Use of natural and artificial radionuclides to determine the sedimentation rates in two North Caucasus lakes

Journal Pre-proof Use of natural and artificial radionuclides to determine the sedimentation rates in two North Caucasus lakes Natalia V. Kuzmenkova, Maxim M. Ivanov, Mikhail Y. Alexandrin, Alexei M. Grachev, Alexandra K. Rozhkova, Kirill D. Zhizhin, Evgeniy A. Grabenko, Valentin N. Golosov PII:

S0269-7491(19)35176-0

DOI:

https://doi.org/10.1016/j.envpol.2020.114269

Reference:

ENPO 114269

To appear in:

Environmental Pollution

Received Date: 10 September 2019 Revised Date:

15 February 2020

Accepted Date: 24 February 2020

Please cite this article as: Kuzmenkova, N.V., Ivanov, M.M., Alexandrin, M.Y., Grachev, A.M., Rozhkova, A.K., Zhizhin, K.D., Grabenko, E.A., Golosov, V.N., Use of natural and artificial radionuclides to determine the sedimentation rates in two North Caucasus lakes, Environmental Pollution (2020), doi: https://doi.org/10.1016/j.envpol.2020.114269. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

1 2 3 4 5

Use of natural and artificial radionuclides to determine the

6

sedimentation rates in two North Caucasus lakes

7 8 9

Natalia V. Kuzmenkova1,2,3*, Maxim M. Ivanov1,4, Mikhail Y. Alexandrin1, Alexei

10

M. Grachev1, Alexandra K. Rozhkova2, Kirill D. Zhizhin5, Evgeniy A. Grabenko6,

11

Valentin N. Golosov1,4

12 13 1

14 2

15 16 17

3

Institute of Geography RAS

Chemistry Faculty, Lomonosov Moscow State University

Vernadsky Institute of Geochemistry and Analytical Chemistry RAS 4

Geography Faculty, Lomonosov Moscow State University 5

18

Laboratory for Microparticle Analysis

6

19

Maykop State Technology University

20 21 22 23 24 25 26 27 28 29

*Corresponding author.

30

e-mail address: [email protected]

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Postal address: : Lomonosov Moscow State University, Chemistry Dep. Leninskie gory 1, bld. 3,

32

119991, Moscow, Russia

33

Abstract The specific activities of natural (210Pb,

34 241

226

Ra, and

232

Th) and artificial (137Cs,

239,240

Pu,

35

and

Am) radionuclides in the sediments of two North Caucasus lakes were determined. The

36

two lakes, Lake Khuko and Lake Donguz-Orun, differ in their sedimentation conditions. Based

37

on the use of unsupported

38

chronological markers, it was established that the sedimentation rates in Lake Khuko over the

39

past 55-60 y did not exceed 0.017 cm y-1. Sedimentation rates in Lake Donguz-Orun were found

40

to be more than an order of magnitude higher. In the latter case, the sedimentation rates for the

41

period from 1986 to the present were over 1.5 times higher than they were for the period 1963-

42

1986. The differences in sedimentation rates were due to differences in the rates of denudation of

43

their respective catchment areas. The specific activities of artificial radionuclides (137Cs, 2600

44

Bq kg-1;

45

Khuko show that their deposition was mainly due to global stratospheric fallout of technogenic

46

radionuclides associated with nuclear bomb testing during 1954–1963—rather than fallout from

47

the Chernobyl accident. Several factors, including the mode of precipitation, features of the

48

surface runoff, and location of Lake Khuko, were responsible for the accumulation of artificial

49

radionuclides.

239,240

210

Pbex and both Chernobyl-derived and bomb-derived

Pu, 162 Bq kg-1; and

241

137

Cs as

Am, 36 Bq kg-1) and their ratios in the sediments of Lake

50 51

Keywords: sedimentation rate, sediment pollution, 137Cs, 239,240Pu , bomb-derived fallout,

52

Caucasus lakes

53 54

Summarizes the main finding

55

It was determined that global warming is the primary reason the denudation rates in the periglacial zone are increasing. The possible sources of the radioactive contamination of the Khuko lake sediment were also determined.

56 57 58 59

Introduction

60

The radionuclides deposited on the Earth’s surface are quickly and firmly fixed by

61

bottom sediments. This makes it possible to trace the history of the influence of radioactivity

62

from both natural and artificial sources on the Earth’s surface. The fallout radionuclides (137Cs,

63

210

64

changes in the levels of radiation in lakes during periods of extensive use of the atom bomb

65

(Appleby 2008; Jweda and Baskaran, 2011). They are also employed in studies of the vertical

66

and horizontal movements of water and air masses and the geochemical transfer through

67

sediments at river-sea, ocean-atmosphere, stratosphere-troposphere, and ocean floor boundaries

68

(Huang et al., 1999; Joshi et al., 1988; Silker, 1972; Waser and Bacon, 1995; Morgenstern et al.,

Pbex) are widely used to reconstruct both sedimentation rates during specific time periods and

69

1996; Buesseler, 1997; Buesseler et al, 1997; Smith et al., 1998; Sayles et al., 1998; Golosov,

70

2002). The efficacy of using a particular radionuclide as a tracer is determined by its source and

71

input function, the characteristics of the geochemical cycle, and its half-life (Froehlich, 2010;

72

Wan et al., 1987). Useful information is obtained by studying the ratio of the radioactivity between the

73 74

members of a natural series. Goldberg (1963) proposed a excess

210

75

(T1/2 = 22.3 y) based on the differences in radioactivity between

226

76

atom –

77

nuclear bomb testing during 1954-1963 (Appleby, 2008; Walling and Navas, 1992; Walling and

