Complex Study of Settlements Dating from the Paleolithic to Medieval Period in the Ural Mountains on the Border of Europe and Asia

Abstract

Soil, geochemical, microbiological, and archeological studies were conducted at eight settlements dating from the Paleolithic to Late Medieval and Modern Ages near the southern Trans-Urals Mountains, Russia. The forest-steppe landscapes, rivers, and abundant mineral resources have attracted people to the region since ancient times. Cultural layers (CLs) are marked by finds of ceramics fragments, animal bones, stone, and metal tools. The properties of CLs include close-to-neutral pH, being well structured, the absence of salinity, enrichment with exchangeable calcium, and anthropogenic phosphorus (0.2–0.4%). The majority of CLs start at a depth of 3–25 cm, extend to 40–60 cm, and contain 6–10% organic carbon (Corg) in the 0–20 cm layer, reflecting carbon input from modern-day processes. At the Ishkulovo site (0.6–0.8 ka BP), Corg decreases to 1.3% because the CL is below 80 cm, and in the absence of fresh organic material input, carbon has been mineralized. The proximity of sites to deposits of copper, chromium, zinc, and manganese in the Ural Mountains creates natural high-content anomalies in the region, as indicated by their abundance in soils and parent rocks. In the past, these elements were also released into CLs from metal products, ceramic fragments, and raw materials used in their manufacture. The sites are quite far (18–60 km) from the Magnitogorsk Metallurgical plant, but industrial stockpiles of S (technogenic coefficient—Ct 30–87%), and, less often, Cr, Mn, and Sr (Ct 30–40%) accumulated in surface layers. These three factors have led to the concentration of pollutants of the first (arsenic, chromium, lead, and zinc) and second (cobalt, copper, and nickel) hazard classes at CLs, often in quantities 2–5 times higher than values for parent materials and geosphere average content (“Clarke” value), and, and less often, more than the allowable content for human health. This may have influenced their health and behavioral functions. Due to the above properties, chernozems have a high buffering capacity and a strong bond with heavy metals. Therefore, no inhibition of microbes was observed. The microbial biomass of the 0–10 cm layer is high, 520–680 µg C/g, and microbes cause the emission of 1.0 C-CO₂ µg/g of soil per hour. During the ancient settlements’ development, a favorable paleoclimate was noted based on the data cited. This contributed to the spread of productive paleolandscapes, ensuring the development of domestic cattle breeding and agriculture.

1. Introduction

In 2022, soil and archeological studies were conducted on the cultural layers of eight settlements of varying ages in the Abzelilovsky district of the Bashkortostan Republic near the southern Ural Mountains, Russia. The cultural layers (CL) formed as a result of human activity at the settlements and consist of various materials: fragments of ceramics and metal products, building ruins, animal remains, etc. The thickness of the CL varies depending on the duration of people’s residence [1,2] (Sycheva et al., 2004; Nowaczinski et al., 2013). The region is located on the border of Europe and Asia and has always served as the most important crossroads between them. With a variety of landscapes, an abundance of rivers and lakes, and a wealth of mineral resources, it has attracted people since the Early Paleolithic. The archeological record includes world-famous sites such as the Shulgantash (Kapova) Cave with Paleolithic paintings and the Mysovaya Paleolithic site, one of the first to be discovered in the region [3]; the large copper mines of Bakr-Uzyak and Kargaly, developed in the Bronze Age [4]; an elite necropolis of nomads of 2500 BP, where 3000 objects were found, including beautiful works of art made of precious metals [5,6]; and the Ufa II settlements with a cultural layer 4 m thick (containing fragments of tools and vessels, remains of buildings, etc.), which functioned from the first centuries AD [7]. The accumulation of heavy metals in the soil profiles and parent rocks near the southern Ural Mountains compared to the Clarke values (the average element content in the geosphere) was established. This indicates that their main concentration is due to native reserves associated with the geological history of the mountains [8,9,10]. Technogenic accumulation of heavy metals occurs mostly within 5–10 km from many industrial enterprises in the region and covers surface soil layers [11,12,13,14,15,16,17].

Archeologists have investigated hundreds of historical monuments of the region [18], though only some of them were examined by the methods of natural sciences [19,20,21,22,23,24,25,26,27,28,29,30,31].

The aim of a complex study of the settlements was to determine the pedological, microbiological, and geochemical properties that characterized the environment in which the ancient population lived and to highlight the causes of the accumulation of heavy metals in the area and resulting soil pollution.

2. Study Area

Pedological and archeological studies were conducted on eight settlements of differing ages in the Abzelilovsky district of the Republic of Bashkortostan near the southern Trans-Ural Mountains in Russia (

Table 1. Characteristics of the sites and the cultural layers.

Settlement	Archeological Culture	Age	Coordinates	CL Thickness, cm, Number of Finds	Distance from Magnitogorsk Plant, km	Salt, %, 90–100 cm
Amangildino	Alakul	3.9–3.4 ka BP	N53°23′07.0″	3–46	60	0.01
			E58°20′02.4″	25
Elimbetovo-7	Gamayun	2.6–2.5 ka BP	N53°31′30.6″	11–38	24	-
			E58°38′20.0″	35
Ishkulovo	Nomads	0.8–0.6 ka BP	N53°14′04.7″	below 80	39	0.004
			E58°55′50.2″	19 **
Kusimovo-8	Karyshkin	136 ka BP	N53°37′08.0″	25–60	33	0.08
			E58°37′17.0″	16
Sabakty-1a	Surtanda	6.3–5 ka BP	N53°37′13.4″	18–162	32	0.05
			E58°38′43.2″	32
Seitkulovo	Multicultu-		N53°25′32.7″	3–13	44	0.06 ***
	ral *		E58°18′33.0″	5
Tashtui-1	Modern	0.1–0.2 ka BP	N53°19′03.6″
	Age		E 58°52′51.1″	3–19; 30	18	0.13
				53; 1
Telyashevo-4	Alakul	3.9–3.45 ka BP	N53°39′43.6″	11–46	36	0.10
			E58°38′42.0″	14

* Mesolithic, Bronze Age, and Medieval periods; ** animal bones; in other CLs—ceramic fragments and stone tools; *** 40–50 cm.

