). The thermomagnetic determination of lepidocrocite (
-FeOOH) followed Vodynitskii's (1989) procedure, which is based on the transformation of the slightly magnetic lepidocrocite into the highly magnetic maghemite (-Fe2O3) by heating at 300 to 350°C.
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RESULTS AND DISCUSSIONS
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The study soils are characterized by reducing conditions with a pH close to neutral or weakly acidic. Water pH of Yedoma deposits is 6.5 to 8.0 and soil horizons have values of 4.8 to 6.0. The direct EH measurements showed that the bottom of permanently frozen part of Yedoma had strong reduced conditions. The EH of the thawed suspension prepared from Yedoma was 120 to 180 mV or less. In comparison, EH of the suspension from upper soil horizon was between 180 to 210 mV.
Table 1 gives selected physical and chemical properties of soils studied.
The clay fraction of Yedoma deposits is dominated by illite (60–65%) with basal reflections at 1.02, 0.5, and 0.33 nm (Fig. 3a)
. In addition, the following minerals are present: Fe-Mg chlorite with basal reflections at 1.4, 0.7, 0.47, and 0.35 nm and kaolinite with reflections at 0.7 and 0.35 nm.
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Fig. 3. X-ray diffraction patterns: (a) Mg-saturated; (b) Mg-saturated ethylene glycol solvated I-illite; Chl, chlorite; K, kaolinite; fsp, feldspar, Qu, quartz. Numbers of samples—see Table 1 and the sampling points on Fig. 2.
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Chlorite besides stable (not affected by glycolation and heating) layers contains unstable interlayers of montmorillonite type (about 5%) which course the decrease of 1.4-nm peak intensity and asymmetry toward the low angle side on glycolation and increase of 1.0-nm peak intensity on heating (Fig. 3, and 4
, Table 2). The clay fraction also contains quartz, feldspar (anorthite), and traces of calcite. The (112) peak of the latter was observed at 2.94 to 2.98 Å. Its position in case of pure calcite is normally at 3.02 to 3.04 Å. Basing on these values we can say about some Mn for Ca substitution in calcite in samples under study.
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Fig. 4. X-ray Diffraction patterns: (a) Sample 11; 1–Mg-saturated; 2–Mg-saturated ethylene glycol solvated; 3–heated to 350°C; 4–heated to 550°C, and (b) Sample 1; 1–Mg-saturated; 2–Mg-saturated ethylene glycol solvated; 3–heated to 350°C; 4–heated to 550°C. Numbers of samples—see Table 1 and the sampling points on Fig. 2.
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The study samples demonstrate slight differences of their XRD patterns with the position within the cryopedon (Fig. 3, and 4). The largest difference exists between the samples from the zone of seasonal thawing (active layer) (Samples 1–6, 14–15) and permafrost zone (7–11, 16–17). Samples from the active layer (Samples 1 and 5 on the Fig. 3 and Sample 1 on the Fig. 4, Table 2) are characterized by lower intensities of 1.4- and 0.7-nm reflections in a comparison with samples from permafrost zone. Chlorite contains minor quantity of unstable interlayers of montmorillonite type (see clay mineralogy of Yedoma deposits). We used the valley/peak (v/p) ratio to describe the changes in symmetry of 1.4-nm peaks after glycolation and smallest value (0.15) was observed for Sample 1 indicating the largest degradation of chlorite. Diffractograms of Samples 1 and 5 (before glycolation and heating) are characterized additionally by the asymmetry of 1.0-nm reflections toward the low angle side (Fig. 3a, 4b). On glycolation and heating till 550°C this asymmetric peak was replaced by more intense and more symmetrical 1.0-nm peak. We regard this structure as a weathering product of illite some layers of which became unstable and behaved like montmorillonite layers. Both underlined features are better observed for the samples under the hummock of the cryopedon (Samples 1–6) than under the interhummock depression (Samples 14–15) (Fig. 5)
. The influence of the relief of permafrost table on the intensity of geochemical processes is supported also by some other properties (Table 1). Samples from the active layer under the hummock of the cryopedon (Samples 1–6) contain visibly less clay fraction in comparison with samples from the permafrost zone (Samples 7–11). Sample 1 is characterized by the smallest content of exchangeable Ca and Mg. Both samples from the active layer of the interhummock depression (Samples 14–15) contain visibly more clay. Upper horizon is enriched in exchangeable Ca and Mg.
Based on these data, we hypothesize the following processes of clay mineral transformation within the active layer: degradation of chlorite and degradation of illite with formation of randomly interstratified chlorite–smectite and illite–smectite respectively.
Iron-Mg chlorite is unstable under soil formation and its degradation is a common soil process taking place in many soil types including Cryosols (Sokolov and Gradusov 1978; Sokolov and Gradusov, 1981). Mica degradation with K release as a consequence of periodic freezing and thawing and wetting and drying has been noted by Goulding (1986) and Robert (1986).