78

He, 1999). The vertical distribution of

79

allowed the determination of the sedimentation rates during two time windows: 0.56-1.1 cm y-1

80

during the 1963-1972 and 0.08-0.9 cm y-1 during the 1954-1973 (Pennigton et al., 1973). Two

81

chronological markers were used for the determination of the sedimentation rates in Lake

82

Michigan. It was also established that using

83

information about sedimentation rates (Robbins et al., 1975). After 1964, the bomb-derived 137Cs

84

fallout rapidly declined, and since 1971, it has stabilized at a low constant level. In 1986, as a

85

result of the Chernobyl accident,

86

fallout significantly changed the pattern and levels of the contamination in many areas of Europe

87

(De Cort et al., 1998; Wallbrink and Murray, 1993; Vanden and Gulinck, 1987; Sutherland and

88

de Jong, 1990). Due to the presence of the Chernobyl-derived and bomb-derived peaks in the

89

137

90

1998; Edgington et al., 1991; Playford et al, 1990; Appleby et al., 2000, 2008; Baskaran et al.,

91

2014; Zapata, 2003; Benoi et al., 2001; Zapata, 2003; Benoi et al., 2001; MacKenzie et al.,

92

2011). It has been established that the profiles of both 210Pbex and 137Cs can be used to determine

93

the sedimentation rates in lakes of various sizes (area and depth) and water regimes (Appleby,

94

2008). Studies of sedimentation rates in mountain lakes in Europe (from the Finnish Lapland

95

(Saanajärvi, 69°5' N 20°52' E) to the Spanish Pyrenees (Redó, 42°39' N 0°46' E)) show that the

96

sedimentation rates are relatively low (approximately 0.01-0.02 g cm-2 y-1) and that there was

97

relatively uniform pre-1963 accumulation (Appleby, 2000). A potential problem with dating

98

mountain lakes is the impact of seasonal changes on the uniformity of supply rates. During

99

winter, the water column is isolated from the natural atmospheric

222

Rn. An artificial radionuclide

137

137

137

Pb (210Pbex) dating method Ra and its daughter radon

Cs was used as a chronological marker following the

Cs in the bottom sediments of five lakes in England

137

Cs and

210

Pb together provides more detailed

Cs was released into the atmosphere, and the corresponding

Cs vertical distributions, they are both used to determine sedimentation rates (Wan et al.,

210

Pb flux. Fallout onto the

100

lake and its catchment during this period is locked up in snow and ice and released only at the

101

time of the spring thaw (Appleby, 2000).

102

Atmospheric fallouts of artificial radionuclides on the European territory of Russia have

103

originated from a number of sources, most of which are from events that occurred during the

104

mid-20th century (Novaya Zemlya, Semipalatinsky, Kapustin Yar, Totsky polygon nuclear tests,

105

and the 1986 Chernobyl accident).The study of isotopic ratios is the main method of identifying

106

sources of radionuclide contamination. A significant amount of data has been obtained by

107

studying the ratios

108

240

109

2000; Cooper et al., 2000; Cagno et al., 2013, Everett et al., 2008).

137

Cs/90Sr,

239,240

Pu/137Cs,

238

Pu/239,240Pu,

237

Np/239Pu,

241

Am/239Pu, and

Pu/239Pu (Sayles et al., 1997; Trapeznikov et al., 1993; Hardy et al., 1973; Cochran et al.,

110

Mountain lakes best reflect the pollution of the upper atmosphere and the enrichment of

111

the chemosphere by toxic elements and radionuclides for the following reasons: 1) their water

112

quality is primarily due to atmospheric fallouts; 2) the effect of atmospheric fallouts on

113

watersheds is large due to slight or absent soil–vegetation cover; 3) low water temperature and

114

its ultra-fresh and oligotrophic nature inhibit the ability of water bodies to self-purify; and 4)

115

there are no other direct sources of water pollution (agricultural or industrial wastewaters). These

116

advantages make it possible to assess the effects of transboundary pollutant transport in the

117

atmosphere on lakes (Moiseenko et al., 2012).

118

The Caucasian mountain range is an important circulation barrier that enables the

119

accumulation of man-made radionuclides, because the latter are transported by air masses and

120

precipitation-induced fallout (Kordzadze et al., 2013). Therefore, it is a natural trap for various

121

pollutants that are transported by air masses from Europe to Asia and from the Middle East and

122

North Africa to Eastern Europe and Western Siberia. However, the radiochemistry of the soil of

123

the Caucasus is poorly understood. The research focus thus far has been on the Caucasus

124

agricultural land (Buraeva et al, 2015; Urushadze & Manakhov, 2017). The studied sections of

125

the Caucasus fall within an area with a contamination density of 10–40 kBq m-2 on the European

126

Chernobyl accident 137Cs contamination map (De Cort et al., 1998). Artificial radionuclides have

127

been found in the glacier cryoconites in Georgia and have been studied in detail recently (Lokas

128

et al., 2018). Natural radionuclides in the Caucasus region have also been used both to study

129

natural geological processes and for earthquake prediction (Feyzullayev et al., 2005;

130

Cherdyntsev et al., 1968; Tsvetkova et al, 2001).

131

The objectives of this study are threefold: 1) evaluation of the sedimentation rates for the

132

two periods (1963-1986 and 1986–2018) in Lake Khuko and Lake Donguz-Orun using

133

technogenic (137Cs) and natural (210Pb,

134

assessment of the specific activity of a number of other technogenic radionuclides (241Am,

135

238,239,240

136

sediments of the studied lakes; and 3) identification of different sources of sediment

137

contamination of Khuko Lake.

