Laure Meynadier

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 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.

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.

Absolute paleointensity from Hawaiian lavas younger than 35 ka

Abstract Paleointensity studies have been conducted in air and in argon atmosphere on nine lava flows with radiocarbon ages distributed between 3.3 and 28.2 ka from the Mauna Loa volcano in the big island of Hawaii. Determinations of paleointensity obtained at eight sites depict the same overall pattern as the previous results for the same period in Hawaii, although the overall average field intensity appears to be lower. Since the present results were determined at higher temperatures than in the previous studies, this discrepancy raises questions regarding the selection of low versus high-temperature segments that are usually made for absolute paleointensity. The virtual dipole moments are similar to those displayed by the worldwide data set obtained from dated lava flows. When averaged within finite time intervals, the worldwide values match nicely the variations of the Sint-200 synthetic record of relative paleointensity and confirm the overall decrease of the dipole field intensity during most of this period. The convergence between the existing records at Hawaii and the rest of the world does not favour the presence of persistent strong non-dipole components beneath Hawaii for this period.

Relative paleointensity across the last geomagnetic reversal from sediments of the Atlantic, Indian and Pacific oceans

Paleointensity records from marine sediments at three locations in the Atlantic, Indian and Pacific oceans show coherent and reproducible signals across the Brunhes-Matuyama reversal. The low-field susceptibility signals are predominantly anticorrelated between the Indo-Pacific and the Atlantic sites while the normalization of the natural remanent magnetization by any rock magnetic parameter yield identical results. We deduce that in these cases climatic components do not induce first order effects in the determination of relative paleointensity. The results establish the worldwide character of the triangular pattern displayed by the decay and recovery phases of the field intensity variations across the Brunhes-Matuyama reversal.

Post-depositional realignment of magnetic grains and asymmetrical saw-tooth patterns of magnetization intensity

Abstract Recent paleomagnetic records suggest that the geomagnetic field intensity has a saw-tooth shape matching the succession of polarity intervals. An alternative hypothesis, that mechanisms linked to the acquisition of magnetization would induce similar saw-toothed records, has been tested by several simulations of post depositional reorientations of magnetic grains. Exponential functions used to date to describe post-depositional magnetization (pDRM) processes do not account for the combination of saw-toothed fluctuations and reasonable delays in the recording of the position of the reversals. At least half of the magnetization must be locked in within a few centimetres below the surface. If not, large delays, which are not observed in the data, are introduced in the stratigraphic positions of the reversals. In addition, the rest of the magnetization must be acquired over depths involving several tens of meters to duplicate a saw-toothed shape. These conclusions are reached with or without incorporating intensity variations across reversals. If the original signal is, in fact, asymmetrical then the pDRM must be very limited to remain coherent with the measurements, since the distortions and the offsets induced by the pDRM smoothing are considerably amplified. We conclude that simulations of saw-tooth patterns of relative paleointensity by pDRM processes have consequences that are difficult to reconcile with our present knowledge of the physical properties of deep-sea sediments. Above all, the hypothesis that the saw-tooth is an artifact of the magnetization acquisition process would have major implications for any sedimentary record of geomagnetic features.

