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 ago(1,2) and is most commonly observed in the lack of a prominent stripe pattern(3) 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 palaeo-magnetic data from igneous(4-6) and sedimentary(7,8) 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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