MicroMorphology of the late and Middle pleistocene paleosols of the central east european plain

. In this paper we focused on the micromorphology of the Late and Middle Pleistocene paleosols exposed in twelve loess-paleosol sequences sections in the central part of the East European Plain. Each studied paleosol complex known as Mesin (MIS 5), Kamenka (MIS 6 (8) - 7 (9)), and Inzhavino (MIS 8 (10) - 9 (11)) pedocomplexes (PCs) consists typically of two members, the earlier – main – phase of the soil development taking place during an interglacial, and the later one – at the subsequent interstadial time. Interglacial paleosols formation is associated with the thermal optimum of climatic macrocycles and corresponds to conditions close to modern in the territory under consideration. Interstadial paleosols formation characterizes the intervals within the glacial period, accompanied by an increase in heat and moisture. However, the heat supply of such intervals did not reach modern level in this region (Velichko and Morozova 2015). As follows from the analysis of the soil micromorphology over the studied area, the soil microstructure experienced notable changes under changing latitudinal zonality. During the interglacial periods clay coatings and Fe-Mn pedofeatures dominated the soil microfabric; in the south loess-paleosol sequences coatings are in negligible quantities, Fe-Mn pedofeatures decrease in amount, and carbonate pedofeatures appear instead. In the microfabric of the interstadial paleosols, Fe-Mn pedofeatures are abundant, but unlike interglacial paleosols, the coatings are rare. Basically, the coatings are humus-clayey in composition, but in the more southern sections coatings are absent.


introduction
The central part of the East European Plain is known as one of the main loess regions (Velichko and Khalcheva 1982;Velichko et al. 1994Velichko et al. , 2006Haase et al. 2007). In the sequences loess alternate with paleosols. Principal phases of the paleosol formation fall in interglacials (MIS 5e; 7 (9); 9 (11)); later on, as the climate became cooler, vegetation changed accordingly and interstadial soils (MIS 5c (5); 6; 8) developed (Velichko et al. 2007). The soil formation process practically stopped during the ice ages, and the loess accumulated on the soil surface. So loess-paleosol sequences (LPS) record and keep information of the soils of past epochs (Velichko and Morozova 2015;Zykin and Zykina 2015;Sycheva and Khokhlova 2016;Sedov et al. 2016;Tabor et al. 2017;Costantini 2017;Panin et al. 2018). Quite a number of studies were aimed at the paleosol sequences on the East European Plain (Morozova 1981;Bolikhovskaya 1995;Velichko 2002;Rusakov and Korkka 2004;Dlussky 2007;Panin 2007;Glushankova 2008;Sycheva et al. 2017;a.o.). The multidisciplinary approach used in those studies (including analyses of the soil morphology and micromorphology, paleomagnetic characteristics, physical and chemical properties, pollen analysis, etc) provided a possibility of reconstructing landscapes and climates of the past. Attempts at reconstructing soils of the Mikulino, Kamenka, and Likhvin interglacials and of the Bryansk Interstadial were made by Morozova (1995), Velichko (2002) and Dlussky (2001Dlussky ( , 2007. The authors, however, did not pay much attention to changes in paleosols under conditions of changing soil zonality in the interglacial and interstadial periods. In paper we focus the attention on the micromorphology of the Late and Middle Pleistocene paleosols. The micromorphological studies of paleosols make it possible to recognize specific features of the soil structure at the micro-level (Matvishina 1982;Bronger and Heinkele 1989;Bronger et al. 1998;Nettleton et al. 2000;Kemp 1999;Kemp 2013;a.o.). In particular, that approach permits to identify pedofeatures of Ca, Fe and Mn, and also other features that may be interpreted as manifestations of certain soil-forming processes (Gerasimova et al. 1992;Fedoroff et al. 2010). Together with physical-chemical characteristics and description of the paleosol morphology, the study of the micromorphology provided an insight into the type of soil formation and made it possible to reconstruct the paleosols of the central East European Plain in the Late and Middle Pleistocene. The purpose of this work is to identify changes in the microstructure and other micromorpho-logical features of paleosols in different latitudinal zones of the soil cover of the center of the East European Plain in the Late and Middle Pleistocene.
Here we use the latest variant of the chronostratigraphic scheme (Table 1) as suggested by A.A. Velichko and his colleagues (Velichko and Morozova 2015;Velichko et al. 2011). There exists, however, another variant of correlation between stratigraphic horizons and the marine isotope stages, also in wide use (Velichko and Morozova 2010). According to it, the Romny paleosol is correlated with MIS 7, and paleosols of Kamenka and Likhvin interglacials -with MIS 9 and MIS 11 respectively.
The sections are exposed in the brickyard quarries or cut in steep valley slopes where surface soils are present -Greyzemic Phaeozems, Haplic and Calcic Chernozems. The modern soils in many quarries have been stripped away; some sections in valley scarps are often devoid of the upper humus horizons (sections Bogolyubovo and Gololobovo).  During the field survey a detailed morphological description (Rozanov 2004) of every section was performed along with sketching, taking photographs, and sampling. Samples for physical and chemical analyses and for micromorphological studies have been taken from every genetic horizon at 15-20 cm intervals. In order to properly interpret the fossil soil genesis, the surface soils were also sampled with the aim of a comparison with samples from paleosols.
Earlier investigations resulted in acquiring abundant materials on physical-chemical characteristics and micromorphological data that gave us an insight into the development, structure, and properties of paleosols, both interstadial and interglacial ones (Velichko and Morozova 1963;Velichko and Morozova 1972;Velichko and Morozova 2010;Velichko et al. 1985;Panin 2007;Panin 2015). It follows from the results obtained that the interstadial paleosols both of Late and Middle Pleistocene developed under grass steppe vegetation under conditions of lesser heat supply than those formed during interglacials. Typically, the interglacial paleosols formed under forests (Velichko and Morozova 2015).
The paleosol micromorphology was studied in thin sections (<30 µm thick) impregnated with polysynthetic resin. In describing thin sections and interpreting the soil formation processes we followed the techniques described in literature (Gerasimova et al. 1992;Gerasimova et al. 2011

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Along with the investigation of the micromorpholology and morphology, the following components were determined in the paleosols: organic carbon, obtained using wet combustion technique developed by Tyurin (Rastvorova et al. 1995); amorphous iron compounds, by Tamm's method (Tamm 1922), and non-silicate iron compounds, by Mehra and Jackson (1960). All the analyses were performed in the Chemical Analytical Laboratory, Institute of Geography, Russian Academy of Sciences.
The genetic horizons in soil profiles are marked in accordance to the FAO (2006) ( Table 2). The names of the surface soils and paleosols are given in according of the IUSS Working Group WRB 2014. results

The Late Pleistocene Mezin paleosol complex
There are two paleosols in the sequence representing the Mezin PC: the Krutitsa interstadial and Salyn interglacial ones. The Krutitsa interstadial soil consists mostly of humus horizons (А and АВ) and is no more than 1 m thick. The soil thickness is slightly greater in the south of the East European Plain (0.54 to 0.80 m in sections Korostelyovo, Sebryakovo-Mikhailovka, Gun'ki) and does not exceed 0.70 m farther north (see Table 2). The paleosol is mostly described as brownish-gray loam of crumby structure. The Salyn interglacial paleosol consists of horizons АВ, AЕ, E, and Вt, the total thickness of the soil reaching 2.4 m (Bryansk section). The Salyn soil is mostly composed of loam and sandy loam with crumby and angular to subangular blocky structure, brown and bright-brown in color. Albic horizon is present as separate whitish lenses of sandy loam. The Krutitsa paleosol is distinguished for a high organic carbon content (up to 1.40%). In the Salyn interglacial paleosol the organic carbon content is less than in the Krutitsa one and does not exceed 0.58%.
In the interstadial Krutitsa paleosol of Ozherelye section hypo-coatings and coatings located in pores are mostly humus-clay in composition (Fig. 3a, b) and large granular aggregates (Fig. 3c, d). In Sebryakovo-Mikhailovka section there are no coatings, while Fe and Fe-Mn nodules appear instead (Fig. 3e). The humus horizon microstructure is well aggregated. Similar granular aggregates are seen as in the interstadial paleosol of the Gololobovo section, only of a smaller size (Fig. 3f ). Large diamond-shaped and ungeometrical crystals (Fig. 3g, h) are distinctly seen, which is typical for gypsum pedofeatures (Kubiena 1938;Gerasimova et al. 1992;Poch et al. 2010). The transitional horizon between the interstadial and interglacial paleosols is free of gypsum. Gypsum pedofeatures are characteristic for paleosols of the Mezin PC in the southern sections of the Azov Sea (Panin et al. 2018).
The microstructure of the Salyn interglacial paleosol sampled in the Gololobovo section (horizon AE@) features large grains of quartz (Fig. 4a) and the appearance of granular aggregates (Fig. 4b). B-fabric of the groundmass is speckled and crystallitic. Albic horizons contain Fe nodules. In the horizon Bt the coatings are mostly of humus-clay composition (Fig. 4c). They are usually of compact structure, dark brown or yellow color, are mostly confined to pores. In the Sebryakovo-Mikhailovka section small-size Fe-Mn nodules occur in horizon Ay@ (Fig. 4d, e). Pedofeatures of gypsum ( Fig. 4f, g) and carbonate (Fig. 4h) were recorded in the horizons Ay and By of the interglacial paleosol. The Middle