Lateral and Radial Differentiation of Cryogenic Soils Chemical Composition in the Vilyuy River valley, Central Yakutia

IntroductionPermafrost rocks in the central part of the cryolithozone are among the oldest and most powerful on the planet: their age reaches several hundred thousand, and maybe 1-2 million years, and the power varies in the range from 500 to 1,500 m.^[7,8,12] Modern climatic changes contribute to the dynamics of permafrost rocks ^[11,15,29], the unevenness of which in space and time creates a variety of cryogenic landscape settings in this region.

^[19] One of the consequences of increasing the capacity of the seasonal melt layer (STS) of modern cryolithozone soils is the involvement of the biological cycle (MBC) in the small biological cycle^[25] chemicals not previously involved in it due to the permafrost state of the rocks containing them. Chemicals released from frozen soil horizons are interesting to study not only because of changes in biogeochemical cycles, but also from the point of view of the actual effect of cryogenesis on the chemical composition and geochemical properties of cryogenic soils. The region of Central Yakutia in the middle reaches of the Vilyu River is one of the unique cryogenic processes developed here – frost cracking, thermosuffusion, thermokarst, thermal erosion, solifluction, frost sorting, ice formation, etc.^[12], therefore, data on the cryogenic factor of formation and dynamics of geochemical properties of local soils are very interesting.

2.      Objects and methods2.1. Physical and geographical conditions of the research area

Geographical location

The research area is located in the valley of the Vilyu River (Central Yakut plain) in its middle course (63 °54'10.92" s.w., 122°33'17.87" v.d.; altitude above sea level about 100 m).

10.5 km to the east of it is the village of Kysyl-Syr of the Vilyuysky district. The research area is 410 km to the northeast from Yakutsk.

Brief physical and geographical characteristics of the research area The Central Yakut Plain is located in the Vilyui syneclise, an upper–order element of the Siberian Platform.

^[31] The geological foundation of the territory is formed by sedimentary deposits of Cretaceous age (K2 – upper section of the Cretaceous system), the thickness of which reaches 150-200 m here.^[9] The banks of the Vilyuya in its middle course are composed of alluvium of different ages (from 39 to 12 thousand years). Low floodplains are sandy material with rare horizontal layering, high floodplains and terraces are more often sandy loam with inclusions of pebbles.^[33] On the right bank of Vilyuya there is a complex of 5 above-floodplain terraces with sharp erosion ledges, the left bank has only up to 3 such terraces, and the width of floodplains on it is 2-3 times less than on the right.^[9] On both shores, large sandy Aeolian massifs with spear–shaped fixed and unfixed dunes - tukulans are quite characteristic.^[17] Permafrost rocks (MMP) in central Yakutia are distributed continuously vertically and laterally, their vertical thickness reaches 320 m in the middle course of the Vilyui.^[23,36] The average temperature of rocks at the boundary of the layer of seasonal fluctuations is from -1 to -3 ° C, the iciness is low – from 0.2 to 0.4%.^[24] The power is seasonally-the melt layer (STS) of surface sediments is very uneven here and depends primarily on their composition: peat bogs thaw by only 0.5 m, while sandy loams and loams – up to 1.5 m, and the STS of sands can exceed 4 m.^[2] The Vilyu River, the left and largest tributary of the Lena River, has a river a basin with an area of 57.3 thousand ^km2, several large tributaries (rr. Chona, Markha, Tung, Olekma, etc.) and a bicarbonate-calcium chemical composition.^[4] Central Yakutia is characterized by the presence of taliks under riverbeds and lake basins, it is known about the spread of subaerial above- and inter-permafrost aquifers on the terraces of large rivers.^[18] Characteristic features of groundwater here are their slightly acidic reaction, low mineralization (up to 0.1 g/l), mainly bicarbonate calcium-magnesium/sodium chemical composition.^[26] The climate of the area belongs to the sharply continental type, the average temperature of the coldest month of the year - January – is -37.8 °C, the warmest - July – +17.9 °C. The average annual air temperature here is about -9.3°C.^[20] The average annual precipitation in Central Yakutia does not exceed 250-300 mm.^[38] Because of this, the snow cover here is generally low, reaches a maximum capacity of 25-30 cm in March, after which it rapidly melts by the end of April.^[22] According to the complex of natural and botanical features, the territory belongs to the Vilyui district of the Central Yakut Middle Taiga subprovincion of the middle Taiga forest subzone.^[37] The vegetation cover is characterized by the predominance of cranberry and bagulnik green-mossy larch forests with pine forests.^[10] Siberian larch (Larix siberica L.) significantly dominates in the structure of the stand – 79.6%, also the common pine (Pinus sylvestris L.) occupies an extensive area – 8.6%.^[5] 9 zonal, 2 azonal and 7 intrazonal soil types were identified in this territory, it is noted that such a high diversity of soil The effect of individual soil-forming macroprocesses on the background of cryogenesis, typical both for Central Yakutia and for the cryolithozone as a whole, is the effect of the soil-forming macroprocesses.^[32] The zonal and most common automorphic soils here are pale yellow and permafrost podzols. According to landscape-geochemical characteristics, the landscapes of the upper and middle parts of the Lena basin have the following formula:

where I is the type of landscape with slow water exchange and the predominance of chemical denudation over mechanical; a is the type of landscape by clay and loamy alluvial deposits (not loess–like); Ca,Na–HCO ₃ – typomorphic elements in the landscape (cations and anions); N, P, K, J ... – deficient elements in the landscape; redundant elements and connections are not marked in the landscape.^[21]

            In more detail, the literature data on the physical and geographical conditions of the territory are presented in the work.^[3]

2.2. Field research methodsField work was carried out at the site in the valley of the Vilyu River in July 2021.

In the course of the research, 12 soil sections were laid in various landscape conditions from the flat surface of the second over-floodplain terrace of Vilyuya to the bottoms of small erosion forms and thermosuffusion funnels (Fig. 1).

Fig. 1. The research area near the village of Kysyl-Syr and the eastern part of tukulan Mahatta. Embedded soil sections are marked with red dots

5 sections out of 12 made up the soil-geochemical catena of the left bank of the Vilyuya (Fig. 2). Soil sections were laid using shovels. In case of reaching the upper limit of the MMP, samples of frozen rocks were knocked out of the thickness with an axe. Samples of seasonally thawed soil horizons were collected using knives in zip-lock plastic bags. The total mass of each sample was 250-500 g. In the field, soil horizons were described according to the scheme proposed for a comprehensive description in the Classification and Diagnostics of soils in Russia^[14] and the Field Soil Determinant^[30]. Field names of soils were also given based on combinations of diagnostic genetic horizons and features indicated in the Field Soil Determinant^[30].

Fig. 2. Schematic representation of the soil-geochemical catena on the left bank of the Vilyu River: 1 – the surface of the II above–floodplain terrace; 2 – the gentle slope of the II above–floodplain terrace; 3 - the surface of the I above-floodplain terrace; 4 - the steep talus slope of the I above-floodplain terrace; 5 – the Vilyuya floodplain; 6 – the line of the landscape profile; 7 – the locations of soil sections2.3. Laboratory research methods

In the packed state, the samples were delivered to the chemical and analytical laboratory of the Ecological and Geochemical Center (EGC) of the Geographical Faculty of Moscow State University (Moscow).

They were dried at room temperature to an air-dry state, after which they were subjected to appropriate sample preparation procedures for further laboratory study of the chemical composition and physico-chemical properties. The following analytical indicators of soils were analyzed in laboratory conditions: acidity of aqueous extract (pH), total content of easily soluble salts (TDS), total content of organic carbon (C _org.), granulometric composition of soils (indicating the class in accordance with N.A. Kachinsky's systematics), concentrations of non-silicate (Fe _hc) and amorphous (Fe _a) iron in soils, as well as the gross content of macro- and microelements.

Acidity of water extractsTo analyze the acidity of soils, the sample was ground with a porcelain pestle in a porcelain mortar, and then sifted through a sieve with a pore diameter of 1 mm.

The acidity was measured with an ion-selective METTLER TOLEDO electrode in a suspension made in a ratio of soil to distilled water of 1:2.5, and in the case of analysis of organogenic horizons, the ratio decreased to 1:25.

Total content of easily soluble salts           

Sample preparation for the analysis of the content of easily soluble salts was similar to that described above for the study of soil acidity. The content of water-soluble salts in soils was studied by a METTLER TOLEDO laboratory conductometer in a suspension made in a ratio of soil to distilled water of 1:2.5.

