Introduction

The Aegean Sea lies in the northeastern Mediterranean, between Ionian Sea and the Levantine Sea. It is bound by the Greek mainland to the north and west, the Turkish coast to the east and the Island of the Cretan Arc to the south as well as its connection with the Marmara and Black Seas through the straits of Dardanelles and Bosphorus [1]. It has a surface area of about 1.8 × 1011 m2, a volume of 8.5 × 1013 m3 and a mean depth of 450 m [2]. Numerous investigations were performed there but the literature on the sediment chronology and evaluation of the element concentration in terms of its origin in Turkish coast of north Aegean Sea is not proportional to the number of investigations which are using nuclear methods. For instance, hydrothermalism of the Aegean Sea was described [3, 4] and origin and distribution of the terrigenous component of the unconsolidated surface sediment of the Aegean floor investigated [5]. Venting fluids are known to be highly enriched in natural radionuclides including 210Po and its grandparent 210Pb and Ca, Al are derived from biogenic debris in hydrothermal areas [6]. In addition, variations in grain size, carbonate content, sand mineralogy of surficial bottom deposits from transition in the Aegean-Çanakkale-Marmara were examined [7]. It is useful knowledge in terms of sorption capacity of radionuclide in sediment and Al, Ni originate from terrigenous supply. Microseismicity of the Milos Island, Greece was investigated to correlate the recorded earthquake signals with venting periodicity and time compositional variability of suspended particle matter in submarine of Milos Bay was studied to better understand role of episodic events [8, 9], microseismicity directly affects the Mn concentration. Distribution and possible sources of metals in the surface sediment of the Gulf of Saros were studied and impact of urban discharges on the levels of metallic contaminants also determined in Mytilene harbor and adjacent coastal area [10, 11]. It is possible to enhance the related samples but it could be enough to give only one sample in different categories. Concerning sediment chronology and natural radioactivity, the cases is that natural radioactivity is widespread in the earth environment and exists in various seafood, marine organisms and geological formations such as soil, rocks, plants, sand, water and air.

In terms of natural and anthropogenic radioactivity, impact of shallow water hydrothermal inputs on the biogeochemical cycling of 210Po and 210Pb was assessed in the coastal marine environment and effects of late quaternary climatic and sea-level changes on sedimentation and paleo bathymetry off the Büyük Menderes River delta in Aegean Sea investigated [12, 13]. Natural and anthropogenic radionuclide levels and some heavy metals were determined in the sea sediments and total alpha and beta activity concentrations of sea water were measured in Aegean Sea [14]. One has determined recent chronology by lead-210 dating models, metal concentrations and hydrothermal impacts on sedimentation in Gülbahçe Bay [15].

The primary aim of the study is to determine the sediment dating of north Aegean Sea via lead-210 models, the second clarify the sedimentation pattern of the region, the third evaluate the pollution degree of study area the last investigate the relation between sedimentation pattern and accumulation of metal concentrations. To achieve the above, sediment chronology was obtained using Constant Rate of Supply and Constant Initial Concentrations mathematical models on the ground that there were profile distributions of 210Pb activities along the cores. Residence time of 210Pb was calculated to clarify the sediment dating and sedimentation pattern of the region. Four pollution index (Enrichment Factor, Anthropogenic Factor, Pollution Load Index, Geo-accumulation Index) were employed to evaluate the pollution degree and evaluation was supported in terms of lead isotopes (206Pb/207Pb ratio). Correlations between the metal concentrations were used to identify the source of the metals. Distribution of metal concentrations and sedimentation patterns were investigated to indicate the potential relationships between them.

Material and method

The present research obtained fifteen surficial sediment samples and two sediment cores cruise of the R/V Piri Reis, of the Institute of Marine Sciences and Technology Dokuz Eylül University, using a Van Veen grab sampler and box corer from the north Aegean Sea in 2014 (Fig. 1). Locations of the stations were from 39°18′6.00′′N, 26°37′0.00′′E to 38°26′15.33′′N, 27°6′6.52′′E In North–south direction, water depths varied between 16 and 132 m, sediment core lengths were 39 and 30 cm and surficial sediments represented the upper 5 cm of the sea floor.

