1. Introduction

Human activity is acknowledged as the sixth factor of soil-formation, or at least as a force capable of modifying other factors (^{Bidwell and Hole, 1965}; ^{Amundson and Jenny, 1991}; ^{Richter and Yaalon, 2012}). Human societies and soils are in a constant interaction and co-influence within the inhabited areas. Human impact progressively increases during the Anthropocene. As ^{Richter and Yaalon (2012, p. 775)} observed: “Changes in soil directly influence the evolution of human societies and cultures. This is a mutual co-genesis, with soil and humanity developing jointly and interactively”.

Within the last update of WRB, soils in archaeological contexts of former settlements are distinguished as Technosols, with a supplementary qualifier Archaic, if there is more than 20% of technic material which is represented mostly by artefacts produced by pre-industrial processes (^{IUSS Working Group, 2015}), or as natural reference soil groups if they contain ≥ 10% artefacts. In the case of soils of long and intensive agricultural use in the past, soils in archaeological contexts can be classified as Anthrosols. Soils of ancient urban or proto-urban areas are very close to (Palaeo)Urban soils (^{Alexandrovskiy et al., 2015}), and a lot of contemporary urban areas are located over thick ancient cultural layers, included in present-day urban pedo- and sedimentogenesis (^{Stroganova et al., 1998}; ^{Alexandrovskaya and Alexandrovskiy, 2000}; ^{Prokof’eva et al., 2001}; ^{Alexandrovskiy et al., 2012}; ^{Mazurek et al., 2016}; ^{Devos et al., 2017}).

Micromorphological studies of cultural layers and habitation floors were introduced in archaeology in the end of the 1950s (^{Dalrymple, 1958}); and exponential development of this research area began at the end of 1980th. Since then, micromorphology is widely applied for understanding archaeological deposits, their facies, and microstratigraphy; as well as the anthropogenic alterations, sedimentary, soil and diagenic processes of site formation and post-anthropogenic transformation (^{Courty et al., 1987}; ^{Courty et al., 1989}). This knowledge is important both for practical geoarchaeology and for the fundamental concept of archaeological deposits as a product of human-environmental interaction. There are hundreds of published case studies containing local-scale information on micromorphology of archaeological sediments all over the world. Such publications have exponentially increased in numbers within the last decades. Studied archaeological sites vary widely in their age, ethno-cultural and environmental contexts, level and type of economy: among others, cave occupation sites of archaic hominins (^{Morley et al., 2019}); Palaeolithic open-air sites (^{Holliday et al., 2007}), Neolithic temporary dwellings of shepherds (^{Díaz et al., 2014}) and medieval urban layers within contemporary cities of Europe (^{Mazurek et al., 2016}; ^{Devos, 2017}); rock shelters (^{Villagran et al., 2016}), and urban environments (^{Sulas et al., 2017}) in tropics, the North European and Canadian Arctic (^{Macphail et al., 2013}; ^{Todisco, Bhiry, 2008}), and so forth. Thus, a huge amount of evidence has been accumulated on the micromorphology of archeological deposits. A majority of knowledge on archaeological micromorphology is accumulated in practically oriented case studies. It is concerned with the reconstruction of specific site formation processes, local environments, and human activities. At the same time, basic concepts of the formation of cultural layers¹ have not been comprehensively developed. In particular, there are no data on the variety and systematization of processes which participate in formation and further transformation of cultural layers, and no understanding how these processes are recorded in their micromorphology. Several approaches to comprehend the formation of cultural layers are contained in key monographs and key manuals on soil, and in micromorphological and geoarchaeological studies of archaeological sites and cultural layers (^{Courty et al., 1989}; ^{Goldberg and Macphail, 2006}; ^{Nicosia and Stoops, 2017}; ^{Macphail and Goldberg, 2018}; ^{Karkanas and Goldberg, 2019}), but there is still no agreement on these questions.