138

226

Ra) radionuclides as chronological markers; 2)

Pu) associated with mid-20th century nuclear tests and accidents that are in the bottom

139

Study sites

140

The two studied lakes are Lake Khuko and Lake Donguz-Orun, which differ in their

141

sedimentation conditions. They are situated within the western (Lake Khuko) and central (Lake

142

Donguz-Orun) sectors of the Caucasus mountain system.

143

Lake Khuko (43°56′18″ N 39°48′12″ E) is a mountain lake situated in the Caucasus

144

natural reserve (Fig. 1) within the mid-latitude belt at 1,740 m above sea level (a.s.l.) The lake’s

145

surface area is 27,500 m2. Its length is 260 m. Its width is 150 m, and its maximum depth is 10

146

m. The area of the lake’s catchment is approximately 120,000 m2 (Fig. 2A). The altitude of the

147

main Caucasus ridge adjoining the lake varies from 1,700-1,900 m a.s.l. The slopes are

148

primarily covered by beech forest. The oval-shaped Lake Khuko is situated within one of the

149

kettles, which are closed distinctive depressions. The lake’s shoreline isn’t indented. The

150

elevation of the surrounding slopes varies from 5–100 m. The microclimate of the lake kettle is

151

rather severe, with relatively low temperatures and high snow accumulation during the cold parts

152

of the year. The snow cover is preserved until the end of June, although ice floes can be still seen

153

in July (Efremov, 1991). The lake’s kettle is a seasonally pronounced deposition environment for

154

atmospheric aerosols.

155

According to the data of the Caucasian Reserve Dzhuga (2000 m a.s.l.) meteorological

156

station, the average annual air temperature on the western part of the Greater Caucasus Mountain

157

Range (GKH) northern slope was 3.70 °С since 1985. There was a decline in the absolute

158

minimum temperature from 1987–2007 and a rise in this parameter from 2008–2015. However,

159

there was little change in the extrema. In the Western Caucasus subalpine belt, the mean annual

160

precipitation was 1320.7 mm and fluctuated between 684.7 mm (1986) and 2755.6 mm (2006).

161

The detail climate information are listed in Supplemental Section 1.

162

Lake Donguz-Orun (43°14'43" N 42°27'51" E) is located in the Central Caucasus in the

163

Elbrus area (Fig. 1) at the riverhead of the left-bank tributary of the Baksan River at an altitude

164

of 2545 m a.s.l. The lake was formed as a result of damming of meltwater from several minor

165

glaciers near the lateral moraine of the Donguz-Orun Glacier. The lake has drainage. There are

166

several inflowing streams and the outflowing Donguz-Orun River. The lake’s surface area is

167

105,000 m2. Its mean depth is 4.5 m and its maximum depth is approximately 14 m. For the

168

observed period 1951–2010, the temperature values recorded at the proximal meteorological

169

station Terskol (4 km north of Lake Donguz-Orun) were as follows: mean annual temperature of

170

2.6 ºC; mean summer temperature (JJA) of 11.4 ºC; and mean winter temperature (DJF) of − 6.3

171

ºC. The average annual precipitation was 948 mm, with monthly sums ranging from 57 mm in

172

January to 100 mm in July (Alexandrin et al., 2018). The lake is fed by several fluvioglacial

173

streams as well as by precipitation, surface wash-off, and the avalanche snow from the slopes.

174

Infiltration through the moraine of the Donguz-Orun Glacier is also likely. The lake’s catchment

175

area is comprised of glaciers, recently deglaciated non-vegetated surfaces, partly vegetated mid-

176

steep slopes, and rock outcrops (steep slopes) (Fig. 2B). Fluvioglacial streams supply most of the

177

terrigenous material. The amount of organic matter supplied by the streams is negligible.

178

Biological productivity in the lake is low due to low temperatures. The sediment grain size is

179

superficially differentiated: coarser material is deposited around the cone formed by the

180

inflowing streams, while finer material is transported further and deposited in the lake itself. The

181

bottom sediment consists of finely laminated beige-brown clay. The lamination is continuous

182

throughout the previously studied sediment cores, and the mean lamina thickness is

183

approximately 1–2 mm (Alexandrin et al., 2018).

184 185

Figure 1. Locations of the study sites in the Caucasus. Legend: 1 – Lake Khuko; 2 – Lake

186

Donguz-Orun

187 188

Figure 2. Yandex view of the lake catchment (dotted lines) and years in which the sediment

189

cores were taken. Legend: A – Lake Khuko; B – Lake Donguz-Orun

190

Materials and methods

191

Sampling technique and preparation of samples

192

Drilling in Lake Donguz-Orun was carried out by the staff of the Glaciology Department

193

of the Institute of Geography of the Russian Academy of Sciences in the summer of 2014. The

194

drilling was performed using a modified piston sampler that was constructed in Atle Nesye,

195

Norway (Nesje et al., 1992) and had a diameter of 110 mm. A specially prepared platform

196

installed on an inflatable catamaran was used to reach the desired location. The thickness of the

197

core was limited by the high density of the bottom sediments (580 mm), which consisted of a

198

series of silt and sand layers.

199

The same technique was used to collect samples from Lake Khuko in 2016. The

200

thickness of the core was 1960 mm. Sediments were composed of loams with well-marked light

201

and dark interlayers. Due to the small size of the catchment area and the absence of permanent

202

watercourses, low rates of recent sedimentation were expected in this reservoir.

