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1.
Summary. The decay of the post-depositional remanent magnetization (post-DRM) during desiccation in magnetic field free space is measured as a function of the loss of water. The decay is ascribed to the drying effect and the time decay of viscous remanent magnetization (VRM). The VRM forms only 10 per cent of the total of loss of remanent magnetization. The decay due to the drying effects depends both on the loss of water and on either the evaporation rate or the period of storage. The percentage of loss of magnetization is independent of its intensity.
A critical drying stage appears (about 60 per cent in water content on a dry basis) which is characterized as a vanishing point of mobile particles or particle units. The mobile particles or units play an important role both in acquisition and demagnetization through physical rotational motion within wet sediments before the critical drying stage. More than 80 per cent of the total loss of the post-DRM is destroyed before the desiccation proceeds to the critical drying stage. The decay of post-DRh4 is concluded to be mainly due to the physically random rotation of the magnetic particles trapped in shallow energy wells which are overcome by the torques caused by the application of the alternating magnetic field less than 200 Oe.  相似文献   

2.
Summary. This paper examines the role of surface tension effects in the post-depositional magnetization of a sediment. After deposition, magnetic grains experience both frictional and magnetic forces but during dessication surface tension forces also become important. It is shown from order of magnitude calculations that surface tension forces will predominate for grains smaller than about 1.3 mm and therefore remanence changes may occur in fine grained sediments upon drying. A simple model predicts that in a moist sediment containing bladed magnetic particles the effect of vertical fluctuations in the level of saturation will be to shallow the remanent inclination.
Remanence changes due to surface tension are demonstrated in a controlled experiment with synthetic sediment. Implications for sediment palaeomagnetism are discussed and alternative drying procedures for sediments are suggested.  相似文献   

3.
The acquisition of a gyroremanent magnetization (GRM) by single-domain (SD) greigite particles during alternating-field (AF) demagnetization is demonstrated. Previous palaeomagnetic studies failed to identify the presence of authigenic greigite in the glacio-marine clays studied. These clays formed the subject of an earlier debate about the validity of a Late Weichselian geomagnetic excursion (the Gothenburg Flip) in southern Sweden. The greigite carries a stable chemical remanent magnetization (CRM), which coexists with a detrital remanent magnetization (DRM) carried by magnetite. AF demagnetization could not isolate the primary remanence in the sediments where magnetite and greigite coexist, due to the overlapping coercivity spectra of the two minerals and the inability to determine the time lag between sediment deposition and CRM formation. Thermal demagnetization removed the CRM at temperatures below 400 C, but this method was hindered by the unconsolidated nature of the sediments and the formation of secondary magnetic minerals at higher temperatures. The results suggest that the low-coercivity DRM carried by magnetite was mistaken for a 'viscous' component in the earlier studies. Hence the former debate about the record of the Gothenburg Flip may have been based on erroneous palaeomagnetic interpretations or non-reproducible results. AF demagnetization procedures applied to samples suspected of bearing SD magnetic particles (such as greigite) should be carefully selected to recognize and account for GRM acquisition.  相似文献   

4.
Summary. Post-depositional detrital remanent magnetization (pDRM) is the primary means whereby many sediments acquire their palaeomagnetic signal. We have studied the acquisition of this magnetization in a variety of natural sediments. Our technique involves determining the magnetic direction recorded by a sediment as a function of the water content present in the sediment when the sediment experiences a change in the direction of the applied magnetic field. Most of the sediments used in this study were collected wet from natural environments and were preserved in their original state until they were used in the experiments. Grain sizes were measured by the settling tube method which led to the determination of the clay, silt and sand fractions in each sediment. Isothermal remanent magnetization acquisition studies indicated that the predominant magnetic carriers were magnetite. In the pDRM acquisition studies two distinct modes of behaviour were found. For sediments with a sand content less than 60 per cent, the original direction of magnetization was preserved regardless of the water content. Such behaviour is not consistent with a theoretical model which assumes that at high water contents the magnetic carriers remain mobile within fluid-filled voids and hence are able to realign along a new magnetic field direction. For sediments with a sand content in excess of 60%, remagnetization along a new magnetic field direction occurred as expected, provided the sediments were sufficiently wet. Studies of natural sediments and corresponding samples of dried and reconstituted sediments have demonstrated that the magnetic characterization of a sediment can be reliably determined even for older, desiccated sediments.  相似文献   