Total organic carbon content     

The concentrations of organic carbon in soils were investigated by the method of I.V. Tyurin with photometric termination. To do this, the soil sample was ground with a porcelain pestle in a porcelain mortar, then sifted through a sieve with a pore diameter of 0.25 mm. A soil sample weighing 1 g weighed on analytical laboratory scales was filled with 10 ml of 0.4 M potassium bichromate (K ₂ Cr ₂ O ₇) and placed in a drying cabinet heated to +150 ° C for 40 minutes. Then the extract was cooled at room temperature and titrated with 0.2 M solution of the Salt of Pestilence (FeSO _?·(NH _?)_? SO _?·6H _? O) in the presence of 5-6 drops of phenylanthranilic acid (C ₁₃ H ₁₁ NO ₂). The end point for this type of redox titration is the change of color of the solution from reddish-brown to dark green. The calculation of the organic carbon content was carried out according to the formula:

where V ₁ is the volume of the Mohr salt solution that went to titrate the blank sample (samples without a sample) of the soil (cm ³); V ₂ is the volume of the Mohr salt solution that went to titrate the sample with a soil sample (cm ³); M is the molarity of the Mohr salt solution; 0.003 is the molar mass of ? S (g/mol); 100 is the conversion factor per 100 g of soil; m is the mass of the dry soil sample.

Granulometric composition The soil sample for granulometric analysis was ground with a rubber pestle in a porcelain mortar and sifted through a sieve with a pore diameter of 1 mm.

The granulometric composition of soils was studied by laser granulometry after grinding the sample in a porcelain cup with a rubber pestle with a solution of sodium pyrophosphate (Na ₄ P ₂ O ₇). Pyrophosphate is used to eliminate the adhesion of soil particles. The analysis of the granulometric composition of soils was measured using a laser granulometer Fritsch Analysette-22 (Germany). To classify soils according to the content of the fraction of physical clay in them, the N.A. Kaczynski scale was used (Table 1).

Table 1. Classification of soils by the content of the fraction of physical clay (N.A. Kachinsky)

Concentrations of non-silicate and amorphous iron

Determination of non-silicate iron compounds (Fe _hc) was performed using the Mera-Jackson method. A sample of air-dry soil sifted through a sieve with a diameter of 1 mm holes, weighing 2 g, was placed in a centrifuge tube with a capacity of 50 ^cm3. 20 ml of 0.3M sodium citrate solution (Na ₃ C ₆ H ₅ O ₇) and 2.5 ml of 1M sodium bicarbonate solution (NaHCO ₃) were poured into the test tube, then 0.5 g of dry sodium dithionite powder (Na2S2O4) was introduced into the test tube and mixed with a glass rod. After that, 5 ml of saturated sodium chloride (NaCl) solution was added to the test tube, stirred and left in a water bath until a flake-like precipitate was formed. Then centrifugation was carried out for 10 minutes at 3000 rpm, the centrifuge was drained through a dry dense filter into a measuring flask with a capacity of 250 ml. The concentration of iron in the resulting solution was determined by spectrophotometry.

Extraction of amorphous iron compounds (Fe _a) from soils was carried out by the F method. Tamma, based on the extraction of crystallized iron using a Tamma buffer solution (0.14 M by H ₂ C ₂ O ₄ and 0.2 M by (NH ₄)₂ C ₂ O ₄). A soil sample filled with a buffer solution weighing 1 g was filtered through a folded paper filter "blue ribbon" with a pore diameter of 0.45 microns. The filtration procedure was repeated 3 times, after which the solution was diluted with distilled water in a 250 ml volumetric flask and poured into plastic tubes. The analysis of concentrations of amorphous forms of iron was carried out by spectrophotometry.

Gross contents of macro- and microelements Concentrations of macro- and microelements were measured in soil samples ground in an agate mortar and sifted through a sieve with a pore diameter of 0.25 mm by X-ray fluorescence analysis (XFA).

For this purpose, a portable mining and geological X-ray fluorescence analyzer Olympus Delta Professional (USA) was used. This device is capable of determining the gross concentrations of 35 chemical elements with atomic masses from Mg to U.  

2.4. Geochemical landscapes and coefficientsDescriptions and characteristics of geochemical features of the landscape are given in accordance with the work of A.I. Perelman and N.S. Kasimov^[28].

To characterize elementary geochemical landscapes, they were given names in accordance with the classification of M.D. Bogdanova and co-authors^[1]. The forms (and elements) of the relief are used here as the main criterion for the allocation of elementary landscapes.

Geochemical coefficients of radial and lateral differentiation were used to characterize the spatial heterogeneity of the chemical composition of soils.^[27,28] The radial differentiation coefficient (R) is used to study the profile (vertical) heterogeneity of the chemical composition by comparing the concentrations of specific substances in soil horizons with those in the horizon of the soil-forming rock:

where R _{x (mountains)} is the concentration of chemical substance x in the soil horizon; R _{x (pores)} is the concentration of chemical substance x in the soil-forming rock.