Some conclusions were drawn concerning the relations between the intensity of human impact and the level of an original soil transformation along with the accumulation of archaeological sediments (^{Alexandrovskij, 1989}; ^{Alexandrovskij, 2019}; ^{Stroganova et al., 1998} for soils of contemporary urban areas including archaeological sediments). A number of generalizations were suggested for a variety of anthropogenic microfeatures and their occurrence in different archaeological backgrounds and in different functional zones of a settlement (^{Macphail and Goldberg, 2010}; ^{Macphail et al., 2017}; ^{Nicosia and Stoops, 2017}; ^{Karkanas and Goldberg, 2019}). Meanwhile there have been only isolated attempts to compare features of cultural layers (including those located within contemporary urban areas) from a geographical perspective. Divergence of local natural soils and soils of palaeo- and contemporary residential areas was suggested together with convergence of soils of urban/palaeo(proto)urban areas in different environmental contexts (^{Stroganova et al., 1998}; ^{Zazovskaya, 2013}). According to this suggestion, features of urban (palaeo-urban) soils located in different climatic conditions are similar, but considerably different from those of zonal background soils. At the same time, it is declared that the composition and morphology of habitation deposits directly depend on the natural conditions of the territory (^{Stroganova et al., 1998}; ^{Alexandrovskiy et al. 2012}); and these deposits are being gradually transformed by pedogenetic processes in correspondence to their natural environmental conditions into zonal soils both in humid and arid temperate regions (^{Alexandrovskiy et al., 2012}; ^{Golyeva et al., 2016}).

There are nearly no studies regarding the variability of micromorphological features of cultural layers in different climatic conditions and local environments. Some very general geographical regularities of the formation of cultural layers were recently discussed in Aleksandrovskij (2019). A few studies mention that a set of micromorphological features and corresponding soil formation processes in urban archaeological sediments is dissimilar to those of original zonal Retisols (^{Prokopf’eva et al., 2001}). Some other works discuss the influence of post-depositional processes, including soil-forming ones, on the preservation of archaeological records, microartefacts and micromorphological features within cultural layers after their deposition, but mostly at a theoretical level (^{Courty et al., 1989}; ^{Campbell et al., 2011}; ^{Nicosia, Stoops, 2017}; ^{Karkanas, Goldberg, 2019}).

This paper offers a comparative study of micromorphological features on four early medieval archaeological sites located in different background environments: humid temperate mixed-forest zone, temperate continental semi-humid forest-steppe zone, and arid desert landscapes of delta areas.

The key aims of this study are: 1. to describe a set of micromorphological anthropogenic and geogenic features and supposed processes responsible for cultural layers formation on studied sites; 2. to estimate the proportional contribution of different environmental conditions and variability of medieval human impact on the described diversity of micromorphological features.

2. Materials and Methods

This study is a first-level generalization based on the evidence obtained by the authors within the last decade from four early medieval archaeological sites of different ethno-cultural background, but to a certain extent similar in general patterns and levels of economic development. The settlements under investigation were significant trading and multi-craft centres with land-use zoning: residential, craft, rural areas etc. All four sites are located in temperate continental climate, but differ in climatic humidity/aridity, landscape conditions, and geological background: (1) humid climate, subzone of mixed forests: floodplain of the Dnieper River (Gnezdovo site); (2) semi-humid climate, subzone of forest-steppe: watershed positions in the Middle Volga region (Muromsky Gorodok, and Malaya Ryazan’ sites); (3) arid climate, cold desert of the Aral region: ancient delta-alluvial plain of the Syr-Darya River (Dzhankent site) (Figure 1 and Table 1). More details on the background of the studied archaeological sites will be given under.

Figure 1 Location and landscapes of archaeological sites discussed in the text. 

3. Regional setting: archaeological and landscape background

Sampling of archaeological deposits/cultural layers was done within ongoing archaeological excavations in stratigraphic sequences of cultural layers, on habitation floors, and within individual archaeological structures and features (household pits, fireplaces, etc.). Sampling was carried out according to the general principles for micromorphological studies in geoarchaeology (^{Goldberg and Macphail, 2003}; ^{Karkanas and Goldberg, 2019}), in archaeological contexts which have been described in detail. Thin sections from different archaeological contexts were studied as follows: 84 in the floodplain area of the Gnezdovo site, 52 for the Dzhankent site, and 27 from five different sections of Muromskij Gorodok and Malaya Ryazan’ (Middle Volga region). Each of the studied thin sections corresponds to a certain stratigraphic or planigraphic unit and records specific environmental conditions including a certain type of local human impact.

Undisturbed samples were impregnated with polymeric epoxy resin and processed according to standard procedure for producing thin sections. The size of thin sections varies between 25 x 30 mm and 35 x 45 mm.Thin sections were studied under a polarizing microscope Nikon E200 Pol, at magnifications of 40x, 100x, and 400x. Micromorphological features were described with special attention to those related to human impact as specifying an original anthropogenic nature of cultural layers and geogenic features related to natural sedimentation and pedogenic processes. Micromorphological descriptions were done according to G. ^{Stoops (2003)}, diagnostics and interpretation of anthropogenic and geogenic features according to ^{Stoops et al. 2018}; ^{Macphail and Goldberg, 2010}; ^{Nicosia and Stoops, 2017}.

