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.
|Original language||English GB|
|Journal||Geophysical Research Abstracts|
|State||Published - 2011|