, Figure 1). The dates of those settlements range from the Paleolithic to the Late Medieval and Modern Ages. The sites are located mainly on the terraces of small rivers in the Ural River basin and are used as pastures with moderate grazing. The altitude above sea level is 400–600 m. The region is in the mountain-steppe, mountain-forest-steppe, and steppe zones of the southern Trans-Urals. The geology of the region is dominated by Devonian rocks, with Quaternary alluvial deposits overlying them in places. In the foothill part, volcanogenic rocks are represented, in particular, diabase rocks. Forests occupy roughly half of the area. Pine and birch-larch forests and virgin mixed grass associations are widespread and mostly plowed. The average regional annual air temperature is 1.4 °C, and the average temperature is +17.6 °C in July and −15.5 °C in January. Lowland areas receive 350–400 mm of precipitation per year; mountainous areas receive 550 mm [32,33]. A large metallurgical plant is located at Magnitogorsk, 18–60 km from the study sites. The CLs developed mainly on Chernozems, Luvic—frequently, Haplic, and, less commonly (Tashtui-1), Mollic Leptosol Eutric—on diabase (Amangildino) and fluvial soil at the Ishkulovo site (Figure 2). In all the CLs studied, pottery fragments and animal bones were the most common finds, while stone tools were found at the Elimbetovo, Kusimovo-8, and Sabakty-1a sites.

Brief Characteristics of the Archeological Cultures of the Sites

All finds from the Elimbetovo-7 site belong to the Gamayun culture, which existed from 2900 to 2400 BP. Their settlements had semi-dugouts and large log houses surrounded by defensive log walls and a ditch. The inhabitants were mainly engaged in hunting but partly in fishing and cattle breeding; they produced ceramics and tools made mainly from stones and less from bones and clay. They smelted copper, which was used to create jewelry [34,35].

Amangildino and Telyashevo-4 are long-term settlements of the Alakul culture, 3900–3450 BP (this chronology is a generalization based on 52 ¹⁴C dates) [36]. The sites of this culture have been found mainly along rivers with some on the watersheds. This population practiced domestic cattle breeding; hoe farming, hunting, fishing, and gathering played a secondary role [37].

The Sabakty-1a site belongs to the Surtanda culture, which existed in the Eneolithic in the southern Trans-Urals. Forty radiocarbon dates for Eneolithic sites in the Trans-Urals fall between 6300 and 5000 cal BP (1σ calibrated ages) or 6500 and 4200 cal BP (2σ calibrated ages) [38]. Unfortified settlements were located near rivers and lakes. The population lived in semi-dugouts and dugouts with fireplaces and walls that were reinforced with stone slabs. Characteristically, there is an abundance of stone tools, decorated ceramic dishes, and isolated items made of native copper. They engaged in hunting, fishing, and cattle breeding [39,40].

3. Methods

The CLs studied begin close to the surface (from 3 to 25 cm) and are affected by modern soil formation processes, except at the Ishkulovo site, where the CL is located below 75–80 cm. At most sites investigated, the CLs end at 40 to 60 cm in depth, although at the Eneolithic site of Sabakty-1a, the CL ends at 162 cm in depth. In the cultural layers of all sites, deep profiles up to 50–165 cm to the parent material were studied and sampled for general soil analyses, and two pits were sampled to a depth of 30 cm for microbiological, Corg, and geochemical analyses. Morphological descriptions of these sections were made. In eight sections, samples were taken from successive layers with a thickness of 0–5 cm down to the parent material. At the Ishkulovo, Kusimovo-8, and Sabakty-1a sites, samples were taken every 10 cm.

Two dates were obtained for the Kusimovo-8 site using optically stimulated luminescence (OSL) in VSEGEI, Saint Petersburg. Soil properties were measured by the following methods: use of a CHNSO elemental analyzer for determining Corg and total nitrogen, potentiometry for measuring pH in aqueous solution (suspension, 1:2.5), the amount of soil exchangeable Ca²⁺ and Mg²⁺ was measured according to Schollenberger, the content of mobile P₂O₅ was evaluated by the photometric method, and the analysis of granulometric composition was carried out by the pipette method with sodium pyrophosphate treatment. Soil salinity was measured as dense residue in an aqueous solution (suspension, 1:2.5) [41]. Laboratory analyses were performed at the Common Use Center of the Institute of Physicochemical and Biological Problems in Soil Science of the RAS.

For microbiological analyses, the soil with field humidity was sifted through a 2 mm sieve, and roots were removed. A 10 g sample in triplicate was moistened to 70–75% of the maximum field moisture capacity and incubated for 7 days at 22 °C in 100 mL vials. The vials were then closed and placed in a thermostat at 22 °C; basal soil respiration (SR) was determined on a gas chromatograph after two days and again after the next two days; Cmic was found in the same samples using the substrate-induced respiration (SIR) method with 10 mg of glucose addition per 1 g of soil. The SR rate was expressed in C-CO₂ µg/g of soil per hour. The formula used was C-mic = 40.04 × V SIR + 0.37 µg C /g of soil (V—rate of SIR) [42].

The soil major and trace elements were determined using a Spectroscan MAX-GV X-ray fluorescence spectrometer (“Spectron” Production Association, St. Petersburg, Russia) at Belgorod University (M.K. Buiya, technician). The concentrations of soil elements were compared with the regional [8] and Clarke values [43].

4. Results

4.1. Morphological Characteristics of the Sites

Amangildino is a long-term settlement of the Late Bronze Age (Alakul culture 3900–3450 BP). The site is located between the slope of the mountain and a swampy forested valley 65 m from the Bolshaya Nayazda River, which today is used as a hay field, but previously was arable land. Artifacts and ecofacts relating to the Alakul culture were found, comprising six fragments of vessels in surface materials, many animal bones, 24 broken pieces of ceramics, and flint flakes at a depth of 3–46 cm, mostly between 3 and 36 cm. Morphological indicators of the sites are given in

Table 2. Morphological indicators of sites.

