Surface sediments of Karelian lakes: their formation peculiarities and chemical composition

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1. Introduction

The lakes of Karelia (63°49’00” N and 33°00’00” E) are young in geological terms. Their age does not exceed 15 thousand years (Subetto, 2009). They were formed at the turn of the Late Pleistocene and Holocene (15-11 thousand years ago) in the process of deglaciation of the territory in the direction from the southeast to the northwest. Since their origin, lakes have developed as freshwater reservoirs with individual characteristics of sediment accumulation, depending on their location on ice-dividing upland, on lake plains, or in zones of denudation-tectonic relief (Paleolimnology..., 2022). Geological and geochemical (Alabyshev, 1932; Биске, 1959; Biske et al., 1971; Perfiliev, 1972; Sinkevich and Ekman, 1995; Lukashov and Demidov, 2001; Demidov, 2003; Demidov and Shelekhova, 2006; Slukovsky and Medvedev, 2015; Slutkovsky and Dauvalter, 2020), paleolimnological (Lac and Lukashov, 1967; Martinson and Davydova, 1976; Davydova, 1985; Devyatova, 1986; Shelekhova, 2006; Shelekhova et al., 2021; Filimonova and Lavrova, 2017; Filimonova, 2014; Subetto et al., 2017; 2020; Lavrova and Filimonova, 2018; Gromig et al., 2019; Hang et al., 2019; Zobkov et al., 2019; Strakhovenko et al., 2020a;b; 2022) and limnological (Semenovich, 1973; Vasilyeva and Polyakov, 1992; Vasilyeva et al., 1999; Belkina, 2017; 2019; Belkina et al., 2018; 2022) studies of the bottom sediments of Karelian lakes have allowed to collect a huge amount of factual material on the limnogenesis of the eastern periphery of the Fennoscandian Crystal Shield in the Lateglacial and Postglacial period. However, little attention has been paid to the issues of studying modern lake sedimentation.

Today, Karelian humid climate has resulted in a unique system of lakes connected by small watercourses to form a single hydrographic network, 96% of which are lakes with an area of less than 1 km². The area of the region occupied by lakes is one of the highest in the world (21%, including lakes Onego and Ladoga) (Lakes..., 2013). The water remains mainly in the liquid phase throughout the year, creating the conditions for many processes of physical, chemical, and biological differentiation of matter at all stages of the lake`s sedimentation.

The aim of the study was to generalize data on the chemical composition of the bottom sediments of Karelian lakes and to identify patterns of lake sediment formation under current conditions.

2. Materials and methods of the research

The paper uses archival materials from studies of the bottom sediments of 139 lakes in Karelia collected at NWPI KarRC of RAS for the period 1965-2020, including the participation of the author. The comparison was performed on 8 indicators of chemical composition: organic carbon (C_org), carbon of humic and fulvic acids (C_ha, C_fa), mineral phosphorus (P_min), ammonium and organic nitrogen (N-_NH4+, N_org), iron, manganese, and ash content (Ash). In the bottom sediments of the model objects (lakes: Yuzhnoye Haugilampi, Vendyurskoye, Korytovo, Polevskoye, Golubaya Lamba, Ladmozero, Shotozero, Padmozero, and Syargozero), the following values were additionally determined: pH and Eh, physical properties: density (d), natural and absolute humidity (Wet_20°С, Wet_105°С), porosity (por), specific gravity (d_s), losses in ignition (LI_550°С), pigment composition of Chl_a,b,c, total phosphorus content (P_tot), daily oxygen demand of the sediment (OD₁) (Belkina et al., 2023).

3. Results and discussion

Currently, Karelian lakes function in a humid climate (marine-continental transition) with relatively warm winters, short cool summers, and unstable spring and autumn weather (Nazarova, 2015). The following climatic features control the formation process of a modern lake. (1) Relatively low temperatures and a significant amount of atmospheric precipitation (600-650 mm per year, 60% of which falls between May and October) ensure the development of surface runoff, which plays an important role in the process of weathering and the transfer of substances of various genesis along the slopes of the terrain to the lakes. (2) Due to the location of the lakes in a temperate climate zone, the surface temperature of the lakes is above 4°C in summer and below 4°C in winter, with significant seasonal variations. As a result of this temperature distribution, the reservoirs are characterized by two complete convective mixing of the water mass throughout the year: in spring and late autumn. The hydrological and thermal regimes of lakes and water dynamics determine the uneven nature of sedimentary matter entering the bottom sediments and changes in its qualitative and quantitative composition depending on the season (Kulik et al., 2023a). (3) The long summer solstice (the longest day lasts about 20 hours) creates a favorable environment in the lakes (low water temperature, sufficient light, and transparency) for the development of phytoplankton, the main supplier of autochthonous organic matter to the bottom sediments. (4) Steady winds from the northwest during most of the year and from the west, south, and southwest during cold periods contribute to the penetration of aeolian dust into the bottom sediments.