Fig. 1
figure 1

Study area and sampling locations

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Polonium-210 and lead-210 radionuclides

There are two stages in preparation of samples for analysis. The first is physical preparation which consists of the following steps. The cores were removed from the PVC pipes and cut in intervals of 1 cm. Before sediment samples were oven dried, wet weights of the samples were noted, then ground and passed through a 63 µm mesh to obtain homogenization. The second which consists of the chemical steps is used for the radiochemical analysis. One completely dissolved 1 g sample and standardized 0.1 Bq mL−1 amount of 209Po (half-life: 103 years) internal tracer (National Institute of Standards and Technology, < 500 Bq in 7% hydrochloric acid) in HF:HNO3 (1:1) and HCl in Teflon beaker. With the addition of the ascorbic acid to the 0.5 M HCl solution polonium was spontaneously plated onto copper discs in it. Ascorbic acid was used to reduce ferrum ions [16]. To measure 210Po concentrations of 5.30 MeV alpha particles emission to stem from 210Po, 210Po-accumulated cupper discs were counted by alpha detectors (Ortec Octete Plus with 450 mm2 ULTRA-AS Detectors). After the initial deposition of polonium, residual 0.5 M HCl solutions were kept for about one year to form of 210Po that was supported by 210Pb [17]. Polonium caused by 210Pb was kept in 0.5 HCl solution was plated again onto discs to determine the 210Pb activity. Counting period was adjusted to keep the standard error under the 10% (relative standard error was approximately 5%). The Certified reference materials, IAEA-300 and IAEA-384, were analyzed to check the sensitivity of alpha spectrometry and recovery rates of standardized tracer for the sediment samples varying between 78 and 91%. Bateman equations and recovery were used to determine 210Po (210Pb) concentrations [15, 18].

In alpha spectrometric studies, 226Ra activity is directly estimated from the layer with the lowest total 210Pb activity. However, gamma spectroscopic studies are greatly likely to measure the concentrations of 226Ra and 210Pb separately for each layer.

The sediment core chronologies were obtained by the Constant Initial Concentration (CIC) and Constant Rate of Supply (CRS) models [19,20,21,22,23,24,25]. It was then evaluated which model was the best considering the model assumptions. CIC model supposes that an increased flux of sedimentary particles from the water column would remove proportionally increased amounts of 210Pb from the water to the sediments, under the assumption of which sediments would have the same initial unsupported 210Pb concentration irrespective of any variations in sediment accumulation rate. If the assumptions of the CIC model are satisfied, the unsupported 210Pb activity will decay with depth in accordance with the below equation,

$$C\left( m \right) = C\left( 0 \right)e^{ - \lambda t}$$
(1)

where C(m) is the unsupported 210Pb concentration at depth m and C(0) the unsupported 210Pb concentration of sediments at the sediment water interface. The age of a sediment layer with 210Pb concentration C(m) is thus obtained from below equation.

$$t = \frac{1}{\lambda }ln\left[ {\frac{C\left( 0 \right)}{C\left( m \right)}} \right]$$
(2)

If the CIC model is applicable, the unsupported 210Pb concentration must show a monotonic decline with depth and the total cumulative residual unsupported 210Pb in sediment cores from the same area would vary roughly in proportion to the mean sediment accumulation rate [26].

However, CRS model assumes that there is a constant rate of supply of 210Pb from the atmosphere to the lake waters resulting in a constant rate of supply of 210Pb to the sediment regardless of any variations which may have occurred in the sediment accumulation rate [26]. Thus the age of each layer is calculated as follows:

$$t = \frac{1}{\lambda }ln\left[ {\frac{A\left( 0 \right)}{A\left( 0 \right) - A\left( m \right)}} \right]$$
(3)

where A(0) is the total inventory in core, λ the decay constant and A(m) the cumulative inventory of unsupported 210Pb concentrations at depth m. Accumulated and total inventory of 210Pb are calculated as follows:

$$A\left( m \right) = \mathop \int \limits_{0}^{n} C\left( m \right)M\left( m \right)dm,\;A\left( 0 \right) = \mathop \int \limits_{0}^{\infty } C\left( m \right)M\left( m \right)dm$$
(4)

where, C(m) is unsupported 210Pb concentration (mBq g−1), M(m) is mass depth of core layer (g cm−2).

$$\emptyset = A\left( 0 \right)\lambda$$
(5)

where ∅ is unsupported 210Pb flux (mBq cm−2 year−1) and calculated as above.

