engleski

ISOTOPE ANALYSES OF THE LAKE SEDIMENTS IN THE PLITVICE LAKES AREA

The analyses of radioactive isotopes 14C, 137Cs, 210Pb, 214Pb and 214Bi and stable isotope 13C were performed in the Plitvice Lakes sediments. Sediment cores, top 40 cm, were taken from four lakes: two big lakes Prošće and Kozjak and two small lakes Gradinsko and Kaluđerovac. Frozen sediment cores were cut into 1-2 cm thick layers, dried and then analyzed. 14C activity in carbonate fraction was measured using liquid scintillation counter technique with benzene synthesis and in organic fraction using accelerator mass spectrometry technique with graphite synthesis. 137Cs, 210Pb, 214Pb and 214Bi were measured by low level gamma spectrometry method on ORTEC HPGe detector with the efficiency of 32%. Distribution of 14C activity in all sediment profiles showed increase of the 14C activity in the top ~15 cm as a response to thermonuclear bomb-produced 14C in the atmosphere in the sixties of the last century. This is also reflected in increased 137Cs concentration in all sediment profiles. Steady increase of the 14C activity and d13C of the carbonate fraction of the sediments from the uppermost lake Prošće to the lowermost lake Kaluđerovac is the results of CO2 degassing and carbon isotope exchange process between atmospheric CO2 and dissolved inorganic carbon (DIC) in water and photosynthetic activity. Different 14C activity of the carbonate fraction (65 – 85 pMC, percent of modern carbon) and organic fraction (77 – 95 pMC) is the result of geochemical and biological processes of the calcite precipitation in the lake waters. The d13C values of both fractions (-10 to -6‰ for carbonate and -34 to -30‰ for organic) are good indicator of origin of carbon and processes involved in lake sediment precipitation. Radioactive fallout 137Cs deposited across the landscape from atmospheric nuclear tests is strongly absorbed on soil particles limiting its movement by chemical and biological processes. Most 137Cs movement in the environment is by physical processes ; therefore, 137Cs is a tracer for studying erosion and sedimentation. The 137Cs technique is used to determine sediment accumulation rates in Plitvice Lakes. The approximately 22 y half-life of 210Pb makes it an ideal indicator of modern sedimentation rates. The analysis of sedimentation rates by the 210Pb method is based on the radioactive decay of unsupported 210Pb with depth in a column of sediment. In lake sediments, unsupported 210Pb can originate from two sources: atmospheric deposition to the lake surface and subsequent scavenging by the depositing sediments or mobilization of sediment from the surrounding watershed. 137Cs is derived from a different source than 210Pb and thus provides a valuable cross-check of the 210Pb results. The presented isotope study of lake sediments from four karst lakes in the Plitvice Lakes formed during the last ~100 year showed that sediments reflect global atmospheric changes in 14C and 137Cs activities caused by anthropogenic influence Differences in isotope composition of sediments from four lakes, particularly between big and small lakes, indicate the influence of geochemical and biological processes on the lake sediment precipitation.

lake sediments; Plitvie Lakes; 14C; 13C; 137Cs; 210Pb

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