1. Introduction

Marine sediments and sedimentation rates have been investigated for decades [1–7], for establishment of chronologies, development of history of erosion and pollution, investigations of ecological and coastal management. Madsen et al. [8] reported that sedimentation rates give information about present trends on the sea level changes. Thus, estimates of mass accumulation rate (MAR) and sedimentation rate (SR) are useful to understanding sediment routes. The main sources of sedimentation are the movement of particles. This movement is mainly related with terrestrial input. In addition to these sources, another way for transport of lithogenic particles is associated with deep-sea sediments [9].

As a member of uranium decay series ²¹⁰Pb (t_1/2= 22.3 y) is an ideal radioisotope to determine sedimentation rates and for dating sediment layers within a period of 150 years. This time period includes both the pre-industrial and industrial eras in the world, hence the importance of the method for studying the environmental changes caused by human activity [10]. ²¹⁰Pb is produced in the atmosphere as a daughter of ²²²Rn [1, 11]. Main sources of ²¹⁰Pb in sediments are supported and unsupported components of ²¹⁰Pb. The supported ²¹⁰Pb is a product of the ²²⁶Ra in sea and lake sediments. The unsupported ²¹⁰Pb is called as excess ²¹⁰Pb which is formed in atmosphere. Therefore, surface sediments have higher ²¹⁰Pb concentration, and ²¹⁰Pb levels decrease with depth. This means sedimentation rates and date of sediment layers can be calculated from vertical profiles of ²¹⁰Pb in cores [11].

The main goal of this study is to determine sedimentation rates in the inner part of Izmir Bay using the vertical profile of ²¹⁰Pb. The bay is adjacent to the third crowd city in Turkey and Izmir harbor is sited on the inner part of the bay. To our knowledge this study is the first attempt to determine sedimentation rates in the inner bay.

2. Materials and Methods

2.3. Radioisotope analysis and sedimentation rate calculation

²¹⁰Pb activity of each sediment layer was determined by alpha spectroscopy of its daughter ²¹⁰Po. Before alpha counting, radiochemical separation and electrochemical deposition process was applied to all samples. In this stage, each sediment sample (1 g of dry sediment) was digested in Teflon beaker with a mixture of concentrated HCl, HF and HNO₃ together with ²⁰⁹Po yield tracer (0.1 Bq/mL, E_α=4.88 MeV, t_1/2=102±5 y) on a hot plate. And then, ²¹⁰Po was spontaneously deposited onto copper discs in 0.5 M HCl in the presence of ascorbic acid to reduce of Fe⁺³ to Fe⁺² [15]. The samples were transferred to 0.5 M HCl medium in the presence of ascorbic acid to reduce of Fe⁺³ to Fe⁺² and ²¹⁰Po in the solution was deposited on a copper disc using magnetic stirrer at 60°C for 4 h [16]. The discs were washed with distilled water and measurements of ²¹⁰Po were performed through its 5.30 MeV alpha particles by using Passivated Implanted Planar Silicon (PIPS) detectors. Collected samples were analyzed two years later [17, 18]. The activities of ²¹⁰Pb in the lowest layers of the cores were assumed to be in equilibrium with parent ²²⁶Ra (²¹⁰Pb_sup). The activities of the layers were therefore subtracted from the total ²¹⁰Pb activities in order to compute the excess ²¹⁰Pb. ²¹⁰Pb geochronology is originated from the radioactive decay of ²¹⁰Pb with depth (m) in a sediment core. The decay of ²¹⁰Pb, A(t), is ruled by the radioactive decay law:

$A_{\text{Pb}-210\text{ex}}=A_{{}^0}{e}^{-\lambda t},$

where A₀ is the ²¹⁰Pb activity (surface) at t=0, and λ = 0.03114 y⁻¹is the decay constant for ²¹⁰Pb. For accepted constant sedimentation rates (CRS), time (t) is linked to depth m through m= wt, where w is the sedimentation rate. Consequently,

$A_{\text{Pb}-210\text{ex}}=A_{{}^0}{e}^{-\lambda m/w}.$

Sedimentation rate is estimated from an exponential decrease of ²¹⁰Pb, using A(m) at depth m [19]. Using this CRS model, the slope of ln(²¹⁰Pb_ex) profile distribution gave the sedimentation rates in cm per year (Fig. 1).