If the CRS model is applicable, non-monotonic profiles may be expected in response to major changes in the accumulation rate and different cores from the same area will have comparable 210Pb residuals despite differences in the accumulation rates [26].

To improve the precision of sedimentation rate values, residence time of 210Pb was calculated. Kumar et al. [27] thoroughly described how residence time of 210Pb is calculated by the first order kinetic relationship.

$$dI_{w} /dt = \varPhi_{w} - \left\{ {\left[ {\lambda_{Pb} + \left( {1/T_{w} } \right)} \right]I_{w} } \right\}$$
(6)
$$dI_{s} /dt = \varPhi_{s} - \left\{ {\left[ {\lambda_{Pb} + \left( {1/T_{s} } \right)} \right]I_{s} } \right\}$$
(7)

where \(\varPhi_{w}\) and \(\varPhi_{s}\) represent the fluxes of 210Pb in water and sediment, respectively. Similarly, Iw and Is represent the inventories, λPb is the radioactive decay constant of 210Pb, Tw and Ts which are the residence times of the 210Pb in water and sediment, respectively. Calculation details and necessary evaluations are in the article by Kumar et al. [27].

Metals and lead isotopes

Role of the heavy metals due to human activities or natural contributions is an important impact on environment. Therefore, researches on pollution are crucial part of geochemical studies [28, 29]. Nieboer and Richardson [30] claim that some heavy metals are constituents inherent in the marine environment. Fe, Cu, Zn, Co, Mn, Cr, Mo, V, Se and Ni are known to be essential to marine organisms which always function in combination with organic molecules in general proteins. However, Ag, Hg, Cu, Cd and Pb are particularly toxic [31]. Thus pollution levels and impacts of elements on environment are generally evaluated by four pollution indices of pollutants which are Enrichment Factor (EF), Anthropogenic Factor (AF), Pollution Load Index (PLI) and Geo-accumulation Index (Igeo). Enrichment factor is an effective tool especially to assess the magnitude of contaminants in the environment. The factor was initially developed to investigate the elements which stem from by precipitation in the atmosphere or seawater then it was extended to other environmental materials for instance soils, sediments, peats, tailings etc. [32]. Calculation of EF is based on the normalization in which Al, Sc, Fe, Zr are widely employed. Al is a common choice [15, 33, 34] to clarify EFs, also compensating the fluctuation on both particle size and sediment matrix in coastal sediments [28]. Fe is widely used in normalization as well as Al [35, 36] but some authors such as Din (1992) [37] and Rubio et al. [38] have argued that Fe originates from anthropogenic pollution [39]. Moreover, Sc and Zr are successfully utilized in the processes [40]. Zr is also known as relatively immobile and not emitted by human activities in practice. EF value is calculated by the formula below [41, 42]:

$${\text{EF}} = \left( {{\text{C}}_{\text{X}} /{\text{C}}_{\text{Fe}} } \right)_{\text{S}} /\left( {{\text{C}}_{\text{X}} /{\text{C}}_{\text{Fe}} } \right)_{\text{C}}$$
(8)

where X is any element, \({\text{C}}_{\text{X}} /{\text{C}}_{\text{Fe}}\) the concentration ratio of X to Fe, index (S) surface layer of sediment column and index (C) Earth crust. In addition, the background values in general could be immediately obtained from the bottom layer [28]. Nolting et al. [43] argues that EF would indicate that sediment is polluted if it is over 10. However, other authors [44, 45] suggest that If EFs varied 0.5 and 2.0 sediments would be independent of human impacts. On the other hand, the fact that they are over 2.0 implies significant anthropogenic inputs [39]. Anthropogenic factor (AF) is used to find the contamination level of a metal and its related values were obtained by the formula below [42];

$${\text{AF}} = {\text{C}}_{\text{s}} /{\text{C}}_{\text{d}}$$
(9)

where \({\text{C}}_{\text{s}}\) is the surface layer concentration and \({\text{C}}_{\text{d}}\) the bottom layer concentration of sediment column [46]. Anthropogenic factor is also known as Contamination Factor (CF) [32, 47], where \({\text{C}}_{\text{d}}\) value identified as the 30th layer of sediment column [15]. Contamination factor generally is used in combination with Pollution Load Index (PLI) in assessing metal pollution. According to Tomlinson et al. (1980) [48]

$${\text{PLI}} = \sqrt[{\text{n}}]{{{\text{CF}}_{1} {\text{CF}}_{2} \,\ldots\, {\text{CF}}_{\text{n}} }}$$
(10)

