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ISSN 0016-7029, Geochemistry International, 2020, Vol. 58, No. 4, pp. 408–422. © Pleiades Publishing, Ltd., 2020. Russian Text © The Author(s), 2020, published in Geokhimiya, 2020, Vol. 65, No. 4, pp. 362–378. Bottom Sediments of the West Siberian Arctic Lakes as Indicators of Environmental Changes Yu. G. Tatsiia, *, T. I. Moiseenkoa, **, L. V. Razumovskiib, ***, A. P. Borisova, V. Yu. Khoroshavinc, and D. Yu. Baranova a Vernadsky Institute of Geochemistry and Analytical Chemistry (GEOKHI), Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991 Russia bInstitute of Water Problems, Russian Academy of Sciences, ul. Gubkina 3, Moscow, 119333 Russia cTyumen State University, Earth Science Institute, ul. Volodarskogo 6, Tyumen, 625003 Russia *e-mail: [email protected] **e-mail: [email protected] ***e-mail: [email protected] Received January 31, 2019; revised May 22, 2019; accepted May 22, 2019 Abstract—Bottom sediment cores from the Langtibeito (Yamal Peninsula) and Gol’tsovoe (Gydan Peninsula) background arctic lakes are studied. The sedimentation rate estimated from 210Pb and 137Cs accounted for 0.17 and 0.20 cm/y, respectively. The element composition, grain-size composition, and loss on ignition were determined layer-by-layer in both cores. The element concentrations in the bottom sediments of both lakes throughout the entire core length are at the background level. The mercury content in the subsurface layers of sediments in both lakes, unlike other elements, was much higher than its local background contents. This is likely related to its atmospheric influx through transboundary transport. The comparison of diatom and grain-size data revealed the rebuilding of lake ecosystems in response to lake shoaling. The bioindication methods are promising for reconstructing the recent climatic changes in arctic regions. Keywords: Arctic lakes, West Siberia, bottom sediments, element composition, mercury, sedimentation rate, grain-size composition, diatom analysis DOI: 10.1134/S0016702920040114 INTRODUCTION Arctic lakes are sensitive indicators of global environmental and climatic changes. At the same time, the state of lake ecosystems is controlled by regional and transboundary transfers of pollutants. The bottom sediments of arctic lakes unaffected by direct anthropogenic impact represent peculiar paleoclimatic and paleogeochemical archives. They bear information on the biogeochemical processes at the catchment and in the lake, which reflect climatic and environmental changes. The vulnerability of Arctic environment causes the high reactivity of ecosystems, which predetermines the informative “record” of these changes in bottom sequences, including changes in biogeochemical cycles, global fluxes of airborne pollutants, as well as climate dynamics and ecosystem state, which are recorded in the geochemical composition and diatom remains in bottom sediments. At present, the studies of diatom assemblages and geochemical composition of lake sediments are regarded as generally accepted and essential method for reconstructing climatic and ecological events. The paleolimnological studies of lakes are focus of large international programs, for instance, of the the Pole-Equator-Pole (pole–equator–pole for Europe and Africa) and CAPE (Circumpolar paleoenvironment – circumpolar environment in the past) programs. Schools of diatom analysis in USA (Stockner and Benson, 1967; Stockner, 1975); Sweden (Tolonen, 1978; Renberg et al., 1978), and Great Britain (Batterbee et al., 2002) play a leading role in studying lake sediments as information sources of the recent changes caused by anthropogenic impact. Diatom algae are microscopic unicellular organisms, which are essential part and important component of lake ecosystems. They are well preserved in sediments owing to the siliceous frustule (valves). A wide spectrum of bioindicator features makes it possible to verify the diatom analysis with chemical, grain-size, and other data on lake sediments. A qualitatively new stage for diatom-based reconstructions was related to the application of isotope method, in particular, the analyzing 210 Pb and 137Cs relations (Lotter and Hofmann, 2003). 408 BOTTOM SEDIMENTS OF THE WEST SIBERIAN ARCTIC LAKES 409 Table 1. Characteristics of the studied lakes (Report…, 2011) Lake Area, km2 lake drainage system Ratio of areas 10.3 8.2 0.8 43.5 3.0 7.71 8.1 2.1 9.6 3.5 7.39 Coordinates Langtibeito 71.063917; 70.321806 Gol’tsovoe 71.423333, 78.849444 3.87 Innovation techniques for reconstructing hydrological parameters on the basis of diatom assemblages were developed to study the anthropogenic-induced changes (Moiseenko and Razumovsky, 2009; Razumovskii, 2012). In Russia, the complex studies of bottom sediments were carried out in the Kola North (Dauvalter, 1995; Moiseenko et al., 1998, 2002; Solovieva, 2002; Razumovskii, 2012), Pechora basin (Walker et al., 2009), as well as in the northern Transurals (Laing et al., 1999), Urals (Cremer et al., 2004; Maslennikova et al., 2014), Siberia (Laing and Smol, 2000), and Chukotka (Kharitonov, 2010). At the same time, only a few works are available on the sediments of the Russian arctic lakes. The northern West Siberia is a region of the intense exploration of hydrocarbon deposits. This region comprises >200 reservoirs annually yielding 290–310 Mt oil and 35–40 billion m3 natural gas. The largest gas fields (Bovanenkovskoe, Kharasaveiskoe, and Tambeyskoe fields) are located in the Arctic zone. The intense development of gas-condensate deposits with oil margins (Novoportovskoe, Tambeyskoe) accompanying by building of gas pipe line and natural gas liquefaction complex (Yamal-SPG) has began on Yamal Peninsula in 2010. This caused the growth of anthropogenic load on the West Siberian Arctic environment. The aim of our studies is the reconstruction of environmental and climatic changes in the Arctic regions of West Siberia, the estimate of long-term element sedimentation dynamics and the state of ecosystem, and revealing a possible anthropogenic input in these changes during industrial activity related to the exploration of petroleum deposits. MATERIALS AND METHODS Characteristics of the area. The study of environmental, including climatic, changes, were carried out in the Arctic zone of West Siberia, in the Yamal– Nenets Autonomous Okrug. In August, 2011, bottom sediment cores were collected from two lakes: Lake Langtibeito in the north of the Yamal Peninsula (Yamal district) and Lake Gol’tsovoe in the northern part of the Gydan Peninsula (Tazovskii district). Both the lakes are located in the Arctic tundra zone, which is characterized by a long-term snow cover (about GEOCHEMISTRY INTERNATIONAL