Hagedorn et at.: Lead-210 Dating and Heavy Metal Concentration in Recent Sediments
371
the calculated mean sedimentation rate from I mm a-I to 0.8 mm a-I.
Besides mobilization processes, 210Pb profiles could also be modified by sediment mixing
due to bioturbation or sediment redistribution. This effect was observed by Nittrouer et al.
(1983, 1984) on the Washington Continental Shelf, and by Robbins and Edgington (1975) in
Lake Michigan. Such mixing processes are particle-selective and could sometimes be identified
by constant 2iOPb activities over the mixing depth and by deep penetration of short-lived
radionuclides like 234Th and 137Cs. In the present sediment core, neither constant 210Pb
activities over greater depth intervals nor macroscopic or microscopic indications of
bioturbation could be observed. So far, a cross check of the 210Pb data by other physical dating
methods (e.g. 14C, 137Cs) is not available.
Trace metal mobilization during organic degradation and redox cycling
Studies of trace metal fluxes across the sediment-water interface in marine waters were
performed by Westerlund et al. (1986). Their results show that Cu, Cd, Ni and Zn were
released from sediments under oxidizing conditions whereas a release of Pb was not observed.
Lapp and Balzer (1993) attributed an observed high pore-water flux of Cd, Cu and Ni in oxic
coastal sediments of Kiel Bight to the aerobic degradation of organic matter. Meyers and
Ishiwatari (1995) stated that degradation of organic matter in lacustrine bottom sediments is
much greater under oxic than anoxic conditions and about 75% of organic carbon can be
released to the hypolimnion due to these processes (Meyers and Eadie, 1993). Therefore the
enrichment of Cd, Cu and Ni in the oxygenized zone of the sediment could be a steady-state
condition (uptake and release by organic matter). The organic carbon contents of the
investigated core are between 1.0 and 0.7 wt.% with a typical lacustrine CIN ratio of 8 in the
upper part of sediment (Figure 7). A decrease of the carbon content from 1.0 to 0.7 wt. %
between 3 and 6 cm depth may reflect degradation processes. On the other hand, since the
accumulation rate also increases at this depth (Figure 3), the low Corg content could be the
result of dilution. Since no change in the CIN ratio occurs at this depth, the lowering in organic
carbon could also be the result of low organic production within the water column.
The most important control on Pb and Zn concentration in lake water are pH and redox
conditions. The partition coefficients of Pb and Zn between water and Fe and Mn oxides
decrease rapidly under acidification, and trace metals are released to the water. In neutral to
alkaline waters like Lama Lake, the partition coefficients of Pb and Zn are high (KD > 1 03;
Tessier et aI., 1985). Furthermore, the reduction of the Mn and Fe oxide carrier phases can
result in the mobilization of bound trace metals. Pore water investigations on various Canadian
lakes indicate that Zn is depleted under anoxic conditions due to precipitation of Zn sulfide (e.g.
Matisoff et aI., 1980; Tessier et aI., 1989). Investigations of Pb mobility in the lacustrine
environments sometimes produce contradictory results. As shown above, some investigations
indicate Pb mobility during anoxic conditions. Based on laboratory experiments, Frevert (1987)
attributed the efficient and
rapid removal of dissolved Pb under anoxic conditions to sulfide precipitation. Following the
results of various field-based studies on Pb and Zn in lacustrine and marine environments, the
sediments acted as a sink more than as a source (e.g. Carignan and Nriagu,1985; HamiltonTaylor and Davison, 1995; Westerlund et aI., 1986). This suggests that the observed Pb and Zn
enrichment can be attributed to pollution rather than redox phenomena and organic degradation.
Since Cu and Ni are more mobile, the negative values of the 210Pb-normalized profiles below
3 cm (Figure 5) could have been affected by organic degradation. On the other hand, Cd
showed the same "enrichment-profile" as the more immobile trace elements Pb and Zn, which
confirms an anthropogenic source of Cd as well. Furthermore, the Cd enrichment seems to be
too high for a steady state due to organic degradation.
