Search

The paper addresses the controversial question of the role of clay-sized dust in the formation of paleosols in coastal eolianites, Israel. At the Habonim type section, the pedocomplex dated by archaeology and luminescence to 45-135 ka ago shows at least three paleosols, not separated by non-soil sediments. The oldest reddish paleosol (apparently related to MIS 5) is magnetically enhanced, leached from carbonates, with signs of bioturbation and strongly aged clay coatings. The reddening is due to very fine, similar to 20 nm, poorly crystallized, super-paramagnetic (SP) hematite, as determined by Mossbauer studies. In subsoil, lithorelics of eolianite are found. Over time, the soil surface aggraded due to accelerated fine dust accumulation alongside local slope wash. On younger materials formed magnetically depleted vertisols, dominated by smectite-type expandable paramagnetic clays. In thin sections, vertisols exhibit strong stipple-speckled and striated b-fabric due to shrink-swell processes, impregnative calcite nodules and Fe-Mn redistribution. The uppermost hydric vertisol shows the strongest expression of juxtaposed features of recurrent calcite and Fe precipitation. This paleosol developed on colluvial soil materials, as evidenced by mixing of clay coated and uncoated grains of quartz and calcite allochems. Mossbauer spectra show high amounts of Fe(III) incorporated in the clay structure, low amounts of SP goethite and absence of SP hematite. Whilst magnetic susceptibility drops in vertisols to minimal values, ferrimagnetic grain sizes increase. The latter along with differences in the hierarchy of microfabric features is taken as indication for lithologic discontinuities which may have resulted from continuous, albeit variable and low-intensity, input of eolian clay from both remote Saharan and local sources, roughly dated to the earlier to middle part of the Last Glacial.

Loess/paleosol sequences in the western Black sea area at Roxolany (48 m thick) and Novaya Etuliya (30 m thick) were studied using paleo- and rock-magnetic methods in conjunction with micromorphology and Mossbauer investigations. Although seemingly complete, the sites show variability in the type and number of loess/paleosol cycles, and inconsistency of the Matuyama/Brunhes boundary (MBB) stratigraphic position. At Roxolany, nine pedocomplexes (PK) and several incipient paleosols, alternating with thick loesses, were observed with the MBB being located between PK 6 and PK 7 . At Novaya Etuliya, 13 PKs occur upon terrace XII with Khapry fauna and with the MBB in the upper part of PK 7 and the Jaramillo subchron within the lower part of PK 8 . The depth functions of rock magnetic properties, organic carbon content C org , both for bulk samples and for clay extracts, along with micromorphology and Mossbauer spectra, allowed objective identification of individual soils within PKs, and maximizing the resolution of paleopedological reconstructions of global climatic stages. The Mossbauer studies demonstrate that paleosols are basically enriched in ferric montmorillonite. Oxidized magnetite, ultrafine iron hydroxides and hematite are intimately associated with clay minerals. In contrast, Fe-compounds in loess consist mainly of ferrous silicates. These data indicate that the paleosol magnetism is basically of bio-chemogenic origin and is related to pedogenic transformation of Fe-compounds in a changing environment, while loess magnetism is primarily detrital. Pedogenic magnetically “soft” and partly oxidized magnetite may also originate during or even after burial. This ensuing transformation of Fe minerals in paleosols seems to be crucial for the understanding of the timing of natural remanent magnetization (NRM) in subaerial sediments. Given that paleosols formed on the similar parent material (loess), their typological variability seems to depend on climate and/or chronology. Hence, the different paleosol types from Roxolany and Novaya Etuliya provide key information for intra- and inter-regional correlation of loess/soil sequences, and represent valid paleoclimatic proxies.

