Natural Remanent Magnetization Acquisition Research Articles

Acquisition of natural remanence in the basaltic laterites of Deccan volcanic province (India): Implications to palaeomagnetic studies in laterites

Lateritization in the Deccan basalt province (India) represents a large spectrum of chemical weathering with up to 99% CIA values and complete ferri- to antiferromagnetic transformations. We combined rock magnetic, palaeomagnetic, XRD, and XRF analyses to investigate the mineralogical transformations and its relationship to the acquisition of natural remanent magnetization (NRM) by sampling a 25 m thick Deccan lateritic profile from the West Coast of India. Significant amount of amorphous hematite (ha), crystalline hematite (hc), goethite and their Al- substituted forms were detected along with gibbsite and kaolinite. The domain size association of ‘SP + SD and ‘SP + SD + MD’ in the ferri- as well as antiferromagnetic forms; and the combination of ha and hc yield strong NRMs amongst these laterites. Low field demagnetization (<5mT) allows removal of haNRM; and the secondary components due to ferrimagnetic oxides can be removed at < 25 mT. Thereafter, the hcNRM continued to decay up to > 150 mT. Palaeomagnetic tests (Koenigsberger ratio, MDF, intensity decay and Zijderveld diagrams) indicate stable Characteristic remanent magnetizations (ChRM) due to hc; and the Al-substitutions does not affect the ChRM stability, depicting these laterites as excellent palaeomagnetic recorders. The stable ChRMs due to Chemical/Crystallization RMs due to ‘hc’ also benchmarks the degree of maturity within the process of lateritization.

Influence of Seasonal Post‐Depositional Processes on the Remanent Magnetization in Varved Sediments From Glacial Lake Ojibway (Canada)

AbstractThe natural remanent magnetization (NRM) of high sedimentation rate sediments provides significant information about paleomagnetic secular variation of the Earth's magnetic field and can also potentially be used for stratigraphy. However, NRM acquisition depends on conditions inherent to the depositional environment. In addition to recording a precise annual chronology, varved sediments reflect marked annual sedimentary changes. The Earth's magnetic field does not vary significantly over such a short period, so magnetic changes recorded by varves are expected to reflect the influence of depositional parameters on the recording process. We focus here on a sequence of 27 ± 1 varves from the former proglacial Lake Ojibway (∼8.5 ka cal BP) from which individual cm‐thick summer and winter beds were sampled. Paleomagnetic, granulometric and geochemical analyses were conducted on each bed. A mean inclination shallowing of 24.3° is observed in winter beds, along with an 11.3° shallowing in summer beds. Magnetic declinations follow, on average, the expected field direction, but differences of up to 20° occur between successive beds. Summer beds are thicker than winter beds and have stronger magnetic susceptibility, higher Ca/Fe ratios and coarser sedimentary and magnetic grains. This grain size pattern reflects the input of coarser detrital particles during summer, while the finer fraction remained in suspension until it was deposited in winter. A combination of differential compaction between the winter and summer beds, seasonally varying physical and magnetic properties of sediments, and delayed NRM acquisition explains the variable and coercivity‐dependent inclination shallowing.

Impact of turbulence on magnetic alignment in sediments

Rapidly deposited layers (RDL) such as turbidites or hyperpycnites are mostly studied for their sedimentological properties, but are carefully avoided in paleomagnetic studies due to the disturbances caused by such sudden and rapid sediment accumulation. Therefore, these layers can also be seen as potential indicators of sediment parameters susceptible of affecting the alignment of magnetic grains and ultimately the acquisition of the natural remanent magnetization (NRM). We have compiled 13 Holocene rapidly deposited layers from core MD99-2222 in the Saguenay Fjord, eastern Canada (St-Onge and al., 2004) with varying thicknesses (from 7.1 cm to 1,510 cm) and 4 Quaternary turbidites of different origins, to document the influence of sedimentary and magnetic parameters on natural remanent magnetization acquisition. We found a logarithmic relationship between rapidly deposited layers thickness on the one hand, and the amplitude of inclination changes and magnetic grain sizes on the other. Inclination and magnetic grain sizes are themselves correlated to each other by a logarithmic law. As there is no relationship between inclination deviation and stratigraphic depth, compaction alone cannot account for such large effects on inclination. Flocculation is grain size sensitive, but it is expected to affect mainly the natural remanent magnetization intensity, rather than its direction. Turbulence that prevails during the rapid deposition of sediments during such events is most likely the dominant factor.