3.1. Russia, Upper Dnieper Region: Gnezdovo

The Gnezdovo archaeological site (including settlement and burial ground) is located in the central western part of the Russian Plain, within the Upper Dnieper river valley, 15-20 km downstream from contemporary Smolensk (Figures 1 and 2a). The Gnezdovo archaeological site is a unique cultural landscape dating to the time of the formation of the Ancient Rus‘ (last quarter of 8th century to beginning of 11th century AD). The site includes a settlement with a total area of more than 30 ha (Figure 2a). The settlement had wooden buildings which have not survived until today. Archaeological investigations over more than 140 years have revealed a spatial arrangement within the settlement: there are residential and craft areas, zones related to shipyard and river harbour activities, locations related to agriculture and to storage and processing of agricultural products. Archaeological material testifies to on intensive and varied human impact including agriculture and husbandry [various crops: wheat, rye, oat, barley, millet, buckwheat, (^{Kiryanova, 2007}; ^{Bronnikova, 2003}); animal husbandry: cattle, pigs, sheep and goats, (^{Kirillova, 2007})], fishing, various types of craft production such as iron and non-ferrous metal working, and bone carving (pre-urban to early-urban centre).

Figure 2 Local topography and key stratigraphic sequences of the sites; a. Gnezdovo (Modified from ^{Panin et al., 2014}); b. Muromskij Gorodok. c. Malaya Ryazan’; d. Dzhankent. 

The climate of the area is moderately continental, temperate humid with moderately warm, rainy summers and cold snowy winters (for key climatic characteristics, see Table 1). Zonal vegetation of the region is mixed forests; zonal soils are mostly Retisols. Sandy river terraces are covered by pine forests over Albic Arenosols Protospodic; the floodplain is today covered with meadows and bogs on a variety of Fluvisols. Natural vegetation and soils of the area are considerably transformed due to contemporary human impact.

The site is situated mostly on the first terrace above the floodplain, but part of the open settlement is on the floodplain buried under a layer of contemporary alluvium of 0.7 - 3.0 m thickness. Micromorphological studies were conducted in the floodplain soil-sedimentary sequences with buried cultural layers. This buried part of the cultural layer is of a key importance for archaeology because of its excellent preservation. The layer was not disturbed by contemporary ploughing comparatively to the one located on the first terrace. The wetland floodplain depressions (Figure 2a-10) are contemporary waterlogged providing conditions for conservation of organic materials. Nowadays, the floodplain is regularly flooded seasonally during spring snowmelt.

Surface Fluvisols are formed over stratified silt, loamy sand and sand fluvic material. Buried Retisols were studied in the escarpment of the high riverbank beyond the area of the medieval settlement in particular in the section 07-01, and the rests of the same Retisols were described under the cultural layers in the excavation plots P8 and DP, in particular (Figure 2a). The buried Retisols indicate that about 1.5 kyr cessation of floodplain sedimentation preceded the accumulation of early medieval cultural layers (^{Bronnikova et al., 2003}). The episode of decreased floodplain sedimentation was dated at Gnezdovo to 2,300 (2,000)-900 cal BP (^{Panin et al., 2014}). This episode is known for many rivers of the East European Plain (^{Sycheva, 2006}).

The cultural layer and buried Retisols vary in texture from loamy sand and silt to sandy loam. The pH values of cultural layers are close to neutral (6.5 -7.2) in comparison with slightly acid (5.5 - 6.5) surface Fluvisols and rests of Retisols under the cultural layers (^{Bronnikova et al., 2003}).

4. Results

4.2. Soil-Sedimentary Sequences and Micromorphological Features

4.2.1. Russia, Upper Dnieper Region: Gnezdovo

Soil-sedimentary sequences including buried cultural layers on the floodplain area of Gnezdovo archaeological complex are classified as a variety of Fluvisols over Urbic Technosols (Archaic, (Fluvic), Gleyic, (Histic), (Lignic), (Pretic)). In some sections Albic Retisols (Cutanic, Siltic) or their Bt horizons were recorded under the Technosols. These remnants of buried Retisols are related to earlier studied 1.5 kyr cessation of floodplain sedimentation preceded the accumulation of early medieval cultural layers (^{Bronnikova et al., 2003}). Floodplain sandy-silty sediments overlaying the cultural layers are 50-100 cm thick.

