Frequent Polarity Reversals in the Cretaceous Normal Superchron

Abstract

The Cretaceous Normal Superchron (CNS), defined as a magnetic quiet zone spanning from ∼126 to ∼84 Myr, is often used to understand the Earth’s geodynamo processes. However, whether there were polarity reversals within the CNS remains contentious. This paper presents the youngest detrital zircon U‐Pb age‐constrained magnetostratigraphic study of the Cretaceous Khorat Group for two sections (i.e., Ban Phonngam (BP) and Ban Nongpen (BN)) in Vientiane, Laos. Both sections are characterized by the prominent long normal‐polarity zones that are correlated to the CNS. Interestingly, six (four) short primary reversed horizons were identified within the BP (BN) section. Integrated with other reported reversed events, there likely at least five globally and two single reported reversed‐polarity events or clusters of events have happened within the CNS. The confirmation of frequent reversed polarities within the two parallel sections will provide a new comprehensive understanding of the Earth’s geodynamic behavior during the CNS.

The Cretaceous Normal Superchron: A Mini-Review of Its Discovery, Short Reversal Events, Paleointensity, Paleosecular Variations, Paleoenvironment, Volcanism, and Mechanism

“…The name of the ISEA event comes from the name of a section of the Umbria Apennines in northern Italy (VandenBerg et al, 1978). Furthermore, a recent study reports multiple geomagnetic reversals during the CNS (Zhang et al, 2021). Using paleomagnetic direction and U-Pb ages, they conclude that samples from two sections of Laos, and other magnetostratigraphy from previous studies indicate that there are at least five global and two single reversals during the CNS.…”

“…Considering them, perhaps we should FIGURE 1 | (A) Geomagnetic polarity time scale for 0-154.9 Ma based on Ogg (2020) and geomagnetic reversal frequency curve calculated using fixed kernel density estimation (Constable, 2000). Possible short geomagnetic reversal events during the CNS are shown as dashed white lines based on the summary of Zhang et al (2021). Note that the duration of the events are not reflected in this figure.…”

“…Constable (2000) calculated the reversal frequency from Harland et al (1990) and Cande and Kent (1995). Pink zone expresses a range of the geomagnetic reversal frequency during the CNS (Chron C34n) calculated from short geomagnetic reversal events during the Superchron suggested by Zhang et al (2021). Dotted gray lines express a time interval of the CNS.…”

Salient Changes of Earth's Magnetic Field Toward the End of Cretaceous Normal Superchron (CNS)

“…Over timescales of ∼10 5 –10 8 years, the geomagnetic field's intensity appears to display an inverse relationship with its polarity reversal frequency (Driscoll & Olson, 2009; Glatzmaier et al., 1999; Hawkins et al., 2021; Tarduno & Cottrell, 2005; Tauxe, 2006; Tauxe & Hartl, 1997; Valet et al., 2005), which may represent a fundamental property of the geodynamo (Smirnov et al., 2017). Although low field intensities during frequent reversals have been widely recognized (e.g., Biggin et al., 2012; McElhinny & Larson, 2003; Tauxe et al., 2013; Tominaga et al., 2021), the geomagnetic field behaviors during long intervals of nearly no polarity reversals, that is, superchrons, remain poorly characterized and understood (e.g., Brandt et al., 2021; Cottrell et al., 2008; Zhang et al., 2021). In particular, it is not well known as to whether geomagnetic field strength stayed persistently strong/weak or exhibited some form of variations during a superchron.…”

Paleomagnetic studies of Jurassic and Cretaceous rocks in the Eastern Qiangtang Terrane, Tibetan Plateau, China

“…However, upon further analysis, this scenario seems unlikely (see section 2.6.3). Furthermore, dual-polarity makes it not likely that the remagnetization has occurred during the Cretaceous Normal Superchron (CNS), even though a very recent study suggests several reverse-polarity events or clusters of events within the CNS (Zhang et al, 2021). Inclination matching and paleolatitude matching were used to determine the remagnetization age.…”