Were chondrites magnetized by the early solar wind?

Chondritic meteorites have been traditionally thought to be samples of undifferentiated bodies that never experienced large-scale melting. This view has been challenged by the existence of post-accretional, unidirectional natural remanent magnetization (NRM) in CV carbonaceous chondrites. The relatively young inferred NRM age [∼10 million years (My) after solar system formation] and long duration of NRM acquisition (1–106 y) have been interpreted as evidence that the magnetizing field was that of a core dynamo within the CV parent body. This would imply that CV chondrites represent the primitive crust of a partially differentiated body. However, an alternative hypothesis is that the NRM was imparted by the early solar wind. Here we demonstrate that the solar wind scenario is unlikely due to three main factors: 1) the magnitude of the early solar wind magnetic field is estimated to be <0.1 μT in the terrestrial planet-forming region, 2) the resistivity of chondritic bodies limits field amplification due to pile-up of the solar wind to less than a factor of 3.5 times that of the instantaneous solar wind field, and 3) the solar wind field likely changed over timescales orders of magnitude shorter than the timescale of NRM acquisition. Using analytical arguments, numerical simulations and astronomical observations of the present-day solar wind and magnetic fields of young stars, we show that the maximum mean field the ancient solar wind could have imparted on an undifferentiated CV parent body is <3.5 nT, which is 3–4 and 3 orders of magnitude weaker than the paleointensities recorded by the CV chondrites Allende and Kaba, respectively. Therefore, the solar wind is highly unlikely to be the source of the NRM in CV chondrites. Nevertheless, future high sensitivity paleomagnetic studies of rapidly-cooled meteorites with high magnetic recording fidelity could potentially trace the evolution of the solar wind field in time.

The effect of speleothem surface slope on the remanent magnetic inclination

AbstractSpeleothems are of interest for high‐resolution reconstruction of the Earth's magnetic field. However, little is known about the influence of speleothem morphologies on their natural remanent magnetization (NRM) record. Here we report on a high‐resolution paleomagnetic study of a dome‐shaped speleothem of middle Holocene age from southern Portugal, with special attention to the anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). To assess the potential influence of the slope of the speleothem surface on the recorded remanent magnetization, we compare magnetic directions and AMS and AARM fabrics from subhorizontal to gradually subvertical calcite growth layers collected in a transversal cross section of the speleothem. A linear correlation is observed between magnetic inclinations, calcite laminae slope, and AARM k1 inclination. The AMS fabric is mostly controlled by calcite crystals, with direction of the minimum axes (k3) perpendicular to laminae growth. Magnetic inclinations recorded in inclined and vertical calcite growth layers are underestimated when compared to a global paleosecular variation (PSV) model. After extrapolating magnetic inclinations to the horizontal, the corrected data better fit the PSV model but are still lower than the predicted magnetic inclinations, suggesting that inclination shallowing affects the entire speleothem. We suggest that speleothem morphology exerts a critical role on the magnetic inclination recording, which is controlled by the Earth's magnetic field but also influenced by particle rolling along the sloping surfaces. These observations open new avenues for reconstructing high‐resolution paleomagnetic secular variation records from speleothems and provide new insights into their NRM acquisition mechanisms.