Horizon, Depth, cm	Color		Munsell Color	Texture	Granulometry	Density
Amangildino
PU 0–25	dark gray		10YR 4/1	lumpy granular		loose
AB 25–60	dark gray		10YR 4/1		medium loam	dense
	and dark brown		10YR 3/3	lumpy
C 60–70	pale brown		10YR 6/3			very dense
Elimbetovo-7
A1 0–40 C 40–50	black gray		10YR 2/1 10YR 6/1	lumpy granular	clay, diabase 5–20 cm, 50% fragmented diabase	loose very dense
Ishkulovo
A1 0–30	gray		10YR 5/1	fine lumpy	sandy loam	loose
A1’ 30–50				lumpy	clay loam	dense
AB 50–80	grayish brown		10YR 5/2	lumpy	clay
80–82	black		10YR 2/1		charcoals	loose
CL 82–120 bur-	grayish brown		10YR 5/2	unstructured
ied	and	blue-gley	5GY 6/4
Cg 120–140	brown	and	10YR 5/3		clay	very dense
	blue-gley		5GY 6/4
Kusimovo-8
A1 0–30	dark gray		10YR 4/1	lumpy granular	sandy loam	loose
AB 30–70	light brownish		10YR 6/2	fine lumpy	gravel of 5–15 cm,	very dense
	gray				50%
Cg 70–180	brown	and olive	10YR 5/3	unstructured	loam
	tinges		5GY 6/4		gravel of 1–2 cm,
					20%
Sabakty-1a
A1 0–50	black dark gray gray dark gray		10YR 2/1	fine lumpy	sandy loam	loose dense very dense dense very dense
A1’ 50–100			10YR 4/1	lumpy	loam
B 100–125			10YR 6/1
[A1] 125–133 D			10YR 4/1		loam
135–180 light gray			10YR 7/1	unstructured	gravel of 2–5 cm, 20%
Seitkulovo
A1 0–30	dark gray light yellowish brown		10YR 4/1	fine lumpy	sandy loam	loose
BC 35–50			10YR 6/4	coarse lumpy	loam, gravel 2–5 cm, 20%	dense
Tashtui-1
A1 0–27	dark gray gray gray-brown pale brown		10YR 4/1	fine lumpy	loam	loose dense very dense
AB 27–70			10YR 5/1	lumpy	clay
BC 70–135			10YR 4/3
C 105–135			10YR 6/3		clay, gravel 1–2 cm, 10%
Telyashevo-4
A1 0–27	very dark gray		10YR 3/1	fine lumpy	loam, gravel 1–2 cm,	loose
AB 27–47	gray		10YR 5/1	lumpy	10%	dense
BC 47–65	pale brown		10YR 6/3	coarse lumpy	clay, gravel 20%	very dense

.

The Elimbetovo-7 site is located 190 m from the Yangelka River, where people of the Gamayun culture lived at 2.6–2.5 ka BP [34,35]. A total of 16 ceramic fragments and 15 stone tools were found at a depth of 11–38 cm. This CL is a thin feature at 40 cm depth, directly underlain by diabase bedrock (Figure 3C).

The Ishkulovo settlement was studied in the eroding bank of the Bolshoy Kizil River. At a depth of about 80 cm, a lens with charcoal marked the upper boundary of the CL. During the initial inspection, it was determined as a nomads’ site of 0.8–0.6 ka BP. A thin charcoal-rich horizon marked the CL (Figure 3B).

Kusimovo-8 is a workshop site that functioned during the Lower Paleolithic c. 136 ka BP. It is located 20 m from Karamala Creek and 1.2 km from Sabakty Lake (Figure 2A,B and Figure 3A). It was arable land until 2005 but is now pasture with many forbs. The settlement area is 175,000 m². During our study of this site, 4 trenches were examined, and 121 stone artifacts were found. They include various stone tools—axes, scrapers, choppers, cleavers, and biface blanks (Figure 3C). Two cultural layers were discovered at a depth of 0.4–0.8 m and 0.8–2 m. Two OSL dates were obtained for the site deposits. A sample from a depth of 1.8 m gave an age of 136 ± 9 ka BP (RGI-1104, VSEGEI, Saint Petersburg), and a sample from 1.0 m gave 258 ± 13 ka BP (RGI-1103, VSEGEI, Saint Petersburg). Obviously, the latter date is overestimated. The forms of the chopping tools recovered are markers of the Lower Paleolithic (125–200 ka BP) of the southern Trans-Urals and were discovered at Mysovaya, Karyshkino-11, and other sites [44]. The water table lies at 170 cm depth; there is sandy loam down to 1 m and then medium-textured loam.

Sabakty-1a. This is a Neolithic workshop site of the Surtandy culture, located near Sabatky Lake in a hollow surrounded by stony slopes and a coastal rampart, which is used as a pasture today. It dates to the period of 6300–5000 ¹⁴C cal BP [38]. In a trench 6 m long, 32 stone tools were found in a layer between 18 and 162 cm, 5 pieces were found between 39 and 45 cm, and 10 pieces were found between 94 and 128 cm (Figure 3D). Some of these tools may have been moved by natural processes from higher locations at the Sabakty-1 and Sabakty-8 sites [39].

Seitkulovo. This settlement is located on the Bolshoy Kizil River terrace. Today, it is grassland that serves as a hayfield. The character of 10 artifacts collected from the surface and 5 found in the 3–13 cm layer indicates that the site was a settlement during three archeological periods: Mesolithic, the Bronze Age, and Late Medieval or Modern times. In addition, eight animal bones were recovered from the surface and five from the 3–13 cm layer.

Tashtui-1. This settlement is located on the sloping summit of a large steppe elevated flatland near a dried-up lake. Twenty-five artifacts were found on the surface and in a layer 3–20 cm below. Two parts of the CL are distinguished: the upper 3–20 cm is marked as a fairly long-term settlement of the second half of the 19th century to the beginning of the 20th century; the lower part relates to a short-term nomads’ camp of the late Sarmatian period (2nd–3rd centuries AD), based on one ceramic find at a depth of 53 cm. Of all the sites considered in this paper, it is the nearest to the industrial center of Magnitogorsk 18 km away. The soil profile was dug in a non-arable area.

Telyashevo-4. This site is a long-term settlement of the Alakul culture (3900–3450 BP), located on former arable land on the low terrace of the Analyk River. Fourteen finds were discovered in a sandy loam cultural layer between an 11 and 46 cm (mostly in 22–38 cm) horizon.

