Introduction

Sediment history in the lagoons and large lakes can provide information about environmental changes, and shows chronological records in sediment layers [1,2,3]. The varve chronologies-based methods are applied to determine sediment chronology. For younger sediment than 100–120 years, Lead-210 (210Pb, T1/2 = 22.3 years) dating method is common. This method include the constant rate of supply (CRS) model and constant initial concentration (CIC) model [4]. Another dating method is 137Cs based method that derived from nuclear fallout studies [5].

The man-made radioisotope of 137Cs (\(T_{1/2} = 30.17\;{\text{years}}\)) produced in nuclear fission reactions has entered into the atmosphere in irregularly varying amounts since 1945 [6, 7]. Nuclear accidents such as the Chernobyl accident and atmospheric nuclear weapons testing of the 1950s released large amounts of fission product like 137Cs into the environment [8].

Considering the 137Cs half-life, makes it’s as measurable tracer in the early 1950s, so sediment profile in a core along with 137Cs detectable activity has chronology application [9, 10]. In practical studies, 137Cs distributions in sediments profile derived from atmospheric nuclear weapons testing in the 1960s and the Chernobyl accident in 1986 were used as time markers to calculate sediment accumulation rates in lakes and reservoirs [11,12,13,14]. Determination of the sedimentation rates with the 137Cs and 210Pb method is suitable for sediments of up to 100 -120 years old [15,16,17].

Today it is accepted that two major challenge of global warming and land use change have caused significant changes of sediment distribution rates within the different parts of the fluvial system during recent decades. Schumm [18] divided a fluvial system into three zones: an upland erosion zone, a transportation zone and a sedimentation zone. Climate changing and/or human activities may result in changes in erosion and transportation processes in the first two zones, and these will influence the sedimentation rate in the third zone [19].

In this research, due to recent concerns about unusual changes of the Anzali Lagoon ecosystem, with emphasizes the effects of short-term natural changes in the coastal lagoons about anthropogenic effects, and demonstrates that rapid change is not only due to human action but also due to rapid sea-level change (Fig. 1). The hypothesis of this research was an investigation of sediment age of the Anzali Lagoon to determine entering and displaced sediment rate in Lagoon. By understanding the annual sedimentation rate, this can provide suitable planning for the lagoon conservation and sediment management. For this reason, we apply the sedimentary analyses method in the Anzali Lagoon. For this purpose, the distribution of the 137Cs and 210Pb in sediment core profiles were measured. The sediment accumulative rates (SAR), sediment chronology and sedimentation rates (SR) after the 1960s were carried out by means of 137Cs and 210Pb based dating methods. Whereas, spatially concentrated on SR and SARs in the last decades. Also, the other purpose was the study of SR and SARs in 1963–1986 and after 1986 periods.

Fig. 1
figure 1

The average monthly mean Caspian Sea level (CSL) changes (1840–2015), [22]

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Materials and methods

Study area description

The study area was selected the Anzali Lagoon with a wet weather marsh ecosystem where this lagoon is one of 10 well-known lagoons in the world located in the western south of Caspian Sea, between 49.25 to 49.5 E and 37.25 to 37.5 N. (Figure 2). The area of the Anzali Lagoon complex covers 150 km2 [20] and the distance from Chernobyl nuclear power plant approximately is 2000 km.

Fig. 2
figure 2

Geographical location of the Anzali Lagoon ecosystem and sampling points

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The average lagoon level is about − 26.2 m. Also, water depth range is between 1 and 3.5 m. Lagoon has a Mediterranean climate with abundant precipitation of 1600 mm (in mean annual precipitation). More than fifteen perennial and ephemeral rivers originating from the Alborz and Talesh Mountains discharge into the lagoon. The geology of the Lagoon basin is characterized by rocks of the Devonian-Carboniferous Gneiss, Precambrian Metamorphic Complex (schist), migmatite and mica schist rocks. These rocks are overlain by middle Triassic basic to ultrabasic rocks. Green micaceous schists and sandstones of Jurassic age, limestone of Cretaceous age, and green and red marls of Pliocene age also form outcrops in the area. The sediment origin of the Lagoon is from three sources: (a) sediments deposited from rivers; (b) erosion of Alborz mountain (coarse sediments); and (c) marine deposition (includes shelly sands) [21].

