Jean-Pierre Valet

Global changes in intensity of the Earth's magnetic field during thepast 800|[thinsp]|kyr

Recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the Earths past magnetic field at numerous field sites. The compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the Earths geodynamo itself. A previous compilation for the past 200 kyr integrated 17 marine records into a composite curve, with the geomagnetic origin of the signal supported by an independent analysis of 10Be production made on different cores. The persistence of long-term features in the Earths magnetic intensity or the existence of long-term periodic changes cannot, however, be resolved in this relatively short time span. Here we present the integration of 33 records of relative palaeointensity into a composite curve spanning the past 800 kyr. We find that the intensity of the Earths dipole field has experienced large-amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction, reflecting the emergence of non-dipole components. No stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the Earths orbital parameters have a direct and strong influence on the geodynamo.

Geomagnetic dipole strength and reversal rate over the past two million years

Independent records of relative magnetic palaeointensity from sediment cores in different areas of the world can be stacked together to extract the evolution of the geomagnetic dipole moment and thus provide information regarding the processes governing the geodynamo. So far, this procedure has been limited to the past 800,000 years (800 kyr; ref. 3), which does not include any geomagnetic reversals. Here we present a composite curve that shows the evolution of the dipole moment during the past two million years. This reconstruction is in good agreement with the absolute dipole moments derived from volcanic lavas, which were used for calibration. We show that, at least during this period, the time-averaged field was higher during periods without reversals but the amplitude of the short-term oscillations remained the same. As a consequence, few intervals of very low intensity, and thus fewer instabilities, are expected during periods with a strong average dipole moment, whereas more excursions and reversals are expected during periods of weak field intensity. We also observe that the axial dipole begins to decay 60–80 kyr before reversals, but rebuilds itself in the opposite direction in only a few thousand years.

Relative variations in geomagnetic intensity from sedimentary records: the past 200,000 years

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Paleointensity of the geomagnetic field during the last 80,000 years

High-resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores. Three duplicate records were used to estimate the regional coherency of the data within a single area (Tyrrhrenian Sea) while the two others document the field variations in the eastern Mediterranean and the southern Indian Ocean. Careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes. The time-depth control was provided by oxygen isotopes, and small-scale variations in the deposition rates were constrained by means of tephrachronology. The synthetic curve calculated from the Mediterranean records provides a continuous record of the intensity variations during the last 80,000 years (80 kyr), which correlates well with the sparse volcanic data available for the period 0–40 kyr. The fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the Pacific Ocean establishes the truly dipolar character of these variations. The dipole field moment is characterized by large-scale changes as shown by the existence of pronounced drops (at 39 and 60 kyr) alternating with periods of higher intensity. The record suggests a periodic nature for these intensity variations; however, the period studied is not sufficiently long to state this conclusively. These results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time.

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 δ18O 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.

High-resolution record of the Upper Olduvai transition from Po Valley (Italy) sediments: support for dipolar transition geometry?

Abstract A detailed record of the Upper Olduvai polarity transition, composed of > 100 transitional directions, has been obtained from the Crostolo section in northern Italy. A careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques, thermomagnetic, scanning electron microscopy, microprobe, X-ray diffraction and Mossbauer analysis, shows that greigite is the main magnetic carrier of the remanence. The correlation of a shift in the magnetic record to a small-scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition, or very shortly after. The virtual geomagnetic pole (VGP) path is largely confined along a great circle over North and South America ∼ 90° west of the site and consists of three stages: first, the VGP moves to southern mid-latitudes, then, after a period of standstill, it comes back to almost true north, and, finally, the south pole is reached in a third step. Although different from a record from the southern Indian Ocean, this path is virtually identical to those obtained for the same transition from North Pacific deep-sea cores, and partly coincides with a North Atlantic record, which suggests that a dipolar component may be present in the transitional field during the Upper Olduvai reversal. A review of the recently obtained records of various transitions shows that in more than two-thirds of the cases the VGP paths are similarly confined along a meridian over the Americas or antipodal to them, irrespective of the sampling site and of the sense of the transition. Although not deterministic, this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages.