A wide variety of anthropogenic microfeatures is identified in the Gnezdovo cultural layers, except for features composed of the most soluble forms of carbonates (calcitic spherulites and eggshells) (Table 2). The most widespread group of anthropogenic features are all kinds of pyrogenic features: charcoals of different sizes and state of preservation, including charcoal dust (particularly incorporated in clay coatings), burnt fragments of clay, and charred stones. Lamination patterns related to sedimentation of anthropogenic material (particularly habitation floors, damping and filling features), trampling features, and micro- heterogeneity occur widely.

Coarse angular rock fragments (mostly granites) which are never found in local floodplain sediments, are common in the cultural layers, and there is a single occurrence of a limestone fragment (Figure 3b). Carbonate-containing features in general are very rare in the Gnezdovo cultural layers although there were several calcareous finds. One is micritic ash within a charcoal-rich spot on the habitation floor, another was the micritic - microsparitic, partly recrystallized filling of a small barrel within the crafts quarter.

Figure 3 Anthropogenic microfeatures; a. Calcareous silt with irregular linear distribution of silicate sandy particles, Gnezdovo, XPL (crossed polarized light); b. Limestone fragment, Gnezdovo, XPL; c. Corroded iron droplet, Muromskij Gorodok, PPL (plane polarized light); d. Articulated cereal phytoliths, Dzhankent, PPL; e. Burnt dung with numerous calcitic faecal spherulites, M. Ryazan’, PPL; f. Burnt dung with numerous calcitic faecal spherulites, M. Ryazan’, XPL. g. Herbivore excrement: possible sheep/goat pellet, M. Ryazan’, PPL; h. Burnt eggshell, biogenic crumb microstructure of the surrounding material, M. Ryazan’, PPL; i. Vitrified vesicular silica (melted phytoliths), grass charcoals, Dzhankent, PPL; j. Micritic ash with grass charcoals, Dzhankent, XPL; k. phosphatized clay and silty groundmass, rich in organic matter, probably developed because of liquid faecal waste intrusion, Gnezdovo, PPL; l. Fen-like, probably phosphatic (iron-phosphatic) coatings and infillings, surrounding crumb biogenic microstructure, Muromskij Gorodok, PPL. 

Partly decomposed plants, especially wood residues, bone fragments and accumulations of phytoliths are also rather common in the Gnezdovo cultural layers. Phosphate accumulations with admixtures of organic matter, iron and clay occur in form of impregnations, intercalations, and nodules. Some of these features are probably developed because of leakage of faecal or midden sludge (Figure 3k).

A variety of redoximorphic features (nodules, intercalations, interlayers within clay coatings) and textural pedofeatures (layered clay, humus-clay, Fe-clay coatings, and silty infillings) are the most widespread pedofeatures within the cultural layers and below (Figure 4c, 4d). Some of these features are anthropogenic (anthropogenically induced), particularly those enriched in dark organic matter and fine charcoal dust (Figure 4c and 4d).

Interlayers with banded distribution pattern, typical for alluvial deposits, occur in the Gnezdovo cultural layers (Figure 4k, 4l). Diatoms are found within interlayers with fine alluvial lamination (Figure 4j). These features indicate episodes of floodplain sedimentation before and within the period of the cultural layer accumulation.

Figure 4 Geogenic microfeatures; a. Biogenic crumb microstructure, excrements of mesofauna, micritic grain coating, bone fragments, M. Ryazan’, PPL (plane polarized light); b. Well-sorted bleached silt infilling, Fe-nodule, crumb microstructure, Gnezdovo, PPL; c. Crescent clay coating enriched with dark organic matter, fine charcoal dust and Fe-oxides, Gnezdovo, PPL; d. Crescent clay coating enriched with dark organic matter, fine charcoal dust and Fe-oxides, Gnezdovo, XPL (crossed polarized light); e. Micritic nodule with biogenic pores, Muromskij Gorodok, XPL; f. Fragment of plant tissue with micritic incrustations, excrements of mesofauna, Dzankent, PPL. g. Gypsum nodules (roses) in highly calcitic crystallitic b-fabric, Dzhankent, XPL; h. Probable hydrogenic aggregate composed of large gypsum crystals, Dzhankent, PPL; i. Fe-oxide hypocoating, Dzankent, deeper horizons, PPL; j. Diatom testifying to alluvial origination of sterile interlayer within the cultural layer, Gnezdovo, PPL; k. Banded distribution pattern of the sterile interlayer within the cultural layer, Gnezdovo, PPL; l. Banded distribution pattern of the sterile interlayer within the cultural layer, Gnezdovo, XPL. 

