A C C U M U L A T IO N R A TES A N D SED IM E N T D E PO SIT IO N IN THE G O K O V A BA Y IN A E G E A N SEA
Aysun (Ugur) TANBAY * , Gungor YENER
Institute o f Nuclear Sciences, Ege University, 35100 Bom ova Izmir
ABSTRACT
Sediment accumulation rates were estimated from the vertical distribution o f excess 210Pb in sediment cores collected at 3 stations in the Gokova Bay Aegean Sea by measuring the a - emission o f 210Po. The Turkish coastal zone o f the Aegean Sea has been heavily industrialized in the last 20 years. “The sediment accumulation rates were calculated to vary from 0.21±0.01cm.y-1 (0.13+0.01 g .c m 'V 1) to O ^TO .O Scm .y1 (0.26±0.03 g .c m 'V 1). The unsupported 210Pb flux was calculated from the accumulated dry m atter o f the examined slices and their unsupported Pb concentrations. The flux o f unsupported Pb varies between 23 ± 4 and 47 ± 3 Bq.m'2y_1.
INTRODUCTION
Suspended particles represent the m ost 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 o f sedimentation rates can give more insight into origin o f particulate matter, particle transport, sediment mixing and the amount o f deposition. 210Pb, the naturally occuring radioactive nuclide has been used intensively as one o f the key tracers to study processes o f chemical scavenging, particle transport and sediment deposition in marine environments. Two principal modes o f supply o f 210Pb to the coastal and shelf waters should be considered: deposition from the atmosphere and production within the water column following decay o f 226Ra. 210Pb is removed from the water column to the sediment by scavenging. Chemical adsorption onto particulate m atter facilitates transport over long distances before its final deposition in the sediment. A fter deposition, 210Pb decays significantly with a 22.3 years half-life. The activity o f 210Pb at the sediment surface is the result o f interplay between accumulation, sediment mixing and radioactive decay. This interplay determines the extent to which particle exchange takes place between the sediments and overlying water column.
The Turkish coastal zone o f the Aegean Sea has been heavily industrialized in the last 20 years, resulting in a considerable input o f wastes to the coastal marine ecosystem. Southern borderland, particularly in the shallow water Gokova Bay has been exposed to fly ash materials
generation o f electrical power using coal, oil or natural gas has the concomitant deleterious effect on the surrounding environment where these plants are operating. The generation o f
1Gwe by a coal power plant (CPP) releases 220Ra, 222Rn, through fly-ash and gases at a rate o f 109 to 1011 Bq.y-1; also generates 108 to 1010 Bq.y-1 o f 210Pb, 210Po, 226Ra, 228Ra, 232Th, 238U (Baxter, 1993). Despite the environmental regulations (trapping filters, etc) an important amount o f the generated radionuclides are often found in airborne particles around each CPP. A t Gokova Bay there are three m ajor CPP, namely, Yatagan in operation since 1982, Yenikoy, since 1988 and Kemerkoy since 1994. Yatagan has 3 units producing 210Mw each, the other two plants in consideration have only 2 units o f similar power. The 210Pb activity in the coal used by these CPP range in 247-297Bq.kg-1 for Yatagan, 518-614 Bq.kg-1 for Yenikoy and 730 863 Bq.kg-1 for Kemerkoy (Yener and Uysal, 1995).
The principal object o f this paper is to present the results o f measurements o f 210Pb in marine sediment cores taken from Gokova Bay Aegean Sea and to determine sediment accumulation rates using this data. Although there is now a large body o f data on 210Pb, most o f the 210Pb work in particular has been concentrated in M editerranean Sea, Black Sea and M armara Sea (see, for example, Tadjiki and Erten, 1994; Buesseler and Benitez, 1994; Gokmen et al., 1996). For 210Pb fall-out there have been very few studies in the Aegean Sea and hardly any at the Turkish coastal zone o f the Aegean Sea.
MATERIALS AND METHODS
Sediment cores from Gokova Bay were recovered using a gravity corer with transparent PVC liners. The locations o f the PCC and the sites o f the samples are shown in Figs. 1 and 2. W ater content was determined by the difference between wet weight and freeze-dry weight. Salt correction was applied to all weights measured. The 210Pb measurements were performed on 1-2 g o f dried, ground sediment placed in 20ml Teflon bombs with a 209Po tracer. The sediment was digested in HF and HNO3 and 210Po and 209Po were plated on to silver discs from 0.5N HCl solution (Flynn, 1968). Polonium activities were measured spectroscopically using surface barrier detectors for which the efficiencies are 29, 31 and 35% using combined source o f 238Pu (630±20dpm) and 241Am (395±15dpm) on a platinum discs. . Chemical yields were measured to range from 77 to 87% from the recovery o f the 209Po tracer, Core samples were stored for a period o f 2-3 half-lives ( 9-14 months) o f 210Po (ti/2=138gun) before analysis to allow build-up o f 210Po from 210Pb. Determination o f the 210Po activity by alpha counting then provided an indirect measurement o f the 210Pb activity.
Sedimentation Rates
Assuming that the flux P (dpm.cm-2.y-1) o f 210Pb to the sediment-water interface has remained constant, it is reasonable to suppose that each layer o f sediment will have the same initial unsupported 210Pb activity. The unsupported 210Pb activity will vary with depth in accordance with the formula
C(m) = C(0) e-Xm/r
Where
X is the 210Pb radioactive decay constant (y-1),
C(0) is the unsupported activity at the surface o f the core,
m denotes the depth in the core (g cm-2),
r denotes the sedimentation rate (g cm-2y-1).