Remagnetization in lava flows recording pretransitional directions

Basaltic lava flows associated with the last reversal in three distinct volcanic sequences of the island of La Palma are characterized by overlapping components with very high unblocking temperatures. A medium high-temperature (MHT) component with normal polarity is resistant up to 500–550°C, while in most cases the high-temperature (HT) primary reverse component cannot be isolated below 520°C. The same situation (with opposite polarities) is observed also in a lava flow associated with the onset of the upper Reunion reversal in the Gamarri sequence in Ethiopia. The presence of both polarities within all these single flows cannot be reconciled with the hypothesis that a full reversal was systematically recorded during their cooling. The direction of MHT is either close or similar to the direction of the HT component of the overlying flows, which indicates that remagnetization occurred shortly after emplacement. Rock magnetic studies, microscopic observations, microprobe analyses and remagnetization experiments have been carried out to investigate the origin of MHT. The rock magnetic parameters do not exhibit specific anomalies. The unblocking temperatures are mostly higher than the simulations involving reheating by the overlying flow combined with thermoviscous overprinting caused by slow cooling. The most plausible scenario is that baking by the overlying flows was accompanied by low-temperature oxidation of titanomagnetite to cation-deficient titanomagnetite. This description also includes the presence of cracks, fractures and other heterogeneities of the lava which induce variability between parallel profiles. Similarities with characteristics observed at Steens Mountain could suggest that the hypothesis of rapid geomagnetic changes recorded by a single lava flow should be considered with caution.

Saw‐toothed variations of relative paleointensity and cumulative viscous remanence: Testing the records and the model

A recent model [Kok and Tauxe, 1996a] that attempts to explain asymmetrical sawtooth patterns of relative paleointensity from sediments [Valet and Meynadier, 1993] as the consequence of cumulative long-term viscosity effects has been tested theoretically and experimentally. A mathematical description of the model demonstrates that only a sharp distribution of relaxation times would be able to match the paleomagnetic record, which implies that all the magnetic grains would be identical. This has direct consequences for the distribution of the blocking temperatures (Tb), which would not exceed 260°C, and for the coercivities of the natural remanent magnetization (NRM). This Tb spectrum derived from the model is inconsistent with the unblocking temperatures of the NRM and with the Tb spectrum of the experimental thermoremanent magnetization (TRM) of the sediment that recorded the sawtooth. New results including thermal demagnetization and Thellier-Thellier experiments across Brunhes-Matuyama confirm the overall stability of the original records. The stability of the older part of the record (2 to 4 Ma) has also been strengthed. The sawtooth is unchanged between 20 mT and 65 mT, and thermal demagnetization in a neutral atmosphere at 300°C and 450°C duplicates the results obtained by alternation field (a.f.) demagnetization. Thus viscous processes, at least within the framework of a “cumulative viscous model” and for the existing data sets, are not responsible for saw-toothed paleointensity records.

A comparison of different techniques for relative paleointensity

Parallel relative paleointensity records that are supposed to display the same geomagnetic variations often show discrepancies. We compared various techniques of normalization with different parameters involving alternating fields (a.f.) and thermal demagnetization of sediment samples spanning the same period from the Indian and the Pacific oceans. The results are basically unchanged using any technique. There is also no causal relationship between changes in rock magnetic properties and mismatch between the two records. Thus, discrepancies between parallel records do not depend on methods for relative paleointensity, and any technique can be used with confidence, provided that the characteristic component has been properly isolated and the sediments are magnetically homogeneous. Complications are more likely due to subtle variations in the sediment remanent magnetization acquisition. These results point to the importance of sedimentological (lithology and physical properties) factors in controling the process of magnetization.

Magnetic properties and origin of Upper Quaternary sediments in the Somali Basin, Indian Ocean

The magnetic concentrations and magnetic fluxes of marine sediments from the Somali Basin show positive correlations with insolation, using oxygen isotopes for calibration. Investigations of the rock magnetic properties indicate an increase in the magnetite/hematite ratio during warmer episodes and the presence of additional fine grains of magnetite. The magnetic susceptibility profiles of several other cores suggest that the same characteristics prevail over a large area within the basin. These features are opposite to the variations observed to the north of this area (DeMenocal et al., 1991) where eolian deposition is dominant. Magnetic measurements performed on the coarse and fine fractions of the sediment show that the magnetic signal is carried by the finer fraction. Analyses of the principal components of clay mineralogy show that river transported sediments are geographically very limited and confirm that there is no indication of significant eolian deposition. It is most likely that the Antarctic bottom currents were responsible for the transport of the magnetic particles and thus for the correlation between the magnetic and climatic records.