The Kirsino 8 settlement is situated in the northwestern part of the East European Plain 54 km to the southeast of the city Saint Petersburg
According to the soil-ecological zonation of Russia (Shishov et al., 2001), the study area belongs to the northern part of the Chudsko-Ladoga province dominated by Retisols and Podzols (according to the classification of the IUSS working group WRB, 2015) developed under southern taiga forests. A forest with a domination of pine spruce with an admixture of small-leaved species, which is typical for zonal representatives of forest landscapes of the southern taiga of the northwest of Eastern Europe, grows within the investigated area.
The study area is represented by an accumulative lacustrine-glacial plain. The flat, monotonous relief of the plain is complicated by the presence of kame hills. The settlement is situated on a flattened surface of a separate kame hill, elevated at ~55 m a.s.l. The climate of the examined region is transitional, from moderately continental to moderately maritime. The mean annual temperature in the studied area is +3.6 °C. The mean annual precipitation is ~600 mm, and the value of the precipitation/evapotranspiration ratio is 1.33.
The study of the Medieval monuments on the territory of the Leningrad region was started in the middle of the 19th century by the archaeologists of the Imperial Archaeological Commission. However, the system of Baltic-Finnish tribes and Slavic population migration has been studied unevenly. The largest arrays of burial mound monuments of the time are known on the territories of the Izhora Plateau (11–15th centuries) and the Southeastern shore of the Ladoga Lake (9–13th centuries). The ground Karelian burials in the Northwestern shore of the Ladoga Lake (12–14th century) are less studied. Burial monuments associated with Izhora discovered in the last decades remain the least studied. The biggest interest lies in a group of burial monuments discovered in 2006, 2009 and 2012 in the watershed territory of the Mga River (left tributary of the Neva River) in the vicinity of the villages Kirsino – Shapki (Fig 1a). The area of archaeological objects located about 20 km from Neva River) (Sorokin 2006, 2008). The 12–13th-century Izhora burials studied to date were inhumations at the horizon, as well as, possibly, in shallow graves. They were surrounded by stone fencings covered with low mounds. The territory was prepared for burial through vegetation burning and surface leveling by means of adding earth.
The four buried soil profiles (profiles K1–K4) were studied (Fig. 1) in order to retrieve environmental trends. It is important to note that both, the buried soil profiles themselves and their fragments, preserved to varying degrees, and the layers of the barrow mound that overlapped them were chosen for this study. The profiles are located in close proximity to each other – no more than 8 m from the central part of the burial mound (where the thickness of the bulk layer is ~1.1 m) to its edge part. In the buried soils in the K2–K4 profiles, Humic horizons of virgin soils are diagnosed. Only the profile K1, which is restricted to the central part of the burial mound, is represented by soil-forming rock.
The two buried soils (the profiles K3 and K4) were classified as Albic Podzols (Lamellic, Arenic) and formed on kame sands. These soils have the following set of genetic horizons, which is typical for Podsols developed on sandy parent material under south taiga vegetation: Ahb–Eb–Bhsb–Cs1b–C(s)2b–Cs3b. The thickness of the Humic horizon in the profiles varies from ~3 cm (profile K3) to 10(12) cm (profile K4). There is a presence of bleached albic material located under the Humic horizons in the profiles of these soils. The Spodic Bhsb horizons of the soils are characterized by an accumulation of blackish organic matter and reddish Fe oxides. The third buried soil (profile K3) was classified as an Entic Podzol (Lamellic, Arenic) (with a set of horizons Ahb–Ahsb–Bsb–Cb–2C1b–2C2b–3C3b), which distinguished it from profiles K3 and K4 by the absence of albic material in the structure of the profile. The thickness of the two Humic horizons of the soil is ~14 cm. The soil formed in the top layer of the burial mound and restricted to the highest flat surface of the mound was classified as a Protic Arenosol (Novic) and has the profile Ah–AC, with a thickness of 6 cm in the Ah horizon.
