engleski

Magnetic mineralogy of soils and sediments from Bokanjačko blato (Croatia)

Mineralogical characteristics of soils and sediments from karst field Bokanjacko blato have been identified by X-ray diffraction and rock magnetic measurement (susceptibility, anhysteretic and isothermal remanences). Bokanjacko blato is a typical karst depression, mainly covered by Holocene lake sediments and Pleistocene red soils. Catchment area is mostly composed of Cretaceous and Eocene karstified carbonate rocks and Eocene flysch deposits. Sediments of the Bokanjačko blato were collected in length of 23 m, where it reaches the limestone bedrock. Rock magnetic studies of sediments and soils rely on the presence of small amounts of ferromagnetic and canted- antiferromagnetis minerals which carry measurable remanent and induced magnetic properties. Mineralogical composition, determined by X-ray diffraction, shows high carbonate content, followed by quartz and rare feldspars in the upper part (~10 m). Clay minerals are illite, kaolinite and vermiculite until 3.2 m, and from 3.2 to 7.8 m appears illite, kaolinite (ordered and disordered), smectite and vermiculite. In the deeper parts of the core, there is no calcite ; dominant mineral phases are quartz, clay minerals kaolinite, illite, vermiculite and Fe-oxides and hydroxides (goethite and hematite), very well noticeable in change in colour, that becomes yellow and red. Dominant clay mineral phase is disordered kaolinite in the deeper sediments (13.5-22.7 m) and together with goethite and hematite are main constituents of the „red clays“, showing similar clay mineral composition to Istrian terra rossa. Magnetic measurement show that there are very low low- field (λ) and anhysteretic susceptibility (λARM) on the top of the sequence (0-4.3 m), followed by increasing values towards bottom of the lake sediments, implying to extensive erosion of the catchment material from 4.3 to 10.5 m. Frequency-dependent susceptibility λ(λFD) is higher from the 13.5 m till the end of the core (22.7 m), referred to ultrafine superparamagnetic magnetite. Very low HIRM(300- 1000 mT)/SIRM values, from 13.5 to 22.7 m are indicative of the presence super-paramagnetic hematite in red clay samples. The mineralogy, magnetic grain size and morphology of magnetic grains all vary according to the origins of the magnetic grains and their subsequent post- depositional sedimentary history and thus variations in rock magnetic properties can give clue about the origins of the magnetic minerals, sedimentary and soil-forming environments and thus climate change, in addition to mineralogical and chemical composition. The grain size dependence of various mineral magnetic parameters has been determined. Supposed sedimentary sequence lake sediments (0-10.5 m), loess sediments (10.5- 12.5 m) and red clays (12.5-22.7 m) is confirmed with magnetic behaviour of the material from Bokanjacko blato.

mineralogy; magnetic properties; lake sediments; red clays

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