Hagedorn et al.: Lead-2JO Dating and Heavy Metal Concentration in Recent Sediments
367
Geochemistry
The geochemical composition of the core is shown in Table 3. The major elements, with the
exception of Mn, had rather constant distribution over the entire core with a mean standard
deviation below 10%. To deduce the source of the sediments, a comparison with the chemical
composition of the CFB formations was undertaken. This formation has been studied in detail
by Lightfoot et al. (1993) and Wooden et a!. (1993). There was good agreement with the
Mokulaevsky (MK) and Nadezhdinsky (Nd3) formations (Figure 5). Therefore, these
formations can be regarded as the geogenic source of the core sediments. Mg and Ca are
depleted compared to the CFBs. This may be the result of weathering processes in polar
regions as discussed by Ugolini (1986). A strong enrichment of MnO (from 0.2 to 0.5 wt%)
was observed at a depth of 6.5 to 7.5 cm (Figure 4). At this depth, a dark brown layer and a
decrease in the water content were also observed. Such layers are reported from many lake
surface sediments and are regarded as the base of an oxidation zone (Kephkay, 1985; McKee et
a!., 1989), where Mn as well as Fe oxides precipitate.
The trace metals Cd, Cu, Ni, Pb and Zn showed a progressive increase towards the sedimentwater interface within the upper part of the core (0 to 5 cm depth; Figure 4). Due to the good
chemical agreement between the CFB formations and the lower part of the core, the geogenic
source of the sediment is well established. This is the basis for the calculation of enrichment
factors for the trace metals Cd, Cu, Ni, Pb and Zn using the mean composition of the lower
part of the core (6-17cm) as a reference for the unpolluted geogenic supply. This calculation is
performed for the uppermost 0.5 cm sediment section:
EFM = (M! Al)sf : (M/ Al)mean
(6)
where EF is the enrichment factor, M represents the metal content (Zn, Pb, Ni Cu), sf refers
to the surface sediment, mean refers to the mean for the lower core sediments, and Al is
aluminium.
In order to eliminate dilution effects, AI-normalization was applied because Al is regarded as a
stable constituent of the mineral matter.
The highest enrichment was found for Cd (EFcd= 2.5) and Pb (EFpb= 2.0), whereas the
enrichments of Cu (EFcu =1.3), Ni (EFNi= 1.3) and Zn (EFzn.= 1.3) were smaller. The heavy
metal enrichment (or supplementary heavy metals) presumably reflects pollution. To evaluate
the "historical" enrichment of the supplementary heavy metal concentrations independent of
variations in the accumulation rate, we calculated the initial 210Pb activity Coz (Bq g-l)for each
depth (z). With this record of Coz, the heavy metal/Coz (~g Bq-l) ratios were determined: 1) for
the total (geogenic + supplementary) heavy metal concentration, and 2) for the geogenic heavy
metal concentration. From the difference, the accumulation- corrected temporal distribution of
the~ supplementary heavy metal/Coz ratios can be calculated. We prefer the 21OPb-normalization
over the AI-normalization because 210Pb is transported by the atmosphere and takes the same
pathway into the lake sediments that we assume for the heavy metals. The 210Pb-normalization
makes the calculation of heavy metal enrichment independent of the accumulation rate. Of
course, this was also achieved by AI-normalization but owing to the similar chemical behavior
of 210Pb and the heavy mctals, the 210Pb-normalization also takes into account further factors,
such as: distribution coefficients between suspended matter and lake water, grain size
distribution of the lake sediments and catchment and water residence times. The results are
shown in Figure 6. The first enrichment of Cd, Pb and Zn occured at 4 cm depth, which
corresponds to the deposition year 1940. Above 3 cm, continuous increases in metal
accumulation up to the sediment surface were evident. In contrast, Ni and Cu were depleted at 4
cm depth but also increased above 3 cm.
367
Geochemistry
The geochemical composition of the core is shown in Table 3. The major elements, with the
exception of Mn, had rather constant distribution over the entire core with a mean standard
deviation below 10%. To deduce the source of the sediments, a comparison with the chemical
composition of the CFB formations was undertaken. This formation has been studied in detail
by Lightfoot et al. (1993) and Wooden et a!. (1993). There was good agreement with the
Mokulaevsky (MK) and Nadezhdinsky (Nd3) formations (Figure 5). Therefore, these
formations can be regarded as the geogenic source of the core sediments. Mg and Ca are
depleted compared to the CFBs. This may be the result of weathering processes in polar
regions as discussed by Ugolini (1986). A strong enrichment of MnO (from 0.2 to 0.5 wt%)
was observed at a depth of 6.5 to 7.5 cm (Figure 4). At this depth, a dark brown layer and a
decrease in the water content were also observed. Such layers are reported from many lake
surface sediments and are regarded as the base of an oxidation zone (Kephkay, 1985; McKee et
a!., 1989), where Mn as well as Fe oxides precipitate.
The trace metals Cd, Cu, Ni, Pb and Zn showed a progressive increase towards the sedimentwater interface within the upper part of the core (0 to 5 cm depth; Figure 4). Due to the good
chemical agreement between the CFB formations and the lower part of the core, the geogenic
source of the sediment is well established. This is the basis for the calculation of enrichment
factors for the trace metals Cd, Cu, Ni, Pb and Zn using the mean composition of the lower
part of the core (6-17cm) as a reference for the unpolluted geogenic supply. This calculation is
performed for the uppermost 0.5 cm sediment section:
EFM = (M! Al)sf : (M/ Al)mean
(6)
where EF is the enrichment factor, M represents the metal content (Zn, Pb, Ni Cu), sf refers
to the surface sediment, mean refers to the mean for the lower core sediments, and Al is
aluminium.
In order to eliminate dilution effects, AI-normalization was applied because Al is regarded as a
stable constituent of the mineral matter.
The highest enrichment was found for Cd (EFcd= 2.5) and Pb (EFpb= 2.0), whereas the
enrichments of Cu (EFcu =1.3), Ni (EFNi= 1.3) and Zn (EFzn.= 1.3) were smaller. The heavy
metal enrichment (or supplementary heavy metals) presumably reflects pollution. To evaluate
the "historical" enrichment of the supplementary heavy metal concentrations independent of
variations in the accumulation rate, we calculated the initial 210Pb activity Coz (Bq g-l)for each
depth (z). With this record of Coz, the heavy metal/Coz (~g Bq-l) ratios were determined: 1) for
the total (geogenic + supplementary) heavy metal concentration, and 2) for the geogenic heavy
metal concentration. From the difference, the accumulation- corrected temporal distribution of
the~ supplementary heavy metal/Coz ratios can be calculated. We prefer the 21OPb-normalization
over the AI-normalization because 210Pb is transported by the atmosphere and takes the same
pathway into the lake sediments that we assume for the heavy metals. The 210Pb-normalization
makes the calculation of heavy metal enrichment independent of the accumulation rate. Of
course, this was also achieved by AI-normalization but owing to the similar chemical behavior
of 210Pb and the heavy mctals, the 210Pb-normalization also takes into account further factors,
such as: distribution coefficients between suspended matter and lake water, grain size
distribution of the lake sediments and catchment and water residence times. The results are
shown in Figure 6. The first enrichment of Cd, Pb and Zn occured at 4 cm depth, which
corresponds to the deposition year 1940. Above 3 cm, continuous increases in metal
accumulation up to the sediment surface were evident. In contrast, Ni and Cu were depleted at 4
cm depth but also increased above 3 cm.