For a site where \({\text{CF}}\) is contamination factor and \({\text{n}}\) number of metals,

$${\text{PLI}} = \sqrt[{\text{n}}]{{{\text{site}}_{1} {\text{site}}_{2} \,\ldots\, {\text{site}}_{\text{n}} }}$$
(11)

where n is the number of sites for a zone [32]. PLI provides an idea to evaluate a site or zone quality. If PLI is 0.0, it shows perfect quality, 1.0 that there is only starting level pollution present and > 1 progressive deterioration of the zone. Geo-accumulation index is used to determine and describe the metal contamination in sediments by comparing present concentrations with pre-industrial ones as below:

$${\text{I}}_{\text{geo}} = log_{2} \left( {\frac{{{\text{C}}_{\text{n}} }}{{1.5{\text{B}}_{\text{n}} }}} \right)$$
(12)

where Cn is the measured concentration of heavy metals in sediments, Bn the geochemical background value in average shale of element n and 1.5 the background matrix correlation [36]. Metal concentrations and stable Lead-isotopes (206Pb, 207Pb) were determined by ICP-MS (Perkin Elmer LN 900) spectrometry for selected samples. Prepared samples were digested with a modified Aqua Regia solution of equal parts concentrated HCl, HNO3 and DIH2O for 1 h in a heating block or hot water bath. Samples were prepared up to a volume with dilute HCl and separated of 0.5 g, 15 g, 30 g to be analyzed in ACME laboratory (ACME Anal. ISO 9002 Accredited Canadian Lab., Canada) (Method Description code is AQ250 2015_1 Bureau Veritas). As a quality assurance, it is declared that DS9, NIST-981-1Y, NIST-983-1Y standards were used in determination of element concentrations.

Results and discussions

Radionuclide concentrations

Vertical distribution of total 210Pb activities ranges from 100.32 ± 7.56 Bq kg−1 to 40.55 ± 4.12 Bq kg−1 along the core IST-8 (Fig. 2b). In a similar way, it ranges from 108.59 ± 6.08 Bq kg−1 to 42.38 ± 3.43 Bq kg−1 along the core IST-2 (Fig. 2a). The total 210Pb activity decreases from layer with maximum value to layer with minimum value, plateauing along the core. The lowest activity of total 210Pb is named 226Ra activity. Thus, determination of dating employs the part of the core between the layer with maximum activity of 210Pb value and the one of minimum value, plateau part of the core is ignored [49]. As aforementioned, 226Ra activities were determined as 40.55 ± 4.12 and 42.38 ± 3.43 Bq kg−1 in IST-8, IST-2 respectively. 226Ra activity is also known as supported 210Pb activity. In determination of dating, unsupported 210Pb activities are used which are calculated by subtracting supported 210Pb activity from the total 210Pb activity in each layer.

Fig. 2
figure 2

Vertical distribution of total 210Pb concentrations and plots of unsupported 210Pb concentrations on a logarithmic scale versus mass depth in IST-2, IST-8 stations. Mass depths were calculated using the relationship with water content sediment pore. \(Mass Depth = D\rho_{w} /\left[ {1/\left( {DW\% } \right) + \rho_{w} /\rho_{dsed} - 1} \right]\), where D indicates the linear depth, DW describes the dry weight of sediment, ρw and ρdsed are density of water and dry sediment, respectively. Values R2 (0.83 for IST_2 and 0.85 for IST_8) seen on the upper right side of the graph c and graph d are lower than 0.95 thus model was not used in calculation of the sedimentation rates CF;CS (Constant flux; Constant sedimentation rates)

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From the vertical distribution of unsupported 210Pb activities in IST-8 and IST-2 stations, it is follows that they do not obey to the exponential decrease from top to bottom along the core since values R2 on the upper right in the Fig. 2c, d are lower than 0.95, in which case CF;CS model is obviously known not to be employed for those stations. Accordingly, applicability of CRS and CIC models were tested for cores. The residence times of 210Pb were also calculated by utilizing profile distribution of 210Pb activities as 2.4 year to correct dating processing. The residence time of 210Pb in study area is lower than that of Mumbai Harbor Bay (40.87 year) [27].