W hen plotted on logarithmic scale the resulting 210Pb activity versus depth profile will appear linear. The mean sedimentation rater can be determined from the slope o f the graph using a least-squares fit procedure.
Measurement o f 210Pb Flux
Measurements o f the atmospheric flux o f 210Pb can be made directly from measurements in rainfall and indirectly from the unsupported 210Pb inventory in soil cores from sites not subject to erosive or other transport processes. The mean flux is estimated from the 210Pb inventory A(0) by
A=XP
W here X = 0.03114 year 1 is the 210Pb radioactive decay constant (Smith et al., 1997).
R E SU LT S AND D ISC U SSIO N
The results o f the analyses are shown graphically in Figs.3, 4, 5. Table 1 summarises a number o f parameters characterising the record in each core, including maximum concentrations, 210Pb inventory and fluxes. Sediment deposition rates have been estimated from a least square fit o f data and the fluxes were estimated indirectly from the unsupported 210Pb inventory in the cores.
In our present study the time elapsed between core recovery and core analysis did not allow the observation o f 7Be. So, the total core recovery, namely the topmost samples o f core A, B, C hasn't been analysed. However, sedimentation rates, calculated by using the 210Pb method that is
not influenced by loss o f material, since the slope o f the decay curve is used to determine the sedimentation rates. A mean "supported" activity o f disintegration per minute per gram (dpm.g-1) corresponding to the whole depth range was calculated from 8 to 13 deepest sediment samples layers o f the cores A, B, C. Thus, the “unsupported” 210Pb activity was found as the difference between the measured activities and the supported mean activity found this way for which the Po activities were observed to be almost unchanged.
Depth (cm)
(a) (b)
Figure 3 . Fallout radionuclide concentrations in Aegean Sea Core A (a) total 210Pb, (b) unsupported 210Pb
Lead-210 Activity
Equilibrium o f total 210Pb activity with the supporting 226Ra, corresponding to c.100 years accumulation, was reached at depths 6 cm in Core A (Fig. 3) and 7 cm in Core B (Fig. 4). Since the values are irregular in core C we can not give a safe depth for this core (Fig. 5).
In core A the unsupported 210Pb activity decreases exponentially with depth (Fig. 3), apart from small displacement o f the profile at a depth o f between 2.5cm and 3.5cm. The 226Ra activity is calculated as 31.1 ± 2.2 Bq.kg-1 from the deepest sample o f core. Total 210Pb activity varied between 113.9 ± 8.0 and 26.7 ± 1.9 Bq.kg-1. The sedimentation rate is calculated to be
0 2 4 6 8 10 12 14 Depth (cm)
(a) (b)
Figure 4 . Fallout radionuclide concentrations in Aegean Sea Core B (a) total 210Pb, (b) unsupported 210Pb.
In Core B the 226Ra activity is calculated as 26.7 ± 1.6 Bq.kg-1 from the deepest sample o f core. The sedimentation rate is calculated to be 0.13±0.01 g.cm-2y-1. Total 210Pb activity varied between 95.7 ± 5.8 and 25.5 ± 1.5 Bq.kg-1, but there was a net decline between the surficial sediments
In Core C the 226Ra activity is calculated as 35.6 ± 1.9 Bq.kg-1 from the deepest sample o f core. Total 210Pb activity varied between 74.63 and 29.58 Bq.kg-1 The sedimentation rate is calculated to be 0.26 ± 0.03 g.cm-2y-1. Mean value o f 226Ra in Core A higher than in Core B, Core C.
0 1 2 3 4 Depth (cm)
(a) (b)
Figure 5 . Fallout radionuclide concentrations in Aegean Sea Core C, (a) total 210Pb (b) unsupported 210Pb
A mean sedimentation rate o f 0.19 ± 0.02 g .cm-2y-1 results from a least square fit through the data. The sedimentation rates in the Gokova Bay seem to be comparable with those reported for the three collected core from M editerranean Sea (0.16 ± 0.01cm.y-1 (0.08 ± 0.01g.cm- 2.y-1 )), ( 0.31 ± 0.05 cm.y-1 (0.13 ± 0.02g.cm-2.y-1 )), (0.08 ± 0.01cm.y-1 (0.04 ± 0.003g.cm-2.y-1 )) (Tadjiki and Erten, 1994). These comparatively high sedimentation rates are probably due to the fact that the cores o f this work were recovered very near the land.
The average flux o f 210Pb computed in southern Turkey from the M editerranean Sea is around 52 Bq.m-2y-1 (Tadjiki and Erten, 1994). The atmospheric flux for core A o f about 47 ± 3 Bq.m- 2y-1 is probably a better representation o f the atmospheric 210Pb flux. There is no possibility to compare the flux determined from the cores due to the lack o f direct rainfall measurements in this site. The unsupported 210Pb fluxes in core B and core C were calculated 27±2 Bq m-2y-1 , 23±4 Bq m-2y-1 respectively. These values are quite lower than those in literature. One o f the reasons may be due to the difficulties in determining the supported 210Pb (226Ra) activity
Table 1. Radionuclide Parameters o f Gokova Bay Aegean Sea Sediment Cores
Unsupported 210Pb
Maximum Inventory Flux
Activity Bq kg-1 ± Bq m-2 ± Bq m-2y-1 ± Core A 82.8 8.3 1533 169 47 3 Core B 69.1 6.0 888 53 27 2 Core C 39.0 4.3 736 51 23 3 Mean values 63.6 6.2 1052 91 32 3 REFERENCES
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