Summary Magnetic properties (IRM, ARM, χin, S‐ratio at 0.3 T, room temperature (RT) hysteresis and thermomagnetic curves) and geochemical data (Fe, S, Mn, Al, Ti, organic C) were studied in two eastern Mediterranean boxcores (ABC26 and BC19) at a resolution of 3–5 mm. The boxcores contain sapropel S1 (9–6 kyr BP) at a few decimetres below seafloor. The magnetic fraction consists predominantly of single‐domain (SD) to pseudo‐single‐domain (PSD) magnetite in the entire cores. The original input of magnetic grains comes from two sources: aeolian dust (both cores) and volcanic ash from the Minoan eruption of Santorini (core BC19 only). Non‐steady‐state diagenesis has changed the magnetic mineralogy considerably in these alternating organic‐rich/organic‐poor sediments. During deposition of sapropel S1, reductive diagenesis and pyritization in and just below the sapropel caused lower magnetic intensities, coarser magnetic grain sizes and partial maghemitization. In thermomagnetic curves two types of pyrite can be identified: one oxidizes below 450 °C and the other above 450 °C. The higher oxidation temperature is predominantly found below the sapropel. This may be related to the microtexture of pyrite, which is euhedral below sapropels and mainly framboidal within sapropels. Since the end of sapropel deposition a downward moving oxidation front has oxidized the upper half (c. 5 cm) of the sapropel. The oxidized part of the sapropel is enriched in diagenetically formed Fe oxides with relatively high coercivity and ARM. The maximum coercivity is found in a distinct layer between the present‐day Mn‐ and Fe‐redox boundaries at the top of the unoxidized sapropel. The freshly precipitated Fe oxides in this centimetre‐thick layer contain a mixture of superparamagnetic (SP) grains and high‐coercivity SD magnetite. Higher in the oxidized zone the freshly precipitated Fe oxides have aged into generally slightly lower‐coercivity SD grains, with relatively high ARM. In addition to the diagenetic formation of Fe oxides at the top of the sapropel, formation of a ferrimagnetic Fe monosulphide may have occurred within the sapropel during later stages of diagenesis, which may have enhanced the ARM signal in the organic‐rich interval in particular.

SUMMARYThe magnetic behaviour of hydrothermally synthesized greigite was analysed in the temperature range from 4 K to 700°C. Below room temperature, hysteresis parameters were determined as a function of temperature, with emphasis on the temperature range below 50 K. Saturation magnetization and initial susceptibility were studied above room temperature, along with X-ray diffraction analysis of material heated to various temperatures. The magnetic behaviour of synthetic greigite on heating is determined by chemical alteration rather than by magnetic unblocking. Heating in air yields more discriminative behaviour than heating in argon. When heated in air, the amount of oxygen available for reaction with greigite determines the products and magnetic behaviour. In systems open to contact with air, haematite is the final reaction product. When the contact with air is restricted, magnetite is the final reaction product. When air is excluded, pyrrhotite and magnetite are the final reaction products; the amount of magnetite formed is determined by the purity of the starting greigite and the degree of its surficial oxidation. The saturation magnetization of synthetic greigite is virtually independent of temperature from room temperature down to 4 K. The saturation remanent magnetization increases slowly by 20-30 per cent on cooling from room temperature to 4 K. A broad maximum is observed at~10 K which may be diagnostic of greigite. The coercive and remanent coercive force both increase smoothly with decreasing temperature to 4 K. The coercive force increases from~50 mT at room temperature to approximately 100-120 mT at 4 K, and the remanent coercive force increases from approximately 50-80 mT at room temperature to approximately 110-180 mT at 4 K.

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The most recent sapropel (S1) in the eastern Mediterranean has been extensively investigated with geochemical and rock-magnetic techniques. Different redox conditions prevailed in different zones of the sediment through time. The oxidized sapropel zone is particularly interesting, because earlier studies indicated that new magnetic material was formed, including possible magnetosomes. Here we utilize component analyses of isothermal remanent magnetization (IRM) acquisition curves and the analysis of first-order reversal curves (FORC) to further investigate the magnetic mineralogy. In the entire box core ABC26, the original input of eolian dust consisted of both magnetite and hematite. In the oxidized sapropel and in the active oxidation zone, an additional magnetite component is present. This magnetite component has a higher coercivity than the eolian magnetite and a very small coercivity dispersion, suggesting a narrow grain size distribution. This is a strong indication that magnetosomes, which are formed in the active oxidation zone, are the magnetic carriers of this coercivity fraction. FORC diagrams support these findings. The S ratio is forwardly modeled for mixed magnetic mineralogies with varying coercivity distributions to explain the down core S ratio behavior: close to 1 in the top sediment and in the oxidized sapropel, a drop in values in the active oxidation zone, and again close to 1 in the (syn)sapropel. The observed S ratio pattern in the oxidized sapropel and in the active oxidation zone can be explained by the recovered IRM components. Whereas both oxidized sapropel and active oxidation zone contain the extra magnetite component, their S ratios are different because of the differences in coercivity characteristics (coercivity and dispersion) of the two magnetites in these zones. Thus, in these zones the S ratio does not reflect variations in the relative contributions of hematite to magnetite but variations in the characteristics of the individual magnetite assemblages. Also in the rest of the core, the S ratio depends on the coercivity characteristics of the magnetite component rather than on the relative contributions of high-versus low-coercivity minerals. The S ratio thus appears to be an unsuitable parameter to describe variations in the magnetic mineralogy, especially when more than two components are present. Therefore the classical interpretation of the S ratio should be treated with caution.

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Summary Low‐temperature magnetic properties of eastern Mediterranean sediments from a box‐core have been investigated. This box‐core contains the present‐day oxic–suboxic boundary that is situated at the top of the relic of the youngest sapropel (S1). The upper half of sapropel S1 has been oxidized, and Fe oxides have precipitated in the oxidized sapropel. Zero‐field‐cooling (ZFC) and field‐cooling (FC) saturation remanent magnetization (Mr; induced in a field of 2.5 T at 20 K after cooling from 300 K with or without the 2.5 T field) was measured during warming to 300 K. An Mr imparted at room temperature (RTSIRM) was cycled to 20 K as well. The difference between ZFC and FC curves around the Verwey transition, as estimated by the parameter δFC/δZFC, suggests the presence of magnetosomes in the oxidized part of the sapropel. We propose a new parameter , where represents the average remanence between 150 and 300 K, to estimate the difference between ZFC and FC curves above the Verwey transition. This is interpreted as being indicative of the number of defects and the extent of partial maghemitization. At the oxic–suboxic boundary where Fe oxides actively precipitate, the magnetite appears to be least maghemitized as inferred from low values for D. Further upward in the oxidized sapropel, in older precipitates, the magnetite is slightly more maghemitized. Just below the sapropel, maghemitization is most pronounced. In addition, the initial slope of ZFC and FC curves indicates that small grains or grain coatings that are superparamagnetic at room temperature (SP), are enriched at the oxic–suboxic boundary. Higher in the oxidized sapropel, the relative contribution of SP grains decreases, presumably because they age to larger grains.

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