Is the Geodynamo Characterized by a Distinct Geomagnetic Secular Variation Regime During the Cretaceous Normal Superchron?

Abstract

The Earth's magnetic field stochastically reversed its polarity over geological time, yet enigmatically interrupted this process during the Cretaceous Normal Superchron (CNS; 84–121 Ma). Two scenarios have been proposed in terms of either gradual or abrupt changes to describe the transition of the geodynamo from the CNS to a period with frequent polarity reversals. To discriminate between these two scenarios, we sampled 136 volcanic flows emplaced 66–73 Ma in the eastern part of the Okhotsk-Chukotka Volcanic Belt (NE Eurasia). The 1,700 collected paleomagnetic samples provide, after correction for serial correlation, a robust data set of 112 independent paleodirections characterizing the behavior of the geomagnetic field at a paleolatitude of $\sim 73°$N shortly after the end of the CNS. Compared to paleomagnetic data from Chukotka and Northern Canada emplaced at similarly high paleolatitude, the dispersion $S_B$ of the virtual geomagnetic poles, used as a proxy for paleosecular variation (PSV), is 19% (resp. 55%) higher at 66–73 Ma than at the end of CNS when transitional directions are discarded (resp. retained). In contrast, our $S_B$ value at 66–73 Ma is, whatever the filtering strategy, statistically indistinguishable from the $S_B$ value at 0–10 Ma derived from the Antarctica and Spitsbergen volcanics. From the viewpoint of PSV observed at high paleolatitude (70–80$°$), our new results are consistent with an abrupt change between two geodynamo regimes. Nevertheless, once the geodynamo starts reversing its polarity, the vigor of PSV does not seem to correlate with the reversal rate.