4.2. Physical, Chemical, and Microbiological Properties

Most of the CLs examined have medium to heavy loamy granulometric composition and contain fractions with a diameter of <0.01 mm (38–56%) and sandy fractions of 1–0.25 mm (less than 20%) (Figure 4). At the Kusimovo-8, Sabakty-1a, and Seytkulovo sites, the upper parts of the profiles are sandy loam with fractions of <0.01 mm 14–20%, and the lower parts are medium loamy (32–45%). In the CLs profiles, a pH value close to neutral was mainly recorded; only in the Tashtui-1 settlement was an alkaline reaction (below 30 cm) observed (Figure 5). Soil and CL profiles are not salinized, with the amount of easily soluble salts at less than 0.1% (Table 1). The exchange complex (= cation exchange capacity) is saturated with calcium up to 74–96% of the total absorbed cations.

The Corg content of the 0–20 cm layer, where the majority of the cultural layers start, is 6–10%, with the exception of the Ishkulovo site at 1.3%, since the cultural layer lies below 80 cm. Corg at a depth of 40–50 cm = 0.9–3.8%, and 90–100 cm = 0.4–3.6%. The highest accumulation of Corg was observed in the Sabakty-1a site due to humus transport along the slope as a result of being located in a hollow. In the 0–10 cm layer, total nitrogen varies from 0.45 to 1.1%, and the C/N ratio is 10.3–11.5; in the Ishkulovo site, the N content is 0.24%, and the C/N ratio is 10.9 = 0.12%.

The microbial biomass (Cmic) of 0–10 cm CLs is high at 520–680 µg C/g of soil and microbes cause emission of 0.2–1 C-CO₂ µg/g of soil per hour in the Ishkulovo site—110 µg C/g of soil and 0.08 C-CO₂ µg/g of soil per h, respectively. In the 10–20 cm CLs, Cmic is still high, at 262–480 µg C/g, decreasing significantly at a depth of 40–50 cm—80–160 µg C/g. The Cmic:Corg ratio was estimated and varies from 0.4 to 1.6 in the 0–10 cm zone of the CLs. At Ishkulovo CL begins below 80 cm, and at a depth of 80–90 cm the Cmic:Corg ratio is 0.9. At this site, in a 0–10 cm layer of modern soil, organic carbon is the most enriched in carbon of microbial origin; despite having the lowest content of Corg, Cmic, and Ntot among the CLs, available phosphorus content is high, which probably determines the high Cmic:Corg value. It is possible that the low Corg content is due to overgrazing at the site.

4.3. Geochemical Properties

Amangildino is a long-term settlement of the Late Bronze Age Alakul culture during 3900–3450 BP; cultural layer thickness was 3–46 cm. A concentration of sulfur in the CL was observed in the 0–10 cm layer, which is above the Clarke value (Figure 7a). The biogenic phosphorus content at a depth of 30–45 cm is 0.20–0.36%, and, in the parent material, it is 0.12% (Figure 6). In the same layer, the concentration of zinc was found to be 1.2–2 times greater than the Clarke value and the parent material. Arsenic was found to be present in the 10–20, 20–30, and 45–55 cm layers and the soil-forming material in quantities higher than the Clarke value. The quantities of copper, chromium, and nickel exceeded the Clarke values by 2–3 times throughout the profile (Figure 7b,c).

Elimbetovo-7. This Gamayun culture settlement dates to 2600–2500 BP. In its cultural layer, the highest biogenic phosphorus saturation was observed among all studied sites: 0.24–0.36% at the 10–35 cm depth interval. At the same depth, the content of zinc and sulfur is 1.7–2.1 times greater than the Clarke value. Significant concentrations of copper, cobalt, nickel, chromium, and vanadium were measured in the 5–10 cm layer, and their concentrations were 1.4–2.3 times higher than the Clarke values. Greater concentrations of rubidium and zircon also occur at a 5–10 cm depth compared to other layers, but they are not higher than the Clarke values. High calcium concentrations are found in all layers except 5–10 cm, reflecting the specific chemical composition of diabase.

Ishkulovo settlement. The nomads’ 0.6–0.8 ka BP cultural layer lies below 80 cm depth. There is no anthropogenic phosphorus accumulation anywhere in the CL or the soil profile above it. In the entire profile of the CL and modern soil, increased amounts of copper, nickel, and chromium are observed, which is 2–3 times higher than the Clarke values. In the soil horizons above the CL, the concentration of sulfur is higher than in the deeper layers but does not exceed the Clarke value. Manganese deposition occurs at 0–50 cm depth (where its concentration is 1.4–1.8 times higher than the Clarke value) and in the parent rock material. The CL is not enriched in other elements.

Kusimovo-8. This is a Lower Paleolithic workshop site. Biogenic accumulation of phosphorus was detected at 0–20 cm depth, which, at 0.08–0.13%, is greater than that observed in the deeper layers. Concentrations of zinc and sulfur were found in the 0–20 cm layer at concentrations 1.2 times greater than the Clarke values. Significant concentrations of copper and chromium occurred in the 30–60 and 100–105 cm layers, 1.6–2.3 times higher than in the lower layers and above the Clarke values.

Sabakty-1a is an Eneolithic settlement (6.5–5 ka BP) of the Surtandin culture, with the CL lying at 18–162 cm in depth. Phosphorus distribution is uniform, accounting for 0.2% in the 0–80 cm layer and 0.1% below 100 cm. Sulfur concentration in the 0–30 cm layer is higher than in the lower layers, and it exceeds the Clarke value by 1.7–2.1 times. Deposition of zinc in the 0–30 cm layer is observed at 1.3–1.9 times higher than the Clarke value. An increased amount of copper and manganese is determined over the whole profile; their concentrations are 1.3–1.5 times higher than the Clarke values.

Seitkulovo. In this settlement, the CL has a depth of 0–13 cm. Phosphorus distribution in the 0–35 cm layer is uniform at about 0.2%. Sulfur concentration occurs in the 0–20 cm layer and is greater than in the deeper layers, exceeding the Clarke value by 1.2–1.7 times. Manganese, an element of the third toxicity category, is evenly distributed throughout the profile, and in all the layers, it is 1.4–1.6 times higher than the Clarke value and the parent material. Increased amounts of copper, nickel, and chromium (3–5 times higher than the Clarke values) occur throughout the profile, increasing down the profile.

