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Magnetic properties of the lake sediments along the eastern Adriatic coast (CROSBI ID 632694)

Prilog sa skupa u zborniku | sažetak izlaganja sa skupa | domaća recenzija

Ilijanić, Nikolina ; Maher, Barbara ; Miko, Slobodan ; Hasan, Ozren Magnetic properties of the lake sediments along the eastern Adriatic coast // 5. HRVATSKI GEOLOŠKI KONGRES s međunarodnim sudjelovanjem/5th CROATIAN GEOLOGICAL CONGRESS with international participation Osijek 23.– 25.09.2015. Knjiga Sažetaka/Abstracts Book / Marija Horvat & Lara Wacha (ur.). Zagreb: Hrvatski geološki institut, 2015. str. 115-116

Podaci o odgovornosti

Ilijanić, Nikolina ; Maher, Barbara ; Miko, Slobodan ; Hasan, Ozren

engleski

Magnetic properties of the lake sediments along the eastern Adriatic coast

Environmental magnetism investigates the magnetic properties of materials which have been formed under the influence of environmental processes. Rock magnetic studies of sediments and soils rely on the presence of small amounts of ferromagnetic and canted- antiferromagnetic minerals which carry measurable remanent and induced magnetic properties. Lake sediments and soils act as sensitive magnetic recorders of paleoclimatic and palaeoenvironmental change. The mineralogy, concentration, magnetic grain size and morphology of magnetic grains may all vary according to the origins of the magnetic grains and their subsequent post-depositional sedimentary history. Such variations can result in shifts in the magnetic properties of the material under study, shifts which can be readily and rapidly identified by highly sensitive magnetic instrumentation. We examined Late Pleistocene and Holocene paleoclimate and environmental information from lake sediments on the Eastern Adriatic coast (Lake Vrana, on the Island of Cres, Bokanjačko blato, Lake Vrana near Biograd, Baćina Lakes), so as sediment sourcing recorded by magnetic proxies. Magnetic measurements include low and high field magnetic susceptibility, anhysteric remanent magnetization (ARM), and isothermal remanent magnetization acquisition (IRM) and they were carried out on powder samples. Magnetic susceptibility was determined using a Bartington MS2B dual frequency susceptiblity meter at the most sensitive (*0.1) range, both on low (0.47 kHz) and high (4.7 kHz) frequencies. Magnetic susceptibility, λlf, indicates the amount of magnetic material. Difference in susceptibilities measured at 2 frequencies reflects the presence of ultrafine ferromagnetic minerals (magnetite) lying in the superparamagnetic (SP) grain size region. Values higher than ~4% are typical of pedogenically enhanced soils (MAHER et al., 2003). Anhysteretic remanent magnetization (ARM), expressed as susceptibility of ARM (λARM), is sensitive to concentration and grain size of magnetite grains and was imparted using a Molspin demagnetizer with ARM attachment at 80 mT, in a 100 mT DC biasing field and then stepwise demagnetized in AC fields of 5, 10, 15 and 25 mT. The stepwise acquisition of isothermal remanent magnetization (IRM), in DC fields of 10, 20, 50, 100 and 300 mT was made on a Molspin pulse magnetizer and at 1 T (regarded as the saturation IRM-SIRM) using a Newport electromagnet. Samples were demagnetized in the same steps as λARM, with an additional ac demagnetisation step of 100mT. Remanent coercivity was determined from IRM measurements using backfield once the SIRM was reached. After a sample has acquired an IRM it is possible to demagnetize the sample which can yield information about the ease of remanence acquisition, or the coercivity of a sample, which depends on the magnetic mineral present. Hard IRM (HIRM) remanence (high coercivity) is indicative of antiferromanetic minerals, mainly hematite and goethite, while soft minerals easily acquire remanence, like magnetite. All remanence measurements were made on a Molspin Minispin fluxgate spinner magnetometer. Magnetic property measurements improved knowledge and understanding of processes involved in paleoenvironmental changes, paleoclimate reconstructions of the karst region of the Adriatic coast, so as lake sediment provenance due to changes in the catchments, soils erosion and dust aeolian flow. Lake sediment core from Lake Vrana on the island of Cres is the northernmost lake and shows different down-core variation of the magnetic susceptibility than the other analysed cores. Variation is in the agreement with other analysis performed on the sediments (grain size, geochemical and mineralogical analysis), but some processes still need to be understood, for example possible authigenic mineral formation in lake sediments which depends on the aquatic productivity and nutrient availability (EVANS & HELLER, 2003). Magnetic measurement of Bokanjačko blato sediment sequence (fig 1) show that there are very low low-field (λlf) 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 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 superparamagnetic hematite in red clay samples. Based on ARM/SIRM ratios versus low magnetic susceptibility, the samples from deeper parts of Bokanjačko blato show similarity to terra rossa soils from the Ravni Kotari region (fig 2). Magnetic properties in sediments from Lake Vrana near Biograd, which are dominantly composed of carbonate sediments, reveal postdepositional changes in lake sediments and possibly formation of secondary sulphide minerals, such as greigite (high SIRM/λlf ). In sediments from Lake Bacina (Crnisevo), tephra was identified by the very high magnetic susceptibility and other magnetic properties.

lake sediments; magnetic properties; magnetic mineralogy; superparamagnetic particles

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Podaci o prilogu

115-116.

2015.

objavljeno

Podaci o matičnoj publikaciji

5. HRVATSKI GEOLOŠKI KONGRES s međunarodnim sudjelovanjem/5th CROATIAN GEOLOGICAL CONGRESS with international participation Osijek 23.– 25.09.2015. Knjiga Sažetaka/Abstracts Book

Marija Horvat & Lara Wacha

Zagreb: Hrvatski geološki institut

Podaci o skupu

5. HRVATSKI GEOLOŠKI KONGRES s međunarodnim sudjelovanjem/5th CROATIAN GEOLOGICAL CONGRESS with international participation

predavanje

23.09.2015-25.09.2015

Osijek, Hrvatska

Povezanost rada

Geologija