Long‐term geometry of the geomagnetic field for the last five million years: An updated secular variation database

Lee [1983] assembled more than two thousand paleomagnetic directional data from lava flows in 65 sampling sites with ages spanning the last 5 million years. Constable [ 1992] recently suggested that the virtual geomagnetic poles (VGP) derived from this data base have been preferentially located within two antipodal bands of longitude. We have reanalyzed and updated the Lee data base, yielding a total of 3179 data from 86 distinct sites. Neither the total data set, nor various subsets of the data unambiguously show any large and clear maxima in the longitude distribution of VGPs. The shape of the common site longitude distribution is far more striking with a pronounced (and robust) minimum close to the common site longitude and secondary maxima about 120 o away from it. The former (first order) feature is equivalent to the far-sided effect discovered by Wilson [1970] and corresponds to a persistent axial quadrupole amounting to about 5% of the axial dipole, as previously found by several authors. The latter (second order) feature could correspond to the VGP biasing effect described by Egbert [1992] although its observed amplitude appears larger than predicted. Part of this effect (an asymmetry in the maxima) may also correspond to Wilsons [1971] right-handed effect. Given only a small axial quadrupolar component and the very uneven actual site distribution, the expected longitude distribution of VGPs can be calculated: this is found to be in good agreement with the paleomagnetic observations (i.e. rather flat with a moderate maximum near 120oE). This study emphasizes problems related to the unsatisfactory distribution of sites. The amplitude and significance of other terms, beyond the dominant axial dipole and significant axial quadrupole contribution, remain to be assessed.

The Blake geomagnetic event: transition geometry, dynamical characteristics and geomagnetic significance.

Abstract Two records of the Blake geomagnetic event have been obtained from marine cores in the Mediterranean. The upper bound for duration of the event, calculated from accurate oxygen isotope stratigraphy and/or tephrochronology, is ∼ 4000 years. One record is very detailed with 70 transitional directions and allows some characteristics of the event to be precisely retrieved. The directional changes do not occur at constant speed, rather a stop-and-go behaviour is observed as in detailed records of reversals. The virtual geomagnetic pole (VGP) path, confined to two bands of longitude over the Americas and antipodal to them, is virtually identical to the only other path of this event obtained from lacustrine sediments in the western USA, suggesting that a dipolar component may be present in the transitional field during the Blake event. The two bands of longitude coincide with those of many recent reversals, suggesting that the physical processes leading to reversals or events are basically the same and that the same mechanism is statistically observed over long periods of time.

Asymmetrical saw-tooth pattern of the geomagnetic field intensity from equatorial sediments in the Pacific and Indian Oceans

Abstract A record of relative paleointensity from marine sediments in the equatorial Indian Ocean spanning the last 4 Ma completes a previous dataset from Ocean Drilling Program (ODP) Leg 138 sites in the equatorial Pacific [1]. The timescale was defined with a precision better than 20 kyr with different and independent methods. Most of the features present in the record from the equatorial Pacific are confirmed: the overall saw-tooth pattern is observed across every field reversal and the short-term fluctuations superimposed on the slow intensity decrease preceding the reversals appear to be similar at both sites. The global character of these features reinforces their interpretation in terms of changes in dipole field intensity and provides new important constraints on models of the geodynamo. A synthetic curve of field intensity changes during the last 4 Ma is proposed as a first paleointensity timescale for future stratigraphic studies.

Simple Mechanism for Reversals of Earth's Magnetic Field

We show that a model, recently used to describe all the dynamical regimes of the magnetic field generated by the dynamo effect in the von Kármán sodium experiment, also provides a simple explanation of the reversals of Earths magnetic field, despite strong differences between both systems. The validity of the model relies on the smallness of the magnetic Prandtl number.