Various plants such as Phragmites, Aquatic macrophytes and Azolla as well as special morphology with some transferring channels to the Caspian Sea make the lagoon a touristic destination. Different uses of the Lagoon by residents such as the application of its water for agriculture, the lagoon plant as forage for domestic animals, its suitable place for birds and fishing makes it essential for monitoring of 137Cs activity concentration in sediment profiles.

Field sampling

A total of four sediment cores were collected from different locations of the Anzali Lagoon, via a vibra-coring unit with portable by boat in September 2014. The core (inside diameter of φ = 12 cm) length was variable from 45 to 57 cm and water depth was from 1.5 to 6 m. The core tubes were transferred vertically to the laboratory where they sliced at 1 cm thick layers.

Sediment analysis

Each sediment sample weighed and dried at ~ 80 °C for 12 h to measuring before and after drying in order to determine the dry weight of the sample. The porosity of samples was calculated using the following equation [23]:

$$\upphi = \frac{{f_{w} }}{{\left[f_{w} + \left( {1 - f_{w} } \right).(\rho_{w} /\rho_{s} )\right]}}$$
(1)

where ϕ is the porosity, \(f_{w}\) is the fraction of water in the fresh sediments (\(1 - \frac{dry \;wt}{fres \;wt}\)), \(\rho_{w}\) is the density of pure water (~ 1 g cm−3), and \(\rho_{s}\) is the density of the dry solids (~ 2.5 g cm−3).

Deeper sediments were a combination of fine and medium grains such as silty, fine and sand. While the surface sediments content was a mixture of silts, clays and mud.

To take into account sediment compaction dating must be performed as a function of mass depth m (kg m−2). For this porpose the mass depth (\(m_{i}\)) of layer (i) (kg m−2), was calculated by:

$$m_{i} = \mathop \sum \limits_{j = 1}^{j = i} \frac{{\Delta m_{j} }}{s}$$

where \(\Delta m_{j}\) is the dry mass of section i (kg), experimentally determined and S is core cross-section (m2).

Gamma spectrometry analysis

The gamma-ray spectra system used to measure of the prepared samples. This system consists of a typical high-resolution gamma spectrometer (SILENA) based on a coaxial P-type HPGe detector with cylindrical shielding, a relative photopeak efficiency of 80% and an energy resolution of 1.80 keV FWHM for the 1332 keV gamma-ray line of 60Co. Before the sample measurements, the environmental gamma background (B.G) in the laboratory site was determined using an empty 100 ml cylindrical beaker under identical measurement conditions. Each sample was transferred to 100 ml sealed polyethylene cylindrical beaker and stored to receiving equilibrium between parent (226Ra) and daughters (214Pb, 214Bi) radionuclides. After then the sample put into the shielded HPGe detector and measured for an accumulated period of 86,000s [24]. Commercial software Gamma 2000 was used for data analysis [25]. The 137Cs, 226Ra and 210Pb activity concentrations were determined for each of the measured samples together with their corresponding total uncertainties. Activities concentration of 137Cs was calculated due to 661.6 keV photopeak energy line. The 210Pb activity concentration was measured with 46.5 keV gamma line energy. Also, the 226Ra was measured using its decay product gamma emitters of 214Pb (295 keV, 351 keV), 214Bi (609 keV). The detector efficiency calibration was performed by using the IAEA-385, IAEA-375, and IAEA-384 for quality assurance reference materials produced by IAEA. The radionuclides concentration was presented in the datasheet with a 95% confidence level and all the radionuclides concentration corrected to the using date.

Activity concentration of radionuclides (A) in Bq kg−1 can be calculated by [26]:

$$A = \frac{N - B}{{t \times p_{\gamma } \times ef \times m}}$$
(2)

where \(N\) is the count rate of the sample, \(B\) is the background, \(t\) is the counting time (s), \(P_{\gamma }\) is the probability of gamma decay (%), \(ef\)—detector efficiency (%), \(m\) is the mass (kg) of the soil or sediment sample.

