Latitude Dependence of Geomagnetic Paleosecular Variation and its Relation to the Frequency of Magnetic Reversals: Observations From the Cretaceous and Jurassic.

Nearly three decades ago paleomagnetists suggested that there existed a clear link between latitude dependence of geomagnetic paleosecular variation (PSV) and reversal frequency. Here we compare the latitude behavior of PSV for the Cretaceous Normal Superchron (CNS, 84–126 Ma, stable normal polarity) and the preceding Early Cretaceous‐Jurassic interval (pre‐CNS, 126–198 Ma, average reversal rate of ~4.6 Myr−1). We find that the CNS was characterized by a strong increase in the angular dispersion of virtual geomagnetic poles (VGPs) with latitude, which is consistent with the results of earlier studies, whereas the VGP dispersion for the pre‐CNS period was nearly invariant with latitude. However, the PSV behavior for the last 5 or 10 million years (average reversal frequency of ~4.4–4.8 Myr−1) shows that the latitude invariance of VGP scatter cannot be considered as a characteristic feature of a frequently reversing field and that a strong increase in VGP dispersion with latitude was not restricted to the long periods of stable polarity. We discuss models describing the latitude dependence of PSV and show that their parameters are not reliable proxies for reversal frequency and should not be used to make inferences about the geomagnetic field stability. During the pre‐CNS interval, the geodynamo may have operated in a regime characterized by a high degree of equatorial symmetry. In contrast, more asymmetric geodynamos suggested for 0–10 Ma and the CNS were evidently capable of producing a very wide range of reversal frequencies. Plain Language Summary: In the geologic past, the changes in the Earth's magnetic field have led to numerous polarity reversals, causing the field directions over the entire Earth to be opposite to those observed today. More subtle changes during periods of stable field polarity are referred to as "secular variation." It is widely thought that the manner in which secular variation changes with geographic latitude provides an indirect way of assessing the field stability with regard to its propensity to reverse. Here we derived estimates of paleosecular variation (PSV) for a long interval of stable polarity in the Cretaceous (84–126 Ma) and the preceding Early Cretaceous‐Jurassic interval (126–198 Ma), during which geomagnetic reversals were frequent. We found that the latitude behavior of PSV during these two intervals was significantly different, but the comparison with PSV estimates for the last 5 and 10 million years showed that a strong latitude dependence of PSV, or its invariance with latitude, cannot be considered as a characteristic feature of a stable or frequently reversing field. Our analysis suggests that models describing the latitude dependence of PSV do not provide reliable proxies for reversal frequency and should not be used to make inferences about the geomagnetic field stability. Key Points: Estimates of geomagnetic paleosecular variation (PSV) in Cretaceous and Jurassic time were derived using a new paleomagnetic databaseDifferent PSV latitude patterns were observed for periods of stable geomagnetic polarity (84–126 Ma) and frequent reversals (126–198 Ma)Comparison with the PSV behavior for the last 10 Ma indicates that its latitude dependence is not a reliable proxy for reversal frequency [ABSTRACT FROM AUTHOR]