Vol. 58 No. 4 Height above sea level, m Average depth, m рН Mineralization, mg/L 354.5 41.15 10 months) and a short cold (1–5°С) summer. The lakes are of glacial origin. Table 1 presents their coordinates and preliminary limnological data. Tundra lakes are fed by atmospheric precipitates and water derived during seasonal thawing of permafrost. In the bottom sediments of the glacial lithogenesis zones, sedimentary material sharply dominate over organics and is represented by clay and loam with a weakly alkaline medium (Strakhov, 1963). The host rocks of the lake basins in tundra of the Yamal and Gydan peninsulas are represented by Quaternary marine and glacial-marine sediments varying from coarse-grained sands to finely dispersed clays. The lake floor is made up either of marine clays or glacial sand–loams, which determined the water mineralization. Sampling. The bottom sediments were collected by researchers from the Tyumen State University in the framework of megaproject no. 220 11G34.31.0036 “Formation of Water Quality under Environmental and Climatic Changes of West Siberia”. The cores were collected from the central parts of lakes by the Edelman Eijkelkamp corer (the cores are up to 70 cm long and diameter 4 cm) for sediment analysis and by C-1 microbenthometer (stratometer) (diameter 4 cm) for diatom analysis. The water depth in sampling points was 3.6 m at Lake Langtibeito and 2.5 m at Lake Gol’tsovoe. The length of cores collected on the Langtibeito and Gol’tsovoe lakes were 57 and 48 cm long, respectively, and cores collected by stratometer, 10 and 23 cm long, respectively. In field conditions, the cores were cut into 1-cm slices, while cores collected by stratometer, into 0.5-cm slices. Each slice was placed in a plastic bag and has preserved in a frozen state up to analysis. Analysis. Under laboratory conditions, the samples were dried up to constant weight at room temperature and crushed. Both the cores were dated layer-by-layer by the measurement of 210Pb and 137Cs isotopes. The measurements were carried out by direct method using a low-background gamma-spectrometric complex with detector made up of the planar-type extrapure germanium BEGe3825 and multichannel pulse analyzer (Stepanets et al., 2010). The loss on ignition (LOI) was determined by keeping samples in a muffle furnace at 550°С for no less than two hours (up to constant weight). 2020 410 TATSII et al. 2000 1950 (а) Dating 1900 1850 (b) Dating 1950 1900 2000 5 25 210 4 0 5 10 15 20 25 Depth, cm 30 35 0 10 0 5 15 20 10 Depth, cm, см 25 30 8 Cs activity, Bq/kg 20 4 137 5 1 Pb activity, Bq/kg 2 137 10 210 3 15 12 Cs activity, Bq/kg 20 30 Pb Cs 210 Pb activity, Bq/kg 137 0 Fig. 1. Distribution of 210Pb and 137Cs activities in the bottom sediment cores from the Langtibeito (a) and Gol’tsovoe (b) lakes and core dating. Individual layers of bottoms sediments were analyzed for grain-size composition by a dry sieving into several fractions: >0.1 (sand), 0.1–0.04 (coarse aleurite), and <0.04 mm (aleurite–pelite). The element composition of bottom sediments (63 elements) after acid digestion was determined using inductively-coupled plasma mass spectrometry (ICP-MS). Mercury was determined by pyrolysis from a solid sample (20–100 mg, n = 3) with the preliminary accumulation on a gold collector and recording on АА spectrometer. The spectrometer was calibrated using saturated mercury vapor (Tatsii and Stakheev, 2001). Sampling and treatment for diatom analysis, preparation of specimens, calculation and identification of diatom frustules were carried out using standard techniques (Davydova, 1985; Renberg, 1990; Razumovskii and Moiseenko, 2009). In addition to the conventional diatom analysis, we also applied a principle of unification of bioindication methods (UBM) (Moiseenko and Razumovsky, 2009). The generally accepted ecological characteristics of species were refined from (Davydova, 1985; Kamenik et al., 2001; Battarbee et al., 2002; Schmidt et al., 2002; Barinova et al., 2006). RESULTS AND DISCUSSION The studied lakes were formed on the watersheds, the hydrogeological conditions of which are mainly determined by the presence of > 300–500 m thick permafrost rocks. The Lake Langtibeito drainage basin is located in the area of the epigenetically frozen Middle Quater- nary glacial and upper Quaternary marine sediments. The lake basin is made up of medium-grained sands with the high content of ferrous iron compounds. The sands are mainly composed of quartz (85–90%), with minor limonite, epidote, pyroxenes and amphiboles, hydromica, and iron hydroxides. The rocks of marine genesis are the sources of carbonates reaching, on average, up to average 0.5%. The total amount of water-soluble salts in local sediments varies from 0.01 to 1% (Cryosphere…, 2013), which explains an elevated water mineralization in the lake. The mineral, grain-size, and chemical composition of sediments of Lake Gol’tsovoe are controlled by permafrost Upper Quaternary sediments of alluvial– marine and marine origin. The lake bottom is made up of ice-bearing sandy–loamy sequence, which contains 60% of feldspar–quartz sands. They are dominated by dust-size varieties (45%), while loams respectively occupy 40%. The loams occur as interlayers and lenses in sandy sequence, while the fine fraction in the loam is represented by limonite and iron hydroxides (Geocryology of the USSR, 1989). Dating. The layer-by-layer determination of activity of disequilibrium 210Pb throughout the studied cores was conducted to date the layers and to determine the average sedimentation rates. The determinations were carried out assuming a constant 210Pb flux (CRS model meaning a constant-rate-of-supply), constant sedimentation rates with time (linear model), and the absence of postsedimentation migration of 210 Pb. The 137Cs activity was measured to control the results of 210Pb determination. Joint variations of 210Pb and 137Cs are shown in Fig. 1. The sedimentation rates in GEOCHEMISTRY INTERNATIONAL Vol. 58 No. 4 2020 Vol. 58 No. 4 1850 1900 1950 5 4 3 2 5 10 15 20 <0.04 mm 25 30 35 40 45 50 55 >0.10 mm 5 10 15 20 25 30 35 40 45 50 55 1 0 10 20 30 40 50 60 70 80 90100 0 Loss on ignition, % Grain-size analysis, % (b) 1850 1900 1950 2000 50 40 30 50 45 40 35 30 25 20 15 10 5 <0.04 mm >0.10 mm 20 10 1 2 3 4 5 6 7 8 9 10 0 10 20 30 40 50 60 70 80 90100 0 Loss on ignition, % Grain-size analysis, % (a) Depth, cm GEOCHEMISTRY INTERNATIONAL 411 Depth, cm the lakes estimated from the isotopic activity accounted for 0.170 ± 0.035 (210Pb) and 0.166 ± 0.033 (137Cs) for Lake Longtibeito and 0.20 ± 0.04 cm/yr for both isotopes for Lake Gol’tsovoe. The grain-size composition and loss on ignition. The cores from both lakes are dark with