The minimum detectable activity (MDA) was calculated using the following equation:

$$MDA = \frac{LLD}{{t \times p_{\gamma } \times ef \times m}}$$
(3)

The lower limit of detection (LLD), was calculated by using the relation \(LLD = 4.66\sqrt {F_{c} }\) where \(F_{c}\) is the Compton background in the region of the selected gamma-ray spectrum with a 96% confidence [27].

Uncertainty calculation

Uncertainty assessment is necessary to each calculation, the activity concentration uncertainty (UA) calculated by the following equation:

$$\frac{{{\text{U}}_{\text{A}} }}{\text{A}} = \sqrt {\left( {\frac{{{\text{U}}_{\text{N}} }}{\text{N}}} \right)^{2} + \left( {\frac{{{\text{U}}_{\text{B}} }}{\text{B}}} \right)^{2} + \left( {\frac{{{\text{U}}_{\upvarepsilon} }}{\upvarepsilon}} \right)^{2} + \left( {\frac{{{\text{U}}_{\text{M}} }}{\text{M}}} \right)^{2} + \left( {\frac{{{\text{U}}_{{{\text{P}}_{\gamma } }} }}{{{\text{P}}_{\gamma } }}} \right)^{2} }$$
(4)

where \({\text{U}}_{\text{N}}\) is sample counting uncertainty; \({\text{U}}_{\text{B}}\), background counting uncertainty; \({\text{U}}_{\upvarepsilon}\), efficiency uncertainty, \({\text{U}}_{\text{M}}\), mass measurements uncertainty and \({\text{U}}_{{{\text{P}}_{\gamma } }}\), gamma line energy uncertainty (for example this uncertainty for137Cs is 0.24% of intensity) [28].

Sediment chronologic models

According to some reports, it is possible to use the 137Cs depth profile within such sediment deposits to identify the first peaks of 137Cs activity corresponding to the year of maximum nuclear weapons testing fallout (1963) and the second peaks of 137Cs activity related to the occurrence of the Chernobyl accident 1986 [29,30,31,32,33,34].

The other models that are used to estimate the chronology in this study are the constant initial concentration (CIC) model by unsupported 210Pbex (excess 210Pb or 210Pb–226Ra) [35] and the constant rate of supply (CRS) model with unsupported 210Pbex [36].

137Cs chronology

It is evident that the magnitude of 137Cs activity has two peaks in all core sampling points. This procedure provides the possibility of calculating sedimentation rates during the periods 1963–1986 and 1986 to the present. However, in order to interpret the 137Cs concentration in sediment profiles need to consider the various factors on the profiles. So, in this method, the evaluation result of sedimentation rates is average on interval times.

Where the 1963 peak could be observed in the 137Cs vertical sediment profile, the average sedimentation rate (SR) \(R_{t}\) (cm year−1) for the period of 1963–1986 was calculated as follows equation [37]:

$$R_{t} = \frac{{D\left( t \right)_{1963} - D\left( t \right)_{1986} }}{{T_{1963 - 1986} }}$$
(5)

where \(D\left( t \right)_{1963}\) is the depth of the 1963 137Cs peak, (cm); \(D\left( t \right)_{1986}\) is the depth of the 1986 137Cs peak, (cm) and \(T_{1963 - 1986}\) is the interval time from 1963 to 1986.

210Pb chronology

The constant rate of supply (CRS) model was used to unsupported 210Pb (210Pbex) inventories evaluated by subtraction of 210Pb supported by 226Ra to generate ages. Whereas, the total 210Pb concentration in sediments including two sections, unsupported which deposited from the atmosphere and supported 210Pb activity that produces inside the matrix of parent radionuclide 226Ra. The CRS model in each of the cores was calculated from the following equation [38]:

$$\sum I_{m} = \left(\sum I_{\infty } \right)(1 - e^{{ - \lambda_{Pb} t}} )$$
(6)

where \(\sum I_{m}\) is the 210Pbex concentration in mass depth (Bq cm−2), \(\sum I_{\infty }\) is the total concentration of 210Pbex in sediment column (Bq cm−2), and \(\lambda_{Pb}\) is decay constant of 210Pb (0.03118 year−1). The age of the sediment layer \(t_{CRS}\) is expressed by:

$$t_{CRS} = - \frac{{\ln \left( {1 - \frac{{\sum I_{m} }}{{\sum I_{\infty } }}} \right)}}{{\lambda_{Pb} }}$$
(7)