203

After collection, all cores were delivered to the laboratory. Each core was divided into

204

two symmetric parts along the central axis, and a sample was taken from its center to avoid any

205

disturbance or contamination that might have occurred during the penetration of the sampler into

206

the strata. The sample was divided into a series of samples 5 mm thick. The slicing was

207

performed along the strata to avoid any cross-contamination. Samples were dried at a

208

temperature of 105 °C, weighed, ground into powder, and placed in plastic containers (Petri

209

cups) for further gamma-spectrometry examination. The mass of the samples that were taken for

210

analysis varied from 1 to 4 g. Small size samples were chosen in case sample dissolution was

211

required. In total, 13 bottom sediment samples from Lake Khuko and 45 samples from Lake

212

Donguz-Orun were studied.

213

Gamma-spectrometric survey

214

Examination of the gamma-active radionuclides was performed using an ORTEC GEM-

215

C5060P4-B gamma spectrometer possessing an HPGe semiconductor detector with a beryllium

216

window (relative efficiency of 20%). The natural and artificial radionuclides that were studied

217

are listed in Supplemental Section 2. The activity of 232Th was estimated by studying the activity

218

of three lines of its daughter radionuclide

219

activity and the uncertainties in the calculated specific activity values were expressed in

220

Becquerel per kilogram of dry weight (Bq kg-1 dry wt.). The time taken to measure each sample

221

was at least 24 h.

228

Ac (Antovic and Svrkota, 2009). Sample specific

222

Alfa-spectrometric survey

223

The bottom sediment samples were ashed (450 °C for 8 h) before plutonium separation

224

and purification; 1 g of each sample was taken for processing. Complete acid decomposition was

225

performed by sequentially adding: 1) concentrated HF; 2) a 3:1 mixture of HF:HNO3; 3) dry

226

H3BO3 mixed with concentrated HCl; and 4) concentrated HNO3 with 30% H2O2. After each

227

addition, the resulting solution was evaporated to obtain wet salts. Finally, the wet salts were

228

dissolved in 7.5 M HNO3. To separate plutonium from the bottom sediment, AB-17 × 8 anionite

229

was used. To stabilize the plutonium isotopes in the IV – valence state, crystalline NaNO2 was

230

added to the stock solution, which was then passed through a preconditioning resin column.

231

Next, the column was washed sequentially with 7.5 M HNO3, 9 M HCl, 7.5 M HNO3, and

232

distilled water. Plutonium isotopes were separated from the anion exchange resin by washing

233

with hydrochloric hydroxylamine that had been heated to 40 °C. To control the radiochemical

234

yield, 10 µl of 236Pu (0.2 Bq) was added to the ashed sample. The plutonium was coprecipitated

235

with CeF3 on a Resolve filter (Eichrom Technologies, LLC) in preparation for alfa-spectrometry

236

measurements. The 236,238,239+240Pu concentration was detected using an ORTEC Alfa-Ensemble-

237

2 α-spectrometer with a vacuum chamber, α- radiation detector (ENS-U900 silicon detector

238

(UL-TRA-AS)), and pulse analyzer. The minimum detected activity (MDA) for Pu isotopes was

239

0.05 Bq kg-1. The Pu fraction after the separation was measured using ICP MS to analyze the

240

240

Pu/239Pu ratio and 241Pu.

241 242

Assessment of sediment age using unsupported 210Pb and radiocesium techniques

243

To determine the age of the selected

210

Pb (210Pbex) samples, their activity was measured

244

using the constant initial concentration (CIC) model. The calculation was made using the

245

following equation (Sapozhnikov et al., 2006): ‫ିߣ = ݐ‬ଵ ݈݊

‫(ܥ‬0,0) , ‫ݐ(ܥ‬, ‫)ݔ‬

246

where t represents the age of the sediment in years; C represents the 210Pbex activity in Bq kg-1; λ

247

is the decay constant of 210Pb; x represents the depth in cm.

248

To determine the ages of the samples using

137

Cs, peaks were identified on their vertical

249

distribution curves: the smaller peak corresponded to the maximum global fallout from the 1963

250

nuclear weapons tests in the Northern Hemisphere, and the larger peak corresponded to the 1986

251

Chernobyl nuclear power plant accident. The bomb-derived and Chernobyl-derived

252

concentration peaks corresponded to the constant sedimentation conditions during the second

253

half of the 20th century. The absence of the 1963 peak indicates that extremely low sediment

254

deposition rates occurred during the second half of the 20th century in Lake Khuko.

137

Сs

255 256

Results and discussion

257

Evaluation of the sedimentation rates.

258

The vertical distribution curves corresponding to the atmospheric component excess 210Pb 210

Pb and 226Ra in given layer/sample) in Lake

259

(the difference between activity concentration of

260

Khuko indicating that this element accumulated relatively uniformly through precipitation

261

(Fig. 3a). The measurement error is no more than 8%.

262 263 264

Lake Khuko’s sediment age distribution is shown in Table 1. The data include maximum sedimentation rates. Table 1. The bottom sediment ages in Lake Khuko using the 210Pb dating model Depth, сm

C(0,0)1, Bq kg-1

C(t,x), Bq kg-1

࡯(૙, ૙) ࡯(࢚, ࢞)

Age, year

0–0.5

2645

918

2.9

34.2

Maximum sedimentation rate, сm yr-1 0.015

0.5–1

2645

615

4.3

47.1

0.021

1–1.5

2645

481

5.5

55.1

0.027

1.5–2

2645

538

4.9

51.5

0.039

2–2.5

2645

479

5.5

55.2

0.045

265

1

2.5–3

2645

244

10.8

77.0

0.039

3–3.5

2645

101

26.2

106

0.033

3.5–4

2645

33.3

79.4

141

0.028

4–4.5

2645

36.5

72.5

138

0.033

4.5–5.5

2645

4.70

561

205

0.027

5.5–6.5

2645

-83.5

-31.7

>220

0.030

– The C(0,0) activity was evaluated using the smooth function The

266

137

Cs dating fully confirms the

210

Pb dating results for Lake Khuko. The maximum

267

137

268

radionuclide’s peak in the vertical distribution curve, the maximum period of formation of this

269

upper layer is estimated to be 30 y (sampling year: 2016), and the maximum accumulation rate

270

during this 30-y period (1986-2016) is 0.017 cm yr-1. The sedimentation rates over the past 55–

271

60 y in Lake Khuko are extremely low. Enhanced radionuclide activity is due to the

272

concentration effect - high specific activity in a very tiny layer.