371
the calculated mean sedimentation rate from I mm a-I to 0.8 mm a-I.
Besides mobilization processes, 210Pb profiles could also be modified by sediment mixing
due to bioturbation or sediment redistribution. This effect was observed by Nittrouer et al.
(1983, 1984) on the Washington Continental Shelf, and by Robbins and Edgington (1975) in
Lake Michigan. Such mixing processes are particle-selective and could sometimes be identified
by constant 2iOPb activities over the mixing depth and by deep penetration of short-lived
radionuclides like 234Th and 137Cs. In the present sediment core, neither constant 210Pb
activities over greater depth intervals nor macroscopic or microscopic indications of
bioturbation could be observed. So far, a cross check of the 210Pb data by other physical dating
methods (e.g. 14C, 137Cs) is not available.
Trace metal mobilization during organic degradation and redox cycling
Studies of trace metal fluxes across the sediment-water interface in marine waters were
performed by Westerlund et al. (1986). Their results show that Cu, Cd, Ni and Zn were
released from sediments under oxidizing conditions whereas a release of Pb was not observed.
Lapp and Balzer (1993) attributed an observed high pore-water flux of Cd, Cu and Ni in oxic
coastal sediments of Kiel Bight to the aerobic degradation of organic matter. Meyers and
Ishiwatari (1995) stated that degradation of organic matter in lacustrine bottom sediments is
much greater under oxic than anoxic conditions and about 75% of organic carbon can be
released to the hypolimnion due to these processes (Meyers and Eadie, 1993). Therefore the
enrichment of Cd, Cu and Ni in the oxygenized zone of the sediment could be a steady-state
condition (uptake and release by organic matter). The organic carbon contents of the
investigated core are between 1.0 and 0.7 wt.% with a typical lacustrine CIN ratio of 8 in the
upper part of sediment (Figure 7). A decrease of the carbon content from 1.0 to 0.7 wt. %
between 3 and 6 cm depth may reflect degradation processes. On the other hand, since the
accumulation rate also increases at this depth (Figure 3), the low Corg content could be the
result of dilution. Since no change in the CIN ratio occurs at this depth, the lowering in organic
carbon could also be the result of low organic production within the water column.
The most important control on Pb and Zn concentration in lake water are pH and redox
conditions. The partition coefficients of Pb and Zn between water and Fe and Mn oxides
decrease rapidly under acidification, and trace metals are released to the water. In neutral to
alkaline waters like Lama Lake, the partition coefficients of Pb and Zn are high (KD > 1 03;
Tessier et aI., 1985). Furthermore, the reduction of the Mn and Fe oxide carrier phases can
result in the mobilization of bound trace metals. Pore water investigations on various Canadian
lakes indicate that Zn is depleted under anoxic conditions due to precipitation of Zn sulfide (e.g.
Matisoff et aI., 1980; Tessier et aI., 1989). Investigations of Pb mobility in the lacustrine
environments sometimes produce contradictory results. As shown above, some investigations
indicate Pb mobility during anoxic conditions. Based on laboratory experiments, Frevert (1987)
attributed the efficient and
rapid removal of dissolved Pb under anoxic conditions to sulfide precipitation. Following the
results of various field-based studies on Pb and Zn in lacustrine and marine environments, the
sediments acted as a sink more than as a source (e.g. Carignan and Nriagu,1985; HamiltonTaylor and Davison, 1995; Westerlund et aI., 1986). This suggests that the observed Pb and Zn
enrichment can be attributed to pollution rather than redox phenomena and organic degradation.
Since Cu and Ni are more mobile, the negative values of the 210Pb-normalized profiles below
3 cm (Figure 5) could have been affected by organic degradation. On the other hand, Cd
showed the same "enrichment-profile" as the more immobile trace elements Pb and Zn, which
confirms an anthropogenic source of Cd as well. Furthermore, the Cd enrichment seems to be
too high for a steady state due to organic degradation.