engleski

Environmental changes inferred from karstic lake sediment geochemistry and clay minerals during the Holocene (Bokanjacko blato and Lake Vrana near Biograd)

The multy-proxy analysis of sediment cores recovered in karstic Lake Vrana (11 m) and polje Bokanjacko blato (7, 8 m and 23 m) located in northern Dalmatia, provided continuous sedimentary record extending back to last 10 000 years. The integration of mineralogical and geochemical analysis, together with age model based on AMS radicarbon dates, enabled reconstruction of the main environmental changes in the region. The clay fraction of both cores results from the erosion of the siliciclastic material (soils, terra rossa, marl or loess) in the catchments. Deeper parts of the Bokanjacko blato (12.5-23 m) are composed of red clay deposits overlying limestones. On red clays there are yellow loess-like sediments, covered with grey lake sediments. Dominant clay mineral in red clays is poorly crystalized kaolinite which does not intercalate with DMSO, while in minor amounts there are illite and vermiculite. In loess like deposits there are more illite and well crystalized kaolinite, together with vermiculite and poorly crystalized kaolinite. In lake sediments from 7.8-3.85 cm there are smectite, vermiculite, illite and kaolinite, which suggest origin from loess/marl deposits. In the upper part of the core, there is dominant carbonate sedimentation, with very little clay minerals. In Lake Vrana, bottom sediments (8.06-11 m) contain smectite, illite and kaolinite, which arise from the erosion of the marl deposits. Higher erosion rates in that interval are result of the environment in which flood plain predominate, with small area permanently covered with water. After 9 100 cal BP the lake could have been formed, after rising the sea level, evident by pure carbonate sedimentation after that period. In the upper sediments (0-8.06 m), there are hydroxyl interlayered vermiculite, illite and kaolinite, which could be attributed to soils (kalkocambisols) and terra rossa dominated in the area. According to palynological data, human impact (deforestation) probably triggered period of increased soil erosion in Lake Vrana near Biograd after 6 100 cal BP.

lake sediments; clay minerals; Holocene

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