Tashtui-1. This settlement dates to the period from the 19th to the beginning of the 20th century AD and has a CL between 3 and 19 cm thick. Samples of the 0–25 cm arable layer, where archeological artifacts occurred, were lost. A small accumulation of biogenic phosphorus in the amount of 0.2% was found at a 25–55 cm depth; sulfur was also deposited at this depth; the concentration of 1.3–1.6 times higher than in the parent rock does not exceed the Clarke value. The concentration of arsenic, an element of the first toxicity category, in the 25–65 cm layer was found to be 1.3–3 times higher than the Clarke value. The abundances of chromium, copper, and nickel, elements of the second hazard category, are observed throughout the profile. Their concentrations are 1.5–2 times higher than the Clarke values.

Telyashevo-4 is a long-term settlement of the Alakul culture (3900–3450 cal BP). The CL occurs at an 11–46 cm depth. Accumulation of biogenic phosphorus of 0.2–0.3% was detected. Sulfur is also a biogenic element; the concentration in the 5–35 cm layer is 1.3–1.5 times higher than the Clarke value and 2.7–3 times higher than that in the parent material. Manganese distribution throughout the profile is fairly uniform; only in the 5–10 cm layer is its content significantly higher (1.7 times) than in the parent material, but throughout the profile and in the parent rock, the Mn content is 1.5–2.4 times higher than the Clarke value. Potassium accumulated in small amounts in the 10–30 cm layer compared to the lower layers. Zinc and lead concentrations are higher in the surface layers and decrease down the profile; they are 1.4–2.4 times higher than the Clarke value and 1.7–2.1 times higher than in the parent material. In the CLs, the concentrations of chromium, copper, and nickel gradually increase down the soil profile and reach their maxima in the soil-forming material, but everywhere, they are 1.2–1.5 times higher than the Clarke value.

The technogenic concentration coefficient (Ct) for each element in the 0-10 cm layer relative to the soil-forming rock was calculated using the following equations [17]: Cci = Ci/Cir, where Ci is the element content in the 0–10 cm layer and Ct = 100 × (Cci − 1)/Ci, where Cir is in the soil-forming rock. The technogenic concentration coefficient was considered significant if its value was greater than 20% [17].

Sulfur concentration in the upper part of all CLs was greater than in the soil-forming rock, and coefficients of technogenic accumulation were 30–87%. Consequently, all industrial metals are chemically bound with sulfur as sulfides. Smaller coefficients of technogenic accumulation of 30–40% were noted for Mn, Cr, Zn, and Sr in the Ishkulovo, Kusimovo-8, Seitkulovo, and Telyshevo sites (

Table 3. Coefficients of technogenic accumulation, %.

Site	Km *	S	Mn	Cr	Zn	Sr	Pb
Amangildino	60	35
Elimbetovo-7	24	46
Ishkulovo	39	42	31	54
Kusimovo-8	33	87	31
Sabakty-1a	32	56			32
Seitkulovo	44	75			27		27
Tashtuy-1	18	36		22		37
Telyasheva-4	36	29	39		21

* Distance from the Magnitogorsk Metallurgical plant.

).

Figure 6. Content of Corg, total nitrogen, and phosphorus; C/N ratio of the sites.

Figure 6. Content of Corg, total nitrogen, and phosphorus; C/N ratio of the sites.

Figure 7. (a) Content of some microelements of the sites. (b) Content of some microelements of the sites. (c) Content of potassium and manganese at the sites. Dashed line means the Clarke value, region—regional elements content accordion (8).

Figure 7. (a) Content of some microelements of the sites. (b) Content of some microelements of the sites. (c) Content of potassium and manganese at the sites. Dashed line means the Clarke value, region—regional elements content accordion (8).

5. Discussion

An interdisciplinary study of soils and cultural layers was carried out for eight sites dating from the Paleolithic to Late Medieval and Modern Ages on the eastern slope of the southern Ural Mountains, at the border between Europe and Asia. CLs are formed as a result of human activity at settlements and consist of various anthropogenic, organic, and inorganic materials. The thickness of the CLs varies depending on the duration of human occupation [1,2].

The CLs begin close to the surface (from 3 to 25 cm) and end in the layer from 40 to 60 cm; in the Eneolithic site of Sabakty-1a, the CL ends at 162 cm depth.

pH value. pH values are almost the same (6–7, close to neutral) throughout the vertical column. This is favorable for vegetation and biota. Only at Tashtui-1 are conditions alkaline (pH 8–8.6), below 30 cm, due to the enrichment of carbonates. However, plant roots and microbes are concentrated mainly in the surface layers where the reaction of solutions is not alkaline. A pH value of 6–7 ensures the stability of many heavy metals, which is important at their high concentrations in soils [45,46].

The cultural layers of most sites have medium and heavy loamy granulometric composition and contain fractions with a diameter of <0.01 mm (38–56%) and sandy fractions with a diameter of 0.1–0.25 mm (about 20%). In the Kusimovo, Sabakty, and Seitkulovo sites, the granulometric composition of the upper part of the profile is sandy loam with <0.01 mm fractions (14–24%), while the lower part is medium loamy, and the percentage of the <0.01 mm fractions is 32–45%. This indicates that layers are heterogeneous in their lithological aspect and probably have a high retention of water in the upper part of the profile. The soil profiles and CLs studied are desalinized, with the content of easily soluble salts less than 0.1%.

Organic carbon. In the CL profiles and soils, the amount of Corg is characterized by a well-defined accumulative distribution, highest in the upper part, decreasing slowly down the profile, which is typical for chernozems [32,47]. The CLs studied contain Corg (6–10%) in the 0–20 cm layer. A large amount of Corg in the regional soils was noted previously [48]. In the Ishkulovo CL, Corg decreases by up to 1.3% since 80 cm of modern soil covers the CL. In other words, there was no input of new organic material, and mineralization of the existing organic matter had occurred. The 0–20 cm layer of modern soil at the Ishkulovo site contains a minimal amount of Corg (2.5%), which is due to a severe reduction in vegetation cover because of cattle grazing and decreased biomass.

The total nitrogen content of the CLs studied varies between 0.45% and 1.1% in the 0–10 cm layer, and the C/N ratio is 10.3% to 11.5%; at Ishkulovo, N is 0.12% and C/N is 10.9%.

The biogenic phosphorus content of the CLs studied reaches 0.2–0.4%, compared to 0.1% in the parent material and the Clarke value of 0.07%. Values greater than 0.1% indicate anthropogenic influence in the formation of the cultural layer. There is no accumulation of phosphorus at the Ishkulovo site.