An integrated natural remanent magnetization acquisition model for the Matuyama‐Brunhes reversal recorded by the Chinese loess

AbstractGeomagnetic polarity reversal boundaries are key isochronous chronological controls for the long Chinese loess sequences, and further facilitate paleoclimatic correlation between Chinese loess and marine sediments. However, owing to complexity of postdepositional remanent magnetization (pDRM) acquisition processes related to variable dust sedimentary environments on the Chinese Loess Plateau (CLP), there is a long‐standing dispute concerning the downward shift of the pDRM recorded in Chinese loess. In this study, after careful stratigraphic correlation of representative climatic tie points and the Matuyama‐Brunhes boundaries (MBB) in the Xifeng, Luochuan, and Mangshan loess sections with different pedogenic environments, the downward shift of the pDRM is semiquantitatively estimated and the acquisition model for the loess natural remanent magnetization (NRM) is discussed. The measured MB transition zone has been affected by the surficial mixing layer (SML) and remagnetization. Paleoprecipitation is suggested to be the dominant factor controlling the pDRM acquisition processes. Rainfall‐controlled leaching would restrict the efficiency of the characterized remanent magnetization carriers aligning along the ancient geomagnetic field. We conclude that the MBB in the central CLP with moderate paleoprecipitation could be considered as an isochronous chronological control after moderate upward adjustment. A convincing case can then be made to correlate L8/S8 to MIS 18/19.

Multiple 10Be records revealing the history of cosmic-ray variations across the Iceland Basin excursion

Cosmogenic 10Be is a proxy of cosmic-ray flux, and its natural records provide vital information about the past intensity variability of the geomagnetic field and solar activity. 10Be records also serve as powerful tools for global synchronization among a variety of paleoarchives and for elucidating sedimentary processes on natural remanent magnetization acquisition. However, high-resolution (multi-decadal to multi-centennial) records of 10Be are scarce, especially those older than several tens of thousands of years. Here we present multiple high-resolution 10Be records of the Iceland Basin geomagnetic excursion interval (ca. 170–200 kyr ago) obtained from sediment cores (authigenic 10Be/9Be ratio) and an ice core (atmospheric 10Be flux). Comparing sedimentary 10Be records with relative paleointensity from the same cores, we found differences in the magnetic lock-in depth, even between adjacent cores. The 10Be-proxy records from the sediment and ice cores exhibit common characteristics: an asymmetric large-scale variation, a ∼7-kyr quasi-plateau around the maximum with a characteristic mid-term depression, and multi-millennial fluctuations in cosmic-ray flux during this interval. Minimal-synchronized and stacked 10Be records show that maximum cosmic-ray flux occurred 188.5–190.0 kyr ago and was double the present flux. A wavelet analysis of the stacked curve reveals dominant 4-kyr and secondary 8-kyr periodicities, both of which can be interpreted as intrinsic geomagnetic cycles. The wavelet spectrum of the high-resolution ice-core record shows a periodicity of 1.7 kyr and somewhat intermingled multi-centennial cycles around the maxima of 10Be, which likely represent solar cycles in this period. High-resolution 10Be records from multiple paleoarchives provide both a robust proxy record of cosmic-ray flux and a valuable tool for detailed global synchronization based on cosmic-ray variations.

Natural remanent magnetization acquisition in bioturbated sediment: General theory and implications for relative paleointensity reconstructions

AbstractWe present a general theory for the acquisition of natural remanent magnetizations (NRM) in sediment under the influence of (a) magnetic torques, (b) randomizing torques, and (c) torques resulting from interaction forces. Dynamic equilibrium between (a) and (b) in the water column and at the sediment‐water interface generates a detrital remanent magnetization (DRM), while much stronger randomizing torques may be provided by bioturbation inside the mixed layer. These generate a so‐called mixed remanent magnetization (MRM), which is stabilized by mechanical interaction forces. During the time required to cross the surface mixed layer, DRM is lost and MRM is acquired at a rate that depends on bioturbation intensity. Both processes are governed by a MRM lock‐in function. The final NRM intensity is controlled mainly by a single parameter γ that is defined as the product of rotational diffusion and mixed‐layer thickness, divided by sedimentation rate. This parameter defines three regimes: (1) slow mixing (γ &lt; 0.2) leading to DRM preservation and insignificant MRM acquisition, (2) fast mixing (γ &gt; 10) with MRM acquisition and full DRM randomization, and (3) intermediate mixing. Because the acquisition efficiency of DRM is larger than that of MRM, NRM intensity is particularly sensitive to γ in case of mixed regimes, generating variable NRM acquisition efficiencies. This model explains (1) lock‐in delays that can be matched with empirical reconstructions from paleomagnetic records, (2) the existence of small lock‐in depths that lead to DRM preservation, (3) specific NRM acquisition efficiencies of magnetofossil‐rich sediments, and (4) some relative paleointensity artifacts.