The CIC model was applied to calculate sediment layer ages in core samples. According to CIC model, the specific activity in the surface deposits will always be constant because due to sediment flux changes, sweeping of 210Pb from the water column will also change [39,40,41]. According to the CIC model, the age of the sediment layer \(t_{CIC}\) is described as follows:

$$t_{CIC} = - \frac{{\ln \left( {\frac{{A_{M} }}{{A_{0} }}} \right)}}{{\lambda_{Pb} }}$$
(8)

where \(A_{M}\) is the 210Pbex activity concentration in cumulative mass depth “M” (Bq g−1), \(A_{0}\) is the 210Pbex activity concentration in the water–sediment interface (Bq g−1) and \(\lambda_{Pb}\) is decay constant of 210Pb.

This model was applied only for comparing the results with the CRS model. The main assumption of this model is that the proportion between 210Pb flux to the sediments and the accumulative mass rates is constant along with the entire core. Whereas the accumulative mass rates will be changed throughout of core, so the results of this model are without good precisions, and we ignore the results of that report.

Results

According to our investigation, there is no study on the radioactivity concentration of 137Cs and 210Pb in the Anzali Lagoon sediments profiles, when this project has done (2018). In the present study, we measured the concentration of 137Cs and 210Pb in the Anzali Lagoon sediments from 0 to 50 cm with dividing into 1 cm intervals. The water depth and other characteristics of sediment sampling points are shown in Table 1. The water depth in four core collections ranged from 1.2 m (Core-4) to 5.8 m (Core-3). The porosity value range and average of core-1, core-2, core-3 and core-4 were 0.72 ± 0.01–0.81 ± 0.02 (0.78 ± 0.02), 0.70 ± 0.01–0.84 ± 0.04 (0.82 ± 0.03), 0.75 ± 0.02–0.85 ± 00.5 (0.79 ± 0.04) and 0.71 ± 0.02–0.88 ± 0.04 (0.84 ± 0.03), respectively are summarized in Table 1.

Table 1 The characteristics of core sampling points
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The activity concentration of 137Cs in selected sediment profiles in depth with an interval of 1 cm was measured (Table 2). The minimum and maximum activity concentration range in Core-1 was 2.51 ± 0.02 - 65.28 ± 4.38 Bq kg−1 corresponding to depth 42 and 24 cm, respectively. This conformity for core-2, core-3, and core-4 were 3.62 ± 0.05 Bq kg−1 (48 cm)—62.15 ± 5.31 Bq kg−1 (22 cm), 3.19 ± 0.02 Bq kg−1 (44 cm)—61.76 ± 4.28 Bq kg−1 (23) and 3.03 ± 0.02 Bq kg−1 (46 cm)—73.50 ± 5.27 Bq kg−1 (25 cm), respectively. The median of activity concentration range was 24.95 ± 2.06 Bq kg−1(core-4) to 32.65 ± 3.01 Bq kg−1 (core-2). 137Cs activity concentration profiles from the four sampling locations are presented in Fig. 3.

Table 2 137Cs and 210Pbex activity concentration in sediment profiles of four core sampling locations
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Fig. 3
figure 3