5.
Recent conceptual models have attributed the weak depositional remanent magnetizations observed in natural sediments to flocculation processes in the water column. Magnetic particles included into flocs have not only to rotate themselves into alignment with the geomagnetic field but also the larger particles to which they are attached, making remanence acquisition an inefficient process. Alignment is hindered further when the magnetization vectors of the particles in any given floc partially cancel, reducing the overall magnetic torque. Existing numerical simulations of flocculation effects on depositional remanence formation have been limited to spherical bodies with translational and rotational motion acting independently of each other. In the case of non-spherical flocs, the translational and rotational motion are coupled and such bodies will describe a complex trajectory through the water column. Calculations will be presented that show the torque exerted on a non-spherical floc by the surrounding water can be orders of magnitude greater than the magnetic torque. Non-spherical flocs will, therefore, align less efficiently with the geomagnetic field and hydrodynamic effects may play an important role in controlling the magnitude of sedimentary remanence.  相似文献   

6.
Laboratory studies of depositional DRM   总被引：3，自引：0，他引：3  
Summary. Acquisition of magnetic remanence in slurries of fme grained organic muds settling in long tubes is investigated using a cryogenic magnetometer. The average settling behaviour of remanence carrying grains relative to the whole sediment gives information about the relative magnetic grain size spectrum, whereas the response of settled deposits to vibration gives an indication of the degree of alignment of particles and their average shape.
Two classes of behaviour are apparent in both the time and field dependence of detrital remanent magnetization (DRM) acquisition. Dilute slurries settling in the Earth's field (analogous to detrital sedimentation) acquire a remanence which reaches a maximum after about 2 day, whereas for concentrated slurries (analogous to slumped or bioturbated sediments) this takes only a matter of minutes. The field dependence of DRM in dilute slurries is in plausible quantitative agreement with Stacey's extension of the classical Langevin expression for the susceptibility of a paramagnetic gas, whereas concentrated slurries show a quasi-linear dependence of DRM on the applied field. Inclination errors are generally absent, but do appear in fields less than about 0.5 Oe, and when the magnetic fraction settles out preferentially. A weak negative dependence of DRM on temperature is found, but the results are too crude to provide a further test of Stacey's theory.
Remanence acquisition in slurries settled in zero field indicates that short term post-depositional magnetization processes are relatively unimportant in slurries that have ceased to compact rapidly. A large increase in remanence from naturally occurring sediments to slowly redeposited long cores, to rapidly settled slurries is best explained as a feature of compaction.  相似文献   

7.
The time and temperature stability of various types of magnetic remanence has been measured in pottery samples containing magnetite and in a clay sample containing manganese ferrite. The time decay of rotational remanent magnetization (RRM), anhysteretic remanent magnetization (ARM) and a low-field isothermal remanent magnetization (IRM) has been measured. While the decay of the last two remanences is easily measurable at about 2 and 19 per cent per decade of time, respectively, the decay of RRM is too small to be measured, being less than about 0.1 per cent per decade of time. Thermal demagnetization of thermoremanent magnetization (TRM), ARM and RRM indicates that RRM is also the most thermally stable. The implications of these experiments are that rocks which exhibit gyromagnetic effects such as RRM contain highly stable particles and therefore are likely to be most suitable for palaeomagnetism.  相似文献   

