Frequency-dependent susceptibility and other magnetic properties of Celtic and Mediaeval graphitic pottery from Bohemia: an introductory study

Frequency-dependent magnetic susceptibility, its anisotropy (AMS), its temperature variation, natural remanent magnetization and time-dependent isothermal remanent magnetization as well as M?ssbauer spectroscopy of a small collection of Celtic and Mediaeval graphitic pottery from Southern Bohemia were investigated. The mineral composition of the pottery is dominated by fragments of quartz, accompanied mainly by various silicates from granitoids and paragneisses, or by calcite, within the plastic component being probably illite but also graphite. No ferrimagnetic minerals were found in optical microscope, among Fe-oxides only limonite was observed, even though the bulk susceptibility of the pottery varies in the orders of 10^?4 to 10^?2 [SI]. This may indicate presence of ferromagnetic particles in the ultrafine (superparamagnetic, SP) state, which is confirmed by frequency-dependent susceptibility ranging from 3% to almost 16%. The low temperature susceptibility vs. temperature curves are only moderately sloped, showing the Verwey transition only in one case. The high temperature curves mostly show presence of two magnetic phases, maghemite and magnetite. Cooling curves show distinctly lower susceptibilities than the heating curves indicating instability of the assemblage of ferrimagnetic minerals, particularly in temperatures slightly under 700 °C. M?ssbauer spectroscopy confirmed the results of the frequency-dependent susceptibility, showing the increase of ferrimagnetic sextets in the spectra measured at 4.2K, likely indicating maghemite as the distinct ferrimagnetic phase. The frequency-dependent AMS indicates preferred orientation of SP1,16 particles, coaxiality between SP_1,16 grain AMS and whole specimen AMS indicate that all grains, ultrafine and coarser ones, were oriented by the same process, i.e. copying the pottery structure created during wheel-turning.     

Recovery in rubble fields: long-term impacts of blast fishing

This paper presents initial results from a study of factors that inhibit or enhance hard coral recovery in rubble fields created by blast fishing in Komodo National Park and Bunaken National Park, Indonesia. Within nine sites monitored since 1998, there was no significant natural recovery. Levels of potential source coral larvae were assessed with settlement tiles in the rubble fields and in nearby high coral cover sites. Rubble movement was measured and shown to be detrimental to small scleractinians, especially in high current areas. In shallow water (2-6 m deep), rubble is often overgrown by soft corals and corallimorpharians, which inhibit hard coral survival. There is increased scleractinian recruitment in quadrats cleared of soft coral, and Acropora nubbins transplanted into soft coral fields suffer greater mortality than those transplanted above the soft coral canopy. Gaining an understanding of the prognosis for coral recovery is essential not only in order to assess the long-term impacts of blast fishing, but also to improve management decisions about protection of intact reefs and potential restoration of damaged areas.