engleski

Environmental changes during Holocene in Ravni Kotari region - records stored in the former lake Bokanjačko blato

Bokanjačko blato is a typical karst depression, mainly covered by Quaternary pond and lake sediments. Together with its catchment area, it belongs to Ravni Kotari region, mostly composed of Cretaceous and Eocene karstified carbonate rocks and Eocene flysch deposits. Numerous springs occur in the catchment. It is today important water supply area for the adjacent town Zadar and its surroundings. Bokanjačko polje was dried out back in 1960-ies, for agriculture purposes. Quaternary lake sediments contain many indicators of environmental changes, including geochemical markers, mineralogical fingerprint of the catchment and pollen remains of ancient land use dynamics. Undisturbed sediments from the lowermost part of the Bokanjačko polje were collected in length of 7, 8 m. Longer core (length 23m) drilled by commercial company Geokon was also included in the investigation. Mineralogical composition, determined by X-ray diffraction, shows high carbonate content, followed by quartz and rare feldspars in the upper part (~10 m). In the deeper parts of the core, there is no calcite ; dominant mineral phases are quartz, clay minerals kaolinite and illite, and Fe-oxides and hydroxides (goethite and hematite). Dominant clay mineral phases are illite and kaolinite, in the deeper sediments, and together with goethite and hematite, are main constituents of the „red soils“. Optical microscopy was used to determine light and heavy mineral fraction. Light fraction is represented mainly by quartz, but in upper parts down to depth 12 m, some gypsum and feldspar, in most cases orthoclase and only few grains of microcline can be found. Share of heavy mineral fraction varies between 23% to only 0, 4% for depths 6 m and 21 m, respectively. It is mainly represented by opaque minerals which content varies between 40% to 96% for core depths 10 and 6 m, respectively. Transparent minerals are represented by epidote group, mainly clinozoisite, garnet and amphibole in upper parts of the core, in the lower parts (>13m) ultra-stable minerals as zircon, tourmaline and rutile are dominant. In virtually all samples staurolite, kianit and galucophane were also found. Boreal period can be evidenced by pollen analysis on 760 cm, which is similar to 910 cm of the Lake Vrana core (mostly herbaceous plants), located in the south eastern part of the Ravni Kotari. Also, 500-600 cm of both core belong to the Atlantic period of the wet and warm climate (mixed deciduous–evergreen forest). Due to its location near the sea, significant changes during the Holocene can be tracked, followed by the sea level rise. Ice melting after Younger Dryas and prolonged pluvial period enhanced erosion in Late Holocene, evidenced by magnetic susceptibility measurements. Until this investigation, it was suspected that the Holocene deposits are making up to 18 m of the deposits, but likely it is not more than 10 m. Based on AMS radiocarbon dating, 760 cm is calculated to 8990 BP. Specific structures occur on the 1050 cm of the core, where evidences of subaeral exposure like desiccation cracks are visible. After that period, the lake could have been formed.

Holocene; Bokanjačko blato; environmental changes

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