Paleogeographic significance of soils with residual relict horizons

Currently, sufficiently large arsenal of methods of retrospective studies is developed in soil science, paleogeography and paleopedology.

Together it allows to give an objective picture of the history of the modern landscape structure formation of a given territory. Ecotones are especially interesting for study with paleogeographic positions. It is territories at the intersection of natural areas for instance the southern half of the territory of VP which is located in touch of forest and forest-steppe biomes.

Traces of several soil formation stages are saved as part of a number of soil cover components. It confirms the high potential of the theoretical and methodological Dokuchaev position about soil as a function of abiogenic and biogenic soil-forming factors and paradigm "soil is the memory of the landscape". Based on the experience of our research, the complex of morphological, genetic, physical, biochemical, radiocarbon, physico-chemical, comparative pedogeographical and other research methods is the most productive. Together, it gives an idea about ​​the conservative - "soil is memory" - and the actual properties - "soil is life".

Soils on covering loams are the most promising for solutions of paleogeographic problems. It is characterized first, the relative homogeneity of initial substantive properties of the mineral phase and, secondly, widespread in the basin of the Kama and Vyatka. It provided the most correct application of the method of the comparative geographical studies of modern soils with polygenetic profile in the zonal aspect.

Morphological and genetic analysis let us to make mediated judgments about leading formatting processes in profiles of studied sod-podzolic and gray forest soils. For example we give the descriptions of the profiles of these soils on VP. It is depositary of a paleogeographic information.

The cut C-8 of sod-weakly podzolic soils with the second humus horizon on the covering noncarbonate loam under virgin fir-wood spruce ferny grassy. Profile laid on top of the vast watershed within Kukarskoe plateau; atmospheric moistening, sufficient (Fig. 1).

The structure of the profile:

Horizon O (0-2 cm): pine needles, leaves, small twigs, cones, wet, blackish-fulvous, weakly decomposed, the transition to the next horizon is clear, straight.

Horizon OAY (2-5 cm): wet, fulvous-black, silty clay loam, fine-grained, friable, crumbly, enriched of highly humified organic residues, with an abundance of young thin roots of light gray and off-white color, transition clear, straight.

Horizon AY (5-17 cm): wet, ash-gray with a whitish hue, silty clay loam, fine lumpy-stratified, friable, a lot of small gray-fulvous ortsteins, many roots, a clear transition (to sharp), noticeable by darkening colors.

Fig. 1. Morphology of sod-podzolic (left) and gray forest with gray humus (right) soils with complex organic profile of Vyatka Prikamye

Horizon AElh (17-25 cm): wet, coaly - gray, with some darker areas, clay loam, coarse-grained - platy (it is platy considerably weaker than in other similar horizons above sections), friable, a lot of small gray-fulvous ortsteins , whitish powder of silica is more intense at the periphery of tongues ​​and pockets in the lower part of the horizon, units are drenched by humus completely, few roots, gradual transition, wavy, with different tongues ​​to a depth of 39 cm.

Horizon Bt1 (25-51 cm): wet, fulvous, with a faint whitish shade and a noticeable humus sheen on the sides of aggregates and root pores, clay, nutty, solid, easily decays into structural units, weak silica powder on upper part, there are small clay kutana (as lenticels) on the aggregates walls, the roots of rare, gradual transition, conspicuous in color and structure.

Horizon Вt2 (51-81 cm): wet, fulvous with very weak whitish silica and with a faint matt-gray humic varnish, clay, finely- coarse nutty, solid with clayey kutans, roots are rare, the transition is gradual, noticeable by changing the structure and viscosity.

Horizon BC (81-103 cm): close to moist, fulvous, with a faint matt-gray and netted humic coating and whitish coating on the walls of large cracks, clay, prism-lumpy, solid, viscous, with a faint structure, roots are rare, a gradual transition, marked by the disappearance of the structure.

Horizon C (103-145 cm): moist, yellowish-fulvous, with rare netted humic coating on the root channels, clayey, unstructured, solid, viscous, few roots.

Morphological analysis shows a clear differentiation of the cut of these soils with complex organic profile on accumulative - eluvial and illuvial part. Upper and lower humus horizons are vary greatly in appearance. Horizon AY in virgin soil has characteristics of typical accumulative horizon that common for zonal sod-podzolic soils: fulvous - brown color, much humus, granular lumpy structure, etc. Arable soil layers lost natural structural condition and characterized by silty lumpy structure.

Pecularities of horizon AElh are in its concurrence in position with the eluvial thickness of profile. Because of this, it is characterized by a contrast combination of features of accumulative and eluvial horizons: dark gray or coaly-black color, fragile platy or granular platy structure, whitish powder of silica on the surface of aggregates which are sodden of dark humus, few roots. Last feature indirectly points to modern biological inertness of AElh horizon. The color intensity of the lower humus horizon is inversely related to the degree of podzolic process. The wavy outlines of the lower limit of the horizon AElh are normal for soils of hydromorphic series or formed in more arid conditions. But it is in some variance with the modern automorphic mode of soils with complex organic profile in southern taiga of VP.

In most cases, the upper part of the horizon AElh contacts with the upper humus horizon directly, although there are options when both horizons are separated by rudimentary clarified horizon El. Its lower part lies above podzolic sub-eluvial ElB horizon often. Both horizons are often located on the same level in arable soils horizon. It alternates with each other due to the wavy shape of its lower bounds.

Morphological face of the ElV horizon is typical for transition sub-eluvial parts of soil profiles of heavy granulometric composition with specific whitish - fulvous color tones, platy or platy - small nutty structure and silica powder. There is the reason to believe that creation of horizon ElB associated with modern soil-forming processes, in particular with filtration of acidic organic compounds to horizon and with the presence of seasonal recovery phenomena at the border of eluvial and texture thickness of profile, accompanied by bleaching of mineral particles.

Thus, the morphology of considered sod-podzolic soils with polygenetic profile of VP bears strong genetic traits of erasing evolution (SHH) with elements of heritable (AY horizon , Ap). In case of strong podzolic process (or when a lower humus horizon is plowed), it approaches in appearance to the typical sod- podzolic soils of the southern taiga. In general, morphological and genetic analysis reveals signs of secondary overlay processes of eluvial nature and varying intensity on the accumulative - eluvial thickness of profile, particularly prominent at the level of SHH (Table 1).

Table 1. Criteria of podzolic process (degradation) of sod-podzolic soils with complex organic profile of Vyatka Prikamye

Data	the level of podzolic process (degradation)
	Low	Medium	High
1. Horizon АElh: a) the color in wet condition	Coaly-black, Dark gray	Gray, steel-gray, mouse-gray	light gray (with a whitish tint)
b) granulometric composition	Clay loam	Clay loam and silty clay loam	silty clay loam
c) structure	granular platy	platy, laminose	laminose, fissile
d) thickness	More than 10 cm; continuous stretch	5–10 см; местами intermittent strike	Less than 5 cm; intermittent strike (spots, lenses, tongues​​)
e) humus content, %	More than 2,0	2,0–1,6	1,6–1,0
2. The presence of other eluvial horizons in the profile	horizon ElВ is absent or poorly developed	horizon ElВ well developed	along with the horizon ElВ the horizon El is allocated; АYand АElh horizons can be separated by a layer of El or АЕl
3. The position of the surface from the lower boundary of eluvial thickness	Less than 30 cm	30–35 cm	More than 35 cm

For comparison, let's consider the morphology of the profile of gray forest soils gray humus, on example of one of the cuts located under the south taiga landscapes in Urzhum plateau called lime tree ramen.

The cut U-27 agro-grey humus gray clay soil on the cover carbonate loam is made on the slightly convex top, close to a plateau-like surface of the watershed. The land is a field which sown with barley; atmospheric moisture, sufficient.

The structure of the profile:

Horizon PY (0-30 cm): wet, brownish-gray, clayey, nutty-lumpy, with signs of dusty, friable, many roots, transition is clear, wavy, noticeable by the change of color and structure.

Horizon AElh (30-52 cm): wet, coaly-gray, with a touch of steel, clay, granular platy, bodied, faint whitish powder of silica on the surface of the aggregates, few roots, transition is clear, wavy, sometimes tongues to a depth of 55 cm.

Horizon ElB (52-62 cm): wet, whitish-fulvous, granular nutty, with signs of platy, the bleached powder of silica on the surface of aggregates, which is especially noticeable on the pores and the root moves (in the form of mesh veins), the roots are rare, clear transition, wavy-tongue.

Horizon Bt1 (62-83 cm): wet, fulvous, whitish hue in the top of the cut, clay, granular nutty, solid, fine-porous, weak whitish powder of silica on the surface of aggregates, roots are rare, gradual transition, conspicuous by the disappearance of silica powder .

Horizon Bt2ca ( 83-104 cm): moist, brown - fulvous, clayey, prismatic - nutty, solid, moderate humus varnished surface on aggregates, at a depth of about 1 m in some places there are local fine-grained calcareous clusters react with HCl , roots are rare, gradual transition, conspicuous by the restructuring.

Horizon BC (116-185 cm): moist, brownish-fulvous, clayey, unclear prismatic, solid, slightly viscous, weak humic varnished surfaces on aggregates, roots are solitary.

Data generalization of morphological descriptions of cuts allows draw some preliminary conclusions. Primarily gray automorphic soils with complex organic profile are characterized by considerable variability of composition of profile, which is highly dependent on geographic and orographic conditions. It boils down to two fundamentally different types of profile structure. First - gray humus- variant is as follows: O + AY + AElh + ElB + Bt1 + Bt2 + BC + C. This is accumulative- eluvial - illuvial type of composition of profile. Second - dark humus - variant of genetic horizons structure of gray soil profile corresponds to the formula: O + AU + (AUB) + Bt1 + Bt2 + BC (ca) + C (ca). This formula conforms to accumulative type of structure. Each option has its own peculiarities of genetic horizons, most contrasting on level of system horizons A.

Gray humus virgin soils have horizon O with thickness up to 3-5 cm. There is humus horizon AY with thickness about 15-25 cm below it. Significant oscillation amplitude of sizes of sod horizon had largely provided due to the prehistory of the soils and vegetation of the region. The thickness of the horizon, as a rule, is less in the virgin forests than in secondary forest. It can be artificially inflated by ploughing of underlying horizons of profile in last. It is part of the horizon AY in modern time. Horizon PY is more stable of its size - about 21-25 cm in arable land. The difference in economic use illustrates variability of color and structural state of humus horizons of the first group of soils. The dominant gray color of AY horizon in soils under virgin forests can acquire additional brownish or blackish color, which is replaced by a light gray or steel gray in soils under secondary forests, due to ploughing of less humus underlying horizons (mainly - AEl (h)). The horizon PY has often a brownish - gray or fulvous - gray tone in the arable soils. Granulometric composition of the humus horizon is silty clay or silty clay loam usually. Granular or fine lumpy - granular structure of AY horizons of forest soils is replaced by dusty granular lumpy and often blocky in horizons PY of mastered gray humus soils due to its transformation in the agricultural processing.

A significant amount of ortsteins are contained in humic horizons, but its amount here is always less than in sod-podzolic soils with complex organic profile. There is admixture of whitish powder of silica besides the iron and manganese segregations in horizons PY, covered by plowing. It gives a lighter color in horizons AY of fallow soil. Sod horizons have the lowest solidity and contain the maximum number of roots for the profile. It generally has straight lower boundary.

Horizons AEl (h) penetrate to deep into an average of 30-35 cm. It has more stable morphological parameters. Among its coaly - gray or steel - gray color, silty clay or clay compozition, platy or granular nutty platy solid structure, more or less quantity of whitish silica on the surface of the aggregates, the abundance of iron-manganese nodules (which is also less than in soils mentioned above), a small amount of roots and wavy lower border. These horizons are often spatially replaced by horizons ElB or it is changed directly to the underlying texture thickness of profile.

ElB horizons have stable whitish-fulvous color, silty clay or clay composition, platy or platy - nutty packed structure, bleached silica, sometimes rare small ortsteins, few number of roots and wavy or wavy - tongue-border, located at a depth of 33-37 cm.

Based on the foregoing, it follows a number of important conclusions.

First, gray humus gray polygenetic soils are virtually identical in composition of profile to conventional, previously known as gray (light gray) forest soils of VP. Gray forest soils can have an absence of the horizon AEl (h) due to natural (degradation evolution) or anthropogenic causes (ploughing, erosion), but analyzed polygenetic soils are "obliged" to have such a horizon. It is some kind of calling card. Secondly, gray humus gray soils with complex organic profile are similar in structure with described above sod- podzolic soils with AElh horizons. This type of profile is more widespread in the northern half of the range of gray soil, where there is a transition from the taiga to the south taiga landscapes, as well as it is widely distributed on the upper, better -drained watersheds elements which are most prone eluviation.

Second dark humus variant tends to the southern half of the area of gray soil (nut-tree ramen) and to lower parts of slopes of watersheds. Dark humus gray forest soils have more extended and less differentiated profile unlike gray humus soils. It tends to absence or weak degree of visually observable of chromatic dismemberment in humic accumulative strata (about 35-40 cm). Sod horizon has dark gray, close to black color, gradually acquiring a brownish tint in the downstream direction (horizon AUB). It has a well-developed granular, granular nutty or lumpy granular structure; the role of dusty, lumpy and blocky parts is increased when plowing. Silica powder and iron- manganese nodules are not typical for humus - accumulative thickness.

The differences between gray and dark humus gray soils are smoothed at the level of texture - transition zone of profile. Another morphological feature is the predominantly inherited carbonate within the lower part of the profile. Carbonate of lime, if available, often represented by illuvial - carbonate growths in the gray humus group of soil. All of this together indicates a lower degree of podzolic process in dark humus gray forest soils, despite the presence of appreciable textural differentiation of profile.

Thus, the southern variant of the composition profile of gray soil, at first glance, has little common with the soils of the "northern " subtype, because of the lower contrast in the structure of genetic horizons. It does not show distinct signs of differentiation of clay mineral strata or secondary overlay of eluvial soil-forming processes to previous - accumulative. It does not contain explicit signs of sharp humus irregularity of profile or eluvial - gley process which are so clearly in the first case. However, the second variant of gray soil structure fully meets the criteria for soils with complex humus profile. At the analytical level, it also reveals (although to a lesser extent) the features of texture and humus differentiation profile, sequence differences of humic substances age in the upper and lower parts of organic accumulative thickness, etc. The "nut-tree" gray soils can be roughly defined as “crypto-polygenetic” formations.

Manifestation processes of humus creation and humus accumulation is typical for both options of the profile structure. It can be estimated in the range from weak (little humus) to moderate (moderate humus) and strong (much humus). The first two levels of humus accumulation are most typical for sod- podzolic soils and the "northern" version of profile structure of gray forest soils, the third - for its "southern" version. However, it is obvious that there are processes of eluviation - illuviation in the studied soils. It is recorded in the sod-podzolic soils in a particularly obvious form and to a much lesser extent - in dark humus soils. The most likely is lessivage, surface gleification and, in some cases, leaching. Evidence of this can be distinct textural differentiation of profile mentioned previously, the presence of iron-manganese nodules in the accumulative - eluvial thickness of gray humus gray soil, humus varnished, illuvial - calcareous formations in Bt2 and BC horizons of both soils subtypes.

In other words, gray forest soils with complex organic profile of VP as and similar sod- podzolic soils show the signs of being in the present conditions at the stage of the erasing evolution with elements of heritable. This conclusion is sufficiently reliable evidenced in the analysis of solid mineral and organic phases of soil ("Soil - memory") and its relevant physical and chemical properties ("soil - life"). At the same time, the morphology of the profile may not always be a reliable indication of polygenetic soils. For example, gray gleyed dark humus soils do not show often visible signs of differentiation organic profile similar to the above-described objects of the VP. It is opened analytically in the study of fractional group composition of humus and especially radiocarbon dating of humic acids.

Particle-size analysis of soil mineral phase is other physical method that helps to indirectly ascertain, first, the probability of reversal of soil over time, and secondly, the zonal differences in the intensity of degradation processes in the event of a change of accumulative developing soil evolution to heritable with elements of erasing or actually evolution of erasing. Its objectivity increases in the comparison of soils developed on homogeneous parent substrates such us covering loam in VP. According to the observations of A.L. Alexandrovskiy (2005) in beneath barrows soils on loess stratum was found that it is enough 2.5-3 thousand years for formation of texture - differentiated profile in terms of southern taiga and broad-leaved forests of Russian plain. This is consistent with our arguments of Late - Holocene time of silt disappearing in top part of the investigated soils profiles.

Judging by the diagrams shown in Fig. 2 sod - podzolic polygenetic soils of the southern taiga are characterized by a high degree of eluviation of physical clay and especially silt in comparison with the same gray forest soils located to south from taiga in VP. Average degree of profile differentiation (S) (taking into account the silt content and solidity of Bt and A horizons, that was defined by a series of cuts) was found to be 6.4 in the sod-podzolic soils and 2.9 in gray forest soils. The first soils are characterized by the negative balance of silt in the profile - about 12%, and the second soils have the positive balance - about 14 %. This fact is consistent with the modern types of water regimes of these soils - wash in the first case and periodically wash - in the second.