Natural and Manmade (137Cs) Radioisotopes In Holocene Sequnce of The Sheresrtobitovsky Raised Bog In The Barabinsk Forest Steppe (West Siberia)

The specic activities of natural ( 7 Be, 210 Pb) and manmade ( 137 Cs) radioisotopes in a suspended matter of snow water have been determined and the density of their fallouts on the surface of the Sherstobitovsky and Ubinsky raised bogs in the Barabinsk forest-steppe has been assessed at the points of snow sampling during winter period. It has been established that the main concentrator of 7 Be radioisotope is a ne fraction of suspended matter (particle size < 0.45 µm) consisting of colloid and dissolved component, 210 Pb atm is a coarse-grain fraction composed of dust particles (> 3 µm in size). As an example of the vertical prole of the Sherstobitosky peat bog, the rst data on the distribution of natural ( 238 U, 210 Pb, 226 Ra, and 40 K) and manmade ( 137 Cs) radioisotopes have been obtained for raised bogs of the forest-steppe zone in Western Siberia. The pattern for 226 Ra and 238 U in the nature of their vertical distribution is the same that is traced for 137 Cs and 40 K. According to our assumption, some increase in activities of these radionuclides is observed in horizons 6–12 cm, as well as in the lower part of the sequence (25–35 cm) is due to the same processes. Migration of 137 Cs deep down the peat deposit is largely governed by the magnitude of peat water level uctuations in both the seasonal and long-term cycle and it is closely related to the surface microrelief of the bog surface. Alongside this, 137 Cs, being geochemical analog of the biophilic 40 K, is absorbed by plant roots from underlying peat horizons during the dry season of a year. An additional contribution to the high activity of 137 Cs in the upper horizons is probably conditioned by input of 137 Cs to the surface of Sherstobitosky peat bog involved into atmosphere as a result of peat and forest res.


Introduction
One of the main migration roads of natural radioisotopes ( 7 Be, 210 Pb,226 Ra, 232 Th, 238 U, and 40 K) and manmade radionuclide 137 Cs is an atmospheric transportation. Radioactive elements transferred by air enter to Earth's surface among dry (dust, aerosol) and "wet" (rain, snow) atmospheric fallouts (Lal at al., 1958;Melgunov et al., 2012Melgunov et al., , 2019a. In recent studies, "atmospheric" radioisotopes 7 Be and 210 Pb are often used as indicators of atmospheric transportation processes in the composition of aerosol and dusty particles. The basis of such studies, as a rule, is determination of contents of these isotopes in the nearsurface air layer and the densities of their fallouts during a certain time interval (Alonso-Hernandez et al., 2014;Baskaran and Shaw, 2001;Mezina and Melgunov, 2019;Persson and Holm, 2014). The origin sources of 7 Be and 210 Pb are different. 7 Be (half-life (T 1/2 ) 53.4 days) is a cosmogenic radionuclide, which is formed in the upper atmosphere layers when the nuclei of 14 N and 16 O atoms interact with high-energy cosmic radiation (Lal at al., 1958). 210 Pb (T 1/2 =22.3 years) is the decay product of 222 Rn, which, in turn, is formed as a decay result of radionuclides of the 238 U radioactive series entering the atmosphere from the Earth's surface. Nearly formed 7 Be and 222 Rn atoms and their decay products are electrically charged and almost immediately after their appearance, these are captured by aerosol and dust particles and all of them fall on the Earth's surface (Melgunov et al., 2019a). Manmade radionuclide 137 Сs (T 1/2 =30.1 year) enters the atmosphere as a result of nuclear weapon tests and accidents at nuclear power plants (Gauhier-Lafaye et al., 2008;Linnik, 2018;Melgunov et al., 2012;Rhikvanov, 2009;Sapozhnikov et al., 2006).
The snow cover is an excellent surveying plane table accumulating information about atmospheric inputs of radioisotopes over a su ciently long time interval from the rst snowfall during the late autumn period to its melting in spring. It can be successfully used for assessment of the deposition density of a number of radioisotopes, including 7 Be and 210 Pb (Mezina and Melgunov, 2019;Melgunov et al., 2019a).