8.
The acquisition of a freezing remanent magnetization (FRM) has been studied in controlled magnetic and thermal environments by successive freezing and thawing (−18 to +20°C) of samples of natural sediments from a frost polygon near Ny Ålesund, Spitsbergen. Successive freeze-thaw cycles cause a significant decrease in the intensity of the initially induced shock remanent magnetization (SRM), associated with directional trends towards the ambient magnetic field direction during the freezing phase. A slow increase in intensity commences after seven to 10 freeze-thaw cycles. The acquisition of a FRM in samples carrying an isothermal remanent magnetization shows a significantly smaller reduction in intensity and only minor directional variations. This result indicates that only a fraction of the magnetic grains in a natural sediment contributes to the natural remanent magnetization. Insignificant changes in lengths and directions of the principal susceptibility ellipsoid axes also indicate that magnetic fabric and remanent magnetization are carried by partly different populations of magnetic grains.
The acquisition of a FRM in nature has yet to be explored. If such a process is confirmed, however, it has the potential for obtaining age estimates of ancient thaw depths and for providing insights into material transport processes in frost polygons.  相似文献   

9.
Magnetic remanence acquisition in Finnish lake sediments   总被引：2，自引：0，他引：2  
Summary. Magnetic studies have been carried out on organic sediments from five Finnish lakes to determine the carrier(s) of the stable NRM and to find how the remanence is acquired. Single or pseudo-single domain 'magnetite' is thought to carry the NRM. It was found that drying and cooling sediment samples resulted in a loss of NRM which was attributed to the misalignment of small magnetic particles. Low-field experiments were carried out on sediment samples in different physical states and from the results of these investigations it was concluded that post-depositional processes are important in the acquisition of an NRM. Stabilization of magnetic grains is thought to be due to the growth of gels in the organic sediment rather than to dewatering.  相似文献   

10.
Summary. This paper describes the magnetic remanence and susceptibility anisotropy of 42 samples of laminated clay from Peak Cavern. The sediments are shown to have acquired a stable remanent magnetization due to the depositional alignment of magnetite grains. The magnetic fabric data confirm the existence of alteration within a reddened surface layer. A record of geomagnetic secular variation in the underlying clay is compared to a dated lake sediment magnetostratigraphy to suggest that deposition occurred around 7000 yr bp . A method for estimating water current directions based on the remanence and magnetic fabric data is critically assessed before deducing the sense of palaeoflow in the cave.  相似文献   

11.
Summary. Measurements of the detrital remanent magnetization (DRM) of redeposited deep-sea sediment of the silty clay grade are described. Variations in the magnitude of an observed remanence inclination error are related to conditions of sediment accumulation, contrasted here as grain-by-grain settling from a dilute dispersion or by settling from a concentrated slurry. For these artificial redepositions post-depositional compaction is shown to be a major factor in shallowing the observed inclination from the ambient field inclination. The term compactive DRM is tentatively assigned to describe such behaviour.  相似文献   

12.
Summary . Detailed thermal demagnetization results combined with vector analysis and study of the convergence point of remagnetization circles reveal that the late Palaeozoic ignimbrites of North Sardinia possess a multi component remanence in addition to having experienced a tectonic rotation. The degree of palaeomagnetic complexity increases with increasing degree of oxidation of the magnetic mineralogy. It is concluded that the rocks were laid down in late Permian time just before the close of the reversed Kiaman geomagnetic epoch. Subsequent oxidation and partial remagnetization basically occurred in late Permian—Triassic time, during a period characterized by alternating field polarity. In the majority of the sites this remagnetization cycle brought about fairly erratic and relatively stable resultant magnetizations which are generally smeared out towards easterly directions. At a later date Sardinia was subject to an anticlockwise rotation of about 45 degrees, after which a minor chemical magnetization, aligned along the direction of the present axial dipole field, seems to have been acquired by some specimens.  相似文献   