gray-green tint and show no clearly expressed thin layerage. Nevertheless, they are clearly stratified. In the Lake Langtibeito core, the upper loam layer near border with water (approximately 1.5 cm) has light brown color owing to the presence of iron oxides and hydroxides. Then up to depth of 8 cm – greenish-brown loam. In Lake Gol’tsovoe, the uppermost 23-cm layer of the core is represented by greenish-brown loam with fine sand admixture. The lower parts of both cores are composed of greenish brown sand, which is supposedly the sandy floor representing a mineral base overlain by finely dispersed sedimentary material, which consists of greenish brown ooze, loam, and greenish brown medium-grained sand. This is also supported by the length of the core collected by the gravity stratometer, which stopped at the denser sandy layer. The visual estimate was mainly confirmed by the grain-size analysis and loss on ignition (Fig. 2). The grain-size composition revealed loam heterogeneity. The uppermost 10-cm of the Langtibeito core is dominated by silty–pelitic fraction (Fig. 2a), and a 7–8 cm horizon contains 70% of <0.04-mm fraction. In contrast, the lower horizons (approximately from 10 cm) show a sharp increase of sand fraction, while the <0.04-mm fraction accounts for less than 10%. The loss on ignition demonstrates the extremely low content of organic matter, but the upper 10 cm of this core have the higher LOI values (4–9%) than the lower sandy horizons (~1%). The content of fine fraction (<0.04 mm) in the uppermost part of the Lake Gol’tsovoe core is much lower and reaches maximum at a depth of 17–20 cm (Fig. 2b), while bottom sediments from the horizon 23 cm become more sandy. The loss on ignition values for this lake are more consistent with grain-size analysis than in Lake Langtibeito, although have lower values (<5%). It is difficult to estimate reliably the content of organic matter on the basis of such low LOI values. The main problem consists in that most of minerals contain structurally bound water, which is gradually released during heating. Therefore, the content of organic carbon and LOI values could be significantly different. With increase of Сorg content, the LOI to carbon ratio approaches 2, but sharply increases at LOI below 10% (Mackereth, 1966). Nevertheless, the LOI values in the upper loam parts of both cores exceed significantly those of the lower sandy portions and unambiguously indicate an extremely low content of organic matter. Geochemical composition. Sampled cores, in spite of definite differences between lakes, are very close in chemical composition and concentrations of both 2000 BOTTOM SEDIMENTS OF THE WEST SIBERIAN ARCTIC LAKES Fig. 2. Grain-size composition, loss on ignition, and dating of sediment cores of the Langtibeito (a) and Gol’tsovoe (b) lakes. 2020 412 TATSII et al. 1 1.5 1.0 2 0.5 0 Al Fe Ca Mg Ti Mn P Li Be S Sc V Cr Co Ni Cu Zn Ga Al Rb Sr Y Zr Nb Mo Ag Sb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Ho Er Tm Yb Lu Hf Ta W Hg Tl Pb Th U Enrichment factor 2.0 Fig. 3. Enrichment factors of elements in the sediment cores from the Langtibeito (1) and Gol’tsovoe (2) lakes. The enrichment factors were calculated for maximum concentrations of elements in the cores relative average concentrations in clay shales with normalization by Sc. typomorphic and trace elements. Both the lakes are characterized by more or less expressed increase of concentrations from lower parts to the core top practically for all elements. For Lake Langtibeito, this increase is better expressed. The concentrations of most elements in the bottom sediment cores of the lakes are approximately at the same level, in spite of the significant differences in water composition. However, even their maximum contents do not exceed the average concentrations for sedimentary rocks (clay shales, and in most cases, sandstones, Table 2). The enrichment factors calculated relative to the average contents in shales (normalized to Sc) are no more than 1.0 for the maximum contents of all elements in the Gol’tsovoe bottom sediment core and 2.0, for the Langtibeito core (Fig. 3). Based on these data, the concentrations of these elements in bottom sediments correspond to background contents, while the lakes can be considered as background lakes. The Re, In, Pt, Au, Se, Rh, Pd, Te, and Cd concentrations are below the detection limits. Distribution of some elements throughout the core length is shown in Fig. 4. The layer-by-layer analysis allowed us to reveal several features in the element distribution along the cores. Both lakes show a gradual upward increase of element concentrations to the surface, which is better expressed in Lake Langtibeito. The uppermost 1.5-cm interval of this lake (light brown clay loam) shows a sharp growth (by 1.5–2 times) of Co, Cr, Cu, Mo, Ni, V, Zn, as well as total sulfur and Fe and Mn oxides (Fig. 4а). Such distribution can be explained by the income of elements in bottom sediments through sedimentation on the Fe and Mn hydroxides, by processes Table 2. Average contents of some elements in the sedimentary rocks (Turekian and Wedepohl, 1961; Ronov et al., 1990) and studied sediment cores of the studied lakes, mg/kg Concentration, mg/kg As Shales Clay, clay shales Sandstone Langtibeito Top. Maximum (loam) Bottom. Average (sand. below 20 cm) Gol’tsovoe Top. maximum (loam) Bottom. average (sand. Below 30 cm) Ba Co Cr 13 580 19 90 6.6 800 20 100 1 n × 10 0.3 35 2.6 681 5.6 31 2.3 578 3.3 648 1.16 563 2.7 15 10 54 3.7 21.6 Cu Hg 45 0.4 57 0.4 n 0.03 8.5 0.075 Mo 0.26 68 0.2 95 0.2 2 0.28 15 3.4 0.004 0.14 23 Ni 5 0.078 0.33 27 4.9 0.005 0.16 Pb 20 20 7 11.5 S Sr Tl V 2400 300 1.4 130 3000 450 1 130 240 20 0.82 20 200 172 0.37 37 8.8 12 6.5 8.4 GEOCHEMISTRY INTERNATIONAL Zn 95 80 15 25 72 126 0.26 20 11 230 214 0.35 79 48 81 132 0.26 Vol. 58 34.7 14.4 No. 4 2020 Depth, cm Depth, cm GEOCHEMISTRY INTERNATIONAL Vol. 58 No. 4 2020 Depth, cm 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50 0 % 1.0 Fe2O3 1.5 La 5 Cr mg/kg 10 V 15 S As Ni Pb mg/kg 4 8 Zn 12 0 Cu 1800 1850 1900 1950 2000 Si Ba 0 5 10 15 20 25 30 35 40 45 50 55 0 2000 5 1950 10 15 20 1900 25 1850 30 35 1800 40 45 50 55 mg/kg 200 400 600 800 Co mg/kg mg/kg 5 10 15 20 25 30 1 2 3 4 5 6 7 8 9 Al2O3 0 5 1950 10 15 1900 20 25 1850 30 35 1800 40 45 50 55 mg/kg 40 80 120 160 200 2000 % Fe2O3 2 3 La 10 Cr Ni Zn mg/kg 10 20 30 40 50 Al2O3 As Pb mg/kg mg/kg 5 15 20 0 2 4 6 8 10 120 V mg/kg 10 20 30 40 50 60 70 80 1 (b) Concentration % 2 4 6 8 10 12 1800 1850 1800 1850 1900 1950 Sr Ba 1800 1850 1900 1950 2000 mg/kg 200 400 600 800 Cu 2000 mg/kg 50 100 150 200 250 S 1900 1950 mg/kg 50 100 150 200 250 2000 Co Fig. 4. Concentration profiles of elements in the bottom sediment cores of the Langtibeito (a) and Gol’tsovoe (b) lakes. 0 mg/kg 5 10 15 20 25 30 35 40 0.5 (а) Concentration % 1 2 3 4 5 6 7 8 9 BOTTOM SEDIMENTS OF THE WEST SIBERIAN ARCTIC LAKES 413 414 TATSII et al. 0 10 (a) Concentration, µg/kg 20 30 40 50 60 70 80 0 10 (b) Concentration, µg/kg 20 30 40 50 60 70 80 2000 2000 5 5 10 10 1950 1950 15 20 20 1900 25 1850 30 Depth, cm Depth, cm 15 1900 25 30 1850 35 35 40 40 45 45 50 50 55 Fig. 5. Mercury in the sediments of the Langtibeito (a) and Gol’tsovoe (b) lakes. on the bottom sediment–water interface, and by sulfide formation. An increase of some elements in the surface layer could be also caused by the intense exploration of the Nizhne-Tambeyskoe gas-condensate field at a distance of a few tens of kilometers from the lake, which is accompanied by gas flaring and the development of wind erosion owing to the disturbance of vegetation cover (Yamal SPG..., 2014). At the same time, it should be noted that the element concentrations in the upper horizon of bottom sediments and elevated contents of some elements in separate horizons remain within background concentrations (Fig. 3а). The distribution of all elements (besides mercury) shows a poor correlation with grain-size and LOI data. The distribution of elements in the bottom sediment core from Lake Gol’tsovoe, in contrast, well correlates with LOI and grain-size data (fraction <0.04 mm) (Fig. 2b). The peculiar feature of the core is an increase of concentrations of practically all elements within 22–16 cm interval. The concentrations of some elements (Bi, Cr, Cu, Li, Mg, Ni, Stot, Th, V, and Zn) in these horizons are over two times higher. Such an increase would be explained by a significant increase of aleurite–pelite fraction (almost up to 50%), but this would hardly affect practically all (>50) elements. An increase of this fraction up to 70% in the Langtibeito core did not cause significant changes of concentrations. The reason and mechanism of such distribution yet remain unclear. Mercury in bottom sediments. The mercury behavior in the bottom sediments of the studied lakes sig- nificantly differ from the distribution of other elements in its excellent correlation with grain size and LOI data for both lakes (Fig. 5), as well as with results obtained for parallel cores (Morozova et al., 2015). Unlike the distribution of other elements in the bottom sediments of Lake Langtibeito (Fig. 5a), the increase in mercury concentrations begins on the sand–loam transition of the core. This transition, at a lesser length of loam section in Lake Gol’tsovoe, has the more expressed character. An increase of Hg content in Lake Gol’tsovoe also begins in the sand–loam transition zone (approximately 27–30 cm), but has a less expressed character (Fig. 5b). For this lake the mercury distribution shows two peaks: at depths of 16–20 cm (as for other elements) and 7 cm. Unlike other elements, a sharp increase of Hg concentrations is observed in the uppermost 2 cm (Fig. 4b). The local background mercury contents in the bottom sediments of the lakes were determined from the mercury contents in sandy portions of the cores: in 10–47 cm layers of Lake Langtibeito and 27–55 cm layers of Lake Gol’tsovoe. These values accounted for 4.5 ± 1.3 and 5.4 ± 1.2 ng/g for the Langtibeito and Gol’tsovoe lakes, respectively. The higher contents of the finest fraction (<0.04 mm) throughout the entire loam part of the Langtibeito core explain the high mercury content in this part. The insignificant contents of this fraction in sandy portions of the cores from both lakes correspond to the low (background) mercury contents. It is characteristic that the mercury GEOCHEMISTRY INTERNATIONAL Vol. 58 No. 4 2020 BOTTOM SEDIMENTS OF THE WEST SIBERIAN ARCTIC LAKES contents in the upper layers of bottom sediments of both lakes reach approximately equal values. The enrichment factor for mercury in the uppermost layers of bottom sediments relative to the local background (mercury content in the lower layers) accounted for 16.9 and 14.4 for the Langtibeito and Gol’tsovoe lakes, respectively, which are much higher than for other elements. This may indicate a similar mercury influx from atmosphere owing to the transboundary transport. A source could be the Norilsk industrial district and petroleum complexes of Yamal (gas flaring). Diatom analysis. The quantitative and qualitative compositions of fresh-water diatom assemblages are tightly related to the chemical composition of water and represent bioindicators, which are able to reflect environmental and climatic changes. They have a well-preserved siliceous frustule of a complex structure, which well preserved in the fossil state. Therefore, the composition of diatoms in bottom sediments can be used to estimate the state of ecosystems and environment in the past and to reveal the dynamics of climate and unfavorable processes. The rate of intraspecies evolution in this algal group is small relative to the formation time of Late Quaternary sediments. The species composition of fresh-water diatom assemblages practically did not change for the last 10–20 thou years. Thus, the diatom algae are a key group in reconstructing the ecological paleoenvironments of fresh-water basins (Razumovskii and Moiseenko, 2009; Moiseenko et al., 2000). For the diatom analysis, the upper (loamy) parts of the columns were used –10-cm of Lake Langtibeito and 23-cm of Lake Gol’tsovoe. The revealed diatom flora of bottom sediments of Lake Langtibeito includes 124 species, which are ascribed to 34 genera of Bacillariophyta. The frustules are well preserved. Planktic diatoms account for 15% of the total taxa. Benthic diatoms amounting up to 82% of total taxa are characterized by the widest species diversity. All these diatoms are fresh-water. Three species of ancient marine redeposited diatoms (Grunowiella gemmata, Paralia sulcata, and Stephanopyxis turris) were identified in samples. The frustules and fragments of Paralia sulcata were found throughout the entire core section, especially many of them in the uppermost sediment layer. The diatom diagram (Fig. 6) shows the distribution of some dominant and indicator diatom species throughout the core, as well as four ecozones distinguished on the basis of diatom assemblages in the Langtibeito core. Within the depth interval of 7–9.5 cm (zone D-I) at the transition from greenish-brown sand to greenish-brown loam, the content of frustules is high. In this zone, the diatom assemblages are dominated by species Staurosirella pinnata. The contents of other species are as follows: genera Achnanthes (Achnanthes calcar) account for up to 50%, Navicula (N. järnefeltii), up to GEOCHEMISTRY INTERNATIONAL Vol. 58 No. 4 415 10%, and genera Staurosira (Staurosira venter), up to 14%. A few frustule specimens from swampy species of genera Eunotia (E. pectinalis var. minor) and Pinnularia (P. microstauron) account for <3%, and Cavinula (C. pseudoscutiformis), up to 5%. Planktic species are mainly ascribed to genus Aulacoseira and account for up to 15% (Aulacoseira italica), while single planktic species Tabellaria flocculosa occur only in this horizon. Upsection, at a depth of 5–7 cm (zone D-II), the greenish-brown loam contains diatom assemblages with the predominance of epiphytic fresh-water– saltish-water Staurosira construens (up to 20%). The dominant bottom species are also Staurosirella pinnata (@до 17%), which is widespread in fresh, sometimes slightly saltish waters (light halophile, oligosaprobe). They are accompanied by foulers Staurosira venter, Achnanthes lanceolata, A. aff. Frigidam, and others. The third zone (D-III) distinguished within the interval of 1.5–5 cm, as compared to the previous zone, shows a growth of diatom abundance. The predominant species is Staurosirella pinnata (up to 39%), which is accompanied with Staurosira venter (up to 7%). The number of epiphytic species increases, primarily through an increase of genus Achnanthes (Achnanthes lanceolata (up to 10%), A. lanceolata spp. dubia (up to 2%), Achnanthes bioretii (up to 5%), and others). The number of planktic species (Aulacoseira alpigena) decreases. The swampy species Pinnularia practically disappear, while species of genus Eunotia occur as single specimens. In the fourth zone (D-IV), within an interval of 0– 1.5 см, the frustules concentration remains high. This horizon is peculiar in the absence of dominant species; the assemblages include 59 diatom species, most of which account for from 3 to 7% of the total taxa. The most abundant species are Staurosira construens (up to 7%), S. venter (up to 6%), Staurosirella pinnata (up to 7%). Achnanthes aff. frigida (up to 6%), A. lanceolata (up to 5%), A. petersenii (up to 3%), A. bioretii (up to 3%), Amphora lybica (up to 5%), Aulacoseira italica (up to 4%), Diploneis elliptica (up to 3%), and others. The diatom assemblages are dominated by bottom species (up to 54%), with significant number of epiphytes (up to 42%). The species composition and categories of ecological groups in the diatom assemblages of Lake Langtibeito are typical of fresh-water cold basins with weakly alkaline or neutral medium. Nevertheless, it should be noted that the diatom assemblages of this lake differ from lacustrine assemblages of the European North Russia in the absence of clearly expressed dominants and the wide species diversity (124 species). In particular, only Staurosirella pinnata reaches 39% in separate horizons. Based on the pH values, the lake was mainly characterized by weakly alkaline and neutral waters. In general, the diatom assemblages throughout the core are indicative of littoral zone with neutral– weakly alkaline medium and the predominance of 2020 5 0 10 0 20 40 70 60 30 50 50 50 40 40 30 4 0 30 0 20 0 50 0 (@морская переотложенная) Paralia suleata Staurosirella pinnata S. venter Staurosira construens Navicula jarhefeltii Gyrosigma kuetzingii Eunotia granulate Diploneis elliptica C. pseudoscutiformis Cavinula cocconeiformis Caloneis bacillum Aulacosera granulata Amphora lybica A. aff. frigida A. petersenii A. lanceolata ssp. dubia A. devei 10 0 7% Ecozones 0 A. lanceolata TATSII et al. Achnanthes calcar 416 D-IV 2010 1 3 4 5 1980 D-II Depth, cm 1990 D-III 2000 2 6 1970 7 D-I 8 1960 9 Fig. 6. Diatom diagram for the bottom sediments of Lake Langtibeito with subdivision into ecozones and dating. benthic (bottom and epiphytic) diatoms; the abundance of planktic diatom algae is relatively insignificant (Fig. 8). Some conclusions can be drawn from obtained distributions. An insignificant growth of epiphytic diatoms in zone D-II as compared to the first sedimentation stage (D-I) may indicate a possible insignificant lake level decrease, while a continuing growth of epiphytic diatoms in zone D-III suggests a further level decrease. The predominance of bottom and epiphytic diatoms at extremely mixed composition of diatom assemblages devoid of clearly expressed dominant in zone D-IV could be caused by instable varying conditions, which prevented significant development of definite species. The bottom sediments of Lake Gol’tsovoe contain scarce but sufficiently taxonomically diverse diatom flora: 107 diatom species and varieties, as well as 5 taxa of marine flagellate alga (silicoflagellate). The freshwater and fresh-water–saltish diatoms comprise 70 taxa, including 8 planktic species, 44 bottom diatoms, and 27 foulers. The saltish-water diatoms surviving light salination of host basin include five species. The bottom diatoms reached the widest species diversity, with the most representative Pinnularia (17 species) and Navicula (7 species). The foulers are represented by 27 species and varieties. These are Eunotia (9 species), Cymbella (7 species), and Fragilaria (3 species). Other genera include the lesser number of species. All frustules of fresh-water diatoms are well-preserved and ascribed to the recent flora. In addition to the recent diatoms, all studied samples contain redeposited marine diatoms and silicoflagellates of Paleogene age. In total, we established GEOCHEMISTRY INTERNATIONAL Vol. 58 No. 4 2020 50 40 0 50 0 50 20 0 60 70 10 0 80 8 0 Paralia sulcata s.l. Synura sphagnicola Tabellaria fenestrata Synedra ulna Staurosira construens Pinnularia interrupta Navicula menisculus Martyiana martyi Hantzchia amphioxis Fragillaria virescens Eunotia praerupt 80 417 80 0 60 % Ecozones 0 Aulacoseira granulata Amphora ovalis var. lybica BOTTOM SEDIMENTS OF THE WEST SIBERIAN ARCTIC LAKES 2010 2 2000 4 1990 6 1980 8 1970 D-IV Depth, cm D-III 10 1960 12 1950 14 1940 16 1930 18 1920 20 1910 22 1900 D-II D-I Fig. 7. Diatom diagram for bottom sediments of Lake Gol’tsovoe with subdivision into ecozones and dating. 