Cs activity is found in the upper layer (0–0.5 cm) (Fig. 3b). Based on the position of this

210Pb

0

300

600

ex,

137Cs,

Bq kg-1 0

900

0-0,5

0-0,5

0,5-1

0,5-1

1-1,5

1-1,5

1,5-2

1,5-2

2-2,5

2-2,5

2,5-3

2,5-3

Depth, cm

Depth, cm

-300

3-3,5

500

1000

1500

2000

Bq kg-1 2500

3-3,5

3,5-4

3,5-4

4-4,5

4-4,5

4,5-5

4,5-5

5,-5,5

5,-5,5

5,5-6

5,5-6

273 274

a)

b) 210

Pbex (a) and

137

Cs (b) concentrations (Bq kg-1) and

275

Figure 3. The depth distribution curves:

276

their standard deviations for the sediments from Lake Khuko.

The density of the dry sediment varies from 0.57 to 0.75 g cm-3, which indicates that

277 278

Lake Khuko sediments are predominantly of organogenic origin.

279

The vertical distribution profile of the 210Pbex in the bottom sediments from Lake Donguz-

280

Orun shows a significant sediment influx from the lake’s catchment (Fig. 4a). The measurement

281

errors range from 8% to 22%. The vertical distribution profile of the

282

concentration decreases with depth, although interlayers with a lower radionuclide content

283

relative to the general trend were found. This distribution was expected because the transfer of

284

radionuclides into the lake is not constant over time. The catchment slopes from which the

285

sediment is delivered to streams has both an area that is not covered by the glacier and an area

286

under the glacier. Intensive rainfall, which produces the surface runoff from the catchment area,

287

does not occur every year. However, this sediment originates from erosion of loose slope

288

deposits, has a lower

289

primary assumption of the most common model used for dating (CRS)—that the transfer of

290

radionuclide into the lake is constant—is violated. Thus, the CRS model is unlikely to be valid

291

when the

292

Baskaran et al., 2014). Nevertheless, the available 210Pbex distribution can be used to illustrate the

293

uneven participation of various sub-catchments in the spatial-temporal pattern of the sediment

294

runoff.

295

210

210

Pbex content than

210

Pbex shows that its

210

Pbex direct fallout from atmosphere does. Thus, the

Pbxs is mostly derived from the catchment area (Appleby and Oldfield, 1992;

Assessment of the

137

Cs vertical distribution curve corresponding to the Lake Donguz-

296

Orun bottom sediments shows that this element has a maximum concentration in the layer whose

297

depth ranges from 9–9.5 cm (Fig. 4b). The

298

radionuclide’s source is atmospheric fallout from precipitation that occurred in May of 1986

299

after the Chernobyl accident. The second peak, which is located at a depth of 13.5–14 cm,

300

indicates that the radionuclide originates from bomb-derived

301

weapons tests (Appleby and Oldfield, 1978, 2000, 2008; Robbins et al., 1975; Gaboury et al.,

302

2001; MacKenzie et al., 2011, Aliyev et al., 2013).

137

Cs activity levels in this horizon indicate that the

137

Cs fallout in 1963 from nuclear

210Pb

0

50

100

150

137Cs,

Bq kg-1 250

0 0

0,5

0,5

1

1

1,5

1,5

2

2

2,5

2,5

3

3

3,5

3,5

4

4

4,5

4,5

5

5

5,5

5,5

6

6

6,5

6,5

7

7

7,5

7,5

Depth, сm

Depth, сm

ex,

0

8

303

200

8,5 9 9,5

150

200

250

300

350

400

450

Bq kg-1 500

8

9 9,5 10

10,5

10,5

11

11

11,5

11,5

12

12

12,5

12,5

13

13

13,5

13,5

14

14

14,5

14,5

15

15

15,5

15,5

16

16

16,5

16,5

17

17

17,5

17,5

18

18

18,5

18,5

19

19

19,5

19,5

20

20

20,5

20,5

a)

100

8,5

10

304

50

b)

305

Figure 4. The depth distribution curves for 210Pbex (a) and 137Cs (b) concentrations (Bq kg-1) and

306

their standard deviation for sediment collected from Lake Donguz-Orun

307

The distribution of

232

Th can also be used to confirm that the transfer of radionuclides

308

into the lake from its catchment area is not constant over time (Supplemental section 3). Thorium

309

does not migrate in a dissolved state. All the thorium minerals are resistant to the effects of

310

natural conditions. Furthermore, chemical processes do not affect the redeposition of thorium

311

and do not lead to its concentration in the form of secondary minerals (Ryabchikov and

312

Holbraich, 1960). More than 100 minerals containing thorium are present in the fine sediment

313

(De Meijer et al., 1985; Seddeek et al., 2005; Bosia et al., 2016; Ivanov et al., 2019). The

314

temporal

315

area reflects the heterogeneity of the geological structure of Lake Donguz-Orun’s catchment.