Sedimentation rate ranges from 0.592 ± 0.022 to 0.036 ± 0.002 cm year−1 along the IST-8 core, displaying the increasing trend in time scale from 1850′s until today though not systematically increasing and also not displaying extremely high or low level sedimentation rates along the core. IST-8 station has a core depth of 30 cm corresponding to time scale between 1850's and 2010's (Fig. 3 and Supplementary material 1). Core inventory was determined as 7809.5 Bq m−2 thus 210Pb flux was also calculated as 242.8 Bq m−2 year−1. Average sedimentation rate is 0.315 ± 0.016 cm year−1 for both CRS and CIC models due to the value R2 close to 0.95. Value R2 is regression coefficient. It indicates the linearity between measured unsupported 210Pb activities and exponential decrease with depth on semi log graph. If value R2 is 1, core will be named as linear and linear cores give the same chronology in both CIC and CRS models [26]. In general, if value R2 is 0.95 for a core, it is assumed as linear. Value R2 of IST-8 is not far away from the 0.95, it is 0.83. In here, it should not be forgotten that according to CRS model, sedimentation rate is different in each layer but average value of CRS is one and according to CIC model average sedimentation rate is one value. It is possible that both models may have same average value. It may be aroused from the increases and decreases in sedimentation rates. Fluctuations on sedimentation rates may compensate the differences on average values and it may fix them to same value.

Fig. 3
figure 3

Age-depth graphs for IST-8 and IST-2 stations

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Sedimentation rate varies between 0.399 ± 0.011 and 0.032 ± 0.001 cm year−1 along the IST-2 core. Sedimentation pattern is similar to IST-8 but is not exactly same. IST-2 station has a core depth of 22 cm corresponding to time interval between 1870′s and 2010′s (Fig. 3 and Supplementary material 1). 210Pb flux (205.9 Bq m−2 year−1) was calculated using core inventory (6621.6 Bq m−2). Average sedimentation rates in IST-2 are 0.251 ± 0.010 and 0.237 ± 0.011 cm year−1 both CRS and CIC models, respectively.

Maximum total 210Pb activities are very close to each other but one core (IST-8) was obtained from Bakircay River mouth and the other one from the Ayvalik offshore in northern part of the Aegean Sea. Measured activities in study area are higher than in Mumbai Harbor Bay (48 Bq kg−1) [27] and San Francisco Bay (33.3 Bq kg−1) [50] but roughly the same as in Ribeira Bay (100 Bq kg−1) [51] and lower than in Thermaikos Gulf (Eastern Mediterranean) (140 Bq kg−1) [52].

Vertical distribution of unsupported 210Pb activities are crucial for choosing mathematical model and pointing out the sedimentation pattern. Sedimentation patterns display almost the same in character on both stations. Therefore, 210Pb fluxes calculated from core inventories are close to each other in study area. Therefore, it is seen that the best model for this study area is CRS because first of all cores display the non-monotonic decrease in depth. The second is that cores from different points within the same general area have comparable total residual unsupported 210Pb content. The last cores have similar 210Pb fallout (210Pb fluxes are close to each other in cores) from the atmosphere.

In general, 210Pb flux around the world changes between 158 ± 113 and 20 ± 15 Bq m−2 year−1 from 0-10 North latitude to 70–80 North latitude, 117 and 2 ± 1 Bq m−2 year−1 from 0-10 South latitude to 80-90 south latitude in coastal, continental, ice-core and ocean samples [53]. 210Pb fluxes calculated from the study area are higher than those from coastal locations in the Mediterranean area (75 Bq m−2 year−1) [54], being roughly within the range of the world value of atmospheric 210Pb flux (165–185 Bq m−2 year−1) [26, 55]. Moreover, 210Pb fluxes from the study area are higher than those of Gülbahçe Bay (173.6, 174.3, 160.5, 189.9, 135.3 and 206.9 Bq m−2 year−1) [15].