13.
Summary. A history of remanence acquisition in the Madison Limestone in north-central Wyoming suggests that many processes affected the primary magnetization, including (1) dolomitization, (2) solution of evaporites during uplift in the Late Mississippian and (3) partial remagnetization in the Late Cretaceous or Cenozoic. The stability and intensity of the magnetization was greatest in the dolomitic limestones, while the dolomites and the less intense fossiliferous limestones contained magnetization of intermediate directional stability. The coarse- and fine-grained limestones gave erratic directions. The preservation of the Mississippian magnetization in the dolomitic limestone from the lower portion of the Madison testifies to the delicate mechanism of the dolomitization process, and suggests that the dolomitization may even have aided in the preservation of the magnetization. The presence of solution features corresponds closely to the presence of unstable directional behaviour. It appears that the formation of these features by solution of the evaporite beds led to the destruction of the primary remanence in the upper portion of the Madison.
Thermomagnetic curves indicate the presence in the lower portion of the Madison of almost pure magnetite. Although thermomagnetic curves from the upper portion are irreversible, alternating field and thermal demagnetization experiments show intensity changes similar to those from the lower portion, suggesting a similar magnetic mineral. Isothermal remanent magnetization acquisition curves also suggest the presence of a large amount of hematite or goethite in over half the samples which apparently did not contribute significantly to the natural remanent magnetization.  相似文献   

14.
An 11 m long sediment core ftorn Lama Lake, Northern Siberia, has been subjected to intense sedimentological, geochemical and rock magnetic analyses. According to a palynologic investigation the recovered sediments cover the whole Holocene and the late Pleistocene reaching back to about 17 ka. IRM acquisition experiments, hysteresis loop and back field as well as thermomagnetic measurements revealed magnetite in the pseudo-single domain range as the only remanence carrier. Sharp rock magnetic boundaries occur at 20 and 745 cm sub-bottom depth that are clearly linked to shifts in the median grain size of the magnetite. These boundaries are close to the present boundaries that bracket an anoxic zone between the subrecent and a late Pleistocene oxic section of the sediments. Within the anoxic section, magnetites are characterized by significantly larger median grain sizes but within a very narrow grain size range. The shift from fine grained magnetite within the oxic sediments to coarse grained magnetite is interpreted as the result of dissolution of the finest magnetite grains within the anoxic sediments. A significant shift of the Ti/Fe-ratio of the bulk sediment at a sub-bottom depth of 735 cm does not correspond to thermomagnetic properties, i.e. Curie-temperatures do not follow the variable Ti-content of the sediment.  相似文献   

15.
Experiments involving the gradual drying out of controlled mixtures of soil and organic lake sediment during storage at room temperature show that this leads to a loss of magnetic susceptibility and isothermal remanence greatly in excess of the initial values for the sediment components of the mixtures. We conclude that during storage in the moist state, soil-derived, fine-grained, ferrimagnetic iron oxides (magnetite and/or maghemite) are transformed to paramagnetic and/or imperfect antiferrimagnetic minerals. The imperfect anti-ferromagnetic component of the initial mixtures, which probably includes goethite, appears to survive and may even increase during storage. The experimental results compare well with the previously documented effects of storing wet sediment from the site, Peckforton Mere, Cheshire, U.K., over a comparable time interval. We conclude that transformation of fine grained ferrimagnets during storage diagenesis may be responsible for many of the examples of loss of magnetic susceptibility and remanence attributed by other authors solely to the oxidation of an iron sulphide such as greigite. Only where greigite is positively identified is it valid to infer a contribution from it to the magnetic properties of lake sediments: loss of susceptibility or remanence during storage is not alone a sufficient basis for such an inference. Early drying of samples will normally avoid the effects of storage diagenesis; and recent sediment samples so treated will, where greigite formation, bacterial magnetite and magnetite dissolution are insignificant, provide a valid basis for source identification on the basis of magnetic properties.  相似文献   

16.
The intensity of rotational remanent magnetization (RRM) acquired by single-domain greigite at a rotation frequency of 5 rps was combined with measurements of anhysteretic remanent magnetization (ARM) to calculate the effective biasing field ( Bg ) that produced the RRM. Samples of single-domain greigite had Bg values between -137 and -84 μT, and a MDFRRM of c. 80 mT. By contrast, a suite of natural and synthetic ferrimagnetic iron oxide samples, including single-domain magnetite and y Fe3O4 tape particles, acquired Bg values between -3 and -14 μT, and MDFRRM ranged between 43 and 68 mT (when RRM was acquired). Multidomain magnetite did not acquire a RRM. Bg values at 5 rps were calculated from previously published data for magnetite particles of different grain sizes, which revealed a minimum Bg value of -24 μT and a MDFRRM of 57 mT for the finest fraction (0.2-0.8 μm in diameter). In a geological example, measurements of Bg and MDFRRM were used to detect the presence of greigite in a 4 m long Late Weichselian sediment core. Variations in inclination, declination and the intensity of the natural remanent magnetization (NRM) correlate with changes in magnetic mineralogy.  相似文献   

17.
Summary. The magnetic viscosity of 334 Upper Tertiary and pre-Bruhnes Quaternary volcanic rocks from the Massif Central (France) and the Steens Mountain (Oregon), and of 40 basaltic cores from DSDP leg 37 has been investigated. Thellier's viscosity index follows a log normal distribution with mean values equal to 6 and 3.5 per cent for subaerial and submarine rocks respectively. For subaerial rocks, the average intensity of the viscous remanent magnetization (VRM) acquired in the Earth's field since the beginning of the Bruhnes polarity epoch ( t = 0.7 Myr) is estimated to be equal to one-quarter of the average intensity of the primary remanence. Alternating field demagnetization of VRMs acquired in low fields for acquisition times t ranging from 2 day to 32 month indicates the resistance to alternating fields is quite different from sample to sample and increases linearly with log t.
Néel's diagnostic parameter of domain structure of the grains involved in magnetic viscosity shows that hard VRM is carried by single-domain grains and soft VRM carried by multidomain particles. Single domain particles carrying hard VRM in subaerial volcanic rocks are almost equant magnetite intergrowths with size near the superparamagnetism threshold, resulting from high temperature oxidation of titanomagnetite. Soft VRM is carried by low Curie point homogeneous titanomagnetite. Unlike his single domain theory, Néel's multidomain theory of magnetic viscosity does not account quantitatively for the resistance of VRM to alternating fields.  相似文献   

18.
Summary. Several tests have been carried out to investigate how the generation of rotational remanent magnetization depended on the frequency of the applied demagnetizing field. The equipment used is described. The investigation was carried out using two specimens, one being a synthetic specimen of magnetite, and the other a cylindrical rock sample. These specimens gave virtually identical behaviour with varying frequency, unlike the differing behaviours reported previously by Wilson & Lomax. For each of the separate alternating field frequencies used (ranging from 50 to 1210 Hz), as the rotational speed of each specimen was reduced from 0.1 cycle s−1, the corresponding rotational remanent magnetization increased to a maximum value when the rotational speed was in each case just a little greater than 0.01 cycle s−1, after which the rotational remanence decreases as the rotational speed decreases.  相似文献   

19.
Summary. Rotational remanent magnetization, RRM, is the magnetization acquired when a sample is rotated during alternating field demagnetization. Although the existence of RRM has been well documented in different laboratories, until now no physical mechanism explaining its origin has been given. We propose that the RRM originates from thermal fluctuations biased by a precessional torque associated with the alternating field. Our theory is consistent with the observation that no directional preference exists in the experimental situation until the sample is rotated relative to the alternating field. Moreover, our theory predicts that the combined sample rotation and precession will produce a RRM that switches direction when the frequency of sample rotation increases from any value below the frequency of the alternating field to any value above that frequency as observed in experiments. Although no precise theory is given for the intensity of RRM, the model presented here can qualitatively explain previous intensity observations.  相似文献   

20.
Summary. The directions of remanent magnetism of samples of the Dunnet Head sandstone from Scotland are very scattered on a scale down to a few millimetres, although an overall mean direction is reasonably well defined. The scattered directions show considerable stability against thermal demagnetization and there is evidence that haematite pigment is an important carrier of the remanence. It is concluded that the origin of the inhomogeneous magnetization is a disturbed ambient field during acquisition of chemical remanence by the pigment.  相似文献   

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