Because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material, much can be learned by monitoring that response through what is known as a “magnetic hysteresis loop”. The measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research. Yet the interpretation of hysteresis loops is not simple. It has become apparent that although classic “single‐domain”, “pseudo‐single‐domain”, and “multidomain” loops described in textbooks occur in natural samples, loops are frequently distorted, having constricted middles (wasp‐waisted loops) or spreading middles and slouching shoulders (potbellies). Such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions. The physics of the problem have been understood for nearly half a century, yet numerical simulations appropriate to geological materials are almost unavailable. In this paper we discuss results of numerical simulations using the simplest of systems, the single‐domain/superparamagnetic (SD/SP) system. Examination of the synthetic hysteresis loops leads to the following observations: (1) Wasp‐waisting and potbellies can easily be generated from populations of SD and SP grains. (2) Wasp‐waisting requires an SP contribution that saturates quickly, resulting in a steep initial slope, and potbellies require low initial slopes (the SP contribution approaching saturation at higher fields). The approach to saturation is dependent on volume, hence the cube of grain diameter. Therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size. (3) We were unable to generate potbellies using an SP/SD threshold size as large as 30 nm, and wasp waists cannot be generated using a threshold size as small as 8 nm. The occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm (±5). (4) Simulations using a threshold size of 15–20 nm with populations dominated by SP grain sizes, that is with a small number of SD grains, produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass. (5) Simulations and measurements using two populations with distinct coercivity spectra can also generate wasp‐waisted loops. A relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp‐waisting resulting from SP/SD behavior.

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S U M M A R YThe principle of a Curie balance was changed by using a sinusoidally cycling applied magnetic field instead of a fixed applied field. This was done with a horizontal translation type Curie balance. By cycling between field values B,,, and B,,,, the output signal is amenable to Fourier analysis. Partial Fourier analysis yields the fundamental harmonic and the second harmonic, termed SIG, and SIGz respectively. These are related to the saturation magnetization (Ms) by M, = (2 SIG, -8 SIG2 [ ( B m a x + B m i n ) / ( B m a x -Bm,,,)]}/[A"(Bmdx -Bm,,,)] and to the paramagnetic susceptibility (xpdr) by xpar = 8 SIGz/[A"(B,,, -B,,,J2], whereby A" is a calibration constant. Through the Fourier analysis continuous drift correction is achieved simultaneously. A personal computer takes care of field control, temperaturecontrol and data acquisition in real time mode, as well as processing the data, to yield SIG, and SIGz. After the experiment, SIG, and SIG, are processed further with a separate transversal filtering program that improves the signal-to-noise ratio. The working temperature range of the adapted horizontal translation type Curie balance is between room temperature and 900°C. Its noise level corresponds to a magnetic moment of 2 x lop9 Am2, making it a very powerful tool for thermomagnetic analysis of weakly magnetic material. Examples demonstrating this potential of the device are shown.

Today's paleomagnetic and magnetic proxy studies involve processing of large sample collections while simultaneously demanding high quality data and high reproducibility. Here we describe a fully automated interface based on a commercial horizontal pass-through ''2G'' DC-SQUID magnetometer. This system is operational at the universities of Bremen (Germany) and Utrecht (Netherlands) since 1998 and 2006, respectively, while a system is currently being built at NGU Trondheim (Norway). The magnetometers are equipped with ''in-line'' alternating field (AF) demagnetization, a direct-current bias field coil along the coaxial AF demagnetization coil for the acquisition of anhysteretic remanent magnetization (ARM) and a long pulse-field coil for the acquisition of isothermal remanent magnetization (IRM). Samples are contained in dedicated low magnetization perspex holders that are manipulated by a pneumatic pick-and-place-unit. Upon desire samples can be measured in several positions considerably enhancing data quality in particular for magnetically weak samples. In the Bremen system, the peak of the IRM pulse fields is actively measured which reduces the discrepancy between the set field and the field that is actually applied. Techniques for quantifying and removing gyroremanent overprints and for measuring the viscosity of IRM further extend the range of applications of the system. Typically c. 300 paleomagnetic samples can be AF demagnetized per week (15 levels) in the three-position protocol. The versatility of the system is illustrated by several examples of paleomagnetic and rock magnetic data processing.

S U M M A R YDuring the Miocene-Pliocene, the Carpathian region represented the westernmost part of the so-called Eastern Paratethys, a palaeobioprovince that covered central and eastern Europe as well as parts of southwest Asia. Previous palaeomagnetic investigations provide a highresolution magnetochronology for the sedimentary sequences of the Carpathian foredeep and indicate a marked transition in magnetic carriers from iron oxides to iron sulphides, in Chron C3r. Here, we demonstrate using detailed rock magnetic investigations and scanning electron microscope (SEM) analyses that the major magnetic iron sulphide mineral is greigite (Fe 3 S 4 ). Thermomagnetic runs indicate an irreversible decrease in magnetization with increasing temperature up to 400 • C and SEM observations indicate octahedral grain morphologies and Fe:S ratios that are indicative of greigite. Hysteresis loops have 'rectangular' shapes, which are typical of single domain behaviour, with coercivity and coercivity of remanence at values of B c = 35-45 mT and B cr = 52-67 mT, respectively. First-order reversal curve diagrams have contours that close around single domain peaks with B c values of 45-90 mT. Isothermal remanent magnetization component analysis reveals a small dispersion for the greigite component (dispersion parameter, DP ∼ 0.15 log units) indicating a narrow grain size distribution. A positive fold test, inclination shallowing and two positive reversal tests are additional arguments for the syn-depositional formation of this greigite. We thus argue that (most of) the greigite was formed under early diagenetic conditions, that is, within 1000 yr of deposition of the sediment in this setting, and that it thus can be considered as a reliable recorder of the palaeomagnetic signal in this situation. The appearance of greigite during Chron C3r (between 6.0 and 5.5 Ma) in the Carpathian foredeep is most likely related to regional tectonic and/or climatic events that reshaped the basin configuration and changed the palaeoenvironmental conditions.