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

Abstract

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.

Geomagnetic field variations between chrons 33r and 19r (83–41 Ma) from sea-surface magnetic anomaly profiles

“…This behavior contrast with the long-term "memory" of the dipole field considered by Cande [41] to account for the anomalous skewness of marine magnetic anomalies, and by Valet and Meynadier [8] to account for the "sawtooth pattern" observed in the paleointensity fluctuations over the past 4 Myr. But it is not in contradiction with the recent update by Valet et al [9], which now argues in favour of a slow decrease in paleointensity only during a period of about 60-80 kyr before reversals.…”

“…Useful insight on the origin of tiny wiggles can however be gained from the few available highresolution magnetostratigraphic paleointensity studies covering the past few Myr (see for instance, [25] for the Brunhes period; [26] for the Matuyama period; [27] for the Matuyama and late Gauss periods; [9] for the past 2 Myr; [8] for the past 4 Myr; [28] for chrons 5n; [29] for chrons 6Bn to 13n; [30] for chrons 12r to 13r). These studies have shown that the magnetic polarity time scale mainly determined from marine magnetic anomalies is almost complete and that only a few tiny wiggles detected so far can be ascribed to additional short polarity intervals (e.g.…”

“…The correlation of these microanomalies, or "tiny wiggles", among distant oceanic basins demonstrate that most of them are related to paleo-variations of the Earth's magnetic field and not to local magnetic sources in the oceanic crust [7,6]. Although often modeled as short magnetic polarity intervals (the so-called "cryptochrons" of Cande and Kent [7,6]), they may in fact be due to strong geomagnetic field intensity variations such as those occurring during excursions [8][9][10]. Up to now, the detection of tiny wiggles has been performed over only a limited number of short intervals and has often been based on favorable profiles from one ocean basin, occasionally complemented by a limited set of profiles from other oceans [3,6].…”

Chaotic Dynamos Generated by a Turbulent Flow of Liquid Sodium

“…We observe that the regimes with given polarity involve an amplitude of the azimuthal field in the range 50-100 G. The amplitude very slowly decays before each reversal and displays a strong overshoot immediately after. Similar features have been reported in some palaeomagnetic recordings of the Earth's magnetic field [10]. Another open question in palaeomagnetism concerns the variation of the mean frequency of reversals over the geological ages.…”

Evidence for the Blake Event recorded at the Eemian archaeological site of Caours, France

“…Jicha et al, 2011). The entire period between~120 ka and~100 ka is characterized by a low intensity of the Earth's magnetic field (Valet et al, 2005) which would make the chance that excursions or even reversals of the field occur higher than for other periods. However, changes in palaeomagnetic directions during 100e120 ka are described only at the older age range (Blake Event) and, occasionally, at the younger age range (post-Blake).…”