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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 moment1,2 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.

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Figure 1: Various stacks obtained for the period 0.6–2 Myr ago.
Figure 2: Composite Sint-2000 record depicting the evolution of the field intensity during the past 2 Myr.
Figure 3: Time-averaged dipole moment calculated within sliding windows of different sizes, as a function of the number of reversals contained within each window.
Figure 4: Field intensity variations across the five reversals occurring during the past 2 Myr.


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This work was supported by the INSU-CNRS programme ‘Dynamique et Evolution de la Terre Interne’.

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Correspondence to Jean-Pierre Valet.

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Valet, JP., Meynadier, L. & Guyodo, Y. Geomagnetic dipole strength and reversal rate over the past two million years. Nature 435, 802–805 (2005).

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