Sedimentation rates in north Aegean Sea are lower than the ones in Mumbai Harbor Bay (0.52 ± 0.10, 0.73 ± 0.21, 1.12 ± 0.24 cm year−1) [27], Thermaikos Gulf (0.88 ± 0.15, 0.75 ± 0.06, 0.48 ± 0.03 cm year−1) [52], higher than in the Ribeira Bay (0.13 ± 0.02, 0.17 ± 0.02, 0.19 ± 0.01 cm year−1) [51]. Sedimentation rates tend to increasing from bottom to top layers of the cores. They do not display continuously increasing trend but sedimentation trend of IST-8 is more stable than the one of IST-2. Average sedimentation rate in Bakircay River mouth is higher than the value of Ayvalik Offshore which is logical since Bakircay River has carried the continental materials down the seabed. Some high sedimentation rates have been seen in Ayvalik Offshore from past to nowadays. Such increases were observed especially in 1956, 1989, 1992, 2000 and 2008 years, which could not be related to the human processes or could not have originated from external inputs from the catchment area, since calculated 210Pb flux in Ayvalik Offshore (205.9 Bq m−2 year−1) is not significantly higher than the world average value of atmospheric 210Pb flux (185 Bq m−2 year−1). Clearly, if calculated value of 210Pb flux there are supposed to be external inputs in the study area is higher than the world average value of atmospheric 210Pb flux there will be outer inputs from the catchment area [56], which could be related to hydrothermal or seismic activity since north Aegean Sea has dynamic hydrothermal and seismic activities [9].

Metal concentrations

Element concentrations were not only measured in vertical sediment cores but also measured in surficial sediment samples (Fig. 4 and Supplementary Material 2) which showed Ca > Al % level, Mn > Ba > Cr > Zn > Ni > Pb > Cu ppm level and Hg ppb level order in surficial samples which consistent of 15 sampling points from Ayvalik to Izmir Bay in north Aegean Sea. A15 (inner part of the Izmir Bay), A14 (Gediz basin), A13 (Uzun Island) sampling points have highest element concentrations in terms of almost all elements, which could be related to maritime traffic or material influxes from Gediz basin, since Izmir Harbor is located in the inner part of the Izmir Bay and Gediz basin could also be responsible for high levels of element concentrations. Conversely in terms of Ba, Cr, Cu, Pb, Hg, A5 (Ayvalik offshore 2) sampling point, Mn, Zn, A7 (Dikili offshore) sampling point, and Al, Ni, A2 (Ayvalik offshore) sampling points have lowest concentrations. This surficial distribution of element concentration implies that A13, A15 stations are rich in Al, Ni and A14, A3 (Karaagac River mouth) stations are also rich in Al but station A2 is poor in them. Elements (Al, Ni) originate from terrigenous supply and it could be said that these stations do not display any pollution in terms of cited elements. Potentially, Karaagac River could be responsible for terrigenous materials. Ca and Ba from biogenic debris. A2, A11 (Foca nearshore), A7, A12 (Karaburun) and A10 (Foca offshore) stations show high level of Ca concentrations and A8 (Candarli nearshore), A15, A14 stations show high level of Ba concentrations which is more probably related to hydrothermal activity since hydrothermal zones have high biogenic affiliations [9]. Therefore, the stations above must be near the hydrothermal vents and Aegean Sea is known to have many hydrothermal areas. Mn is responsible for a group of micro earthquakes. A15, A10 (Foca offshore), A9 (Candarli offshore), A13, A14 stations have high but A7 (Dikili offshore) station has low level in Mn concentrations, which is reasonable because Aegean Sea has evidently high level of seismic activity. Cr and Cu are known as less mobile during the rock weathering because of their lower solubility. Thus they are immobile and Cr is transported in refractory minerals such as quartz, ilmenite, chromite, zircon whereas Cu is in the organic matter or clay minerals. A14, A15, A13 and A12 stations have high Cr and Cu concentrations but A5 station is poor in such element concentrations. Pb and Hg are known to be toxic in particular. Stations A15, A2, A12 and A8 are rich in Hg whereas those A14, A15, A13 and A11 are high in Pb concentrations both of which are lower than the other in A5 (Fig. 5). Pb input to the aquatic environment is an important problem and it should be identified in terms of natural or anthropogenic. 206Pb/207Pb ratio is utilized for differentiation and determination of the Pb input which could be natural or anthropogenic. 206Pb/207Pb ratio is 1.2 in geologic sediments but is lower than the 1.2 in anthropogenic sediments [57, 58]. In addition Zn, Cu, Cr and Ni are known as essential for marine organisms which always function in combination with organic molecules in general proteins. Stations A14, A15 and A13 have high level of Zn concentrations whereas A7 has low level concentration of Zn. According to surficial distribution pattern of element concentrations, stations A2 and A12 display the same trend but they differ from the others in terms of pollution.