An important diagnostic morphological feature of the studied buried soils and parent material (with the exception of profile K3) is the presence of non-concretionary new Fe formations, represented by pseudofibres (lamellas) which reflect intrahorizon migration and segregated accumulation of Fe oxides. Iron lamellae are non-cemented bands of illuvial iron < 2.5 cm thick and reflect seasonal pulling of groundwater. In all investigated buried soil profiles of buried soils there are inclusions of charcoal fragments of different sizes, the content of which varies by volume from 2 to 10%. We noted that the inclusion of charcoal fragments was observed only in the allochthonous material of the burial mound and the preserved Humic horizons of the buried soils. We can assume that 1) only the upper part of the soil cover under the construction of the burial mound (profiles K2–K4) was affected by anthropogenic influence or 2) the soil profile was completely cut off within the central, initially relatively high part of the kame hill (profile K1).
The grain size distribution pattern is in good agreement with the morphological observations. All studied buried soils are formed in loamy sand and sand sediments (Table 1), with medium and fine sands fractions are significantly predominating. One exception can be observed in the K2 profile, in which, at a depth of 85–105 cm, there is a presence of a silt loamy interlayer. The surface soil, unlike buried soils, is characterized by sandy loam texture in the upper 0–70 cm, whereas the basal part is sand to loamy sand in texture. The earth material of the burial mound is similar in granulometric composition to the material of the surface soil (Tabl. 1), suggesting that the upper and middle parts of the previous soil profiles have been taken from the nearest surrounding during the burial mound construction. This is proved particularly clearly by textural change of the allochthonous layers and the original parent material in the profile K1.
All horizons of buried and surface soils as well as the material of the burial mound are mostly strongly acidic (Tabl. 1), which is typical for the Podzols of south taiga areas of the East European Plain.
Fig. 1. Kirsino Archaeological site 8. K1–K4-buried soil, K5-surface soil.
To retrieve environmental trends the surface soil (profile K5), which formed on both the same surface and lithology and at approximately the same hypsometric level, was studied. The surface soil of the chronosequence (profile K5), – Entic Podzol (Lamellic, Arenic), – is morphologically close to profile K2 but is characterized by a relatively thick thickness of Humic horizons (35 cm). Also the surface soil, unlike buried soils, is characterized by sandy loam texture in the upper 0–70 cm, whereas the basal part is sand to loamy sand in texture.
In the surface soil, the bedding of sandy loam rocks with deposits of heavy boulder loam is also noted. Only in this section the loamy material is wedged at a depth of 37-52 cm. The surface soil is characterized by the presence of well-defined topical pseudofibre layers in the soil-forming rock at a depth of 74-110 cm. Therefore, we can talk about a similar water regime (seasonal pulling of groundwater).
Investigations of the composition of the buried soil formed on the surface of the kame hill to which the medieval Izhora burial mound Kirsino 8 was tied showed that the original soil cover was represented by combinations of automorphic Entic Podzols (Lamellic, Arenic) and Albic Podzols (Lamellic, Arenic). These soils are also characteristic of modern biogeocenoses, which indicates a practically changeless trend of pedogenesis in the average time interval.
It was determined that during the construction of the mound and leveling of its surface the soil cover was scalped to a significant degree: almost completely (in the central part of the kame hill) to several cm and tens of cm (in the peripheral part of the mound). The degree of diagenetic changes in soils during the time of their burial (~ 800 years) was determined, primarily in the reduction of the TOC content by dozens of times compared to surface soil.
Based on the data of phytolith and spore-pollen analyses it is quite possible to reconstruct the following sequence of development of the territory. The original coniferous forest was cut down, partly burnt. Then the site with ash was plowed for cereal crops (slash-and-burn type of management). After a short time (3–5 years), when the fertility of the soil had decreased, the site was abandoned and began to overgrow with forest (from small-leaved species to coniferous). At this stage, the forest was rebuilt for burial mounds.
Because of their relative (up to 10 m) elevation above the terrain surface which excludes ground swamping, the kame hills – composed mostly of sandy and water-permeable parent material – were a convenient place for the construction of barrow medieval burial grounds.
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