engleski

Distribution of authigenic and allogenic fraction in recent lake sediment: isotopic and chemical composition

Uppermost layers of lake sediments in Lake Kozjak, the Plitvice Lakes National Park, Croatia, were used to study the environmental changes in the area. The process of calcium carbonate precipitation in form of the lake sediment is very intensive in the lake. Sediment cores, top 40 cm, were taken at three locations in the lake: (1) near the confluence of Rječica stream to Lake Kozjak, (2) ~50 m far from the point (1), (3) ~100 m far from the point (1). At the location (1) intensive growth of macrophites was observed. The depth profiles of the following quantities were obtained: organic and carbonate fraction, C-H-N analyses of organic fraction, 14C activity of organic and carbonate fraction, 13C/12C composition of carbonate fraction and trace element analyses. The concentration of organic fraction decreases from the outflow of Rječica stream towards the open lake: at location (1) 15-44%, location (2) 6-35%, and location (3) 12-26%. The C/N ratio is the highest at (1), 12-17, follows (2), 10-15, and at (3) it is 10-12, indicating mainly terrestrial origin of organic material, particularly at point (1). 14C activity of organic fraction is similar for all three locations: 106-111 percent of modern carbon (pMC) for upper layers (top 10-15 cm) and 92-97 pMC for lower layers. 14C activity of carbonate fraction increases towards the open lake: at (1) 37 pMC (mean value), at (2) 58 pMC and at (3) 65 pMC.  13C values of carbonate fractions show variations between -3‰ and -8‰ indicating different contribution of authigenic and allogenic carbonate in the sediment. Trace element concentrations show no changes along depth profile. The higher concentration of phosphorus at location (1) (500-1500 mg/kg) could be the consequence of intensive plant growth at this site. The isotopic and chemical results of sediments at 3 locations show significant influence of the material transported by the Rječica stream to the composition of lake sediment. Increasing of the allogenic fraction in the carbonate sediment, e.g. organic material, can produce contamination of the natural origin causing growth of macrophites in the lake.

lake sediment; 14C; 13C; 15N; 18O; CHN; Croatia; Plitvice Lakes

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