The geomagnetic field during the Cretaceous Normal superchron from marine magnetic anomalies

Superchrons are remarkable geomagnetic field features during which the polarity remained stable for several tens of Myr. The latest interval to experience steady polarity, the Cretaceous Normal superchron (CNS, between ~121 and 83 Myr ago), is best expressed above the oceanic crust where sea surface magnetic anomalies lack a prominent stripe pattern. Here we show, using the first deep-tow magnetic profile encompassing the entire CNS, together with widely-distributed sea surface magnetic anomaly data, that the variability of the dipolar geomagnetic field increased at the beginning of the superchron, leading to a period of highly fluctuating field between 110 and 100 Myr ago. A transition back to a more stable field resulted in a subdued magnetic signal in the last 9 Myr of the superchron. This long-term pattern requires that the conditions at the core-mantle boundary have significantly varied during the superchron. Besides their geomagnetic implications, our results provide new time markers to re-evaluate seafloor-spreading history during the CNS, when important plate reorganizations took place and ultrahigh spreading rates have been speculated but not directly confirmed.