232

Th fluctuations in the mineral composition of sediment from the lake’s catchment

316

According to the data from the studied cores, the increase in the sedimentation rates

317

during the second half of the 20th and early 21st centuries was 2.74 kg m-2 y-1 during 1963-1986

318

and 4.34 kg m-2 y-1 during 1986-2014.

319

It is likely that the increase in the sedimentation rates after 1986 is due to the increasing

320

rate of denudation of the catchment area and the reduction in glaciation due to global warming,

321

both of which are extremely sensitive to climatic changes (Houghton et al., 2001; Dyurgerov et

322

al., 2003; Strokes et al., 2006). The reduction in glaciation in the Caucasus mountain region

323

began in the mid-19th century and continued throughout the 20th century, with small periods of

324

growth in the 1910s, 1920s, and 1970-1980 (Solomina et al., 2016). It is noteworthy that glacier

325

growth during the latter period coincides with lower sedimentation rates (Fig. 6). The period

326

from 1990–2010 is comprised of the two warmest decades of the 150 y period of instrumental

327

meteorological observations (IPCC, 2013). Results of satellite images interpretation show that

328

the Elbrus glaciers decreased approximately 4.9 ± 1.2% during the period from 1999–2012

329

(Shahgedanova et al, 2014). The surface area of the glacial lakes increased significantly. The

330

area of some lakes increased by two or more times during 1985–2000, although there did not

331

appear to be any change in the surface area of Lake Donguz-Orun (Strokes et al., 2007). It is

332

noteworthy that previous work on the sediment core collected in 2012 from Lake Donguz-Orun,

333

during which XRF-derived geochemical markers were used to detect varve thickness, revealed a

334

relatively consistent sedimentation rate in the lake (mean 1.82 mm y-1) throughout the studied

335

period from 1922–2010 (Alexandrin et al., 2018). The discrepancy in the sedimentation rates

336

between the late 20th and early 21st centuries is yet to be studied in detail but will be addressed in

337

the near future.

338 339 340 341

Evaluation of the causes of the high concentration of artificial radionuclides in the bottom sediments of Lake Khuko and the identification of their possible sources. Data obtained from the analysis of Lake Khuko’s bottom sediments indicate that a 137

342

significant fraction of the total

343

Intensive precipitation on the windward northern Caucasus macroslope (particularly its Western

344

Black Sea sector) is typical. The discovery of high concentrations of

345

sediment (more than 3 kBq kg-1) was very unexpected. This indicates the presence of a powerful

346

source of radioactivity. Unfortunately, the low rates of sedimentation do not allow for a

347

stratigraphic determination of possible sources of the radionuclides.

348

However, together with

Cs in the sediment is associated with the Chernobyl fallout.

137

137

Cs in the Lake Khuko

Cs, a detectable amount of the technogenic

241

Am (Fig. 5a),

349

which could be the result of nuclear bomb tests or part of the Chernobyl atmospheric deposition,

350

was discovered.

351

weapons tests (Hirose and Povinec, 2015). Due to the short half-life of the parent 241Pu (14.7 y),

352

the concentration of 241Am (half-life 433 years) in the environment increases every year.

241

Am is a 241Pu decay product, and its main sources are the 1952-1963 nuclear

241Am,

0

10

20

239,240Pu,

Bq kg-1

30

0

40

100

150

Bq kg-1 200

0-0,5

0-0,5

0,5-1

0,5-1

1-1,5

1-1,5

1,5-2

1,5-2

Depth, cm

Depth, cm

50

2-2,5

2-2,5

2,5-3

2,5-3

3-3,5

3-3,5

3,5-4

3,5-4

4-4,5

4-4,5

353

a)

354

b) 241

355

Figure 5. The depth distribution curve of

356

sediments and the corresponding standard deviations. The significant amount of

357

241

Am (a) and

239,240

Pu (b) in the Lake Khuko bottom

Am in the samples indicates the possible presence of other

358

plutonium isotopes—with the exception of 241Pu, which was not found. A relatively high level of

359

the

360

analysis of the Pu activity in two series of samples (sediment layers 0.0–0.5 and 0.5–1.0 cm). In

361

the surface layer, the specific activity of the 239,240Pu isotopes varied from 162.3 to 191.2 Bq kg-1

362

in the two series of samples, while in the 0.5-1 cm layer, its concentration was as low as 7.9-13.7

363

Bq kg-1. To separate the isotopes (239 and 240), ICP-MS analysis was conducted. The mass

364

spectrometric analysis results confirmed the plutonium isotopes’ activity concentration levels,

365

revealing a 239Pu level of 85 ± 2 ppt and a 240Pu level of–14 ± 2 ppt. Plutonium isotopes 241 and

366

238 were not detected, i.e., in both cases, the

367

were below the detection limit.

239,240

Pu isotopes’ total activity was discovered (Fig. 5b) through alpha-spectrometric

238,241

Pu specific activity and concentration levels

368

High technogenic radionuclide activity was also recorded in the cryoconites of the Adishi

369

glacier, which is near Lake Khuko (Lokas et al., 2018) (Supplemental Section 4). In fact, the

370

absolute artificial radionuclide activity levels in the bottom sediments coincide with those in the

371

cryoconites. Both appear to be effective at trapping technogenic radionuclides. The mechanisms

372

of technogenic radioisotopes accumulation in the cryoconite, which may occur during its

373

formation, do not parallel the processes of accumulation of the technogenic radionuclides in the

374

sediments of Lake Khuko. For example, the activity of cyanobacteria on the glacier’s surface

375

(Lokas et al., 2018) can influence the concentrations of the natural radionuclides within it. The

376

values of natural

377

bottom sediments of Lake Khuko due to the microbial communities present in the cryoconities. It

378

can be argued that the high concentrations of radionuclides in the upper layers of the Lake

379

Khuko bottom sediments are due to atmospheric radioactive fallout received from the catchment.