The enrichment of total phosphorus, sulfur, and manganese—biogenic elements—in the top layers compared to the bedrock occurs due to their absorption by roots from the lower horizons into plant leaves and subsequent deposition on the surface. It accounts for 20% of the total element content [17]. Phosphorus can be supplied to CLs as a result of ancient people’s activities: organic household and food wastes, excrement, and mineral input with ashes and bones [49,50].

The significant enrichment of CLs with phosphorus and non-carbonate calcium at settlement sites is related to the input of these elements from animal bones containing stable calcium phosphates–apatite. This is poorly soluble and not carried away in solution. Phosphorus can also enter a CL from wood, food residues, and household waste. The accumulation of phosphorus in a CL indicates that the way of life was settled, and human occupation was lengthy. Phosphorus deposition in the CLs of the settlements is a distinctive feature [51,52,53]. This is because more than 50% of anthropogenic material enriched with organic compounds that contain phosphorus can be brought into long-term CLs, as found in sediments from ancient cities in Crimea (Tanais, Fanagoria) [54].

Potassium increase is not observed in the CLs studied compared with soil-forming material. An elevated level of potassium occurs only in a thin charcoal layer at an 80–82 cm depth in the Ishkulovo site on fluvial deposits. The source of potassium in the CL is wood ash, which accumulates near fireplaces and in ashy layers. In addition to potassium, it has been found that ash deposits typically contain Cu, As, Si, Al, and Pb [55].

Manganese concentration in the CLs was also found to be 1.3–3.1 times higher than the Clarke value, except at the Tashtui-1 site. This can be explained largely by the fact that the region contains much manganese ore. During the Second World War, manganese was mined near the ancient settlements of Elimbetovo, Kusimovo-8, and Sabakty-1a. At the same time, Mn can enter CLs from ceramics, bronze smelting waste, and ash deposits from hearths and stoves. In previous studies, Mn was found in the CL in pottery fragments (600–700 mg/kg), slag from bronze production (200–5000 mg/kg), and plant ashes (700–4800 mg/kg), as well as places where there were fireplaces (up to 6500 mg/kg), especially during the burning of trees (alder, willow). Peculiar behavioral reactions have been noted in people who were constantly near the fireplace and inhaled Mn with smoke [56].

Sulfur accumulation was detected compared to soil-forming material. In some CLs, it exceeded the Clarke value. Sulfur can enter the CLs with the products of human activity as a result of the decay of ceramic fragments and from ore and slags from bronze production [57]. Sulfur and phosphorus are elements of intensive biological accumulation by plants, and Mn is an element of biological capture [58]. They are deposited in the upper horizon of soils after the plants’ death. It has been shown that with climatic drying of steppes, the amount of wormwood and goosefoot increases, and K, Ca, P, and S are deposited in soil, while, during humidification graminoid, species spread and the concentrations of all these elements in the soil increase [59].

The upper part of all CLs contained more sulfur than the soil-forming rock, with the coefficients of technogenic accumulation of 30–87%. Consequently, all industrial metals are chemically bound to sulfur as sulfides. Smaller coefficients of technogenic accumulation of 30–40% were noted for Mn, Cr, Zn, and Sr at the Ishkulovo, Tashtuy-1, and Telyshevo sites.

5.1. Reconstruction of the Holocene Climate Based on the Results of Paleosol Studies

It is possible to reconstruct the climate during the formation of CLs, although they lie close to the surface and are influenced by modern soil processes. The formation of dark-colored soils, then chernozems of Siberia, began in the early Holocene between 11.65 and 8.3 ka BP [21,60,61,62,63]. During this period, there was an increase in humidity, warming, and a decrease in the continentality of climate. This was established on the basis of a comprehensive (palynological, malacological, macrobotanical, and other methods) study of dated paleosols, sediments of lakes, and swamps of the forest steppe of the southern Urals and adjacent regions [19,64,65,66,67,68,69].

In the steppe Trans-Urals in the Arkaim museum reserve (south of the modern Chelyabinsk region), 7 fluvial paleosols of the Holocene Age were studied with 15 radiocarbon dates and palynological analysis. The following was found: in 200–2000 BP, the Little Ice Age, and arid warm “Viking time-period” were noted; in 2000–4200 BP, sharp fluctuations in atmospheric moisture, the predominance of aridity, and increasing continental climate were reflected; in 4200–5500 BP, humid, warm, climax forest areas accounted for 67% of the phytocenosis area; in 5500–8000 BP, arid, warm, forests accounted for 15–20% of the phytocenosis area; and in 8000–9500 BP, Mucky Gley soils reflect sharp fluctuations in climate [62].

In the steppe southern Cis-Urals in the period of 6800–5200 BP, the climate was arid, and chernozems (Calcic) and Kastanozem developed under mixed grass and gramineous steppes with elements of arid flora. The maximum aridity occurred between 5900 and 5400 BP. Later, until 4200 BP, climatic humidification occurred, with the most pronounced humidity between 4600 and 4200 BP. The soils were transformed into Haplic chernozems, formed under typical mixed grass and gramineous true steppes. This was confirmed by pedological and phytolith studies of cultural layers in the Turganik settlement and 20 paleosols from several necropolises [70].

The study of the chronosequences of paleosols in the southern Urals revealed humidification in the Middle Bronze Age (4.0–3.4 ka BP) and a change from an arid climate to a wetter one during the Early Iron Age (3.2–2.5 ka BP) [24].

Extensive palynological spectra were obtained for the south of Western Siberia along with 150 radiocarbon dates. Between 7700 and 5000 BP, the climate was warmer than today, and humidity decreased, except for increased humidity from 6300 to 6100 BP. In the period of 5000–4500 BP, it became colder, and wetness was similar to the present day. In the period of 4500–3200 BP, the climate was warm and arid, with periods of humidification between 3700 and 3450 BP. In the interval of 2600–2500 BP there was a warming and humid phase. During the last 1000 years, the landscapes have been similar to the present [71,72].

In the southern Urals forest steppe, a study of palynological spectra from ten settlements found that in ca. 3500 BP and ca. 2500 BP, the climate was warm and humid [73].