4.2.2. Russia, Middle Volga Region: Muromskij Gorodok and Malaya Ryazan’

Both settlements are located in watershed steppe landscapes. Soils in the studied sections of the cultural layers of Muromskij Gorodok are classified as Luvic Chernozems (Loamic, Stagnic, Technich, Turbic) on loess-like loams. The M. Ryazan settlement is located on outcrops of limestone along the Volga River high bank. The thickness of the studied sections is 90 to 100 cm. Two studied sections were classified as Stagnic Chernic Rendzic Phaeozems (Loamic Technic, Turbic).

All varieties of anthropogenic microfeatures besides plant residues occur in these cultural layers. Phytoliths, particularly articulated (chained as they were inside plants), including phytoliths of vegetative parts of cereals, bones, faecal pellets of herbivores (Figure 3g), easily dissolved calcareous features such as calcitic faecal spherulites (Figures 3e, 3f), calcareous ash, fragments of eggshell (Figure 3h), and fragments of mollusk shells were recorded in these layers.

Phosphatic coatings and infillings are particularly frequent in the cultural layers of Muromskij Gorodok. Fen-like patterns are specific to these features (Figure 3l).

Fragments of rammed-earth materials occur in both sites. The rare find of a metal droplet was recorded in the cultural layer within a craft context of Muromskij Gorodok (Figure 3c).Pedogenic features related to biogenic activities are most frequent: zoogenic pores, structure and excrements of mezofauna (Figures 3h, 3l, 4a and 4e). Carbonate pedofeatures are also very common, variable and well-shaped in the cultural layers of the forest-steppe zone where soils originally have carbonaceous geochemistry and accumulate secondary carbonates. There are impregnations, coatings on pore walls, incrustations and pseudomorphs of plant residues and nodules (Figure 4e).

Textural pedofeatures are also rather typical for the cultural layers of forest-steppe zones formed at the base of Luvic Chernozems.

Redoximorphic features occur in lower horizons of the cultural layers and underneath them as the cultural layers at both sites are quite heavily textured. These features are more common for Malaya Ryazan’ where the cultural layer is underlain by massive limestones.

4.2.3. Kazakhstan, Aral Sea Region: Dzhankent

The Dzhankent cultural layers are classified as Urbic Technosols (Archaic, Aridic, Calcaric, Loamic/ Clayic, Salic).

The cultural layers of this site are extremely rich and variable in anthropogenic microfeatures (Table 2). Various remains of earthen architecture (mud-bricks, adobe, daub, wall plaster) are the most common macro- and microartefacts here. There are abundant pyrogenic features, both in local hearths and fireplaces and within several stratigraphic layers related to fire events. Those are charcoals, in particular grass charcoals (Figure 3i, 3j), micritic ashes (Figure 3b), vitrified silica of melted phytoliths (Figure 3i), burnt bones, dung, and mineral material.

Thick layers stuffed with plant remains and phytoliths are common at Dzhankent. Phytoliths of Phragmites are among the most abundant. There are frequent accumulations of articulated phytoliths, which means in-situ decomposition of big amounts of plant materials. In particular, phytoliths of vegetative parts of cereals are quite frequent (Figure 3d).

Trampling features are very common, both in mineral layers and in interlayers rich in plant residues.

Bone fragments are widespread while phosphate pedofeatures are not so common.

Pure gypsum, in plates of several square centimeters as well as soft powdery pebbles, is found in the cultural layers. These materials were imported from the surroundings beyond the settlement boundaries.

A set of geogenic microfeatures is typical for the desert soils and sediments. Carbonates are present everywhere, mostly in form of micritic impregnation: calcitic crystallitic b-fabric (Figure 4g). Micritic incrustations along plant tissues occur in the deeper horizons (Figure 4f). These features may have originated from precipitation of calcite in a capillary-moisture zone.

Highly variable forms of gypsum are found in the cultural layer of Dzhankent. We suppose that most gypsum features are pedogenic but related to different stages and conditions of soil formation. Thus, densely packed microcrystalline gypsum infillings were presumably formed by crystallization from solutions saturated in Ca²⁺ and SO₄ ^2-. Aggregates of lens-like crystals with radial growth (druses, or gypsum ‘roses’) often reveal partial gypsum substitution for carbonates which corresponds to somewhat decreased aridity (Figure 4g). Large idiomorphic crystals and their aggregates most probably were accumulated due to slow crystallization from dilute solutions within a water-saturated zone (Figure 4h).