Fig. 4
figure 4

Distribution of lead isotopes and lead isotope ratios in stations

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Fig. 5
figure 5

Element concentrations in surficial samples

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When glimpsed to the distribution of surficial 206Pb/207Pb ratios, it is seen that there are fifteen surficial stations along the Turkish coast in northern part of the Aegean Sea. In terms of 206Pb/207Pb ratio, five of them with error bars (A2, A4, A6, A11 and A15) have anthropogenic surficial sediments. A15 station displays high toxic (Pb, Hg) and biogenic (Ba) pollution. 206Pb/207Pb ratio also supports the sediment pollution in A15. In relation with the high level toxic and biogenic debris, A2, A4, A6, A11 stations have also displayed sediment pollution (lower values than 1.2) in terms of 206Pb/207Pb ratio.

In addition to surficial samples, vertical samples IST-2 and IST-8 were obtained from the Ayvalik offshore and the Candarli nearshore (Bakircay River mouth), respectively (Fig. 6). Considering Enrichment Factor (EF) values, Ayvalik offshore core has rich Ca (3.89), Mn (1.84), Zn (1.57), Pb (1.69), Hg (4.53) concentrations (Table 1) but in terms of Contamination Factor (CF) IST-2 station displays Ca (1.92) and Hg (2.40) pollutions (Table 2) whereas Bakircay River Mouth (IST-8) interestingly shows only Ca pollution in both contamination (1.78) and enrichment (1.98) factors. Enrichments were extensively seen in 2012. Furthermore, Ca displayed high EF values between 1968 and 2012 in IST-2 station but Ca displayed high EF values between 1995 and 2012 in IST-8 station too. Both Ayvalik offshore and Bakircay River Mouth have shown a starting level pollution from past until today while IST-2 showed an progressive deterioration in 2004 in terms of Pollution Load Index (PLI). In terms of Geo-accumulation Index, any metal concentration is evaluated in such a way that if Geo-accumulation Index is equal or under the zero, sediment will not be polluted by cited metal and if Geo-accumulation Index is between zero and one, sediment will be classified as “unpolluted to moderately polluted” [36]. It could be said that both Ayvalik offshore and Bakircay River Mouth stations are unpolluted by the above elements except by Ca and Hg. In terms of, Hg (0.68) IST-2 station has had the “unpolluted to moderately polluted” pollution degree since 2012, which has two time scales which correspond to “unpolluted to moderately polluted” pollution degree from 1984 to 2004 and from 2012 to nowadays in terms of Ca. Similarly, station IST-8 has also had “unpolluted to moderately polluted” pollution degree since 1995 (Table 3).

Fig. 6
figure 6

Element concentrations in time scale for IST-2 and IST-8 stations

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Table 1 Enrichment factors (EF) for IST-2 and IST-8 stations
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Table 2 Contamination Factor (CF) and pollution load index (PLI) values for IST-2 and IST-8 stations
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Table 3 Geo-accumulation Index (Igeo) for IST-2 and IST-8 stations
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It is known that, Al is the lithogenous component of the sediment. Ni (0.941**), Mn (0.933**), Cr (0.882**), Cu (0.804**), Zn (0.880**), Pb (0.779**), Hg (0.879**) distributions have a correlation with Al distribution in Bakircay River Mouth but in Ayvalik offshore only Ba (0.973**), Ni (0.961**), Cr (0.917*), Cu (0.967**) show positive correlation with Al (**p < 0.01, *p < 0.05). From the correlations with Al, it is clear that concentrations of all elements are directly controlled by continental weathering except Ca and Br in station IST-8 (Table 4) which is an expected situation since Ca and Ba originate from biogenic debris. In addition to Ca, concentrations of some other elements are also not directly controlled by continental weathering, for instance Mn, Zn, Pb, Hg in station IST-2. It is also reasonable because potentially Bakircay River may bring the continental weathering materials from stream bed to river mouth. Concentration of Ca interestingly displays negative correlations with the rest in both stations. The trend for Ni has closely been followed by the trend for Cr and Cu in the same way but it has no correlation with Zn, Pb and Hg in Ayvalik offshore. Ni, Cr and Cu are probably derived from the same source or sources, which is necessary because they are known to be essential to marine organisms. Pb and Hg are particularly toxic so their sources are apart from the Ni, Cr, Cu ones in Ayvalik offshore whereas Pb and Hg show correlations with Ni, Cr and Cu in station IST-8 which indicates that Bakircay River contributes to concentrations of Pb and Hg in aquatic environment. In general, element concentrations have decreased from past to nowadays in both IST-2 and IST-8 but elevated element concentrations have not been seen. However, Ca concentrations have increased since 1917 in station Bakircay River Mouth and Ca, Hg concentrations have increased since 1944 in station Ayvalik offshore.

