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Geomagnetic field variability during the Cretaceous Normal Superchron


Prolonged periods of stable polarity in the Earth’s magnetic field are termed superchrons. The most recent of these intervals, the Cretaceous Normal Superchron, lasted from approximately 121 to 83 million years ago1,2 and is most commonly observed in the lack of a prominent stripe pattern3 in the sea-surface magnetic anomaly above the oceanic crust formed during this period. The exact behaviour of the geomagnetic field during this interval, however, remains unclear, as palaeomagnetic data from igneous4,5,6 and sedimentary7,8 sections yield conflicting results. Here we report a deep-tow magnetic profile from the Central Atlantic Ocean, African flank, spanning the entire Cretaceous Normal Superchron. We suggest that this profile, along with widely distributed sea-surface magnetic anomaly data, records the rising variability of the dipolar geomagnetic field at the beginning of the interval, which culminates in a highly fluctuating field between 110 and 100 million years ago. We interpret the subdued magnetic signal in the last 9 million years of the superchron as the return to a more stable geomagnetic field. This variability allows us to define two internal time markers valuable for plate reconstructions. Based on the degree of variability observed, we conclude that geodynamo models that call for low field variability may provide an oversimplified view of superchrons.

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Figure 1: Location map and deep-tow magnetic results.
Figure 2: Comparison of inverted deep-tow profile and widely distributed sea-surface magnetic anomalies.
Figure 3: Comparison of geomagnetic field variability during the CNS.


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We thank the captains and crews of RV Le Suroît and the scientific parties for their dedication at sea on cruises Magofond 3 (2005) and Magofond 3b (2008). IPGP, CNRS-INSU, IFREMER and GENAVIR are gratefully acknowledged for their financial and technical support at various stages of the project. R.G. was supported by fellowships given by IPGP and the city of Paris. This is IPGP contribution 3255.

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J.D. and Y.G. designed and led data acquisition experiments. R.G. processed the data, interpreted them, and wrote the paper with contributions from both co-authors.

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Correspondence to Roi Granot.

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The authors declare no competing financial interests.

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Granot, R., Dyment, J. & Gallet, Y. Geomagnetic field variability during the Cretaceous Normal Superchron. Nature Geosci 5, 220–223 (2012).

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