To elucidate the way in which past climatic changes are encoded magnetically by wind-blown silt, we have investigated two stratigraphic sections in the Chinese Loess Plateau (Xiagaoyuan, in the cool, dry western plateau and Houzhuang, in the warmer, wetter central plateau). In view of the anticipated importance of magnetic particle size, we have determined grain-size distributions from > 50 μm down to ∼10 nm by sequential sieving, sedimentation and centrifugation of typical loess and palaeosol material from the two sites. For the essentially unaltered loess, the main susceptibility contribution lies in the 20–50 μm fraction, with only ∼20% of the signal residing in the 1 (corresponding to oxygen isotope stage 5) at Houzhuang, and a peak value of 4.8 × 10 8 atoms/g was observed, almost identical to that found by other workers for the same horizon at Luochuan. A similar profile at Xiagaoyuan reveals three peaks corresponding to palaeosols S 1 S 1 (2.6 × 10 8 atoms/g), S 1 S 2 (3.3 × 10 8 atoms/g) and S 1 S 3 (3.3 × 10 8 atoms/g). Conversion of these concentrations into 10 Be fluxes indicates low dust accumulation during warmer climate episodes. At these times pedogenic susceptibility enhancement is favoured, the amounts of which imply palaeoprecipitation slightly higher than today for S 1 S 3 at Xiagaoyuan, but somewhat lower than today for S 1 at Houzhuang.

A detailed rock magnetic investigation of loess/palaeosol samples from the section at Lingtai on the central Chinese Loess Plateau (CLP) is presented. Thermal demagnetisation of isothermal remanent magnetisation (IRM) and Curie temperature measurements suggest the presence of magnetite, maghemite and hematite as remanence carrying components. Bulk and grain size fractionated samples have been analysed using coercivity spectra of remanence acquisition/demagnetisation curves, which identify four main remanence carriers in different grain size fractions of loesses and palaeosols. A linear source mixing model quantifies the contribution of the four components which have been experimentally derived as dominating endmembers in specific grain size fractions. Up to two thirds of the total IRM of the palaeosols are due to slightly oxidised pedogenic magnetite. Two detrital components dominate up to 90% of the IRM of the loess samples and are ascribed to maghemite of different oxidation degree. Detrital hematite is present in all samples and contributes up to 10% of the IRM. The iron content of the grain size fractions gives evidence that iron in pedogenically grown remanence carriers does not originate from the detrital iron oxides, but rather from iron-bearing clays and mafic silicates. The contribution of pedogenic magnetite to the bulk IRM increases with the increasing degree of pedogenesis, which depends in turn on climate change.

A simple magnetic mineral identification technique is proposed which measures isothermal remanent magnetisation (IRM) acquired in different field strengths at 293 K and 77 K. Magnetic parameters are introduced that use these IRMs in order to distinguish concentration and type of ferromagnetic minerals in different magnetic states. The method determines easily contributions of superparamagnetic ferrimagnetic minerals in environmental magnetism studies. In addition, high coercivity goethite and possibly superparamagnetic hematite are identified. The analysis of the magnetic properties of a soil profile on a Late Pleistocene terrace from the Jarama river in central Spain is given as an example.

Most rock magnetometers measure the bulk remanent magnetization of 1-inch cylindrical samples (centimeter scale) or even larger volumes of drill cores. High- Tc superconducting quantum interference device (SQUID) magnetometers are able to measure magnetic fields on rock surfaces at higher resolution on a millimeter scale: the fields of discrete rock-forming magnetic mineral grains or fine magnetic rock textures and structures can be observed when scanning across selected surface areas. The stability problems of a commercially available high- Tc SQUID magnetometer have largely been solved by improving the magnetic shielding and reducing the noise due to turbulent boiling of liquid nitrogen. Magnetizations as weak as 5×10−4 A/m can now be discriminated with a resolution of 1 mm. A software package has been developed to eliminate measurement errors arising from instrumental drift. The program also calculates the downwards continuation of the field data and offers full inversion for the vertical magnetization component. A synthetic sample demonstrates the performance of the SQUID sensor and the inversion software. The potential of high-resolution magnetic imaging is shown by measuring three rock samples with very different magnetic properties.

The 10Be flux onto a loess plateau is composed of two different components: the atmospheric flux and the dust flux. A simple 10Be-susceptibility model shows that the magnetic susceptibility flux can be also divided into a dust related component and an in situ produced pedogenic component. From comparison with the dust 10Be flux, we can separate and quantify the detrital (FD′) and the pedogenic (FX) magnetite contributions to the total magnetic flux (FS). FX/FS varies between 0% and 80% with low values in loess layers and high values in palaeosols and weathered loess. The pedogenic magnetic susceptibility flux could be considered to be linearly related to palaeoprecipitation and may be used as a quantitative measure for regional palaeorainfall in the Chinese loess plateau.