The Sabakty-1a CL was formed in the period of 5.0–6.3 ka BP. By this time, 2–4 thousand years had passed since the chernozem’s development began, and a significant part of their profile and Corg reserves had already been formed. It was previously revealed that the maximum rate of soil formation (2–4 mm/year) occurred in the first decades of their development. After 1250 years, it slowed down, and, after 3750 years, there is a minimal rate [74]. More than four thousand years have passed since the initial soil development of other sites, and the chernozem profile had time to form. The relevance of these results may be summarized as follows. In our study area, between 6300 and 6100 BP, the climate was moist and warm, and between 6100 and 5000 BP, conditions were drier and warmer than today (Sabakty-1a; site), but the proximity to the lake of the same name created a favorable microclimate for people. In the period of 3900–3450 BP (Amangildino, Telyashevo-4), the climate was warm and humid, while between 2700 and 2500 BP (Elimbetovo-7), the climate was cool and moist.

The favorable past climate supported chernozem formation and productive biomes near settlements, fostering the development of domestic livestock farming. Additionally, fertile soils contributed to the prosperity of hoe farming, which was developed by some ancient communities.

5.2. Heavy Metals Concentration in the CLs Since Ancient Times

Arsenic is an element of the first toxicity class; its concentration is higher than the Clarke values and soil-forming material, and it is found in the 10–20, 20–30, and 45–55 cm layers of the Amangildino settlement of the Alakul culture (3900–3450 BP). Arsenic was probably accumulated during the stages of human activity intensification. An increase in arsenic levels in the soil at the Tashtui-1 site was observed (lower the CL) at 25–30 and 45–100 cm depth, its content was 1.3–2 times higher than values of the parent material and Clarke, and, in the 55–60 cm layer, its content (11.3 mg/kg) even slightly exceeds the healthy permissible value. The content of As in soils of southern Western Siberia is 13 mg/kg [45,46], in the mining and ore region of the Ural Mountains, it is 96 mg/kg, and, in the northern forest steppe of the Cis-Ural area, it is 20 mg/kg [8]. These authors believe that the permissible value for arsenic is underestimated; for example, the world’s soils are considered uncontaminated when their content is 50–95 mg/kg [75].

Zinc accumulation among toxic pollutants of the first hazard class was also found compared to values of the soil-forming material and Clarke in the Amangildino and Telyashevo-4 (3900–3450 BP), Sabakty-1a (6300–5000 BP), and the multitemporal site of Seytkulovo. In these CLs, the Zn amount was higher than its range limits in soils of the Trans-Urals mining region (Zn 80–95 mg/kg) [8], but less than 220 mg/kg is a value that is dangerous for human health (Kloke, 1980; see also [45]).

In the 0–10 cm layer of all CLs and soils, anthropogenic contamination with zinc, lead, copper, chromium, manganese, and, less often, arsenic, is possible. This occurs because the objects are located 18–60 km from the Magnitogorsk Metallurgical plant, and the Kusimovo-8, Sabakty-1a;, and Elimbetovo-7 sites are located near manganese deposits.

In some CLs, the content of copper, chromium, and nickel, elements of the second toxicity category increases gradually down the profile and reaches their maximum in the soil-forming material, but everywhere in the profile, their content is 1.2–4 times higher than the Clarke value. Consequently, the reserve of these elements is formed due to the geochemical anomaly of the Ural Mountains region, where large reserves of copper, chromium, and manganese ores exist.

It is worthy of note that beginning with the Bronze Age, the contamination of CLs with many pollutants was found in settlements. Whereas in the Neolithic and Eneolithic CLs, for example, at the Kochegarovo-1 settlement (forest-steppe zone, Kurgan region, the south of Eastern Siberia), these pollutants were not detected, but an accumulation of phosphorus, potassium, calcium, magnesium, manganese, and strontium was observed. They serve as indicators for the reconstruction of the economic activity of the ancient population [49,50,54,76]. Observation of the sources of soil and water contamination with heavy metals and their harmful effects on human health was carried out, and biological and chemical methods of soil and water recovery were proposed [77].

With the slag and ore of bronze smelting production, pollutants of hazard class I and II, such as As, Pb, and Cr, as well as Zn and Mn, enter the CLs. In addition, As, Cr, Pb, and, to a lesser extent, Zn enrich the CL due to the destruction of large amounts of pottery fragments and materials of residential and defense constructions. The composition of ore, slag, and ceramics, as shown by the CL of the Berezovaya Luka settlement during the Bronze Age, points to the sources of these elements [78].

Since the Bronze Age, contamination of the settlements’ CLs with many pollutants was established in different regions of the world mainly due to the bronze smelting. In the southern Urals, around 4000 BP, people began to smelt arsenic bronze in the fortified settlements of Arkaim, Kuysak, Kamenny Ambar, and Ustye [79]. In China, the accumulation of Cu, Ni, Pb, Zn, Cr, and As was revealed in 22 sites created in c. 4000 BP in the Hexi corridor due to intensive metal smelting [80].

In the Neolithic and Chalcolithic CLs of the Kochegarovo-1 settlement (forest-steppe zone, southern Western Siberia), these pollutants were not detected, but an accumulation of P, K, Ca, Ca, Mg, Mn, and Sr was found. They serve as markers for the reconstruction of the economic activity of the ancient population [51,76]. In the multilayer settlement of the Golden Horde and Medieval Bolgar on the Middle Volga, an excess of Cu and Pb of 10–37 times, and Ni and Zn of 2 times compared to the background area, was found due to metal smelting [81]. In the ancient cities of southern Russia, CLs are locally enriched with Zn, Cu, Pb, and As, which is due to paints, products from non-ferrous metals, slags, and ores from metal production. The concentrations of pollutants are greater in the organic CLs of ancient cities of the forest zone than in CLs of the dry steppe [54].

5.3. Modern Industrial Stockpiles near the Metallurgical Plants

Did the accumulation of heavy metals in CLs have an anthropogenic cause?

In the upper part of all CLs, sulfur concentrations were higher than in the soil-forming rock; the coefficients of technogenic accumulation were 30–87%. Consequently, all industrial metals are chemically bound with sulfur as sulfides. Smaller coefficients of technogenic accumulation of 30–40% were noted for Mn, Cr, Zn, and Sr in the Ishkulovo, Tashtuy-1, and Telyshevo sites.