22 diatom taxa and 5 flagellate taxa. The redeposited diatoms are dominated by Paralia sulcata and Aulacoseira sp., which were found in all samples with high quantitative estimates. Less abundant Coscinodiscus payeri and Stephanopyxis turris occur in most samples. Fragments of species Grunoviella gemmata and Hemiaulus sp. were found almost in all samples, while other diatom species and silicoflagellates are very scarce. The most abundant silicoflagellates are Dictyocha transitoria. The studied core is characterized by the presence of golden algae (Chrysophyta) in some samples. The species was determined as Synura sphagnicola Korschikov. It is ascribed to the holarctic planktic algae typical of stagnant fresh waters. Synura sphagnicola was identified in 10 samples of the considered core, and scales of GEOCHEMISTRY INTERNATIONAL Vol. 58 No. 4 this alga reach over 75% of total algae amount in sediments within 10–10.5 cm interval. The diatom diagram (Fig. 7) demonstrates the distribution of some dominant and indicator diatom species throughout the section. Based on the composition and character of the diatom assemblages, we distinguished four zones reflecting the change in sedimentation conditions. In zone D-I within the depth interval of 22.5–18 cm, bottom sediments are represented by greenish-brown loam with numerous dark brown spots of organics. It is characterized by poor diatom assemblages, which are dominated by foulers Fragilaria virescens (up to 20%), Tabellaria fenestrata (up to 8%), Staurosira construens (up to 7%), and Eunotia praerupta (up to 6%). Bottom species Amphora ovalis and Navicula menisculus are ascribed to the subdominants. Upsection, the 2020 418 TATSII et al. (а) Content, % 0 20 40 60 80 Grain-size composition, % 0 50 100 2010 100 0–1 D-IV 1–2 2000 2–3 D-III 1990 Ecozones Horizon, cm 3–4 4–5 5–6 1980 D-II 6–7 1970 7–8 D-I 8–9 1960 9–10 (b) Content, % 20 40 60 0–1 1–2 2–3 3–4 4–5 5–6 6–7 7–8 8–9 9–10 10–11 11–12 12–13 13–14 14–15 15–16 16–17 17–18 18–19 19–20 20–21 21–22 22–23 80 100 D-IV 2000 1980 D-III Ecozones Horizon, cm 0 Grain-size composition, % 0 50 100 1960 1940 D-II 1920 D-I 1900 Plankton Bottom Foulers <0.04 mm >0.1 mm Fig. 8. Diagram of distribution of ecological diatom groups, grain-size composition, and dating, and ecozones in the sediments of the Langtibeito (a) and Gol’tsovoe (b) lakes. amount of foulers within the interval decreases from 70 to 23%. The upper part of the interval is marked by the appearance of Synura sphagnicola, as well as by the population peak of planktic Aulacoseira granulata. Redeposited diatoms varies from 20 to 65% of total diatoms. In Zone D-II (17.5–12 cm, loam with numerous inclusions of organic black-brown ooze), diatom assemblages are dominated by Fragilaria virescens (up to 18%), Martyiana martyi (up to 18%), Staurosira construens (up to 11%), while Tabellaria fenestrata pre- GEOCHEMISTRY INTERNATIONAL Vol. 58 No. 4 2020 BOTTOM SEDIMENTS OF THE WEST SIBERIAN ARCTIC LAKES dominating in the lower layer becomes subdominant. A list of subdominants is supplemented by epiphytic diatoms of genera Eunotia, which reached 8% in the upper part of the interval. This zone contains bottom species (P. interrupta, P. viridis), which are absent in the underlying sediments. Synura sphagnicola sporadically occur. The amount of foulers decreases from 87 to 18% upsection within the interval. The redeposited diatoms amount from 35 to 60%. In Zone D-III (11.5–6 cm), the number of inclusions of black-brown organic ooze decreases upward. This assemblage demonstrates a decrease of population and species diversity of fresh-water diatoms. Most samples are dominated by Synura sphagnicola, reaching 75%. In the fresh-water assemblage, the role of indifferent species (Hantzchia amphioxis) and halophiles (Synedra ulna – up to 7%) increases, while the amount of halophobes (Eunotia praerupta, Fragilaria virescens, Pinnularia interrupta, planktic species Aulacoseira granulata), decreases. This zone preserve a large number of Staurosira construens (8–9%). This interval is characterized by the lowest content of redeposited diatoms (varies from 10 to 30%). The abundance of foulers within the interval decreases upward from 55 to 35%, except for 10–10.5 cm interval. The wide abundance of Synura sphagnicola suggests a possible increase of the basin area and an increase of its trophic status. The upper zone D-IV (5.5–1 cm) yielded poor scarce diatom assemblages. The dominant species are Martyiana martyi (up to 18%), as well as Fragilaria virescens, Staurosira construens in the lower part of the interval. Subdominants are Eunotia praerupta, Navicula menisculus, and N. pupula. The amount of foulers upward the interval decreases from 55 до 35% (except for 2–2.5 cm interval). Synura sphagnicola was found in two horizons (4–4.5 and 2–2.5 cm). Horizon 0–0.5 cm contains sporadic redeposited diatoms. The amount of redeposited diatoms increases relative to the underlying sediments. In terms of composition and relations between diatom species, zone D-I is close to zone D-II, but differs in extremely poor diatom assemblage. The studied diatom assemblages of the Gol’tsovoe core make it possible to reconstruct the conditions in the lake basin during the accumulation of the uppermost 22.5 cm of bottom sediments. Characteristic feature is the low number of diatoms, the constant presence of redeposited marine Paleogene species, as well as golden algae found in most samples. Four sedimentation stages were distinguished on the basis of relations between dominant species, as well as between diatom groups with different habitat. A change in proportions between bottom species and foulers with upward decrease of the latters within each stage suggest lake-level variations, with some increase of the lake depth at each successive sedimentation stage. At the first stage (D-I), the diatom assemblages characterize the conditions of formation of littoral zone with neutral–weakly alkaline medium. The secGEOCHEMISTRY INTERNATIONAL Vol. 58 No. 4 419 ond stage (D-II) was characterized by an increase of eutrophication of the basin. The third stage (D-III) is marked by the appearance of golden algae in most horizons, the number of which exceeded that of freshwater diatoms. The diatom assemblage shows an increase of species – indifferents and halophiles. This is likely related to the increase of the lake area and simultaneous increase of its trophic status. At the fourth stage (D-IV), the low abundance of diatoms further decreases. This is likely caused by the deterioration of conditions, in particular, cooling and salination of the basin. During the entire studied sedimentation period, redeposited diatoms represented by marine Paleogene (likely Eocene) species were involved in sediments. The distributions of ecological algal groups throughout the cores from the Langtibeito and Gol’tsovoe lakes are shown in Fig. 8. The correlation of diatom data with grain-size composition showed that the grain-size composition of lake sediments of the studied lakes in 1950–1960s was characterized by a clear decrease of fine dispersed fraction and increase of coarse-grained fraction. This was related with a pronounced restructuring in ecosystems of both lakes, which is also characterized by a significance decrease of relative abundance of planktic species and an increase of percentage of foulers and bottom species. Such processes of rebuilding of ecosystem are usually related to lake shoaling. Both these processes