The Mössbauer spectra obtained at room (298°K) and liquid N (90°K) temperatures represent doublets with the superposition of lines of Fe2+ and Fe3+, which occupy different structural positions in minerals. Iron-57-Mössbauer spectroscopy is specific for Fe so that analyses may be conducted of Fe bearing compounds in low concentration and without spectral interference from other minerals. It can be used to study finely divided or poorly crystalline materials. The Mössbauer spectrum of a Fe mineral is characteristic of that mineral and, with proper interpretation and under suitable conditions, can be used as the fingerprint. Three parameters, the isomer shift (
), quadrupole splitting (
EQ), and magnetic hyperfine field (Bhf) are commonly extracted from the Mössbauer spectra of Fe compounds. Isomer shift values for Fe2+ and Fe3+ usually fall within characteristic but separate ranges so that this parameter may by used to determine the oxidation state of Fe in a compound. The numerical values of the Mössbauer spectra parameters depend on the type and magnitude of hyperfine interactions that exist between the charge distribution of 57Fe nuclei and extranuclear electric and magnetic fields. The significant distinctions in parameters allow with confidence to distinguish Fe2+ in silicates and Fe3+ in silicates and Fe hydroxides. The analysis of Fe bulk distribution was based on Mössbauer spectroscopy data. The Fe concentration in a test sample is proportional to the area under the spectral curve (Mössbauer spectra squares) standardized for the background and equal sample mass.
High Fe2+/(Fe2+ + Fe3+) ratio—30 to 40% (up to 50%, in frozen sediments) reflects the low intensity of weathering. Fe2+ occupies the structural positions in chlorite and illite (Fig. 6)
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Fig. 6. Profiles distribution of Fe2+ in silicates, percentage of total Fe in (a) clay fraction and (b) bulk sample.
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There is less Fe2+ in the structure of silicates in the active layer and significantly more (twice as much) total Fe in clay fraction on the boundary with permafrost zone are observed (Fig. 7) .
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Fig. 7. Profiles distribution of total Fe, relative units (Mössbauer spectroscopy data). (a) Clay fraction and (b) bulk sample.
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Goethite, hematite, and other hydroxides were not observed by means of Mössbauer spectroscopy. The results of thermomagnetic analysis indicate the presence of lepidocrocite (- FeOOH). The maximal concentration of lepidocrocite—0.34% by weight of a sample in a clay fraction and 0.16% for a sample as a whole—are observed at the permafrost table (Fig. 8)
. In a permafrost zone the concentration of lepidocrocite is noticeably lower, 0.01 to 0.03%.
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Fig. 8. Lepidocrocite profiles distribution, percentage from sample weight (a) clay fraction and (b) bulk sample.
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Lepidocrocite is less stable than goethite and occurs mainly in soils. Wet soil and gley horizons are the most enriched in lepidocrocite. The formation of lepidocrocite requires an adequate amount of Fe2+ in solution. It is most abundant in clay of noncalcareous soils with periodical and local anaerobic conditions. Not only the reduction, but also the slow oxidation of Fe2+ is important for its formation (Schwertmann and Taylor, 1989; Schwertmann, 1993; Vodyanitskii, 1989; Alekseeva and Alekseev, 1997).
The conditions necessary for lepidocrocite synthesis are observed in an active layer. As a result of the seasonal dynamics of temperature, moisture, pH and EH, the highest concentration of lepidocrocite exists in the bottom of the active layer. The high content of lepidocrocite at the boundary with permafrost in the microdepression is, probably, associated with accumulation of enrich with Fe 2+ solution with its sedimentation and oxidation. Permafrost blocks the infiltration of surface waters and leads to concentration of solutes in the active layer. Both low temperature and evaporation cause the development of reductomorphic conditions and slow-rate oxidation. In a course of chlorite degradation in active layer the Fe2+ is mobilized from its structure and ingress of Fe2+ to the soil solution with its subsequent oxidation results in the lepidocrocite formation.
The samples studied are very weakly magnetic (10 x 10-8–30 x 10-8 m3 kg-1). The clay fraction has a lower magnetic susceptibility than the soil as a whole (5 x 10-8–10x10-8 m3 kg-1) (Fig. 9) . Fine-grained magnetically susceptible minerals such as Fe oxides do not form in the active layer and particles of paramagnetics contribute mainly to susceptibility. An increase of magnetic susceptibility in the permafrost seems to be associated with the content of Fe silicates, such as Fe chlorites.
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Fig. 9. Magnetic susceptibility profiles distribution, x 10-8 m3 kg-1 (a) clay fraction and (b) bulk sample.
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Statistical multivariate classification techniques such as cluster analysis has been used to group samples from different zones. Software package, STATISTICA (StatSoft, Inc) provides a comprehensive range of cluster analysis option. Ward's method was used to group or agglomerate samples. In the analysis (Fig. 10)
were included all described above mineralogical parameters for clay fraction (14 parameters). At the broad level samples cluster into three main groups—samples from active layer, samples of transition layers, and permanently frozen zone.
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Fig. 10. Dendrogram (cluster analysis) for samples from different zones using the mineralogical parameters of clay fraction (Fe bulk, Fe2+ content, magnetic susceptibility, content of lepidocrocite, clay fraction content, mineralogical composition and XRD peaks parameters).
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CONCLUSIONS
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The investigators underline the extremely weak intensity of chemical weathering of Yedoma deposits because of their specific mineralogy, continental origin, cold and arid climate conditions (Grinenko and Sergeenko, 1980). The illite-chlorite association of the parent Yedoma deposits dominates the clay fraction. Degradation of illite and Fe-containing minerals (chlorite) with further Fe migration within the active layer and its crystallization in the form of lepidocrocite are the main processes of mineral transformations in the Kriozems under investigation. The highest concentration of lepidocrocite is associated with the boundary between the active layers and permanently frozen ground which is an important geochemical barrier.
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ACKNOWLEDGMENTS
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The International Association for the promotion of cooperation with scientists from Independent States of the Former Soviet Union supported this work (Projects INTAS-93-0266 and INTAS 01-2329).
Received for publication January 18, 2000.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