Currently, many metallurgical and thermal power plants have been built in the south Urals mining region. At a distance of less than 5–10 km from them, the accumulation of gross and mobile forms of the ore and related toxicants was established [82]. So, in the 0–10 cm layer of soils near the Magnitogorsk Metallurgical plant, they exceed the permissible amount for human health, reaching for arsenic 3–21 times, zinc 2–4 times, lead 1–5 times, copper 1.5–2 times, and manganese 1.5–2.5 times. Winds with dust and smoke also carry pollutants over long distances [9].

In Sibai city (90 km north of Magnitogorsk), where sulfur dioxide, copper, and zinc concentrates are produced, the ecological state of the 0–30 cm soil layer is acceptable and moderately hazardous, based on the total pollution index for several elements. Strong soil contamination by the mobile forms of copper and zinc was noted [13]. Near the Sibai tailing storage, dust saturated with Cu, Cd, Pb, Sb, and Zn has sharply increased their content in the upper soil horizons.

Also, near Sibay, the concentration of heavy metals in the river water and bottom sediments exceeds two orders of the Clarke value, though they accumulate less in the soil. The absorption of these elements by floodplain and watershed vegetation is comparable to that of the phytocenosis in an unpolluted area [14,16,83].

Similar results were obtained for herbaceous plants from the site contaminated by Zn, Pb, Cu, Co, V, Ni, Mn, and Cr (Trans-Urals, Sverdlovsk region, 90 km west of Magnitogorsk). The accumulation of heavy metals by grasses was close to the unpolluted area [10]. So, phytocenosis serves as a phytobarrier in the circulation and stabilization of the technogenic flow and reduces pollutant input in the food chain (animals, people) [45,46].

It is established that humic acids (which are the main components of soil Corg) can absorb trace elements. Their adsorption by humic acids from the Trans-Urals chernozems decreases in the following series: Cu > Mo > Cr > Zn > Zn >Ni > Pb > Ti > V [84]. Humic acids form strong complexes with Zn, Cu, and Ni [45]. It was shown that in the presence of citric acid and lysine, clay minerals such as Na-bentonite adsorbed more Pb [85].

In other countries, soil pollution also occurs near industrial centers [86,87]. Thus, in China, near mining and industrial areas, the accumulation of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn has been noted at 4–27 times higher than the background level and 12% of sites demonstrated a carcinogenic risk, based on an analysis of 118 articles [88].

5.4. The Reasons for the Concentration of Metals in CLs Beyond the Permissible Limits and Clarke Values

• The geological background is the main reason for the metal concentration in CLs since they are located near the Ural Mountains enriched in copper, chromium, manganese iron, etc. So, they concentrate in CLs, soils, and soil-forming rocks, at levels significantly greater than the Clarke value. The native regional heavy metals accumulation in soils has been documented by many scientists.
• Industrial accumulations of pollutants near the Magnitogorsk Metallurgical plant occur mainly in the upper layers. The distance of 18–60 km of CLs from Magnitogorsk is large for there to be a significant concentration of technogenic elements in CLs. It is worth noting that no influence by a remote, abandoned lead-smelting plant was found in six soil profiles in Brazil, and a dependence on atmospheric lead deposition and its content in parent rocks was not established [89].
• Metals accumulated in CLs due to ancient people’s activities, the presence of metal products, and fragments of ceramics enriched in many elements; upon decay, they accumulated in the soils and CLs.
• All these elements in microquantities are needed by plants, but when they are in large concentrations, they become pollutants entering plants and water. Animals and people use them, and that can cause certain diseases.

6. Conclusions

The factors that contributed to the appearance of ancient settlements in this region were the form of relief and their location near the Ural Mountains, which served as defenses and sources of a vast number of mineral resources, access to water, fertile soils, and the presence of wild animals living in the nearby forests.

From the artifacts found, it is shown that the depths of cultural layers vary from 3 to 25 cm to 40–50 cm; in the Eneolithic site of Sabakty-1a, the CL was recorded as up to 162 cm deep, while at Ishkulovo, it is less at 80 cm deep. Favorable chemical characteristics of the cultural layers that formed in chernozems and fluvial soils (Ishkulovo) were described.

At present, soil processes continue to affect CLs. The structure and properties of CLs are determined by pedogenic, chemical, and geochemical processes and intensive human activity over relatively small areas. In the CLs studied, the pH was mainly close to neutral, and good structure, saturation with exchangeable calcium, and an absence of salinization were found. There is a high enrichment of organic carbon of 6–10% in CLs (0–20 cm layer) except at Ishkulovo, where it is only 1.3%, as the CL is below 80 cm and mineralization occurs in the absence of fresh organic material input. The Corg content of the 40–50 cm layer is 0.9–3.8%, and, at 90–100 cm it is 0.4–3.6%. Animal bones were found in the CLs; their decay and the accumulation of organic products of animal and human activity caused the deposition of biogenic forms of phosphorus and calcium.

The proximity of the archeological sites studied to deposits of copper, chrome, zinc, and manganese in the Ural Mountains and the Magnitogorsk Metallurgical plants has created geochemical anomalies in the regional soils, as described in the references cited. Anthropogenic factors are superimposed on natural ones—the release of pollutants into the CLs from bronze products, ore, ceramic fragments, and raw materials. This explains the concentration of several pollutants of the first (arsenic, chrome, lead, and zinc) and second (cobalt, copper, and nickel) hazard classes in the CLs studied. These elements are often found in quantities higher than the Clarke values, and, in rare cases, their levels are beyond the permissible limits, which can have negative effects on human health.

However, the CLs and chernozems, due to the above-mentioned properties, are characterized by high buffering capacity and cause strong adsorption of heavy metals. Most steppe plants weakly absorb these elements, as noted in the cited publications. The favorable conditions are evidenced by the active microbiota as confirmed by the microbial biomass of 520–680 µg C/g in the 0–10 cm layer; microbes cause the emission of 0.2–1.0 C-CO₂ µg/g soil per hour.

The publications cited in this paper have provided information about the climate during the periods of development of the settlements discussed. It has been shown that from 6300 to 5000 BP (Sabakty-1a;), conditions were generally drier and warmer than today but moist and warm between 6300 and 6100 BP. In the period of 3900–3450 BP (Amangildino, Telyashevo-4), the climate was warm and humid, while between 2700 and 2500 BP (Elimbetovo-7), the climate was cool and moist. Thus, the climate was favorable to human habitation. This, along with good soil properties, contributed to the development of forests and meadow-steppe vegetation and the existence of productive pastures.