likely have climatic reasons, because the lakes have different size and are sufficiently far away from each other. Since both lakes are located in the permafrost zone, it is reasonable to suggest a short-term climate aridization, with simultaneous erosion of surrounding rocks and their subsequent influx into shoaling lakes. The assumption of regional climatic reasons of these time-limited processes is also supported by the different duration and the rate of “response” of ecosystems of these lakes. In larger Lake Langtibeito, changes in the grain-size composition occurred slightly later, and phytoplanktic assemblages did not disappear completely. In Lake Gol’tsovoe, the finely dispersed fraction disappeared slightly earlier, while phytoplanktic species are absent in the diatom assemblages. The multi-factor nature of these processes, including different distance of lakes from coastal marine bays, also should be taken into account. Estimate of environmental dynamics and the latest lake evolution. In addition to the conventional versions of diatom analysis, we applied a principle of unification of bioindication method (Moiseenko and Razumovskii, 2009). According to the new method, the pH values are reconstructed using the evaluation procedure of saprobity index (S) according to Slàdeček (1973). S = 2020 ∑s × k, ∑k i 420 TATSII et al. (a) 0.9 0 S 1.1 1.0 1.2 pH 6.9 7.0 7.1 7.2 7.3 1.3 15 16 t, °C 17 18 19 20 D-IV 1 2 Diatom ecozones Depth, cm 3 4 5 6 D-III D-II 7 8 D-I 9 10 t, °C 9 10 11 12 13 14 15 16 17 Diatom ecozones D-IV Depth, cm (b) pH S 0.8 1.0 1.2 1.4 1.6 1.8 2.0 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 D-III D-II D-I Fig. 9. Variations of parameters of hydroenvironments: saprobity, pH, and temperature in the Langtibeito (a) and Gol’tsovoe (b) lakes. where si is an individual value of saprobity for each indicator taxon; k is the coefficient of relative abundance of each indicator taxon calculated on a six-stage scale (Manual…, 1992). Correspondingly, the values of pH and temperature (Тo) were calculated by analogy to the evaluation of saprobity index S: pH = ∑ ph × k ; ∑k i T° = ∑t × k , ∑k i where phi and ti are the individual values for each indicator taxon. If there is data for given taxon in the form of numerical interval, its average value was calculated: phi = ( phmin + phmax ) (t + t ) ; ti = min max . 2 2 The starting data base for pH and Тo calculations was taken from (Barinova et al., 2006). GEOCHEMISTRY INTERNATIONAL Vol. 58 No. 4 2020 BOTTOM SEDIMENTS OF THE WEST SIBERIAN ARCTIC LAKES It should be especially emphasized that the main advantage of our technique is the reliable reconstruction of rates and trends of pH and Тo variations instead of calculation of definite values. In samples of bottom sediments from Lake Langtibeito, 57 saprobity indicator taxa were identified. The pH indicator group includes 71 taxa; while the temperature indicator group includes 13 taxa. The calculation showed that paleotemperatures varied within 19.7–15.8°С (Fig. 9а). This indicates a strong heating of water column during “open water” periods and the degradation of ice sheet. Significant changes of saprobity index (S) were not recorded (1.24–0.99). The lake is characterized as an oligosaprobic basin, which is typical of these latitudes. The calculation of pH values confirmed the conclusions based on the conventional diatom analysis that the basin water was slightly weakly alkaline or neutral, with pH values varying within 6.93–7.2. No significant correlations were found between the saprobity, temperature, and pH values. Samples from the bottom sediments of Lake Gol’tsovoe yielded 47 saprobity indicator taxa. The pH- and temperature-related groups include, respectively, 70 and 17 taxa. Calculations of paleotemperatures showed that the lake was a cold water basin (16.75–10.0°С) (Fig. 9b). For the cold transpolar lake, it has a sufficiently high trophic status: the average value of saprobity index (Sav) is 1.46, which practically falls on the boundary between oligosaprobic and β-mesosaprobic zones. The pH values vary within 7.34–6.16. Changes in pH values may be determined by transgressive effects on the lake area. Variations of saprobity, temperature, and pH show no any significant correlations. However, a certain relationship is observed between ecozones distinguished by classical methods of diatom analysis and nature of variations of reconstructed hydrological parameters. The studies of lake sediments demonstrated the high prospect of diatom-based bioindication analysis for reconstructing the recent climatic changes in the Arctic areas of West Siberia. CONCLUSIONS The study of bottom sediment cores from two arctic lakes, Langtibeito (Yamal Peninsula) and Gol’tsovoe (Gydan Peninsula), showed that the core lithology represented by loam in the upper part and sand at the base shows no significant effect on the element distribution. The studied bottom sediments are characterized by the extremely low content of organic matter (based on loss on ignition results) even in the upper loam portion. The enrichment factors calculated relative to the average contents in clay shales are <2 even for maxiGEOCHEMISTRY INTERNATIONAL Vol. 58 No. 4 421 mum concentrations, indicating that the studied lakes can be regarded as background. The mercury content, unlike other elements, is much higher than the local background in the subsurface layers of bottom sediments of both lakes. This can be related to the atmospheric influx of the metal through transboundary transport. The sedimentation rate estimated from 210Pb and 137 Cs activity accounted for 0.17 and 0.20 cm/y for Langtibeito and Gol’tsovoe, respectivelyThe comparison of the diatom analysis and grain-size data revealed the rebuilding of lake ecosystem, which is recorded in a significant change of proportions of different diatom groups. This can be related to the lake shoaling in response to climatic changes. The diatom-based bioindication methods is promising tool for reconstructing the recent climatic changes in arctic regions. FUNDING The studies were financially supported by the Russian Foundation for Basic Research (project nos. 18-05-60012 and 17-05-00673/19). REFERENCES S. S. Barinova, L. A. Medvedeva, and O. E. Anisimova, Biodiversity of Algae as Environmental Indicators (TelAviv, 2006) [in Russian]. R. W. Battarbee, J. A. Grytnes, R. Thompson, P. Appleby, J. Catalan, A. Korhola, H. J. B.Birks, E. Heegaard, and A. Lami, “Comparing palaeolimnological and instrumental evidence of climate change for remote mountain lakes over the last 200 years,” J. Paleolimnol. 