Geochronology of the southern Baltic Sea sediments derived from 210Pb dating

Journal Pre-proof Geochronology of the southern Baltic Sea sediments derived from

210 Pb dating

Tamara Zalewska, Paweł Przygrodzki, Maria Suplińska, Michał Saniewski PII:

S1871-1014(19)30049-4

DOI:

https://doi.org/10.1016/j.quageo.2019.101039

Reference:

QUAGEO 101039

To appear in:

Quaternary Geochronology

Received Date: 17 April 2019 Revised Date:

13 November 2019

Accepted Date: 13 November 2019

Please cite this article as: Zalewska, T., Przygrodzki, Paweł., Suplińska, M., Saniewski, Michał., 210 Geochronology of the southern Baltic Sea sediments derived from Pb dating, Quaternary Geochronology (2019), doi: https://doi.org/10.1016/j.quageo.2019.101039. 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. © 2019 Published by Elsevier B.V.

Geochronology of the southern Baltic Sea sediments derived from 210Pb dating

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Tamara Zalewska1, Paweł Przygrodzki1, Maria Suplińska2, Michał Saniewski1

3 4 1

5

Institute of Meteorology and Water Management – National Research Institute, Waszyngtona 42, 81-342 Gdynia, Poland

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2

Central Laboratory for Radiological Protection, Konwaliowa 7, 03-194 Warsaw, Poland

8 9 10

Corresponding author*: e-mail: [email protected], phone: 48 58 62 88 266, Fax: 48 58 62 88 163

11 12

Keywords: geochronology, bottom sediments, southern Baltic Sea, maps

13 14

Abstract

15

Based on the vertical distribution of

16

accumulation rates (LAR) and mass accumulation rates (MAR) were determined for 31

17

locations in the southern Baltic Sea region by applying models: Constant Flux Constant

18

Sedimentation Rate (CF:CS) and Constant Rate of Supply (CRS). The dating of sediment layers

19

in selected locations was also carried out. The reliability of the results was verified using

20

radiotracers:

21

development of maps of continuous distributions of LAR and MAR values in the areas where a

22

particular type of bottom sediments were found: silt-clay, sandy silt, silty sand and mixtites

23

covering the region of the southern Baltic. The maps constitute a tool supporting basic and

24

applied research. Data for marine open areas were supplemented with sediment accumulation

25

rate data for the areas of the Szczecin Lagoon and the Vistula Lagoon. The linear accumulation

26

rates of bottom sediments of the southern Baltic varied from 0.61 to 3.54 mm y-1 while mass

27

accumulation rates ranged from 390 g m-2 y-1 to 910 g m-2 y-1, largely reflecting the LAR values.

137

210

Pb activity concentrations in bottom sediment, linear

Cs and, for the first time,

90

Sr. The results obtained were the basis for the

28 1

29

1. Introduction

30

Marine bottom sediments play a huge role in studies of the marine environment (eg. Szefer

31

et al., 1995; Bełdowski and Pempkowiak, 2003; Glasby et al., 2004; Zajączkowski et al., 2004;

32

Szefer et al., 2009; Hutri et al., 2013; Zaborska, 2014; Zaborska et al., 2014). One of the main

33

pieces of information that can be obtained from sedimentary profiles is the history of

34

environmental change in terms of specific parameters or phenomena. At the same time, to be

35

able to use any information derived from the studies of sediment, the key issue is to determine

36

the rate of sediment accumulation specific to the studied areas. Information on sediment

37

accumulation rate and the age of the sediment is key to understand the dynamics of sediment

38

accumulation and the formation of bottom sediments. (Rubio et al., 2003; Roos and Valeur,

39

2006; Szmytkiewicz and Zalewska, 2014; Zaborska, 2014). Such information can be used to

40

determine the inflow of chemical substances, with particular emphasis on hazardous substances

41

associated with human activities, and consequently to identify sources of contamination.

42

(Pempkowiak, 1991; de Carvalho Gomes et al., 2009; Diaz-Asencio et al., 2009; Hille et al.,

43

2006; Kading et al., 2009; Mulsow, 2009; Li et al., 2012; Brady et al., 2014; Huang et al., 2014;

44

Zalewska et al., 2015). Information on the rate of sediment layer formation is also crucial to the

45

strategy of monitoring the marine environment and assessing contaminants deposited in bottom

46

sediments. Sediment accumulation rate data also allows us to assess when we will observe the

47

effects of decreases in environmental contaminants following the implementation of mitigating

48

actions to improve the environment. An example may be actions resulting directly from regional

49

conventions such as the Convention on the Protection of the Marine Environment of the Baltic

50

Sea Area (Helsinki Convention) and developed the Baltic Sea Action Plan (HELCOM, 2007a).

51

Other activities result from the need to implement legal acts in force in the European Union,

52

such as the Marine Strategy Framework Directive (Directive of the European Parliament and of

53

the Council 2008/56 / EC of 17 June 2008 establishing a framework for Community action in

54

the field of marine environmental policy).

55

Sediments begin to accumulate following the transportation of suspended matter to the

56

bottom of the sea floor. Deposits include both material of biological origin (including products 2

57

of marine organism decomposition, zooplankton, phytoplankton, fungi, bacteria) and mineral

58

material (various types of mineral particles originating from eroded rocks, atmospheric dusts,

59

cosmic dusts, volcanic dusts as well as the bed load carried by rivers into the sea). The speed at

60

which the sediment material falls to the bottom depends on its size and shape, as well as on the

61

specific density and viscosity of the water. The movement of sediment material is also

62

influenced by its concentration and whether or not a fixed sediment layer is formed is

63

determined by other hydrodynamic processes and the seabed.

64

According to Mc Kee et al. (1983), sedimentation is defined as the overall process of

65

particle transport to, emplacement on, removal from and preservation in the seabed. This

66

definition discerns certain phases/stages of the sedimentation process; the first stage is

67

deposition defined as temporary emplacement from and preservation on the seabed and it

68

pertains to this relatively short time of sediment formation. Sediment accumulation is the stage

69

pertaining to a decidedly longer period; it is the result of particle deposition and removal and

70

leads to preservation of the strata. Particle removal may be due to several mechanisms eg.

71

physical erosion, biological resuspension and chemical dissolution (Mc Kee et al., 1983).

72

The formation (in terms of types) of surface sediments in the Baltic Sea areas, including

73

the area of the southern Baltic Sea depends on many factors (Uścinowicz, 2011), not least the

74

sea-bottom relief of the seabed, the sea depth and the development of the shoreline. In addition,

75

hydrodynamic processes, frequency of occurrence, intensity and direction of movement of water

76

masses are also very important. Differentiation of hydrodynamic processes, and the shape of the

77

bottom affects the existence of areas (zones) of domination of specific lithographic processes

78

(Uścinowicz, 2011). Above the pycnocline there are sand-gravel deposits and sands.

79

Hydrodynamic processes in this layer prevent permanent deposition of silt-clay sediments. The

80

fraction smaller than 0.063 mm is below 1% or even 0.5%. Hydrodynamic processes are

81

particularly important for shallow water areas. Water from the North Sea is characterized by

82

much higher salinity than the Baltic Sea waters, which also affects its higher density. This is the

83

reason why water from the North Sea is introduced by the Danish Straits as a bottom layer. The

84

main route of transport of bottom-water masses from the North Sea goes through the Arkona 3

85

Basin, then through the Bornholm Gate (the area between Bornholm and the Swedish coast),

86

then enters the Bornholm Basin. The next stage is the Słupsk Furrow, from where waters with

87

less intensity melt into the Gotland Basin and the Gdańsk Basin. Such a flow path of dense

88

waters from the North Sea in the bottom layer enables the formation of layers of sediments in

89

the areas of so-called quiet sedimentation, which include the Gdańsk Basin, the Bornholm Basin

90

and the Eastern Gotland Basin. The bottoms of these areas are often characterized by the

91

occurrence of strong oxygen deficiency, anaerobic conditions and laminated deposits without

92

bioturbation structures reflecting the annual sedimentary rhythmicity. The accumulation rate of

93

the silty-clay can vary in a relatively wide range from 0.5 to 2 mm y-1 (Uścinowicz, 2011).

94

Higher rates of sediment accumulation are recorded in the central areas of sedimentary basins,

95

while on their outskirts the sediment accumulation rate is lower.

96

One of the common methods used for the determination of sediment accumulation rate

97

is the isotopic method based on analysis of the activity concentration changes of naturally

98

occurring radionuclides in sedimentary profiles, including the use of the

99

(Goldberg, 1963; Appleby and Oldfield, 1992; Appleby, 1997; Zajączkowski et al., 2004;

100

210

Pb isotope

Zaborska et al., 2007; Suplińska, 2008; Diaz-Asencio et al., 2009; Mulsow et al., 2009).

101

A significant part of both natural and artificial radionuclides introduced into sea waters

102

is ultimately deposited in bottom sediments. This is the result of the adsorption of radioactive

103

isotopes into the organic and inorganic matter particles that are part of the sediments. The main

104

source of anthropogenic radionuclides in the bottom sediments of the Baltic Sea is the

105

atmospheric deposition associated with nuclear weapons tests carried out in the 1950s and

106

1960s, and with the Chernobyl power plant accident. The radioactive cesium isotopes, 137Cs and

107

134

108

importance, due to the short half-life time: 2.06 years, in contrast to 137Cs, which has a half-life

109

of 30.05 years and is still detected in marine environment elements. The total amount of

110

introduced into the Baltic Sea after the Chernobyl accident is estimated at 4700 TBq, which is

111

as much as 82% of the total amount present in the Baltic Sea (HELCOM, 1995; Nielsen et al.,

112

1999; HELCOM, 2003). It was estimated that the total amount of

Cs, had the largest share in the atmospheric deposition from Chernobyl.

137

134

Cs is now of less

137

Cs

Cs accumulated in 4

113

sediments up to 1998 is 2210 TBq (HELCOM, 2007b). This is about 8 times more than

114

throughout the 1980s (277 TBq). In the area covering the Polish economic zone of the Baltic

115

Sea, the amount of

116

Chernobyl accident. Therefore,

117

concentrations.

137

Cs accumulated increased from 11 TBq to 46 TBq as a result of the 137

Cs can be used to verify dating based on

210

Pb activity

The aim of the study was to use the dating method based on the analysis of changes in

118 119

210

120

first time, 90Sr activity concentration changes to determine the sediment accumulation rates and

121

dating of sediment layers in a systematic manner throughout the entire southern Baltic. The

122

results obtained were used to develop maps. These will be applicable to other areas of research

123

and act as an alternative to individual results presented in various publications. Data for marine

124

open areas were supplemented with sediment accumulation rate data for the areas of the

125

Szczecin Lagoon and the Vistula Lagoon. The results were also used to identify the ages

126

corresponding to particular layers of bottom sediments.

Pb activity concentrations in sedimentary profiles verified by analysis of

137

Cs, and, for the

127 128

2. Materials and methods

129

2.1 Sampling

130

Samples of bottom sediment were collected in the area of the southern Baltic Sea, in

131

areas of sediments’ occurrence with the structure of silt-clay, sandy silt, silty sand and mixtites

132

(Uścinowicz, 2011). The geological maps of the Baltic Sea bottom, developed by the Polish

133

Geological Institute, Marine Geology Department in Gdańsk, were used to determine the

134

bottom sediment sampling stations (Pikies, 1990a, Uścinowicz and Zachowicz, 1990b,

135

Kramarska, 1991, Uścinowicz and Zachowicz, 1992, 1994, Pikies and Jurowska, 1995a, Pikies,

136

1995b). Twenty-seven stations in the open sea and coastal areas were initially designated (TZ1-

137

TZ27), representative of certain types of bottom sediments. Two stations (TZ17 i TZ21) were

138

excluded during sampling, due to the inability to take up stratified bottom sediments. Finally, in

139

2010 and 2011, sediment samples were collected at 25 stations located in the Bornholm Basin,

140

Eastern Gotland Basin, Gdańsk Basin, Słupsk Furrow and in Vistula Lagoon and Szczecin 5

141

Lagoon (Fig.1a,b). The Bornholm Basin, the Eastern Gotland Basin and the Gdańsk Basin, and

142

especially their deep-water regions, belong to the areas of silt-clay sediments containing

143

fractions finer than 0.063 mm at the level even higher than 75% (Uścinowicz, 2011). Such

144

sediments are also present in the Gulf of Gdańsk to a much lesser extent in the areas of Słupsk

145

Furrow. On the outskirts of these areas, there are sand-silt deposits or mixtites that occupy a

146

significant area of Słupsk Furrow.

147

Considering the data above, it should be assumed that silt-clay sediments in the areas covered

148

by the research are well-laminated, and therefore reliably reflect changes occurring in the

149

environment. Low oxygen conditions do not favour macrozoobenthic organisms, which are

150

often associated with bioturbation activity. As shown in the studies conducted under the Polish

151

State Environmental Monitoring in 2011, oxygen conditions in the deep-water zone, from the

152

Bornholm Basin (stations P39, P5) through Słupsk Furrow to the eastern Gotland Basin (station

153

P140) and to the Gdańsk Basin (station P1) were unfavorable to macrozoobenthic organisms

154

(Zalewska et al., 2012). From January to June 2011, oxygen concentrations in the bottom layer

155

were maintained in the range from 0.34 to 1.17 cm3 dm-3, with values below 1 cm3 dm-3

156

considered a strong oxygen deficit. However, it should be emphasized that these data refer to

157

the period preceded by the North Sea inflows in 2010, which brought saline and oxygenated

158

waters in the bottom layer. In the second half of the year, hydrogen sulphide appeared in the

159

depths, which confirms the presence of anaerobic conditions. This does not refer to a single year

160

and is specific to these areas. Oxygen conditions directly affect the presence of

161

macrozoobenthic organisms, the number of which in the studied areas in 2011 was negligible.

162

In samples taken in June in the Bornholm Deep, only 4 taxa were identified, in the area of the

163

Eastern Gotland Basin - 3, while in the Gdańsk Deep there was no taxon (Zalewska et al., 2012).

164

The samples were taken with a Niemistö corer with an inner diameter of 5 cm. Three

165

parallel sediment cores were acquired at each station. The cores were divided into 1 cm wide

166

slices down to 5 cm depth and deeper into slices of 2 cm width. This yielded the following

167

sediment layers: 0-1, 1-2, 2-3, 3-4, 4-5, 5-7, 7-9, 9-11, 11-13, 13-15, 15-17, 17-19, 19-21, 21-

168

23, 23-25, 25-27, 27-29, 29-31 cm. The corresponding slices/layers from the 3 parallel cores at 6

169

the sampling station were integrated to produce a single analytical sample. These samples were

170

initially deep-frozen onboard the ship and freeze-dried and homogenized in the land laboratory

171

prior to analysis. In 2009, sediment samples were also collected at six stations: P110, P116, P1,

172

P140, P5 and P39 — part of the network of monitoring stations, in which samples for the

173

activity of selected radioactive isotopes are collected each year. Analyses of monitoring samples

174

are carried out by the Central Laboratory for Radiological Protection in Warsaw and

175

commissioned by Polish Atomic Agency. Bottom sediment samples at these stations are

176

collected and divided in the same way as described above, with the difference that six cores are

177

acquired in parallel. The monitoring data from 2010-2014 from the P1 and P116 stations was

178

used to verify the results obtained in this study. Additionally, for the purpose of verifying the

179

accuracy of dating, in 2015 samples of bottom sediments were taken at the P1 station, which

180

were divided into layers of 1 cm thick to 30 cm deep and in which 137Cs and 90Sr activities were

181

determined (90Sr was used as a date marker for the first time). In 2013 and 2012, stratified

182

samples of bottom sediments of 50 cm length were taken from areas of the Vistula Lagoon and

183

the Szczecin Lagoon to determine the sediment accumulation rate. These were divided into

184

layers with a thickness of 2 cm.

185 186

2.2 Models for 210Pb sediment dating

187

210

Pb identified in sediment samples originates from two sources. One fraction is the

188

result of the radioactive decay of radium (226Ra) and it is called supported

189

activity along the vertical sediment profile does not change practically. The other source is

190

atmospheric fallout. The activity of

191

atmospheric deposition, decreases with sediment depth. This activity forms the basis for

192

determining rates of sediment accumulation: mass accumulation rate — MAR — and linear

193

accumulation rate — LAR — and, in the particular sediment layers, an age determination. The

194

210

195

subtracting the activity of one of the products of

196

determinations of sediment accumulation rates and sediment age along the vertical profiles were

210

210

Pb (210Pbsupp); its

Pb unsupported or excess (210Pbex), originating from

Pbex activity concentration is determined from the total activity of the isotope (210Pbtot) by 226

Ra decay, e.g.

214

Bi or

214

Pb. In this study,

7

197

done using two models (Robbins, 1978; Appleby and Oldfield, 1992; Appleby, 1997; Boer et

198

al., 2006; Diaz-Asencio et al., 2009; de Carvalho Gomes et al., 2009; Szmytkiewicz and

199

Zalewska, 2014). The first model – the Constant Rate of Supply (CRS) model – is based on the

200

assumption that the supply of

201

accumulation rate might vary. This model seems to apply to most sedimentary systems where

202

sediment supply may vary in response to climatic or anthropogenic changes. The second model

203

– the Constant Flux Constant Sedimentation Rate (CF:CS) model – assumes a constant dry-

204

mass sedimentation rate (Szmytkiewicz and Zalewska, 2014).

210

Pb to the sea surface is constant, while the sediment

205

In order to verify the results of age determination by the 210Pb method it is necessary to

206

apply an additional tag whose concentration changes in the marine environment can be easily

207

traced back to specific events. In the case of the Baltic Sea, the most obvious tag is the

208

anthropogenic isotope of cesium –

209

based on

210

since 1945 with maximum deposition recorded in 1963 and the accident at the Chernobyl power

211

plant in 1986) should be identifiable as an increase in the isotope along the sediment core.

212

Simultaneously, the results have to be interpreted cautiously, taking into account the complexity

213

and large number of processes affecting the final result – the presentation of 137Cs distribution in

214

the sediment vertical profile. Therefore, the isotope will be most useful for verifying sediment

215

chronology when post-depositional processes are negligible (Diaz-Asencio et al., 2009).

137

137

Cs. In the verification of the age determination method

Cs it is assumed that historical events (e.g. testing of nuclear weapons performed

216 217

2.3 Analysis of gamma emitters

218

Frozen samples of bottom sediments were dried by freeze-drying, homogenised and

219

then placed in cylindrical containers with a diameter of 40 mm, identical to those used while

220

preparing calibration gamma mix standards (mixture of gamma-emitting isotopes - “mix

221

gamma” (Isotope Production and Distribution Center, Swierk, Poland, BW/Z- 62/27/07 was

222

used for the calibration).

223 224

210

Pb,

226

Ra,

214

Bi,

214

Pb and

137

Cs content in marine sediments were analyzed by high

resolution gamma spectrometry using a HPGe detector with a relative efficiency of 40%, and a 8

225

resolution of 1.8 keV for peak of 1332 keV of 60Co. The detector was coupled with an 8192-

226

channel computer analyser (GENIE 2000). The samples were placed in plastic containers of a

227

geometry identical to those used for calibration. After reaching equilibrium between

228

its daughter nuclides (214Bi, 214Pb) the samples were ready for measurements. The measurement

229

time was 80 000 s for each sample.

230

226Ra was determined by the emission of its daughter nuclides 214Pb and 214Bi at 352 keV and

231

609 keV respectively and 137Cs was measured via its emission at 661.6 keV.

232

The reliability and accuracy of the applied method was verified by the measurement of certified

233

sediment material IAEA-300 (Tab. 1).

210

226

Ra and

Pb was determined by gamma emission at 46.5 keV,

234 235

2.4 Analysis of 90Sr

236

Before analysis, sediment samples were ashed at a temperature of 450°C in a muffle

237

furnace. The sample was then digested with concentrated nitric acid on a hotplate to decompose

238

most of the organic matter. After digestion, the residue was collected on filter paper and

239

discarded. The filtrate was diluted with distilled water to 150 ml. The reagents: 100 ml of 8%

240

oxalic acid, 20 mg of natural strontium, and ammonium (to raise pH to 4-4.5) were added to the

241

diluted filtrate. The solution was heated to 80°C in order to completely precipitate the strontium

242

oxalate. The precipitate was collected on hard filter paper and allowed to dry in ambient

243

conditions. The oxalate was then converted to carbonate at 650°C in a muffle furnace. Next, the

244

strontium carbonate was separated from calcium carbonate with 65% HNO3. Radium removal

245

was done by precipitation with BaCrO4 in the presence of a buffering agent (pH= 5.5). 20 mg of

246

stable yttrium was added, and the samples were allowed to stand for 21 days to reach complete

247

equilibrium between 90Y and 90Sr. Beta activity of the samples was measured using Low-Level

248

Beta Counter FHT 7700T (ESM Eberline) with the background count rate of 0.01 counts s-1 and

249

the lowest detectable activity of 3 mBq per sample.

250 251

2.5 Mapping

9

252

Mapping was based on linear accumulation rate data and mass accumulation rates from all

253

stations located in the open area as well as Szczecin and Vistula Lagoons. Data for the outer

254

Puck Bay (Szmytkiewicz and Zalewska, 2014) was also used. The map is limited to areas in

255

which physicochemical and hydrological conditions allow the formation of sediments reflecting

256

geochronology. This means that the maps cover only areas of the occurrence of silt-clay, sandy

257

silt, silty sand and mixtites (Uścinowicz, 2011). It should be emphasized that in areas on the

258

outskirts of well-laminated silt-clay sediments, i.e. in areas with sand-silt deposits or mixtites

259

that occupy a significant area of Słupsk Furrow, mapping is subject to greater uncertainty due to

260

the smaller amount of data.

261

The measurements have been interpolated using ArcGIS ver. 10.0 software, with Spatial

262

Analyst toolbox. The interpolation method had been chosen to enforce the best predictive values

263

(LAR, MAR) for cells in a raster. The Topo to Raster tool was used because the interpolation

264

technique is specifically designed to create a surface that more closely represents a natural

265

environment. The interpolation method is designed to take advantage of the types of input data

266

commonly available and the known characteristics of elevation surfaces. The algorithm is based

267

on ANUDEM, developed by Hutchinson et al. (2009; 2011) at the Australian National

268

University.

269 270

3. Results and discussion

271

3.1 Dating and data verification

272

Changes in

210

Pb activity concentrations, both total and excessive, in sedimentary

273

profiles at all stations were exponential. An example of 210Pbex activity concentration changes as

274

a function of the linear depth as well as cumulative mass depth at the station TZ4 is given in

275

Fig. 2a. It is shown that linear correlations between the

276

linear depth and cumulative depth presented on a logarithmic scale are statistically significant

277

(210Pbex vs depth: R = -0.9150, p = 0.00000;

278

0.00001). For the same station, the dependence of the age of particular sediment layers on the

279

depth determined on the basis of the CRS model is presented, which was also statistically

210

210

Pbex activity concentration and the

Pbex vs cumulative depth: R = -0.8633, p =

10

280

significant (Fig. 2b). The error of dating of individual layers of sediments was determined by

281

the error propagation method. As expected, it increases with the depth of sediments, reaching a

282

maximum value of 50 years at a depth of 30 cm. In order to verify the accuracy of dating, the

283

137

284

CF:CS models (Fig. 2c). The results obtained indicate very good compatibility between the two

285

models. In addition, the dating of individual sediment layers was carried out, taking into account

286

the

287

concentrations was recorded around 1940, which corresponds to the start of nuclear weapons

288

testing era. Their intensification falls in the 1950s and 1960s, which was reflected in the peak

289

recorded in this period. The largest increase in the activity concentration of

290

however, after the Chernobyl nuclear accident in 1986. An analysis of dependence of sediment

291

age on depth and verification with 137Cs was carried out for all locations.

292

Additionally, for the first time in the southern Baltic, another anthropogenic isotope - 90Sr, was

293

used to verify the models, alongside

294

events (Fig. 2d). At station P1, as in the case of TZ4, an increase in 137Cs activity concentrations

295

was recorded around 1945. Further growth in the following years was related to the continuation

296

of nuclear weapons tests and, later, Chernobyl. After 2000, a visible decrease in the activity

297

concentrations of this isotope is observed, resulting primarily from an intense decline in activity

298

in seawater (Zalewska and Suplińska, 2013). The pattern of changes in

299

concentrations on the timeline is slightly different from that observed for 137Cs, but also remains

300

in accordance with the events that resulted in this isotope being introduced into the Baltic Sea

301

environment. A noticeable increase in activity concentrations is observed during the nuclear

302

weapons tests, whereas the most visible change in the activity concentration of 90Sr to 40 Bq kg-

303

1

304

bottom sediments and the utility of using 90Sr as a radiotracer.

Cs activity was analysed with the depth replaced with a time axis based on two CRS and

137

Cs activity concentrations in the vertical profile. The first increase in

137

Cs. The presence of

90

137

137

Cs activity

Cs occurred,

Sr is associated with the same

90

Sr activity

d. is in the form of a peak recorded after Chernobyl. The results confirm the dating of layers of

305

To determine the dispersion of results on linear accumulation rates and mass

306

accumulation rates, the results obtained at two stations located in the Gdańsk Basin over 6 years

307

were compared (Tab. 2). At station P1, the LAR values were in the range from 1.67 mm y-1 to 11

308

1.98 mm y-1, giving an average value of 1.83 mm y-1 with a standard deviation of 0.12 mm y-1.

309

The median value was nearly identical - 1.82 mm y-1. In the case of P116 station LAR changed

310

in a slightly wider range from 2.0 mm y-1 to 2.41 mm y-1. The average value was 2.26 mm y-1,

311

and the standard deviation was 0.19 mm y-1. The MAR values varied respectively in the ranges:

312

320 - 400 g m-2 y-1 at P1 station and 330 - 530 g m-2 y-1 at P116 station. The average values were

313

390 g m-2 y-1 and 520 g m-2 y-1, and standard deviations remained at 14 and 16%.

314 315

3.2 Linear accumulation rate - LAR and mass accumulation rate - MAR

316

In the open sea region, the linear accumulation rate at TZ and P stations varied from

317

0.61 to 3.54 mm y-1 (Fig. 1b, Fig. 3). The lowest linear accumulation rate was observed, as

318

expected, in the areas of sandy silt and silty sand occurrence, which are characterized by a large

319

diversity of fractions and a thickness not exceeding 20 cm, or, in some areas, even 10 cm, and

320

where undisturbed sedimentation is significantly impeded. The lowest values were recorded at

321

the TZ18 station (0.61 mm y-1) determining the beginning of the Słupsk Furrow on the western

322

side and at the TZ14 station (0.75 mm y-1), constituting its eastern end (Fig.1b, Fig.3). At other

323

stations in Słupsk Furrow, in the areas of occurrence of silty sediments, the linear accumulation

324

rate was at the level of 1.2 - 1.3 mm y-1. The largest values, exceeding 3 mm y-1, characterize

325

the Bornholm Deep area where the linear accumulation rate drops to values slightly below 3

326

mm y-1 at the eastern and western borders (2.95 mm y-1 at TZ27 station and 2.97 mm y-1 at

327

TZ19 station). In the southern direction, the decrease in the LAR value is much more visible at

328

the values of 1.36 mm y-1 at TZ26 and 1.11 mm y-1 at TZ22 stations, despite the fact that they

329

are still areas of occurrence of muddy sediments. In the Eastern Gotland Basin, LAR values

330

were in the range from 2.09 mm y-1 at TZ11 station to 2.81 mm y-1 at TZ9 station. Values at a

331

similar level characterized the area of the Gdańsk Deep (2.27 - 2.80 mm y-1), while the LAR

332

values at stations located on the border of the Gulf of Gdansk (the line connecting Cape

333

Rozewie with Cape Taran) were slightly lower: 2.06 mm y-1 - TZ4. Comparatively lower

334

sediment accumulation rate of 1.67 mm y-1 characterized the Puck Bay (Szmytkiewicz and

12

335

Zalewska, 2014). The LAR values in the areas of the Szczecin and Vistula Lagoon were at a

336

similar level and amounted respectively to 3.5 mm y-1 and 3.3 mm y-1.

337

The values of the linear accumulation rate are generally in agreement with data from the

338

southern Baltic presented in other publications, despite these being point data (Suplińska and

339

Pietrzak, 2008; Zaborska, 2014; Zaborska et al., 2016), although for some locations there were

340

visible differences. It should be emphasized that the data presented here concerns the average

341

values of linear accumulation rates over the entire period covered by the dating, while taking

342

into account the possibility of changing conditions, mainly in the field of hydrodynamics and

343

the size of suspended matter (main factors determining the sedimentation conditions) (Zaborska,

344

2014; Zaborska et al., 2014), and sedimentation rate could have changed over the period.

345

Changes in MAR values largely reflected changes in the LAR values (Fig. 1b, Fig. 4). It

346

was shown that the linear correlation between the two parameters is statistically significant (R =

347

0.5804, p = 0.0003). At the same time, some deviations occurred, for example at TZ12 station

348

the highest MAR value was equal to 910 g m-2 y-1, despite the fact that the LAR value was not

349

the largest and was 2.78 mm (Fig. 1b, Fig. 4). For similar linear accumulation rate in other

350

areas, MAR values remained at levels 650-690 g m-2 y-1. Disproportionately high MAR values

351

of 690 g m-2 y-1 and 600 g m-2 y-1 were noted, against expectations, at TZ14 and TZ18 stations

352

respectively, where the linear accumulation rates were the lowest. This situation applies to the

353

areas where sandy-silt and silty sand occur, and the structure of the sediments may determine

354

their density and thus the greater mass accumulation rate. Linear accumulation rates in the outer

355

Bay of Puck, in the areas of silt-clay, were slightly lower than the values observed in the open

356

sea regions with similar characteristics. However, the value of MAR was one of the largest (820

357

g m-2 y-1) (Szmytkiewicz and Zalewska, 2014). The rates of mass accumulation in the areas of

358

the Szczecin and Vistula Lagoon were respectively 900 g m-2 y-1 and 740 g m-2 y-1. Relatively

359

high MAR values result largely from a large amount of suspended matter, especially of organic

360

origin (increased blooms - areas under high river water pressure and thus a significant inflow of

361

biogenic substances). Maps presenting continuous information on linear accumulation rate and

362

mass accumulation rate developed on the basis of discussed discrete values are largely in 13

363

compliance with the lithological map of bottom sediments developed by the Polish Geological

364

Institute - National Research Institute (PGI-NRI) (Figs. 3, 4).

365 366

3.3

Dating layers

367

Based on the CRS and CF: CS models, ages were assigned to individual sediment

368

layers, with the range of dating limited to the depth corresponding to the isotope measurements,

369

at levels guaranteeing measurement with a certain accuracy (Figs. 5a, 5b). This means that

370

extrapolating dating outside of the measurement range has not been performed and at the same

371

time, when interpreting the results, it is necessary to take into account the dating errors that

372

grow in depth, the scale of which is shown in Fig. 2b. In the Bornholm Basin, at TZ19, TZ23,

373

TZ25 and P5 stations characterized by a linear accumulation rates at the level of 3 mm y-1 and

374

above, layers at a depth of 19 cm were created about 50 - 60 years ago, with the deepest layers

375

at a depth of 25 cm corresponding to 70 - 80 years ago. A slightly different situation occurs at

376

the TZ27 station, where the LAR value was also close to 3 mm y-1, and the deepest layer 25 cm

377

was created around 112 years ago. In the case of P39 station (LAR - 2.41 mm y-1), the layer at a

378

depth of 19 cm was created about 70 years ago. A significantly longer time of bottom sediment

379

formation was valid for stations characterized by a lower sediment accumulation rate. In the

380

Bornholm Basin, such stations were TZ26, in which the layer at a depth of 13 cm was formed

381

about 90 years ago and TZ22, where the layer at the level of 15 cm corresponded to 150 years

382

ago.

383

In the area of the Eastern Gotland Basin, TZ8, TZ9, TZ10, TZ11 and TZ12 stations

384

were characterized by a relatively narrow range of LAR values (2.09 - 2.78 mm y-1). The

385

deepest layers of 25 cm were created between 80 to 130 years ago. In the Gdańsk Basin, at TZ3

386

and TZ6 stations, large LAR values (2.7 - 2.8 mm y-1) and the deepest 25 cm layers were

387

created 80 - 90 years ago. In the case of TZ2 and TZ4 stations, where the rate of linear

388

accumulation is at the level of 2 mm y-1, the creation period of the deepest layer is older at 110 -

389

120 years. The formation of layers at a depth of 19 cm at stations P110, P116 and P1

390

corresponds to the same period. 14

391 392

4. Conclusions

393 394

1.

In order to determine linear accumulation rates - LAR and mass accumulation

395

rates – MAR, and to assign age to particular layers of bottom sediments in the

396

area of the southern Baltic, a dating method based on analysis of

397

isotope changes in sediment cores using Constant Rate of Supply – CRS and

398

Constant Flux - Constant Sedimentation - CF: CS models was applied. Its

399

applicability in areas of clays, silt clays, sandy clays, clay silts, mules, sandy

400

silts, clay sands, sand - silt, silty sands and sand, was demonstrated.

401

2.

210

Pb lead

The reliability of the method used has been verified with use of a radiotracer,

402

(137Cs) and for the first time

403

activity concentrations to relevant and known historical events. In addition, a

404

very good reproducibility of results obtained in six consecutive years at two

405

stations located in the Gdańsk Basin has been demonstrated. At station P1,

406

the LAR values were in the range of 1.67 mm y-1 to 1.98 mm y-1, giving an

407

average value of 1.83 mm y-1 with a standard deviation of 0.12 mm y-1. In the

408

case of P116 stations, LAR varied from 2.0 mm y-1 to 2.41 mm y-1 giving an

409

average value of 2.26 mm y-1, with a standard deviation of 0.19 mm y-1.

410

3.

90

Sr, by assigning specific changes in their

The linear accumulation rate of bottom sediments of the southern Baltic

411

varied from 0.61 to 3.54 mm y-1, with the smallest values characterizing the

412

areas of sediments with morphology of sandy silt and mixtites due to

413

sedimentation, which is mainly hindered by hydrodynamic processes. The

414

highest rates of linear accumulation were specific to the areas of calm,

415

undisturbed sedimentation processes, i.e. deeper areas.

416

4.

Mass accumulation rates ranged from 390 g m-2 y-1 to 910 g m-2 y-1, largely

417

reflecting the LAR values. Discrepancies in some areas may be related to the

418

amount and structure of suspended matter. 15

5.

419

Maps presenting linear accumulation rate and mass accumulation rate largely

420

reflect lithological areas of bottom sediments in the southern Baltic region

421

and are an excellent tool to support the study of processes taking place in

422

marine areas, as well as supporting research and assessment of the Baltic

423

Sea's environmental status.

424

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Characteristics of selected elements of the environment). IMGW, Warszawa (in Polish)

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560

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561

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562

Figure captions

563

Fig. 1a Coordinates of sampling stations.

210

Pb geochronology, Journal of Environmental

137

Cs in

21

564

Fig. 1b Linear accumulation rates (mm y-1) - upper values, mass accumulation rates (g m-2 y-1) –

565

lower values at sampling stations (names in brackets) located in the southern Baltic Sea; red line

566

marks areas of occurrence of silt-clay, sandy silt, silty sand and mixtites.

567

Fig 2

568

depth and age of sediment layers at station TZ4 – b, changes in 137Cs activity concentrations in

569

dating with CRS and CF:CS models sediment profile at station TZ4 – c, changes in

570

(squares) and 90Sr (circles) activity concentrations in dating with CRS model sediment profile at

571

station P1 – d.

572

Fig. 3 Map showing linear accumulation rates (mm y-1) in the southern Baltic Sea areas of

573

occurrence of silt-clay, sandy silt, silty sand and mixtites (red line).

574

Fig. 4 Map showing mass accumulation rates (g m-2 y-1) in the southern Baltic Sea areas of

575

occurrence of silt-clay, sandy silt, silty sand and mixtites (red line).

576

Fig 5 a, bMaps showing the age of sediment layers in the southern Baltic Sea areas of

577

occurrence of silt-clay, sandy silt, silty sand and mixtites (red line).

578

Tables

579 580

Table 1. Results of certified reference materials analysis

210

Pb activity concentrations in the sediment core at station TZ4– a, correlation between

Certified reference material IAEA – 300, Radionuclides in the Baltic Sea

581 582 583 584 585 586 587

Analysed element 137

Cs

210

Pb

137

Certified value

Confidence interval

Mesured value

-1

1046-1080

1065 ± 24 Bq kg on reference date: 1.01.1993 -1 350 ± 40 Bq kg on reference date: 1.01.1993

1056.6 Bq kg on reference date: 1.01.1993 -1 360 Bq kg on reference date: 1.01.1993

339-395

Cs

-1

Table 2. Comparison linear sedimentation rates (LAR) and mass accumulation rates (MAR) determined at two station (P1 and P116) located in the Gdańsk Basin in years 2009-2014. Station

P1

P116

2009

LAR -1 mm year 1.78

MAR -2 -1 g m year 320

LAR -1 mm year 2.00

MAR -2 -1 g m year 330

2010

1.86

460

2.32

520

Year

22

2011

1.98

410

2.41

520

2012

1.95

350

2.36

490

2013

1.72

430

2.06

510

2014

1.67

400

2.43

530

Mean

1.83

390

2.26

480

Median

1.82

400

2.34

520

SD

0.12 (6%)

55 (14%)

0.19 (8%)

77 (16%)

588

23

Fig. 1a

Fig. 1b

Fig. 3

Fig. 4

Fig. 5a

Fig. 5b

Highlights • • • • •

Maps presenting geochronology of the southern Baltic Sea region were developed LAR and MAR were determined with 210Pb method combined with CF:CS and CRS models The age of the particular layers of bottom sediments were presented on the maps LAR of the southern Baltic varied from 0.61 to 3.54 mm y-1 MAR of the southern Baltic ranged from 390 g m-2 y-1 to 910 g m-2 y-1

Conflict of Interest Declaration

On behalf of all authors I declare that no undisclosedrelationship that may pose a competing interest exists and no undisclosedfunding source that may pose a competinginterest exists.

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