Coseismic magnetization of fault pseudotachylytes: 1. Thermal demagnetization experiments

AbstractFault pseudotachylytes form by quenching of silicate liquids produced through coseismic frictional melting. Here we show that in natural pseudotachylytes the main carrier of magnetic remanence blocked in during cooling of the frictional melt is fine‐grained magnetite. This confirms previous studies on friction melt experiments. Stoichiometric magnetite, produced during earthquakes by the breakdown of ferromagnesian silicates, records the ambient magnetic field during seismic slip. We find that most fault pseudotachylytes exposed in the Santa Rosa Mountains, southern California, a classic pseudotachylyte locality, acquired their natural remanent magnetization (NRM) upon cooling of the frictional melt through the range of magnetization blocking temperatures of the magnetite grains and this primarily constitutes a thermal remanent magnetization. NRM intensities typical of most pseudotachylyte veins range from 1 to 60·10−4 Am2/kg. A few specimens, however, contain magnetizations significantly higher than that caused by the Earth's field as well as magnetization directions that are highly variable over short distances. Other magnetization processes, possibly related to coseismic electric currents, may be involved during the seismogenic process to control NRM acquisition.

Magnetotaxis and acquisition of detrital remanent magnetization by magnetotactic bacteria in natural sediment: First experimental results and theory

The widespread occurrence of magnetotactic bacteria (MTB) in several types of marine and freshwater sediment, and the role of fossil magnetosomes (magnetofossils) as main remanent magnetization carriers therein, has important paleomagnetic and paleoenvironmental implications. Despite numerous studies on MTB biology and on magnetofossil preservation in geological records, no detailed information is yet available on how magnetotaxis (i.e., the ability to navigate along magnetic field lines) is performed in sedimentary environments, and on how magnetofossils possibly record the Earth magnetic field. We provide for the first time experimental evidence for these processes. MTB living in sediment are poorly aligned with the geomagnetic field, contrary to what is observed in water. This can explain the seemingly excessive magnetic moment of most MTB. The observed alignment is sufficient for supporting magnetotaxis across the typical thickness of chemical gradients. Experiments with magnetofossil‐rich sediment suggest that a natural remanent magnetization (NRM) is acquired by magnetofossils in the so‐called benthic mixed layer, where natural MTB populations usually occur. The acquired NRM is proportional to the applied field at least up to ∼160 µT, and its intensity is compatible with values observed in nature for same sediment types. Therefore, if fossil magnetosome chains are not subjected to further alteration by early diagenetic processes, they can provide useful relative paleointensities. We propose a preliminary model to explain early stages of magnetofossil NRM acquisition as the result of a dynamic equilibrium between magnetic torques and randomizing forces due to sediment mixing.

The fidelity of paleomagnetic records carried by magnetosome chains

Magnetotactic bacteria (MTB), which use aligned chains of magnetosomes (magnetic crystals) as a navigation tool, are found in a wide range of modern day marine, river and lacustrine environments and their fossilized remains are being increasing recognized in geological records. Despite an increasing realization that biogenically derived magnetic particles may play a key role in sedimentary magnetizations, little is known about the influence that they may have on the fidelity of paleomagnetic recordings. Using cultured Magnetospirillum magneticum strain AMB-1, we have conducted simple 2D (i.e., zero-inclination) deposition experiments to assess the efficiency with which magnetosome chains align along magnetic field lines and the implications that this has for paleomagnetic records. Our results indicate that the natural remanent magnetization (NRM) acquired by deposited MTB is near linear with applied field (0–120 μT), but that NRM acquisition does not perfectly follow the assumed linear trend and over a six-fold increase in applied field only a factor ∼5.5 increase in NRM intensity is observed (i.e., ∼9% lower than the assumed trend). Both anhysteretic remanent magnetization (ARM) and saturation isothermal remanent magnetization (SIRM) normalized relative paleointensities (RPIs) can successfully recover field strength variations under identical situations. When the MTB concentration of the initial solution is varied (up to a factor 8), NRM carried by MTB responds in the expected fashion (i.e., a doubling of bacteria produces a doubling of NRM intensity). Both ARM and SIRM, however, do not respond to as expected: A doubling of concentration corresponds to an increase of ∼1.76 in SIRM intensity and an increase of only ∼1.63 in ARM intensity. Both are influenced by magnetostatic interactions that arise from the close packing of bacterial cells or subchain interactions within a single bacterial cell. First-order reversal curve (FORC) diagrams, however, are too insensitive to indicate the presence of paleomagnetically relevant magnetic interactions. Remanence based methods (i.e., the ARM ratio) are more appropriate to detect such interactions.

Paleomagnetic record of the late Pleistocene reef sequence of Tahiti (French Polynesia): A contribution to the chronology of the deposits

Rock- and paleomagnetic investigations were performed on the late Pleistocene reef sequence of Tahiti, and take part of a multidisciplinary study of the Holocene and Pleistocene reef sequences, carried out by the IODP (Integrated Ocean Drilling Program) Expedition 310. The objectives were to characterize the natural remanent magnetization and its mineral carriers, in order to identify high-resolution magnetostratigraphic markers of the late Brunhes chron. A series of 85 specimens subsampled within a 79 m-thick silty-carbonate sequence cored at the Maraa site (SW of Tahiti) display a strong and stable natural remanent magnetization (NRM) carried by titanomagnetite inherited from the basalts which form the bulk of the island. The average negative inclination is assignable to the field generated by the normal polarity axial dipole field at the site latitude. A few specimens (103–119 m below modern sea level) carry a reversed primary magnetization acquired in a weak excursional field. Based on subsidence rate estimates ranging from 0.25 to 0.4 m/kyr, this suggests the identification either the Laschamp excursion (∼ 40 ka) or the Blake event (115–120 ka). Radiometric dates of ∼ 131 to ∼ 134 ka at 117 m below modern sea level confirm that these layers were deposited during the marine isotope stage 5.5 and recorded the direction and intensity anomaly associated with the Blake event. Another sea-level highstand reef unit, occurring deeper in the Pleistocene sequence has seemingly recorded another paleomagnetic intensity anomaly which is not yet definitely identified. Reef systems, which developed around volcanic islands, contain geological records of excursions. But, it is necessary to improve the understanding of NRM acquisition and the role of carbonate diagenesis in order to establish radiometrically dated records of geomagnetic excursions.

Magnetostatic interactions in deep‐sea sediments inferred from first‐order reversal curve diagrams: Implications for relative paleointensity normalization

Understanding magnetostatic interactions in sediments is important for paleomagnetism and rock magnetism. Magnetostatic interactions are known to affect significantly the efficiency of anhysteretic remanent magnetization (ARM) acquisition, and hence the ratio of ARM susceptibility (χARM) to saturation isothermal remanent magnetization (SIRM), a magnetic grain‐size proxy widely used for environmental applications, can be controlled by magnetostatic interactions. Relative paleointensity estimations may also be influenced by magnetostatic interactions because ARM is often used as a normalizer to correct for the difference in magnetizability of sediments. In this study, characterization of magnetic grains in deep‐sea sediments from the North Pacific Ocean was conducted using first‐order reversal curve (FORC) diagrams and IRM acquisition curves. The FORC diagrams indicate that the magnetic grains consist mainly of a noninteracting single‐domain (SD) component and an interacting SD component, and additionally of a multidomain (MD) component. The relative abundances of these components were semiquantitatively estimated by curve fitting of cross sections along a line parallel to the axis of local interaction fields (Hu) and through a peak in the coercivity (Hc). Three components were fitted assuming a Gaussian distribution of interaction fields (Hu). The IRM acquisition curves can be described by two dominant components assuming a log‐Gaussian distribution. The mean coercivities of the two components are ∼40 and ∼100 mT, respectively. It is estimated that the former component is carried by biogenic magnetite, and roughly corresponds to the noninteracting SD component derived from the FORC diagrams, and that the latter component is carried by eolian maghemite, corresponding to the interacting SD component. The χARM/SIRM ratio decreases with increasing concentration of the interacting SD component. This implies that the χARM/SIRM ratio is significantly affected by magnetostatic interactions and that it does not necessarily reflect magnetic grain size. The effect of magnetostatic interactions on natural remanent magnetization acquisition in sediments is not well understood, but if it is similarly sensitive to the SIRM, normalization by ARM in relative paleointensity estimations would overcompensate for the concentration of magnetic grains and cause the significant coherence between the normalized intensity and the normalizer that has been reported in the literature.

Effects of nanoscale exsolution in hematite–ilmenite on the acquisition of stable natural remanent magnetization

To investigate the acquisition mechanism of high and stable natural remanent magnetization (NRM) in rocks of the Russell Belt, Adirondack Mountains, New York, we examined the exsolution microstructures and compositions of magnetic minerals using three samples with different magnetic properties. The samples contain titanohematite with ilmenite lamellae, end-member hematite without lamellae and rare magnetite as potential carriers for the NRM. Transmission electron microscopy (TEM) observations and element mapping by energy-filtered TEM (EFTEM) of the titanohematite indicated that very fine ilmenite lamellae with a minimum thickness ∼2 nm are abundant between larger ilmenite lamellae a few hundreds of nanometers thick. The ilmenite lamellae and titanohematite hosts, with the compositions of Ilm 90–100Hem 10–0 and Ilm 7–16Hem 93–84, respectively, share (001) planes, and the abundant fine ilmenite lamellae have coherent, sharp structural and compositional interfaces with their titanohematite hosts. Comparison between samples shows that the magnetization is correlated with the amount of fine exsolution lamellae. These results are consistent with the lamellar magnetism hypothesis, suggesting that the acquisition of high and stable NRM is related to the interfacial area between fine ilmenite lamellae and their host titanohematite. End-member hematite with a multi-domain magnetic structure only contributes a minor amount to the NRM in these samples.

Rock-magnetic properties of multicomponent natural remanent magnetization in alluvial red beds (NE Spain)

An earlier study of the cyclic Miocene red bed sequence of La Gloria (Spain) by Krijgsman et al. indicated complicated behaviour of the natural remanent magnetization (NRM) in parts of the section, particularly close to reversal boundaries. We resampled part of the section with high resolution and used extensive rock-magnetic analyses to characterize the magnetic remanence carriers. Below a conspicuous hydromorphous layer, the magnetic contributions of haematite and magnetite are approximately equal, while in the brown layers on top of the hydromorphous layer the contribution of magnetite is drastically reduced. This is probably related to a change in hydrological conditions. The NRM is characterized by: (1) a low-temperature (200–360 °C) overprint of unknown age, (2) a medium-temperature (360–580 °C) component, interpreted as the characteristic remanent magnetization (ChRM); and (3) a high-temperature haematite component (600– 680 °C), carrying the same directions as the medium-temperature component. There is no perceivable delay in NRM acquisition between the medium- and the high-temperature components; they are both regarded as primary. The behaviour of the NRM seems to be similar in the cyclic lithologies. The acquisition of NRM thus seems to be independent of lithology in the cyclic part of the section. The higher sampling resolution yielded the detection of a new polarity zone, which probably represents a geomagnetic feature according to rock-magnetic properties. However, the more detailed magnetostratigraphy of the resampled part of the section indicates that the earlier correlation to the geomagnetic polarity timescale is no longer tenable. Therefore, the La Gloria section should no longer be used as a magnetostratigraphic dating tool of mammal biochronology.

Paleomagnetism of Paleogene basalts from the Tien Shan, Kyrgyzstan: rigid Eurasia and dipole geomagnetic field

Several conflicting explanations, invoking persistent non-dipole fields or tectonic motion on various scales, were suggested to account for anomalously low paleomagnetic inclinations in Cenozoic rocks of Central Asia. In order to better understand this problem we undertook a paleomagnetic study of Paleogene basalts from a part of the Tien Shan close to the China–Kyrgyzstan border. Stepwise thermal demagnetization showed that only a single-component remanence of reversed polarity is present in these rocks above 200–260°C. The overall mean direction of this remanence ( D=194.6°, I=−54.0°, α 95=3.8°, n=18 sites) agrees well with the Eurasian reference directions for the Paleogene which is in sharp contrast with most earlier published Cenozoic inclinations from Central Asia which are 20–30° shallower than the expected values. Good agreement of the reference inclinations and those from basalts rules out models which invoke non-dipole fields and/or large-scale tectonic motions for explanation of the Cenozoic inclination anomaly. Instead, a mechanism related to natural remanent magnetization acquisition in redbeds is strongly indicated. The above conclusion also implies that the differences between the reference data and measured inclinations in Cretaceous redbeds from Central Asia are also of non-tectonic origin and a revision of our views on evolution of this region is required.

‘Sawtooth’ variation in magnetic intensity profiles and delayed acquisition of magnetization in deep sea cores

Recent records of normalised intensity of natural remanent magnetization (NRM) obtained from deep-sea sediments document an asymmetrical saw-tooth pattern, associated with geomagnetic reversals. This observation may reflect a fundamental aspect of the geodynamo time evolution or, alternatively, delay in acquisition of the NRM. It is demonstrated here that a simple model, in which a large fraction of the magnetic grains acquire NRM at deposition time while remaining grains reorientate or acquire magnetization after deposition, is able to produce a ‘saw-tooth’ profile strikingly similar to that observed. Interestingly, this mechanism of NRM acquisition does not damp out high frequency oscillations of the field, such as those observed in the deep-sea records exhibiting the saw-tooth profile. It provides an accurate record of field directions, except for periods of very low field intensity.

Acquisition of Natural Remanent Magnetization for Dirt-Snow Containing Rock Dust.

Magnetization process of dirt-snow containing fine-grained magnetic minerals was investigated in order to understand the acquisition mechanism of natural remanent magnetization (NRM) by dirt-ice layers in Antarctica. Dirt-snow acquired NRM from the direction of geomagnetic field (GMF) direction at rooms -10°C and -20°C, and inclination becomes shallower than the GMF after 50 days.The following two types of NRM acquisition mechanisms in the GMF are proposed; (1) magnetic grains align along the GMF when they drop onto the surfaces of underlying snow due to vaporization of snow; (2) the magnetic grains are rotated to align in the GMF direction on the snow surfaces by the torque with the GMF and the NRM. The compaction of the natural snow at formation of depth hoar works to flatten the NRM direction.Probably the Antarctic dirt-ice has acquired NRM during the periods of accumulation of snow and magnetic grains and formation of the depth hoar. The NRM of dirt-snow and -ice are usuful for understanding the glacial dynamics and paleomagnetism of the Antarctic ice sheet, although many problems still remain.

Reversed magnetization in pyroclastics from the 1985 eruption of Nevado del Ruiz, Colombia

One of the fundamental principles of palaeomagnetism and rock magnetism states that the direction of natural remanent magnetization (NRM) is acquired parallel to the applied field. However, ferromagnetic mineral phases with complex microstructures may violate this rule1,2 and produce a magnetization vector which is antiparallel to the direction of the external field. Andesitic pumice, which was hurled several kilometres during the disastrous 1985 eruption of the Nevado del Ruiz volcano (Colombia), carries a stable but reversed NRM with southerly declination and negative inclination. Heating experiments show that this magnetization is due to a self-reversal mechanism which also induces a reversed thermoremanent magnetization (TRM) in the laboratory field. Thus we have used the Nevado del Ruiz 1985 pumice, which was magnetized in the present well-known geomagnetic field, to demonstrate for the first time that self reversal has actually controlled the process of NRM acquisition.