Distribution of 137Cs activity concentration in four core sediment profiles

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The total activity concentration of 210Pbex were ranged from 11.26 ± 0.07 Bq kg−1 to 218.42 ± 12.51 Bq kg−1 and 226Ra activity concentration was ranged from 3.18 ± 0.16 Bq kg−1 to 34.52 ± 2.77 Bq kg−1 in the Core -1 samples, from 9.40 ± 0.24 Bq kg−1 to 230.42 ± 11.38 Bq kg−1 and 226Ra activity concentration was ranged from < MDA (\(MDA_{226Ra} = 1.2\) Bq kg−1) to 28.15 ± 1.27 Bq kg−1 in the Core-2 samples, from 5.64 ± 0.04 Bq kg−1 to 191.28 ± 15.36 Bq kg−1 and 226Ra activity concentration was ranged from 1.53 ± 0.28 Bq kg−1 to 17.27 ± 1.28 Bq kg−1 in the Core-3 samples and from 5.89 ± 0.03 Bq kg−1 to 305.22 ± 14.08 Bq kg−1 and 226Ra activity concentration was ranged from < MDA to 27.13 ± 1.55 Bq kg−1 in the Core-4 samples. 210Pbex activity concentration profiles from the four sampling locations are presented in Fig. 4.

Fig. 4
figure 4

Distribution of 210Pbex activity concentration in four core sediment profiles

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Sedimentation rates estimation

According to our investigation, the first and second peaks of 137Cs concentration in sediment profile ranged from 37 to 40 and 22 to 25 cm, respectively (Table 3). The first peak concern to significant fallout on a global scale began after the initial test in the early 1960s from nuclear weapons testing and then declined because of the performance of the international treaty banning like tests. The substantial second peak originated from Chernobyl accident fallout in 1986.

Table 3 The average sedimentation rates (SR) and average sediment accumulative rates (SARs) calculated using 137Cs and 210Pbex based in the Anzali Lagoon samples
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The analysis of 137Cs activity concentration in sediment profile led us to the calculation of SR and SAR in the Anzali Lagoon. The calculation results are chronologically divided into two sections from 1963 to 1986 and after 1986. In the section from 1963 to 1986 interval, the range of SR and SAR were 0.52 ± 0.08 to 0.86 ± 0.15 cm year−1 with an average of 0.69 ± 0.07 cm year−1 and 0.08 ± 0.02 to 0.14 ± 0.07 g cm−2 year−1 with an average of 0.11 ± 0.05 g cm−2 year−1, respectively. Whereas, the SR and SAR values range after 1986 were 0.78 ± 0.11 to 0.89 ± 0.15 cm year−1 (0.84 ± 0.13 cm year−1 on average) and 0.16 ± 0.01 to 0.18 ± 0.03 g cm−2 year−1 (0.17 ± 0.02 g cm−2 year−1 on average), respectively. (Table 3).

The results of SR and SAR with 210Pbex based inventories in sediment profile calculated and shown in Table 3. The value of SR was 0.61 ± 0.18 cm year−1 in core-2 and 1.03 ± 0.27 cm year−1 in core-3 with average 0.76 ± 0.12 cm year−1. Also, the SAR value calculated by CRS were reported in Table 3. The CRS obtained values were ranged from 0.12 g cm−2 year−1 in core-2 to 0.21 g cm−2 year−1 in core-3.

Discussion

In this research, the sediments porosities taken from four sampling points have not any unusual event along to sediment profiles. Porosity varies between 0.72–0.81 from bottom to top levels in core-1, between 0.70–0.84 levels in core-2, between 0.75–0.85 levels in core-3 and between 0.71–0.88 levels in core-4. So, the sediments accumulated in the study area basement was homogenous. The relatively lower porosity at the bottom layers of cores might be due to sediment consolidation or compaction [6, 42, 43].

The result of 137Cs and 226Ra activity concentration in surface sediment (1–2 cm) were ranged between 9.74 to 19.34 Bq kg−1 and 3.85 to 34.11 Bq kg−1, which is comparable with the last report from the Anzali wetland surface sediment 0.43- 63.35 Bq kg−1 and 18.06 to 33.36 Bq kg−1, respectively. [44].

137Cs activity concentration in all sediment profiles samples showed two peaks in layers. These peaks were observed in depth 40 cm and 24 cm, 42 cm, and 22 cm, 39 cm and 23 cm, and 37 cm and 25 cm for sample core-1, core-2, core-3, and core-4, respectively. Pulley and Foster et al. identified these peaks in Silsden sediment profiles at 28 cm and 13 cm depth [45]. Which these results were compared with other worldwide reports.