Modern sedimentogenesis in the lakes of the basin is mainly determined by geological factors. The geomorphologic features of the territory (relief, horizontal and vertical dissection of the surface, thickness of the covering rocks, lithologic composition of Quaternary deposits) determine the conditions of water runoff and sediment transport to the lakes. Tectonic structures, Archean and Proterozoic rocks of the crystalline Fennoscandian Shield, loose Quaternary sediments, and Upper Devonian and Lower Carboniferous rocks of the Russian platform, which differ in structure and time of formation, determine the composition of the mineral part of the bottom sediments. Thus, the predominance of silicate rocks in the basin ensures the accumulation of silicon in the bottom sediments, which enter as part of a terrigenous suspension, and also creates conditions for the presence of silicic acid in the lake water, stimulating the development of a diatom complex of phytoplankton with the subsequent accumulation of biogenic silicon in the bottom sediments. The relatively low phosphorus content in rocks determines the limitations of aquatic ecosystems of phosphorus and consequently affects the quantitative and qualitative composition of organic matter in the bottom sediments.

Podzolic soils play an important role in the immobilization and transfer of metals to lakes. Thus, the high content of Fe and Mn in rocks capable of complexation reactions with soluble humic organic matter formed in the soil is the reason for their active migration into natural waters and accumulation in the bottom sediments (Belkina et al., 2018).

The influence of the anthropogenic factor on the accumulation of lakes is mainly manifested in reservoirs with direct anthropogenic impact (domestic and industrial wastewater discharge, surface runoff from residential and agricultural areas, fish farming, aeolian inputs from open-pit mining, shipping, building of hydraulic structures, etc.). The effects of forestry on sedimentogenesis are indirect. A change in the water balance of the lake’s catchment area as a result of deforestation leads to a change in the hydrological regime. This, of course, affects the inflow of water into the lake and, as a result, the processes in the lake itself.

All of the above factors determine the functioning of the hydrographic network. The location of the territory in the zone of excessive moisture, where precipitation prevails over evaporation, determines the presence of surface and underground runoff. The hydrological regime of this system is greatly determined by tectonics and topography. A significant part of the substances involved in the deposition process in lakes are transported from the catchment area by water in dissolved form. Drainage from the catchment area of surface and underground river runoff is significantly influenced by different depths of bedrock under the thickness of Quaternary sediments, extremely fragmented relief and the proximity of watersheds to erosion bases. Surface runoff is not typical only for reservoirs with an area of less than 1 km².

The predominant type of groundwater in the region is calcium bicarbonate, which is formed independently of the composition of rocks in the zone of active water exchange. This is due to the fact that leaching of rocks under conditions of saturation with carbon dioxide (the source of which is the soil) already at the first stage leads to the formation of solutions of the siliceous-calcium type, which are characterized by a nonequilibrium state with primary aluminosilicates and carbonates, but they are already saturated with respect not only to kaolinite, but also to other secondary aluminosilicates (montmorillonite, illite, pyrophyllite). The conditions of short-term residence of water in rocks and short distances from the aquifer feeding area to groundwater discharge zone provide general geochemical features of the surface waters of the region (Borodulina, 2011). Low-mineralized calcium-siliceous groundwater enters the lakes and watercourses of the basin, which are the product of leaching of primary aluminosilicates enriched with organic substances and carbon dioxide washed out of the soil.

The main features of surface waters formed in Karelia are low mineral content, high color, and noticeable iron content. The waters of the region are ultra-fresh (the average value for Karelia is 31 mg/l). Most of the studied lakes have a mineralization of less than 50 mg/l, a hardness of 0.2-0.4 mg-eq./l. Ca²⁺ prevails among cations, Mg²⁺ is rare, and K⁺ is even rarer. Alkaline earth metals prevail over alkaline ones. Among the anions, the lowest content was noted for Cl^- (1.7 mg/l) and SO₄^2- (3.5 mg/l), especially in humified waters, where their concentrations are lower than in atmospheric precipitation. Bicarbonates predominate in the anionic composition. The alkalinity ranges from 0 to 276 mg/l, but for most reservoirs and watercourses, its value is less than 30 mg HCO_3-/l. The concentrations of organic acid anions range from 0.01 to 0.4 mmol/l. As a rule, lake water is characterized as medium–humus (color is 35-80 degrees, COD_KMnO4 is 8-15 mgO/l), while river water is characterized as high-humus. The CO₂ content is usually 2 times higher in rivers than in lakes and ranges from 0 to 46 mg/l. In the water column, its concentration increases with depth, unlike oxygen, whose concentration decreases with depth. The pH value varies greatly (from 4.07 to 8.34) depending on the content of HCO₃^-, CO₂, organic acids, and their salts, and generally rises with increasing alkalinity of the water. Most lakes have slightly acidic (5.5-6.5) and neutral (6.5-7.5) waters. Rivers with heavily swampy areas and small reservoirs with atmospheric feeding have the lowest pH. Concentrations of iron, phosphorus and manganese vary widely (from 0 to 4.6 mg/l Fe, from 6 to 26 micrograms/l P and from 0 to 2.1 mg/l Mn). Their content depends on the alkalinity and the presence of organic matter of a humic nature (The current..., 1998; Lozovik et al., 2020). As mentioned above, humic acids of soils contribute to the transfer of metals to surface waters. Organic acids enhance the leaching of carbonates and phosphates of Ca and Mg, which, in turn, leads to higher concentrations of phosphorus, and carbon dioxide in surface waters with a high humus content compared with surface waters with a low humus content. Part of the phosphorus can bind to soluble forms of iron (organometallic complexes) or be sorbed on iron hydroxo compounds migrating as part of suspended solids, which leads to the entry of phosphorus into bottom sediments not only as part of detritus but also as part of iron humus suspension.

The content of other chemical elements in surface waters, including silicon (in rivers, its concentration ranges from 1.2 to 4.9 mg/l, in lakes, it is from 0.2 to 2.6 mg/l), is quite stable and does not strongly depend on the type of water and season. The content of suspended solids in river waters depends on the season and ranges from 0.2 to 13 mg/l. The waters of the vast majority of rivers are of the alkaline type with a high humus content. Most lakes have alkaline waters, with an average humus content; small reservoirs have slightly alkaline waters, with a high humus content (Lakes..., 2013).

Geomorphological differences in the catchment areas of the lakes in the basin largely determine the diversity and uniqueness of the sedimentation regimes of the small lakes. The uneven distribution of river flow into large reservoirs, combined with the complex morphology of the basins and the irregularity of the coastline, leads to the existence of local basins (limnic areas) with different sedimentation patterns (Belkina, 2021). A significant part of the mineral component in the bottom sediments of the lake is formed due to suspended matter containing detrital rock material of the catchment area. In conditions of humic, low-mineralized surface waters, poorly soluble compounds of silicon, iron and manganese are formed and deposited directly in the reservoir itself, and as a result, phosphorus or metals are co-deposited (as a result of sorption processes on hydroxo-compounds) (Kulik et al., 2023b). Insoluble humates deposited in the bottom sediments are formed in the water mass during the biochemical oxidation of dissolved organic matter.

Biological communities in lake ecosystems are suppliers of organic matter to the bottom sediments. The main factors affecting ecosystem productivity are water temperature, its salt composition, and the presence of biogenic elements (Konstantinov, 1986). The growth of aquatic organisms is limited by a short vegetation period and a low water temperature. The flora and fauna of the lakes of the region are based on cold-loving representatives: diatom plankton, deep-sea relict crustaceans, coregonine, and salmonid fishes in ichthyocenoses. The dominance of the most taxonomically diverse diatoms, green, blue-green, and golden algae in the algoflora of lakes (93.5% of the total list) is a zonal feature of the northwestern territories. Quantitative indicators of phytoplankton development (abundance and biomass) change significantly during the growing season. In spring and autumn, diatoms are quantitatively predominant in the lakes, and mixed plankton develops in the summer. Phytoplankton biomass increases significantly with an increase in the trophicity of lakes. The average annual production of phytoplankton is from 11 g C/(m²/year) (Succozero, oligotrophic) to 160 g C/(m²/year) (Svyatozero, eutrophic), and in most of the lakes it does not exceed 50 g C/(m²/year) (average of 45, median 38) (Chekryzheva, 2011).

The structure of the zooplankton community also depends on the trophic status of the reservoir and varies depending on its thermal and dynamic modes. In the early spring period (early June), infusoria occupy a dominant position in the community. As the water warms up and the food conditions change, rotifers become the main complex. In the summer, cladocerans dominate. In autumn, the role of rotifers increases again. Winter zooplankton is mainly represented by copepods and rotifers. The biomass of zooplankton ranges from 0.18 to 27 g/m³, and the number ranges from 1 thousand to 5 million individuals/m³ (Lakes..., 2013).

The modern fauna of the seabed is very diverse in taxonomic terms and, according to recent data, has more than 1,000 species and forms of invertebrates. The dominant complex of benthic cenoses is formed by three systematic groups: chironomids, oligochaetes, and mollusks. The lakes of South Karelia are more productive: the average benthic biomass of the lakes of the Shuya River basin is 4.36 g/m², the Vodla River basin is 2.26 g/m², and the lakes of the Zaonezhye Peninsula are 3.92 g/m² (Lakes..., 2013).

Macrophytes make a significant contribution to the formation of organic matter in the bottom sediments only in small, shallow eutrophic reservoirs with a developed littoral zone. Lakes of tectonic and glacial-tectonic origin, whose littoral is characterized by stony-boulder, rocky, stony-sandy, or sandy bottom, are unfavorable for the growth of aquatic plants. The increased content of humic substances in the water also suppresses their development. The value of annual production varies from 0.5 to 6 g С/(m²/year¹) and usually does not exceed 1g С/(m²/year¹) (Lakes..., 2013). The values of biomass and algae abundance decrease as the pH level declines (Komulainen et al., 2006).

Low water mineralization is important for regulating the water-salt balance of aquatic organisms (Konstantinov, 1986). It affects both the number of species and the biomass of phytoplankton, as well as the presence of marine glacial relics in deep lakes. The most sensitive to salt deficiency are mollusks, whose shells become thin and their sizes become smaller (Kalinkina et al., 2013). For aquatic invertebrates, the low hydrogen index also acts as a toxic factor that violates the integrity of cell membranes (Kalinkina et al., 2017). The high color of the waters affects the structure and vertical distribution of microalgae. In meso– and polyhumous reservoirs, the photic layer narrows due to the weakening of the penetration of photosynthetically active radiation into the water column, which reduces the productivity of plankton. Accordingly, an insufficient food supply provides low natural fish productivity in lakes – 10 kg/ha (Lakes..., 2013).

Part of the phosphorus entering reservoirs, which is necessary for the energy metabolism of organisms, is bound in complex with humus and iron and, therefore, is in a form that is of low accessibility to aquatic communities. The gradual transformation of humic substances as a result of their photooxidation and the activity of heterotrophic microflora (which, with a lack of easily mineralized organic substances, uses them as a substrate) requires additional time, which is also a deterrent to production processes. The amount of bacterioplankton is a fairly stable indicator and ranges from 1.5-2 million/ml increasing in contaminated areas (Gashkina et al., 2012). The amount and biomass of bacteria during the growing season vary by 1.5-2.5 times depending on the trophicity. For most reservoirs, there are two peaks in the seasonal development of bacterioplankton: spring and summer. The bulk of bacteria in the water column is in the form of single cells, whose vital activity is based on complex organic substrates (actinomycetes, oligotrophic bacteria). In reservoirs exposed to anthropogenic influence, groups of bacteria reflecting one or another type of contamination (nitrifying, cellulose-destroying, oil-oxidizing, phenol-oxidizing, coliform) have a noticeable development. The biomass of bacteria in its raw form in the summer reaches values of 0.1-0.5 mg/l, expressed in carbon: 10-54 mkg C/l, in contaminated reservoirs, its values increase 1.5-2 times. The dark assimilation of carbon dioxide as an indicator of the biosynthetic activity of bacterioplankton in the summer in oligotrophic reservoirs does not exceed 0.5, in mesotrophic reservoirs it has limits of 0.8-3.1, in eutrophic reservoirs it reaches 7.8 mkg C/(l/day) and above (Lakes..., 2013).

Thus, temperature, salt composition, humus content, and alkalinity of surface waters are key environmental factors limiting the development of living organisms in the lakes of the eastern margin of the Fennoscandian Shield. Phytoplankton is the main source of autochthonous organic matter in the bottom sediments. Apparently, the low productivity of lakes is the reason that the bulk of easily oxidized organic matter is actively consumed and mineralized in water, and difficult-to-oxidize, little biodegradable organic matter accumulates in the bottom sediments. The quantitative and qualitative composition of organic matter in lake water, which depends on the production of phytoplankton and the mass of humus coming from the catchment area, determines the rate of accumulation and intensity of the transformation processes of organic matter in the bottom sediments. The significant contribution of higher aquatic vegetation to the organic matter of the bottom sediments is typical for small, well-heated, productive lakes with a developed littoral zone.

Allochthonous organic matter enters lakes with the river runoff in the form of dissolved humic substances formed in the soils of the catchment area, and in the form of leaf litter of terrestrial vegetation of the shores.

The intensity of the soil formation process, which depends on the chemical and granular composition of the parent rocks, controls the flow of amorphous silicon, humic substances, and chemical elements into surface waters prone to the formation of complex compounds and colloidal systems with silicon and humic acids, which affects both the chemical composition of the water and the biological characteristics of lakes and, ultimately, the composition of bottom sediments. The role of soil cover in the formation of bottom sediments increases with the growth of the lake’s catchment area. It should also be noted that since 1989 in Russia there has been a steady excess of the norm of the average annual air temperature (Gruza and Rankova, 2012). An increase in the duration of the vegetation period and an increase in precipitation inevitably lead to an increase in the intake of allochthonous organic matter into the lake from the catchment area, an increase in the production of the lakes themselves, and, as a result, an increase in the intake of organic matter into the sediments.

Analysis of the chemical composition of the surface sediments of Karelian lakes has shown that lakes with different types of accumulation (accumulators of mineral and organic matter by different genesis) are found in the region (Table 1). Differences in the hydrological and morphometric characteristics of lakes, various areas, and the composition of catchment rocks, as well as the different trophic status of reservoirs, determine differences in the chemical composition of sediments. Most lakes in the region are characterized by an uneven distribution of sedimentary material at the bottom of the reservoir, which is naturally controlled by the morphology of the basin and the dynamics of the waters: sandy-gravel bottom sediments form the littoral zone, and clay silts prevail in deep-water zones.

Table 1. – Generalized chemical composition of the surface layer (0-5 cm) of the bottom sediments according to the data of 139 small Karelian lakes, % (Belkina, 2021).

Note: «*» – By the prevailing granulometric fraction (sand 0.05-2 mm; silt 0.005-0.05 mm, clay < 0.005).

The variety of sedimentological trends in the lakes of Karelia is associated with the local landscape conditions of the catchments. Its most significant characteristics are equally important: the area and relief of the catchment, the morphology of the lake basin, and the chemical composition of the water (Belkina, 2021). It is difficult to draw a clear conclusion about the predominance of a certain type of lake accumulation, depending on the position of the reservoir in the relief. The chemical composition of precipitation in accumulation zones shows that in large lakes (S_mirror > 10 km²), the mineral type prevails, and in small lakes (S_mirror < 1 km²), the organic type of accumulation prevails. Both mineral and organic sediments are found in lakes with S_mirror of 1 to 10 km² (Fig. 1).

Fig.1. Distribution of lakes with different types of sediments: green is organomineral (ash content<80%), blue is mineral (ash content >80%); I is iron-silicon-humus, II is iron-humus-silicon, III is humus-iron-silicon) depending on the location (H is altitude above sea level, m) and size (S is lake mirror area, km²)

As a rule, the content of organic matter in the bottom sediments increases in proportion to the trophic level of the reservoir, from oligotrophic to eutrophic (Belkina, 2021). Organo-accumulating lakes are found in all modern landscapes of the region (terminal moraine uplands, ice-dividing accumulative uplands, lake plains). According to the macrocomposition, the deposits of such lakes are characterized as iron-silicon-humus. One illustrative example is Yuzhnoye Haugilampi Lake (West Karelian upland, 63°33 N, 33°20 E), an eutrophic, shallow lake (average depth 4.1 m) with a well-developed coastal zone. The altitude above sea-level is 153 m. S_catchment is 0.329 km², S_mirror is 0.276 km²). It has been functioning as an independent reservoir for about 12,000 years. The surface deposits are represented by brown silts. The mean values of the chemical composition are as follows: LI_550°С is 60%, С_org is 26%, Ash is 38%, OD₁ is 4 mgO₂/(g/day), ∑Chl_a,b,c is 1000 mkg/g, pheophytin is 1200 mkg/g, N_org is 1.9%, N_NH4+ is 0.02%, Р_tot is 0.3%, Р_min is 0.2%, Mn is 0.04%, and Fe is 2.5%. The redox cycle of iron and manganese controls the decomposition of organic substances in the bottom sediments. The distribution of biogenic elements (N_org, N_NH4+, Р_tot, Р_min, Fe, and Mn) along the vertical column is nonmonotonic (Fig. 2). The chemical composition of the water in Yuzhnoye Haugilampi Lake corresponds to the mesohumic medium-alkaline neutral weakly alkaline bicarbonate type of the waters of the calcium group. The mineralization of the water in the lake is high (90 mg/l). In the ionic composition, bicarbonates predominate among the anions (95%), calcium (54%) among the cations, alkalinity is 71.12 mgHCO₃^-/l, pH is 7.1, color is 25 deg. The ratio of PO is 2.96 mgO/l and COD is 12.2 mgO/l and indicates an autochthonous origin of organic matter (Lakes..., 2013).