380

Evaluation of the possible sources of artificial radionuclides in Lake Khuko’s sediments

381

was performed using isotopic ratio analysis. The isotope ratio method allows scientists to

382

distinguish between tropospheric and stratospheric sources of radioactive contamination. The

383

stratospheric events correspond to the so-called “global or bomb-derived fallout” (atmospheric

384

nuclear tests), whereas the tropospheric events correspond to ground-based tests and accidents

385

(Hirose and Povinec, 2015).

386 387 388 389

210

Pb in cryoconites are an order of magnitude higher than they are in the

In this study, the isotopic ratios of the artificial radionuclides in Lake Khuko sediments were determined (Table 2). Table 2. Comparison of artificial radionuclide activity ratios in the surface layer of the Lake Khuko bottom sediments with results of the studies Source

Activity ratios 239,240

Pu/137Cs

241

Am/239,240Pu

240

Pu/239Pu

Lake Khuko (0.0–0.5 см)

0.050–0.062(0)*

0.21–0.28(0)

0.13–0.15(0)

Global fallout

0.008–0.042(1,6)

0.36–0.42(6)

0.17–0.34(3,4,8)

Chernobyl fallout

2.5 × 10-6 – 3.6 × 10-

0.08–0.88(7,11)*

0.35–0.42(4,8)

0.12–0.50(5)

0.04–0.05(1)

0.5–100(9)

0.03–0.14(2,9)

0.05–0.29(10)

0.03–0.16(10)

4(7)

Semipalatinsk site

6.0 × 10-4 – 1.1 × 102(5)

Plant ‘Mayak’

3.1 × 10-4 – 2.5 × 102(9)

Novaya Zemlya (bottom sediments)

0.12–0.80(10)

*(0)

This study; (1)Beasley, 1998; (2)Cagno, 2013; (3)Dahlgaard, 2001; (4)Hirose and

390 391

Povinec, 2015; (5)Kadyrzhanov, 2005; (6)Kershaw, 1990, 1995; (7)Kirshner, 1988; (8)Muramatsu,

392

2001; (9)Skipperud, 2004; (10)Smith, 2000; (11)Lujaniene, 2009. * These ratio increased till now 2.5 times because of americium ingrown from decay of

393 394

241

Pu (T1/2=14.5 y) The

395

239,240

Pu/137Cs ratio corresponding to the mainly during the 1950s and early 1960s

396

global fallout events is equal to 0.012. This universal ratio is confirmed by numerous soil

397

analyses (Everet et al., 2008; Sayles et al., 1992; Wan et al., 1987). The bottom of the Techa

398

River, which is near Mayak, was subjected to severe local radioactive contamination, that’s why

399

the ratio of

400

(Trapeznekov et al., 1993). The dependence of this ratio on latitude has also been investigated:

401

for 60-70° N, for instance, it equals 0.040 ± 0.005 (Hardy et al., 1973). The salinity of the

402

reservoir can have a strong effect on the isotope ratio 239,240Pu/137Cs. It is known that the ratio is

403

an order of magnitude larger in the bottom sediments of saline water bodies than it is in the

404

bottom sediments of freshwater bodies. Values of

405

freshwater and seawater, respectively (Sanschi et al., 1983). This is due to the presence of

406

dissolved cesium in seawater, which contains high concentrations of salts (Sanschi et al., 1983;

407

Livingston and Bowen, 1979). The ratio 239,240Pu/137Cs changes significantly over time due to the

408

short half-life of 137Cs (30.2 years).

239,240

Pu/137Cs for the silts is significantly lower than the global fallout value

The rarely used

409

241

239,240

Pu/137Cs are 0.01-0.05 and 0.1-0.4 for

Am/239,240Pu isotopic ratio is an indirect confirmation of the isotopic

410

input due to the Chernobyl accident (Lujaniene et al., 2009). The amount of americium in

411

environmental objects is directly related to the amount of its parent isotope

412

14.3 years), and due to its short half-life, it is increasing over time. For example, in the vicinity

413

of Mayak, this ratio varies significantly for different reservoirs due to the presence of dissolved

414

radioactive chemicals, and the americium content is often much higher than that of

415

(Christensen et al., 1997; Kuzmenkova et al., 2017). It is difficult to realistically assess the

416

source of radioactive fallout based on the isotopic ratios of radionuclides having different

417

chemical properties. Therefore, the ratio of different isotopes of plutonium is considered to be

418

the best marker, as it includes only one element. The

419

0.50 for the Chernobyl fallout (Lujaniene et al., 2009). It was not possible to reliably determine

420

238

421

of 0.05 Bq/sample, indirectly ruling out the Chernobyl fallout source. The

422

most often used to determine the source of plutonium, which may be global or local (Cagno et

423

al., 2013). The isotope ratio of plutonium entering to the environment during nuclear tests varied

424

greatly depending on the power and type of nuclear bomb tested (Hirose and Povinec, 2015). For

238

241

Pu (half-life is

239,240

Pu

Pu/239,240Pu ratio ranges from 0.44 to

Pu in the bottom sediments of Lake Khuko. The sample activity was below the detection limit 240

Pu/239Pu ratio is

425

an explosion of about 4 kilotons, atomic ratios of 0.0326 and 0.0011 for plutonium isotopes

426

240

427

2004) (Supplemental section 5).