There are not so many data on the variety of readily soluble salt accumulations because they are strongly disturbed during impregnation procedure. Nevertheless, infillings of pores with fine crystals of soluble salts (mostly NaCl as based on the data of bulk chemical composition and XRD-analysis in the electron microscope) were recorded in the cultural layers of Dzhankent.

Biogenic features are rare in the desert cultural layers of Dzhankent. Fine aggregation is mostly related to a high content of readily soluble salts. Nevertheless, excrements of mesofauna (Figure 4f), elements of zoogenic structure, fungal mycelium and fruits, eggs of mesofauna, and other biogenic features do occur here.

5. Discussion

Studying the cultural layers (archaeological sediments, anthropogenic soils) of archaeological sites and the regularities of their formation, we deal with the historical legacies of soil-human interrelations and local human impacts in the past. Cultural layers are multi-component objects, a product of complex multifactor and multiphase interactions. These interactions include geogenic (geomorphic, erosion-related and sedimentary, syn-sedimentary pedogenic and post-sedimentary pedo- and diagenic) processes as well as a variety of anthropogenic processes: input, translocation, turbation, processing and transformation of natural materials, and production of anthropogenic materials: ceramics, glass, mortars, alloys etc. (^{Butzer, 1982}; ^{Sycheva, 1995}; ^{Alexandrovskaya, Alexandrovskiy, 2000}; ^{Alexandrovskiy et al., 2012}; ^{Alexandrovskiy et al., 2015}; ^{Karkanas, Goldberg, 2019}).

The present study was conducted on archaeological sites of the same respective period and with a more or less similar intensity of human impact. All settlements concerned (except for Malaya Ryazan’) were large pre-urban or urban, densely populated centres with well-developed infrastructure, and evidence of various householding and economic activities. It is obvious already from classification issues and approximate thickness of cultural layers that not only the intensity and duration of human impact, as suggested earlier (^{Alexandrovskij, 1989}; ^{Stroganova et al., 1998}), define the depth, extent and direction of an original soil transformation and formation of cultural layers. The present thickness of cultural layers and their artefact contents are also a result of the quality of accumulated substrates and sediments, the conditions for their further preservation in a post-sedimentary period, and the transformation by post-sedimentary geogenic (natural erosion-sedimentation, soil and diagenic) processes.

In the specific case of Gnezdovo, the cultural layers in the floodplain in the humid temperate climate of the boreal mixed forest subzone are about 1 - 1.5 m thick and rich in artefacts (particularly due to the good preservation of wooden materials in a waterlogged environment), so that these soils meet diagnostic criteria of Technosol. (Palaeo)Technosols were overlain by floodplain sediments soon after the end of the settlement, and in some cases underlain by remains of Retisols which predate the early medieval settlement. Therefore, cultural layers here were classified as buried Technosols over buried Retisol.

Cultural layers on the watershed sites of the forest-steppe zone in subboreal semiarid climate have a similar thickness, but they are exposed to contemporary soil formation with high bioclimatic potential. These cultural layers are well aerated, balanced in water supply and biologically active. The settlement has a huge area and contains very little anthropogenic organic matter, so the cultural layers are not very thick, and poor in artefacts. The lack of organic materials, in this case, is explained, first of all, by good conditions for its mineralization.

The Dzhankent cultural layers were classified as the surface Technosols. These were accumulated in the delta-alluvial plain in arid desert landscapes. The contemporary capillary fringe is close to the lower boundary of the layers and possibly it was even closer to the surface in early medieval times. These layers are about 5-8 meters thick due to a large component of earthen building materials, big quantities of weakly decomposed plant materials in the lower part of the layer, and a low bioclimatic potential resulting from high aridity and high level of salinization.

The sets of geogenic micromorphological features are different and strongly environmentally dependent for every studied cultural layer. Clear zonal regularities are evident in the distribution of pedogenic features and their abundance. Thus, textural pedofeatures are typical of cultural layers in a humid boreal climate. The cultural layer of Gnezdovo settlement was studied in the floodplain part of the settlement, but as it was shown earlier, the settlement area was not affected by seasonal floods which were low and irregular (^{Bronnikova et al., 2003}; ^{Panin et al., 2014}). Zonal soil formation proceeded in the floodplain at that time, and textural pedofeatures within the cultural layer and beneath are the relicts of this phase (^{Bronnikova et al., 2003}).