Fig. 1Full-screen View

Map of the study area

3. Results and Discussion

The ²¹⁰Po activity concentration measured by alpha spectrometry corresponded to the total ²¹⁰Pb activity. ²¹⁰Pb_tot (total ²¹⁰Pb) activities of all cores were plotted against the depths (Fig. 2).

Fig. 2.Full-screen View

²¹⁰Pb (total ²¹⁰Pb) activities of all cores against the depths.

The ²¹⁰Pb activity that remained constant below a certain depth is considered as supported ²¹⁰Pb (existence of ²²⁶Ra in the sediment), and the unsupported ²¹⁰Pb(²¹⁰Pb_ex) profile distribution was obtained from the differences between the total ²¹⁰Pb and supported ²¹⁰Pb values. The ²¹⁰Pb_sup activities determined for each core varied between 32 and 50 Bq/kg. The depth profiles ln(²¹⁰Pb_ex) in sediments for eight cores are shown in Fig. 3.

Fig. 3.Full-screen View

The depth profiles ln(²¹⁰Pb_ex) in sediment profiles

In this study sedimentation rates varied from 0.10 to 0.52 cm/y. Our results and literature data are given in Tables 2 and 3, respectively. The sedimentation rates obtained for Izmir Bay are in the ranges of the reported values, and agree well with the two locations in the Mediterranean Sea shown in Table 3. The highest sedimentation rates were found at C7 (0.52 cm/y) and C8 (0.41 cm/y). The two sites are in the southern portion of the bay and are affected by the Poligon stream which undergoes flooding every year. The lowest sedimentation rate was at C5 (0.10 cm/y) where the water is the deepest of all the sites, and it is located in the middle of the study area, being the farthest from river and/or streams.

Figure 4 shows the spatial distribution of ²¹⁰Pb_tot in surface sediments of the bay. ²¹⁰Pb_tot activities on sediment surfaces ranged from 56 to 268 Bq/kg. The distributions show that ²¹⁰Pb_tot was concentrated near at C1. This part of the bay has an enclosed environment and two streams drain into the area. Duman et al. [13] reported that sediment contamination level is extremely high in the inner part of the bay.

Fig. 4.Full-screen View

Spatial ²¹⁰Pb_tot distribution of surface sediments.

Our results show that the highest ²¹⁰Pb_tot concentrations in surface sediments. But, in some sediment cores, the highest levels were detected in subsurface layers. It may be linked to effects of natural process like turbidity, surface water circulation, and intense harbor activities. Nevertheless, ²¹⁰Pb concentrations decreased exponentially with depth in all cores except at C3, where the ²¹⁰Pb_tot distribution was nearly linear; ²¹⁰Pb_tot concentrations in every layer of this core being almost the same. Bioturbation can cause mixing that result in the linear ²¹⁰Pb distribution noted. In addition, water depths of the inner part of the bay are quite low. This is a problem that blocked large container ships from entering the bay, thus the harbor area was dredged two times between 1976 and 1990. Collected sediments were dumped on the sea-bed at Göztepe Bay Dump Site and Hekim Island Dump Site, respectively [28]. These dredging activities may be another reason for the non-exponential distribution of ²¹⁰Pb in the core.

4. Conclusion

This study is planned to present preliminary data on sedimentation rates of Izmir Bay (inner part) and levels of ²¹⁰Pb of surface sediments. The study area (Izmir Bay) is surrounded by heavily populated landscape and intense industrial activity. Also, harbor activities make that bay an important location. Our results showed that highest sedimentation rates were obtained at C7 and C8, in the southern portion of the bay.They are affected by the Poligon stream which undergoes flooding every year. Low sedimentation rates were found at C4 (0.19 cm/y), C5(0.10 cm/y) and C6(0.18 cm/y), which are far away from the streams.