Ages calculated using 210Pbex and 137Cs technique are plotted against cumulative mass depth for each of the four cores (Fig. 5). In the core-1 profiles, among the CRS and CIC models, the CRS plot seems to have good agreement with 137Cs technique-based chronology. The first peak of 137Cs is 40 cm (1963), whereas according to the CRS model 40 cm is 1958. The second peak of 137Cs indicated 24 cm (1986). Hence the CRS model provided 1982. The good agreement between the CRS model and 137Cs technique-based chronology was reported by [46,47,48,49,50,51].

Fig. 5
figure 5

Vertical profiles of sediment ages by cumulative mass in four cores obtained from CRS, CIC models and 137Cs technique

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The 137Cs technique with CRS and CIC models’ plots has significant overlap in core-2, whereas in this core, 137Cs was located in between the CRS model. The first peak of 137Cs is 42 cm (1964), whereas according to CRS and CIC models 42 cm fit in 1970 and 1954, respectively. The second peak of 137Cs indicated 22 cm (1986), while the CRS model provided 1985 and CIC model fit in 1988. So, three methods have good agreements with together in core-2. The core-3 and core-4 results in CRS and CIC models are more distributed. There is no uniform consonant for the second peak of 137Cs between cores. This inconsistency has been reported elsewhere [38, 52,53,54,55]. Only in cores 1 and 2 there is noticed a rapid trend alteration 45 and 55 years before, respectively.

The 137Cs-peak based sediment accumulation rates (SAR) is generally based on one curve fitting method, based on the 137Cs first peak corresponding to 1963 date. Also, the appearance of the 137Cs radionuclide in sedimentary profiles returns to 1952 can be used as a time scale [56]. The 2nd peak of 137Cs in sediment profiles is corresponding to Chernobyl accident fallout in 1986 [57, 58]. The average SR and SAR were calculated in two intervals. Whereas the average SR and SAR value in after 1986 interval was 22% and 54% more than 1963–1986, respectively. Where our value of 0.66 cm year−1 in 1963–1986 interval has good agreement with their report results. Also, the results of this study are within the range of SR value of 1.999 cm year−1 to 0.036 cm year−1 which reported in [59]. Eleftheriou et al. reported the SAR value between 0.21 and 0.6 cm year−1 (CRS model) and 0.13–0.42 cm year−1 (CIC model) in north-east Mediterranean [60]. As shown in Table 3, our average SR value in the Anzali Lagoon were higher than those measured in Greece (Lavrio) [61], Poland (Baltic Sea) [62]. Moreover, the results of this this study is lower than the last study in the Anzali Lagoon with the same characteristics with 137Cs tracer technique [63].

Conclusions

This study has focused on recent concerns about unusual changes in the Anzali Lagoon ecosystem. For this purpose, the sediment porosities were computed and an exponentially decreasing trend observed in the porosity of upper sediment layers to deeper layers in all core sampling of the study area. Also, the distribution of 137Cs and 210Pbex concentration in sediment profiles were measured in core samples. The 137Cs technique and 210Pbex based models (CRS and CIC models) were applied to sediment chronology. The results of CRS, CIC models, and 137Cs technique were compared. The CRS model has good compatible with the 137Cs-peak-based technique in core-1 and core-2, whereas the CIC model only was agreed with results of 137Cs-peak-based technique in core-2.

In this study, the SR and SAR of Anzali Lagoon were studied using natural 210Pbex based models and artificial 137Cs-peak-based technique. Average obtained results of SR in the North area (core-4) of the lake was higher than the South area (core-2). This can be due to the gradient of the land surface of the Lagoon because the slope of the land is southern in this area. Also, the 137Cs peak-based technique of SR values was calculated in two intervals of 1963–1986 and after 1986 years. The SR value of after 1986 was higher than 1963–1986 interval on average. Besides, this trend is also true with the results of SAR. Whereas the results showed that the average of SR and SAR values in after 1986 interval were 22% and 54% more than between 1963 and 1986, respectively. It could be due to floods and storms are occurring in the study area and human activities. Also, due to the lack of any available date in this area, the results of the present study will provide available and comparable data for the next studies.