Pu/239Pu and 241Pu/239Pu, respectively, were observed in atmospheric precipitation (Skipperud,

428

It can be argued based on the isotopic relationships (Table 2) that the distributions of

429

artificial radionuclides in the Lake Khuko sediments are the result of both global and local

430

stratospheric depositions, as they are in the case of the Caucasus cryoconites (Lokas et al., 2018).

431

It is also obvious that part of the recent 137Cs activity in the bottom sediments of Lake Khuko is

432

associated with the Chernobyl fallout. Lake Khuko, located close to the path along which air

433

masses from the accident site pass through the Caucasus, must have been influenced by the

434

Chernobyl accident (Israel, 1998). It should be noted that a large amount of precipitation is

435

typical for the subtropical climate of the Black Sea in the Caucasus. During the period from the

436

end of April to the end of June 1986, a total amount of 162.6 mm of precipitation was recorded

437

at the Sochi weather station, i.e., 9.7% of the annual average. Lake Khuko is located 43 km north

438

of that weather station.

439 440

Conclusions and perspectives

441

The two investigated Caucasus lakes differ in their sedimentation conditions due to the

442

different rates of exogenous processes in their catchments. The Lake Khuko catchment of the

443

middle mountain belt, whose soil surface is densely covered with vegetation, currently

444

experiences sedimentation rates of approximately 0.017 cm yr-1. In Lake Donguz-Orun, whose

445

catchment has large areas devoid of vegetation and which is located in the high-mountain belt in

446

the periglacial area, the sedimentation rate is much higher. Moreover, the present sedimentation

447

rates in Lake Donguz-Orun are more than 1.5 times higher than they were during 1963-1986.

448

The increase in the rates of accumulation is due to global warming, which contributes to an

449

increase in the rate of glacier melting and a consequent increase in sediment inflow from the

450

slopes to the watercourses. This latter phenomenon is due to the draining of the catchments of

451

lakes and nearby streams that formed under the glaciers due to the intensification of glacier

452

melting. The vertical distribution of thorium, unsupported

453

sediments of Lake Donguz-Orun are used as markers for identifying the contributions of and

454

spatial and temporal changes in various sediment sources from the catchment area of the lake.

455

The data obtained indicate that the extremely low sedimentation inflow from the catchment area

456

promotes the concentration of radionuclides from radioactive fallout in the sediments of Lake

457

Khuko. The levels of these radionuclides significantly exceed background values.

210

Pbex, and other tracers in the

458

It was not possible to unambiguously determine the contributions of different

459

atmospheric sources of technogenic radionuclides to the upper layers of the Khuko Lake

460

sediment by calculating the ratios of their isotopes. However, it is possible to conclude that the

461

plutonium and americium in the bottom sediments are the result of atmospheric fallout from the

462

period after the open-atmosphere nuclear bomb tests. The sources of these fallouts include global

463

stratospheric depositions and the Semipalatinsk, Kapustin Yar, and Totsky tests. To study the

464

possible sources of radioactive fallout in more detail, it is necessary to isolate uranium isotopes,

465

search for possible “hot” particles in the bottom sediments of Lake Khuko, determine the extent

466

of pollution, and create landscape-geochemical maps of the catchment of Khuko Lake. To

467

determine the forms of constituent artificial radionuclides, it is necessary to conduct a detailed

468

analysis of the physicochemical properties of the sediments of Lake Khuko and determine the

469

organic matter content. It is also necessary to study the chemical and radionuclide composition

470

of lake water.

471 472 473 474 475 476

Acknowledgements. Initial work with the cores (sample preparation, etc.) was performed with support from the Russian Foundation for Basic Research (grant No. 17-05-01170). The remaining work was conducted with support from the ongoing Russian Science Foundation project No. 19-17-00181: “Quantitative assessment of the slope sediment flux and its changes in the Holocene for the Caucasus mountain rivers.”

477 478

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List of the figures Figure 1. Locations of the study sites in the Caucasus. Legend: 1 – Lake Khuko; 2 – Lake Donguz-Orun Figure 2. Yandex view of the lake catchment (dotted lines) and years in which the sediment cores were taken. Legend: A – Lake Khuko; B – Lake Donguz-Orun Figure 3. The depth distribution curves:

210

Pbex (a) and

137

Cs (b) concentrations (Bq kg-1) and

their standard deviations for the sediments from Lake Khuko. Figure 4. The depth distribution curves for 210Pbex (a) and 137Cs (b) concentrations (Bq kg-1) and their standard deviation for sediment collected from Lake Donguz-Orun Figure 5. The depth distribution curve of 241Am (a) and sediments and the corresponding standard deviations.

239,240

Pu (b) in the Lake Khuko bottom

Highlights Both 137Cs and 210Pbex were used for evaluation of sedimentation rates in the two Caucasus lakes Sedimentation rate in Donguz-Orun lake located in proglacial zone increase in 1,5 times since 1986 Global warming is the main reason of increasing denudation rates in periglacial zone High concentrations of technogenic radionuclides were determined in sediments of Lake Khuko The possible contribution of different sources of Khuko lake sediment contamination is determined

Author Statement

Natalia V. Kuzmenkova – the corresponding author, was involved in planning and supervision the work, performed activity measurements as well the radiochemical researches Maxim M. Ivanov - was involved in planning and supervision the work, performed activity measurements, co-wrote the paper Mikhail Y. Alexandrin - field investigations Alexei M. Grachev - field investigations Alexandra K. Rozhkova - performed activity measurements as well the radiochemical researches Kirill D. Zhizhin - performed ICP measurements Evgeniy A. Grabenko – supervised the field investigations Valentin N. Golosov - supervised the work, co-wrote the paper All authors discussed the results and contributed to the final manuscript

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: