Relative Palaeointensity Research Articles

A detailed palaeomagnetic record for the last interglacial period

A palaeomagnetic investigation of a late Quaternary loess-palaeosol sequence in central China has revealed two episodes of anomalous geomagnetic field behaviour in the lower and upper parts of soil complex S1, which spans the last interglacial interval. Comparison of the magnetic susceptibility record with the deep-sea oxygen isotope stratigraphy provides a chronology which has been used to estimate the age and duration of these episodes. The age of the younger episode has been estimated at 89-75 ka, comparable with the age of a geomagnetic event recognised from sediments in the Norwegian-Greenland Sea and Arctic Ocean. The older episode is associated with a broad low in relative palaeointensity which has been assigned to the time interval 130-110 ka. This is consistent with existing age estimates of the so-called Blake Event. In contrast to a published record of the Blake Event from the loess at Xining, which shows three short episodes of almost full reversed polarity, in the current record the Blake Event is represented by a loop in the VGP path which passes through shallow southern latitudes. Although both episodes occur within the same part of soil complex S1, the difference between the two records implies a greater degree of smoothing of the geomagnetic signal in the Huanxian record. This is probably a reflection of the lower accumulation rate at Huanxian and may also indicate a greater time constant for the remanence aquisition at this site.

Fuzzy c-means cluster analysis of early diagenetic effects on natural remanent magnetisation acquisition in a 1.1 Myr piston core from the Central Mediterranean

The influence of early diagenesis on the natural remanent magnetisation (NRM) in sediments from the Calabrian ridge (Central Mediterranean) is analysed with the help of fuzzy c-means (FCM) cluster analysis and non-linear mapping (NLM). The sediments are variably coloured: white, beige, purplish, greenish and grey layers occur with occasionally intercalated sapropels. The NRM acquired depends on both depositional conditions and diagenetic processes. To describe these, FCM was performed with χ in , ARM, CaCO 3, Ba, Mn and S as variables. An eight-cluster model was derived with the clusters belonging to two categories: one expressing mainly diagenetic processes, i.e. dissolution and precipitation, and the other expressing mainly depositional conditions. The impact of diagenesis on NRM acquisition is profound and not restricted to the close vicinity of the anoxic sapropelitic layers. As a consequence, the influence of diagenetic processes on the NRM should be thoroughly assessed when selecting samples, e.g. for the determination of the relative palaeointensity of the geomagnetic field. Application of multivariate classification techniques appears to be useful because it links rock magnetic parameters to the geochemical environment. In the present piston core, three short reversed geomagnetic events in the Brunhes chron are preserved and, indeed, occur in clusters expressing no or minor diagenesis. The recording of the Blake event, however, has been prevented by later precipitation of magnetite in the corresponding interval.

Middle/Late Pleistocene relative palaeointensity of the geomagnetic field from lacustrine sediments, Lake Chewaucan, western United States

SUMMARY Detailed palaeomagnetic and rock magnetic studies of a 15 m succession of MiddlelLate Pleistocene lacustrine sediments from ancient Lake Chewaucan, southern Oregon, western United States, indicate that the remanence-bearing grains are sufficiently uniform to be applicable to relative palaeointensity studies. We have used ARM, SIRM and x for normalization of the NRM. All three parameters give essentially identical results in their relative stratigraphic variations, which indicates that the normalizations efficiently remove the effects of variation in magnetic mineral concentration. Patterns in grain-size variation, as indicated by small-scale quasi-cyclic fluctuations in hysteresis parameters, may be due to environmental changes such as lake-level variation. However, these fluctuations are within the acceptable range of grain sizes for palaeointensity studies and cannot be correlated with any of the features of the normalized remanence record. We therefore conclude that the large-scale variations in the normalized remanence record are due to geomagnetic palaeointensity fluctuations. Parts of the normalized remanence record, where firm chronological constraints exist, may correlate with features of relative palaeointensity records from deep-sea sediments. Our results also confirm the observation that low geomagnetic field intensities dominate during geomagnetic excursions. Further studies of relative palaeointensity of the geomagnetic field may enable the development of an independent time-scale which would make possible the direct correlation of palaeoclimate records from deep-sea and continental environments.

Relative palaeointensity records from the Ontong-Java Plateau

SUMMARY Rock magnetic, palaeomagnetic and oxygen isotopic results are presented from core RNDB 75p, which was recovered from the Ontong-Java Plateau (OJP). A high degree of uniformity in magnetic properties characterized by relatively small changes in concentration and grain size in the upper 4 m of the core, combined with a lack of coherence between the normalized remanence and rock magnetic data suggests that the natural remanence normalized by saturation remanence reflects variations in relative palaeointensity of the geomagnetic field. The record from RNDB 75p replicates other Ontong-Java records spanning the last 400Ka and extends the record back to some 700Ka. Spectral analysis of the Ontong-Java record suggests periodic behaviour in the relative palaeointensity record with a dominant period of between 30 and 40Ka, which appears not be be an artefact of lithologic variability. This dominant period lies between functions describing climatic precession and obliquity changes in the Earth’s orbit. Comparison of the normalized remanence record with astronomical precession (26 Ka period), however, is much more favorable. None the less, ‘tuning’ the palaeointensity record to that of astronomical precession appears inconsistent with existing isotopic age constraints derived from the SPECMAP time-scale. Based on these data, we must choose between assuming that the Earth’s orbit controlled ice volume (inherent in the SPECMAP time-scale) and assuming that the Earth’s magnetic field is driven by astronomical precession. The former assumption has a substantial theoretical and observational base and we prefer to interpret the data presented here as suggesting that the Earth’s orbit has not played a detectable role in the modulation of the magnetic field. Plots of saturation remanence and magnetic susceptibility are very sensitive to quite subtle changes in magnetic grain size. A slight shift within the pseudo-singledomain grain-size range toward the multidomain (or superparamagnetic) field was detected at about 4m in RNDB 75p. This change in grain size may reflect a diagenetic alteration of the magnetite (such as dissolution) and may be related to the phenomenon responsible for the loss of magnetic remanence at depth detected in other cores from the region.

Geomagnetic moment variations in the last 70,000 years, impact on production of cosmogenic isotopes

On various time scales, the intensity of the Earth's magnetic field presents large amplitude oscillations due to secular variation of the non-dipole field (short term regional influences) or to variations of the moment of the main dipole (long term global influence). Here we introduce variations of the relative intensity of the local geomagnetic field reconstructed from a sedimentary sequence deposited during the Late Pleistocene in a maar crater lake located in Western Europe: Lac du Bouchet, Massif Central, France. The comparison of these results with absolute palaeointensities and virtual dipole moment (VDM) values obtained from volcanic rocks, as well as with relative palaeointensity variations measured on marine sedimentary sequences, lead to conclude that the geomagnetic dipole was drastically reduced between 43 and 30 ka. In the second part of this article, we discuss the relation of this geomagnetic crisis with the increase of atmospheric concentration of cosmogenic isotopes, recently documented for the relevant time interval. The modulation of the magnetospheric shielding by geomagnetic dipole fluctuations can be retained as the probable origin of such global atmospheric changes.

Relative geomagnetic intensity of the field during the last 140 ka

Relative magnetic field palaeointensities have been obtained from three marine cores in the Somali basin (western Indian Ocean). The correlation between the records based on changes in the low-field susceptibility and the inclination is extremely detailed. A time versus depth correlation has been established from the δ 18O record. The remanence intensity was normalized with respect to the anhysteretic remanent magnetization with special attention to changes in grain sizes, coercivities and mineralogy. The synthetic record of relative palaeointensity has been extended to cover the last 140 ka. The quasi-cyclic pattern observed during the last 80 ka confirms the results previously obtained in the Mediterranean sea and thereby establishes its dominantly dipolar character. Between 140 and 80 ka the field intensity exhibits a much longer cycle. Spectral analysis of the intensity records shows the presence of two dominant periodicities, but the length of the signal is too short to document the presence of a stationary process.

Olduvai termination: detailed palaeomagnetic analysis of a north central Pacific core

SUMMARY A high-resolution record of the termination of the Olduvai (normal to reverse) subchron (1.77 Myr) polarity transition was obtained from a fully azimuthally oriented deep-sea sediment piston core, recovered at a northern mid-latitude site (3722.4'N, 179O36.l'W). The N-R Olduvai reversal was 'continuously' sampled at 3 to 5 mm intervals across 130 cm of section. This 'continuous' sampling was achieved by staggering the 1 cc plastic boxes incrementally, so that offset segments corresponding to sampling windows of 3 to 5 mm were obtained. The reversal is well defined, and at least 10 truly intermediate directions have been recorded, displaying a definite directional change of MOO, with a pronounced decrease in intensity of magnetization. The decrease of magnetization and the directional change coincide closely. The inclination record during the transition is characterized by a brief steepening trend early in the reversal passing through high positive values, followed by a gradual shallowing trend, this time passing through the horizontal. From this point on, the inclinations show a final settling about the reversed polarity mean. This record is characterized by full normal and reverse polarity directions which are in good agreement (f2) with those predicted by the geocentric axial dipole field model for the core site latitude. The estimated duration of the directional change was about 10 OOO yr whereas that of the intensity decrease was between 12 OOO and 16 OOO yr. Anhysteretic Remanent Magnetization (ARM) studies and thermomagnetic analyses indicate that a uniform magnetic mineralogy characterizes the transition zone of the core. This further suggests that the J10/ARM70 ratio may represent a relative palaeointensity estimate and may thus indicate a decrease of the intensity of the geomagnetic field during the reversal. The virtual geomagnetic pole (VGP) path of the record is far-sided. Directions in projected (D', 1') space indicate that the N-R Olduvai reversal transition field geometry had contributions from both non-axisymmetric and axisymmetric terms. The results based on this single record show that: (1) The palaeofield at this locality during the Olduvai transition was characterized by contributions from both zonal and non-zonal terms but the contribution of non-zonal quadrupolar terms was predominant. This is in contrast to the observation by Herrero-Bervera et al. (1991), based on four deep-sea records in which the VGPs show general confinement within the longitude band of the Americas and their antipode, that the transitional field may be dominated by a dipolar component. (2) The field intensity dropped to about 22 per cent of its initial or final recovered value during the transition. It stayed at or close to this minimum for approximately half or less of the overall intensity change.

Palaeomagnetic study of the Holocene and Upper Pleistocene sediments from Lake Barombi Mbo, Cameroun: first results

The analyses of the stable (deposition and post-depositional) remanent magnetization carried by the lacustrine sediments contained in a 24 m sequence collected from the bottom of the maar lake, Barombi Mbo (Cameroun) yield well-defined declination and inclination oscillations which can be attributed to palaeosecular variations of the local geomagnetic field. The transformation to a time-scale, based on the interpolation and extrapolation of 14C ages, allows comparison of this record with contemporaneous records obtained in other parts of the world. The successive inclination peaks can be individually identified with those found in western Europe, in spite of small chronological discrepancies which are in the range of the standard deviation of the ages. Normalization of the NRMs by anhysteretic magnetization provides a profile of ‘relative palaeointensities’ which shows similarities with that reconstructed from the same time interval in a sequence from western Europe although an important phase shift appears.

Variations of the relative palaeointensity of the geomagnetic field in western Europe in the interval 25-10 kyr BP as deduced from analyses of lake sediments

Summary. Two independent methods have been applied on late glacial and glacial homogeneous lacustrine sediments in order to obtain an evaluation of the geomagnetic field palaeointensity in the interval 10 000-95 000 yr BP. The anhysteretic remanent magnetization (ARM) normalization method and the stirred remanent magnetization (StRM) normalization method yield relative palaeoiiitensity curves which can be critically compared. The St RM normalization curve, considered as the most reliable, shows successive oscillations with periods ranging from 1500 to 1 1 000 yr. The lowest values of the field intensity occur at 10000 and 31 000 yr BP and the highest value occurs at 15 000 yr BP.

10Be contents of Mono Lake sediments: search for enhancement during a geomagnetic excursion

Summary We report the 10Be and Be contents of 22 sediment samples of lacustrine silt from the Wilson Creek section of Mono Lake, California. The section spans about 20–35 kyr bp and contains a record of a geomagnetic excursion for which the relative palaeointensity is estimated from the ratio of natural remanent magnetization to anhysteretic remanent magnetization. The 10Be contents of the excursion samples are somewhat higher than the average of non-excursion samples (0.24 ± 0.02 versus 0.15 ± 0.02 dpm kg−1). It is clear however that geomagnetic effects are not the sole cause of the observed variations.

Behaviour of the earth's magnetic field as recorded in the sediment of Loch Lomond

A palaeomagnetic record of geomagnetic secular variation during the last 7000 years has been obtained from the sediments of Loch Lomond, Scotland. The magnetic direction fluctuations repeat well between cores and show greater detail, especially over the last 5000 years, than other European records. A time scale has been derived from 14C analyses on the Lomond sediment and comparison with other 14C-dated sediments. Investigation of relative palaeointensity determination methods has shown that the widely used normalization parameter of partial ARM is insensitive to even small sediment grain size fluctuations. The new high-fidelity direction record and improved time scale show that geomagnetic field changes have not followed a simple oscillatory pattern during the last 7000 years. The record enhances the application of palaeomagnetism to dating recent sediments, as the main declination swings are now characterized by fine detail, and paired inclination data are also available. The problem of mismatching swings when correlating with other paired directional records is thus reduced. The palaeomagnetic record agrees well with some archaeomagnetic results. It confirms the period of anticlockwise motion of the geomagnetic field vector, between 1000 and 600 years B.P., which was first documented by English archaeomagnetic investigations. Clockwise motion is shown to predominate during the remainder of the last 5500 years. The VGP path does not correlate with that of Japanese archaeomagnetic results nor North American sediment data from 2000 to 0 years B.P. This suggests that the secular changes are dominated by local non-dipole sources rather than wobbling of the main geomagnetic dipole.