Fig.2. Vertical distribution of chemical, physical and physico-chemical characteristics in the surface layer of the bottom sediments of Lake Yuzhnoye Haugilampi. 1– Eh, mV; 2 – pH; 3 – solid matter mass in 1 ml of wet soil, g/ml; 4 – Wet_20°С, %; 5 – Wet_105°С, %; 6 – por; 7 – ds, g/cm³; 8 – С_org, %; 9 – LI_550°С, %; 10 – OD₁, mgO₂/g; 11 – N_NH4+, %; 12 – N_org, %; 13 – Fe, %; 14 – Mn, %; 15 – Ash, %; 16 – P_min, %; 17 – P_tot.

We should note that organic matter of various origins accumulates in small lakes, regardless of the features of the landscape, the trophic status of the reservoir, and the chemical composition of the waters. For example, a high content of organic matter is observed in the eutrophic lake Korytovo (S_catchment is 0.1 km², S_mirror is 0.003 km², LI_550°С is 86%, Ash is 11%) and in mesotrophic lake Polevskoye (S_catchment is 31.8 km², S_mirror is 0.45 km², LI_550°С is 66%, ash content is 33%), which was formed 12-11 thousand years ago within the lake-glacial plain, and in oligotrophic lake Golubaya Lamba (S_catchment is 0.21 km², S_mirror is 0.04 km², LI_550°С is 89%, and Ash is 9%), which was formed on the Vokhtozersk upland with an area of 7.9 thousand years ago. Sedimentation rates in lakes Korytovo, Polevskaya, and Golubaya Lamba differ by more than an order of magnitude (10, 1, and 0.1mm/yr, respectively). The sources of organic matter in the bottom sediments are also different. In Lake Polevskoye, the main source is humic substances coming from the catchment area: in Lake Korytovo, there is higher aquatic vegetation, and in lake Golubaya Lamba, there is terrestrial and higher aquatic vegetation and phytoplankton. These small lakes have different chemical compositions of water (mesohumus, bicarbonate class of the calcium-magnesium group with ions ∑_ion is 60 mg/l in Lake Polevskoye; carboxylate class of potassium group waters with ∑_ion is 30 mg/l in Lake Korytovo; oligohumic sulfate class of the calcium group with ∑_ion is 3 mg/l in Golubaya Lamba) (Lakes..., 2013).

In lakes where mineral deposits are formed, the most common type of accumulation is iron-humus-silicon. A typical representative of such a reservoir is mesotrophic lake Vendyurskoye (Vokhtozerskaya upland, 62°13 N, 33°16 E, S_catchment is 79.8 km², S_mirror is 10.1 km², altitude above sea level is 143.8 m, average depth is 6.1 m). A reservoir of accumulative-residual genesis, the total capacity of Quaternary deposits is 3.50 m. Current bottom sediments are formed in the conditions of oligohumus waters of the bicarbonate class of the Ca group and are represented by gray-brown silt (LI_550°С is 29%, Ash is 68%, and Fe is 7.8%). The range of fluctuations of physico-chemical parameters along the vertical surface layer (up to 40 cm) is one pH unit (from 4.3 to 5.6) and 600 mV Eh (from 25 to +600 mV). The variability of the Eh values, in our opinion, is determined by seasonal oxygen deficiency in the bottom waters, which causes diagenetic restructuring of the surface layer due to the development of anaerobic processes of organic matter transformation. As a result, interlayers with different metal content and quantitative and qualitative composition of organic matter and, consequently, with different microflora processing, this organic matter is formed and buried. The LI_550°С value changes slowly and monotonously down the column, which indicates a significant transformation of organic matter in the water column of the reservoir before it reaches the bottom. OD₁ values are low (1-1.8 mgO₂/ (g/day). The distribution of phosphorus along the vertical axis of the sediments correlates with the distribution of iron and manganese, with values ranging from 0.06 to 0.3% (Fig. 3).

Fig.3. – Vertical distribution of chemical, physical and physico-chemical characteristics in the surface layer of bottom sediments of Lake Vendyurskoye. 1– Eh, mV; 2 – pH; 3 – solid matter mass in 1 ml of wet soil, g/ml; 4 – Wet_20°С, %; 5 – Wet_105°С, %; 6 – por; 7 – ds, g/cm³; 8 – С_org, %; 9 – LI_550°С, %; 10 – OD₁, mgO₂/g; 11 – N_NH4+, %; 12 – N_org, %; 13 – Fe, %; 14 – Mn, %; 15 – Ash, %; 16 – P_min, %; 17 – P_tot.

Monotypic (siliceous or ferruginous) deposits are rarely found in the lakes of South Karelia compared to the northern part. Humus-silicon sediments were found in lakes on the territory of the Zaonezhye Peninsula (Lakes Nizhnee Myagrozero and Syargozero), in small reservoirs of the Shuisky lowland (Lake Lindozero), and in the northern part of the Onega Lake catchment (Lake Munozero, Lobskoye settlement area) (Demidov and Shelekhova, 2006). For example, diatom sediments of the mesotrophic lake Sargozero (S_catchment is 17.4 km², S_mirror is 0.65 km²) have a light green color and are characterized by high porosity values (0.94) throughout the thickness, low specific gravity (1.1 g/cm3), and very low iron content (0.5%). The ratio of organic matter (С_org is 21%, LI_550°С is 45%, N_org is 1.35 % and P_tot is 0.05 %, С:N is 18, C:P is 1029) and the mineral part of the sediment (ash is 51%) is close to unity. The distribution of pH, Eh, and elements of chemical composition along the vertical of the sediment in the surface layer is monotonous.

Humus-iron-silicon bottom sediments, as a rule, are formed in an oxidizing environment in reservoirs with a developed littoral zone and a large catchment area (significantly swamped, where illuvial-humus-ferruginous podzols are common). Deposits of this type are formed in oligotrophic lakes with low color of water and deeply embedded basins (Lake Ladmozero, S_catchment is 120 km², S_mirror is 24 km², H_max is 52 m, water class is bicarbonate) and in shallow, high-flowing reservoirs with high color of water (Lake Shotozero, S_catchment is 5540 km², S_mirror is 74 km², H_max is 10 m, carboxylate water class is carboxylate). A feature of such reservoirs is the accumulation of lacustrine iron ores in the form of crusts, nodules, oolites, and coins in the littoral zone (at a depth of 1 to 5 m) (Perfiliev, 1972). These biogeochemogenic sediments, containing up to 40% Fe and 2% Mn, are mineral mixtures of a non-crystalline structure (Strakhovenko et al., 2020a; Ovdina et al., 2018; Belkina et al., 2018). The sediments consist mainly of iron hydroxides (goethite, lepidocrocite), manganese oxides, and also contain a small amount of clay minerals, quartz, and rarely carbonates. They are formed in the presence of oxygen during the deposition of suspended and colloidal substances containing excess iron. Bacteria contribute to the deposition of colloids. The thickness of ore deposits lying in the coastal zone of lakes up to 300 m wide ranges from 1 cm to 1 m. At the same time, the iron content in silt sediments lying in deep-sea zones is usually lower than the Clarke value. The accumulation of iron in silts (up to 40%) is also typical for small, shallow forest lakes with a swamped catchment area. In Karelia, until the 19th century, humans employed the use of lacustrine iron ores for iron mining (Kuleshevich et al., 2010).

Current carbonate bottom sediments are rare in the territory of Karelia. For example, in Lake Padmozero (S_mirror is 10 km², S_catchment is 78 m², H_max is 15 m, tectonic-glacial basin, Onego ice Lake relict), located in the eastern part of the Zaonezhye Peninsula. The reservoir accumulation zone contains light, cream (beige) silts formed by clastic weathering products of carbonate rocks common in the catchment area. The content of organic matter, nitrogen, and phosphorus is relatively low (LI_550°С is 13.9%, С_org is 7.1%, OD₁ is 0.32 mgO₂/ (g/day), N_tot is 0.80%, Р_tot is 0.13%, С:N is 10, Chlа is 0.5 mg/g). The physico-chemical conditions and chemical composition of the lake water differ from other reservoirs of the peninsula in gas composition and higher pH and mineralization values (CO₂ varies from 0.8 to 20 mg/l, pH is 8, HCO₃^- is 89 mg/l, ∑_ion is 150 mg/l (Lakes..., 2013)), however, do not imply the formation of chemogenic calcium and iron carbonates in the reservoir. Carbonate sediments can be deposited in the bottom sediments as a result of subaqual discharge of groundwater, as is observed in Lake Rahoylampi (Vokhtozersk upland).