The approximately 50-m thick loess/palaeosol sequence at Paks brickyard (Hungary) was deposited largely during the Brunhes normal polarity chron and its magnetic properties resemble those of similar sections in Europe, Central Asia and China. In order to get information on the grain size dependence of some of the magnetic parameters, two loess and two palaeosol samples were separated into a number of grain size fractions. The separation method employs consecutive sedimentation and centrifugation steps performed on bulk samples suspended in water and diluted with Calgon. Ten grain size fractions were obtained, ranging from 50 μm down to 10 nm. Several magnetic parameters of the different grain size fractions have been measured, at room temperature (low field susceptibility, hysteresis parameters) and at low temperature (susceptibility and thermoremanent magnetisation). They show a strong grain size dependence for the ferromagnetic minerals. Low temperature measurements allow discrimination of the paramagnetic vs. the ferromagnetic signal and give information on the grain size distribution of the very fine ferromagnetic minerals and on the mineral types carrying the magnetic signal. Strong relationships between weathering processes, destruction of paramagnetic minerals and possibly iron-hydroxides, and enhancement of the ferromagnetic signal are observed.

Saturation magnetization and saturation remanence of 139 loess and paleosol samples from Tajikistan, China and Hungary increase linearly with low field susceptibility in all sections investigated. Thus magnetic enhancement is caused by increasing quantities of ferrimagnetic minerals. The observed variations of high field susceptibility χh are attributed to variations in the amount of clay minerals as χh is linearly correlated with the < 2 µm grain size mineral fraction. The magnetic enhancement process is compared with a theoretical mixing model based on two magnetic mineral assemblages with different but invariable coercivities. The two-component model provides a good approximation to the data from Hungary and the magnetically enhanced paleosol samples in Tajikistan and China. Discrepancies between model calculation and data arise in the behaviour of the loess samples in Tajikistan. Here a three-component model improves the agreement between data and mixing calculation. The magnetic enhancement in the loess in Tajikistan (and China) is different from the one in the paleosols and the link between paleoclimate and susceptibility is not as straightforward as in the Hungarian loess-paleosol sequence.

Samples of metallurgical dusts and fly ashes from coal power plants and iron works in Upper Silesia as well as soil profiles in the close vicinity of these plants and in Ojcow National Park (ca. 25 km east of the industrial area) have been studied magnetically and mineralogically. The metallurgical dusts and fly ashes are highly enriched in ferromagnetic minerals. The topsoils from profiles collected near the plants have very high values of magnetic susceptibility while susceptibility in the fermentation and humic subhorizons in soil profiles from Ojcow National Park is considerably increased. The magnetic properties of the metallurgical dusts and fly ashes such as frequency dependence of susceptibility, saturation remanence or coercivity are similar to those observed in the top horizons of the soils. They are mostly related to the occurrence of large (multidomain) grains of non-stoichiometric magnetite ranging from 1 to 20 µm. The similarity of the magnetic particles in the soils is taken as evidence of an anthropogenic origin. They are responsible for the high soil susceptibilities in Upper Silesia and in adjacent areas. Some of the magnetic particles carry substantial quantitities of trace elements such as Pb, Ni, Zn and Cu. Field and laboratory susceptibility measurements can therefore be used as a simple and costeffective method of detecting the presence of heavy metals in the soils of this area.

Magnetic analyses of hippocampal material from deceased normal and epileptic subjects, and from the surgically removed epileptogenic zone of a living patient have been carried out. All had magnetic characteristics similar to those reported for other parts of the brain [6]. These characteristics along with low temperature analysis indicate that the magnetic material is present in a wide range of grain sizes. The low temperature analysis also revealed the presence of magnetite through manifestation of its low temperature transition. The wide range of grain sizes is similar to magnetite produced extracellularly by the GS15 strain of bacteria and unlike that found in magnetotactic bacteria MV-1, which has a restricted grain size range. Optical microscopy of slices revealed rare 5–10 micron clusters of finer opaque particles, which were demonstrated with Magnetic Force Microscopy to be magnetic. One of these was shown with EDAX to contain All, Ca, Fe, and K, with approximate weight percentages of 55, 19, 19, and 5, respectively.

The stable component of the natural remanent magnetization (NRM) of the Upper Triassic “Calcari con Selce” limestone formation at Marsico Nuovo (Lagonegro Basin, Southern Italy) has an inclination which is about 45° too steep for the expected paleogeographic position. In order to explain this abnormal magnetization direction, several rock magnetic properties such as the dependence of susceptibility on temperature and alternating field frequency, anisotropy of low field susceptibility, anisotropy of anhysteretic remanent magnetization (ARM) and characteristics of acquisition and thermal decay of isothermal remanent magnetization (IRM) have been investigated. Strong inverse fabrics of both susceptibility and ARM have been recognized, which are due to magnetite in the superparamagnetic state on the one hand and in the stable single domain grain size on the other hand. Although internal deformation in the investigated rocks has not been reported in the geological literature, this may be the cause of the observed anisotropies. The anisotropy is probably responsible for the anomalous NRM directions of these limestones.

SUMMARY The low field susceptibility variations in the Chinese and Tajik loess sequences reflect the palaeoclimatic fluctuations of the Quaternary on the continents in a very direct and complete way. It can be demonstrated that the fine-grained ferromagnetic mineral fraction is largely responsible for the climatically controlled susceptibility enhancement in the loess sediments. In order to gain more information about the origin and quality of the susceptibility enhancement we are scrutinizing the F-factor (F,) which is conventionally used as a measure of the frequency dependence of the susceptibility. For qualitative estimates of the grain sizes present, F, seems to be a rather misleading parameter because it is flawed when variable amounts of minerals with both frequency-independent and frequency-dependent susceptibility are present. We introduce a new concentration-independent F-factor (K) which takes the influence of the mineral fractions with frequency-independent susceptibility on the total susceptibility into consideration. F, is fairly constant in all the investigated sections. This result is explained theoretically following NCel's (1949) fine-particle theory and Stephenson's (1971a) modelling of various single-domain grain distributions. It is found that the grain-size distribution is essentially the same throughout all loess sections investigated and the upper and Iower grain-size limits are essentially invariable. In addition, susceptibility and its frequency dependence were measured at low temperatures on selected loess samples in order to determine the maximum grain size present in the fine-grained magnetic fraction.

SUMMARY The origin of the Milankovitch palaeoclimatic signal so dramatically recorded in the Chinese loess remains uncertain. At least five hypotheses have been put forward to explain the well-documented enhancement of magnetic susceptibility in palaeosols which formed when conditions were warmer and wetter. We report here a magnetic study of a prominent loess-soil couplet (L4-S3) at three widely separated sites where present-day mean annual rainfall and temperature range from ∼350mm to ∼700mm and from ∼6°C to ∼13°C. the properties determined are laboratory remanences (isothermal, IRM; anhysteretic, ARM; viscous, VRM, including viscous decay), room-temperature susceptibility, and thermomagnetic behaviour. At fields beyond 100 mT, the IRM spectra are all essentially identical, both in shape and in absolute magnitude, suggesting a uniform magnetic component widespread across the Plateau. Subtracting the signal of this component isolates a second component, the amount of which varies by an order of magnitude between sites. the magnetic properties of this latter material are remarkably similar to those reported for bacterial magnetite in geological sediments and modern soils. Viscous decay of VRM takes place in two distinct stages, one of which falls to zero in < 10min. We attribute magnetic enhancement-which is the source of the palaeoclimatic signal to in situ pedogenetic formation of magnetite, which we characterize in our interpretation as two separate forms of bacterial magnetofossils. the most magnetic sample requires a mass fraction of about half of one per cent of this biogenic material.

Pronounced differences in concentration, alignment and grain-size distribution of the magnetic minerals have been identified between loess and palaeosol samples from the section at Xifeng (Central Chinese loess plateau). Superparamagnetic magnetite formed in situ plays an important role in controlling magnetic low field susceptibility. The variable brown-red colouration of the loess-palaeosol sequence is due to a significant hematite component produced by pedogenesis. The magnetic fabric of the palaeosols is weaker than that of the loesses. This is taken as evidence that the formation of ultrafine ferromagnetic minerals by pedogenesis is the most likely explanation for the observed susceptibility enhancement in palaeosols (4 to 10 times higher than in loesses). It is argued that variations of frequency dependent susceptibility are sensitive recorders of second order palaeoclimatic variations during glacial periods, whereas bulk susceptibility seems to be an appropriate measure of palaeoclimate change during interglacial periods.

The temperature dependence of magnetic domain pattern in titanomagnetite and titanohematite of self-reversing andesites from the 1985 eruption of the Nevado del Ruiz volcano (Colombia) has been observed using the Bitter technique. During heating/cooling cycles in the laboratory, the natural remanent magnetization loses the self-reversal properties between 110°C and 180°C. The titanohematite domains disappear in the same temperature interval. Large grain regions without visible colloid contrast are observed between room temperature and 180°C. Utilizing the high resolution of a magnetic force microscope, it is found that these regions contain either domains with wall spacing of about 3 µm or a buckling pattern. Along the phase boundary between a titanomagnetite and a titanohematite grain but within the titanohematite, a 2 µm wide chemically different domain has been observed. It has a higher iron content than the main titanohematite. The two titanohematite phases represent two phases with higher Curie temperature, Tc, but lower spontaneous magnetization, Ms, and lower Tc but higher Ms, respectively. Negative interaction between these phases is called upon to explain the self-reversing mechanism.

Based on palaeomagnetic measurements a three-stage model for the formation of lateritic duricrusts that postulates a para-autochthonous origin of the duricrusts, is proposed. The directions of the natural remanent magnetization (NRM) which reside in lithogenic magnetite contained in the duricrust matrix, have no preferential orientation. The NRM directions of in situ formed hematite contained in the micronodules of the duricrusts, are randomly distributed. The random distribution of the hematite magnetization directions suggests a para-autochthonous origin of the ferriferous micronodules which is explained by erosion and reworking prior to consolidation of the duricrusts. These processes lead to a mechanical iron accumulation at the top of the lateritic profiles.

The natural remanent magnetization of andesitic pumice emitted during the 1985 eruption of the Nevado del Ruiz volcano (Colombia) has a direction opposite to the present geomagnetic field. The self-reversing mechanism can be re-activated in the laboratory during cycles of heating and subsequent cooling in air and zero magnetic field. Laboratory-produced thermoremanent magnetization is dominated by the same self-reversal process in fields up to several mT. Microchemical, optical and Curie temperature analyses indicate that the ferromagnetic minerals are members of the magnetite-ulvospinel and hematite-ilmenite series with average compositions of Fe2.73Ti0.27O4 and Fe1.38Ti0.62O3, respectively. In analogy with the magnetic behaviour of synthetically grown antiferromagnetic-ferromagnetic FeMn-FeNi films, the self-reversal can probably be interpreted in terms of an exchange field acting between a Ti-poor canted antiferromagnetic and a Ti-rich ferrimagnetic phase in the hemoilmenite grains.

The natural remanent magnetization of andesitic pumice erupted from the Nevado del Ruiz volcano (Colombia) in 1985 is reversed with respect to the present geomagnetic field because of self-reversal. Laboratory thermoremanent magnetization acquired in fields of

Remanent coercivity spectra derived from IRM acquisition curves and thermal demagnetization of the IRM indicate that magnetite, haematite and minor amounts of goethite determine the magnetic properties of the Pliensbachian limestones at Bakonycsernye. These limestones have been sampled at approximately 7-cm intervals along a 10-m stratigraphic section which covers the whole Pliensbachian stage (Lower Jurassic) without any recognizable break in sedimentation. The primary natural remanent magnetization (NRM) is carried by detrital particles of magnetite and haematite, but it is seriously overprinted by a normal magnetization which originates from secondary haematite with a wide range of blocking temperatures. This haematite is believed to have formed diagenetically during one of the Mesozoic periods of normal polarity. However, the reversal pattern obtained after NRM thermal demagnetization at temperatures ≥450°C is thought to be characteristic of the Pliensbachian stage.

Goethite and hematite are the main magnetic phases contained in two ferriferous middle Jurassic beds which are exposed within the Mandach structure, the major tectonic structure of the eastern tabular Jura of northern Switzerland. The iron oxidic phases in a reddish brown sparry limestone known as Spatkalk were formed diagenetically, whereas these phases are weathering products in a stratigraphically slightly younger oolitic bed. The hematite in the oolitic bed was magnetized after the Jura main tectonic event in the Miocene whereas the hematite in the Spatkalk was magnetized before the Miocene. The goethites in the oolitic bed carry a pretectonic and in the Spatkalk a syntectonic magnetization. Physical and chemical data show that the goethite which was resistant to remagnetization contains an Al-substitution of 3.4 mol% resulting in a Neel temperature of 90°C. The remagnetized goethite has an Al-content of 10.7 mol% and a Neel temperature of 70°C. The different goethite properties indicate that the tectonic formation of the Mandach structure occurred under low-temperature conditions below 90°C.

The anisotropy of magnetic susceptibility of Triassic red sandstones from northern Bavaria (Plattensandstein Formation) is very weak within the maximum susceptibility plane. The anisotropy resulting from the shape and size of cylindrical red bed samples can exceed and obscure the lineation of the sedimentary fabric when measured with a spinner magnetometer. This frequently observed error can be eliminated when the cylindrical specimens are drilled perpendicular to bedding. Alternatively, cubic samples which are measured on a susceptibility bridge, are not affected by this error.

Summary A furnace for the progressive method of demagnetization is described. The furnace is heated electrically. The heat transfer between the heating elements and the chamber is effected by a circulating gas current. An example of thermal treatment of a granite specimen is given.

The organic top horizons of forest soils in the vicinity of industrial centers in Upper Silesia, Poland, are characterized by remarkably high magnetic susceptibility. The unusually strong soil susceptibility does not result from weathering or pedogenesis or from deposition of natural detrital ferrimagnetic minerals but is due rather to the influx of anthropogenic magnetic particles contained in industrial dusts and fly ashes. The magnetic particles are iron oxide spherules which originate during the combustion of fossil fuels (brown and hard coals) and during iron or steel production. Heavy metals, such as zinc, lead, or cadmium which stem from the same pollution sources, are often associated with the magnetic and other dust particles and cause soil contamination, which is potentially hazardous for plants, animals, and humans. Because the regional magnetic susceptibility distribution pattern is closely correlated to measured dust fall and largely parallels the distribution of heavy metals, measurements of magnetic susceptibility, which can be conducted easily both in the laboratory and in the field, can be used as a fast and sensitive tool to trace and monitor soil contamination in industrialized regions.

Numerous paleomagnetic investigations have been carried out on limestones, but their rock magnetic properties have often been neglected. In this review, geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized, and laboratory techniques for the identification of the magnetic mineralogy are evaluated. The conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence (NRM), probably by a postdepositional alignment process. The effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation. Other remanence components can be acquired during diagenesis, in the soft sediment right after deposition or much later even in the indurated limestone. The magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration; magnetic techniques are more convenient. Direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract. A particularly useful technique involves combined observations of isothermal remanent magnetization (IRM) acquisition in strong fields up to at least 4 T and the subsequent destruction of this IRM during stepwise or continuous thermal demagnetization. The magnetic minerals most commonly identified by these techniques are magnetite, goethite, hematite, and maghemite. These magnetic minerals occur in different combinations in the marine limestones of central Europe and the Mediterranean realm. Pyrite is common in certain limestones, where it can be the precursor of an unwelcome late goethite. Pyrrhotite may be responsible for unstable magnetization in the Swiss Helvetic limestones but otherwise is rare. Thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures (300°–400°C) due to the breakdown of a metastable mineral such as maghemite, goethite, or pyrrhotite. At high temperatures (above 500°C), pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite. This necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent (VRM) behavior of the newly formed magnetite. VRM is a widespread component of NRM in many limestones, but it can usually be removed effectively by standard magnetic cleaning techniques. The susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles. The susceptibility is anisotropic and shows a classical ‘depositional’ pattern of the distribution of principal axes. However, the probable postdepositional mechanism of NRM acquisition (especially in bioturbated sediments) implies that the anisotropy pattern results from postdepositional compaction of the sediment.