Earth's Magnetic Field Strength and the Cretaceous Normal Superchron: New Data From Costa Rica

Abstract

Constraining the long‐term variability and average of the Earth's magnetic field strength is fundamental to understanding the characteristics and behavior of the geomagnetic field. Questions remain about the strength of the average field, and the relationship between strength and reversal frequency, due to the dispersion of data from key time intervals. Here, we focus on the Cretaceous Normal Superchron (CNS; 121‐84 Ma), during which there were no reversals. We present new intensity results from 41 submarine basaltic glass (SBG) sites collected on the Nicoya Peninsula and Murcièlago Islands, Costa Rica. New and revised 40Ar/39Ar and biostratigraphic age constraints from previous studies indicate ages from 141 to 65 Ma. One site with an age of 135.1 ± 1.5 Ma (2σ) gave a reliable intensity result of 34 ± 8 µT (equivalent to a virtual axial dipole moment, VADM, value of 88 ± 20 ZAm2), three sites from 121 to 112 Ma, spanning the onset of the CNS, vary from 21 ± 1 to 34 ± 4 µT (53 ± 3 to 87 ± 10 ZAm2). These results from the CNS are all higher than the long‐term average of ∼42 ZAm2 and data from Suhongtu, Mongolia (46–53 ZAm2) and are similar to the Troodos Ophiolite, Cyprus (81 ZAm2, reinterpreted in this study). Together with the reinterpreted data, the new Costa Rica results suggest that the strength of the geomagnetic field was approximately the same both before and after the onset of the CNS. Therefore, the data do not support a strict correlation between polarity interval length and the strength of the magnetic field.

High‐Latitude Paleointensities During the Cretaceous Normal Superchron From the Okhotsk–Chukotka Volcanic Belt

“…Using strict selection criteria combined with a careful analysis of the remanence carriers, we determined two robust paleointensity estimates at the end of the CNS before analyzing the distribution of geomagnetic dipole moments during the CNS. Based on an updated version of the API database, we emphasize that determinations based on nonglassy whole rocks, submarine basaltic glasses, and single crystals are mutually incompatible, in accord with previous observations (e.g., Kulakov et al., 2019) suggesting a separate analysis of their distributions; dipole strength estimates over the Cretaceous time window (145–66 Ma) are statistically indistinguishable before and after the onset of the CNS, in accord with recent studies (e.g., Di Chiara et al., 2021) refuting a strict correlation between chron duration and dipole moment; and the distribution of dipole moments during the CNS may be bimodal, in accord with previous observations (e.g., Shcherbakova et al., 2012) suggesting possible distinct geomagnetic behavior during the CNS (e.g., Lhuillier et al., 2016). …”

“…dipole strength estimates over the Cretaceous time window (145–66 Ma) are statistically indistinguishable before and after the onset of the CNS, in accord with recent studies (e.g., Di Chiara et al., 2021) refuting a strict correlation between chron duration and dipole moment; and…”

“…(2020) and Di Chiara et al. (2021). This subset presently consists of 526 average estimates of dipole strength, predominantly determined using double‐heating paleointensity protocols (330 for Thellier‐style experiments, 13 for Wilson‐style experiments) or less frequently using Shaw‐style (102) or multispecimen (35) protocols.…”

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

“…I summarize previous paleointensities during the CNS of submarine basaltic glass (SBG) (Pick and Tauxe, 1993;Selkin and Tauxe, 2000;Riisager et al, 2003;Tauxe and Staudigel, 2004;Tauxe, 2006;Di Chiara et al, 2021), subaerial volcanic whole rocks (Thomas et al, 2000;Riisager et al, 2001;Tanaka and Kono, 2002;Zhu et al, 2002;Zhu et al, 2004aZhu et al, , 2004bZhao et al, 2004;Shi et al, 2005;Shcherbakova et al, 2007;Shcherbakova et al, 2008;Zhu et al, 2008;Qin et al, 2011;Shcherbakova et al, 2011;Shcherbakova et al, 2012), baked contact (Shcherbakova et al, 2008;Shcherbakova et al, 2009), and gabbro (Granot et al, 2007). Virtual (axial) dipole moments (V(A)DMs) in the CNS have been reported from 1.1 to 19.9 × 10 22 Am 2 (Figure 1B).…”

“…The CNS, Cretaceous Normal Superchron. (B) Virtual (axial) dipole moments during the CNS (Cottrell and Tarduno, 2000;Pick and Tauxe, 1993;Juarez et al, 1998;Thomas et al, 2000;Riisager et al, 2001;Tarduno et al, 2001;Tanaka and Kono, 2002;Tarduno et al, 2002;Zhu et al, 2002;Riisager et al, 2003;Tauxe and Staudigel, 2004;Zhu et al, 2004a;Zhu et al, 2004b;Zhao et al, 2004;Shi et al, 2005;Tauxe, 2006;Granot et al, 2007;Shcherbakova et al, 2007;Shcherbakova et al, 2008;Zhu et al, 2008;Shcherbakova et al, 2009;Tsunakawa et al, 2009;Qin et al, 2011;Shcherbakova et al, 2011;Shcherbakova et al, 2012;Di Chiara et al, 2021). The long-term median value and the medians of 5 Myr bins for the last 200 Myr by Tauxe et al (2013) and Tauxe and Yamazaki (2015) are displayed by the solid blue line and the blue stars, respectively.…”