Pleistocene Kamenka PC includes two paleosols -late Kamenka interstadial paleosol and early Kamenka interglacial one (Table 2). According to morphological descriptions, the interstadial paleosol consists of a humus horizon 0.22 m (Ozherelye section) to 1.10 m (Korostelyovo section) thick. As is seen in Fig. 5, the Kamenka PC in sections Mikhnevo (Fig. 5a) and Strelitsa-2017 (Fig. 5b) is heavily cryoturbated. Horizon A@ of the late Kamenka interstadial paleosol is present in the cryogenic wedge-like structures penetrating into the early Kamenka interglacial paleosol.
The maximum organic carbon content in the interstadial paleosol is 0.55% (Korostelyovo section). The interglacial paleosol profile shows lesser proportions of organic carbon (0.43%) as compared with the interstadial soil. In common with the interstadial paleosol, the maximum content of both is found in the Korostelyovo section.
The late Kamenka interstadial paleosols are mostly characterized by gray-brown groundmass of humus-clay composition. The paleosol described in Ozherelye section is noted for a predominance of humus-clay coatings around channels (Fig.  6a, b). There is also Fe-Mn nodules present in the soil. The groundmass is penetrated by cracks, which led to the destruction of the coatings (Fig. 6c, d). In terms of micromorphology, the humus horizon in the Sebryakovo-Mikhailovka section is characterized by brownish-gray color and aggregate structure (Fig. 6e, f ). The groundmass is silty-clay. The b-fabric is striated and speckled.
The early Kamenka interglacial paleosols differ noticeably from the interstadial ones in the coating quantity and variability of their composition. In the horizon EBt dominant are laminated light brown and brown coatings, mostly clayey (Fig. 7a) in composition (Ozherelye section). There are Fe-Mn pedofeatures present here (Fig. 7b, c). In the horizon Bt the clayey coatings are confined to ancient pores (Fig. 7d). The groundmass both in upper and lower parts of the interglacial soil profile is light-brown. In the Sebryakovo-Mikhailovka section the interglacial paleosol microstructure is typically well aggregated (Fig. 7e). The soil in thin section is brownish. Carbonate and Fe-Mn nodules occurs over the entire interglacial soil profile (Fig. 7f, g, h).

The Middle Pleistocene Inzhavino paleosol complex
The Inzhavino PC lies stratigraphically below Kamenka PC; in common with the latter it consists of two paleosols: the late Inzhavino interstadial paleosol and the early Inzhavino interglacial one. Cryoturbations are clearly distinguishable in the Inzhavino PC in all the sections (Gololobovo section, see Fig. 8). Interstadial pa- leosols have brownish-gray humus horizon composed of clay loam. The thickness of the paleosols varies from 0.14 to 1.04 m ( Table 2). The maximum organic carbon content in the interstadial paleosol is 0.39% (Korostelyovo section).
The albic horizon is present in the profile of interglacial paleosols studied in sections Gololobovo, Ozherelye, Strelitsa-2017 and Likhvin. In the interglacial pa-leosols the dominant colors are dark and bright-brown argic horizons; albic horizons composed of sandy loam are whitish.
In sections Korostelyovo and Sebryakovo-Mikhailovka there is no albic horizon in the soil and the profile consists of argic horizons only.
In the interglacial paleosol studied in the Gololobovo section there is a distinctly pronounced albic horizon where iron   Table 3).