Table 4 Correlation coefficients between element concentrations for surficial and core samples
Full size table

When glimpsed to the relation between EF of some elements and profile distribution of 206Pb/207Pb ratio in core IST-2, it is seen that EF values for Ca, Ba, Ni, Mn, Cr, Cu and Hg are higher than 1 in 1968 and also 206Pb/207Pb ratio indices sediment pollution in same year. It is possible to see similar relations between EF values (Table 1) and 206Pb/207Pb ratios (Supplementary Material 2) in 2012–2009, 2007–2006, 2002–1992, 1990–1983, 1982–1918 years respectively for core IST-8. However there is no relation between profile distribution of unsupported 210Pb activity (Fig. 2) and 206Pb concentrations (Supplementary Material 2) in both IST-2, IST-8 cores.

Conclusions

Present study has tried to describe historical evaluation of north Aegean Sea in Turkish coast. To achieve a general idea for study area, first of all vertical distributions of 210Pb activity concentrations were determined by alpha spectroscopy, which shows that maximum unsupported 210Pb concentrations are the same in each core and they compatible with other part of the Aegean Sea [15]. It could be said that 210Pb depositions to the Ayvalik Offshore and Bakircay River Mouth stations are systematic and do not display exponential decrease from top to bottom layers. Thus in calculation of sedimentation rates, applicability of CRS and CIC mathematical models were tested and it is decided that the best model is CRS for study area. 210Pb fluxes from core inventories are close to each other and they are almost within the range of the global atmospheric 210Pb flux value. However, 210Pb flux from Bakircay River Mouth is a little higher than the Ayvalik Offshore and the global atmospheric fluxes, which is logical since if measured 210Pb flux is higher than the atmospheric supply rates, there would be external inputs from the near catchment area [56]. It could be argued that 210Pb flux has been accounted for by the material from the river. In addition, sedimentation rates along the cores display irregular increases and decreases in stations of Ayvalik Offshore and Bakircay River Mouth. Station Bakircay River Mouth has two effects which may be related to irregularity on sedimentation pattern. The first one is Ba concentration. It is known that, high level of Ba concentration indicates the biogenic debris (possible hydrothermal activity) [8]. The second one is the inputs from the river to sediment accumulation area. Thus, average sedimentation rate (0.315 ± 0.014 cm year−1) in Bakircay River Mouth station is higher than in the Ayvalik Offshore one (0.215 ± 0.009 cm year−1). However, station Ayvalik Offshore has no obvious effect related to irregular sedimentation pattern. The residence time of 210Pb was also calculated by profile distribution of 210Pb activity concentrations in the core which was found to be 2.4 years and sediment dating was corrected via residence time of 210Pb.

Pollution level in the study area was evaluated by means of four pollution index and results were supported in terms of lead isotopes (206Pb/207Pb ratio). Some element concentrations have high level in the surficial sediment samples, more or less half of the study area is polluted in terms of measured elements. In addition, Lead-isotope ratio supports the polluted decision in surficial samples especially stations A2, A4, A6, A11 and A15 (Fig. 4b). As to core samples, some elements display high level concentrations but Ca and Hg pollutions exist in Ayvalik Offshore station and only Ca pollution exists in Bakircay River Mouth. While Ayvalik Offshore displayed Ca pollution between 1968 and 2012, Bakircay River Mouth showed Ca pollution from 1995 to 2012 in terms of enrichment and contamination factors. Moreover, profile distributions of 206Pb/207Pb ratio support the sediment pollutions between mentioned years in both cores. Under the Pollution Load Index, Ayvalik offshore and Bakircay River Mouth have both shown a starting level pollution through decades but Ayvalik Offshore displayed an progressive deterioration in 2004. Considering Geo-accumulation Index, both stations have Ca and Hg pollutions. While Ayvalik Offshore station has had the “unpolluted to moderately polluted” pollution degree since 2012, Bakircay River Mouth has had the “unpolluted to moderately polluted” pollution degree since 1995.