Accumulation rates and sediment deposition in the Gökova Bay in Aegean Sea Turkish Coast

Abstract

Sediment accumulation rates were estimated from the vertical distribution of excess ²¹⁰Pb in sediment cores collected at six stations in the Gökova Bay near the heavily industrialized Turkish Coastal zone of the Aegean Sea. Total ²¹⁰Pb was determined by measuring ²¹⁰Po activities. The sediment accumulation rates were calculated to vary from 0.32±0.01 cm yr⁻¹ (0.17±0.01 g cm⁻² yr⁻¹) to 1.92±0.20 cm yr⁻¹ (1.13±0.10 g cm⁻² yr⁻¹). The unsupported ²¹⁰Pb flux was calculated from the accumulated dry matter of the examined slices and their unsupported ²¹⁰Pb concentrations. The flux of unsupported ²¹⁰Pb varies between 62±5 and 105±8 Bq m⁻² yr⁻¹. The average depositional flux was found to be considerably higher for cores from the Mediterranean.

Introduction

Suspended particles represent the most important vehicle for chemical elements and compounds, both natural and anthropogenic. Sediments may act as a sink or as a source according to their nature and the environmental conditions. Determination of sedimentation rates can provide more insight into the origin of particulate matter, particle transport, sediment mixing and the amount of deposition. ²¹⁰Pb, the naturally occurring radioactive nuclide has been used intensively as one of the key tracers to study the processes of chemical scavenging, particle transport and sediment deposition in marine environments. Two principal modes of supply of ²¹⁰Pb to the coastal and shelf waters should be considered: deposition from the atmosphere and production within the water column following the decay of ²²⁶Ra. ²¹⁰Pb is removed from the water column to the sediment by scavenging. Chemical adsorption onto particulate matter facilitates transport over long distances before its final deposition in the sediment. After deposition, ²¹⁰Pb decays significantly with a 22.3 year half-life. The activity of ²¹⁰Pb at the sediment surface is the result of the interplay between accumulation, sediment mixing and radioactive decay. This interplay determines the extent to which particle exchange takes place between the sediments and the overlying water column.

The Turkish coastal zone of the Aegean Sea has been heavily industrialized in the last 20 years, resulting in a considerable input of wastes to the coastal marine ecosystem. Southern borderland, particularly in the shallow water, Gökova Bay has been exposed to fly ash materials from three major uraniferous coal-fired power plants (CPP) since 1982 (Tanbay et al., 1999). The generation of electrical power using coal, oil or natural gas has a concomitant deleterious effect on the surrounding environment where these plants are operating. The generation of 1 GWe by a coal power plant (CPP) releases ²²⁰Ra, ²²²Rn, through fly-ash and gases at a rate of 10⁹–10¹¹ Bq yr⁻¹; also generates 10⁸–10¹⁰ Bq yr⁻¹ of ²¹⁰Pb, ²¹⁰Po, ²²⁶Ra, ²³⁸Ra, ²³²Th, ²³⁸U (Baxter, 1993). Despite the environmental regulations (trapping filters, etc.) an important amount of the released radionuclides are often found in airborne particles around each CPP. At Gökova Bay, there are three major sources of CPP, namely, Yataǧan in operation since 1982, Yeniköy, since 1988 and Kemerköy, since 1994. Yataǧan has three units producing 210 MW each, the other two plants in consideration have only two units of similar power. The ²¹⁰Pb activity in the coal used by these CPP range in 247–297 Bq kg⁻¹ for Yataǧan, 518–614 Bq kg⁻¹ for Yeniköy and 730–863 Bq kg⁻¹ for Kemerköy (Yener and Uysal, 1996).

The principal object of this paper is to present the results of measurements of ²¹⁰Pb in marine sediment cores taken from Gökova Bay Aegean Sea and to determine sediment accumulation rates using this data. Although there is now a large body of data on ²¹⁰Pb, most of the ²¹⁰Pb work in particular has been concentrated in Mediterranean Sea, Black Sea and Marmara Sea (see, for example, Tadjiki and Erten, 1994; Buesseler and Benitez, 1994; Gökmen et al., 1996). For ²¹⁰Pb fallout, there have been very few studies in the Aegean Sea and hardly any at the Turkish coastal zone of the Aegean Sea.