**4.4 Analyses of isotopes and diatoms**

The absolute chronology of the sediments is also based on available isotope measurements of 137Cs, 241Am and 210Pb (Fig. 9). The peak radiation of 137Cs and 241Am at a core depth of 100 cm is attributed to the 1963 maximum of bomb fallout which started in 1954. Americium clearly demonstrates bomb fallout because there was no emission of americium during the Chernobyl disaster. The peak at 40 cm core depth is assigned to the Chernobyl fallout in 1986.

Analyses of microfloral and faunal remains confirmed the system change between the upper and lower parts of the reference core (transition at 190 cm). However, a very high frequency of diatoms was found in the upper part of the core. Samples taken from core ML 14/03 from the sediment surface down to 166 cm core depth (location see Fig. 3), indicate that about 80-90 % of the individuals are planktonic and the remaining 10-20% are benthic diatoms. This uniform palaeolimnological stratification is interrupted by one distinct event at a depth of 66-76 cm, where the proportion of planktonic individuals drops to 15 % and the benthic forms increase to a peak of 85 %. This event indicates a high sediment inflow during a major flood. The analysis of the runoff data indicates that this big event relates to the extreme magnitude of the flood in 1978.

Lake Mladotice in the Western Czech Republic – Sediments as a Geoarchive

variations in sedimentation rates between 6.7 and 1.8 cm/a.

the sediment surface (2003).

for Flood Events and Pre- to Postcommunist Change in Land Use since 1872 317

between the layers results in a possible error of ± 5 years. Owing to the alternation between event-dependent high sediment inputs and annual sediment layers, there are substantial

Above 160 cm core depth, there is a clearly bedded diatom mud that can be dated relatively accurately by various sediment analyses. The start of bomb fallout in 1954 provides a time marker (120 cm core depth). The sedimentation rate between 1920 and 1954 was calculated at 2.1 cm/a. Maximum fallout at 100 cm core depth occurred in 1963 (sedimentation rate 2.2 cm/a). The next time marker is the flood of 1978, shown by a distinct event layer and a change in diatom composition. The sedimentation rate from 1963 to 1978 was calculated at 2.7 cm/a. Until the fallout from Chernobyl in 1986, the sedimentation rate fell only slightly to 2.5 cm/a. The rate is 2.4 cm/a between 1986 and

Fig. 10. Results of thin section analyses, temporal resolution and calculated sedimentation

Data obtained from various sediment analyses yield a high temporal resolution of the sediment stratigraphy. In 1920 the sedimentation rate was 2.1 cm/a; then came a slight rise,

rates on the basis of the reference core ML 18/03.

**5. Conclusions** 

Fig. 9. Isotope contents in reference core ML 18/03 (137CS, 241Am, 210Pb).

#### **4.5 Agrochemical analyses**

Agrochemical analyses of four samples in core ML 18/03 do not show the presence of pesticides such as DDT or any of its metabolites. Nor were any polychlorinated biphenyls (PCBs) found in the sediment samples. Fractionated qualitative and quantitative analyses reveal large quantities of polycyclic aromatic hydrocarbons (PAH). These compounds are evidently left over from incomplete combustion processes, and their presence in the sediment is due to atmogenic input into the sediments. The highest concentration is detected at a depth of 45 – 50 cm. The levels are comparable with concentrations in the sediments of other mountain lakes in central and southeastern Europe (Fernandez et al. 1999, Muri et al. 2003).

#### **4.6 Thin sections, temporal resolution and sedimentation rates**

Thin sections give an additional chronology, in some cases with an accuracy of one year. The new sediment data on geochemistry, isotopes, diatoms and thin sections especially of reference core ML 18/03 and partly of core ML 14/03 (diatoms) yield the following interpretation (see Fig. 10):

The 1872 landslide impounded the lake, and sedimentation began. Thin section analyses show that clastic sediments were deposited in annual layers above the base. In some cases the boundaries between the layers are blurred, resulting in erroneous ages for the lower part of the core. It was possible to count the layers up to 1883 with an error of ± 2 years. The average sedimentation rate was 1.8 cm/a.

This was followed by a 50 cm thick, homogeneous sequence of unbedded sediment. This sediment is interpreted as having been deposited during an event or a phase of events prior to 1890; the average sedimentation rate is about 9.1 cm per year. The material comes either from the still unvegetated mass failure area at the southern end of the lake (Fig. 3) or from flood input by the Mladoticky creek.

Up to 190 cm core depth, thick unbedded sequences alternate with annually bedded sediments. A layer count dated this depth to 1920. Partial blurring of the boundaries

Fig. 9. Isotope contents in reference core ML 18/03 (137CS, 241Am, 210Pb).

**4.6 Thin sections, temporal resolution and sedimentation rates** 

Agrochemical analyses of four samples in core ML 18/03 do not show the presence of pesticides such as DDT or any of its metabolites. Nor were any polychlorinated biphenyls (PCBs) found in the sediment samples. Fractionated qualitative and quantitative analyses reveal large quantities of polycyclic aromatic hydrocarbons (PAH). These compounds are evidently left over from incomplete combustion processes, and their presence in the sediment is due to atmogenic input into the sediments. The highest concentration is detected at a depth of 45 – 50 cm. The levels are comparable with concentrations in the sediments of other mountain lakes in central and southeastern Europe (Fernandez et al. 1999, Muri et al. 2003).

Thin sections give an additional chronology, in some cases with an accuracy of one year. The new sediment data on geochemistry, isotopes, diatoms and thin sections especially of reference core ML 18/03 and partly of core ML 14/03 (diatoms) yield the following

The 1872 landslide impounded the lake, and sedimentation began. Thin section analyses show that clastic sediments were deposited in annual layers above the base. In some cases the boundaries between the layers are blurred, resulting in erroneous ages for the lower part of the core. It was possible to count the layers up to 1883 with an error of ± 2 years. The

This was followed by a 50 cm thick, homogeneous sequence of unbedded sediment. This sediment is interpreted as having been deposited during an event or a phase of events prior to 1890; the average sedimentation rate is about 9.1 cm per year. The material comes either from the still unvegetated mass failure area at the southern end of the lake (Fig. 3) or from

Up to 190 cm core depth, thick unbedded sequences alternate with annually bedded sediments. A layer count dated this depth to 1920. Partial blurring of the boundaries

**4.5 Agrochemical analyses** 

interpretation (see Fig. 10):

average sedimentation rate was 1.8 cm/a.

flood input by the Mladoticky creek.

between the layers results in a possible error of ± 5 years. Owing to the alternation between event-dependent high sediment inputs and annual sediment layers, there are substantial variations in sedimentation rates between 6.7 and 1.8 cm/a.

Above 160 cm core depth, there is a clearly bedded diatom mud that can be dated relatively accurately by various sediment analyses. The start of bomb fallout in 1954 provides a time marker (120 cm core depth). The sedimentation rate between 1920 and 1954 was calculated at 2.1 cm/a. Maximum fallout at 100 cm core depth occurred in 1963 (sedimentation rate 2.2 cm/a). The next time marker is the flood of 1978, shown by a distinct event layer and a change in diatom composition. The sedimentation rate from 1963 to 1978 was calculated at 2.7 cm/a. Until the fallout from Chernobyl in 1986, the sedimentation rate fell only slightly to 2.5 cm/a. The rate is 2.4 cm/a between 1986 and the sediment surface (2003).

Fig. 10. Results of thin section analyses, temporal resolution and calculated sedimentation rates on the basis of the reference core ML 18/03.