28, 161–179 (2002). H. Cremer, A. Andreev, and H. W. Hubberten, “Paleolimnological reconstruction of Holocene environments and climate from Lake Lyadhej-To, Ural Mountains, Nortern Russia,” Arctic Antarctic Alpine Res. 36 (2), 147–155 (2004). Cryosphere of the Oil–Gas–Condensate Fields of the Yamal Peninsula. Volume 2. Cryosphere of the Bovanenkovskoe Oil–Gas–Condensate Field, Ed. by Yu. B. Badu, N. A. Gafarov, and E. E. Podborny, (OOO “Gazprom Ekspo”, Moscow, 2013) [in Russian]. V. A. Dauvalter, “Heavy metal concentrations in bottom deposits of lakes of the Kola Peninsula as indicator of contamination of aquatic ecosystems,” Problems of Chemical and Bilogical Monitoring of Ecological State of Aqueous Objects of the Kola North (KNTs RAN, Apatity, 1995), pp. 24–35 [in Russian]. N. A. Davydova, Diatom Algae as Indicators of Natural Conditions in Basins during Holocene (Nauka, Leningrad, 1985) [in Russian]. Geocryology of the USSR. West Siberia (Nedra, Moscow, 1989) [in Russian]. C. Kamenik, R. Schmidt, G. Kum, and R. Psenner, “The influence of catchment characteristics on the water chemistry of mountain lakes,” Arctic Antarctic Alpine Res. 33, 404–409 (2001). 2020 422 TATSII et al. V. G. Kharitonov, “Diatom algae in the deposits of three mountainous oligotrophic lakes of the Amguema River Basin, Chukotka,” Sibirsk. Ekol. Zh., No. 4, 609–622 (2010). T. E. Laing and J. P. Smol, “Factors influencing diatom distributions in circumpolar treeline lakes of northern Russia,” J. Phycology 36 (6), 1035–1048 (2000). T. E. Laing, R. Pienitz, and J. P. Smol, “Freshwater diatom assemblages from 23 lakes located near Norilsk, Siberia: a comparison with assemblages from other circumpolar treeline regions,” Diatom Res. 14 (2), 285–305 (1999). A. F. Lotter and G. Hofmann, “The development of the lateglacial and Holocene diatom flora in Lake Sedmo Rilsko (Rila mountains, Bulgaria),” Aspects of Paleoecology, Ed. by S. Tonkov (Pensoft Publishers, Sofia– Moscow, 2003), pp. 171–183. F. J. H. Mackereth, “Some chemical observations on postglacial lake sediments,” Phil. Trans. R. Soc. B250, 165–213 (1966). A. V. Maslennikova, V. N. Udachin, and V. V. Deryagin, Paleoecology and Geochemistry of the Holocene Lacustrine Sedimentation of the Urals (RIO UrO RAN, Yekaterinburg, 2014) [in Russian]. T. I. Moiseenko and L. V. Razumovskiy, “A new technique for reconstructing the cation–anion balance in lakes by diatom analysis,” Dokl. Biol. Sci. 427 (1), 325–328 (2009). T. I. Moiseenko, V. A. Dauvalter, and I. V. Rodushkin, “Mechanisms of the cycle of natural and human–introduced metals in surface waters of the arctic basin,” Water Res. 25 (2), 212–224 (1998). T. I. Moiseenko, V. A. Dauvalter, B. P. Il’yashuk, L. Ya. Kagan, and E. A. Il’yashuk, “Paleoecological reconstruction of the anthropogenic load,” Dokl. Earth Sci. 370 (1), 102–105 (2000). T. I. Moiseenko, V. A. Dauvalter, A. A. Lukin, L. P. Kudryavtseva, B. P. Il’yashuk, L. I. Il’uashuk, S. S. Sandimirov, L. Ya. Kagan, O. M. Vandysh, Yu. N. Sharova, I. N. Koroleva, and A. N. Sharov, Anthropogenic Modifications of the Lake Imandra Ecosystem (Moscow, Nauka, 2002) [in Russian]. N. V. Morozova, N. S. Larin, T. P, Kotova, and D. Z. Shaikhutdinova, “Geochemical distribution of bulk mercury content in the bottom deposits of the West Siberian lakes,” Vestn. Tyum. Gos. Univ. Ekol. Prirodopol’zov. 1 (1), 65–73 (2015). L. V. Razumovskiy, Assesment of Transformation of Lacustrine Ecosystems using Diatom Analysis (GEOS, Moscow, 2012) [in Russian]. L. V. Razumovskiy and T. I. Moiseenko, Estimation of spatiotemporal transformations of lake ecosystems by the method of diatom analysis, Dokl. Biol. Sci. 429 (2), 514–517 (2009). I. Renberg, “Paleolimnology and varve counts of the annually laminated sediment of Lake Rudetjärm, Norten Sweden,” Early Norrland 11, 63–92 (1978). I. Renberg, “A procedure for preparing large sets of diatom slides from sediment cores,” J. Paleolimnology 4, 87– 90 (1990). Report on the Scientific Study no. 11.G34.31.0036 from November 25, 2010 (intermediate 2nd stage) “Water Quality under Anthropogenic Load and Climatic Changes in the West Siberian Regions” (Tyumen, 2011). https://ecotoxlab. files.wordpress.com/2011/11/result2011.pdf A. B. Ronov, A. A. Yaroshevsky, and A. A. Migdisov, Chemical Structure of the Earth’s Crust and Geochemical Balance of Major Elements (Nauka, Moscow, 1990) [in Russian]. R. Schmidt, K. A. Koinig, R. Thompson, and C. Kamenik A multi proxy core study of the last 7000 years of climate and alpine land-use impacts on an Austrian mountain lake (Unterer Landschitzsee, Niedere Tauern),” Palaeogeogr., Palaeoclimatol., Palaeoecol. 187, 101–120 (2002). V. Slàdeček, “System of water quality from biological point of view,” Arch. Hydrobiol. Ergeb. Limnol. 7 (7), 1–218 (1973). N. Solovieva and V. J. Jones “A multiproxy record of Holocene environmental changes in the central Kola Peninsula, northwest Russia,” J. Quaternary Sci. 17, 303–318 (2002). O. V. Stepanets, A. P. Borisov, A. V. Travkina, G. Yu. Solov’eva, M. V. Vladimirov, and R. A. Aliev, “Application of the 210Pb and 137Cs radionuclides in the geochronology of modern sediments at the storage sites of solid radioactive wastes in the arctic basin,” Geochem. Int. 48 (4), 398–402 (2010) J. G. Stockner, “Phytoplankton heterogeneity and paleolimnology of Babine Lake, British Columbia, Canada,” Verb. Internat. Verein. Limnol. 19, 2236–2250 (1975). J. G. Stockner and W. W. Benson, “The succession of diatom assemblages in the recent sediments of Lake Washington,” Limnol. Oceanogr. 12, 513–532 (1967). N. M. Strakhov, Types of Lithogenesis and their Evolution in the Earth’s History (Gostoptekhizdat, Moscow, 1963) [in Russian]. Yu. G. Tatsii and Yu. I. Stikheev, “Determination of mercury contents in samples of complex composition,” Partnery Konkurenty, No. 4, 29–34 (2001). Textbook on the Hydrobiological Monitoring of Fresh-Water Ecosystems, Ed. by V. A. Abakumov (Gidrometeoizdat, St. Petersburg, 1992) [in Russian]. M. Tolonen, “Paleoecology of annually laminated sediments in Lake Ahvenainen, S. Finland III. human influence in the lake development,” Ann. Bot. Fen. 15, 223–240 (1978). K. K. Turekian and K. H. Wedepohl, “Distribution of the elements in some major units of the earth’s crust,” Geol. Soc. Am. Bull. 72, 175–192 (1961). T. R. Walker, P. D. Crittenden, V. A. Dauvalter, V. Jones, P. Kuhry, O. Loskutova, K. Mikkola, A. Nikula, E. Patova, V. I. Ponomarev, T. Pystina, O. Rätti, N. Solovieva, A. Stenina, T. Virtanen, and S. D. Young, “Multiple indicators of human impacts on the environment in the Pechora Basin, north-eastern European Russia,” Ecol. Indic. 9, 765–779 (2009). Yamal SPG. Estimate of the Environmental and Social Impact: a Brief Report of Results (ENVAIRON, 2014) [in Russian]. GEOCHEMISTRY INTERNATIONAL SPELL: OK Translated by M. Bogina Vol. 58 No. 4 2020