For retrospective estimates of atmospheric intake of radioisotopes during the pre-industrial epoch and throughout the XX century, the most suitable subjects of investigations are peat deposits of raised bogs (Bobrov et al., 2019;Efremova et al., 2002;Old eld et al., 1979;Shevchenko et al., 2008). Soil aeolation can speci cate the saturation of air masses with terrigenous matter, which is most affected by areas of arid zones of lands with weak soil and vegetation cover or its absence (deserts). Here upward of ne mineral matter (dust particles) takes place and this matter is transported by wind currents over considerable distances. In the twentieth century, the pollution of the atmosphere and, accordingly, the ow of aerosols to the Earth's surface increased sharply due to the intensive development of industry, the fuel and energy complex and performance of weapon tests. The thinnest micron (µm) fraction of aerosol micro particles contributes to the capture of chemical elements and radioisotopes (Lisitsyn, 2001). Peat deposits of raised bogs intensively accumulate mineral matter of aerosol and dust supplied from the atmosphere acting as natural lters. Buried deposits of sphagnum peat retain their radioisotope composition in near-surface strati ed horizons re ecting the geochemical characteristics of atmospheric precipitation in the past and present (Bobrov et al., 2019).
Pine-dwarf shrub-sphagnum raised bogs of the forest-steppe zone in the Western Siberia are located on the south border of the range of distribution of raised bogs where they are most affected by the climate change and anthropogenic impact. The raised bogs were formed at the end of the Sub-boreal period predominantly under wetted and cold conditions (Khazin et al., 2016;Orlova and Volkova, 1990). Currently, raised bogs of the forest-steppe zone in the Western Siberia are developed under climatic conditions of the unstable moisture zone and they have the pattern of fragments of the raised bogs embedded into eutrophic mires (Khotinsky, 1970;Stepanova and Volkova, 2017). Within the Novosibirsk Region, raised bogs are located on the area of 1800-2000 hectares and they are rare relict complexes.
The urgency of ecogeochemical and radiogeological studies of raised bog complexes in the forest-steppe zone of Western Siberia is quite evident. Previously, the authors assessed the contamination degree of biogeocoenosis components of raised bogs with heavy metals in the forest-steppe zone of the Western Siberia using geochemical criteria (Leonova et al., 2018) and with manmade radionuclide 137 Сs (Melgunov et al., 2019b). Data on atmospheric intake of 7 Be and 210 Pb radioisotopes and manmade 137 Сs radionuclide on the surface of raised bogs of the Barabinsk forest-steppe are not available in the recent publications. The data on the distribution of radioisotopes in sections of highmoor peat in the south of the forest zone in the Western Siberia are limited (Gauhier-Lafaye et al., 2008; and they are completely absent for the raised bogs in the forest-steppe zone of Western Siberia. This fact led the authors to start similar studies. The aim of this work is research study of atmospheric input of 7 Be, 210 Pb and 137 Cs to the peat deposits surface of the Sherstobitovsky and Ubinsky bogs of the Barabinsk forest steppe and the distribution of natural ( 238 U, 210 Pb,226 Ra and 40 K) and manmade 137 Cs radioisotopes.
The following tasks were solved: 1) To collect snow samples on the Sherstobitovsky and Ubinsky raised bogs, to determine the speci c activity of 7 Be, 210 Pb and 137 Cs in snow water, and to assess the density of radioisotope deposition on the surface of the studied bogs at points of snow sampling.
2) To examine the distribution of natural radioisotopes ( 238 U, 210 Pb,226 Ra and 40 K) and manmade radionuclide 137 Cs in the vertical pro le of the Sherstobitovsky bog peat deposits.
3) To elucidate speci c activities of natural and manmade ( 137 Cs) radioisotopes in geobiocoenosis components of the Ubinsky burnt raised bog, i.e., in drained peat, litter of forested phitocenoce of raised bog (litter), green and sphagnum mosses, and leaves of birch and cowberry.

Material And Methods
The most indicative, key areas of the Sherstobitovsky raised bog being in the natural, undisturbed by res, state, and the Ubinsky burnt raised bog, which is anthropogenically disturbed and having signs of in uence of amelioration and res have been selected.
The Ubinsky burnt raised bog is represented by pyrogenic birch-aspen-dwarf shrub phytocoenose. The dense arborescent stratum was formed by young aspens (Populus tremula L.) and birch trees (B. pubescens Ehrh.). The grass-dwarf shrub stratum with the total projective coverage up to 70% was formed by Ledum polustre L.) (30%), cowberry (Vaccinium vitis-idaea L. (25%), and, to a lesser extent, by bog blueberry (Vaccinium uliginosum (L.), and sedge (Carex limosa L.). The moss cover of hypnaceous (green) mosses was fragmentary. The main peat-forming plants in layers of the high moor peat in the Ubinsky burnt and in Sherstbitovsky raised bogs were sphagnum mosses: Sphagnum fuscum and S. magellanicum. In the Ubinsky burnt raised bog, the ground vegetation of sphagnum mosses is currently disappeared under the in uence of anthropogenic factors (amelioration and res).