fig. 8. Morphology of the Inzhavino PC: horizon А@ of the interstadial paleosol, horizons E@ and B@, BCg@ of the interglacial paleosol
The microstructure of the interstadial paleosols sampled in the Gololobovo section displays granular aggregates in horizon А@ (Fig. 9a, b). Coatings of humus-clay and clay composition are recognizable in thin sections under microscope, together with Fe pedofeatures (Fig. 9c, d). Coatings located in pores up to 1 mm in diameter are composed of humus and clay particles, laminated. In the Sebryakovo-Mikhailovka section the interstadial paleosol is characterized by brown coloration in thin section, granular aggregate microstructure, silty-clayey groundmass with admixture of coarse sand (Fig. 9e, f ). Packing voids are well pronounced, as well as chambers.
The interglacial paleosol exposed in the Gololobovo section (horizon E) contains large Fe-Mn nodules (Fig. 10a, b), though no coatings. Lower in the profile, in horizon Bt, coatings, mostly clayey, appear and increase in number downwards (Fig. 10c, d). The b-fabric is speckled. The groundmass of the interglacial paleosol shows a silty-clay composition in the Sebryakovo-Mikhailovka section (Fig. 10e, f ). There are also large (more than 1 mm in diameter) carbonate concretions and Fe-Mn nodules (Fig. 10g, h).

discussion
Synthesis of paleoenvironmental data obtained as a result of integrated studies of the loess-soil sequences in the central East European Plain made possible to trace the principal stages in the soil evolution in the region over ~400 000 years, that is, since the Middle Pleistocene to the present days. Table 4 summarizes the results of the micromorphological studies of the Loess-Paleosol Sequences. A relative abundance (Stoops 2003) of Fe-Mn, carbonate, and gypsum pedofeatures, as well as coatings is given.
According to Urusevskaya (2011), the main processes in the Albic Retisols (latitudinal zone I) are lessivage, humus accumulation and mineral acid hydrolysis. The leading soil-forming processes in the Greyzemic Phaeozems (latitudinal zone II) are ground leaf litter formation, humus accumulation, acid hydrolysis of minerals and lessivage. The microstructure of both Greyzemic Phaeozems and Albic Retisols are distinguished by an abundance of Fe and Mn pedofeatures (Table 3), indicative of wetting (Gerasimova et al. 1992;Zaidel'man and Nikiforova 2010;Lindbo et al. 2010).
In the Chernozems (III latitudinal zone) development the leading processes were as follows: ground leaf litter formation, humus accumulation, active structuring due to biogenic processes and flocculation, and carbonate redistribution due to eluvial and illuvial processes (Chernova 2011). The microstructure of Chernozems is noted, besides Fe-Mn pedofeatures, for carbonate nodules present in abundance.

