engleski

Clay-mineral record in Vrana Lake sediments: The Holocene and Late Glacial transition

Clay mineral composition was determined on long cores (approx. 7 m long) from karstic lake Vransko jezero (Lake Vrana). It is formed in a karst depression within kastified carbonate rocks of Upper Cretaceous age on Cres Island (Croatia). The lake has no inflow or outflow rivers. A chronology for the lake sedimentation record during the Holocene has been developed based on AMS 14C- dating (Schmidt et al., 2000). The transition of the Late Glacial to the Holocene can be tracked. Surface sediments (0-30cm) are siliciclastic, whereas deeper sediments reflect alternating successions of sedimentation ranging from dominantly siliciclastic to dominantly calcareous. Basic studies included change in clay composition between glacial and interglacial sediment samples assuming transformation processes in the soils from the catchments. Analysis included X-ray diffraction (XRD) for bulk and clay mineralogy, magnetic susceptibility measurement and pollen analysis. Dominant clay mineral phases are illite, chlorite and kaolinite. Clay mineral abundances and clay-mineral ratios are used to reconstruct the weathering conditions. Mineralogical results are compared with palaeomagnetic-derived time scale. Clay data are further compared with diatom and pollen core material. The cold and dry periods during the glacial times were responsible for physical weathering and thus contribution of higher amounts of chlorite and illite, as they are less sensitive to chemical weathering. The abundance of kaolinite is good indicator of warm and humid interglacial period caused by extensive chemical weathering (Chamley, 1989). Lakes accumulate sediments from their surrounding environment and so sediment cores can provide a record of sediment source. The clay mineral composition of both potential lake sediment source soils and lake sediments were used to reconstruct the soil erosion history across the lake catchment during the Holocene.

Lake sediments; clay minerals

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