Hagedorn et al.: Lead-2JO Dating and Heavy Metal Concentration in Recent Sediments
365
from the 2lOPbunsupported, the measured 21OPbtotal have to corrected:
210Pb
- 210Pb
_ 210Pb
unsupported -
total
supported
(I)
whereas 21OPbsupported can be estimated from core depths at which the age of sedimentation
deposition is greater than 5t1l2 of 210Pb. This is the time at which the 21OPbunsupported has
decayed and the measured 210Pb results from 238U decay in the mineral matter only. This depth
was reached below 8 cm, where the 210Pb activity had a constant value of 0.041 ± 0.006 Bq gI (Figure 2). As a further control, gamma-spectrometrical measurements of 226Ra and 214Pb,
which are in secular equilibrium with 210Pbsupported, were carried out (Table 2). Within the
error margin, the measured 226Ra and 214Pb activities agreed with the estimated 21OPbsupported
activities from the lower core sections (Figure 2). Therefore a mean value of 0.04 ± 0.01 Bq gI activities was estimated for the 21OPbsupported and assumed to be constant for all depths.
Table 2: 21OPb, 2l4Pb and 226Ra activities and calculated accumulation rate (r) of' core PO 1111-1.
cumulative
depth
mass
2lOPb
±
r
2l4Pb
226Ra
(cm)
(g cm· 2 )
(Bq g.l)
(Bq g.l) (g cm·2 y.l) (Bq g.l)
(Bq g.l)
0.25
0.14
0.300
0.058
0.046
0.75
0.44
0.258
0.029
0.049
l.25
0.76
0.214
0.040
0.049
l.75
l.07
0.217
0.009
0.039
2.25
l.36
0.210
0.032
0.030
3
l.89
0.118
0.011
0.044
0.03
4
2.64
0.078
0.006
0.053
5
3.39
0.070
0.005
0.036
6
4.21
0.051
0.003
0.026
7
5.18
0.033
0.002
8
5.95
0.050
0.003
9
6.69
0.048
0.003
0.03
0.06
10
7,43
0.045
0.003
II
8.19
0.039
0.001
12
9.02
0.032
0.002
13
9.73
0.043
0.003
14
10.43
0.040
0.003
15
11.18
0.039
0.003
16
1l.92
0.037
0.002
17
12.67
0.034
0.003
0.04
0.05
The sediment accumulation rates and ages were determined using the CRS model which
assumes a Constant Rate of 210Pb S,upply. The applicability of this model was tested by
Appleby et al. (1979), who compared 210Pb profiles to annually laminated sediments, and
found a good agreement between varve counting and CRS derived 210Pb data. The sediment
accumulation rate and age determinations were calculated according to the CRS model as:
A z = Ao e -A t (z)
Az = z=o f' p (1-<1>x) Cx dx
<1>x = V w(x) / V w(x)+Ms(x)/ps
(2)
(3)
(4)
365
from the 2lOPbunsupported, the measured 21OPbtotal have to corrected:
210Pb
- 210Pb
_ 210Pb
unsupported -
total
supported
(I)
whereas 21OPbsupported can be estimated from core depths at which the age of sedimentation
deposition is greater than 5t1l2 of 210Pb. This is the time at which the 21OPbunsupported has
decayed and the measured 210Pb results from 238U decay in the mineral matter only. This depth
was reached below 8 cm, where the 210Pb activity had a constant value of 0.041 ± 0.006 Bq gI (Figure 2). As a further control, gamma-spectrometrical measurements of 226Ra and 214Pb,
which are in secular equilibrium with 210Pbsupported, were carried out (Table 2). Within the
error margin, the measured 226Ra and 214Pb activities agreed with the estimated 21OPbsupported
activities from the lower core sections (Figure 2). Therefore a mean value of 0.04 ± 0.01 Bq gI activities was estimated for the 21OPbsupported and assumed to be constant for all depths.
Table 2: 21OPb, 2l4Pb and 226Ra activities and calculated accumulation rate (r) of' core PO 1111-1.
cumulative
depth
mass
2lOPb
±
r
2l4Pb
226Ra
(cm)
(g cm· 2 )
(Bq g.l)
(Bq g.l) (g cm·2 y.l) (Bq g.l)
(Bq g.l)
0.25
0.14
0.300
0.058
0.046
0.75
0.44
0.258
0.029
0.049
l.25
0.76
0.214
0.040
0.049
l.75
l.07
0.217
0.009
0.039
2.25
l.36
0.210
0.032
0.030
3
l.89
0.118
0.011
0.044
0.03
4
2.64
0.078
0.006
0.053
5
3.39
0.070
0.005
0.036
6
4.21
0.051
0.003
0.026
7
5.18
0.033
0.002
8
5.95
0.050
0.003
9
6.69
0.048
0.003
0.03
0.06
10
7,43
0.045
0.003
II
8.19
0.039
0.001
12
9.02
0.032
0.002
13
9.73
0.043
0.003
14
10.43
0.040
0.003
15
11.18
0.039
0.003
16
1l.92
0.037
0.002
17
12.67
0.034
0.003
0.04
0.05
The sediment accumulation rates and ages were determined using the CRS model which
assumes a Constant Rate of 210Pb S,upply. The applicability of this model was tested by
Appleby et al. (1979), who compared 210Pb profiles to annually laminated sediments, and
found a good agreement between varve counting and CRS derived 210Pb data. The sediment
accumulation rate and age determinations were calculated according to the CRS model as:
A z = Ao e -A t (z)
Az = z=o f' p (1-<1>x) Cx dx
<1>x = V w(x) / V w(x)+Ms(x)/ps
(2)
(3)
(4)