The Late Pleistocene Mezin paleosol complex
The presence of humus-clayey coatings, together with a relatively high humus content in the Krutitsa interstadial paleosol (exposed in Suvorotino, Bogolyubovo, Gololobovo, Mikhnevo, Ozherelye, Likhvin, Bryansk, Arapovichi sections), suggests a   Gerasimova et al. (1992), such crystals may develop under conditions of continuous, or long-term, presence of water in the soil, which makes possible the crystal growth. Most likely gypsum was introduced by eolian processes and formed crystals of rhomboid forms in the process of soil-formation. The considered soils may be classified with Gypsic Chernozems.
In the Salyn paleosol microstructure there is a clearly traceable albic horizon that contains Fe-Mn nodules, clayey coatings are present in the argic horizon (Suvorotino, Bogolyubovo sections). The described characteristics permit the soil to be classified as texture-differentiated soils. In the Gololobovo, Ozherelye, Likhvin, Bryansk, Arapovichi sections Fe-Mn nodules become less pronounced in the Salyn paleosol microstructure, while the coatings gain in abundance. There are clayey coatings of illuviation, undoubtedly indicative of lessivage (transportation of clay matter). According to Novenko et al. (2008), at the beginning of the Mikulino Interglacial the studied region was dominated by spruce communities, pine and birch-pine forests; later on, oak forests appeared with elm, ash, and maple. As suggested by the pollen analysis data, that time was marked by an increase in moisture supply and the stagnant water persisting on the land surface. The considered Salyn paleosol described in Suvorotino and Bogolyubovo sections may be classified as Albic Retisols, in the Gololobovo, Ozherelye, Likhvin, Bryansk, Arapovichi sections the paleosol is assigned to Greyzemic Phaeozems. The results obtained are in a good agreement with conclusions by Sycheva (1998) on Retisols being also formed near the Aleksandrovsky Quarry (51°36'09"N, 36°06'51"E) in the Interglacial.
In Sebryakovo-Mikhailovka, Korostelyovo, and Gun'ki sections the Salyn soil micromorphology studies revealed the presence of large rhomb-shaped pedofeatures of gypsum and carbonates. Similar forms of gyp-sum were described in the Salyn paleosol in the Azov Sea region (Velichko et al. 2017a;Velichko et al. 2017b;Panin et al. 2018); that gives ground for suggestion on a similarity in the soil-forming processes all over the south of the East European Plain. Gypsum is present in the soil as a concentration of small and larger crystals. Presumably, it could be related to fluctuations of the humidity, when the soil profile was repeatedly dried up (Minashina and Shishov 2002;Poch et al. 2010). As compared with northern regions, the soil profile is richer in organic carbon (0.49%), its thickness is 1.32 m. Taking into consideration the presence of gypsum and carbonates in the soil and well aggregated groundmass, one may suggest that the paleosol development followed the Chernozem type. The considered soils may be compared, therefore, with modern Gypsic Chernozems or Gypsic Kastanozems. According to Glushankova (2012), soils at the Korostelyovo section vicinities at the Mikulino Interglacial were represented by a combination of forest and meadow-chernozem soils.

The Middle Pleistocene Kamenka paleosol complex
The interstadial paleosol known as late Kamenka has much in common in its genesis with the Krutitsa interstadial paleosol. In the northern sections (Gololobovo, Ozherelye, Mikhnevo) studies of the soil microstructure revealed humus-clayey coatings, along with Fe nodules and carbonate pedofeatures. There are traces of cryoturbations recognizable in the paleosol profile. It may be concluded that the Kamenka interstadial paleosols are practically identical to those of Krutitsa in their characteristic. In the Gololobovo, Mikhnevo, Ozherelye sections in the studied region the paleosols are comparable with imperfectly developed modern Gleyic Phaeozems (Panin 2007(Panin , 2015Velichko and Morozova 2015 (Bronnikova and Targulyan 2005). The main soil-forming processes that took part in the paleosol profile development are lessivage, clayization, humus accumulation and illuviation. As follows from the micro-biomorphic analysis, the early Kamenka interglacial paleosol developed under broadleaf forests. Those paleosols may be classified as Cambisols, developed under broadleaf forests (Glushankova and Agadjanian 2015). The modern analogs of those soils are found in Central Europe, in particular, in the Vistula River basin (Świtoniak and Charzyński 2014).
In the Sebryakovo-Mikhailovka and Korostelyovo sections, the early Kamenka interglacial paleosol is aggregated in microstructure; there are pedofeatures of carbonates and Fe nodules, indicative of warm and wet climate at the time of soil development (Kovda et al. 2016). The groundmass is well aggregated, with granular structure, the soil profile thickness is 1.08 on average, organic carbon content is 0.25%. Based on those characteristics, the soil may be placed in the group of Chernozems.