It is important to note that the chemical composition of the bottom sediments of small forest lakes with small catchment areas has changed little over the past 100 years, as evidenced by the profiles of chemical characteristics in the columns of the bottom sediments that do not change vertically, as well as the results of periodic observations, for example, on the lakes of the Zaonezhye Peninsula, studies on which have been conducted since 1929 (Belkina and Kulik, 2019). Lakes with settlements and agricultural lands in their catchment area are characterized by a higher share of the terrigenous component in the bottom sediments as well as the presence of toxic substances. For example, in the bottom sediments of Lake Suoyarvi, the content of oil products in the urban area (0.55%) exceeds background values by 2 orders of magnitude or there are high concentrations of heavy metals in the reservoirs of the town of Petrozavodsk (Chetyrehverstnoe, Sulazhgorskaya lamba), etc. (The current..., 1998; Slukovsky and Medvedev, 2015). Pronounced accumulators of sediments are reservoirs located within residential areas or reservoirs with a high coefficient of water exchange that are part of lake-river systems (for example, Lake Logmozero). Most of the territory of Karelia is occupied by forests: therefore, no significant anthropogenic anomalies were found in the bottom sediments of the lakes.

4. Conclusion

Climatic conditions and the composition of rocks of the Fennoscandian Crystal Shield determine the chemical characteristics of the bottom sediments common to all lakes in the region (macro composition): silicon, humus (organic matter), and iron make up the bulk of the substance of the current bottom sediments, and their ratio determines the type of lake accumulation.

In the lakes of the southeastern part of the Fennoscandian Crystal Shield, the bottom sediments of a mixed type are currently being formed: iron-humus-silicon, iron-silicon-humus, or humus-iron-silicon. There are small lakes where the bottom sediments mainly accumulate either silicon (diatomites), iron (lake ores), or organic matter.

The general patterns of the sedimentary process in the region are: (1) geological and geomorphological conditions and the area of the catchment determine the entry of the mineral component of bottom sediments into the lake; (2) deposition of sediments occurs mainly in conditions of bicarbonate-calcium waters; (3) morphogenetic characteristics of lake basins determine the accumulation of organic matter in bottom sediments; (4) the entry of iron into the sediments determines the direction of the processes of early diagenesis in the sediment itself.

Acknowledgements

The study was carried out within the State assignment No. FSZN-2021-0006 of Northern Water Problems Institute of KarSC RAS.

Conflict of interest

The authors declare no conflict of interest.

About the authors

N. А. Belkina

Author for correspondence.
Email: bel110863@mail.ru
Russian Federation, Aleksander Nevsky Str., 50, Petrozavodsk, 185030

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2. Fig.1. Distribution of lakes with different types of sediments: green is organomineral (ash content<80%), blue is mineral (ash content >80%); I is iron-silicon-humus, II is iron-humus-silicon, III is humus-iron-silicon) depending on the location (H is altitude above sea level, m) and size (S is lake mirror area, km2)

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3. Fig.2. Vertical distribution of chemical, physical and physico-chemical characteristics in the surface layer of the bottom sediments of Lake Yuzhnoye Haugilampi. 1– Eh, mV; 2 – pH; 3 – solid matter mass in 1 ml of wet soil, g/ml; 4 – Wet20°С, %; 5 – Wet105°С, %; 6 – por; 7 – ds, g/cm3; 8 – Сorg, %; 9 – LI550°С, %; 10 – OD1, mgO2/g; 11 – NNH4+, %; 12 – Norg, %; 13 – Fe, %; 14 – Mn, %; 15 – Ash, %; 16 – Pmin, %; 17 – Ptot.

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4. Fig.3. – Vertical distribution of chemical, physical and physico-chemical characteristics in the surface layer of bottom sediments of Lake Vendyurskoye. 1– Eh, mV; 2 – pH; 3 – solid matter mass in 1 ml of wet soil, g/ml; 4 – Wet20°С, %; 5 – Wet105°С, %; 6 – por; 7 – ds, g/cm3; 8 – Сorg, %; 9 – LI550°С, %; 10 – OD1, mgO2/g; 11 – NNH4+, %; 12 – Norg, %; 13 – Fe, %; 14 – Mn, %; 15 – Ash, %; 16 – Pmin, %; 17 – Ptot.

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