Magnetic spherules in sediments of the karstic Dobra River (Croatia)

PURPOSE: The paper describes an unusual finding of magnetic particles in stream sediments of the karstic Dobra River (Croatia). It aims to investigate the magnetic susceptibility properties of river sediments and to find links with their mineralogical and chemical composition. The goal of our research was to suggest the possible origin of magnetic particles in the river sediments within the Dinaric karst region, which to our knowledge was not exposed to metal manufacturing processes. MATERIALS AND METHODS: A preliminary, exploratory sediment sampling programme was undertaken. Sediments were collected near the channel bank (from the top layer) at 16 representative stations distributed along the length of the Dobra River (110 km). Sediments were air-dried and passed through 2-mm and 63-μm sieves, and analysed for magnetic susceptibility (and Curie temperature), isothermal remanent magnetism (IRM), stereo-microscopy of separated magnetic grains, mineralogical analysis (using X-ray diffraction), and chemical analysis (using inductively coupled plasma–mass spectrometry). RESULTS AND DISCUSSION: Increased magnetic susceptibility and IRM values were observed mostly in the sediments of the Upper Dobra. Thermomagnetic curves show a distinctive Curie-point of magnetite at 580 °C. Additional transformation observed at 520–560 °C derives from titanomagnetite. There was no significant correlation between magnetic susceptibility and Fe. Magnetic particles from the Dobra River sediments contain pyroxene, plagioclase, hematite and quartz, in addition to magnetite. White spherules within magnetic grains are also present. The major constituent of five separated magnetic spherules is Fe; the minor constituents are Al, Ca, Mg and Si, and there are numerous trace elements (Ba, Cr, K, Mn, Na, Ni, Ti and V). The ratio Ni/ Fe versus Cr/Fe suggests that the magnetic spherules are impactites. CONCLUSIONS: Magnetic spherules were discovered for the first time in stream sediments of the sinking karstic Dobra River, a region where anthropogenic sources for that contribution are absent. Preliminary results point to a possible impactite, formed either by a shock event caused by a meteorite impact or by volcanic processes. The presence of magnetic spherules in the fluvial sediments of the Upper Dobra River represents a new and exciting finding and deserves further field work and laboratory research.