  The lateral differentiation coefficient (L) is used to study the catenary (subhorizontal) heterogeneity of the chemical composition of the landscape. In this case, a comparison is made between the concentration of chemical substance x in the soils of subordinate and autonomous (eluvial) elementary geochemical landscapes. This indicator is intended to characterize the lateral migration of chemicals in soils connected by single streams of moisture moving along the relief from top to bottom under the influence of gravity. The soil-geochemical catena acts as a model area when using L:

where L _{x (sub)} is the concentration of chemical substance x in a subordinate elementary geochemical landscape; L _{x (auth.)} is the concentration of chemical substance x in an autonomous (eluvial) elementary geochemical landscape.

       When analyzing the catenary heterogeneity of the chemical composition of soils in the Vilyuya Valley, data on the L coefficients of seasonally thawed and permafrost soil horizons are presented in parallel.

3.      Results3.1. Classification, diagnostics and properties of soils

The studied soils, according to the taxonomic levels accepted in the classification and diagnostics of soils in Russia ^[14], belong to the trunks of syn- (alluvial serohumus) and postlitogenic soil formation (other soils).

The rates of accumulation of river sediments in alluvial soils are comparable to the rates of soil formation, which leads to a cyclical nature of soil formation in them. The soils of the postlithogenic trunk belong to the following types: sod-podzols and sod-podburs (soils developing on elevated terrain areas – above-floodplain terraces and their slopes), peat–gleezems (in wet bottoms of small erosive relief forms - ravines and gullies), gray humus, including those buried under Aeolian sediments on tukulans, as well as stratozems on river floodplains and psammozems – on swampy and watered areas inside thermosuffusion funnels. In more detail, the results of classification and diagnostics of soils, as well as descriptions of the morphology of soil profiles are presented in our work.^[3]

Fig. 3. Diagrams "box with moustache" showing statistical indicators of soil properties (a) and the content of granulometric fractions (b): 1 – minimum values; 2 – quartile 25%; 3 – arithmetic averages; 4 – quartile 50%; 5 – quartile 75%; 6 – maximum values

The acidity of soils varies in the studied profiles from 2.8 to 7.6 units, which corresponds to gradations from strongly acidic to close to neutral. Basically, the acidity is regulated by the content of organic matter as a source of easily decomposable organic substances, which are a source of free hydrogen ions during destruction. Surface organogenic (T, O and AY) horizons, as well as buried peat horizons ([O], etc.) differ by more acidic reactions. Moreover, if the pH of the AY gray humus horizons differ slightly from the underlying horizons of mainly mineral phase composition (E, BF, etc.) – by about 0.5–1.0 units, then peat horizons are the most acidic (pH drops to 2.8). In general, the soils are characterized by an eluvial-illuvial type of profile distribution. The average pH value is 5.7, and the spread of standard deviations is ± 0.9 with a coefficient of variation of 0.2 (Tables 2, 3).

The total content of easily soluble salts and the associated electrical conductivity are distributed relatively evenly in soils, while the salt content is low, it rarely exceeds 10.0 mg/l, respectively, EC rarely takes values above 20.0 µs/cm. The average salt content in soils is 14.7 ± 19.3 mg/l, and the coefficient of variation is 1.3. Organogenic horizons, as a rule, are slightly enriched with water-soluble salts (by 3-5 mg/l), however, there are also very high values (up to 83.7 mg/l) associated with peat and buried coarse humus horizons (Table. 2). In the profiles, the EC and TDS values are distributed with maxima in the upper part and with monotonic distributions in the lower ones.           

It is obvious that the surface horizons of soils contain the highest (up to 11.3%) concentrations of _{organic matter.} In deep horizons of soils slightly affected by soil–forming processes, the contents of _organic matter decrease and are further distributed evenly (concentrations of about 0.3-1.0%), with the only exceptions that in buried organic horizons (from 1.0 to 11.3%) these values increase significantly. With an average concentration of organic carbon equal to 1.5%, the standard deviations are ± 2.4%, and the coefficient of variation is 1.6 (Table 2).

            Among the iron fractions considered, the non-silicate fraction is concentrated in the studied soils in concentrations 11 times greater than the amorphous fraction. The concentration range of the Fe _hc fraction is 0.35%, which is about 2.5 times higher than the range of Fe _a concentrations. Presumably, the proportion of amorphous iron fraction in the composition of a non-silicate group of compounds is so low due to the shallow depth of soil formation, as a result of which the organic carbon content is low (especially in the mineral part of soil profiles).

Fig. 4. Diagrams "box with whiskers" showing statistical indicators of macro- (g/kg) (a) and microelement (mg/kg) (b) composition of seasonally thawed (c), seasonally thawed cryogenic (d) and permafrost horizons of soils (e): 1 – minimum values; 2 – quartile 25%; 3 – arithmetic averages; 4 – quartile 50%; 5 – quartile 75%; 6 – maximum values

            In almost all cases considered, the contents of all the studied chemical elements vary more widely than their contents in cryogenic and permafrost horizons (Fig. 4). This is most likely due to the increased contents of elements in the surface organogenic horizons of soils, which is characteristic especially for elements with an increased tendency to associate with organic matter. substance and sorption on colloids of organic nature – Ca (up to 1.3 g/kg), Mn (up to 0.14 g/kg), Fe (up to 4.3 g/kg), etc. It is these peaks that are associated with the increased maximum concentrations of these metals, reflected in Fig. 4, and they are also associated with the fact that the boundaries of the quartiles of the Fe, Mn, V, Zr and Zn contents are shifted to the lower limit of the concentration amplitude.

            Many average element contents in the cryogenic horizons of the seasonally thawed soil layer are lower compared to the seasonally thawed horizons (see Fig. 4). The only exceptions are the average concentrations of Si, Al, Sr and Rb, which, apparently, are mainly part of the primary, weathering-resistant minerals. Low (Ca – 0.7 g/kg on average, Ti – 0.2 g/kg on average, Mn – 0.03 g/kg on average, Y – 1.4 mg/kg, Pb – 1.7 mg/kg on average) contents of these elements in mineral horizons are primarily associated with the destruction of mineral grains when the soil freezes in winter. Cryogenic destruction of minerals was noted earlier in various areas of the cryolithozone.^[16,34,35] After destruction by cryogenic processes, many of these elements, which are part of the soil particles, move down the profile with water flows, after which they accumulate in the permafrost and permafrost horizons (Ca – 0.9 g/kg, Ti – 0.3 g/kg, Fe – 1.6 g/kg, Mn – 0.1 g/ kg (Fig. 4a), Zr – 16.2 mg/kg, Zn – 3.9 mg/kg (see Fig. 4, b) on average).

Si and Zn vary most widely in permafrost horizons of soils (18.2 – 22.3 g/kg and 1.7 – 3.2 mg/kg). The remaining elements are distributed in permafrost horizons within much narrower limits. The accumulation of elements in the horizon of permafrost rocks is facilitated by both thermodynamic (frozen state of the horizon) properties and the restorative environment arising in connection with them.^[13] Thus, permafrost performs the biospheric function of a radial geochemical barrier,^[28] on which Al, Si, Ca, Mn, Fe are deposited.

3.2. Soil profiles and radial differentiation of soil chemical composition

Various soil properties create a variety of profile distributions of coefficient values R. Most of the studied elements are distributed in profiles (Fig. 5) of automorphic soils (see Fig. 5, d and 5,e) are distributed with accumulation in mineral horizons, and they are actively removed from the surface organogenic horizon of the sod-podzole illuvial-glandular lingual (values R vary in the range from 0.1 to 0.8), except Si, the increased value of which R (see Fig. 5, e) is most likely due to the low thickness of the AY horizon itself and its mixing with the underlying sand. The most strongly accumulated elements in soil profiles are Ca, Ti, Mn, Fe, Zn, Zr, whose R values reach 20.0 (see Fig. 5, l). The washing type of the water regime of sandy and sandy loam soils of granulometric composition provides active radial water migration of substances in the profiles of most of the studied sections. This explains the reduced content of elements in the horizons E (as well as with the sign e) and C, this is especially evident in the horizon C of the sod-podbura (see Fig. 5, g), in which almost all the elements considered are scattered (R varies from 0.1 to 0.9), with the exception of Si, whose high content is it is also associated with sand in the granulometric composition, which has silica (SiO ₂) at its core. The remaining elements, especially Mg, Ti, Fe, V and Pb, accumulate in the permafrost dark-colored horizon C’ (R = 1,1–1,5). Pseudofibre horizons of alphegumus soils are often distinguished by similar morphology and physico-chemical properties with higher chemical element contents compared to non-pseudofibre ones. Thus, the supposed role of iron-containing particles in the translocation of ionic forms of other metals was noted. The increased content of both gross and amorphous iron, the phase of which is characterized by the ability to associate with trace elements (RTi = 1,0–1,8; RMg = 1,0 – 1,4; RMn = 1,0–33,0, etc.). Ca among all other chemical elements demonstrates one of the most stable trends towards the surface-accumulative type of distribution. Apparently, Ca concentrations are in direct correlation with the depth of soil formation. The RCa in the horizons W, O and AY are equal to 0.5 – 10.0. The loamy granulometric composition, manifested, for example, in the RYaq water-accumulative horizon, contributes to the accumulation of all of the studied elements, except Si, Ca, V and Zn. The high content of organic substances (Sorg. = 2.57%) and increased electrical conductivity (EC = 140 µs / cm) in combination with the weighted granulometric composition lead to the fact that R here is 1.3–3.7 (see Fig. 5, and). Permafrost horizons of soils are characterized by increased concentrations of almost all elements, especially macronutrients, in comparison with super-frozen ones (see Fig. 5,b and 5,m). At the same time, most likely, the degree of development of the organogenic horizon and its decomposition affects the intensity of sorption of metals and metalloids by organic matter. Thus, the coarse humus horizon W in the permafrost gleevate psammozem passes most of the elements entering it (R = 0.5–1.0), and the highly decomposed T in the peat-gleezem fine peat accumulates most of the incoming chemicals, especially Si, V and Zr (R = 1.5, 1.7 and 2.0).

Fig. 5. Values of the coefficient of radial differentiation (R) of soil profiles in the valley of the Vilyu river: a) alluvial gray humus; b) sod-podzole illuvial-glandular; c) Peat-gleezem fine peat; d) buried serohumus; e) sod-podzol illuvial-ferruginous lingual; f) sod-podzol illuvial-ferruginous pseudofibre; g) sod-podbur illuvial-ferruginous; h) serohumus ozheleznennaya; i) stratozem serohumus; k) psammozem water-accumulative; l) psammozem gleevaty; m) gley permafrost psammozem; 1 – seasonally thawed non-cryogenic horizons; 2 – seasonally thawed cryogenic horizons; 3 – permafrost horizons

3.3. Soil-geochemical catena and lateral differentiation of soil chemical compositionThe soil-geochemical catena laid down on the left bank of the Vilyu River consists of five conjugate elementary geochemical landscapes (HCL) (Fig. 6). A detailed description of the natural conditions of the laying of the sections and the soil profiles opened by them is presented in ^[3].

Fig. 6. Values of the lateral differentiation coefficient (L) in elementary geochemical landscapes of the catena in the valley of the Vilyu River: 1 – first–order eluvial HCL; 2 – transeluvial with weak HCL removal; 3 – second–order eluvial HCL; 4 – transeluvial with active HCL removal; 5 – superaqual transit HCL; 6 – aquatic landscape of the Vilyu River; 7 – relief profile; 8 - locations of embedded soil sections; 9 - seasonally thawed soil horizons; 10 – cryogenic soil horizons

            In the studied soils, the limits of variation of the coefficients of lateral differentiation of chemical elements are very wide. Ca catenes are most widely distributed in soils (LCa = 0.3 – 1.8; LMn = 0.1 – 2.0; LFe = 0.6 – 2.1; LZn = 0.9–2.9 and LY = 0.3–1.4). Many elements from the studied ones, such as Al, Ti, Fe, Zn and Zr, accumulate in cryogenic eluvial hardened and pseudofibre horizons of podzols on the gentle slope of the first above-floodplain terrace, which may also be associated with an increased content of iron and its amorphous forms (see Table 2). An increased content of iron and aluminum in these horizons are also explained as high (relative to medium-profile) contents of large granulometric fractions with diameters of 250-1000 microns up to 89.2%, since primary minerals containing Al and Fe in their crystal lattices, slightly destroyed during cryogenesis, are contained here in high quantities. The accumulation of trace elements – Zn, Y, etc. – they are most likely explained by the presence of a sorption lateral geochemical barrier, since the BFff horizon contains up to 26% of the fraction of physical clay with increased sorption capacity. Most of the chemical elements in the considered soil-geochemical catena are characterized by concentration in the upper part as a result of the predominance of radial migration of substances there, as well as the degree of development of the humus horizon acting on the migration path as a radial sorption-biogeochemical barrier. A large amount of Ca, Mn and Ti - LCa = 1.8, LTi = 1.5 and LMn = 2.0, respectively, are concentrated in the sod–podburs on the first over-floodplain terrace of Vilyuya, especially in cryogenic horizons. This is also due to high concentrations of non-silicate and amorphous Fe along with rather high (up to 50.8%) physical clay contents. Lower down the slope of the catena, horizons with signs of cryogenesis are not common due to the almost complete absence of a screening surface of vegetation cover (on the slope of the first terrace of Vilyuya) and the warming influence of river water (in the superqual HCL of the floodplain). In the gray-humus soil on the slope of the terrace, many elements are scattered (see Fig. 6), for example, LMg = 0.8, LCa = 0.3, LMn = 0.1, etc. Mn is especially weakly concentrated in the horizons of this soil. Apparently, the very light granulometric composition of the soil, the low thickness of horizons, shallow penetration of soil formation and other factors do not contribute to the development of contrasting geochemical barriers here. Most of the elements in the soils of the Vilyuya floodplain are dispersed (L = 0.4 – 0.9), Mg, Si and Ti are distributed either evenly or with insignificant accumulation in a powerful water-accumulative organogenic horizon and the horizons lying below, also enriched with _organ (concentrations up to 3.4%) (see Table. 2) and clay particles (up to 52.8%) (see Table 3).

4. Discussion4.1. Profile distributions of geochemical properties of soils

Similarly to the results obtained by us, T.I. Vasilyeva obtained the following results of chemical analysis of the soils of Central Yakutia: the highest concentrations among macronutrients have such elements (reduced to one-and-a-half oxides) as Si (60.8–84.2%), Al (8.3–16.9%), Fe (1.1–5.8%).

Ca is characterized by similar Fe concentrations (0.9–5.0%), which is explained by the ratio of the studied soils to the types of fawn and chernozems, which are characterized by development on carbonate soil-forming rocks. Accordingly, the main carbonate compound included in their composition is CaCO ₃ – calcium carbonate. The Ca content in the soils studied by us, mainly belonging to alfegumus soils (sod-podzols, sod-podbura), rarely exceeds 2%, which is due precisely to the reduced calcite content in the soil-forming rocks. The main part of the elements, with the exception of Ca, mainly concentrated in the lower horizons (Bca, C, Cca, etc.), is distributed evenly in the soil profiles, which especially characterizes Si, which makes up the main part of rock–forming minerals - silica (_SiO2). Also, very high concentrations of Si (SiO ₂) and Al (Al ₂ O ₃) – up to 84.2 and 15.5%, respectively, were noted by A.P. Chevychelov^[32]. Profile distributions of trace elements in the background soils of the diamond–producing provinces of Yakutia are characterized by one common feature – active accumulation of Li (R = 3.3 – 3.6), Ti (R up to 16.6), V (R = 3.3 – 3.6), Cr (R = 2.2-4.9), etc. in the permafrost and permafrost horizons. According to Ya.B. Legostaeva and A.G. Gololobova, the lowest values of the R coefficient are characterized by Sn (in most cases) and Pb– R reaches 1.5–1.7. Organic substances bind to complexes with Pb (R in the organogenic horizon = 1.2–1.8) and As (R = 1.1–10.0), which is the most a frequently observed pattern in the profiles of cryozems of the Hannya-Nakyn interfluve. Ni, Cd and Cu are mainly removed from the soil profile, demonstrating a weak ability to sorption by organic matter (R according to A.G. Diaghileva vary in the range from 0.4 to 0.9).4.2. Catenary distributions of geochemical properties of soils

In the taiga-forest-steppe soil-geochemical catena in the Prilensky district of the Central Yakut lowland, the following characteristics of the lateral distribution of elements were revealed: Si is distributed relatively evenly in the catena soils (0.8 < L < 1.3), Al has a weak removal from the soils of autonomous upland landscapes, aluminum accumulation occurs in the soils of slope and superqual geochemical landscapes with a coefficient of L = 1.1 – 1.4. Fe demonstrates intensive removal from the soils of autonomous landscapes (content from 0.5 to 2.7%) with accumulation in the lower part of the soil-geochemical catena (concentrations up to 5.4–5.8%) – the values of the R coefficient reach 2-5 units.

Moreover, the values of R in the lower horizons of soils are usually much higher than in the upper ones. The catenary distributions of Ca and Mg are generally similar to the patterns of Fe distribution, the difference is that the upper, especially organogenic, horizons of soils enriched with calcium, in comparison with the lower mineral horizons. Ti and Mn are distributed relatively evenly in the soils of the landscape-geochemical catena (0.7 < L < 1.2).^[32]

5. ConclusionThe authors studied the regularities of profile (radial) and catenary (lateral) distributions of geochemical properties of cryogenic soils, the sections of which were laid in the valley of the Vilyu River in its middle course.

Geochemical coefficients of radial and lateral differentiation – R and L - were used for this purpose.

Different soil properties create in the studied soils a wide variety of variants of profile distributions, irreducible to uniform patterns. Most of the elements considered in soils are characterized by an eluvial-illuvial distribution in the profiles of automorphic soils. They are characterized by values of the radial differentiation coefficient R from 0.1 to 0.8. The exception is Si. The most strongly accumulated in soil profiles are Ca, Ti, Mn, Fe, Zn, Zr, whose R values in soils reach values of about 20.0. Eluvial horizons as a whole will differ in the removal of most of the elements (R = 0.1–0.9), and Si is again an exception, since it is part of silica – the mineral basis of soil particles in sandy soils. Mg, Ti, Fe, V and Pb accumulate in the permafrost dark–colored horizon C’ (R = 1,1-1,5). Pseudofibre horizons of soils differ in similar morphology and physico-chemical properties by higher contents of chemical elements. Thus, the supposed role of iron–containing particles in the translocation of ionic forms of other metals was noted: Ti, Mg, Mn (RTi = 1,0 – 1,8; RMg = 1,0 – 1,4; RMn = 1,0 - 33,0, etc.). The loamy granulometric composition of the RYaq water-accumulative horizon in the stratozeme on the Vilyuya floodplain contributes to the accumulation of all of the studied elements, except Si, Ca, V and Zn. Permafrost horizons of soils are characterized by increased concentrations of almost all elements, especially macronutrients.

The limits of variation of the coefficients L of chemical elements are very wide in the studied soils. The widest limits of variation are Ca, Mn, FeZn and Y (LCa = 0.3 – 1.8; LMn = 0.1 – 2.0; LFe = 0.6 – 2.1; LZn = 0.9 – 2.9 and LY = 0.3 – 1.4). Elements such as Al, Ti, Fe, Zn and Zr accumulate in the cryogenic horizons of the podzols on the gentle slope of the first above-floodplain terrace (TE1 landscape), which is presumably associated with an increased content of iron and its amorphous forms. The accumulation of trace elements – Zn, Y, etc. – is explained by the presence of a sorption lateral geochemical barrier. Most of the chemical elements in the considered soil-geochemical catena are characterized by concentration in the upper part. A large amount of Ca, Mn and Ti - LCa = 1.8, LTi = 1.5 and LMn = 2.0, respectively, are concentrated in the sod–podburs on the first over-floodplain terrace of Vilyuya, especially in cryogenic horizons. In the gray humus soil on the slope of the terrace (TE3 landscape), many elements are scattered, for example, LMg = 0.8, LCa = 0.3, LMn = 0.1, etc. Low power horizons, shallow penetration of soil formation and other factors do not contribute to the development of contrasting geochemical barriers here.

The use of lateral and radial differentiation coefficients seems to us to be a promising method for compiling characteristics of the soil-geochemical structure of territories – determining the characteristic features of the profile distribution of element concentrations in soils, distribution of elements in elementary geochemical landscapes, indication of vertical and horizontal geochemical barriers, etc. Nevertheless, in remote areas of the cryolithozone of Yakutia, including in the Central Yakut lowlands, such works are few. The gross chemical composition of permafrost soil profiles has been studied more often than the elemental composition of soil-geochemical catenae. At the same time, there are practically no studies differentiating soil horizons into permafrost and non-permafrost, thus, the influence of cryogenic processes on the chemical composition has been very poorly studied. A more active use of the methodology proposed by us in this article can expand modern ideas about the cryogenic differentiation of the gross composition of chemical elements in soils. Also, a detailed analysis of the fractional (group) composition of individual elements may have diagnostic value in the study of soil cryogenesis and its effect on soil formation.

ThanksThe authors express their gratitude to the professor of the Geographical Faculty of the Lomonosov Moscow State University, Doctor of Biology M.I. Gerasimova and M.N. S. J.Yu.

Vasilchuk for assistance in the diagnosis and classification of soils, E.V. Terskaya, a researcher at the EGC of the Faculty of Geography of Moscow State University, and L.V. Dobryneva, an engineer, for assistance in carrying out laboratory work, N.V. Torgovkin, Ph.D., for organizing a summer field school seminar, as well as A.D. Belik and E.G. Egorov for assistance in field work.