Biogenic features, accumulations of different forms of secondary carbonates, and clay coatings are the most common pedogenic features for cultural layers on the watersheds of the forest-steppe zone. All these features are typical for natural Chernozems and Phaeozems IUSS (^{Working Group WRB, 2015}; ^{Stoops et al., 2018}).

A variety of carbonate, gypsum, and soluble salts microfeatures are characteristic of the cultural layers of the desert zone. Despite the location on the delta-alluvial plain and the influence of ground waters, these features could be regarded as zonal because the geochemistry of the groundwaters and their fluctuations are also climatically dependent.

Along with zonal regularities, the sets of geogenic features are controlled by local geological backgrounds (geomorphology, hydrology and parent materials). Floodplains and delta alluvial areas in any climatic zone form in response to geomorphic and hydrological factors which define additional water supply related to seasonal inundation and ground waters. This results in permanent or semi-permanent waterlogging (^{Cook et al., 2009}). Gnezdovo cultural layers are located now in the seasonally inundated floodplain. Besides that, cultural layers of the wetland depression are located in the zone of groundwater discharge; these positions are constantly waterlogged. This explains that redoximorphic features are most common in the Gnezdovo cultural layers, especially within wetland depressions. Within these layers and underneath them, floodplain and lacustrine facies of fluvial materials were described.

Redoximorphic features are also common in the lower horizons of Dzankent cultural layers. These layers are not directly related to ground waters today but lay within the capillary fringe or close to it. Fluvial sedimentary patterns do not occur within the layer of Dzhankent but are always found underneath.

The cultural layers of the forest-steppe zone are located in watersheds having no connection with groundwaters. Some special aspects related to soil-forming substrates are observed here. Loamy texture and especially limestone bedding underlying the cultural layer of Malaya Ryazan’ result in seasonal water stagnation and development of redoximorphic features in lower parts of these cultural layers. Carbonate features are more abundant and variable due to the highly calcareous parent material. A high diversity of anthropogenic features was observed in all the studied cultural layers located in different environmental backgrounds. Nearly all groups of anthropogenic microfeatures mentioned in manuals of archaeological micromorphology were present in medieval cultural layers (^{Macphail and Goldberg, 2010}; ^{Karkanas¿ and Goldberg, 2019}). These are features related to input, output, turbation and compaction: features disturbing the original horizonation and lamination, creating new sedimentary patterns and lateral macro- and microheterogeneity; features related to input and processing of mineral and organic materials: extraneous fragments of rocks and sediments, plant residues, biomorphs, livestock excrements, faecal spherulites, bones, mollusc shells and eggshells; and neoformed phosphatic features, or ones composed of organophosphates with iron hydroxides, and clay admixture; various pyrogenic and technogenic products.

The sets of anthropogenic micromorphological features have much in common though they are not the same for all studied archaeological sites. Thus, features of the cultural layers located in different climatic conditions are similar in being considerably different from those of zonal background soils. This result confirms the convergence of features for thick and intense (proto)urban cultural layers in different environmental backgrounds, as suggested earlier (^{Stroganova et al., 1998}; ^{Zazovskaya, 2013}). In summary, the most obvious conclusion is that some described anthropogenic features are common for all studied objects. And regarding all accumulated knowledge on the occurrence of anthropogenic features in habitation deposits, we consider these features to be universal for cultural layers in general. Such features are related to (a) turbations: disturbance of original soil and sedimentary patterns of arrangement at all morphological levels, and replacement by newly formed anthropogenic horizonation, layering and other types of heterogeneity; to (b) anthropogenic sedimentation: accumulation of different mineral and organic materials, of earthen building materials or destroyed constructions; and to (c) pyrogenic processes. Most of these ‘universal’ features occur in archaeological deposits of any age, irrespective of cultural or environmental contexts (^{Macphail and Goldberg, 2010}; ^{Karkanas and Goldberg, 2019}). The universal occurrence of these anthropogenic features in cultural layers is explained, first of all, by some universal anthropogenic processes related to life-supporting practices. As an example, the inevitable significance of fire in human history has resulted in the widest distribution of various pyrogenic features in soils and sediments of archaeological contexts. A high level of anthropogenic production of features is related to fire and to phosphor-containing materials. Since most ancient times, there have been two cornerstones of human existence: the human use of fire; and the human use, transformation, emittance, and accumulation of organic matter of animal origin. The second reason for the wide distribution of the above-mentioned features is the high resistibility of most of them in most environments. Carbonate ashes are an exception because they are theoretically unstable in acid or even in close to neutral intra-soil environment. They are widespread possibly due to the sheer quantities in which they are produced during residential, craft and other human-related activities, and because of local soil alkalinization in the burning process.