A peat core with undisturbed strati cation up to the depth of 40 cm was obtained on the Sherstobitovsky raised bog using BTG-1 sampling drill. The peat core hermetically packed into a plastic pipe was stored in the cold storage facility until chemical analyses were carried out.
In mid-March 2018, representative snow samples were collected in order to assess the level of atmospheric intake of radioisotopes 210 Pb atm ,7 Be and 137 Cs on the surface of studied raised bogs. The snow samples were taken to the entire depth of the snow cover from a xed sampling area. This made it possible to obtain averaged information on atmospheric precipitation over the 4-month winter period of stable snow accumulation.
The volume of these samples was 40 liters of snow, from which, up to 20 liters of snow water were obtained during melting. The sample preparation for analysis was in settling the snow water until precipitation of a coarse fraction of the suspended substance with subsequent decantation (draining the solution from the precipitate). Then, subsequent ltration of the decanted solution was performed in order to separate the ne solid fractions trough two lters: "blue ribbon" (average pore size was 3 µm) and the size of membrane lter was 0.45 µm. This made it possible to separate two components of suspended matter represented dust particles of different sizes of more than 3 µm and from 3 to 0.45 µm. The separation of a fraction containing nanoparticles with the sizes less than 0.45 µm of colloidal and dissolved components was carried out by evaporation of the solution ltered through a membrane lter to a dry residue.
For radioecological studies on the Ubinsky burnt raised bog anthropogenically disturbed by res, indicator species of plants (green and sphagnum mosses, birch and cowberry leaves) were selected. Sampling of litter and drained peat soil (upper 5 cm) was carried out with standard steel ring (82 mm in diameter, 50 mm in height, and 264 cm 3 in volume).
Activity determination of the uranium series radioisotopes ( 238 U ( 234 Th), 226 Ra, and 210 Pb), 137 Cs, and 40 К in peat samples (in the suspended matter of snow water, 7 Be in addition) was performed by high-resolution semiconductor gamma-ray spectrometry using HPGe of low-background well-type detectors EGPC192-P21-R (EURISYS MESURES) and GWL-220-15 (ORTEC). The workspace of detectors was ~ 200 cm 3 , the resolution was better than 2.1 keV with the release of energy 1.33 MeV. The measurements were carried out at the Analytical Center for multi-elemental and isotope research SB RAS. The technique described in detail in (Gavshin et al., 2005;Melgunov et al., 2003) takes into account the in uence of a number of interfering factors: measurement geometry, density of an analyzed sample, the overlap of interfering peaks and the effect of cascade summation of analytical gamma lines. The following analytical gamma lines were used for determination of radioisotopes: 210 Pb for 46.5 keV, 226 Ra for 186.1 keV, 238 U ( 234 Th) for 63.3 keV, 137 Cs for 661.7 keV, 40 К for 1460.8 keV, and 7 Be for 477.6 keV. The lower detection limit detection of these radioisotopes was 0.02 Bq. The duration of measurement of a single sample varied from 12 to 48 hours and it was chosen proceeding from the need to obtain a statistical determination error of the areas of analytical photo peaks no worse than 5%. The accuracy and reproducibility of the analysis were controlled by measurements of national geological reference samples SG-1A, SG-3, SG-2, DVG, DVT, ZUK-1, BIL-1, ST-1A (Govindaraju, 1994).

Results And Discussion
Radioisotope composition of atmospheric fallouts (snow water) Based on the obtained analytical data on the content of 210 Pb atm ,7 Be and 137 Cs in the studied snow samples, the speci c activities and densities of radioisotope fallout at the sampling points on the Sherstobitovsky and Ubinsky burnt raised bogs were calculated. The results obtained correspond to the total atmospheric input of radioisotopes during the 4-month winter period of the stable snow accumulation. All data are linked to the date of snow sampling on March 11, 2018 (Table 1).
Тable 1 Speci c activities of suspended matter of snow melt water and the fallout density of radioisotopes at the snow sampling points on the Sherstobitovsky and Ubinsky burnt raised bogs 1 -coarse-grained fraction (matter particles > 3 microns retained after decantation); 2 -fraction consisted of particles > 3 microns in size (the matter settled during ltration of snow melt water on a "blue-ribbon" lter); 3 -highly dispersed fraction consisted of particles 0.45-3 microns (the matter settled during successive ltration trough membrane lte; 4 -highly-dispersed fraction consisted of particles < 0.45microns (colloidal and dissolved components); * -snowmelt water.
Speci c activities of 210 Pb atm ,7 Be and 137 Cs in snow water with respect to the contribution of all granulometric fractions of suspended matter for snow samples taken at Sherstobitovsky raised bog are as follows: 345, 505 and 5.77 mBq/l, and at Ubinsky burnt raised bog 383, 484 and 2.08 mBq/l, respectively.
The calculated values of 210 Pb atm precipitation densities on the Sherstobitovsky (36.9 Bq/m 2 ) and on the Ubinsky burnt (39.3 Bq/m 2 ) raised bogs, as well as 7 Be on the Sherstobitovsky (53.9 Bq/m 2 ) and the Ubinsky burnt (50.0 Bq/m 2 ) raised bogs were similar in values despite the farness distance of these raised bogs from each other by 110 km. This suggests that the input of these radionuclides in the composition of snowfalls was fairly uniform in the area. With respect to the short half-life period of 7 Be, the real density of its precipitation during winter period will be signi cantly higher than the values given in table 1. As it may be supposed conditionally that the input of 7 Be was uniform in time, then the density of its precipitation for entire period of snow accumulation with reference to radioactive decay, it can be estimated according to the technique (Aba et al., 2016) at the level of 112 Bq/m 2 for the Sherstobitovsky raised bog and 104 Bq/m 2 for the Ubinsky raised bog. It should be noted low speci c activities of 137 Cs in the suspended matter of snow water of the Sherstobitovsky and Ubinsky burnt raised bogs such as 5.77 and 2.08 mBq/l respectively as compared with speci c activities of 210 Pb atm and 7 Be. This indicates insigni cant contemporary intake of this manmade radionuclide from the atmosphere to the Earth's surface. It should be noted that speci c activities of 137 Cs in the studied samples are close to the detection limit of the used analytical method. Consequently, the error of their determination often exceeds the value of 20-30%, which makes it impossible to perform statistical interpretation of the data for 137 Cs in conjunction with 210 Pb atm and 7 Be.
The share contribution of granulometric fractions of suspended matter of snow water into the common speci c activity of 210 Pb atm and 7 Be has being studied ( Table 2). 1-2 is sum of fractions, where 1 is the matter composed of particles with sizes > 3µm remained after decantation and 2 is the matter settled during ltration of snow water on a lter "blue ribbon"; 3 is ne fraction of particles with sizes 0.45-3 3µm (matter settled during sequential ltration through membrane lter); 4 -ne fraction with particles < 0.45 3µm (colloidal and dissolved components).
The comparative analysis of contributions of granulometric fractions showed that: 1) Most of 7Be (46-57%) is associated with a ne fraction of suspended matter consisted of particles with sizes of less than 0.45 3µm. In this fraction, the speci c activity of 7 Be is 5.8-6.2 times higher than that of 210 Pb atm (Table 1). The share of the coarse-grained fraction of a suspended matter in the total activity of 7Be is 38% for Sherstobitovsky raised bog and that is 48% for the Ubinsky burnt raised bog.
2) The share of the fraction of suspended matter consisted of particles with sizes from 0.45µm to 3 3µm is so small just as for 210 Pb atm , and so 7 Be in all studied samples of snow water and it is no more than 11%.
3) The main share of 210 Pb atm (77-79%) in the studied samples of suspended matter in snow water is in the composition of a coarse-grained fraction mainly represented by dusty particles of various sizes.
Thus, the main concentrator of the 7 Be radioisotope is a ne fraction consisted of colloids and a dissolved component (fraction particles < 0.45 µm), and 210 Pb atm is concentrated mainly in a coarse-grained fraction of suspended matter consisted of dust particles (> 3 3µm).
The important value characterizing the source of radioisotope intake on the Earth's surface is the 7 Be/ 210 Pb atm ratio (Melgunov et al., 2019a). The data presented in Table 1  0.45 µm (6.2 and 5.8). With respect to the short half-life of 7 Be, it should be assumed that such particles succeed to the initial values of 7 Be/ 210 Pb atm ratios, which are inherent to them initially when they are located in the upper horizons of the soil cover.
Based on the data given in Tables 1 and 2, it can be stated that the separation according to granulometric fractions of the suspended matter in snow water shows that the studied radioisotopes are present in all isolated fractions (from the largest particle sizes > 3 µm to the smallest ones < 0.45 µm associated with nanoscale aerosol particles, colloidal and dissolved component). Speci c activities of 137 Cs in suspended matter of snow water of both the Sherstobitovsky and Ubinsky burnt raised bogs (5.77 and 2.08 mBq/l respectively) are signi cantly less as compared to those 210 Pb atm and 7 Be.This indicates the resent insigni cant contents of this radionuclide in the atmosphere.
The distribution of radioisotopes in the vertical pro le of the Sherstobitovsky burnt bog For the rst time, the data on distribution of natural ( 238 U, 210 Pb,226 Ra, and 40 K) and manmade ( 137 Cs) radioisotopes in the section of the Sherstobitovsky peat bog have been obtained (Table 3). Its peat core is composed of highmoor peat. The upper layer is formed mainly by remains of sphagnum mosses (S. magellanicum and S. angustifoliun) to the depth of 25 cm, the layer 25-35 cm is formed by pinesphagnum peat (pine remains make up 30%), and the layer 35-40 cm is represented by pine peat. Mass contents of U and K were calculated in terms of speci c activities of 238U and 40K judging from the following ratios: activity 1 mg U corresponds to 12.432 Bq of U 238 ; 1% K corresponds to 303.1 Bq of 40 K.
The shape of the 210 (Appleby et al., 1997;Bao et al., 2010;Bobrov et al., 2019;Clymo et al., 1990;El-Daoushy et al., 1982;Gavshin et al., 2004;Holynska et al., 1998;Old eld et al., 1979). 210 Pb (half-life T 1/2 =22.3 years) is a natural radioactive element included into the 238 U radioactive series. The source of Pbex in the atmosphere is 222 Rn. Radon ( 222 Rn), being a decay product of 226Ra and gas, emits from the Earth's surface. Having a half-life (T 1/2 ) of 3.82 days, 222 Rn decays into 210 Pb over a short time period through a series of short-lived isotopes. At the moment of formation, radon atoms and their decay products are electrically charged. This leads to their rapid sorption by aerosol and dust particles in the atmosphere. In the composition of these particles, Pbex gradually is settled on the Earth's surface. This process is continuous. Settled on the surface of strati ed formations such as lake sediments or peat deposits, Pbex is gradually superposed by younger formations. In the absence of processes leading to vertical migration, the further behavior of buried Pbex is governed by the law of its radioactive decay. This provides the basis of methods for dating strati ed formations by nonequilibrium 210 Pb. As a rule, the model of permanent stream of Pbex (CRS is Constant Rate of Supply) is used for dating the upper horizons of ombrotrophic peat bog deposits (Appleby, 2002;Appleby and Old eld, 1978). The CRS model, as applied to the dating of peat deposits, is assumed the presence of two main initial conditions, namely: 1) The rate of Pbex precipitation from the atmosphere is the constant magnitude throughout the entire period of material accumulation; 2) There is no signi cant vertical redistribution of Pbex after its burial. The model takes into account the possibility of changing the rate of biomass growth over the entire dating period.
According to the CRS model, the age of the lower surface of the horizon being dated is calculated by the formula (Appleby, 2002;Sanchez-Cabeza and Ruiz-Fernández, 2012): where i is the number of the dated horizon, λ is the decay constant of 210 Pb ( . The cumulative supply is calculated through speci c activities of 210 Pb de ned for each horizon of the dated section by applied analytical method with reference to the data on the density of dry matter in the studied peat bog material. The activity of a nonequilibrium (atmospheric) Pbex is equal to the difference between the total activity of 210 Pb in the sample and the activity of 210 Pb being in balance with the parent 226 Ra. Complementary to dating the upper horizons of a peat deposit, the CRS model provides an estimate of dry material accumulation rates, which can be determined using the formula: where C(i) is the speci c activity of Pbex on the surface separating i and i + 1 dated layers.
The results of calculation of age-related characteristics for the upper part of the Sherstobitovsky peat bog are given in Table 4 and in Fig. 2.  Fig. 2, there is observed a peak of activity in the lower part of the section, the upper limit and maximum of which is quite likely corresponds to the age-related verge.
Some blurring of the peak is probably associated with redistribution mechanisms of 137 Cs in connection with signi cant uctuations in the groundwater level (GWL) of both seasonal and annual ones, which are speci c for a zone of the unstable heat and water supply as described in (Bogs of Western Siberia…, 1976) According to data from the Barabinsk weather station, the increase in water cut of a bog under conditions of the directed decrease in temperature and the increase in precipitation during the period from 1967 to 1986 years could contribute to the displacement of this peak, that is, the diffusion of 137 Cs into the underlying layers of the peat deposit (All-Russian Research…, 2020). Under conditions of cooler and wetter climate, the duration of nding the upper horizons in a wet state increases, which should contribute to diffusion of 137 Cs into the lower layers of the peat. The diffusion of 137 Cs occurred in the layer of sphagnum peat, sharply decreasing, judging by its activity, in the layers of pine-sphagnum and pine peats.
In Holarctic is peculiar to the peat layer deposited during the period of mass nuclear tests (Clymo, 1984). In the southern taiga subzone of Western Siberia, under conditions of a more humid climate compared to the forest-steppe, this peak of 137 Cs (accepted as in 1963 year) is located at different depths of peat deposits.
In this case, the tendency for dependence of its depth occurrence on the degree of stability of the water regime and activity of peat accumulation is identi ed. On native actively growing bogs, 137 Cs is found at depths of 31-43 cm on naturally and anthropogenically drained ones at 16-21 cm, and on highly dehydrated ones at 9-12 cm , which is similar to the upper peak of 137 Cs in peat deposits of the Sherstobitovsky raised bog. This similarity is explained by the fact that the core of this raised bog was sampled: 1) on the peripheral part of the bog, the water regime of which was also somewhat disturbed by the building a bypass channel of the reclamation system, 2) in the depression between sphagnum hummocks, where the peat growth was usually very low. However, the difference of this core from those of the south taiga is the presence of two 137 Cs peaks, which does not allow us to date the upper peak at 8 cm to 1963.
It is known (Efremova et al., 2020) that the effect of uctuations of groundwater levels is the main reason governing the boundary of vertical migration of 137 Cs in peatlands. According to the data from the Barabinsk weather station (All-Russian Research…, 2020) during the period from 1987 to 2009 years, air temperatures were increased and the amount of liquid precipitation, just as average annual and so summer ones was decreased. Under conditions of climate aridization, uctuations in the groundwater level (GWL) were intensi ed and this level was sharply decreased during warm seasons of the year. In contrast to the cooler and wetter period in 1967-86 years, when transpiration by helophytes (evaporation of water through leaf stomata) was less active, the sharp increase in transpiration under hot and dry conditions causes active suction of peat water and biophilic transfer of trace elements by plant roots from the lower peat horizons. It is common knowledge that 137 Cs replaces biophilic 40 K element in biochemical cycles of plant organic matter production (Kabata-Pendias and Pendias, 1989). This replacement process is especially active in the raised bogs, which are characterized by potassium de ciency. Therefore, it is likely that the peak of 137 Cs at the depth of 8 cm is largely due to the transfer of the main amount of 137 Cs deposited in 1949-1963 to the overlying horizons. This also explains the low activity of the lower 137 Cs peak. This replacement process is especially active in the high-moor peats for which the potassium de ciency is intrinsic. Therefore, it is likely that the peak of 137 Cs at a depth of 8 cm is largely due to the transfer of the main amount of 137 Cs deposited in 1949-1963 years into the overlying horizons. This explains the low activity of the lower peak of 137 Cs. According to this assumption, the different nature of 137 Cs redistribution in the peat deposits of the Barabinsk forest-steppe during various time periods is largely due to the difference of their climatic parameters.
The assumption of a signi cant impact of uctuations of a peat water level on the 137 Cs migration is supported by two more facts.
Firstly, according to the data obtained with the help of the Automatic UPV Recorder (AKR4-logers), signi cant seasonal uctuations of the groundwater level (GWL) of the Sherstobitovsky raised bog from September 2017 to September 2018 were revealed. During the snowmelt period, the GWL was raised from 80 cm (March 2018) to 35 cm (April) and it continued to rise to 10 cm from the surface (May). In summer, the GWL was reduced with speci c sharp rises after precipitation and gradually decreased due to evaporation and runoff up to 30 cm. The highest values of GWL standing in the Sherstobitovsky raised bog up to 10 cm from the surface were recorded during the spring period at the peat core sampling point, which corresponds to the upper peak of 137 Cs.
Secondly, the nature of the vertical distribution of the 40 K isotope (Table 1 and Fig. 2) is a geochemical analog of Cs, which almost completely coincides with the distribution of 137 Cs. Here, also, at the general low level of potassium contents (< 25 Bq/kg of 40 K, with conversion to K < 0.08%) in horizons 4-12 cm and 22-30 cm, increased 40 K activities reaching in the peak 114 Bq/kg and 49 Bq/kg, respectively, are observed. During summer period, when increasing transpiration (evaporation) of water through stomata of plant leaves takes place, the suction of peat water and its active transport by plant roots occurs along with biophilic 40 K and 137 Cs from the lower peat horizons.
Thus, signi cant GWL uctuations stipulate just as blurring, so displacement of the 137 Cs activity peaks, that should be taken into account when they are used for dating peat deposits.
In addition, frequent res should be contribute to the increase of the speci c 137 Cs activity under conditions of climate aridization. It is known that almost all the raised bogs of the Barabinsk forest-steppe were subjected to multiple res. Increasing ash-contents of the upper 15-cm peat layer in the Sherstobitovsky raised bog from 4.3 to 6.0-9.6%, in the absence of embers as traces of res on the site of the raised bog, indicates the contamination of the Baraba atmosphere. This cannot be explained only by global anthropogenic pollution of the atmosphere as since 1985 it began to be puri ed as well as over Western Siberia . The signi cant burning of peat in the bogs of the Barabinsk foreststeppe, in particular in the Ubinsky burnt raised bog (Leonova et al., 2018), is indicated by an increase in the ash content in the surface layers of burnt peat in 0-22 cm from 3.7 to 14.5%, on average, to 6.4%, respectively. This is also evidenced by the occurrence of peat layers with the maximum activity of 137 Cs  Bq/kg) directly on the surface (0-6 cm) in the burnt area of the Ubinsky burnt raised bog.
Therefore, an additional contribution to the high activity of the 137 Cs in the upper peak in a core from the Sherstobitovsky raised bog is quite likely due to secondary fallout of this radioisotope from the atmosphere as a result of res.
The and soil, are usually, as a rule, greater than 1 (Aleksakhin et al., 1990;Markose et al., 1993). Moreover, it was previously shown (Noskova and Shuktomova, 2010) that the highest BAC values are observed in woody species: mountain ash (Sorbus aucuparia) and pubescent birch (Betula pubescents), the concentrations of radionuclide in which can exceed its concentration in the soil up to 1575 and 399 times, respectively. For herbaceous species, the BAС are lower, but they can reach values signi cantly greater than 1. For example, for reweed (Chamaenerium angustifolium), BAС can reach up to 14.8 and for wild angelica (Angelica silvestris) up to 1.98. Thus, vegetation, especially some of the woody species, can be attributed to concentrators of 226 Ra. For 238 U, BAС > 1 are not typical. The BAC of 226 Ra exceed that of 238 U by 1-2 orders of magnitude for herbaceous vegetation species and by 2-4 orders of magnitude for woody species. From all of the above, it follows that under natural conditions, vegetation will accumulate 226 Ra much more intensively than 238 U. Table 5 shows the results of determining the contents of radioactive elements in the biogeocoenosis components from the Ubinsky burnt raised bog sampled on the background (unburned) site. The data given in Table 5 show that radioactive disequilibrium in favor of 226 Ra in the coupe U-Ra is observed for all studied samples. In the forest oor and in the upper sod horizon of soils, 226 Ra is 1.59 times greater than its equilibrium contents. Still greater excess of 226 Ra is observed in green moss Polytrichum commune Hedw. (1.7) and in sphagnum moss Sphagnum magellanicum (2.13). As it was expected, the highest values of Ra/U ratios are found in terrestrial vegetation: this ratio in birch leaves is obviously greater than 10, and that in red bilberry leaves is 14.7. Thus, in all components of the phytocoenosis, the remains of which form a peat deposit, the signi cant excess of 226 Ra is observed relative its equilibrium contents. Having regard to the long half-life period of 226 Ra constituting ~ 1500 years, such state can be kept in the course of very extended time period (5-6 half-life periods of radium), provided that there are no processes leading to signi cant migration of uranium and radium. It is seen from the data given in  210 Pb situated in this horizon is extremely related to the radioactive decay of 226 Ra. The assemblage of these facts shows that during the last 250 years at least, there were no processes leading to signi cant removal or input of radioactive elements within the region under study. It should be noted that for both 226 Ra and 238 U in the nature of their vertical distribution, the same regularity is traced as it is for 137 Cs and 40 K. In the horizons 6-12 cm, as well as in the lower part of the section (25-35 cm), the increase of activities of these radionuclides is observed. This indicates that the curvatures of the vertical distribution of these radioactive elements in the Sherstobitovsky peat bog were formed at the expense of the same processes.

Conclusion
The speci c activities of 210 Pb atm ,7 Be, and 137 Cs in snow water with reference to the contribution of all granulometric fractions of suspended matter for snow samples selected on the Sherstobitovsky raised bog amount to 345, 505, and 5,77 mBq/l, and those on the Ubinsky burnt raised bog amount to 383, 484, and 2.08 mBq/l, respectively. The speci c activities of 137 Cs are signi cantly less as compared with those of 210 Pb atm and 7 Be, which testi es the insigni cant present-day contents of this radionuclide in the atmosphere.
The calculated values of 210 Pb atm fallout densities on the Sherstobitovsky (36.9 Bq/m 2 ) and on the Ubinsky burnt (39.3 Bq/m 2 ) raised bogs as well as 7 Be on the Sherstobitovsky (53.9 Bq/m 2 ) and the Ubinsky (50.0 Bq/m 2 ) raised bogs are close to each other in the values despite the vast distance at 110 km. This suggests that the ingress of these radionuclides in the composition of snowfalls was fairly uniform over the area.
The equity contributions of granulometric fractions of the suspended matter from snow water to the total speci c activity of 210 Pb atm and 7Be have been established. The most part of 7Be (46-57%) is associated with a ne fraction of suspended matter consisting of nanosized aerosol particles (< 0.45 µm), colloids, and a dissolved component. The main share of 210 Pb atm (77-79%) is concentrated mainly in the coarsegrained fraction of suspended matter composed of dust particles (> 3microns). The share of suspended matter with sizes of particles from 0.45 to 3 µm is small just as for 210 Pb atm , so for 7 Be in all studied samples of snow water is no more than 11%.
The age estimation of the upper part of Sherstobitovsky bog performed with respect to nonequilibrium 210 Pb allowed the fact of redistribution of 137 Cs and 40 K with the shift to the upper horizons to be established. Such redistribution can be associated with three processes: seasonal GWL uctuations, suction of water trough leaf stomata by roots of helophytes with increasing transpiration (evaporation) of water through the leaf stomata in the dry year season, and additional intake of 137 Cs implicated into the atmosphere as a result of peat and forest res.
For the studied peat deposit of the Sherstobitovsky raised bog, there is observed a violation of the radioactive balance between 238 U and 226 Ra in favor of the latter. In depth of the studied section, the 226 Ra/ 238 U ratio has values greater than unity varying within the range from 1.3 in the horizon 0-2 cm to 11.2 in the horizon (18-20 cm). The same values of the 226 Ra/ 238 U ratios are speci c also for the primary components of the biogeocoenosis forming the peat deposit.

Declarations
Authors Contributions Galina Leonova: material preparation, sample and data analysis, and writing the original manuscript. Michael Melgunov and Yulia Preis: review and editing and supervision. Ksenia Mezina: led investigation and sample analysis. Anton Maltsev: review and editing. Alexei Shavekin and Maxim Rubanov helped in sample collection and chemical analysis.
Availability of data and materials Data and material is available for research purpose and for reference.
Funding Work is done on state assignment of IGM SB RAS with partial nancial support of the regional project of the RFBR No. 17-45-540063 r_a "Integrated geoecological monitoring of the raised bogs in the Barabinsk forest-steppe of Western Siberia (Novosibirsk region)". The analytical work was carried out at Analytical Center for multi-elemental and isotope research SB RAS.
Ethics approval This manuscript has not been published or presented elsewhere in part or in entirety.
Consent to participate Not applicable.
Consent for publication All authors agreed to publish this article in Environmental Science and Pollution Research.
Competing interests The authors declare no competing interests.

Figure 1
Reconnaissance map of the study region (Barabinsk forest steppe, Novosibirsk Region). 1 -Ubinsky burnt raised bog (Ubinsky District), 2 -Sherstobitovsky raised bog (Chulym District) Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.