The Middle Pleistocene Inzhavino paleosol complex
The microstructure studies performed on the late Inzhavino interstadial paleosol exposed in Gololobovo and Ozherelye sections revealed the dominance of humus-clayey coatings, and the presence of Fe nodules. The entire paleosol profile is 0.36 m thick on the average, organic carbon content amounts to 0.17%. The main soil-forming processes that account for the soil appear- The early Inzhavino paleosol is dated to the Likhvin Interglacial. Its profile is 1.5 m thick on the average, organic carbon is 0.17%. A whitish albic horizon is clearly seen in the soil profile. In common with other interglacial paleosols, there are numerous coatings (clayey and silty-clayey ones) in the Inzhavino paleosol microstructure. Our data gave grounds to assign the soil to texture-differentiated group, under coniferous-broadleaf forests (Glushankova and Agadjanian 2015). Among the modern soils Albic Retisols in combination with Greyzemic Phaeozems may be considered as their analogs.
In the Korostelyovo and Sebryakovo-Mikhailovka sections the early Inzhavino soil profile is, on average, 0.66 m thick, organic carbon content is 0.21%. The microstructure features carbonates, the groundmass is well aggregated, silty-clayey in composition, contains Fe-Mn nodules. The Likhvin interglacial paleosols in the region may be classed with Chernozem ones. The present-day analogs of the paleosols are Chernozems under meadow steppe (Glushankova and Agadjanian 2015).
As can be seen from Table 4, the quantity of Fe-Mn pedofeatures and coatings decreases from north to south, while the quantity of carbonates increases to the south. First of all, this could be due to a change in the amount of precipitation. So for surface soils of I and II (Fig. 11) zones, the average annual precipitation is 540 -650 mm, and for III zone is 300 -560 mm per year. The same II -Chernozems zone trend is typical for the Salyn paleosol (MIS 5e), where in more southern sections, in addition to carbonates, gypsum appears. According to Velichko et al. (2004) during the Mikulino interglacial the paleosols of the I and II zones (Table 4, Fig. 11) developed at an average annual precipitation of 600 -650 mm per year, while the paleosols of the III zone (Table 4, Fig. 11) developed at 580 mm per year (Ryskov et al., 2008). The average annual precipitation in Kamenka (MIS 7 (9) and Likhvin (MIS 9 (11)) interglacials varies between 600 and 750 mm, but the greatest amount of precipitation is typical for the Likhvin interglacial (Velichko et al., 2004;Ryskov et al., 2008). As can be seen from Table 4, the maximum content of Fe-Mn pedofeatures and coatings is limited to the Inzhavino paleosol of the Likhvin interglacial, which confirms the degree of moistening of the paleosols of this period. Therefore, according to the microfabric of paleosols, it can be seen that in the territory of the center of the East European Plain the most humid conditions are characteristic for the Likhvin interglacial (MIS 9 (11)) with their gradual decrease towards the Holocene period.
According to Velichko et al. (2004) during the interstadial periods of the Late and Middle Pleistocene the amount of precipitation was 220 -450 mm per year. In the microfabric of interstadial paleosols, there are Fe-Mn pedofeatures and coatings, and in more southern sections carbonate and gypsum nodules, but their quantity does not change significantly during the considered Pleistocene periods, which suggests similar pedogenic conditions for their formation. In the interstadials periods, considered soil complexes, the climate did not vary greatly from the Middle to Late Pleistocene, but the soil cover during these periods had latitudinal zonality, which consisted of two zones. conclusion 1. As follows from the data on micromorphology, the interstadial and interglacial paleosols permit tracing the changes in climate of the time intervals when the paleosols developed in the center of the East European Plain. The soil microstructure dominated by Fe-Mn nodules and coatings is typical of the soil formation in humid climate, while the presence of carbonate pedofeatures and gypsum suggests the climate aridity.
2. In the northern part of the East European Plain interglacial paleosols of the Mikulino and Inzhavino age had a texture-differentiated profile, while paleosols dated to the Kamenka interglacial lacked of albic horizon. In the southern regions the profile of the interglacial paleosols feature blackish chernic surface horizon and with secondary carbonates.
3. During interstadials of the Late and Middle Pleistocene the latitudinal zonality was fairly well pronounced, two zones being distinguished within the considered region. Interglacial paleosols -Mikulino permitted to identify three latitudinal zones, the Kamenka and Inzhavino interglacial soils were also distributed over two latitudinal zones.

acKnowledgeMents
The study was funded in part by Russian Foundation for Basic Research to the research projects № 18-04-00145, 17-55-53035. Micromorphological studies of the Sebryakovo-Mikhailovka section were carried out in the framework of the State Task № 0148-2019-0005 (Registration Number 01201352491).
The author is thankful to Spasskaya I.I. for her help in language editing. The authors would like to thank the anonymous reviewers for their helpful suggestions.