There are two inferences from the above-mentioned facts. As universal anthropogenic features occur in every cultural layer, they do not contribute much to a closer specification of human activities. On the other hand, the exact level of abundance of these features reflects the intensity of anthropogenic pressure.

Meanwhile, some deeper studies of these universal features also can give very important information on the local environment, some aspects of human impact and the important events in the settlement history. As an example, pyrogenic materials were studied in detail in the Dzhankent cultural layers. Stratigraphic, micromorphological and microstratigraphic investigations combined with anthracological studies and dating allowed us to identify three fire events recorded in the cultural layers, and to define the difference between the patterns related to fires (conflagrations) and local fireplaces.

In addition to the universal features, there are features of rare occurrence found only in some of the studied objects, and in certain stratigraphic layers or sectors of a site. The occurrence of such rare features is determined not only by the occurrence of related specific human activities, but also by the local intrasoil environment. For example, no faecal spherulites and metal droplets were found in the Gnezdovo cultural layers although there is evidence at the site for animal husbandry, and some of the studied objects were located directly within metal-working zones. The humid climate and the floodplain water regime of the location, leading to high groundwater levels, and slightly acidic pH values do not favour the preservation of unstable biogenic calcite nor of small metal artefacts. It is known that faecal spherulites are generally more soluble than geogenic calcite (^{Canti, 1999}; ^{Karkanas and Goldberg, 2018}). Nevertheless, some calcareous material has been found there, in the form of carbonate ashes and limestone fragments. The reason for this is location conditions: alkalinization during burning, and the presence of large quantities of limestone.

Generally, the more intensive and variable the human impact has been, the more abundant and varied the anthropogenic features are. In the cultural layers of investigated pre-urban and urban medieval settlements with clear zoning and high intensities of human impact, anthropogenesis has changed (or even destroyed) the original soil profile, suppressed in some respects certain soil-forming processes (mostly due to changed geochemistry), and introduced a wide variety of anthropogenic processes and features. This is well known for urban and pre-urban archaeological sediments (^{Stroganova et al., 1998}; ^{Prokof’eva et al., 2001}; ^{Mazurek et al., 2016}; ^{Howard, 2017}; ^{Devos et al., 2017})

In medieval towns, the human impact considerably changed the geochemistry of local soil/sediments within habitation areas (primarily due to an input of phosphorus, organic matter, carbonate, sulphur, and nitrogen-containing materials). These changes are generally more advanced in humid/semi-humid conditions than in arid regions.

Despite intensive residential and craft-related human impact in the medieval past which formed the cultural layers in pre-urban or urban local environments, zonal regularities and the impact of geological background are of great importance for the occurrence and distribution of micromorphological features. Geogenic features in the studied cultural layers vary depending on the background landscape and climatic conditions. In addition to the described set of variable anthropogenic features, the studied cultural layers were characterized by a set of pedofeatures typical of zonal soils and some other geogenic features related to the geological setting.

6. Conclusions

The set of anthropogenic features reflects the past human impact in the locality: the more varied and frequent anthropogenic features are, the more intensive and variable has been the human impact which occurred in the past. At the same time, the occurrence of certain anthropogenic features may indicate not only human-related processes during their formation, but also a current soil environment which may be favourable or destructive for these features. For instance, calcareous features, especially delicate ones such as faecal spherulites or egg shells, are not normally preserved in conditions of humid climate and active leaching processes; metal droplets certainly would be easily dissolved even in a seasonally waterlogged environment. On the other hand, as a practical guide on environmental archaeology realistically commented: “Predicting the survival of environmental remains in archaeological deposits is not an exact science” (^{Campbell et al., 2011}). For example, micritic and microsparitic ash accumulations and large quantities of softened and partly re-crystallized, but not considerably dissolved limestone was recorded in the Gnezdovo cultural layers, on a site which is located in a humid climate in a seasonally flooded location.

The set of anthropogenic features found in a certain stratigraphic unit or sector of settlement deposits is conditioned both, by the type and intensity of local human impact and by the current local soil environment. Thus, intensive and variable human impact in a medieval settlement would have strongly changed the geochemistry of local soil/sediments, transformed or (more often) destroyed the original soil profile, changed input and output of compounds and materials, widened the set of soil and sedimentary processes, and suppressed some of the original soil processes. Nevertheless, cultural layers contain data of the climatic and landscape conditions of their formation and transformation, in a set of features which is related to the local background environment.