engleski

Holocene evolution of the Vransko jezero catchment (Croatia): A reconstruction based on clay mineralogy and geochemistry

Vransko jezero (Lake Vrana) is formed in a karst type depression (doline) within kastified carbonate rocks of Upper Cretaceous age on Cres Island (Croatia). The shape of the lake reflects geomorphic attributes of a karst polje and 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). Several dramatic changes in lake sediment composition related to both natural and human influences (deforestation) occurred during the Holocene. Surface sediments (0-30 cm) are siliciclastic, whereas deeper sediments reflect alternating successions of sedimentation ranging from dominantly siliciclastic to dominantly calcareous. The present deep-water lake es¬ ; tablished at about 8.5 ky BP (Schmidt et al., 2000). To reconstruct the paleohydrology dynamics during the Holocene sediment cores from the littoral to the profundal zones as well as soil profiles of terra rossa, loess, and slope deposits were analyzed for bulk geochemistry, their clay content, heavy mineral composition, and pollen. Dominantly siliciclastic sediments consist mainly of calcite, dolomite and quartz, with smaller amounts of micas, chlorites, K-feldspars and plagioclase. In the rest of the profile calcite is dominant, while dolomite, quartz, micas and chlorites occur in small amounts. The terra rossa, consists dominantly of quartz, small quantities of mica, chlorites and plagioclase. Samples from a littoral core (550 cm long, from present to 6650 y BP) have a consistent composition of clay mineral fraction dominated illite, vermiculite and a weak 14 Å ; peak indicating Fe rich chlorites. The Pleistocene loess soil (24 ky BP 14C dating) in the catchment contain dominantly dolomite, quartz, and smaller amounts of calcite, chlorites and illite. Mineral composition of palaeosoils and sub-surface samples of point out to the same origin (transported and partially rubified soluble residue of limestone and dolomite mixed with loess). The <2μ m fraction in the same samples contain kaolinite and illite. Those are dominant minerals in clay size fraction in Istrian terra rossa (Durn, 2003). 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.

Holocene; Lake sediments; Terra rossa; Loess

nije evidentirano